Depth profiling of low energy ions implanted into metals using the field ion microscope/imaging atom probe

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Depth profiling of low energy ions implanted into metals using the field ion microscope/imaging atom probe
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xi, 314 leaves : ill. ; 28 cm.
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Walck, Scott Douglas, 1954-
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Field ion microscopes   ( lcsh )
Ion implantation   ( lcsh )
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Thesis:
Thesis (Ph. D.)--University of Florida, 1986.
Bibliography:
Includes bibliographical references (leaves 307-312).
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Scott Douglas Walck.
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Typescript.
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Vita.

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DEPTH PROFILING OF LOW ENERGY IONS IMPLANTED INTO METALS
USING THE FIELD ION MICROSCOPE/IMAGING ATOM PROBE










By


SCOTT DOUGLAS WALCK


A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL
OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF
THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY




UNIVERSITY OF FLORIDA


1986
































To my wife and son, Karen and Adam Walck,

the "frog",

and my mother, Drucilla Linn















ACKNOWLEDGEMENTS


The author wishes to thank the members of his supervisory committee:

Dr. Christopher D. Batich, Dr. Robert T. DeHoff, Dr. Paul H. Holloway,

and Dr. Gary G. Ihas. This author is extremely grateful to his advisor

and friend, Dr. John J. Hren, who furnished an atmosphere in which the

author was allowed to find his own way and discover his full potential.

There are several members of the field-ion community without whose

help and assistance the FIM/IAP at the University of Florida could not

have been built. The author is deeply indebted to Dr. John A. Panitz of

Sandia National Laboratories, whose generosity in terms of advice, aid,

and criticisms to a struggling graduate student is nothing short of

remarkable. The author also wishes to thank Dr. Gary G. Kellogg, also of

Sandia Laboratories, for his advice and many stimulating discussions.

Many thanks go to Gerry Fowler and Larry Karkeweitz, who shared with the

author the knowledge of the details of the FIM/IAP instrumentation.

Encouragement from Dr. Sidney S. Brenner of the University of Pittsburgh

and the learning experience that he jammed into three months is greatly

appreciated. Dr. Michael K. Miller of Oak Ridge National Laboratories is

also thanked for his technical and scientific discussions.

Many technical and support groups at the University of Florida

should also be thanked. Dr. Anthony Buonoquisti is thanked for the many

fruitful discussions and scholarly advice he has given over the years.









Machinists Ralph Jones and Herman Bailor were indispensable with respect

to designing and building the FIM/IAP. Many thanks go to Robert Owen

whose late night discussions and help with electronics made impossible

tasks a little easier. Mr. Don Sanford in the Department of Physics is

thanked for his help in designing the new FIM/IAP specimen holder and

TIG-welding the copper braid. The author wishes to extend thanks for the

technical assistance given to him by Dan Steadham and Temel Buyuklimanli.

He also expresses his gratitude to his fellow students who shared the

same boat and helped to infuse into this degree some of the philosophy

that goes with it. The author would like to thank the local chapters of

the American Vacuum Society in Florida and New Mexico for the scholar-

ships they have given him to attend meetings and short courses which were

invaluable to him with respect to his research progress. Special thanks

go to Norman Parsons of the Florida Chapter for his efforts to help the

students

The author also would like to thank Tektronix employees: Dave

Jackson, Bob Messer, John Loggee, and Richard Knapp for solving and

helping to get the Tektronix equipment on-line as an integral part of the

Imaging Atom Probe. They were invaluable in solving both software and

hardware problems. These gentlemen not only stand behind their product,

but represent the Tektronix motto, "Committed to Excellence."

The author wishes to thank the Department of Energy for funding of

this effort. Gratitude is also extended to the University of Florida

College of Engineering for the SEED money to start the project, the Major

Analytical Instrumentation Center (MAIC) for technical and instrumental

support, and the Department of Materials Science and Engineering for









supporting the author on the Center of Excellence program for an extended

period of time.


















TABLE OF CONTENTS


ACKNOWLEDGEMENTS . . . .


ABSTRACT . . . . .




CHAPTER I
LITERATURE SURVEY . .


Introduction . . . .
FIM Principles . . .
Field Ionization . . .
Field Evaporation . .
Atom Probery . . .
Applications . . .
Field-Induced Stresses . .
Microstructure Of Implanted Metals .
Hydrogen In Metals . . .
Hydrogen Embrittlement . .
Trapping Of Deuterium . .
Nitrogen Implantation In Metals .
Tribological Studies . .
Ion Beam Carburization . .
Applicability Of FIM Techniques To


. . . .
. . ..
. . ..
. . ..
. . ..
. . ..
. . . .
. . . .
. . . .
. . . .
. . . .
. . . .
. . . .

Nitrogen In Metals


CHAPTER II
INSTRUMENTATION . . . . .

Field Ion Microscope/Imaging Atom Probe (FIM/IAP)
FIM/IAP Description . . . .
Chamber . . . . .
Sample Holder . . . .
General Description Of The IAP . .
TOF Mode . . . . .
Time-Gated Mode . . . .
Computer And IEEE-488 Instrumentation .
7912AD . . . . .
Scientific Instruments 5500 . .
Bertan 205A-30R . . . .









IAP Data Collecting And Handling Programs . . 55
Ion Gun . . . . . 58
Mass Identification in the Colutron Ion Gun . . 65
Transmission Electron Microscopy . . . 67
FIM/IAP "Bulk" Holder . . . . 68
FIM/TEM "Double-Tilt" Holder . . . 70
FIM/TEM "Field-Effect" Holder . . . 72



CHAPTER III
EXPERIMENTAL CONSIDERATIONS . . . . . 79

Depth Profiling Using The FIM/IAP . . . 79
Ring Counting Experiments . . . . 80
Specimen Geometry . . . . . 82
IAP probed-volume considerations . . 84
Volume calculation . . . . 85
Determination of 0 . . . 89
Determination of KD for nickel . . 91
Experimental procedure . . . 92
Determination of geometric parameters . . 101
Calculated curve . . . . .. 105
Sample selection criteria . . . 105
Effects Of Field Ionization Imaging And Evaporation . 112
Field Induced Stresses . . . . 112
Field Evaporation Observations ... . .... 118
Dependency of evaporation field on imaging gas 118
Implantation lowering of evaporation field . 119
Low field desorbed species . . . 120
He signal . . . . . .. 122
Special Problems Associated With Hydrogen/Deuterium . 123
The Deuterium Background (Di) 124
Deuterium Implantation Conditions . . . 129



CHAPTER IV
RESULTS OF IMPLANTATION EXPERIMENTS . . . . 132

Specimen Preparation . .. . . . 132
Nitrogen Implantation In Nickel . . . . 134
Nitrogen Implantation In Tungsten . . . 148
Deuterium Implantation In Nickel . . . 159
Carbon In Tungsten And Nickel . . . . 167









CHAPTER V
DISCUSSION OF RESULTS . . . . . 172

Instrumentation . . . . . . 173
FIM/IAP . . . . . . 173
FIM/TEM Specimen Holders . . . . 175
Methods/Techniques . . . . . 175
Discussion Of Experimental Results . . . 177
Control Experiments . . . . 177
Field effects . . . . . 178
Ring counting experiments . . . 179
Automated data collection . . . 179
Background gases . . . . 180
Deuterium background . . . . 181
Nitrogen Implantation In Nickel . . . 183
Nitrogen Implantation In Tungsten . . . 186
Applicability Of The TRIMML Code To Low Energy Ions 189
Reproducibility Of Profile Results . . . 189
Curve shapes and total ions detected . . 189
Error analysis of geometric model . . 191
Deuterium Implantation In Nickel . . ... 192
Carbon In Tungsten And Nickel . . . ... 194




CHAPTER VI
CONCLUSIONS . . . . . . . 196

Depth Profiling Techniques . . . . 197
Field-Induced Stresses . . . . . 198
Nitrogen Implantation . . . . . 200
Nitrogen In Nickel . . . . . 200
Nitrogen In Tungsten . . . . 200
In Situ Implantation Results . . . . 201
Deuterium Embrittlement Of Nickel . . . 202
FIM/IAP Observations . . . . . 202




CHAPTER VII
SUGGESTED FUTURE WORK . . . . . 204

Nitrogen In Nickel . . . . . 204
Nitrogen In Tungsten . . . . . 205
Carbon In Metals . . . . . 205
Deuterium In Metals . . . . . 206
Deuterium Implanted Nickel Emitter Failure . . 206
Deuterium Redistribution . . . . 208
Cryogenic/room temperature experiments . 208
Temperature-controlled experiments . . 209


viii









Field-Induced Stresses . . . . . 210
Implantation Depths . . . . . 211
Orthogonal Axis Implantation . . . . 212
Implantation Lowering Of Evaporation Field . . 213




APPENDIX A
IMPROVED FIM SPECIMEN HOLDER WITH TEMPERATURE CONTROL . 215



APPENDIX B
ALIGNMENT OF THE COLUTRON ION GUN . . . . 218



APPENDIX C
MEASURED TIME DELAYS . . . . . . 222



APPENDIX D
FIM/IAP TEK BASIC PROGRAMS . . . . . 225

IAPRING.HV . . . . . . 225
IAP2TAPE . . . . . . 246
PEAKPICKOFF.FAST . . . . . 268
PEAKCUMULATOR . . . . . . 274
DATAREDUCER . . . . . . 275
CUMVSRING . . . . . . 280
FIMSUPPLY . . . . . . 286
TIPREDUCER . . . . . . 291
PROFILER . . . . . . 296



APPENDIX E
INDEXED FIM MICROGRAPH MONTAGE . . . . 305



RFERENCES . . . . . . . 307


BIOGRAPHICAL SKETCH . . . . .


. 313














Abstract of Dissertation Presented to the Graduate School
of the University of Florida in Partial Fulfillment of
the Requirements for the Degree of Doctor of Philosophy



DEPTH PROFILING OF LOW ENERGY IONS IMPLANTED INTO METALS
USING THE FIELD ION MICROSCOPE/IMAGING ATOM PROBE


By


Scott Douglas Walck

August, 1986



Chairman: John J. Hren
Major Department: Materials Science and Engineering


The implantation depth is an important parameter if properties are

to be modified in a controllable and reproducible fashion by ion

implantation. The Field Ion Microscope/Imaging Atom Probe (FIM/IAP) is

superb for depth profiling, having depth resolutions on the order of a

crystal lattice spacing of the metal under investigation. This study

involves the application of the FIM/IAP in obtaining depth profiles of

low energy ions (<20 keV) implanted into metals.

Two technologically important ion species were used, nitrogen and

deuterium. It was shown that the deuterium partial pressure was not a

problem when a properly differentially pumped ion gun was used. Specific

problems of applying FIM/IAP to these species were reported. The results

of nitrogen implanted into nickel showed that the nitrogen resists field









evaporation even though the nickel was removed. Range and straggling

values for nitrogen implanted into tungsten with energies of 2.5-9 keV

were found and compared to Monte Carlo computer calculations.

Two special Transmission Electron Microscope (TEM) sample holders

were developed for use with FIM specimens. Using these holders, it was

shown that there was little or no influence of the high electric field on

the implanted defect microstructure of the specimen. However, nickel was

seen to be embrittled by deuterium implantation, but not by helium or

nitrogen.

The use of TEM was critically important in obtaining the geometric

parameters used to determine a depth scale from implanted emitters

profiled under ultra-high vacuum conditions. The volume probed was

related to the cumulative matrix signal. The geometric parameters were

used to obtain the proportionality constant between the cumulative matrix

signal and the probed volume (a function of depth).

Several critical observations for proper interpretation of FIM/IAP

results were reported. Foremost of these was the lowered evaporation

field of implanted emitters. This study established a methodology for

applying the FIM/IAP to the depth profiling of ion implanted species into

metals.
















CHAPTER I
LITERATURE SURVEY


Introduction

In the field of materials science, there is a wide range of interest

in the use of ion implantation. This interest extends from basic and

applied research to commercial applications. Ion implantation has been

used extensively in semiconductor processing for a number of years.

Recently ion implantation is proving to be highly beneficial in the

modification of surface properties of certain alloys. Reduced friction

and wear and/or increased corrosion resistance have been reported in tool

steels, cemented carbides, and titanium-based surgical prosthesis alloys.

Radiation damage in alloys caused by the hostile environments of nuclear

reactors has long been simulated by ion implantation. Ion implantation

can cause amorphization of surface regions and impart special properties

or created supersaturated concentrations of insoluble species not

achievable by conventional methods. In fact, it can provide an almost

limitless modification of surface composition, allowing the formation of

phases and compositions without regard to thermodynamic stability.

The implantation depth is an extremely important parameter when ion

beams interact with metals. Very little data are available for the

ranges of light, low energy (<40 keV) ions into metals; conventional

analytical techniques such as secondary ion mass spectrometry (SIMS),








2

nuclear reaction analysis (NRA) and Rutherford backscattering (RBS)

suffer from a limited spatial resolution for the shallow depths.

Although it is not widely recognized outside the FIM community, the atom

probe and imaging atom probe are superb for compositional depth

profiling. With an imaging gas present, depth resolutions of one atomic

plane are expected. The imaging atom probe (IAP) has several advantages

over the conventional atom probe (AP) for profiling: 1) it offers a

larger area of analysis (approximately 102-103 larger), 2) crystallo-

graphic effects are minimized, 3) a complete mass spectrum is collected

with each pulse, and 4) all atoms within the probed volume will strike

the detector. However, there are two disadvantages in using the IAP.

The IAP employs an extremely short flight distance (z.15m) as compared to

the AP (=lm) and mass resolution suffers accordingly. Because of this,

material systems must be chosen such that the mass-to-charge ratios are

sufficiently resolved temporally. The other disadvantage is the

difficulty in the determination of ring collapses under ultra high vacuum

(UHV) conditions (i.e. no imaging gas present). In the AP an increase in

signal occurs when the edge of a ring sweeps by the probe hole [1,2].

(See below for a discussion of the AP.) This periodic increase

corresponds to the collapse of a ring and can be used to assign the depth

scale. In the IAP, distinguishing between an increase in signal and an

increase due to a voltage increase is difficult to do because the signal

from a single set of planes is small compared to that coming from the

entire probed surface. This particular disadvantage will be discussed in

detail and a solution put forth.








3

This study involves the application of the IAP toward the depth

profiling of nitrogen and deuterium into field emitters. Specific

instrumental problems will be addressed. These include those attributed

directly to implantation into field emitters and those associated

specifically with the implanted species. Deuterium and nitrogen are the

primary implantation ions used in this study. The study of the

interaction of hydrogen and its isotopes to defects in metals may well be

one of the most important problems in materials science. A full

understanding of the trapping of hydrogen at defects is essential for the

operation of future fusion reactors. Essentially all of the energy

sources of tomorrow will have hydrogen interacting in some way with the

materials system. Nitrogen is now playing a major role in the surface

modification of commercial alloys. Nitrogen, sometimes in conjunction

with other elements such as boron and titanium, is the most widely used

element for modifying surface properties by ion implantation. The

advantages of ion implantation over conventional methods of surface

treatment include relaxed heat treatment requirements, stability of final

shape, controlled composition with depth, and the possibility of creating

beneficial metastable phases.

Although the effects that deuterium has on materials may be

deleterious and those of nitrogen may be beneficial, finding the range

for low energies of these elements is a common problem. Each has

particular instrumental difficulties which arise when the IAP is used to

analyze for them. These difficulties will be discussed and solutions

presented for overcoming them. A knowledge of the complexities for

implanted nitrogen and deuterium in applying the IAP to studying them








4

will be invaluable for its general use to other types of ions. Many of

the problems for these interstitial-type species (i.e. nitrogen and

deuterium) will not be present for substitutional-type elements, but it

is essential that the experimentalist be aware of the potential problems

when a new system is first investigated.

The rest of this chapter will be divided into four main sections: 1)

a brief introduction to FIM principles, techniques, and terminology,2)

the microstructure of implanted specimens, 3) a literature survey of

hydrogen in metals, and 4) a literature survey of nitrogen implanted in

metals.



FIM Principles

This section will be used to describe some of the fundamental

aspects of FIM techniques for the reader who is not thoroughly familiar

with them already. For a more complete discussion of FIM theory and

practices, several excellent books are available. The three which this

author recommends are: 1) Field Ion Microscopy edited by Hren and

Ranganathan [31, 2) Field Ion Microscopy, Principles and Applications by

MUller and Tsong [4], and 3) Field Ion Microscopy in Materials Science by

Wagner [5], The last of these is the most recent and includes operating

principles of both the atom-probe and imaging atom probe types of

instruments.

Field Ionization

A basic FIM in comparison with today's sophisticated instruments is

relatively simple. It employs no lenses, no primary beam, and requires

no high power consumption. It consists of a vacuum chamber (usually









5
UHV), a cryogenically cooled specimen holder that can be biased to a high

voltage (0-20 kV), and a leak valve which allows backfilling of the

vacuum with an inert gas such as helium or neon to a pressure of

approximately 1x10-5 Torr. Figure 1-1 shows a schematic of a basic field

ion microscope. The specimen is normally electrochemically polished to

an emitter endform (having a radius of curvature of about 200 to 1000 A)

When a voltage is applied to it in the microscope, the shape of the

emitter causes a high electric field that ionizes the gas molecules

(field ionization) above the surface step sites where the planes of atoms

emerge at the surface. The ionization process is due to quantum

mechanical tunneling of an electron from the gas atom through a potential

barrier between the metal surface and the atom. The ions are then

repelled away from the positive tip and strike a phosphor screen assembly

(a channel electron multiplier array (CEMA)/phosphor screen combination)

where the image is photographically recorded. This process is

illustrated in Figure 1-2. Besides the apparent resolution of the image,

crystallographic information can be obtained directly, i.e. without

diffraction, but lattice parameter information cannot.

Field Evaporation

When the voltage is raised such that the fields exceed those

normally used for imaging conditions, the atoms themselves can be

positively ionized (termed field evaporation). This process can be

considered to be the result of the electric field lowering the energy

barrier for sublimation of the surface atoms. Because of this, field

evaporation is a thermally activated process. Every metal has a

characteristic field evaporation strength which is also a function of


















High Voltage
++


Liquid
Nitrogen


To Pumps
and
Gas Supply


Screen


Schematic diagram of a simplified field ion microscope.


Figure I-1
































Field
aIonization
Ionization Zone






O"Average"

/ ^/ /f/' ^Surface
/ / FIELD ION TIP ,S a








Figure 1-2 Schematic diagram of the filed ionization process. The gas
atom is polarized by the high electric field and is attracted towards the
tip, strikes the cold surface several times, loosing some of its kinetic
energy each time. At a critical distance above an ionizing site the gas
atom looses an electron to the metal by a quantum mechanical tunneling
process and is repelled from the tip towards the viewing screen where the
image is photographically recorded.









8

temperature. The process of field evaporation can be controlled very

precisely by varying either the applied field with the voltage or by

changing the specimen temperature, and forms the basis for AP instru-

ments. Atom probe instruments rely on one of these two methods to

initiate field evaporation in a very short time in order for the ions to

be measured temporally at a detector, thus forming a time-of-flight (TOF)

mass spectrometer. A high voltage pulse with a subnanosecond risetime

and 10-20 nsec width and having an amplitude of 1-3 kV is used in the

conventional AP and imaging atom probe (IAP). The pulsed-laser atom

probe (PLAP) is basically an IAP employing a nitrogen laser to

instantaneously heat the surface of the emitter to cause field

evaporation. The latter instrument enjoys the benefits of a reduced

electric field and the capabilities of analyzing specimens with poor

electrical conductivities, such as silicon, and glasses. Because the

removal of atoms from the surface of the emitter is seen as a collapse of

the characteristic rings which form the FIM image, depth resolutions of

one atomic plane are expected.

It should be noted that field evaporation is the final step in the

specimen preparation technique. When the voltage is applied for imaging

in the FIM, local protuberances are field evaporated. The tip continues

smoothing in this manner until the ring structure appears, and a stable

image occurs. A particular radius of curvature can be "dialed-in" by

continued field evaporation from the "as-polished" condition to a preset

voltage which corresponds to that radius at the given temperature,

providing, of course, that the initial radius is less than that of the

desired radius.










Atom Probery

The conventional AP is designed with a 1-2m flight tube behind a

CEMA/phosphor screen assembly that has a 4-6mm hole through it. Surface

atoms which are aligned with and pass through this probe hole are then

focused with an einzel lens to strike the detector. Electronic timing

circuits interfaced with a computer are used to determine the flight time

which in turn is used to calculate the mass to charge ratios. The

detector most often used is a Chevron assembly, constructed using two

CEMA's. The mass resolution, M/AM, of the AP is on the order of 200 [6].

The source of the poor resolution is due to energy deficits inherent in

the field evaporation process [6]. The energy-compensated AP uses a

Poschenreider lens [7] to increase the resolution to as high as 5000 [6].

In both instruments the apparent size of the probe can be changed by

varying the tip to screen distance. Compositions of precipitates with

dimensions on the order of 10 A have been analyzed routinely. In

addition, since the AP is a TOF instrument, all elements can be detected,

including isotopes of hydrogen. It should be noted that the metallur-

gical applications of the AP did not advance at a rapid pace until the

widespread use of fast electronics and computerized interfacing allowed

the collection and reduction of large number of data. The early AP's

employed a storage scope with the results of each trace needing to be

analyzed by hand.

The IAP (and PLAP) are constructed with a Chevron assembly (which

can be curved) mounted .1-.15m from the tip. The assembly is used as a

detector and viewing screen. Since the IAP has such a short flight

distance, extremely fast digitizing scopes are employed to capture the









10

resulting waveforms. The IAP, although having poor mass resolution

(=50), does have the advantages of having a full spectrum result with

each pulse and a large probe area (the entire imaging surface). The

Chevron assembly can be gated to be turned "on" to coincide with the

arrival a particular mass at the detector. In this mode of operation,

the results of many pulses are integrated photographically to form a

time-gated image. The resulting image can be compared with the FIM image

in essentially the same way as an X-ray elemental map is compared to a

SEM micrograph. The results can be very dramatic, for example showing

the segregation of impurities to grain boundaries [8]. For metallurgical

applications, the AP and IAP together in one instrument can be a powerful

combination.

The FIM/IAP also suffers no problems of minimum detectability. In

principle, it has single-atom detectability. Another important point to

be made is that the IAP resolves deuterium from other low-mass peaks such

as H* and He+ very well, despite the flight distance being extremely

short (.15 m).

Applications

The FIM and AP techniques are being applied to diverse areas of

interest. The atomically clean surface of an emitter caused by field

evaporation make them attractive techniques for surface science

applications. Studies are being done in the areas of surface segrega-

tion, surface diffusion, adatom adsorption, precipitation, radiation

damage, radiation-induced precipitation and catalysis. Segregation of

elements to interfaces are the forte of the AP. Many different materials

have been examined, including: steels, nickel-based superalloys,









11

aluminum-copper alloys including those which develop Guinier-Preston

zones, alloys which undergo order-disorder transformations, glasses, and

semiconductors. The review by Wagner discusses in detail the fundamen-

tals and applications of FIM techniques in materials science [5].

FIM techniques are by no means the solution to everyone's problems.

Not all specimens that one would like to examine in the FIM can be

prepared into the required shape. And if they can, it doesn't mean that

they will withstand the effects of the high electric fields. One major

disadvantage of FIM techniques is the extremely small volume of material

which can be examined. The volume of a field emitter examined is on the

order of 100 times smaller than that of a TEM specimen. This means that

finding features of interest, even with the aid of TEM, can be quite

difficult and care must be exercised when interpreting results from such

a small sampling.

Field-Induced Stresses

Also of primary importance to the success of applying FIM techniques

to the study of implanted species with defects is the influence that the

high electric field has on them. Extremely high tensile and shear

stresses are induced within the emitters due to electrostatic (coulomb)

forces present at the surface of the emitters. These stresses can be

visualized by imagining a sphere of the same radius of curvature as a

field emitter to which the same voltage is applied (Figure I-3a). The

stresses induced in this imaginary sphere are purely tensile and at the

surface are equal to F2/8n where F is the electric field at the surface.

The electric field is equal to V/r, where V is the applied voltage and r

is the radius of the sphere. Within the volume of this sphere, there is
























































gure 1-3 Field distributions for various shaped emitters.
Idealized perfect sphere, b) spherical cap on cone shaped shank,
parabolic shaped tip, d) tungsten-like specimen with flattened apex.









13

no shear component of stress because of symmetry. Now imagine the model

of an idealized field emitter (Figure I-3b), a spherical cap superimposed

onto the apex of a cone. One can see that the field and hence the

stresses taper off sharply down the shank where the local radius of

curvature increases. Figures I-3c and I-3d show the added complexity of

determining the electric field distribution and hence the stress

distribution when samples deviate from this ideal model of Figure I-3b.

This leads to significant stress gradients with both tensile and shear

components. The large shear components can lead to massive plastic

deformation and destruction of the specimen (known as flashing).

FIM samples are completely compatible with TEM when the proper

holders are designed and used. In addition when wire samples are used

having a preferred orientation, the crystallography becomes very

simplified. TEM observations of tips are used routinely at many FIM

locations for checking specimen quality.

Two especially designed transmission electron microscope (TEM)

sample holders have been designed and constructed to be used to observe

the influence of the applied field on FIM specimens containing defects.

The first of these two is a "double-tilt" holder used for characterizing

the defects using conventional TEM diffraction contrast. The second is a

"field-effect" holder which is capable of having up to a 5 kV voltage

applied to a FIM specimen in the TEM. The use of these holders is of

considerable importance in understanding how the techniques developed in

this study can be applied to studies of more general types of defects.

These holders facilitate the search for a particular type of defect, and

help to indicate whether it will be feasible to study such defects using








14

FIM techniques (i.e., is the defect stable under the field-induced

stress?). The TEM has proven to be indispensable with in establishing a

depth scale for specimens analyzed with the IAP.



Microstructure Of Implanted Metals

The microstructure of ion-irradiated metals is dependent on several

variables. These include: ion species, energy, temperature, target

composition, crystallographic orientation, and post-irradiation heat

treatment. The combination of these experimentally controllable

parameters determines the implantation depth, the amount of damage

present in the lattice, the concentration profile, the size and type of

defects, and the precipitation of second phases (if possible). A

complete discussion of these parameters is outside the scope of this

study. The discussion which follows will concentrate only on those

aspects of ion implantation which relate to this study.

The most important parameter which determines the amount of damage

in an irradiated target is the maximum energy, Em, transferred to a

lattice atom in a head on collision with the projectile ion. For

non-relativistic particles, Em is given by



Em = 4Emlm2/(mi+m2)2 1-5


where mi is the ion particle mass, m2 is the lattice atom mass, and E is

the kinetic energy of mi. There are two critical values of Em which

separate implantation studies into three regimes. The first is Ed, which

defines the minimum transferred energy which can produce a stable Frenkel








15

pair with a probability of i. (A Frenkel pair is a self-interstitial

atom (SIA) and its vacancy.) For values of Em < Ed, no stable damage is

produced. In this regime, the implanted species would be interstitial in

nature (if diffusionally stable). The second critical energy, Ec,

denotes the threshold energy required to form a displacement cascade

(with probability of i) [9-12]. When Em > Ec, the energy is transferred

to a primary knock-on atom that creates the displacement cascade,

characterized by a center depleted zone of high vacancy concentration.

The interstitials are ejected from the zone by either a channeling

process or a focused replacement collision sequence [13,14]. In either

case, the SIA's are found relatively far from the depleted zone.

Seidman and coworkers [14-24] have used the FIM to study depleted

zones in tungsten which are produced by a single incident heavy ion.

They photographically recorded each FIM image after a field evaporation

pulse which removes on the average of one atom from a (222) plane. After

the analysis of numerous frames (on the order of several hundred

thousand), a three dimensional reconstruction of the depleted zone was

accomplished with the aid of the computer. These depleted zones have a

high fraction of clusters (approximately 83%) that are composed of more

than six first-nearest neighbor vacancies, while only about 13% appear as

monovacancies having no first-nearest neighbors [16]. The depth from the

surface of the depleted zones depends on energy and mass. For 20 keV W

into tungsten it is about 20 A [24]. Unfortunately, FIM results on the

structure of depleted zones created by light elements such as hydrogen

and helium have not been achieved to date.








16

For Ed < Em < Ec, mainly isolated Frenkel pairs are created which

may or may not agglomerate into defect clusters, depending on tempera-

ture, energy,fluence impurity content, etc. For Em just over Ed only

single Frenkel pairs are created. For high dosages, separate-event

interstitials and vacancies can annihilate each other. For increasing

transferred energies, both the number of Frenkel pairs per incident ion

and the clustering of vacancies also increases.

The radiation induced point defects described above will not be able

to be resolved with TEM unless they agglomerate by diffusional processes

with sizes greater than a few nanometers. Here, size refers to either

the physical geometrical size or the extent of the strain field of the

defect. The types of defects which nucleate due to the migration of the

point defects are both vacancy and interstitial dislocation loops, voids,

gas bubbles, etc. In addition, pre-existing lattice imperfections can

trap the point defects and hence change their local contrast. Any type

of lattice inhomogeneity or defect that modifies the local diffraction

conditions will result in contrast in the micrograph. The types of

inhomogeneities which produce contrast include the following:

(1) A locally varying strain field.

(2) A planar defect, e.g., a stacking fault tetrahedron.

(3) A void or gas bubble within the volume observed.

(4) A local change in the extinction length, e.g., due to local

variation in composition.

In general direct interstitial defects are difficult to find,

because they are much more mobile than vacancies at low temperatures. On

the other hand pure vacancy type defects can exist in equilibrium at








17

relatively high temperatures. The image contrast from small dislocation

loops which are visible in the TEM characteristically have a complex

black-white contrast pattern. The contrast pattern depends on several

factors, material, loop type (vacancy or interstitial), Burgers vector of

the loop, beam direction, extinction distance, and relative position

within the thin foil. The correct identification of small loops from TEM

images therefore requires the aid of computer simulation. The computer

simulation programs use a column approximation to the foil in which the

intensity is found solely by the diffraction conditions within this

volume. The various models used for the calculations for the volumes

containing the defect and strain fields are reviewed by Wilkens [13] and

Sykes [25]. Sykes et. al [26] have compiled a catalog of computer

simulated TEM images of small dislocation loops in FCC and BCC metals.

This catalog allows the microscopist to directly compare an experimental

image for a loop with the catalog. The required parameters for the loop

are the depth in the foil, foil thickness, and beam condition. It should

be noted that all the loop contrast patterns are calculated under

dynamical conditions, i.e., "two-beam" conditions. Therefore, all

micrographs compared to simulated images should be made under identical

conditions.

It must be emphasized that TEM observation of radiation induced

defects is not feasible without an accumulation of the point defects and

the formation of voids and/or loops of sufficient size ( 3 nm).

Thorough discussions of the conditions required to identify such defect

clusters are given by Wilkens [14], RUhle [27] Venables and Thomas [28],

and Sykes [25]. As mentioned earlier, Seidman and coworkers [14-24] have









18

used the FIM to study the distribution of clusters in the depleted zone

of a displacement cascade. Their methods can also be used to determine

both types of radiation induced point defects, vacancies and intersti-

tials, and to map their spatial distributions within the emitter. The

FIM is the only microscopic technique which can image point defects at a

temperature at which no recovery occurs. Seidman and coworkers have

utilized the FIM to its fullest in these studies and have contributed

substantially to our present understanding of the spatial arrangement of

point defects in irradiated metals.



Hydrogen In Metals

Most often, the presence of hydrogen in metals is deleterious.

Since metals are frequently exposed to hydrogen, (e.g. containment) the

harmful effects of hydrogen on metals are an important technological

problem. As a result, interest in hydrogen in the field of materials

science extends to both basic and applied research. Because hydrogen has

relatively low-solubilities and extremely high diffusivities in most

engineering alloys the study of the interactions of hydrogen with crystal

defects is of basic importance. The term defect, as used here, means any

microstructural feature differing from a perfect crystalline solid.

Typical defects include: grain boundaries, dislocations, precipitates,

interphase boundaries, vacancies, etc. The analysis of hydrogen

associated with these defects cannot be studied by conventional electron

probe techniques, such as analytical electron microscopy (AEM), energy

dispersive x-ray spectroscopy (EDXS), electron energy loss spectroscopy

(EELS), scanning electron microscopy (SEM), and Auger electron








19

spectroscopy (AES), because these techniques are incapable of detecting

it. Surface analytic techniques employing mass spectrometers for

example, secondary ion mass spectrometry (SIMS) can detect hydrogen, but

so far lack sufficient spatial resolutions. This spatial resolution

includes both the lateral probe diameter and depth profiling resolution.

The quantity of hydrogen which can cause harmful effects to alloys

can be extremely small and hydrogen defect interactions can be on

extremely fine scale. Thus the techniques which can analyze hydrogen

must have high sensitivity for it and adequate spatial resolution in

order to study the mechanisms of degradation.

It has long been known that field ion techniques are extremely

sensitive to the presence of hydrogen. In fact, this sensitivity has

made it difficult to apply FIM and atom probe techniques to studies of

hydrogen. In general all hydrogen studies using the FIM must overcome

the same problems: 1) hydrogen's high diffusivity, even at low

temperatures, 2) the apparently low solubility of hydrogen in the absence

of defects, and 3) hydrogen is ever-present in the experimental

background. Experimentally this means that hydrogen must be introduced

into the metal, kept there for sufficiently long time to be analyzed, and

confidently distinguished from pervasive background hydrogen.

Background hydrogen is always a consideration in mass spectrometric

techniques because it is a major constituent in all normal vacuum

systems, since it is difficult to pump. With FIM techniques the

background hydrogen problem is further aggravated by the phenomenon of

field adsorption. Field adsorption occurs because of the polarization of

the hydrogen gas molecule under the influence of the applied field on the









20

emitter. The rate of adsorption in the presence of a field and at a

given partial pressure of hydrogen is several orders of magnitude higher

than without the applied field. The consequence of this is that a

hydrogen background desorption peak is present in all atom probe studies.

To circumvent this problem, deuterium is used in this study instead of

hydrogen. The deuterium and hydrogen signals are well differentiated in

the IAP.

The introduction of hydrogen ex situ (with respect to the FIM/IAP

chamber) requires very complex quenching and cryogenic/UHV transfer

techniques. This difficulty is in addition to the elaborate control

experiments required to determine the adsorption of hydrogen in all the

steps preceding introduction into the FIM/IAP chamber. For these reasons

in situ implantation of deuterium was chosen for the present study. This

method offers major advantages since 1) both the deuterium and the

defects can be reproducibly and controllably introduced into the lattice,

2) the background deuterium can be controlled by differential pumping and

valving, 3) the number and complexity of the control experiments can be

greatly reduced, and 4) the deuterium can be easily distinguished from

the hydrogen background.

There are major questions which must be answered before the FIM/IAP

can be applied to studies of general types of defects (i.e., those not

induced by ion implantation). These questions will be addressed and

answered in this study. The most important of these will be the

feasibility of implementing a differentially-pumped ion gun for in situ

implantation with respect to the deuterium field adsorption. Any ion gun

coupled to the FIM/IAP and used for ion implantation of deuterium will









21

contribute a deuterium partial pressure to the FIM/IAP chamber. The

question arises, can the deuterium partial pressure be kept sufficiently

low so as not to interfere with the implanted deuterium as detected with

an IAP? This study will show that this question can be answered in the

affirmative when special precautions are taken.

Hydrogen Embrittlement

A major impetus for studying hydrogen in metals is the phenomenon of

hydrogen embrittlement in commercial alloys, especially steels. Because

of hydrogen's extremely high diffusivity, even at low temperatures,

failures involving hydrogen embrittled alloys usually occur catastrophi-

cally. Unfortunately, the mechanisms of hydrogen embrittlement are still

little understood. In steel at least five different mechanisms for

hydrogen embrittlement have been proposed. For example, Zapffe and Sims

[29] developed a pressure-expansion theory in which very large gas

pressures within internal micro-cracks and voids cause either plastic

deformation or cleavage. This leads to a coalescence of the micro-voids

and to eventual failure. However, this mechanism is inadequate to

explain hydrogen embrittlement which occurs with hydrogen gas pressure of

one atmosphere or less. Another proposed mechanism, by Petch and Stables

[30,31], suggests that hydrogen adsorption lowers the energy of the

surface created in crack propagation, which in turn lowers the work of

fracture, according to Griffith's criterion. There are several

criticisms of this mechanism as well [32-34] and it seems not to be

generally applicable.

The decohesion theory proposes that high concentrations of hydrogen

lower the intrinsic cohesive force between atoms [35,36-38]. It is









22

postulated that the stresses developed at crack tips promote the

accumulation of hydrogen to very high concentrations and thus substan-

tially lower the cohesive forces. Unfortunately, the dependence of the

maximum cohesive force on hydrogen concentration, interface structure,

and alloy composition is not known. Still another theory proposed by

Westlake [39] also centers on the processes occurring near a crack tip.

It is postulated that hydrogen concentrates near the tips of cracks

because of the hydrostatic stresses present there. A high enough hydrogen

concentration, in turn, leads to hydride formation. Embrittlement then

is thought to occur due to hydride cracking near the crack tip. In fact,

Gahr et al. have shown that the hydrostatic stress field of a crack tip

can stabilize hydrides in niobium which are otherwise unstable [40].

They also demonstrated embrittlement due to hydride cracking. For iron

and steel, no hydride is stable up to 2 GPa [41] and Westlake's model is

not considered applicable [32-42]. Nonetheless, calculations of the

hydrogen atmosphere at a crack tip show that very high concentrations can

occur there and that the hydride formation model could be appropriate

[43,44].

Trapping Of Deuterium

Kellogg and Panitz [45] have directly observed the trapping of

deuterium at a grain boundary in tungsten. In this study seven tungsten

emitters were implanted with low energy (200 ev) deuterium ions. The

specimens were imaged with the TEM and then transferred to a FIM/IAP and

analyzed. The FIM/IAP can give a time gated image which can map the

distribution of deuterium with depth. (The FIM/IAP and its modes of

operation will be discussed further in Chapter II.) The only sample that








23

retained any deuterium was one in which a grain boundary was present, the

gated IAP image showing the deuterium associated with the grain boundary

visible in the FIM micrograph. In a more recent set of atom probe

experiments, Macrander and Seidman [46] have shown that 200 eV H; ions

implanted into Tungsten emitters completely diffuse out of the samples at

a temperature of 290K. Their results indicate a diffusion coefficient at

290K that is greater than 10-17 cm2/sec. Since the implanted specimens

in Kellogg's and Panitz's study were stored at room temperature for

several days and were heated during imaging somewhat in the TEM, grain

boundaries are apparently deep traps for deuterium.

In the second study, Panitz [47] demonstrated the importance of

using control experiments to distinguish the difference between the

implanted and the background deuterium signal in the IAP spectrum.

Implantation was done in the specimen pre-chamber of the IAP and not

in situ. Complex control experiments were required because the

implantation was performed outside the IAP chamber. This suggests the

possible benefits which could be derived if in situ implantation were

performed.

The results of Panitz's study indicate that the 80 eV ions are

implanted to a depth of 49 A at 3000K and that adsorbed surface species

selectively scatter some of the deuterium into preferred channels. These

results are at odds with those of Macrander and Seidman [46] which

indicate that the deuterium should have diffused out of the sample since

no implantation damage can occur with 80 eV deuterium ions implanted into

tungsten. The implantation was carried out at 3000K where the solubility

of hydrogen is extremely small and the diffusivity is high. With these








24

considerations the implanted deuterium should have diffused to the

surface where it would have been field desorbed. A possible explanation

for the discrepancy is that the background deuterium has not been

accounted for adequately. There is the possibility of surface migration

of deuterium up the shank during the profiling experiment. This

deuterium would appear in the IAP spectra as an implanted ion. A change

in evaporation rate due to a voltage increase during the analysis can

change the arrival rate of the deuterium to the analyzed portion of the

tip. Another factor which could affect a possible background signal is

the time interval between field desorption pulses. These factors were

not reported in this study [47].

Hren [48] confirmed the necessity for in situ studies when he

performed several ex situ experiments preliminary to this dissertation

using the facilities of Myers* and Panitz and Kellogg A discussion of

the design requirements for coupling an ion gun to a FIM/IAP will be

presented in Chapter II. The control experiments associated with in situ

implantation of deuterium are relatively simpler than for ex situ

implantation and will be presented in Chapter III.



Nitrogen Implantation In Metals

Tribological Studies

Nitrogen is by far the most widely used element for the modification

of the surface properties of alloys to improve their wear resistance.

*Ion Implantation Facility, Sandia National Laboratories,
Albuquerque, NM.
**Surface Physics Division, Sandia National Laboratories,
Albuquerque, NM.









25

There is a great deal of research being done in the area of determining

the mechanisms for the improvement of wear resistance in a variety of

alloy systems. It should be noted that not only the wear characteristics

are improved by implantation, but also often are such properties as

fatigue, corrosion (both atmospheric and aqueous), toughness, strength,

and high temperature oxidation. Wear resistance, however, is the common

goal of applying ion implantation to alloy systems. Wear can be improved

by increasing the surface hardness, by reducing the friction coefficient,

by modifying the subsurface composition and structure to resist fracture

and debris formation, and/or by increasing the wetability of the surface

to lubricants. This lack of understanding of the basic mechanisms of

wear improvement has not stopped the spread of industrial applications of

ion implantation of nitrogen. The major thrust of commercial use of

implantation involves tool steels and Co-cemented WC [49-52]. The area in

which ion implantation is very attractive is where little or no

dimensional changes and low processing temperatures are desired. It is

also of interest where delamination of coatings applied by more

conventional techniques may be a problem [51]. Basic research efforts

are directed towards understanding the wear reduction mechanisms in

technologically important alloy systems such as stainless steels [53-55],

hard chromium plate [56], Ti-6AI-4V [57-60], and Co-cemented WC [61-63].

The increases in wear resistance are usually attributed to nitride

formation and/or Cottrell atmosphere hardening. The nitride formation by

implantation is usually in the form of finely dispersed, hard

second-phase precipitates. The Cottrell atmospheres arise from nitrogen

being attracted to dislocations and pinning them preventing them from








26

being mobile and thus hardening the material. Depending on the material,

one of these may be more predominant than the other. For example, mild

steels without alloying elements having strong affinities for nitrogen

such as Al, Cr, Mo, V, and Ti do not show dramatic increases in wear

resistance upon nitrogen implantation [64]. One disadvantage to nitrogen

implantation to improve the wear characteristics of an alloy is that the

benefits are not retained at elevated temperatures. High operating

temperatures must be avoided and this sometimes precludes its use in

particular applications such as tools for chip forming. This is thought

to be due to the instability of nitrogen-defect structures which are

responsible for the improved wear characteristics [51,62]. This is

supported by experiment. For example, Dose Santos et al. have seen

nitride and carbo-nitride formation upon implantation and then subsequent

dissolution at higher temperatures [65].

The understanding of the wear mechanisms are complicated by studies

in which the nitrogen is seen to still be retained beyond the original

implantation depth. The depths that the nitrogen is still modifying the

surface behavior can be 10-50 times the implanted range [51]. Clearly

this nitrogen retention is happening during the wearing of the material

and microstructural changes must be occurring. There are several

proposed ways for this to happen. During wear, the rubbing of asperities

on the two surfaces causes localized heating. This thermal heating is

thought to be responsible for the dissolution of the nitride phases and

diffusion down the steep temperature gradient [66]. This mechanism would

agree with the observations that nitrogen implantation does not give good

results at high service temperatures. These local temperatures may be as









27

high as 10000K (e.g. Fe2N has been shown to be stable in the electron

microscope to 9000K [67]). This same local heating could also cause, the

nitrogen, decorating dislocations to diffuse deeper into the material

[68]. Another possibility for the retention of nitrogen is the

continuous transfer of nitrogen back and forth between the surfaces as

the wear front advances [51].

Ion Beam Carburization

Several investigators have reported improvements in properties in

alloy systems with the implantation of inert elements such as neon, argon

[51], and zirconium [59]. Others report that no effects are seen when

elevated temperatures (500-6000C) are used during implantation [51]. The

probable explanation of this is due to beam carburization of residual

hydrocarbons in the vacuum system decomposing under the effects of the

irradiation during implantation. Elements with high affinities for

carbon tend to form carbides giving a characteristic improvement in

surface hardness. It is more likely that at high temperatures the

decomposed hydrocarbons are not stable and reform rather than the carbide

phases being unstable. The resulting carbide phases would necessarily be

quite small and finely dispersed.

Applicability Of FIM Techniques To Nitrogen In Metals

A variety of techniques have been applied to study the composition

and morphology of implanted surfaces. These include secondary ion mass

spectroscopy (SIMS), Auger electron Spectroscopy (AES), scanning electron

microscopy (SEM) with energy dispersive analysis of X-rays (EDAX),

Rutherford backscattering (RBS), nuclear reaction analysis (NRA), and TEM

(and analytical electron microscopy, AEM). With the exception of TEM,









28

all of these techniques have the ability to use the same samples which

are used for the wear tests. Several of them can also analyze small

particles within the wear tracks of the samples.

Field ion microscopy techniques offer some appealing features for

studying nitrogen (and other elements) implanted into metals. It is a

surface microscopic technique and the results can be directly compared

with that of some of the above technique. It has the capability of

resolving both the defect structure caused by implantation on an atomic

scale and small precipitate phases which form. It has the ability to

dissect these features as a function of depth with an accuracy of

d-spacing of the material. The same method for determining the depth

scale can be used with atom probe analysis, both AP and IAP, to give

chemical information as a function of depth. Because the FIM samples are

completely compatible with the TEM, the implanted samples do not require

any further sample preparation to observe them there. In other words,

the implanted structure is accessible to two high resolution microscopic

and analytical techniques with no treatment to them after they have been

implanted other than to introduce them into the specific instrument.

The delicate nature of a FIM specimen in terms of its geometry

precludes it from also being used in wear test apparatus. However, the

relationship between the microstructure and macroscopic properties of

materials is one of the principal goals in materials science. The FIM

sample lends itself to investigation by two microscopic techniques, FIM

and TEM, without further specimen preparation after implantation. This

study of the implantation depth of implanted nitrogen into tungsten and

nickel in is the first step in applying the FIM/IAP to the future use in









29

using it to observe and analyze the small nitride, carbides, and

carbo-nitrides responsible for wear reductions in many ion-implanted

samples. These types of precipitates in emitters have proven to be very

amenable to FIM and AP analyses in the past.















CHAPTER II
INSTRUMENTATION


The instrumentation employed in the experimental work can be divided

into three categories: FIM/IAP, ion gun, and transmission electron

microscope. This chapter will describe the unique features or techniques

which have been developed in utilizing these instruments for the present

study, as well as document the instrumentation.



Field Ion Microscope/Imaging Atom Probe (FIM/IAP)

Until recently, no commercial FIM/IAP has been available and each

instrument has been custom-built and usually designed with a specific

purpose in mind. The FIM/IAP used in this study is no exception, and

several unique features have been incorporated in it which will be

discussed. It was designed to be a versatile instrument for metallur-

gical applications. The electronics and timing circuitries are based

primarily on those of Panitz [69]. The vacuum chamber, specimen exchange

mechanism, specimen holder/manipulator, and chevron/phosphor screen

assembly have been designed primarily by this author. The construction

of the FIM/IAP proceeded in stages. Each was a major design or

developmental stage in the instrument leading to its present form. Most

of the problems associated with each of these stages have been solved by

this author. Several features have been included because of considera-

tions to the present study or anticipated future studies involving









31

hydrogen. Many of the design features or specifications within each

stage are either common to both or intimately related. Figure II-1 is a

diagram showing these major stages and the relationships between them.

The arrows in this diagram indicate these relationships. A double-headed

arrow represents the fact that the one stage is intimately related to the

other and must be compatible. A single-headed arrow represents an

independence, but that some of the same requirements in the one stage are

also requirements in the other. The details of the design and

interactions of the various stages represented in Figure II-1 will be

described in each of the following sections. This chapter will provide

the bulk of the documentation required for someone to learn to use this

instrument.



FIM/IAP Description

Chamber

The FIM/IAP used in this study differs from other instruments in

that the sample can be directed towards two, six-inch ports separated by

1500 (Figure II-2). One of the six-inch ports is used as the FIM/IAP.

It has a chevron assembly [70] mounted 15 cm from the chamber's axis.

The assembly uses flat microchannel plates from Galileo Electro-Optics

and was designed by this author. The other port is a utility port.

Various options are available for use with this port which provides the

versatility of this instrument. Originally, a conventional atom-probe

[71] was planned to be attached to it after completion of the IAP. For

the present study a Colutron Research Corp., model G-2-D ion gun was

coupled to this port (Figure 11-3). This gun was obtained as a kit, and























































Figure II-1 Block representation of the major development stages of the
FIM/IAP. The double-headed arrows indicate sections whose parts or
design requirements must be compatible. The single-headed arrows
indicate stages with sections having similar requirements as to those of
the section of the arrow's origin.




































Figure 11-2 View of the FIM/IAP shown from the IAP side. To the right
is the associated electronics. The ion gun can be seen in the
background.


Colutron ion gun coupled to the HFM/IAP.


Migure 11-3









34

then assembled, tested and made operational by the author. This port has

also been used as a conventional FIM with a 7 cm tip-to-screen distance.

Other uses for this port are anticipated, including an evaporation source

and an electron gun for heating the sample.

Midway between the two ports described above is a mini-flange port

(1.33 in. 0.0.) used for the exchange of the specimen. Figure 11-4 is a

photograph of the exchange chamber. Huntington Mechanical Laboratories

supplied the major hardware for the exchange system, including an MS-075

straight-through valve, a VF-169-10 magnetically-coupled linear/rotary

drive, the special 6-way cross, and special 1.33 to 2.75 in., 0.D.

nipple. The minimum inner diameter is .625 in. which is that of the

valve. A positioning device coupled to a bellows is used to align the

insertion rod with the sample stage in the main chamber. A viton gasket

is used for the blank flange used for the specimen exchange. A special

clamp is used in lieu of bolts to seal this flange. A 170 1/s Balzers

turbo-pump used on the ion gun is also shared with the exchange

mechanism. An all-metal, bakeable valve separates the two volumes. An

absorption pump first roughs the exchange valve to approximately 1x10-3

Torr. The valve is opened very slowly to the turbo-pump and an ultimate

pressure of about 1x10-6 Torr is achieved in approximately 10 minutes.

The chamber is roughed by two absorption pumps used in series. A

pressure of about 2x10"- Torr can be achieved with these. A Perkin-Elmer

80 1/s ion pump is used as the primary pump to attain UHV pressures.

This pump has special elements for higher pumping speeds for hydrogen.

It is assisted by a Perkin-Elmer, liquid nitrogen cooled titanium

sublimation pump. Additional pumping arises from the Air Products



























































Figure 11-4 Close-up photograph of the FIM/IAP showing the introduction
of a sample into the chamber using the Huntington rotary-linear,
magnetically coupled feedthrough.









36

Displex helium closed cycle refrigerator used for cooling the sample. A

pressure of about 1x10-9 Torr can be achieved with the ion/Ti sublimation

pump combination. The cryopumping usually brings the system pressure to

about 5x10-10 Torr.

Although all the components in the vacuum chamber are bakeable to at

least 300C, this system has not been baked to achieve ultimate vacuum

conditions. The system is, however, subjected to a procedure which does

lower the pressure considerably. The conductance between the ion gun and

FIM/IAP is approximately .01 1/s. Dry nitrogen from liquid nitrogen in a

dewar flask is bled into the system through a leak valve. The valve

between the ion gun and the main chamber is opened. The heater on the

ion pump is turned on. By properly adjusting the leak valve, a pressure

of about 1x10-1 Torr in the chamber and 9x10-4 Torr in the ion gun is

achieved. These conditions lead to an intermediate flow regime for the

nitrogen which helps drag difficult-to-pump gases such as He, Ne, and

water vapor out of the main chamber. This is done for eight hours, after

which the system is pumped down to 1x10-5 Torr by the turbo-pump, the

valve closed, and the ion pumps started.

An imaging gas, either He or Ne, is admitted into the system by leak

valves after the ion pump is turned off. The gas lines from the

cylinders are carefully baked out when cylinders are changed. When He is

admitted into the chamber, the pressure is monitored carefully until the

desired pressure is achieved. The valve is immediately turned off.

Helium is not pumped effectively by the cryopumping of the Displex

refrigerator and the pressure is stable. The Ne is, however, pumped by

this system and the valve must be opened very carefully to a set point to








37

maintain a stable pressure. It was found that this set point is more

easily achieved if the valve in the beam line to the ion gun is opened

and a steady state condition is found. Furthermore, it was found that if

the Ne leak valve is turned off, the turbo-pump will pump the main

chamber to approximately 1x10'6 Torr. Closing the valve to the ion gun

and turning the ion pump on achieves a lower ultimate pressure than if

the ion pumps were used alone from the imaging pressure.

Sample Holder

There are extreme requirements imposed on the design of the FIM/IAP

specimen stage. For a relatively simple holder with no manipulation,

there are three major and somewhat conflicting requirements: i) the

sample must be cooled to cryogenic temperatures, ii) a high voltage of up

to 15 to 20 kV must be applied to the sample and iii) all materials must

be UHV compatible, which includes being bakeable to at least 3000C. For

good thermal conduction, OFHC copper is used. For poor thermal

conduction, 304 stainless steel with a thin cross-sectional area is used.

For electrical insulation with good thermal conduction, two materials can

be used, either boron nitride or sapphire. Because sapphire is extremely

difficult to machine, boron nitride was utilized for all holders designed

for this instrument.

For metallurgical applications using the FIM/IAP, manipulation is

also required further complicating the design. Two orthogonal tilt axes

located at the specimen's apex allow the specimen to be examined thus,

the disadvantages of the limited viewing area due to the increased

tip-to-screen distance are overcome. This helps in crystallographic

indexing of the sample and orienting a particular feature of interest for








38

analysis. Linear motion in the X, Y, and Z directions is needed for

alignment with the exchange rod and also with the ion gun. The two

six-inch ports in the chamber, separated by 1500, add another complica-

tion in that both ports have the same degree of motion associated with

them. A design requirement for this instrument was that the specimen

should have a maximum of 300 in the extreme rotation position for each

port, thus giving a total of a 2100 rotation angle that the holder

requires.

Figure 11-5 shows a photograph of the specimen stage and manipu-

lator. The stage consists of a cylindrical outer conductor of copper and

stainless steel. The stainless part is attached to the manipulator so

that the vertical rotation axis is in-line with the specimen. This

connection also serves as the fulcrum for the up/down tilt supplied by a

rod at the end of the cylinder. Copper braid (Beldon wire 5558) is

silver-soldered to a collar on the copper portion of the outer cylinder.

The other end of this braid is silver-soldered to a copper block (button)

which is fastened to the end of the Displex refrigerator with a bolt.

The silver-soldering of this braid proved to be the weak stage in this

design. An improved braid/cold finger arrangement is described in

Appendix A. There is an inner concentric copper rod into which a copper

sample holder is screwed. This inner copper piece is held in place by

two boron nitride insulators and tightening a copper nut on the threads

of it. The boron nitride insulators are machined so that a high voltage

breakdown path is not straight across the face of the insulator but is

blocked by concentric ribs in them. When the system is cooled, the parts

are contracted, making the fit even tighter. Since the heat path is from






















(A)





























Figure 11-5 Manipulator/Sample holder, a) is a photograph of the
manipulator and sample holder outside of the FIM/IAP. Cooling braid is
not attached. b) shows the holder in situ through one of the six-inch
ports.








40

the sample, through the boron nitride, through the outer cylinder,

through the braid, the outer cylinder will be colder than the specimen,

providing some cryopumping directly around the tip. The stainless steel

in the outer cylinder proved not to be the best design. An improved

design (Appendix A) has an all copper outer cylinder. There is no

radiation shield for this holder but one has been incorporated in the

improved design.

The hardware attaching the manipulator to the cylinder provides the

motion and the thermal insulation from room temperature. The vertical

cylinder in Figure 11-5 is 304 stainless steel. There is a .010 in.

thick thin-walled section in this piece. This thin wall and the length

of the prongs going down to the pivot points provides most of the poor

thermal conduction. This section has been improved upon in the new

design (Appendix A). The up/down tilt is provided by the coaxial-linear

drive in the rotary feedthrough of the Huntington manipulator being

coupled by a rod to the outer cylinder. The length of travel in this

drive is approximately .5 in. At room temperature this provides about

300 to the sample holder. At cryogenic temperatures, the increased

stiffness of the copper braid resists the relaxation of a spring inside

this manipulator and thus limits the upward motion of the specimen stage.

A stiffer spring was tried but with little success.

The high voltage leads to the sample come down through this

stainless steel cylinder. A thin-walled stainless steel tube is

insulated with a glass tube and goes through a 2100 crescent hole cut in

the connecting flange between the large vertical cylinder and smaller

diameter rod. A flexible, stainless steel, catheter guide connects this








41

lead to a stainless steel pin screwed into the inner conductor for the

sample stage. This electrical path is well insulated both electrically

and thermally and is mechanically stable.

The following are the operating characteristics of this holder. The

ultimate temperature is 650K. The breakdown voltage of this stage is in

excess of 20 kV. During operation breakdown first occurs in the external

connection of the high voltage lines. A full 2100 rotation is available

which includes the 30 at the extreme positions. About +50 to -350 is

available from second tilt axis. A new sample introduced through the

sample exchange port is cooled within 30 min. Sufficient manipulation of

the XYZ and tilt on the Huntington manipulator is present to center the

sample in the beam of the ion gun. The alignment procedure for the ion

gun with the sample holder is described in Appendix B.

A field-limiting aperture [69] is conspicuously absent from this

specimen stage. Eaton and Gibson [72] have shown through finite-

element calculations that the electric fields decrease rapidly away from

the tip. In atom probes employing a field-limiting aperture, the

mass-to-charge ration, m/n, is found by using the equation,



m/n = 1930-(Vdc +a*Vp)t2/D2 II-1



where Vdc is the applied d.c. voltage in kV, Vp is the pulser charging

voltage in kV, a is the fraction of this voltage which reaches the tip, D

is the tip-to-screen distance in m, and t is the time-of-flight in

microseconds [69]. Masses found with this equation and also by taking

the ratios of the squares of a known and unknown mass agree quite well.










General Description Of The IAP

The physical appearance of an IAP is very similar to that of a

simple FIM. The most obvious differences are the longer tip-to-screen

distance and the increased number of cables and instrumentation attached

to the electron multiplier channel plate/phosphor screen combination.

The IAP's flight distance is usually between 10 and 45 cm. The channel

plates, instead of a single plate, are incorporated into a chevron

configuration, and have a maximum gain of approximately 1x107 The IAP is

a time-of-flight mass spectrometer with two operating modes. Ionization

of the surface is initiated by high voltage pulses superimposed on the

applied d.c. voltage to induce field evaporation. The detector of the

system is the chevron assembly (including phosphor screen), which also

serves as the imaging screen for the instrument in the FIM mode, but is

operated at full gain. In the TOF mode, the instrument is essentially an

atom probe with a very short flight distance and very large defining

aperture. All atoms viewed in the FIM image will strike the detector in

the IAP's TOF mode. The spectra obtained from the IAP are recorded with

very fast storage scopes or transient waveform digitizers. The

time-of-flight of the field evaporated ions are dependent on their

specific mass-to-charge ratio, m/n, by equation II-1. The analysis of

the TOF spectra is, therefore, straightforward. It should be noted that

the mass resolution is extremely poor, approximately 20 to 50, because of

the short flight distance.

The time-gated mode of the IAP is used to form an image using a

selected mass in the TOF spectrum. The image formed is caused by the

actual atoms from the tip striking the detector. These ions are selected









43

by "switching on" the detector to coincide with the arrival time, and

then the detector is "switched off" again. Only those ions arriving

within this time window will result in an image. The gating on and off

of the detector is accomplished by putting a high voltage pulse (formed

in the same manner as the desorption pulse, except the high voltage

switch employs a Krytron tube) onto the chevron assembly. The time

window's duration is determined by the pulse width, which is determined

by the charging cable length. In the instrument used in this study, the

pulse width is 50 nsec. The selection of the particular mass is

accomplished by delaying the triggering of this pulse to coincide with

displayed TOF peak. Usually, several pulses are integrated into one

micrograph by putting the camera in the "bulb" mode. The resulting

micrographs, when compared with the FIM image, are capable of displaying

with the same spatial resolution very sharp concentration gradients

across interphase boundaries, and grain boundaries [73].

TOF Mode

The heart of the IAP is the Tektronix 7912AD transient waveform

digitizer. It is extremely fast and has a digitizing rate of up to 100

GHz. The 7912AD is used primarily for collecting time-of-flight (TOF)

data and setting up the proper conditions for the time-gated imaging

mode. It also proved to be indispensable in determining cable and

instrumental delays as well as system characteristics such as pulse

shapes. The 7912AD can be operated in a limited way from the front

panel, but is designed for operation via the General Purpose Interface

Bus (GPIB or equivalently IEEE-488 interface). An important feature of

the digitizer is that several waveforms can be added internally in a









44

"signal average" mode. Three benefits are derived from this: i) the

signal to noise ratio is improved, ii) collection of data can proceed at

a much higher rate, and iii) the demand for computer storage of the

waveforms is relaxed.

Figure 11-6 is a block diagram of the electronics and computer

system of the IAP in the TOF mode. All cables are terminated with 50 0

impedances. There are two pulse pickoffs on the desorption pulse line.

A Tektronix model CT-3 is the first in line on the desorption pulse line.

It triggers the digitizer to sweep. The second pickoff is located just

before the pulse line enters the vacuum system. The design of this

pickoff was suggested by Miller*. It serves two purposes: i) it

terminates the pulse with a 50 0 impedance and ii) it picks off the pulse

signal reduced by a factor of 21 to send to channel 1 of the digitizer.

The coaxial cable leading from this pickoff to the digitizer is the same

length as the one leading from the chevron plate detector to channel 2 on

the digitizer. When the two channels are summed, the flight time is

exactly that between the displayed pulse and the particular

mass-to-charge species because all time delays due to cables have been

considered. It should be noted that the time for conversion from the

incident ion to the output in the chevron assembly is considered

negligible [70].

Figure 11-7 shows an analyzed spectrum from an Fe-Mo-N system

previously studied by this author [74]. Several features of the spectrum

should be discussed. The spectrum is the result of having the digitizer

"signal-average" 16 waveforms. The lower left corner gives the d.c.


Miller, M. K., private communication.












45





5-

4-

















*-
0.
.- C





















ao
S- E













-u 0


0 -0
4- .








x< O 0.0



-O

4- )



I
4* 0



Ec
) 0


r-







O I-

E 40
04-











SC 4-
00U)




r- o








I(
rO -
*r-0































I. E-3 V FeMoN 84022813 *0
15., ,i


0.59 03/19/84


KVDC=12.22 KVP=4.44 16 Pulses TFILE* 26


Figure 11-7 Analyzed spectrum of an Fe-3at% Mo specimen which has been
internally nitrided. The field evaporation voltage was 13.99 kV
(12.22 d.c. + 0.4 x 4.44 pulse) at a temperature of 650K.









47

voltage of 12.22 kV and the pulse charging voltage of 4.44 kV. With the

pulse voltage being 40% of the charging voltage, the voltage for field

evaporation was 13.99 kV. The time base of the digitizer was .1 sec/div,

giving 1 sec full scale. The vertical amplifier was set at 5 mV/div,

indicated by the step between solid vertical lines multiplied by the

scale factor, 1x10-3 V. Note that the seven naturally occurring isotopes

of Mo (A=92, 14.8% relative abundance; 94, 9.1%; 95, 15.9%; 96, 16.7%;

97, 9.5%; 98, 24.4%; and 100, 9.6%) are not resolved but their presence

is reflected in the widths of the peaks. The two peaks indicated with

the "*" are due to the combination of a ringing in the detector induced

by the high voltage pulse and the truncation of the negative-going part

of the ringing when the digitizer was used on the 5 mV scale. This

condition has been remedied since this system has been studied and no

longer interferes with the analysis of spectra.

Time-Gated Mode

The timing considerations for the time-gated mode of operation are

rather more involved than the TOF mode. Figure 11-8 is a block diagram

of the IAP in the time-gated mode. In this mode all delays must be

determined precisely. Once all the possible delays in the instrument are

accounted for, the operation of the IAP in the time-gated mode is

straightforward. To set it up in this mode, the cable from the CEMA

output to channel 2 of the digitizer is disconnected and the voltage on

the second plate is reduced to 1 kV. Since the voltage across this plate

is zero, the gain of the chevron assembly is also zero. A Minolta XG-7

SLR 35 mm camera using Kodak 2475 Recording film is opened in the "Bulb"

mode. The number of pulses for the integrated photograph is set on the















U)






0
r-o















4-



4-
0



















c-
0

ot
0)







4- U

4-)






-E
t"-







0E
C *
or- "


a) =
0
4- E




EC
0>
- *t-

0 -0
01
0






E4-

4o) S-
0


*- E





4-)










(A) TOF Mode


(6)


(1) Pulser fires. 1st pickoff pulse goes to trigger scope.
(2) 1st pickoff triggers scope. (delayed sweep with second time
base)
(3) HV Pulse arrives at tip. 2nd pickoff pulse sent to channel 1
of scope.
(4) 2nd pickoff pulse displayed on scope.
(5) Ionic mass species strike detector.
(6) Mass signals inverted and displayed on scope.


(B) Time-Gated Mode


H(2) (3) 1I
I n n I I I k \



(1) Pulser fires. 1st pickoff goes to delay generator
and to trigger scope.
(2) Pickoff pulse triggers delay generator and scope.
(3) Delay generator sends delayed pulse to gate pulser.
(4) HV pulse arrives at tip. 2nd pickoff pulse sent to scope.
(5) 2nd pickoff pulse displayed on scope.
(6) Gate HV pulse arrives at channel plates coinciding with the
arrival of mass species. A gate pickoff was sent to "Z-in" on
scope when gate pulser fired and is not shown for clarity.
(7) Trace on scope is intensified, outlining the portion of
spectrum in which plates are active.

Figure 11-9 Synchronization diagrams of the IAP in a) TOF mode and b)
in time-gated mode. The larger pulses correspond to high voltage pulses,
small pulses to 5v logic pulses, and exponentially decaying pulses to
real and displayed signals.









50

control panel of the desorption pulser. The sequence of events for

gating the channel plate is initiated by each desorption pulse. Figure

11-9 shows the timing sequence and the instrumental and intentional

compensating delays which occur in the IAP. Comparison of this sequence

with Figure 11-8 and the description which follows will help the reader

follow what occurs during the operation in the time-gated mode. When the

mercury-wetted reed switch of the desorption pulser fires, the pulse is

sensed by a signal pick-off immediately. The high voltage pulse is

intentionally delayed by a 53.2 m cable (Figure 11-8, label G) to

compensate for instrumental delays.

The pick-off pulse goes to the Berkeley Nucleonics Corp. Model 7050

digital delay generator through cable (B). The delay generator (C)

supplies two pulses, an initial pulse and a delayed pulse. The initial

pulse output triggers the digitizer trace through cable (J). The delayed

pulse triggers the gate pulser through cable (D). The gate pulse is sent

through cable (F) to the second channel plate in the chevron assembly.

The channel plate is turned on for 50 nsec, the duration of the pulse.

This cable is carefully terminated into 50 0. The delay generator is set

to the time-of-flight of the desired mass specie. A cable (I) from the

gate pick-off to the "Z"-intensification on the digitizer enables the

mass to be visually selected. The delay generator is changed until the

intensified portion of the trace overlaps the mass peak's position. The

shutter of the camera is closed when the pulser stops.

Computer And IEEE-488 Instrumentation

The computer coupled to the IAP is a Tektronix model 4052A desktop

computer. It uses an extended form of the BASIC programming language,









51

especially designed for graphics and instrumentation control over the

IEEE-488 interface bus. This is called the General Purpose Interface Bus

(GPIB) by Tektronix. There are four instruments on the GPIB: i)

Tektronix 7912AD transient waveform digitizer, ii) Scientific Instruments

5500 temperature indicator/controller, iii) Bertan 205A-30R D.C. high

voltage supply, and iv) Tektronix 4662 Interactive digital plotter. The

7912AD and the SI 5500 are both microprocessor controlled and can operate

independently of the 4052A, thus freeing the computer for other

operations.

A Tektronix 4909 File Manager system is also on the GPIB. The 4909

is a hard disk storage drive with a total of 96 Mbytes of memory.

Although it is on the GPIB, the interface is transparent to the user

because it has no address and is controlled through a ROM pack inserted

into the 4052A. The 4909 has been proven to be indispensable when the

large number of waveforms in a depth profile are collected. During

profiling, each waveform is stored while the digitizer is collecting the

next waveform. This procedure minimizes the dead time between the

collection of waveforms. Minimal data reduction is done to further

increase this efficiency. Additional storage is provided by magnetic

tape cartridges for the 4052A and an internal 256 Kbytes extended file

manager. With the extended file manager is also a serial communications

interface. There are four ROM slot positions. The third position must

contain the 4052R10 advanced file manager ROM pack for the 4909. Two ROM

packs are required for the operation of the IAP. The first is a TransEra

732-BCD I/0 ROM pack. It has 40 lines, 32 of which are configured by

software to be either input or output for binary coded decimal (BCD)









52

information. Four digits each from the meters of the high voltage supply

and the pulser's charging supply are read by the computer over these

lines. Each of these meters has 4-1/2 digits. Since each meter has only

16 lines, the least significant digit is disregarded. This ROM pack has

four control lines and four status lines, the latter not being utilized.

Two control lines are used to start and stop the pulse control unit.

The second ROM pack required by the IAP control software is the

Tektronix 4052R07. This ROM pack facilitates the handling of dimensioned

arrays. Waveforms from the 7912AD are 512 point arrays. The ROM pack

programs include MIN, MAX, CROSS, INT, DIF2, DIF3, and DISP. The MIN,

MAX, and CROSS commands find the indices of the minimum, maximum, and

where the waveform crosses a particular value. The INT command performs

a numerical integration of the array while the DIF2 and DIF3 commands

perform a two or three point numerical differentiation. The DISP

displays the complete waveform on the 4052A's screen very fast.

7912AD

The Tektronix 7912AD is the fastest digitizer which exists. It can

capture a single trace within a 5 nsec window into a 512 point array with

full 9-bit resolution. It utilizes Tektronix 7000 series plug-in

modules. The plug-in modules used in the IAP are the 7A24 dual time base

and the 7B28 dual channel amplifier. The dual time base was chosen

because a portion of the waveform can be expanded. The dual channel

amplifier allows the pulse pickoff to be displayed in addition to the

output from the chevron assembly. The 7912AD can be operated in a

limited fashion from the front panel. In the non-storage or TV mode, the

waveform is displayed on a monitor in real time. The monitor is used to









53

set the beam intensity and to set the proper operating conditions such as

time base and amplifier settings. When the digitizer is used from the

front panel in the digitize mode, a dual parallel trace is displayed on a

Tektronix 2213 oscilloscope. It is important to understand how the

digitizer works and why the dual trace occurs because of the importance

of setting the beam intensity properly. There are two electron beams in

the digitizer, a "writing" and "reading" which face each other. The

"reading" beam charges a solid state array chip and is scanned vertically

from left to right. The "writing" beam discharges part of the array.

When the slower reading beam scans these areas again, a current is sensed

indicating the results of the writing beam. The two traces displayed are

two arrays where the reading beam senses the change of state from written

to non-written, i.e. the top and bottom of the trace. A command which is

sent from the computer to the 7912AD is the "ATC" for average-to-center.

This command causes the 7912AD to find the center value between the two

arrays and fill in any missing points, and then store this new array in

the digitizer's memory. It also displays it on the oscilloscope as a

single trace. If the main intensity is too high, i.e. the trace is too

thick, after the "ATC" command, the displayed peak will be smaller than

it actually is. This means that in setting up the digitizer care must be

taken in adjusting the intensity to make sure the digitized signal is the

same as the monitored height. The pulse pickoff signal is used for this.

Another feature which the 7912AD has is the ability to average

several traces together, thus increasing the signal to noise ratio. The

signal averages 2,4,8,16,32 or 64 traces by adding each waveform to what

is memory. Because each waveform has 9-bit resolution and each value is









54

sent to the computer as a 2-byte word, no precision is lost in this

averaging mode. This mode is important because both time and memory

storage are saved by compressing the signal-averaged traces into one

waveform. Care must be exercised in choosing the number of pulses per

waveform because of the possibility of washing out the signal from a

minor constituent in the sample.

Scientific Instruments 5500

The 5500 is a microprocessor-based, low temperature, indicator/

controller. It is configured to use a chromel-gold w/.07% Fe

thermocouple. The maximum heater output is 90 watts. All calibration

and control can be done equivalently either from the front panel or over

the computer interface. It has three modes: i) Stop, ii) Manual, and

iii) Run. In the Stop mode, the heater power is shut off and the system

will go to the lowest possible temperature. In the Manual mode the

controller will try to maintain the set point temperature. In the Run

mode, up to a 30-point temperature profile program stored in the 5500's

memory is run. In all three modes, an actual temperature profile can be

graphed on the screen and also output to the Tektronix plotter.

The 5500 was used with the specimen holder used in this study to

determine the cool-down time and ultimate temperature. A heater was not

used. The time required to reach this temperature of 650K is

approximately two hours. The thermocouple in this holder broke after a

short time and was not repaired because of the need to disassemble the

manipulator system. The new holder described in Appendix A has an

improved cryogenic design and a heater is installed.









55

A program listed in Appendix D was written to input a program into

the 5500, graph the temperature profile data, and change the set point

temperature in the manual mode. The program name is "PROFILER" and it is

stored in the public directory of the 4909 file manager.

Bertan 205A-30R

The Bertan 205A-30R is a 0-30 kV d.c. power supply. It can be

controlled by the computer when used with the Bertan model 200-C488

interface controller. The controller accepts commands over the bus and

converts them to a 0 to 5 volt output which is fed into the remote

programming input for the 205A-30R. When the local/remote switch in the

rear of the 205A-30R is put in remote, control is done over the

interface. In the local mode, the output is the sum of the panel switch

settings. Since the front panel settings on this supply are discrete and

not continuous, a program was written to control the voltage with the

user-definable keys of the computer. This program is also listed in

Appendix D. The program is "FIMSUPPLY" in the public directory of the

4909. Because the supply is controlled by the computer, the IAP

data-handling program had to be modified to include the voltage controls

with the user-definable keys.

IAP Data Collecting And Handling Programs

The programs described in this section which collect and reduce the

waveforms from the IAP are based on the Tektronix demo programs which

were supplied with the 4052A and 7912AD. The programs evolved from

having the waveform acquisition, storage on magnetic tape, and data

manipulation done with single keystrokes of the user-definable keys to

automatic data acquisition and storage on the 4909 file manager. The









56

basic program used is "IAPINDEX_4909." This program stores waveforms on

indexed files in the 4909. New indexed files can be created at any time.

Single waveforms can be collected with one keystroke. Waveforms

collected can be chosen to be automatically stored in the next available

index within the file being used. The m/n ratios of peaks are calculated

in two ways: 1) by taking the ratio of the squares of the flight times

of an unknown and a known mass multiplied by the known mass; 2) by

straight TOF calculation using the equation II-1, previously defined.

The values obtained for the selected flight times are compared in table

format. The best results are obtained when the largest mass-to-charge

ratio known is used as the reference.

The "IAPINDEX_4909" program is written in independent sections, each

section being a subroutine which can be addressed either from the

user-definable keys or from within a controlling program. Two programs

which utilize a control program using these subroutines are "IAPRING" and

"PEAKPICKOFF." All control sections start at statement 60000. The

"IAPRING" program collects and stores waveforms continuously. The

"PEAKPICKOFF" program retrieves each waveform which was stored and finds

the Ni signal peak height. "IAPINDEX_4909" is listed in Appendix D. The

two controlling sections are also listed. The modified "IAPRING" program

having the high voltage supply control sections is listed. Its name is

"IAPRING.HV." In this program, all sections of "IAPRING" not required

for data acquisition were removed and sections of "FIMSUPPLY" needed for

controlling the high voltage supply were added. In addition all sections

which are used most frequently are controlled by user-definable keys

1-10; the others required the shift key for 11-20. Table II-1 gives the














User-defined
key #


Lower 1

2

3

4

5

6

7

8

9

10


Upper 11

12

13

14

15

16

17

18

19

20


57

Table II-1
User-Defined Key Functions

IAPINDEX 4909 & IAPRING.HV
IAPRING


FIMSUPPLY


I 9 .9


Acq. waveform

Move cursor left

Move cursor right

Light cursor

Read wfm. from file

Graph wfm. @ plotter

Calc. masses (eqn.)

Graph @ screen

Calc. masses (ratio)

Mark TOF times


Change current file

Set left marker

Set right marker

Marker's time dif.

Record wfm. on file

Auto-store on/off

Graph@ plot. w/markers

Ground (DC offset)

Graph@ scrn. w/markers

Reset flight-times


(same)

Increase V by vinc

Reset power supply

Increase V lOOv

(same)

Shutdown supply

Decrease V by vinc

(same)

Decrease V lOOv

(same)


(same)

Move cursor left

Move cursor right

Set V to Ov

(same)

Volt. ramp on/off

Incr. vinc by 10%

(same)

Calc. masses(ratio)

(same)


Read V,I

(same)

not used

(same)

Reset supply

Incr. inc 10%

(same)

not used

(same)

Shutdown


Poll devices

not used

Limit current

not used

Set V to Ov

Decr. inc 10%

not used

Limit voltage

not used

not used


Note: Some of these functions can be used when the control sections
(the portions of the IAP-series programs starting at statement number
60000), are running and looping. The result of pressing the key will
not occur until an appropriate point in the program. These points are
normally when the program is going from one subroutine to another.
Non-critical subroutines can be interrupted, but the program returns to
the point where it was interrupted and continues from there.









58

user-definable keys and their functions for the programs "IAPINDEX_4909",

"IAPRING," "FIMSUPPLY," and "IAPRING.HV."

Ion Gun

The ion gun employed in this study is a model G-2-D system from

Colutron Research Corp. This kit comes complete, assembled in a vacuum

housing. It must be configured by the customer with ancillary equipment

such as vacuum pumping system, power supplies and associated insulation

housing, and cooling system for source and electromagnet. This gun was

chosen for this study for several reasons, the most important being

vacuum considerations. The in situ implantation of deuterium to overcome

oxidation and cryogenic transfer problems presents a major difficulty

with Field-Ion techniques, enhanced field adsorption of hydrogen. That

is, during implantation with deuterium, the deuterium partial pressure in

the FIM/IAP chamber must be kept sufficiently low in order that during

subsequent IAP analysis field adsorbed deuterium does not interfere with

the implanted deuterium signal. The ion gun system was designed in order

to keep the FIM/IAP chamber pressure below 5x10 9 Torr during implanta-

tion. Figure II-10 shows the ion gun that is coupled to the FIM/IAP with

associated hardware and electronics. With the experimentally determined

operating parameters of this gun, the pressure in the chamber is

approximately an order of magnitude better than the design limit.

The gun operates with a throughput of approximately 10-' Torr-1/sec.

With the Balzers TSU 170 1/sec turbopump, the pressure during operation

is 5-8x10-6 Torr in the gun and 4-6x102 microns in the source. These

values are typical with the gaseous charges used in the source. Although

the gun is capable of using solid charges, for this study it has only
























































Figure 11-10 Overall view of the Colutron ion gun and associated
hardware, power supplies, and vacuum equipment. The anode and filament
supplies, thermocouple gauge, and ion source float at high potential and
must be isolated from ground. Cooling of the ion source and the velocity
filter is accomplished with a 1/2 h.p. refrigeration system. Only the
refrigerant lines can be seen in the photograph.









60

been configured for gases. The pressure loads for solid charges will be

considerably higher and the beam currents will also be less. Additional

differential pumping is done by a 20 1/sec Perkin-Elmer DI ion pump and

titanium sublimation pump connected to the beam line with a flexible

metal hose. Conduction limiting is also done with a 2 mm hole in a

double-sided blank flange mounted before the straight-through valve which

connects to the FIM/IAP chamber. Although the gun is capable of being

baked-out, this is not done due to the difficulty involved with removing

the freon cooling lines to the magnet and ion source.

Another important parameter of this gun which needed to be

considered was current density. A reasonably short time is required for

the implantation, again, because of the above arguments. For gaseous

charges this gun produces ion currents up to about 10 uA. With the

einzel lens in the system, current densities of 1-10 mA/cm2 are easily

achieved. The ion gun also incorporates vertical deflection plates and a

velocity filter. The velocity filter is an ExB type, or Wien filter,

with a horizontal deflection plane. Normal operation for aligning and

selecting an ion of a particular mass-to-charge ratio is done by changing

the voltage on vertical deflection plates and varying the power to the

magnet. The voltage on the horizontal plates are set at maximum supply

output for maximum dispersion. The Colutron filter [75] differs from a

simple ExB velocity filter in that the electric field is shaped by

electrically biased guard rings about the horizontal deflection plates.

The reason they are needed is that the Wien filter is a strongly

focussing lens and produces a line image with a very short focal length.

This is due to the fringe field shapes of the finite sizes of the magnet







































MAGNET 1 1 "
MAGNET U 2 I -
MIDDLE SHIM 3
PLATE 4 -- -- 4 PLATE
SHIM 3 5 3 SHIM 3
SHIM 2 6 BEAM 2 SHIM 2
SHIM i 7 SHIM 1
MAGNET 2 8

-- S
















Figure II-11 Schematic diagram of Colutron velocity filter. A desired
beam shape is obtained by carefully balancing the voltages on the shims.
The guard ring control unit supplied with the filter is a set of parallel
voltage dividers, each delivering a fraction of the deflection plate
voltage to its respective ring.









62

and deflection plates. By carefully balancing the potentials on the

various guard rings, the filter can operate in a non-focussing mode in

which a circular beam is produced, or in a line mode with a very narrow

line image. The resolution, M/AM, of this filter is approximately 400.

Figure II-11 is a schematic of the velocity filter.

The use of this velocity filter provides several benefits to this

study. Ions with different mass-to-charge ratios can be separated very

easily. This allows less pure charges or mixtures to be used. In fact

for the gases hydrogen, deuterium, and helium a mixture of 2% argon

stabilizes the plasma and reduces the pressure in the source considerab-

ly. The filter separates the same species with different masses and

charge states. For example, hydrogen can be ionized into H%, H;, and H+

and argon can be ionized in two states, Ar* and Ar*2. The choice of ion

complex, of course, will affect the relative implantation depths. For

example, the depth for H; would correspond to that of H+ at half the

energy.

The ion gun was mounted to the FIM/IAP in such a way that it can be

decoupled and used independently. A special holder for FIM specimen is

made from a MDC electrical feedthrough mounted on a Conflat flange. This

holder, shown in Figure 11-12, monitors the current density during

implantation. The grounded shield has a .062" diameter hole in it so

that the current measured multiplied by 50 (the reciprocal of the

cross-sectional area in cm2) is equal to the current density. When the

gun is coupled to the FIM/IAP, the current is measured by disconnecting

the high voltage lead and connecting it to an electrometer. It should be

noted that the current measured at this point is not the actual ion










































Figure 11-12 Faraday Cup/Sample Holder. This holder is used on the ion
gun when it is not coupled to the FIM/IAP. If the beam is larger than
the grounded outer shield aperture (piece to right), the beam current
density (A/cm2) is obtained by multiplying the current by 100.


DEFLECTION PLATES
SUPPLY


ISOLATED FROM GROUND


VERTICAL
DEFLtCTION
ACCELERATION & FOCUSSING PLATES
SYSTEM %


ANJ i I
SUPL, ALLELLEATIUN
SUPPLY


Figure 11-13 Schematic diagram of
target.


the Colutron ion gun, beam line, and


FILAMENT
SUPPLY









64

current, but also includes secondary electron and ion currents. Both

arrangements, ex situ and in situ, of the FIM/IAP can accommodate both

FIM and TEM samples. SEM samples can also be used but there is a

restriction on their physical size because of specimen transfer

mechanisms.

The beam line has a transmission phosphor screen with a hole and a

mirror for viewing, selecting and aligning the beam to be used. There is

a Faraday cup which swings into the beam and can monitor the current and

current density. The hole diameter of the tantalum grounding front

shield is .140" such that if the beam is centered and larger than this,

the current at the cup is multiplied by a factor of 10 to give the

current density. This cup is used to optimize the beam conditions. The

ion source pressure and anode voltage is adjusted for maximum beam

current.

Figure 11-13 shows a schematic diagram of the ion gun, beam line,

and target. The filament and anode power supplies are Lambda model

LK-351 and two LQ-524 (connected in series). These supplies together

with the thermocouple gauge controller for the source are isolated from

ground by a Colutron isolation transformer. This transformer is rated

for 10 kV at 1 kVA. Field testing of this transformer has shown it to be

capable of operation up to 22 kV. This value is the break-down voltage

between the ion source flange and the ion gun body. Two Brandenburg 30kV

power supplies provide the acceleration and einzel focussing voltage. A

Colutron power supply contains three separate variable, regulated voltage

supplies: a -100 to +100 V supply for the vertical deflection plates, a

0 to 300 V supply for horizontal plates, and a 30V-30A magnet supply.









65

For low Z ions with energies greater than 9 keV, an additional 45 V is

required for the vertical plate voltage. A box containing five 9-volt

transistor batteries in series is inserted in series with the above

supply. The currents at the target and cup are measured by Keithley

model 600B electrometers.



Mass Identification in the Colutron Ion Gun

A major advantage of the Colutron ion gun with the velocity filter

is that gas mixtures may be present in the gun whether deliberately mixed

for better performance or as the result of changing gases without

prolonged purging of the lines. In selecting a beam with a particular

mass to charge ratio by using the velocity filter, there sometimes is

confusion as to the identity of the various beams. The low end masses

are readily sorted out at a given energy by comparing the beams of the

helium/argon and deuterium/argon mixtures. The He* and D; signals have

the same mass to charge ratio and therefore the same magnet setting.

Both gas mixtures have the Ar+ and Ar+2 signals when hydrogen and

deuterium are present in the ion source, four beams are present having

m/n = 1,2,3 and 4 (H+, H2 or D', H+ or HD, and D0). In addition, the

Ar peak is always present when the gas mixtures are used, and the Ar

peak is usually the largest mass present, having the highest magnet

setting.

The masses are tuned with the velocity filter by changing the

magnetic field. The following expression holds true for determining the

masses in the velocity filter,









66

B,/B, = (m2/m1)1/2 II-2



where Bi is the magnetic field to balance the filter for mass, mi. The

magnetic field is proportional to the magnet current which in turn is

proportional to the dial setting on the Colutron magnet power supply.

Because of hysteresis effects, there is a constant offset current which

needs to be added to the measured current. This setting which is the sum

of the value read and the offset value is referred to as the true

setting, S. Thus the following can be used in place of equation 11-2,



S2/S, = (m2/m1)1/2 11-3



It should be noted that the difference between two true settings and the

measured settings is the same. Therefore rearranging equation 11-3,



S2/SI SI/Sl = (m2/m1)1/2 1 11-4

or

AS/SI = (m2/ml)1/2 1 11-5



By knowing two or more masses in the beam, a reference true setting for

one of the masses can be found,


S, = AS/(m2/ml)1/2-1 11-6


The true setting for mass, mi, can be found with two masses, or an









67

averaged S, could also be found using other known masses. If mi is

chosen as HI (mi = 1), then equation 11-6 reduces to


SH = Rm2-RH/(m2)1/2-1, 11-7



where Rm2 and RH are the measured settings of m2 and H, respectively.

Once SH is found, unknown masses are easily identified by

rearranging equation 11-7 and solving for mx, leading to


mx = [Rmx-RH/SH + 1]2, 11-8



where mx refers to the unknown mass at the measured setting Rmx.

The program, "COLUMASS", is available on the Tektronix 4052A which

will find both SH for a given energy and an unknown mass for a particular

setting. If more than two masses are known an averaged SH value is

found. Table 11-2 gives the values of SH at several energies and the

differences between the mass setting values for the indicated ions and

the setting for H*.



Transmission Electron Microscopy

Three different holders have been designed and used for TEM imaging

of field emitters in this study. The use of the TEM is indispensable for

obtaining the geometric parameters required for the depth profiling

programs which will be described in the next chapter. It also provides

an independent means of checking the total depth which has been field

evaporated during a profiling experiment. In addition, the TEM is











unsurpassed in obtaining crystallographic information about defects on a

fine scale with the use of diffraction contrast. In the following

sections, each of the three different holders will be presented and their

uses with respect to FIM studies will be discussed.



Table 11-2
Hydrogen and Relative Magnet Settings for
Several Ions and Energies

Difference Between Ion And H* Setting


Energy
(keV)


SH-HO H2,0D


(HD)*, H3


D2, He+


5 50.1 24.1 22 36 48 198



6 45.6 22.5 19 35 43 -- 184



7 44.1 25.1 18 35 41 -- 176



8 39.4 24.9 14.5 30.5 41.5 -- 159



9 30.0 26.0 13 22 30 126 156



10 31.3 18.3 12.6 22 32.5 -- 133




FIM/IAP "Bulk" Holder

Figure 11-14 shows a simple holder for use with the JEOL-SCSH bulk

holder. This holder provides a single tilt axis about the specimen's

































Figure 11-14 FIM/TEM "BULK" specimen holder. This holder has only a
single axis of rotation. Shown also is the sample insert which actually
holds the specimen.


Figure 11-15
two orthogonal
information is


FIM/TEM "Double-tilt" specimen holder. This holder has
tilt axes and is required when specific crystallographic
needed.











axis. This holder is useful when routine observation is desired, such as

checking specimen quality after preparation or after implantation. When

several of these holders are available, specimen throughput in terms of

the cycle time required to obtain a two-beam imaging condition, record a

micrograph, and insert a new sample is on the order of 15 to 20 minutes.

A two-beam imaging condition is important when imaging FIM specimens

because the penetration of the electrons is deeper due to dynamical

conditions. The value of this holder is limited when crystallographic

information is required. This is due to having only a single-tilt axis

available. Because most FIM specimens are prepared from drawn wire, they

have a preferred orientation along this axis. The orientation is a

usually a low-index pole parallel to this axis. For example, nickel

(FCC) has a (111) orientation, platinum (FCC) has a (001), and tungsten

(BCC) has a (011). The attainment of a two-beam condition is relatively

easy to achieve because of this, but the choice of the imaging condition

is determined purely by chance. Studies such as the determination of

defects are impossible with this type of holder because of the lack of a

second, orthogonal, tilt axis.

FIM/TEM "Double-Tilt" Holder

The above problems with defect identification using diffraction

contrast have been remedied by the construction of another TEM/FIM holder

which incorporates the second tilt axis [76]. Figure 11-15 shows this

specimen holder. It is a modified standard JEOL-BST double-tilt holder.

Both the original cups were removed and some machining of the rod was

done to accommodate a single-elongated holder for FIM samples. The

double-tilt is achieved by two rods applying friction to the cup as they









71















































Figure II-16 Partial kikuchi map of a Ni specimen showing the extent of
tilt available with the "Double-tilt" holder of Figure 11-15.











move in and out thus tilting the cup. Figure 11-16 is a portion of a Ni

Kikuchi map illustrating the tilt capabilities of this holder.

Approximately 270 on the second axis is available with this holder.

Two-beam conditions of various different orientations, can be obtained

and defect characterization can be done by using the *-b=0 criteria.

This type of analysis was done on a dislocation found in a Ni specimen

[77]. Figure 11-17 shows two two-beam conditions, 2=(111) and (111) of

this sample. In Figure II-17b, *-b=O and the dislocation is not visible.

The (111) case was imaged close to the [112] zone axis while the (111)

case was imaged close to the [011] zone axis. This illustrates the

usefulness of this holder in obtaining the desired crystallographic

information.

FIM/TEM "Field-Effect" Holder

The third TEM/FIM holder is shown in Figure 11-18. This

field-effect holder has only a single axis of tilt, but is capable of

having up to approximately 5 kV applied to the sample in situ in the TEM.

This holder was designed and constructed by Panitz*, but was first used

by this author [77] to study the effect of field-induced stresses in

emitters. This holder was found to have some peculiar operating

characteristics. If the holder is used without a cable connecting the

high voltage supply, the image would oscillate slowly. This was probably

a charging effect because when a grounding connector was used on the end

of it, the oscillations ceased. If the cable is attached and the power

turned off, the image is again unstable and drifts. Turning the supply

on and applying approximately 10 v stabilizes the image. Care must be


*Panitz, J. A., private communication.













4. C)
II
a .. i



S0r 0

S4l.a- -

*r- oC
M 4- s- i
10) +. aU e





v h ,C-Ir- U
I- 3"*"- E 0Io




S l -,- 4-













^^^1 ^ --- 0 ----af
to 44 4)
a) 4-- w 0
t' E


o o




11 O L:. *r- n
E 4- 0 U4




LL moa)
--~ 0 4--






= 04
0

C II- *a
.0 U'













U X .4-.) II (0
-- 4- M -
I

.00)



S-. .0 m

0.0
4-- In oi.-
o -t
w r- C U


0 U w-
4P U r-

0 E L. 4-
a) a) Cm
X- -0 -II r
0-.1 4.-

X4) 1


r-






L4




























































Figure 11-18 FIM/TEM "Field-Effect" holder. Voltages between 0-5 kV
can be applied to FIM specimens in situ TEM with this holder. Only a
single rotation axis is available.









75

taken that the weight of the excess cable does not affect the mechanical

stability of the stage. When applying relatively low voltages (up to 1.0

kV) to the emitter, the tip can still be imaged, but both mechanical and

beam positioning must be done and extreme beam settings (e.g.,condenser

and stigmater lens) must be done. Above approximately 1.1 kV an apparent

"hole" forms at the apex of the specimen due to the electric field. As

the field is increased, the "hole" gets larger and the specimens appear

to disappear into it. Figure 11-19 is a sequence of increasing voltage

depicting this. Because of this, the results of applying the field to

the sample in the TEM can not be seen while the voltage is on. Because

field evaporation is a thermally activated process, the application of

3 kV to a specimen in this holder will result in a field evaporation

voltage of about 13 kV if it would be returned to the FIM. For this

reason, voltages above 3 kV should be avoided in the TEM. Figure 11-20

is an example of the results of applying 3 kV to the Ni specimen

containing the dislocation in Figure 11-17. It is seen that the

field-induced stresses have caused the dislocation to sweep through the

sample. A faint slip trace can be seen in Figure II-20b and approxi-

mately 20 nm has been field evaporated.

The field effect holder will surely prove to be a valuable aid in

future metallurgical applications of FIM techniques. Studies using this

holder will be able to answer the most often asked question, what is the

effect of the applied field to the sample?

Another major use for this holder will be to expose features of

interest such as grain boundaries or precipitates to the surface for

imaging in the FIM. This holder, in conjunction with back-polishing


















-4 4- 5-~"
I 0= :3 U)

oo




(n r-- 24-
*- 0. -0 0





dC -C 0 -
S a) >-4












SM-*- O U --
r-- > --4 (U>
S *r-


-0 ( 4-40)



4- E W-
S- 4 U ),.

) CU. (0
--- O -" r)
0 > 00
00 0 0-i












C r 0- 4

C .' -
U *- E g

4- N0 4J-








0) Q-( 0 0


1 E 0
0- U






E 0"0 40
S i. **- ( I-q
Sit r- 40 a(
4-> 1 0



U C .- .4- 4-
0- 0 0a



-r- C
04 a, -C
0.- MWO

.4-- > -
C) 0- r 0.0
--
ar














-0 C)
0 0L
r- C >
o tO 4
I EO
0) *- E
= OX
E 0

CL
4- 0) C




E c



0 4-- 1o


to Em
to a


L 3 ,
4- -- I D
0 O
C 03 4-> C




to m
( E= 0
O cn
f- cc 0
4-i E -- CD
I *- 4- U
1- (0 0
-LAU U "-









0 O.
a Q.





U C- CL
0- CL0 0-
u- (0t







wE L- 0

C 4-
o (0






0 0 .'

0 .- Q.
()J 4-
*- 10 M 0

04a) c.
-C 0 V a0
4- 0)
0> -0



D-.--
4) 0 4- 4-
-.- .- J0
4- 0) C
4- CX 0





LUI (A u C


E
CA c
cm
F--
LL V lLo









78

techniques developed by Nord6n et al. [78,79], will help overcome one of

the major disadvantages of FIM techniques, specimen size. Because of the

extremely small volume accessible to analysis, features of interest are

extremely difficult to find and, in general, many samples are required to

be examined. This is true even when these features are present in large

quantities, e.g., samples with high dislocation densities or extremely

fine grain size for grain boundaries. In general, the geometry of FIM

specimens is ideal for observation in the TEM. In addition, this

field-effect holder can create experimentally controllable stress states

which are similar to the conditions found in the volume just ahead of a

crack tip in an embrittled alloy.

















CHAPTER III
EXPERIMENTAL CONSIDERATIONS


Depth Profiling Using The FIM/IAP

This chapter deals with experiments designed to establish the

validity of the procedures developed in this study. Without such

experiments, a complete FIM/IAP study of the ion implantation into

materials, especially the trapping of deuterium to implanted defects,

would not be feasible or believable. In addition to these control

experiments, the procedures for reducing the raw IAP data from implanted

specimens will be presented. The actual results of implanted specimens

will be given in Chapter IV.

This chapter will be divided into three main sections: 1) "Depth

Profilng Using The FIM/IAP", 2) "Effects Of Field Ionization And

Evaporation", and 3) "Special Problems Associated With

Hydrogen/Deuterium." Each section is important with respect to the

proper operation of the FIM/IAP and interpretation interpretation of

experimental results. The section on depth profiling in the FIM/IAP

will demonstrate a method for establishing a depth scale for extended

profiles when an imaging gas is not present. Such studies have not been

possible to date and are critical for emitters with relatively low field

evaporation fields. The section on the effects of high fields gives some

of the answers to questions which are always raised about their effect on









80

defect microstructures. There is reason for considerable optimism that

some defects induced by implantation are unaffected by field induced

stresses. The section on the special problems of hydrogen/deuterium will

show that in situ implantation is indeed feasible. Field adsorbed D can

be reliably distinguished from implanted D. This section will also

present some observations of a more general interest to FIM studies but

which also of importance to this study. Some of these are important to

proper operation of the FIM/IAP and/or analyzing the data correctly. It

should be born in mind that there are still fewer than twenty of these

instruments worldwide, nearly all individually designed and built.

Therfore, it is important that this type of information be disseminated.

Ring Counting Experiments

It is extremely important to be able to accurately depth profile

implanted species. Depth profiling is the forte of FIM in conjunction

with the atom probe. During evaporation, with an imaging gas present,

each ring of atoms constituting an atomic plane, appears to collapse and

disappear as evaporation proceeds. Each ring collapse of a particular

set of planes denotes a precise distance probed equivalent to that

d-spacing. This is the most accurate profiling method which exists,

providing, of course, that one can observe these ring collapses implying

that an imaging gas is present. In investigations which require UHV

conditions during analysis (i.e. without an imaging gas), such as the

present one, the rings can neither be seen nor directly counted. It was

thus necessary to develop another method of ascertaining the depth.

Further complications arise with respect to the IAP spectra, since

the signals vary in strength even when the voltages are held constant.









81

For shallow depth profiles a sudden increase in peak height indicates a

ring collapse. Counting these gives the same accuracy as with counting

the rings visually. This method is impractical for larger depths, such

as those needed for ion implantation studies, typically 100 nm or more.

This is because the voltages need to be raised many times in such a

profiling experiment, and it is extremely difficult to differentiate

reliably an increased pulse height due to a ring collapse or from an

increased evaporation rate. Furthermore, up to 30 thousand pulses may be

required to evaporate to the required depth on the order of 100 nm or so.

It would be very tedious to sort through such a mountain of data in order

to determine all the occurrences of ring collapses.

In Chapter II major requirements were cited for establishing a

suitable procedure for extended depth profiles with the IAP. The first

is that the method must be able to reduce the number of spectra to

manageable levels. Consider a typical example: the amount of storage

required if 30 thousand spectra are to be stored. (Because of the

requirement for rapid pulsing the spectra must be collected and stored;

analyzing them in real time requires too much time.) Each spectrum takes

about 5 kbytes of disk storage. Thus for the 30 thousand spectra, over

100 Mbytes of storage is required! The signal-averaging mode of the

Tektronix 7912AD alleviates this problem somewhat. The 7912AD can

average 2,4,8,16,32 or 64 traces, thus reducing the storage requirement

by the factor chosen. The second point, is that the method chosen for

depth profiling must be insensitive to the field evaporation rate.

During an extended profiling experiment the field evaporation rate will

of course change widely, both while the voltage is being held constant









82

(giving a decaying rate of field evaporation) and when the voltage is

increased (leading to an increased rate of field evaporation).

In the remainder of this chapter the method chosen for the depth

profiling requirements of this study are described in detail. It relies

upon the geometry of field emitters observed by TEM, the basic principles

of operation of the FIM, and a particular calibration procedure.

Specimen Geometry

The tip geometry assumed to apply for this study is a conical shank

with a spherical cap at the apex. The spherical cap joins the shank

tangentially, i.e. a radius vector from the center of curvature to this

point is perpendicular to the shank. In the FIM only a portion of the

specimen is imaged. Figure III-1 shows the assumed geometry of a

specimen. The cross-hatched region represents the probed volume of our

instrument with a subtended viewing half-angle of 200. With

this model, the radius of curvature is given as


R= (h+10o)sina/(1-sina)


III-1


where 1l is the distance from the initial surface, a is the shank

half-angle and h is the distance probed from the initial surface to the

final surface. Figure III-2 is a montage of an evaporation sequence of a

Ni specimen performed with the "field-effect" (FIM/TEM) holder. This

figure illustrates two points, the geometric model is quite valid and the

distance which the FIM/IAP is capable of probing is considerable (>1 um).

The computer-generated figure was done using equation III-1. The

parameter, a, was measured from the montage. Using the Tektronix 4662









83



1000 A


-LO I ---h


Figure III-1 Assumed geometry of emitter. Cross-hatched region
indicates volume probed by IAP. The distance h is the depth probed from
the initial surface to the second surface.


FIELD EVAPORATION OF A Ni EMITTER TO 3kV in-situ TEM













0 0.9 1 .3 1 7 2.1 2.5 3 0

0 7 1 1 5 1 9 2.3 2.7

APPLIED VOLTAGES (kV)



Figure III-2 Montage of TEM micrographs field evaporated in situ TEM in
the FIM/TEM "Field-Effect" holder. Also shown for comparison is a
computer-generated sequence using the model for the tip geometry
discussed in text.









84

plotter, each radius was drawn at the measured distance from the apex of

the cone (equal to 10 in equation III-1). The computed radii at the

various voltages in Figure III-2 are all very close to the actual radii

from the micrographs except one, the radius drawn at the indicated

voltage of 3 kV. This radius was measured from the micrograph and drawn

at the distance given by equation III-1. When it was drawn by the

computer, it did not match the location in the montage. After examining

the montage more carefully, it was found that one micrograph, taken at

2.9 kV was missing. The last micrograph in the sequence had been

positioned a little too far to the left. The offset between the surface

drawn and the one in the montage is approximately the displacement due to

this error. It should be pointed out that this error was not found until

after the application of equation III-1 in generating the figure. This

suggests a precision of better than 10 nm.

IAP probed-volume considerations

For the Chevron plates used as the detector in the IAP, it has

already been shown that the detector efficiency and gain is constant over

the energy range and for the types of ions employed here [70].

Furthermore, it is the detector which limits the width of the area probed

indicated in Figure III-1. If all the atoms within that volume strike

the detector and the detector efficiency is constant, the integrated

signal should be proportional to the number of atoms contained within the

probed volume. The number of atoms within this volume is simply related

to the volume by the density of the specimen. This model is the

foundation for the method used to determine depths in this study. That

is, it is assumed that the integrated bulk signal from the Chevron









85

detector is proportional to the volume probed. The integrated signal is

easy to measure, since it is just the cumulative matrix signal from the

spectra collected. The accumulation of spectra approximates an

integration very closely because of the small step involved with a ring

collapse, and that it can take many pulses to evaporate one layer. The

one assumption made with this method is that the concentration of the

element used for the accumulation process is constant and is proportional

to the probed volume. There may be situations for which this assumption

is not valid, e.g. ion implantation of massive ions or the presence of

precipitates, but for ion implanted deuterium into nickel, this

assumption is certainly reasonable.

The problem of depth profiling using the model just described is

reduced to finding the functional dependence of the volume with depth

and geometrical parameters of the specimen. Then to compare these with a

calibration experiment in which the rings are counted and the cumulative

signal is found from the spectra.

Volume calculation

The calculation of the probed volume is straight-forward.

Figure III-3 shows a representation of the cross-hatched volume in

Figure III-1. The volume of interest is the shaded region bounded by the

two spherical caps representing the initial and final surfaces, and the

sides of the cone representing the limit of the area of the emitter

surface projected onto the detector. The bases for the spherical caps

are indicated by dotted lines. This volume can be seen to be


V = V, + V2 V3


III-2
























h-


Figure 111-3 Representation of the volume probed by the FIM/IAP in an
emitter. The shaded area corresponds to the cross-hatched area in
Figure III-1.









87

where V, is the volume of the frustrum of the cone bounded top and bottom

by the dotted lines, V2 is the spherical cap of radius Ro, and V3 is the

spherical cap of radius R. The value h is equal to the number of planes

evaporated normal to the emitter's axis, (N dhkl). Using equation

III-1, the radii of curvature can be expressed as


Ro= lo*sina/(l-sina) III-3a

and

R= (h+10o)sina/(1-sina) III-3b


The equation for the volume of a spherical cap subtended by an angle,

3, and radius, Rc is



Vs.c.= *Rc(2-3cosB +cos30) III-4


Using this formula, V2 and V3 are given as



V2= *Ka 3l (2-3cosp +cos3 ) III-5a
and



V3= 3 *K3.[h3+3h210 +3hl2 +13]*(2-3cosp +cos3P) III-5b


where Ka= sina/(1-sina), and 1o is defined above. Using equations III-5a

and III-5b,


V2-V3= 3 -K3-[h3 +3h210 +3hl2]*(2-3coso +cos30) III-6









88

The volume of a frustrum of a cone is given as



VF= *Z-(RT +RB +RTRB) 111-7


where RT and RB are the radii of the top and bottom areas, and Z is the

height of it. For V1, RT= RosinB, and for V3, RB= RsinB. Using

equations III-3 and substituting into equation III-7 and doing a little

algebra, V, is given as



V= -[K2sin2 +K3(1-cos3)sin23]*[h3 +3h210 +3hl2] III-8



Substituting III-8 and III-6 into III-2, and simplifying the coefficient

terms,



V= .[K2sin2 -K3.(1-cosp)2].[h3 +3h210 +3hl] III2-9

This is the probed volume to which the cumulated matrix signal is

proportional. The parameters needed for each specimen to be found are the

shank half-angle, a, and lo, the initial surface to apex of the cone made

by extending the sides of the emitter to their point of convergence. The

system parameters to be determined are the detector viewing half-angle,

3, and the proportionality constant between the volume and the

accumulated signal. This constant, KD, has several other constants

within: the emitter density, the efficiency of the detector, the gain of

the detector, the gain of the amplifier, etc. Determination of KD will

be discussed in a later section.











Determination of P

For the depth evaporated in an IAP experiment the viewing

half-angle, 0, is assumed constant. The reason for this can be explained

by considering the FIM image. The image is a projection of the surface

of the tip onto the screen. This projection is similar to a stereo-

graphic projection in that the projection point is close to that used in

a stereographic projection [80]. With this pseudo-stereographic

projection, the angular positions between poles of crystal planes are

still approximately constant at all radii. This is true even though the

radius of curvature is increasing as the emitter is field evaporated

since the radius of the emitter is much smaller than the radius of the

sphere of projection. It can be understood by considering the distance

the projection point changes with depth (on the order of 1 um) as

compared to the projection distance (on the order of 10 cm). The only

difference between low and high voltage images of the same tip is the

appearance of higher-indexed planes as the radius increases. The

invariance of the motion of the relative positions of the major poles

present at all radii can be observed by d.c. field evaporation. The

value of 3 can be determined in the same manner as FIM micrographs are

indexed [80]. Figure III-4 shows an indexed micrograph of a Ni emitter.

The viewing screen angle can be found from the following relationship,



20 = (Dscreen/D(111),(113))'8(111),(113) III-10


where Dscreen is the measured diameter of the total viewing area of the

micrograph, D(111),(113) is the measured distance and 8(111),(113) the