Sputtering and ionization by helium and argon in the microsecond pulsed glow discharge using time-of-flight mass spectrometry

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Sputtering and ionization by helium and argon in the microsecond pulsed glow discharge using time-of-flight mass spectrometry
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Thesis:
Thesis (Ph. D.)--University of Florida, 2001.
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Includes bibliographical references (leaves 203-207).
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by Matthew Mohill.
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SPUTTERING AND IONIZATION BY HELIUM AND ARGON IN THE
MICROSECOND PULSED GLOW DISCHARGE USING TIME-OF-FLIGHT MASS
SPECTROMETRY











By

MATTHEW MOHILL


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


2001











TABLE OF CONTENTS

Page

ACKNOWLEDGEMENTS........................................................................v

A B ST R A C T ... ................. ........ ... .. ... ................................. ................vii

CHAPTERS ...........................................................................................

1 INTRODUCTION................................................. ................. .........

Solids Elemental Mass Spectrometry .....................................................1
Solids Analysis Techniques for Mass Spectrometry ..................................3
Glow Discharge and GDMS History ....................................................5
Scope of this Dissertation ..................................................................7

2 THE GLOW DISCHARGE ...............................................................9

Gaseous Discharges ......................... .......... ........................ ...... 9
The Abnormal Glow Discharge ........................................................... 12
Regions of the Glow Discharge .................................................... 14
Glow Discharge Processes ......................... ....... ...........................18
Glow Discharge Sputtering ................................................... 18
Mass of sputtering ion ..................................................23
Angle of incident ion bombardment .................................. 23
Incident ion energy ........................ ........ ..................25
The target material ................................................... .....29
Glow Discharge Excitation/Ionization .... .......... .........................31
Collisions in Gases ............ .......................................... ......... 31
Sputtered Species Excitation .............................................................35
Sputtered Species Ionization ............................................................ 38
Secondary Excitation/Ionization Processes ...........................................42
Types of Glow Discharge Devices .....................................................44

3 EXPERIMENTAL CONSIDERATIONS ...........................................48

Po%\ering the Glow Discharge ..........................................................48
D irect C current ......................................... ................... ....... 48
RadioFrequency ......................................................... ........ ..50
The Microsecond Pulse ..........................................................51







Pulsed Glow Discharge Mass Spectrometry ....................................... 51
DC vs p.s-Pulsed Experimental Considerations ....................................... 52
Sputtering and Ionization in the Microsecond Pulsed GD ...........................53
Analytical Advantages of DC GD ................................................... 54
Analytical Advantages of Microsecond Pulsed GD ...................................59
Atomic Emission Signal Enhancement ..................... ................59
Sputtered Spcies Ion Signal Enhancement ....................... .........59
Temporal Consideratinos in GDMS ............................................61
Glow Discharge Time-of-Flight Mass Spectrometry ...............................68
Introduction ........................................... ........................ .. 68
Time-of-Flight Mass Spectrometry ...........................................73
T theory ................ ........................................ ... ........ 73
Outpur Characteristics ........................................... ....... 74
Performance Limitations and Potential Capabilities ...............77
Inherent Advantages of TOFMS ...................... .............78
Material Transport and Instument Design ...........................................80
Ion Source .................................. ................................ 80
Direct Insertion Probe ........................................................80
Discharge Cell ...................................... ................. 82
Ion Extraction ........................................................... 84
Ion Optics and TOF Sampling ..........................................87
Electronics and Timing .............................................. 88
V acuum System .................................. ............ ........... 90
Ion Detection ............................................................. 90
General Procedure and Data Analysis .................................................91

4 CHARACTERIZATION OF THE MICROSECOND PULSED GD PROCESSES
IN HELIUM .............................................................. ......... 93

Introduction ............................................ ............................ ... .. 94
Background ............ ........................................ .................... 96
Microsecond Pulsed Glow Discharge Processes: Comparing Ar and He .........99
Discharge Initiation ............................. ....................99
Electron Ionization .................................. ... .........100
Voltage effect ... ... .................................. 105
Pressure effect .................................... ............ 107
Sputtering of Atoms ........... ....................... ................07
Incident ion energy .............................................113
Pressure effect .............................................. 115
V oltage effect ....................... ........... .............. 17
Diffusion of Atoms .................................................... 119
Ionization of Atoms ................................................. 126
Sam pling ........................................................... ..... 127
Sum m ary ................................................................. ................ 134







5 THE HELIUM DISCHARGE ANALYTICAL CHARACTERISTICS ........... 138

Introduction .......................................................... .. ..............138
Identifying Interferences .................................... ............................138
Interfering Ion Formation ..................................................... ....... 139
Rem oval of Interferences ................................ ................................ 141
Determination of Calcium in the Presence of Argon .................................147
Relative Sensitivity Factors ............................................................ 151
Relative Ionization Factor ............ .................... ................. ......... ... 160
Summary ...................................... ................ ....... 163

6 CLUSTER FORMATION AND IONIZATION IN A iS-PULSED GD .........164

Introduction ....................................... ................................. ...164
Background .......... ....................................................... 165
Sputtering Theory ................... .......... .... ...............................167
Cluster Form action Theory ................................................ ........ ........ 170
Direct em mission ............................ ............................... 170
Atomic Combination (AC) Single Collision .............................173
Atomic Combination by Double Collision ................................. 174
Theory Application to a Glow Discharge ................... ................... 176
Diffusion, Dissociation, and Ionization of Polyatomics .............................180
Matrix Species Distribution: A Cathode-to-Orifice Distance Study ..............188
The Effect of Pressure on Polyatomic Formation ................................. 195
The Effect of Voltage on Polyatomic Formation ...................................97
Sum m ary ............................... ................... ....................... ... 197

7 CONCLUDING REMARKS AND FUTURE DIRECTIONS ....................200

8 REFERENCES .......................................................... ....... 204

9 BIOGRAPHICAL SKETCH ................................. ........................ 210














ACKNOWLEDGEMENTS

It is with much pleasure and much sorrow that my graduate career is coming to an

end. I am indebted to many people who have not only been mentors but also friends. I

have the utmost respect for my advisor, Dr. W.W. Harrison, who has taught me scientific

integrity and discipline, and has guided me through the process of writing this

dissertation.

The work here is but one of many that have been performed in the Harrison/

Winefordner group laboratory, and would not have been possible without the support and

help from my peers. Special recognition goes out to Dr. Ben Smith. He has been an

active part of the success of my laboratory work and has enriched my time here at

University of Florida. Dr. Wei Hang and Dr. Kristofor Ingeneri introduced me to GDMS

and taught me much about the technique. I must thank Kris for many late night talks. Dr.

Chenglong Yang deserves special attention. Chenglong was a great laboratory mentor

and friend. His hard work and help in the lab were deeply appreciated. I must extend

thanks to the rest of the members of the Harrison and Winefordner group, new and old. I

will miss their friendship and lively banter.

I also wish to thank Dr. Christopher Barshick. He has been nothing short of a

great friend over the last five years, and was the one who convinced me to join the

Harrison group. I thank him.






A final word must be said about my family and close friends. I would not be the

person I am today without the love and support of my parents; through the very lows and

very highs. they have kept me on an even plane. I will forever be indebted to them for

this and I honor them with this dissertation. Thanks, Mom and Dad!!!

This work has been supported by the US Department of Energy, Basic Energy

Sciences, Hewlett-Packard Laboratories, and LECO Corporation.









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

SPUTTERING AND IONIZATION BY HELIUM AND ARGON IN THE
MICROSECOND PULSED GLOW DISCHARGE USING TIME-OF-FLIGHT MASS
SPECTROMETRY

By

Matthew Mohill

MAY 2001


Chair: Willard Harrison
Major Department: Chemistry

The focus of this dissertation is the study of the sputtering and ionization

phenomena in the microsecond pulsed glow discharge using helium and argon as plasma

gases. The glow discharge is an electrical plasma which directly atomizes and ionizes

solid metal samples. In this dissertation, a short, intense voltage pulse is applied to the

sample creating temporal aspects that are both fundamentally and analytically useful. An

in-house built reflectron time-of-flight mass spectrometer is used in these experiments in

which several ion source configurations can be used, including a direct insertion probe

and a Grimm-type device.

The ionization phenomena in helium and argon are studied. The high helium

metastable atom energy results in higher ionization capabilities. The pts-pulse mode must

be used in order to produce higher analyte intensities, that are not achievable in the DC

mode. The temporal advantages and selective ionization of sputtered species versus ga.i







species afforded by argon are lost when helium is used as the plasma gas. The removal

of mass peaks in critical parts of the mass spectrum, without losing sensitivity, is a

benefit of the use of helium as the plasma gas.

The impact of argon and helium ions, and the resulting collision cascade, were

studied relative to mass spectra observed for a copper matrix. Polyatomic clusters of the

copper matrix form and interfere with several elements in the mass spectrum. The

formation, dissociation, and ionization of these species are detailed in this dissertation.

The analytical advantages of using helium as the plasma gas to effect detection of

iron, selenium, arsenic, tellurium, gold, and lead are discussed. For lead and tellurium, a

reduction in the formation of polyatomic clusters results in the removal of these mass

interference. Relative sensitivity factors are calculated for both helium and argon,

showing that the higher metastable atom energies of helium make it a less selective

ionization agent resulting in signal enhancement of hard-to-ionize elements such as As,

Se, and Au. A relative ionization factor is defined which measures the sputtering and

ionization efficiency of helium compared with argon. In general. argon ions are efficient

at sputtering a copper target, but argon metastable atoms are only moderately able to

ionize the atom population produced. Helium is an inefficient sputtering agent. but an

excellent ionizing agent for the species present in a glow discharge source.













CHAPTER 1
INTRODUCTION

Solids Elemental Mass Spectrometry

Elemental analysis of solids employs a wide variety of detection techniques, of

which mass spectrometry is one. A mass spectrometer requires the production of ions for

analysis and subsequently separates them by their mass-to-charge (m/z) ratio. Inorganic

elemental analysis has traditionally been dominated by optical methods in which the

excitation and subsequent radiative relaxation of the analyte produce photons

characteristic of the energy difference between the accessible excited and relaxed states

of the analyte atoms. These photons are measured using spectrometers, diode arrays, and

other similar detection devices that convert photons to electrons. The difficulty for optical

methods is not in the analysis of pure materials, but rather in the analysis of multi-

element materials in which transitions of several elements may have similar energy such

that an overlap of wavelength occurs. This spectral complexity, with many transitions for

a single element, is quite opposite of what is seen in mass spectrometry, where the

elements are typically singly ionized and typically have few isotopes. Mass spectrometry

also suffers from mass interference; however, there are many schemes that combat

isobaric interference in glow discharge mass spectrometry (GDMS). In general, spectral

simplicity in MS is advantageous.

Mass spectrometry has been utilized since the early 1900's \whcn J.J. Thomson

observed "positive rays" from a low-pressure electrical discharge.' What we call positive










ions were studied by Thomson for isotopic abundance measurements, material analy sis.

and the physics of the electrical discharge. The electrical discharge used by Thomson

continued to be developed as an ion source for material and isotopic analysis, and

eventually a need for improved precision and higher sensitivity in mass spectrometers

arose. The ability to detect lower abundance isotopes of various elements was satisfied by

the continuing development of mass spectrometers, in particular the quadrupole.2 The

development of quadrupole mass spectrometry, in the 1950s, would become important

for organic analysis. Inorganic analysis would also benefit from the maturation of the

technique and become a popular mass spectrometric technique in the field.

With improvements in mass spectrometers, the construction of ion sources more

suitable than the Thomson electrical discharge were eventually developed. Aston created

an ion source for low-volatility solids such as refractory metals and metal oxides,3 and

small improvements continued with the Thomson electrical discharge.4 The big

breakthrough in ion sources for solid samples was the vacuum spark.56 The spark is

produced by a high voltage at high frequency between two electrodes (typically wires) to

vaporize and ionize solid material from one of the electrodes. The spark was originally

used as an optical emission device, and was slowly developed into a source of ions for

mass spectrometry. The spark source had the advantage of covering the full elementIal

range with typical detection limits of 0.1 ppm reported by Boumans and Gijbels.78












Solids Analysis Techniques for Mass Spectrometry

Techniques for the direct analysis of solids must fulfill three basic requirements:

1) vaporization/destruction of the sample; 2) atomization of the sample; and 3) ionization.

The techniques that are capable of performing all three in a single environment are ones

in which there is interaction of a plasma and or gas ion with a solid surface, where

particle bombardment at the surface and particle collisions in the gas phase satisfy the

requirements.

Energy is needed in order to effect requirement #1 above. The energy applied in

plasmas occurs in two distinct forms: electrical energy and optical radiation. Glow

discharges, spark sources, microwave and ICP plasmas all use electrical energy, while

pulsed lasers use optical energy produced by photon flux. The energy in particle beam

sources is strictly kinetic. Secondary ion mass spectrometry (SIMS) and fast atom

bombardment (FAB) mass spectrometries both are particle beam sources that take

advantage of surface chemistry to produce ions. A discussion of kinetic energy transfer

via elastic collisions follows in Chapter 2.

As is the case for the development of the spark source for mass spectrometry, the

development of new ion sources is driven by the status of material science, which

requires precision and accuracy in analyzing a sample, particularly for sample impurities.

Surface techniques used in materials science have also become critical areas of study in

analytical chemistry. Requirements for knowing the amount and spatial distribution of

impurities on the surface of a solid material have created the need for better










characterization techniques. In developing these new ion sources, the complexity of the

mass spectrometer system and type of mass spectra observed are all determined by the

analysis sought and the ion source used. These entities are then responsible for the

calibration capabilities and the quality of the mass spectra observed. In the development

of these sources, research has focused on the decoupling principle in which the

atomization and ionization steps are physically separate. For example, in using a laser for

solid sample analysis, the laser is used to vaporize the sample; the sample particles are

then transported into an ICP plasma for subsequent ionization. Ionization in direct laser

analysis does occur, but an ICP is more often used because of the wide range of ion

kinetic energies that are produced during laser analysis. In the case of a glow discharge,

the atomization and ionization steps don't occur at the same time even though they take

place in the same plasma volume in which no physical transport of the atoms is required

for ionization to occur.

The spark source mass spectrometer system was the first big breakthrough in

solids analysis. The source was originally coupled to the Mattauch-I lrzog double

focusing mass spectrometer, capable of energy and angular focusing in a single plane.

Due to the wide energy distribution and the erratic nature of the spark source, this mass

spectrometer system was the only one capable of reducing this energy distribution for

accurate analysis of the ions produced simultaneously.

In the late 1940s and earl\ '50s, secondary ion mass spectrometry (SIMS) \\a.

developed to solve the lack of precision and accuracy of the spark source.9 J.J. Thomson

x as the first to observe the ejection of secondary ions from a surface after primary ion










bombardment,9 and the first instrument of this kind was developed by Herzog and

Viehbock in 1949.10 In the 1970s, development of SIMS as a method for detecting

organic compounds on a surface became the primary focus and continues to be one of the

leading applications. 1-12 The development of SIMS was most likely headed in the

direction of organic analysis and left the inorganic elemental analysis field open for

improvement.

Plasma source development included the work of Fassel13 with the ICP and later

its combination with a quadrupole mass analyzer for the elemental analysis of solutions.

The ICP combined with mass spectrometry has developed into the most powerful

elemental analysis technique. Microwave induced plasmas (MIP) were also developed

for solution-based elemental mass spectrometry,14 yet the MIP didn't develop into a

commercial success. The MIP is still developing new directions including the plasma

torch15 and other configurations that take advantage of its excitation capabilities as

opposed to its atomization and ionization capabilities. Over the same time period, glow

discharge development led to its use in the plasma mass spectrometry field. The

developmental history of glow discharge into an analytical technique follows in the next

section. The use of a glow discharge as an ion source was a critical point in this history

and is important for this dissertation.



Glow Discharge and GDMS History

Glow discharge devices have long been used in atomic spectroscopy and are

routinely used for analysis of solid materials. Hollow cathodes were used by Paschen to










investigate atomic structure by atomic emission and absorption.16 The use of hollow

cathodes was limited to analytical atomic emission as a means of determining low

concentrations of elements across the periodic table, including nonmetals. In the five

decades that followed Paschen's work, the applications of GD devices in chemistry

developed slowly, mainly due a lack of instrumental advancement and a poor

understanding of the analytical capabilities of the glow discharge. The last 30-40 \ ears

has seen the slow growth and maturation of the glow discharges as a technique for solids

analysis, including the development of commercial instrumentation. A need for more

routine analysis resulted in the development of "restricted" glow discharges by Grimm,'7

in which flat samples were used directly as the cathode material. Just as hollow cathodes

grew in popularity when Paschen first introduced them, so did the Grimm-type GD

device. It has become the metal industry standard for bulk analysis and particularly for

depth profiling by atomic emission spectrometry.



Glow Discharge Mass Spectrometry

Analytical glow discharge mass spectrometry has been around since the early

1970s, \%hcen Coburn reintroduced the glow discharge as a solids ion source for the

analytical community.'8 Coburn was able to show the utility of monitoring the ions

produced by a sputtered target in both direct current (DC) and radiofrequency (RF)

discharges. In the mid-to-late 1970s, the development of a quadrupole-based GDMS

s stem was undertaken by the Ilarrison group at the Uni\ersity of Virginia. in whichh bulk

solid and solution residue analysis were the main focus. Radiofrequency (RF) and direct










current (DC) modes are the traditional and more commonly used modes of operating the

glow discharge. RF is primarily used for non-conducting materials, which cannot be

analyzed in the DC mode. The DC mode is more widely used, especially in the metals

industry, where trace analysis is important. Pulsed discharges have been around for many

years, primarily for enhancing signal from hollow cathode lamps. Many improvements

have been undertaken to develop better GD ion sources for mass spectrometry. One of

the most important developments for elemental analysis is the resurgence of time-of-

flight mass spectrometry (TOFMS). With rapid analysis time of the full elemental range,

the TOFMS system has been shown to be quite advantageous in analytical chemistry.19-20

Chapter 3 will describe the operation of the glow discharge with respect to analytical

performance and its coupling to the time-of-flight mass spectrometer.



The Scope of this Dissertation

Developmental and fundamental studies of the pulsed glow discharge have been

of primary interest in the Harrison group over the last twelve years. With the advent of

the microsecond pulsed source a new series of studies have become of interest.21'23 A

thorough comparison of the helium gas characteristics in the microsecond pulsed plasma,

including sputtering and ionization, has yet to be accomplished. The focus of this

dissertation is the characterization of the [ts-pulsed glow discharge using argon and

helium as plasma gases. The following chapters describe the differences observed from

the use of helium and argon as discharge gases for mass spectrometric purposes.

Microsecond pulsed discharge processes are considered for each gas. with special





8



attention given to atomization and ionization. One of the problems facing DC and RF

GDMS is the high background from the discharge gas and related species. One goal of

using different gases and the microsecond pulse mode is to alleviate these interference

problems reducing and removing interference in key parts of the mass spectrum, thereby

facilitating element detection.












CHAPTER 2
THE GLOW DISCHARGE

Gaseous Discharges

When a sufficiently high voltage is established between two electrodes in a

gaseous medium, a breakdown of the gas forms positively charged ions and free

electrons. The positive anode and the negative cathode create a voltage gradient in the

plasma, such that an electrical circuit has been created where the discharge carries the

current. This is what is called a gaseous discharge, and these discharges can be defined

by the current at which they operate. Figure 2-124 shows a schematic diagram of the

characteristics of gaseous discharges based on current and voltage. The three main

discharges shown in the diagram are: 1) the Townsend discharge, 2) the glow discharge,

and 3) the arc discharge.

The Townsend discharge operates in the sub-millitorr pressure range and is

characterized by the production of very few ions and electrons, or low current. This

discharge is low in luminosity due mainly to the low production of excited species which

give a gas discharge its "glow" characteristic. The Townsend discharge is not self-

sustaining, but relies on external X-rays and UV light to produce electrons and ions in the

plasma. The Townsend discharge has been used in analytical mass spectrometry in two

ways: first as a way of ionizing the fill gas instead of high energy electrons emitted from

a hot filament;25 and second as a chemical ionization source.26 The transition from the

Townsend discharge to the glow discharge is distinguished as a region in \which electrical






10









T titT
I I I l
I ,, ( I t
C C C tr
)I I
b .o4I I I ,-
> v) ( I -I I
C &4 I I i
c X, E0 03 3
o., 2. oo u0





9 -7 -5 -3 C-1
01 0-4 W W












10 10 10 10 10 10
0




Current (







Figure 2-1. The current-voltage characteristics of gaseous
discharges showing the plasma coverage on the sample surface.
Vb= breakdown voltage, Vn= normal operating voltage,
=operating voltage of the ar discharge [Ref. 24
10 10 10 10 10 10
Current (A)


Figure 2-1. The current-voltage characteristics of gaseous
discharges showing the plasma coverage on the sample surface.
Vb= breakdown voltage, V = normal operating voltage,
Vd=operating voltage of the arc discharge [Ref. 24]










current increases as the discharge voltage decreases. This increase in current and decrease

in voltage are mostly due to the increase in collision energy exchange at higher gas

pressure.

The glow discharge has two modes, a normal glow discharge and an abnormal

glow discharge. The normal glow discharge is characterized by its luminous glow.

Increases in current do not result in a current flux change because the plasma hasn't as

yet covered the surface of the cathode entirely (see Figure 2-1). No voltage increase,

therefore, is required to sustain the plasma, as long as current increases. Once the entire

surface is covered by the plasma due to increasing current, the current density undergoes

change and a transition to the abnormal discharge occurs. Increases in current result in

increased current density in the abnormal glow discharge region. Increases in voltage

result in increases in current, and help sustain the plasma. The abnormal glow discharge

region, where plasma atomic spectroscopy is performed, is the region of gaseous

discharges used in this body of work. The following sections will be devoted to

describing this region.

As the current is increased, thermal vaporization of the cathode occurs due to the

high current density and increased bombardment by the filler gas. The availability of

high analyte number densities affect the potential fields, and the current-voltage

characteristics become "normal", such that current increases, but the maintenance voltage

decreases. This defines the transition from the glow discharge to the DC arc.

The arc discharge is commonly operated at atmospheric pressure, has large

currents (10-1000 A), and a bright plasma. The high current heats the surface so much










that thermionic electron emission becomes a prominent current carrying mechanism. Gas

temperatures are typically as high as 2 x 104 K and charged particle densities about 1016

cm-3. The arc discharge has been used in atomic emission analysis of solid metals27 and

in vapor deposition of thin films.28 Recent papers have reviewed this old technique for

new applications.29



The Abnormal Glow Discharge

The glow discharge source is a simple device. At its simplest, it consists of an

anode, a cathode, a fill gas at low pressure, and a power supply (Figure 2-2). In our case

the cathode consists of the material to be sampled, the anode is the glow discharge

chamber housing, and the power supply is operated in either a continuous DC, RF, or

pulsed DC mode. When a negative voltage is applied to the cathode, electrons released

from the surface, or spontaneously emitted free electrons, undergo acceleration to\ ard

the relatively positive anode chamber housing. During this acceleration, electrons collide

with fill gas atoms producing positive gas ions. These ions experience the attraction of

the cathode at high negative potential and are accelerated toward it and eventually

bombard the cathode sample. If the gas ion has enough energy (greater than the binding

energy of the atom in the matrix) it will cause the release of several species from the

cathode, including neutral atoms and molecules, ionized atoms and molecules, photons.

and electrons. The electrons will continuously be accelerated toward the anode while the

voltage is on, thereby sustaining the flow of current, creating a "self sustained glow

discharge." The glow discharge can operate at a pressure from 0.01 50 Torr depending










Power Modes


Pulsed DC


DC /


Figure 2-2. A simple glow discharge device showing the various power modes


RF










on the discharge gas, and has average currents in the low milliampere range in DC mode

and less than 1 mA in pulsed mode (depending on duty cycle).



Regions of the glow discharge

A glow discharge consists of 7 distinct regions: the Aston dark space (ADS), the

cathode layer (also called cathode glow, (CG) ), the cathode dark space (CDS), the

negative glow (NG), the Faraday dark space (FDS), the positive column (PC), the anode

dark space (ANDS), and the anode glow (AG). The research in this dissertation requires

operation in the abnormal glow discharge range, as described above. Atomic mass

spectrometry and other analytical spectroscopies probe the collision rich negative glow

for analytical information about the sample. The close proximity of the cathode and the

anode result in the disappearance of the faraday dark space, the positive column, the

anode dark space, and the anode glow particularly in MS when cathode to anode

distances are less than 1 cm. When the cathode and anode are brought closer and closer

together, the cathode dark space remains unchanged in thickness. The positive column

shrinks until it and the Faraday dark space (FDS) are consumed. Figure 2-330 is a

representation of the glow discharge and its distinct regions and their electrical

characteristics. Figure 2-4 shows the abnormal discharge and the regions that are most

important for mass spectrometry and how the negative glow will be probed in this

dissertation.

Nearest the cathode surface is the Aston dark space, which is a largely non-

luminous region, due to the lack of collisions that cause excitation to occur. This is an







ADS = Aston dark space
CG = Cathode glow
CDS = Cathode dark space
NG = Negative glow
PC = Positive Column
ANDS = Anode dark space
AG = Anode glow


PC


HANDS



li


Figure 2-3. The regions of the normal glow discharge and
the corresponding electrical properties [Ref. 30]


C(
ADS
\V



II


SCDS


./^*


AG


[G






Sampler (Anode)

I Ion Skimmer


To Mass Spectrometer


: >


Figure 2-4. The abnormal glow discharge setup for mass
spectrometric detection


n
C11
0
CP
CD










area of negative space charge due to the secondary emission of electrons from surface

collisions of bombarding gas ions, as well as the presence of electrons released upon

neutralization of gas ions impinging on the target. Regardless, the electrons are

accelerated away, and current is primarily carried by positive ions in this region. A short

distance later the space charge becomes positive and remains that way through the

cathode dark space (see Figure 2-3).

The cathode glow or cathode layer separates the Aston dark space from the

cathode dark space. The low luminosity is due to the recombination of incident ions with

slow electrons. The cathode dark space is the next zone of little luminosity. The negative

glow is an electrically neutral zone; therefore, the voltage difference between the cathode

and the anode is reduced in the small region between the cathode and the negative glow,

this being the cathode dark space. This region is often called the cathode fall region, due

to the decrease of the voltage over this small area. The negative glow is the region with

the most analytically useful information. It is a collision-rich, field-free environment in

which excitation and ionization processes occur. It is this region in which glow discharge

ionization studies occur. The relative luminosity of these regions is determined by the

radiative relaxation processes occurring within that region. Hence, a "dark' region has

few excitation processes, while a "glow" region has many. The lack of luminosity in the

dark space is due to the small number of electron-atom collisions. The electrons have

been accelerated by the cathode potential and the efficiency with which discharge gas

ionization occurs decreases. Hereafter, all discussion will pertain only to the use of the

abnormal glow discharge, and mainly its negative glow region.










Glow Discharge Processes

The mechanisms that lead to atomization, excitation, and ionization in a glow

discharge occur within discrete zones of the discharge structure. Ionization of discharge

gas occurs in both the negative glow and in the cathode dark space. Atomization occurs

at the surface of the cathode, a result of the ionization and subsequent bombardment of

the discharge gas. Excitation and ionization both occur in the field-free negative glow as

a result of collisions with electrons, ions, or excited atoms with sufficient energy. Figure

2-5 is a diagram summarizing the processes occurring in the glow discharge and the

zones with which they occur.



Glow Discharge Sputtering

The approach or acceleration of an ion toward a cathode surface can lead to 1 of 5

possible interactions called collectively the collision cascade: 1) the ion may be reflected

or scattered away, 2) the impact of the ion may cause the release of an electron from the

surface (necessary for maintenance of the self-sustaining glow discharge), 3) the ion can

bury or implant itself permanently in the crystal lattice, 4) the ion may cause

rearrangement of the top few layers (called "radiation damage"), and 5) the impact of the

ion will eject a target atom. This last one is known as sputtering and is the most

important analytically. For any ion bombardment event, more than one of these

interactions occur.

The set of interactions above imply that an ion is the only species impinging on

the surface, when in fact either an ion or a neutral species can do so."' It is easiest to use









































Figure 2-5. Summary of GD processes










an ion in the model, because it can be accelerated across the dark space via the attraction

of the negative cathode, whereas explaining how a neutral species is attracted to the

surface is a good bit more difficult. It is believed that the Auger emission of an electron

from the surface neutralizes a majority of the incident ions just prior to striking the

surface. With all that can occur during the collision cascade, it is not surprising that

sputter ejection comprises only 1% of all the energy transferred at the surface of the

cathode.30

Sputtering occurs when an ionized (or neutralized) gas atom bombards the surface

of the cathode, embedding itself in the lattice of the solid, and then transferring enough

energy to create a "shock wave" cascading through the lattice. This shock wave has been

compared to the break in the opening of a game of billiards except in three dimensions"3

(See Figure 2-6). Three species representative of the sample can be sputtered upon

bombardment; a sputtered ion, a sputtered neutral atom, or a polyatomic cluster. In the

case of glow discharges, the negative voltage on the cathode will immediately attract the

sputtered ion for redeposition onto the cathode surface, leaving only a "cloud" of

sputtered neutral species to diffuse, without any field effect, across the cathode dark

space and into the negative glow. The sputtered ions produced upon bombardment of the

gas ions are called "secondary ions", and are the species used for analysis in the surface

technique called secondary ion mass spectrometry, or SIMS. The sputtered atoms are of

interest for spectroscopic purposes, as they are the primary species that leave the surface

during the sputtering process. The release of atoms from one medium (a solid surface)

and the subsequent dissolution into a gaseous medium gives glow discharges and



















------------- T












Cathode
Surface






Figure 2-6. Schematic of sputtering in a glow discharge based
on a billiards break










particularly GDMS a relative freedom from matrix effects, which often plague other

surface and solid analysis techniques. While not completely true, theoretically the

atomization and ionization processes are separate in the glow discharge plasma and

ensure the lack of chemical memory for the sputtered species once they leave the surface

of the cathode.2023

Sputtering yield is the primary measure of sputter efficiency in a glow discharge,

and is usually measured as the number of atoms sputtered per incident ion. Two

equations exist in order to determine this efficiency. The first, developed by Sigmund,32

takes advantage of the energy/mass transfer term (bold in the equation). In Equation 2-1,

the sputter yield (S) is given by:

S = (3/4)7T2(c)[4MiM2/(MI + M2)2](E/Uo), (Eq.2-1)

where a is a function of the relative masses and the angle of incidence of the incoming

ion, MI and M2 are the respective masses (g) of the ion and the sputtered atom, E is the

incident ion energy (eV), and Uo is the surface binding energy that must be overcome for

sputtering to occur (eV).

Experimentally, Boumans33 developed an expression for the sputter yield (S)

represented by Equation 2-2:

S = 9.6 x 104 (W/M i t), (Eq. 2-2)

where W is the weight loss (g), M is the relative atomic mass of the sputter species (g), it

is the ion current (A), and t is the sputtering time (s). The bombarding ion current i+ is

rclactd to the total discharge current i by Equation 2-3:

i+= i/(1 + Yi) (Eq. 2-3)










where yi is the number of secondary electrons released, on the a\ crage, by one ion. The

following subsections will detail the parameters used in these two equations.



1. A. h .s.s of Sputtering Ion

One might assume that as you increase the mass of the sputtering ion, that the

sputter yield would increase because more momentum can be delivered to the surface.

The mass transfer term predicts that sputtering is dependent upon both the sputtering ion

and the target atom, and that sputtering efficiency will reach a maximum when the value

of Mi/M2 is unity. This would make Ar+ an ideal sputter ion for the first row transition

metals and Kr+ for the second row. In Sigmund's theory (Equation 2-1), the alpha (a)

factor offsets the mass transfer term such that the Ar ion is a better overall sputtering

agent than other inert gases such as krypton and xenon even though these two heavier

gases would deliver more kinetic energy and momentum to the surface under similar

conditions. The mass of the sputtering ion is critical to this work. The lighter mass of

helium will be an important factor as a sputtering agent in GDMS.



2. Angle of incident Ion Bomnhardichnt

The collision cascade describes a variety of sputtering ion-surface interactions

that can occur, and it makes sense that sputtering would be dependent upon the angle at

which the gas ion bombarded the surface, thus maximizing momentum and energy

transfer to the top few layers of the surface. Figure 2-734 shows the effect of angle of

incidence in ion beam sputtering on sputter yield. It should be noted that ion beam












>25 E=I OkV




Eo 105/ e ,V





S(dree)





sputter yield at normal incidence[Ref. 34]
0 30 60 90
s a (degrees)


Figure 2-7. The effect of ion angle of incidence (a) on sputter
yield (atoms/ion). Y(a) = sputter yield at angle a, Y(O) is
sputter yield at normal incidence[Ref. 34]










sputtering is much different than glow discharge sputtering and is used here solely as a

means to explain sputtering in the glow discharge system. Figure 2-8 sho m two possible

paths for an incident ion in a glow discharge upon application of the discharge voltage.

One argument for the incident nature of ions is illustrated in the picture of a glow

discharge sputtered surface (Figure 2-935). Microscopic cone-like spires have formed on

the surface of the cathode. The most likely way formation of such structures could occur

is for the impinging ion (or atom) to have a bombardment angle 90 degrees to that of the

surface proper, such that "holes" are dug into the surface. This means that regardless of

the trajectory of an incident ion, in the region between the cathode surface and the

negative glow, the field attraction interacts strongly with the ions, that any effect of its

initial trajectory is eliminated. So, while ion bombardment studies may indicate relative

sputter patterns in a vacuum, they are not necessarily indicative of what occurs in a low

pressure discharge.



3. Incident Ion Energy

The energy of the bombarding ion also plays an important role in sputter yield.

Sigmund's theory predicts that the sputter yield of a given target material is proportional

to the bombarding ion energy. Carter and Colligan have shown that proportionality exists

over only a small ion energy range.36 Figures 2-10a37 shows the effect of low incident

ion energy on sputter yield for a Cu target with noble gas ions. It has been predicted that

glow discharge ions bombard the surface with energies that lie in this linear region of the

curve. The sputter yield line in the figure shows a range of proportionality from about

















03

>


X


Y


Figure 2-8. Possible paths for incident ion (A). X = No voltage
effect on trajectory. Y = Voltage controls trajectory













* d1.,4



1- A


f 4 j


Figure 2-9. A copper target sputtered by a 1 torr argon discharge
showing microscopic cone formation [Ref. 35]


ft. 41E





















20
E 20-
0
Lineor region
8 A

C"

I-0- A

A


'Knee' of curve


0 500 1000

Id on energy (eV)
Threshold
energy
(ET)


Figure 2-10. a) The effect of low incident ion energy on sputter yield
for a copper target and argon ions. Triangle and circles are various data
points[Ref. 36]










100 eV to 750 eV. The range of this proportionality will likely change basIed on the size

of the incident ion. Figure 2-1Ob36 shows a higher energy range for sputter yield \ eruM

incident ion energy. As ion energies get much higher, ion implantation occurs and a

maximum of energy transferred to the top few layers of the lattice is reached, and the

number of sputtered atoms reaches a maximum.



4. The Target Material

The target material is the one parameter in glow discharge sputtering that is the

most variable, as there are many matrices that can be analyzed. Therefore, multi-element

analysis can be affected by the matrix. The Sigmund theory suggests that sputter yield is

dependent on the ion to atom mass ratio. Wehner3 believed that other processes are

involved aside from just the mass relationship. Having produced charts of the sputter

relationships with a 400 eV Ar+ gun, he found a dependence of sputter yield on d-shell

filling. The explanation says that as the d-shells are filled with electrons, the atomic radii

decrease, thereby increasing the atomic density in the matrix. The increased atomic

density prevents the bombarding ion from implanting or penetrating too deeply into the

matrix. The crystallographic orientation of the matrix most likely does not change with

the increase in density as the orientation of the atoms does not change, only their size.

The maximum amount of energy is then transferred to the top few layers of the matri\.

increasing sputtering. The difference in sputter yield between iron and copper is argued in

this manner. The mass of copper is greater than that of iron, and according to the mass

transfer term, the sputter yield of iron would be larger than copper if argon \\ re used as














Copper


Almei & 3ruce
(1961)


- Xe


. 0


"- N e


_/


10 20 30 40 50 50 70 k.V

Incident Ion Energy (keV)



Figure 2-10. b) The effect of high incident ion energy on
sputter yield for various gases [Ref. 37]


o / -- -- N
i i i










the bombarding gas. The sputter yield of iron is less because copper has more d-shell

electrons that, ultimately, increase the density of the matrix. Wehner's plots for argon and

helium are shown in Figure 2-11 a and b. The different sputter yields amongst elements

may cause a problem when analyzing alloys. Preferential sputtering3 will lead to an

enrichment of the lower sputter yield material at the surface. Due to the inherent

sputtering characteristics of the glow discharge, an eventual steady state will be

established and the gas phase population will be representative of the bulk sample.



Glow Discharge Excitation/Ionization

For analytical glow discharges, the primary concern is the excitation and

ionization of the sputtered species, and one would hope that these were the only

processes that occurred in the plasma. However, ionization of the plasma gas and its

impurities, and interference from sample impurities, can make analysis difficult. It is

important to understand the processes that maximize analyte excitation and ionization

and minimize discharge gas excitation and ionization. First a brief discussion of the

nature of collisions in the glow discharge is given below.



Collisions in Gases

Collisions in a glow discharge are the most essential processes that occur and are

relied upon to ionize sputtered species in the negative glow region of the plasma. Belo\\

are definitions taken from Howatson:24 Elastic Colli.-ion. are simple, mechanical. gas-

kinetic collisions in which the energy is always kinetic; Inelastic colli. ion. are those in












2-6
2.4- Argon
2-2-
2-0-
1-8
1-6
1-4

1-0-
08^ *AI

0 e4 SI
0-2- C
0 10


---




Cu 'Pd

Cr NI
Co
Fe &Ge Ru
Mo
V Zr b
Ti Nb


I I ____ ,


20 30 40
Atomic


50 60 70
number


80 90


\0.24- Heeium
-22- *Be
-20-

0-16
0-14- A
0-12-
0-10-
0-08-
0-06- .C
0-04-
0-02-

0 10


t



T


Au


20 30 40 50 60 70 80 90
Atomic number


Figure 2-11. Wehner sputter yield plots for a) argon
and b) helium at 400 eV incident ion energy [Ref.39]


I










which some of the energy of the collision is transferred into internal energy of the particle

struck, thus producing a reduction of kinetic energy of the system. Elastic collisions can

be compared to the collision of two billiard balls, much like that shown in Figure 2-6 for

sputtering. Figure 2-1230 shows schematically the two collision possibilities of an

electron and an atom. The electrons in orbit around an atom give an approximate uniform

field about the nucleus, and when an electron approaches, it can be repelled or deflected

in a way that is similar to the deflections of two billiard balls or two ideal elastic spheres.

The fundamentals for perfectly elastic collisions comes from the Maxwell-Boltzmann

kinetic theory, in which each atom has a definite radius and cross-sectional area such that

the probability of the two atoms colliding can be calculated.40 However, in such a

collision, changes in internal energy are not considered. For example, if an electron with

energy less than the ionization potential of helium were to strike a helium atom at zero

energy, then the helium atom would have to sustain the shock of the electron collision

(via kinetic energy) because there is no mechanism to transfer that energy (via potential

energy) under the theory of elastic collisions.

Now consider an electron with exactly the amount of energy needed to remove an

electron (IE of the atom). In order for the atom to be converted into a positive ion, the

electron must have enough energy to not only remote e the electron, but also some excess

energy to remove the electron and to compensate for recoil from the atom. Therefore,

electrons with energies that don't exceed that of the ionization potential of the atom they

are striking will result in an elastic collision.














a.








b.







Figure 2-12. a) Elastic collision of an electron and an atom.
b) Electron ionization [Ref. 30]










In the negative glow region of the analytical glow discharge, collisions that excite

and ionize are inelastic. Inelastic collisions fall into two categories: collisions of the first

kind and collisions of the second kind. Collisions in the negative glow plasma with

electrons of various kinetic energies are collisions of the first kind. Collisions of the

second kind occur when atoms and massive particles collide, resulting in potential energy

transfer. Table 2-140 lists the most likely excitation and ionization phenomena in the glow

discharge. A number of factors determine the relative role of these processes including

discharge conditions and source geometry.



Sputtered Species Excitation

Atoms that have diffused across the dark space into the negative glow, face a

series of inelastic collisions by electrons, metastable atoms, and other ions (gas and

sputtered). The glow discharge plasma is an electrically neutral plasma with a high

population of electrons. Electrons in the negative glow are very low in energy (Figure 2-

1341) as the Electron Energy Distribution Functions show under various conditions.

Hence, excitation mechanisms that involve energy transfer to levels at, or less than 3-4

eV (above ground state) will be preferentially populated over those that exist at higher

energies.41 Transitions from non-ground state energy levels to higher levels do occur, but,

without the populations as most of those that are ground state resonance transitions.

These ground state transitions are mostly responsible for the atomic transitions that occur

in the UV-VIS (k = 200-700 nm) regions of the electromagnetic spectrum, and are

responsible for the primary emission from the negative glow.















I. Primary excitation/ionization processes
A. Electron impact
Me + e- (fast) M* + e~ (slow)/M' + 2e-
B. Penning collisions
Me + Ar* -* M* + Ar/M + Ar+ e-
II. Secondary processes
A. Charge transfer
1. Nonsymmetric
Ar+ + M0 -, M' (M+*) + Are
2. Symmetric (resonance)
X+ (fast) + XO(slow) -, Xo(fast) + X+ (slow)
3. Dissociative
Ar+ + MX -* M+ + X+Ar
B. Associative ionization
Ar*, + MO -* ArM+ + e
C. Photon-induced excitation/ionization
Me + hv -, M*/M+ + e-
D. Cumulative ionization
Me + e- -* M* + e- -+ M+ + 2e-


Table 2-1. Glow discharge excitation and ionization phenomena




















AA ** a
AA A
0 o oo a 5 mA
= o o 8 mA
030 o*0 & 12 mA
30 0 &
u. so o o 15 mA
um 20 % *% A 18 mA
*1 00 0 0

20 6.0
10 0 o ca'--


0.0 1.0 2.0 3.0 4.0
Electron energy, eV


60- a---
o 00oo b
s50- 0
0 *e o
S40 o* 10 mA
" vi0eo- o 12 mA
30 o % o 15mA
So 18 mA
U 20 MC oa o a 21 mA

10 0 o

0-
0.0 1.0 2.0 3.0 4.0
Electron energy, eV




Figure 2-13. Electron energy distribution function
(EEDF) for the negative glow region [Ref. 41]










Sputtered atoms are not the only species that are excited in the negative glow.

Excitation of ionized sputtered atoms can also occur. As shown in the list of processes,

the collision of an electron and a gas atom can produce a metastable gas atom that has

enough energy to ionize and or excite sputtered atoms. These ionized sputtered atoms

can undergo similar collision with electrons and metastables that produce excited state

ions. The collision must overcome the energy required to raise an electron above the

ground state ion energy (IE) to the next available energy level.



Sputtered Species Ionization

As previously mentioned, the quantification of an total ion number density via

mass spectrometry is much easier than by atomic emission because relatively few

isotopes exist for each element; whereas multiple transitions represent the excitation of

the ion population. Therefore, fewer measurements are needed in mass spectrometry to

analyze trace elements. The energy of such ions is not entirely important, except when

sample focusing for mass spectrometry is considered. For quantitative analysis in MS, a

maximization of analyte ionization, while maintaining matrix representation, increases

the informing power of the technique. As Table 2-1 illustrates, several collision

imcchaniiisms exist for analyte ionization. The most likely sputtered species ionization

mechanisms are by electron ionization and Penning-type collisions.

For electron ionization, the electrons must have enough energy to ionize the

analyte atom. However, the electron energy distribution function (Figure 2-1341), shows

that very few electrons have enough energy to ionize most metals with I.P.s in the 5-10










eV range. Electron and ion densities have been measured in a RF glow di'chlirge plasma,

based on electrons in the 0-5 eV range.30 The density measured, 10'I cm-3, is 3-4 orders

of magnitude higher that electron densities measured in the 5-10 eV range.30 The lower

electron population in the 5-10 eV range, would produce very low relative analyte ion

populations compared to what have been typically measured in the negative glow of

argon plasmas.30 Vieth and Huneke42 have studied the production of an electron via the

Lovett rate model in which a 7.1-7.6 eV electron is produced that could ionize many

metals. This model, based on electron impact and three-body collisions, proposes a

double Penning collision as shown below in Equation 2-4.

Arm* + Arm* -- Ar+ + Aro + e- (7.1-7.6 eV) (Eq. 2-4)

No experimental evidence has been provided to assume that this mechanism occurs for

analyte ionization as the probability of this process is very low; even though Penning-

type collision (inelastic, with transfer of potential energy) are assumed in many

experiments as the primary analyte ionization mechanism in the negative glow. Table 2-2

lists the low lying metastable energies of various noble gases. The energies are well

above the I.P.s of most of the solid periodic table, and for noble gases from argon to

krypton, the energy is below that of most gas impurity I.P.s. Metastable gas atoms are

formed when inelastic collisions raise the energy to this long lived excited state (11.5 and

11.7 eV for argon).

Quantitative estimations of Penning ionization in a glow discharge have been

performed and have given numbers anywhere from 40-95% depending on discharge

conditions.43 A variety of reasons exist to support Penning ionization as the primary









Gas Spec IE (eV)
02(dimer) 12.0697
H20 12.621
OH 13.017
O 13.618
H 13.598
Ni 14.53
Ar2(dimer) 14.5
N2(dimer) 15.581
Ar 15.759
He2(dimer) 22.22
He 24.587


Ar met 11.5
11.7
He met 19.8
20.6


Table 2-2. Gas species ionization energies and helium and
argon metastable atom energies










mechanism of sputtered atoms. Mass spectra from argon discharges have analyte ion

signals higher than those of the discharge gas species even though the gas species are in

much higher quantities than the sputtered species.44 This reasons is applicable to an argon

DC discharges and not a helium DC discharge because the energies of the helium

metastables are higher. It's been postulated that preferential ionization of the sputtered

species is the reason for this observation. The metastable energy levels of argon are high

enough to ionize a majority of the elements in the periodic table, whereas the metastable

energies of argon lie below the ionization potential of any impurities from air leaks. The

discrimination in energy is one of the main reasons why argon has become the primary

discharge gas in most analytical discharge mass spectrometric instruments. Chapters 3

and 4 will discuss the role of metastable energies on analyte and gas species ionization

further relative to the use of helium as discharge gas.

The best evidence for the Penning ionization mechanism arises from laser

depopulation studies in which lower ion signals were observed for species with ionization

energies less than the metastable level of argon at 11.72 eV, while those with higher

energy were unaffected.45 The same result was observed for a neon discharge except that

the species with ionization energy above 16.71 eV were unaffected.46-49 Experimental

methods of determining the role of Penning ionization, including varying the discharge

conditions,46-49 and quenching the metastable states with methane46-49 have supported the

claim that Penning ionization plays a prominent role in sputtered species ionization. In

Chapter 3, a discussion of the pulsed glow discharge will show even more dominating










ex idence for metastable ionization based on the nature of the events that occur in the

pulsed discharge.



Other secondary excitation/ionization processes

Sy)Immeric charge e.\change is thought to be limited to the region closest to the

cathode such that argon ions that are normally accelerated across the dark space collide

with neutral atoms close to the surface. The incident argon ion continues on its path to the

cathode where it contributes to sputtering. Asymmetric charge exchange also contributes

to ionization in the negative glow. These mechanisms are summarized as equations in

Table 2-1 and shown in Figure 2-14.30 The contribution of each charge exchange process

varies depending on the ion source. This is particularly true for hollow cathode and

Grimm-type glow discharges,50 where asymmetric charge exchange is thought to play a

greater role than symmetric charge exchange. As described before, the ionization

provided by Penning-type collisions is preferred, because of the strong uniformity of the

ionization among elements (RSFs), and the selectivity against background gas species.

Associative inization is the primary mechanism for the formation of polyatomic ions that

contain a di,,charge gas and sputtered species. This process usually takes place within the

negative glow and its contribution in the mass spectrum is in some proportion to the

population of the sputtered species. For a typical diode discharge, the relative role of each

of the ioni/ation mechanisms is uncertain, except that Penning ionization appears to be

the primary ionization mechanism for the sputtered species. As discharge parameters






















Electrode


Ion strikes neutral,
charge exchanges 0, Ion enters dark space
Sfand accelerates

+O +

tt



Forms fast
neutral and slow 0.:*:A: ~
Ion G

Sheath Glow
Region


Figure 2-14. Charge exchange processes [Ref.30]










change. so do the magnitude of these mechanisms; whereas Penning-type collisional

ionization is considered to be the favored process regardless of the discharge conditions.



Types of Glow Discharge Devices

Pin-type/Disc or coaxial (Figure 2-15a): This device is the most commonly used for trace

analysis of metal and environmental samples (usually non-conducting soils) \where a

conducting binder is required. In the case of the pin type, the ease of using the chamber

as the anode makes it the most versatile of these devices. The pins are 1-2 mm in

diameter, with about 5 mm in length exposed to the plasma. The high density of atoms

and ions above the tip of the pin or disc sample have made this device common amongst

commercial glow discharge devices; one example is the VG 9000 Double Focusing Mass

Spectrometer. The disc samples are mounted using a holding device that exposes only

the flat front surface to the plasma, via shielding that surrounds the diameter of the top

surface (typically 4 mm). The work described here uses these small flat discs for metal

analysis.

Planar diode (Figure 2-15b): This is the simplest of the glow discharge devices yet, it is

not used very much. Coburn used an RF power supply with a planar diode for

imcasurement, via mass spectrometry, of ion populations in a sputter deposition

chamber. 8 This is also the basis for the de\ elopment and production of the Grimm-type

glow discharge source.

Grimm Discharge Lamp (Figure 2-15c): The Grimm-type GD device was first introduced

by Grimm in 196817 and has gained in popularity mostly because of the ease of sample










interchange and the ability to analyze thin films.51 The Grimm discharge is often called

an obstructed discharge because the anode is positioned within the dark space distance of

the cathode. This limits any sputtering to the area within the anode shape. It has become

very popular for bulk analysis. Similar sources that resemble the Grimm source have

been constructed for use in atomic emission and mass spectrometries.30

Hollow Cathode (Figure 2-15d): The hollow cathode lamp is probably the best known

and one of the most widely used GD devices. It is commonly used in atomic absorption,

emission, and fluorescence studies. This device is characterized by low voltages and

higher currents compared with other glow discharge devices, due to the constricted nature

of the plasma within the cathode and the static nature of the discharge gas. Called the

"hollow cathode effect," the plasma constriction produces an increase in the radiation

emitted from the negative glow, making this source attractive for optical spectroscopy.52

Hollow Cathode Plume (Figure 2-15e53): The hollow cathode plume was developed by

the Harrison group, and was primarily used for atomic emission studies. The plasma was

formed by the constriction of a hollow cathode discharge to a 1.5 mm orifice in the base

of a normal hollow cathode. Sample cathodes were machined to fit in the base of graphite

cylinders. At a certain pressure, the plasma plume would form through the top end of

the cylindrical hole. Typical argon pressure was about 1 Torr, and currents betw cen 50

and 200 mA at 1000 V were used.







MHV
connector







Silicone
fiberglass
tubing


MACOR
probe
tip





Sample
pin
High-vacuum
feedthru


0-=


CATHODE


Sample


Q,2mm thick
teflon sheath
y to pump 1


DISCHARGE CHAMBER


d.


+
D.--


Figure 2-15. GD devices, a) coaxial cathode b) planar diode c) Grimm
discharge lamp d) hollow cathode lamp


a.


b.


ANODE












CTr
CL
Q-


Figure 2-15 (cont). e) The hollow cathode plume


e.











CHAPTER 3
EXPERIMENTAL CONSIDERATIONS

Powering the Glow Discharge

Direct Current (DC)

DC is the most familiar and most widely used method of powering glow

discharge plasmas because of the availability of inexpensive, stable power supplies that

are very easy to use. The direct current glow discharge plasma is a self-cleaning plasma

in which little sample preparation is required before discharge initiation. Elemental

analysis in the DC mode usually consists of a preburn period in which contaminants from

the sample surface are sputtered off, such as oil from the users hands, oxides from the air,

and any non-uniformity of the sample surface. The DC plasma can be compared to an

electrochemical cell in which the cathode material is eroded or dissolved into a solution.

In the case of an argon glow discharge, the atoms dissolve into what is essentially an

argon gas-phase solution.

The roles of atoms, ions, and metastables have been extensively modeled by

Bogaerls and Gijbels,54-56 and have shown a good correlation with experimental data.

Their results show that mctastable ionization and electron impact ionization are the

primary mechanisms for analyte ionization, of which electron ionization can contribute

up to 40%. In their models, fast argon atoms are also considered to be a sputtering agent,

not just argon ions. The models define the role of each species present in the plasma and

\which ones are most important for the analyst. The most important feature of the glow










discharge is its ability to provide a stable concentration of atoms and ions for analysis,

the advantage being that in most GDMS instruments, the analyzer always samples the

same region of the plasma, leading to higher precision and higher accuracy. For GDMS

chemists, the DC glow discharge is an excellent ion source because of its 100% duty

cycle, which, with the appropriate choice of mass spectrometer, is a great advantage in

elemental analysis.

DC glow discharges have two disadvantages; one is low power compared to

techniques such as inductively coupled plasma, electrothermal vaporization, and laser

induced plasmas; and the other is the need for an electrically conduction material. The

lack of a conductivity requirement for these other techniques allows universal sampling

of any sample or matrix, giving these techniques a great advantage over the DC glow

discharge which for now is limited to conducting metal matrices. In order to sustain a

current in a DC discharge, the sample being analyzed must at least have semi-conducting

properties, and even then accurate analysis is not guaranteed. The conductivity

requirement has not prevented the analysis of non-conducting samples in the DC mode.

The use of a conducting matrix binder has been very successfully used in the analysis of

non-conducting powders."7 The trouble faced in these analyses is heterogeneous

distribution of analyte in the matrix. A secondary cathode has also been emplo) ed to

sample non-conductive solids. A "dummy" hollow ring cathode is positioned above a

non-conductive surface allowing accelerated atoms and ions to strike the non-conductive

target. Sputtering of the non-conductive surface results in the ejection of material into the










plasma formed in the middle and above the surface of the conducting ring cathode,

effecting excitation and ionization of the non-conducting material.



Radiofrequency (RF)

With radiofrequency powered glow discharges, most concerns about sample non-

conductivity are minimized. The RF glow discharge was created by Wehner, who

proposed the use of a rapidly oscillating voltage that would cause a DC bias at the surface

of the cathode thereby creating a plasma much like that in the DC mode. The ability of

the RF glow discharge to analyze both conductive and non-conductive samples makes it

the most versatile of the sampling modes of the glow discharge. The RF discharge is less

affected by surface contamination and reaches a stabilization point faster than the direct

current mode of operation.8

The main drawbacks of RF glow discharges include expense and power

efficiency. RF power supplies are significantly more expensive than DC power supplies

and require a matching network so that efficient coupling of the power to the sample

occurs. Often, reflected power can dissipate the effectiveness of RF po\cer and the

analyst usually tries to minimize or eliminate it. Additionally, if shielding of the source

and sample is not complete, the source can act as a radiotransmitler and disrupt the

capabilities of other detecting devices such as ion counters and microchannel plates on

mass spectrometer devices. Lo\ er sputter rates have been reported for the RF glow

discharges compared to the DC glow discharge, although increases in ionization have










been observed compared with the DC discharge.59,60 There are several comparisons of

the fundamental and applied aspects of these two powering methodes.58-60



The Microsecond Pulsed Mode

Thus far the use of a glow discharge device has been described in a continuous

fashion where voltage and current are maintained throughout the analysis. While there

are advantages to such analysis, pulsing the glow discharge can create additional

advantages for elemental analysis. A pulsed cycle consists of a short on time, eg. 10 Its,

followed by a long off time, before another 10 uts pulse is initiated. In a one second time

period, the frequency of this cycle is 100 -1000 Hz for the work in this dissertation. It is

difficult to maintain a plasma at frequencies less than 20 Hz and often instability occurs

at frequencies higher than 1000 Hz. Sampling the sputtered species in mass spectrometry

occurs anywhere from 80-300 uts after the pulse period. This delay will be referred to

here as the "deflection time", and corresponds to the time between the glow discharge

pulse and time that ions are deflected into the time-of-flight mass spectrometer. There are

several analytical advantages to using the glow discharge in this mode and they are

covered in the next few sections.



Pulsed Glow Discharge Mass Spectrometry

Pulsed GDs have been studied by the Harrison group for about 30 years, starting

mainly with hollow cathode lamps. The goal of the experiments was to obtain higher

emission signals by decreasing the on time and increasing the peak pot~er. At that time,










pulsed periods \\ere milliseconds long. In the last 6-7 years, the pulsed discharge has

moved from the millisecond to the microsecond time regime. A variety of techniques

have been used to probe the microsecond pulsed plasma, including atomic emission (AE),

atomic fluorescence (AF), and mass spectrometry (MS).61'62 The immaturity of the us-

pulsed plasma in analytical chemistry necessitates some explanation of the fundamental

and analytical aspects that distinguish this power mode from the more conventional

continuous modes of operation.



DC vs. las-pulsed Experimental Considerations

The microsecond pulse is characterized by a peak voltage of about 2 kV, and a

peak current of about 200 mA (avg. 0.4 mA at 200 Hz). The DC discharge typically

operates at about 1 kV and 4 mA average current. The high peak current in the uts-

pulsed mode causes an instantaneous heating of the surface, followed by a long relaxation

period. In the DC mode, there is no cooling or off time, and sample over-heating can be

a problem, causing the sample to melt despite the low average current. The average

current in the pulsed discharge mode, approximately 0.4 mA, is less than that in the DC

mode, even though the peak current is 50 times higher. The lower a\ eragc current

reduces overheating in the pulsed mode and extends the operation time. Sample

overheating in the DC mode often results in the need to cut short analysis, which is why

many GDMS instruments have cryogenic cooling s) stems installed. The long-term

stability of the pulsed discharge is advantageous for bulk and trace anal\ si.s. With a

longer operation time, diagnostic studies, like those in this dissertation, are more easily










performed. In addition, the time consuming cooling or replacing of an overheated sample

is not needed in the pulsed mode.

Different instruments are required to operate in the two modes. The power

supplies are different, in that an internally pulsing unit is required to create the short

transient signal desired in the pulsed mode. Leading edge rise times must be fast, on the

order of 30 ns, in order for the pulsing unit to be effective. The maturity of the pulsed

discharge is dependent on decreasing these rise times so that shorter pulses can be used

with much higher power. There are mass spectrometers that are better suited for the DC

mode than for the pulsed mode. An ideal MS system for the DC mode might be a Fourier

Transform Ion Cyclotron Resonance Mass Spectrometer (FT-ICR-MS) where inter-

element and isobaric interference can be eliminated with extremely high resolving

power.63 In the pulsed mode, taking advantage of every pulse for mass analysis is

advantageous. A mass spectrometer that accepts short transient signals and analyzes all

masses simultaneously would be the best fit. The time-of-flight mass spectrometer

(TOFMS) is the right choice. The system offers full elemental coverage and is fast with a

linear dynamic range of about 5 orders of magnitude. A later section will discuss the

fundamentals of the TOFMS system and cover the advantages of using it as a detection

device for the microsecond pulsed mode.



Sputtering and Ionization in the Microsecond Pulsed GD

The sputtering mechanism doesn't change between the continuous and pulsed

modes. Argon ions and fast argon atoms are still the primary sputtering agents.










However, the time regime and the energy with which sputtering occurs do change. While

the DC mode results in continuous injection of atoms into the plasma, the tps-pulsed

mode supplies the plasma with atoms in a transient fashion, allowing for certain temporal

advantages. The sputtering ions and atoms will have a greater energy when bombarding

the surface in the pulsed mode as compared to the DC mode, owing to the higher

instantaneous power for pulsed operation. This leads to higher instantaneous sputter rates

in the pulsed mode as compared to the DC mode. In the ps-pulsed mode, analyte

ionization most likely occurs by Penning ionization. Argon metastable atoms are the

dominant ionizing agent existing after the discharge is extinguished. Electrons rapidly

dissipate after the 10 pIs pulse period, and by 100 p.s (analyte sampling time) electrons

have most likely thermalized. This means that electron impact makes little contribution

to analyte ionization unlike in the DC mode.

In the DC mode, there is always a constant ion flux toward the cathode surface,

and there is always a constant electron flux away from the cathode surface, which leads

to a dark space larger than in the pulsed mode and forces the optimal sampling distance to

be longer in the DC mode than in the pulsed mode. This may contribute to the lower ion

signal observed in the DC versus pIs-pulsed modes for mass spectrometry since the

atomic ion concentration is greater at 3 mm abo\ e the sample surface as opposed to 7 mm

due to the concentration gradient of atoms away from the cathode surface.










Analytical Advantages of the Direct Current GD

DC glow discharge analysis is a proven technique. The constant source of atoms

and ions is reliable and increases the precision and accuracy for trace elemental analysis.

It is a good atom generator despite its relatively low power compared with other

atomization techniques such as the ICP. The DC discharge is inherently self cleaning,

therefore, requiring little sample preparation to the surface. This is one of the advantages

that can be very useful for samples that readily oxidize. With the constantly applied

voltage and continuous bombardment of the sample surface, oxides are less likely to

form, allowing for greater sputtering of the surface. Figures 3-la and 3-lb show a

comparison of a DC and a uts-pulsed glow discharge mass spectra for a tantalum cathode.

The [ps-pulsed spectrum shows a series of oxides beyond the elemental species while the

DC spectrum has only an elemental peak. The elemental peak is greater in the DC mode

than in the pulsed mode showing that oxides are masking the sputtering of the tantalum

elements in the pulsed mode, due to their formation during the relaxation period. If TaO

formed in the area above the surface, then its contribution would be observed in the DC

mass spectrum. However, it is not observed, and it must be assumed that TaO is

sputtered off the surface in the p[s-pulsed mode, likely being formed immediately after

perturbation of the surface by a voltage pulse.

DC glow discharge operation is simple, requiring few fast electronics to gate

signals for collection. For analytical use, it has very good sensitivities for all elements

and allows detection by multiple analytical techniques (Figure 3-261'62). DC glow






0.35 +


0.30


0.25


0.20

0 -
'r 0.15


= 0.10


0.05


0.00
I I I i I
0 100 200

m/z

Figure 3-1. a) DC mass spectrum of tantalum at I kV and 1 torr Ar





















0.05 -




0.00 -


100


200


m/z


Figure 3-1. b) gs-pulsed mass spectrum of tantalum
at 2 kV and 1 torr Ar


0.20-




0.15-


0.10-


>-
(I,
a,
C
CD
c-,
.


Lj--,









Glow Discharge Spectrometries




MASS SPECTROMETRY


+
+

+


hv


ATOMIC
FLUORESCENC


ATOMIC
ABSORPTION


hvo
E



Po P

-I I-,


RIMS = Resonance Ionization Mass
Spectrometry
OGE = Optogalvanic Energy


t
OGE
_t,


Figure 3-2. Glow discharge analytical techniques


ATOMIC
EMISSION


LASER
RIMS OGE


r J










discharges have become very useful for thin layer analysis providing excellent depth

resolution with multi-element capabilities.51



Analytical Advantages of the Microsecond Pulsed Mode

Atomic Emission Signal Enhancement

There is a two-fold advantage in performing AES in a microsecond pulsed GD

plasma: first, there are instantaneously more atoms produced at the surface than there are

in a DC plasma. Secondly, the microsecond pulsed GD produces electrons having more

energy than do DC GD electrons, resulting in greater excitation of atomic states, allmo ing

for better detection limits and increasing informing power. Furthermore, the wide range

of populated states can be used to determine how energetic microsecond pulsed GD

electrons are compared with DC and RF GD discharges. More energetic electrons also

lead to a greater population of metastable argon atoms. Metastable atoms are capable of

ionizing gaseous metal atoms, and have excitation capabilities. These factors result in

emission signals up to 20X greater in the pulsed versus DC modes for certain elements.62

It has also been shown that the signal enhancement experienced by the sputtered atoms is

much greater than that experienced by the fill gas. This leads to better signal-to-noise

and signal-to-background measurements.



Sputtered Species Ion Signal Enhancement

Figure 3-3 shows the comparison of a DC signal at 1 kV and a pis-pulsed signal at

2 kV. The more energetic electrons produced in the pulsed mode lead to an increased


















Cu'


I I I I I a
20 40 60 80 100 120

Mass/Charge





Figure 3-3. Glow discharge mass spectra for copper in
argon; a) ls-pulsed mode, b) DC mode










production of Ar ions available for sputtering. Therefore, a greater instantaneous

population of atoms occur during the pulsed period, or a greater sputtered atom density at

the surface than produced in the DC mode. The more energetic electrons also serve to

create a greater metastable argon atom population, increasing the ionization of sputtered

species. All these factors lead to the 3-4 fold signal enhancement seen in Figure 3-3 for

the copper ion. The signals for Sn at 400 ppm in a NIST brass sample are shown in

Figure 3-4. Figures 3-3 and 3-4 show that about the same signal enhancement is obtained

in the pulsed mode compared the DC mode for both bulk and trace signals. One concern

was that the transient nature of the ion source would cause high noise. However, a

reduction in noise compared to the DC mode is observed because of the long relaxation

of the discharge before sampling occurs.



Temporal Considerations in GDMS

One of the important temporal aspects of the pis-pulsed mode is the ability to

separate the various ionization events that occur during the pulse cycle, beginning with

the ionization of the discharge gas species by electron ionization. In the 10 Ps pulse

period, electron impact ionizes the discharge gas and its impurities (Figure 3-5). Once

the discharge has been turned off, mass spectra show that no significant ionization occurs

for about 100 ats. There is no significance to 100 p.s time period, however, during this

period, electrons sustaining the plasma current completely dissipate. This means that little

or no electron impact ionization of gas species or sputtered species occur. After the 100

ps relaxation period, other ionization events such as charge exchange occur just prior to
















700-






0-


100-


0-


I
116


I
118


I
120


vZ


Figure 3-4. Mass spectra of tin at 460 ppm for a) p[s-pulse b) DC


I
12D)








600 @







- 300







0 J ----

0 50

m/z


Figure 3-5. Electron ionization spectrum
of argon at 1 torr and 10 ps delay time










the maximum signal for the sputtered species (Figure 3-6). Associative ionization is

mostly responsible for the observed polyatomic and molecular species present in the

plasma and is the result of metastable association with sputtered species. Penning

ionization dominates ionization processes and is considered the primary mechanism for

sputtered species ionization at deflection times greater than 100 p.s (Figure 3-7). Figure

3-7 shows an ion counting mass spectrum where copper ions have saturated the detector,

and showing the small presence of gas species as compared to Figures 3-5 and 3-6.

Associative ionization will last as long as there are metastable atoms present in the

discharge volume. The separation of electron and Penning ionization events is due in

part to the temporal attributes of pulsing the glow discharge. When the discharge is not

pulsed there are contributions in the mass spectrum from all of these ionization

mechanisms. Figure 3-8 is a DC spectrum showing the combination of all the above

ionization processes that occur separately in the ups-pulsed mode (Figures 3-5 through 3-

7) and simultaneously in the DC mode.

Sputtered species from the cathode diffuse toward the anode and are c\ cnlu.tLlly

deposited. In this case, sputtered species diffuse into the negative glow region, are

excited and ionized, and then sampled in the same region. In a DC glow discharge, the

negative glow region is always present and ionization of gas and metal species is always

occurring. This means that species containing the discharge gas and gas impurities will

always be in the mass spectrum. In the pLs-pulsed mode, the negative glow only exists for

a 10 p.s period and is then extinguished due to the rapid loss of electron population when

the pulse period ends. During the short on-period, long lived metastable atoms are formed










800-


*i--





- 400-








0


U


M/Z


Figure 3-6. Associative and Penning ionization for
the analysis of copper in argon at 90 ps delay time


+









2000


+
C U
C
1000 +

0





0 50


m/z

Figure 3-7. Penning ionization of copper atoms in argon
for the ps-pulsed GD at a delay time of 140 gs


















50 100


M/Z


Figure 3-8. DC GD mass spectrum of a copper sample in
argon


30000 -



20000 -


10000-



0-


JI-


i 1LJ


i-jj,-X-j










and become the ionizing agent after the pulse period ends. These metastables (in argon)

have enough energy to ionize the metal species but not the discharge gas or impurity

species, including those molecular in nature. This results in spectra dominated by the

discharge gas species at early sampling times and dominated by sputtered species ions at

later deflection times (after 100 lts). By selecting an appropriate deflection time,

reduction, but not removal, of these gas discharge interference can be effected. Figures

3-9 a and b show the schematic diagram of how the separation of gas and sputtered

species is effected in tis-pulsed GD-TOFMS. The first figure shows that gas species are

sampled prior to sputtered species in the glow discharge cell. Then by using a short or

long deflection time, a mass spectrum either rich in gas species or rich in analyte species

is obtained. Figure 3-10a shows the mass spectrum of an aluminum sample. At early

delay times, very little aluminum and lots of discharge gas species and impurity ions are

observed. By delaying the ejection of the ions into the TOFMS, the spectrum shown in

Figure 3-10b can be obtained; a spectrum consisting mainly of the aluminum ion. There

is no exact value to which the deflection time can be set to perfectly maximize the signal.



Glow Discharge Time-of-Flight Mass Spectrometry

Introduction

Time-of-flight mass spectrometry (TOFMS) is a well known technique in

biomolccular chemistry for the analysis of large biomolcculcs. It began in this field

mainly because other types of mass spectrometers suffered from limited mass range. The

time-of-flight has a theoretically unlimited mass range. As long as an ion is produced and






Electron
Ionization


Sputtering




Diffusion and
Ionization


"bu.n
O4 O

u


Cathode
I j1


*4 C


Sampler


Figure 3-9. a) Electron ionization, sputtering, diffusion and ionization
in the ps-pulsed GD


Ar+


Cuo



Cu+








Discharge Gas Sampling I f


0

0


Cu+


Repelling Pulse


Ar+


Sputtered Species Ion
Sampling


Cathode
I v


Repelling Pulse

"--- ---
Cu+
Sampler


Figure 3-9. b) TOF Sampling. Short delay times sample gas ions. Longer
delay times sample analyte ions







0.05


0.04


0.03


0.02


0.01


0.00

0 20 40 60

m/z


Figure 3-10. a) TOFMS spectrum of aluminum at 70 is





0.07


0.06


0.05


0.04


0.03


0.02


0.01


0.00


m/z


Figure 3-10. b)TOF delayed (230 jis) mass spectrum of aluminum


1 I ,~--


m l


I i 1 I 1 ----










can be accelerated into the flight tube, mass analysis can be accomplished. At that point.

detecting the ions is the limiting factor in the success of a particular analysis. GDMS has,

typically been dominated by quadrupole scanning systems and double focusing magnetic

sector instruments. These provide high sensitivity and fairly rapid analysis and peak

hopping capabilities, allowing the selective monitoring of se% eral peaks in a short time

period. In the last decade, TOFMS systems have been explored for elemental analysis,

and this dissertation uses TOFMS for ion analysis and for diagnostic studies, namely to

measure the effect of discharge gas composition on the uts-pulsed GD mass spectra and

general GD phenomena.



Time-of-Flight Mass Spectrometry

Theory

TOFMS works on the principle that if all ions are given the same energy then the

time it takes for them to travel a certain distance is determined by their mass. Thus,

equations can be used, starting with the kinetic energy equation, to calculate the time it

takes for an ion to reach the detector at some distance beyond the ion sampling point.

The kinetic energy equation states that

KE = 1/2mu2 (Eq. 3-1)

where m is mass (g) and u is velocity (m/s). By rearranging this equation, we can

calculate the velocity of an ion in the time-of-flight tube

v = (2zeV/m)/2 (Eq. 3-2)










where z is the charge on the ion and eV is the kinetic energy the ion is given upon being

accelerated into the TOF. Ion flight time is given by

TOF = L/u = (m/2zeV)/2 D (Eq. 3-3)

where D is the distance the ion must travel before being detected. A heavier ion will

travel slower and take longer to reach the detector. Ions having multiple charges will

experience more acceleration and will hence have greater velocity. This equation is the

basis for calibrating the mass spectrum (see section on Data Analysis) and will be used in

the next section to address mass resolution and signal broadening.



Output Characteristics

The resolving power of the TOF instrument can be considered a function of the

flight tube length. The resolving power of any mass spectrometer is given by

m/Am = t/At (Eq.3-4)

where t is the calculated flight time and At is the time difference of when the first and last

ions of a particular mass reach the detector. By increasing the flight tube length. t

increases. However, the separation of the ions in the mass bundle changes negligibly,

only because At will be determined by their initial energy distribution before acceleration.

Therefore as the flight time (t) increases, so does resolution. The resoli ing power is

affected by the accelerating voltage and the spatial and energy distribution of the ion

bundle. Figures 3-1 la and 3-1 lb64 show the problems of ion splaial and energy

distributions in the TOF s) stcm. Ions that are separated in space can experience a

different accelerating voltage. This affects its flight path and/or its flight time. Ions in

















SSource S-c


- Drift rion I)---


Ei-- =2s -



E=V/s E=O


Figure 3-11. a) Spatial distribution effects in TOF [Ref 64]


--I
1-


- -


@f-






















Source S 14







S=\V/s


D)rit region 1)







E= 0I
^V,


Figure 3-11. b)Energy distribution effects in TOF [Ref. 64]


.0 :


4










which flight path is affected result in noise accumulation at different parts of the nma-.

spectrum, resulting in poorer S/N and detection limits, or they simply hit the flight tube

walls and are neutralized. Ions in which flight time is affected result in poor mass

resolution. Ions closer to the accelerating plate will experience more voltage that those

further away. This results in a small difference in drift velocity, causing the faster ion to

reach the detector sooner than the less affected ion. By reducing this spatial distribution,

an increase in resolving power can be effected. Broadening of the mass peak generally

occurs when there are thermal energy variations in the incoming ion packet, and/or there

are spatial orientation differences caused by either space charge effects or as a result of

ion optic misalignment. These broadening factors can be reduced by judicious tuning of

the ion optics.



Performance Limitations and Potential Capabilities

The spectral production rate is a function of the slowest ion. For example, with a

2 m flight tube and accelerating voltage of 2 kV, an ion of mass 500 has a flight time of

90 jis. This means that the next pulse of ions into the TOF cannot occur until this one has

reached the detector because overlap of the ion packets may occur. Therefore the

repetition rate must not exceed 11 kHz. For the case of the microsecond pulsed glow

discharge, this factor is of no concern. Operating frequencies are generally not higher

than 1 kHz for the [is-pulsed GD source.

Wiley and Mclaren65 developed a TOF system that would increase the resol, ing

power over typical TOF systems of their time. The hardest factor to overcome was the










spatial distribution of the ions in the accelerating region. Figure 3-11 a shows that the

ions closest to the accelerating plate will experience a greater field than those further

away, creating an energy distribution and affecting different flight times once in the drift

region. Wiley and McLaren65 solved this problem by creating and separating the energy

and spatial distributions prior to the packet reaching the accelerating region by turning

the ions around after a certain flight distance so that the ions moved in a V shape. The

reflectron resulted in the distribution of ions being reduced in the turn around (reflectron)

region. This is achieved by applying positive and negative voltages in a manner such that

faster ions will spend more time in the reflectron than slower ions within a single mass

packet. Faster ions will require a longer flight path to slow them down and turn them

around as compared to slower ions. Thus, as Figure 3-12 shows, the ions of each mass

packet will reach the detector within a much shorter time and as a "tighter" ion packet.

and much reduced velocity distribution. The spatial and velocity distributions could be

ultimately minimized by using the reflectron. The TOF instrument used in this

dissertation has a mass resolution of 1600-1700 FWHM (full width half max) in the

reflectron mode.



Inherent Advantages of TOFMS

Cotter et al.66 studied many aspects of TOFMS and listed several advantages of

time-of-flight \ crsus all other mass spectrometric techniques.

1. The TOFMS is much more conservative of the sample in that every ion in an

ion packet is detected






























(b) / Spat






)Deflector



SI -,---
V


c-focus plane


, !4-..-d1
, I


-- I
_I .4d.]
L'o


_______ C-L2 *.....-..~

111111 /


Figure 3-12. Reflectron TOF with spatial and temporal
correction by the reflectron lens [ReT. 64]


----.,2


I










2. The open flight tube with the absence of slits presents a very wide aperture to

the source of ions

3. The TOFMS has no fundamental m/z limit, other than detectability, on the

range of m/z values analyzable. Additionally the mass/charge scale can be

calibrated accurately based on equations 3-1,2, 3, and 6

4. The process of creating an ion packet or bunching can result in low sample

utilization via a low duty cycle and can improve mass resolution

5. Very fast electronics are required for detection of transient detector outputs.



Instrument Design

GDMS requires the physical invasion of the plasma in order to measure the ions

produced. The extraction of ions from a glow discharge plasma would benefit from a

design that maximizes the number of ions sampled. Optical techniques have some

advantage in that the measurement of a photon does not require invasion of the plasma.

However, mass spectrometry is considered to be a much more sensitive technique for the

detection of ions. The following section describes the design of the prototyc GD-TOFMS

instrument used in this dissertation.



Ion Source

Direct Insertion Probe (DIP)

The direct insertion probe is a generic device used in GD devices. The essential

parts are shown in Figure 3-13. An insulated stainless steel rod serves as the interlace







insulated
wire


solder
connection


copper
sample
holder

( ,,T,


stainless steel
body


Figure 3-13. Direct insertion probe


[I


sample


macor
shield


electrical
feedthru










between the powered lead and the sample. A small handle (not shown in Figure 3-13) is

used to couple the power to the back of the probe and is used for easy removal and

position manipulation of the sample in the glow discharge chamber. The sample is

mounted into a small holding chuck that connects the sample and high voltage lead at the

probe tip. The holding chuck is covered with teflon tape so that no surface at high

voltage, other than the sample surface, is exposed. The samples were machined in the

University of Florida Chemistry machine shop so that they could fit into the loading

chucks. The top surface of the sample has a 4 mm diameter. The entire probe tip and

loading chuck are protected by a Macor shield that minimizes surface exposure to only

the top of the sample surface. The shield was not in contact with any part of the sample

or any other high voltage surface to prevent shorting the power supply. These shields

were also machined here at the University of Florida. To prevent arcing when gas creeps

inside of the shield, the loading chuck was wrapped with teflon tape.



Discharge Cell

Two discharge cell designs have been used in this dissertation. The first as a

modified six-way cross with a 450 viewing port that allowed viewing of the sample while

in operation. The source was large and bulky, containing several vacuum interfaces so as

to minimize the amount of contaminant leaks in the system. Figure 3-14 shows a picture

of this source. The coupling of this source to the mass spectrometer had one severe

problem. There was no way during sample exchange to protect the 2"d and 3rd vacuum

stages necessary for ion detection. This is the primary reason why various vacuum







































Figure 3-14. Photograph of direct insertion
probe source #1










interlocks were needed during sample interchange or source cleaning and maintenance.

Hence, the need for vacuum protection led to the use of a slide valve that would ensure

the vacuum integrity while sample interchange or source cleaning was required. This

slide valve was nearly identical to the one constructed for the interface of the quadrupole

GDMS system in this laboratory. The new slide valve interface was designed around the

development of a Grimm-type ion source. This meant that the six-way cross design

could no longer be used. A simple source plate was developed to fit the Grimm-type

designed interface and allowed the same operation of the direct insertion probe. The

interface design is shown in Figure 3-15. Regardless of design, the interface of the probe,

sample, and mass spectrometer is the same for both sources. The only severe limitation

of the modified source is its exposure to air during sample exchange. In our efforts to

maintain vacuum integrity and create source simplicity, we sacrificed cell exposure to air

and its contaminants.



Ion Extraction

Ions are extracted from the glow discharge plasma by moving the sample surface

(and plasma) within 8 mm of a sampling orifice. The sampling orifice is 1 mm in

diameter and is the first of two high vacuum interfaces. The source is operated at

approximately 1 torr; beyond the sampler, the pressure is at 10-4 torr. As ions pass

through the sampling orifice, the rapid drop in pressure causes a jet expansion of the

plasma into a field free region. A skimmer cone placed beyond the sampling cone

samples approximately 1% of the ions produced in the expansion jet and leads to the ion







































Figure 3-15. Direct insertion probe source #2. Designed
for exchangability with a Grimm source interface










focusing lenses. The skimmer cone lies at a distance, 0.7 cm, equal to the mach disc of

the jet expansion of the plasma. At the mach disc distance, ions have reached their

maximum velocity. The skimmer cone also serves as the interface between the jet

expansion and lens and ion optic region that lie at a pressure of 105 torr. Figure 3-16

shows a schematic of the source interface and ion optics region.



Ion Optics and TOF Sampling

The ion optics for the time-of-flight system were constructed at the University of

Florida. The sampling plate interface could either be grounded or floating during ion

extraction. The choice was dependent on the mode of operation. A higher signal was

obtained for the jps-pulsed discharge when the sampler was left floating. Once beyond

the skimmer cone, ions were focused via a set of ion optics including a set of steering

plates. The ions passed through a plate with a small slit. The plate has a small potential,

which minimized the spatial distribution of the ion packet just prior to TOF sampling.

The function of the slit was to reduce the spatial distribution of the ions just before they

reach the deflecting region where ion distribution can severely affect resolution. This

opening measured 10 x 2 mm. The steering plates controlled the position of the ion beam

and adjusted it into an elliptical cross section from its round cross section, allo\\ ing a

greater number of ions to be pulsed into the TOF accelerating grid and creating a

"tighter" ion packet. Sampling in the TOF system occurred orthogonally to the

production of ions and ion focusing. The ions focused in the ion optics filled a repelling

region in which a 2-5 jis long, 70-80 V pulse deflected ions into the TOF accelerating




Samy


Direct
Insertion
Probe

1zi

Samp
1 torr


)ler
Skimmer
Slit
Steering
j Plates


- c- -
Optics


2x104 torr


8x106 torr


1.5


Repeller -


SOV '~i'" G

-2000V 25 mm G


m


Flight Tube


L2 L3
II0


To Detector


Figure 3-16. Schematic of the source interface and ion optics region


1


Faraday
- Detector

2

__Deflectc
Plates





_Microchanr
Plates










grid. A constant 1.8 kV voltage accelerated the ions into the linear portion of the flight

tube. After a 1.5 m flight, ions were detected. For the studies here the ions traveled into

a reflectron lens system in which they were reflected into the reflectron flight tube for an

additional 0.97 m flight. At the end of the reflectron flight tube, ions were detected by a

microchannel plate.



Electronics and Timing

The temporal aspects of the pulsed GD have been discussed. To take advantage

of these aspects, coordination of the GD pulse and TOF sampling must be optimized. In a

DC glow discharge source, the need for fast electronics is minimal because signal

variation and timing are insignificant. Once transient signals are produced, especially in

the microsecond regime, the need for fast electronics in order to capture the signal is

critical. Triggering the time-of flight mass spectrometer to sample the ion population at a

selected time after the pulse of the glow discharge source is necessary (Figure 3-17).

Additionally, in order to study the ionization processes outlined above, the delay betw een

GD pulse and TOF sampling (deflection time) must be adjustable. Therefore fast

electronics were needed. Figure 3-17 shows an example of the timing scheme necessary

to obtain spectra rich in analyte with reduced or no contribution from the discharge gias

species.







A
Applied voltage







B:
Ion profile




C:
Repeller signal


Figure 3-17. ts-pulsed GD timing scheme










Vacuum System

Plasma sources can operate anywhere from the millitorr to atmospheric pressure

range. This means that differential pumping is required in order to maintain the

necessary pressure to transport the ions from their source to the detector. For this system,

source pressure is about 1 torr, second stage at 10-4 torr, and third stage at 10-6 torr. The

vacuum pressure in the 2nd and 3rd stages allow the drift of ions from the source to the

deflecting region, 14 cm, and then from the deflecting region to the reflectron detector

2.5 meters in distance. Vacuum integrity is important for all MS systems. The ability to

maintain a high vacuum in the 2nd and 3rd stages will determine the 1st stage or source

pressure. Our GD-TOFMS system uses 3 turbo pumps. A 1000 L/s turbo pump

(Tubovac 1000 C, Leybold, Export, PA, USA) is connected to the second stage, and a

250 L/s (Varian Vaccum) and a 170 L/s (Varian Vaccum) turbo pump are connected to

the third stage flight tube. All the turbo pumps are backed by 13 1/s mechanical roughing

pumps. The flow rates correspond to the maximum pumping capabilities for nitrogen at

the operating pressure for the GD.



Ion Detection

TOFMS systems commonly use an electron multiplying device such as a

microchannel plate as the detector. Many types of software are used to obtain spectra and

perform elemental analysis. A dual-chevron microchannel plate (MCP) (Galileo,

Electrooptics, Sturbridge. MA, USA) is located at the end of the linear section of the

flight tube behind the reflectron lens and at the end of the reflectron flight tube. The










linear mode was not used in the work presented here. Much better resolution can be

obtained in the reflectron mode due to the longer flight distance of the ions.19

Two detection methods exist that can take full advantage of the multiplex

capabilities of the TOF mass analyzer. The first is a real-time averaging mode in which

the ions from many pulses are averaged and viewed on an oscilloscope, which allows the

user to view real-time spectra and analyze the fundamental processes occurring in the

glow discharge. This is a running average in which every pulse is averaged to the 99

pulses before it. This mode is also used for ion lens focusing and signal optimization.

The second method is an ion counting or accumulation method, commonly used to

measure trace elements in samples. EG&G has developed a Picosecond Time Analyzer

(PTA) (6109, Eg&G Ortec) that can accommodate the transient signals that the

microsecond pulsed GD-TOFMS system produces. The bulk and trace mass spectra

presented in this dissertation were obtained using these two detection schemes.



General Procedure and Data Analysis

Samples were prepared for analysis as described in a previous section. Once the

DIP was in the source, evacuation by a 13 L/s roughing pump ensues. The discharge gas

was then added and the pressure was measured via a wide range capacitance manometer

(WV-100-2 Varian Vacuum). After pressure stabilization, the source was opened to the

2nd and 3rd stage by opening the slide valve. A 3 or 5 kV pulse power supply (IRCO,

MD) was used for discharge powering. The sample was sputtered for 10-15 minutes, to

effect sample surface cleaning and stabilization of the discharge. Spectra from the










oscilloscope were obtained using software developed in our lab from a National

Instruments Data Acquisition program. Trace spectra were taken using the PTA program

software. Generally, 50,000 pulses were accumulated using the PTA counting mode.

Spectral data were downloaded into Microcal Origin*(TM), where files were conm erted

into mass spectra. Mass scaling was completed using Origin according to the equations

shown in the TOF theory section such that.

Mass = at2 + b (Eq. 3-6)

where a and b are constants that are determined when two separate masses are used to

calibrate the mass spectral scale, and t is the flight time of the ion.

Sputter rates were determined by sputtering a copper sample for 30-45 minutes.

Samples were weighed on a milligram analytical balance, with a range of 0.001 mg to 20

mg 0.0001 mg. Sputtering, by pulse, was calculated by dividing the weight loss in mg

by the number of pulses over the given sputtering time. At least three replicates of each

gas were performed.

The gases used in all experiments were research grade 5, 99.999 % pure from

Spectra Gases and from BOC Gases.




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