A CYLINDRICAL HELIUM CAPACITIVELY COUPLED
MICROWAVE PLASMA: DIAGNOSTICS AND
DETERMINATION OF SILICON
BILLY MAC SPENCER, II
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
TTKTNTTI/C DTT T V f P3T fDTTh A
I would like to give special thanks to my research advisor, Professor James D.
for his patience and advice during the past several years.
great help in the preparation of this manuscript.
I appreciate Dr.
to take on this research project in conjunction with Texaco Inc.
Their are many group members who have given much help.
I appreciate Dr. Ben
Acknowledgement is given to
which are exhibited in chapter
for taking the photographs of the
Special thanks must go to Dennis Hueber for
his help with the research and especially for dealing with the computers.
Others in the
group include, but is not limited too, Giuseppe Petrucci, Anil Raghani, Mike Wensing,
MaryAnn Gunshefski, Donna Robie, and Wellington Masamba.
My appreciation goes to
Texaco Inc. for allowing me the opportunity to return
Texaco has provided the financial support that made this research possible.
In addition, I thank Dr.
Cliff Mansfield who has given me exceptional advice and for
help in establishing the
Texaco fellowship which I received.
Additional help has been
provided by Dr.
Bhagwandas Patel and Dr. Abdalla Ali.
My love and gratitude go to my parents,
support for all my endeavors.
who have always given their time and
I want to thank my sister and her husband, Shonna and
Robert Gage, and their family, who have given their support and understanding.
I have missed my niece and nephew,
Vanessa and Derrick, during the time I have been
Ravanelli, for their guidance and assistance.
Most of all, I
understanding and su]
to return to school.
would like to thank my wife, Kimberly,
for her love, patience,
She has given up part of her life to allow me the opportunity
I am always grateful for her help during the past several years.
TABLE OF CONTENTS
S. ~ S S S 6 S S S V11
CHAPTER 1 A HISTORICAL PERSPECTIVE OF PLASMAS
Plasmas in Elemental Analysis
. S S S S S S S S S S S S
I S S S S S S S S S . S S S S S
* I 9 5 .
* S S 6
* S S S 5 9 .
History of Capacitively Coupled Plasmas . .
Capacitively Coupled High- Frequency Plasmas
Capacitively Coupled Microwave Plasmas ..
Microwave Radiation Safety
Goals of This Research
* S S S S 5
* S S 9 5 S S S S S
CAPACITIVELY COUPLED MICROWAVE PLASMA
Waveguides . . .
Instrumentation Used in This Research
S I S S S 15
Plasma Configuration .
* a .
f f S S
CHAPTER 3 DETERMINATION OF SILICON IN ORGANIC SOLUTION
USING PNEUMATIC NEBULIZATION
Experim mental . . . * * * *
D1ata Ioints . * * * * .
Wavelength s .. ..... .... .......
Cylindrical and Spherical Plasmas . . . . .
Tandem Electrode .
i tmit of Detection
Results and Discussion
Cylindrical and Sp
Effect of Power
Molecular Gas Addition
Figures of Merit
. S S S U 9 9 S 9 9 S S
U U U U S S U S 9 4 5 5 0 U 9 9
* 9 9 9 U 9 9 5 U S S 0 0 9 S S 9 6 2
S . . a a .. 67
S a a a S 9 *. 81
CHAPTER 4 DETERMINATION OF SILICON IN ORGANIC SOLUTION BY
Results and Discussion
SU U 9 9 S 83
* U S S S S S S U S U S U U S U U U 9 5 5 U U U S S S L83
* U S S S S S U U U a U a S S S S S S U S 5 U 9 4 5 5 U 8 8J\-
S. S 5 9 9 S S S . S 91
. S 91
Temporal Nature of Silicon
Effect of Ashing Time .
Effect of Ashing Power
Effect of Atomization Power
Electrode Cup Materials .
Figures of Merit .
S . . . * 10 1
. .* . . . 106
* * . . . 101
* . * * . 1 16
. * *. . 107
CHAPTER 5 DIAGNOSTICS IN THE CYLINDRICAL CMP
Introduction . . . . . .
Electronic Excitation Temperature, Tex . .
Rotational Temperature, Trot. . . .
S S S U 0 9
. a a U U
Electron Number Density, n . . .
lonization-Recombination Temperature, Tion .
Experimental * * .
Electronic Excitation Temperature, Tc .
Electron Number Density, n . .
Ionization Recombination Temperature,Tio.
Limits of Detection, LODs
Results and Discussion
Electronic Excitation Temperature, Texc
Rotational Temperature, Trot . .
Electron Number Density, n . .
Ionization Recombination Temperature,
Limits of Detection, LODs
S 5 S S S S S S S S S S 13 4
St ft S .. 5 135
.* . . 5 135
S t ft 142
. .. .. ........ 149
T io n S f S f f f f S f S S 1 5 1
. .. ... .152
S S S 5 ft ft S S 15 8
CHAPTER 6 CONCLUSIONS
Summary of This Work
. . 163
APPENDIX ABBREVIATIONS AND SYMBOLS
. . . 170
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
A CYLINDRICAL HELIUM CAPACITIVELY COUPLED MICROWAVE
DIAGNOSTICS AND DETERMINATION OF SILICON
Billy Mac Spencer, II
microwave plasma (He-CMP) and is applied to the determination of silicon in organic
Samples are introduced by nebulization and thermal vaporization.
profiles of Si emission, microwave power, helium plasma gas flow rate, and the addition
of molecular gases are studied.
Electrodes are fabricated from graphite, titanium, and
background and noise levels, compared to the graphite electrode.
W electrode is visible after 150 hours of operation.
No deterioration of the
A limit of detection (LOD), 3cr, of
introduction by thermal vaporization, are made from graphite,
titanium, and tungsten.
The optimum ashing power and ashing time are
135 W and 10 s, respectively.
levels or longer ashing
the Si signal,
probably due to
formation of silicon carbide.
with a RSD of
The LOD (3cr) for thermal vaporization is 0.03 pg mL-'
temperatures and electron number densities, are determined in the
cylindrical He- CMP.
Electronic excitation temperature (Tex) is 3400 K.
temperatures (Trot) are
=1600 K and
electron number density (ne) is 4
= 1900 K, using OH and N1+
Trot, and ne are similar
for both aqueous and organic solutions.
Ionization-recombination temperatures (Tion)
Background spectra of the cylindrical He CMP, while introducing aqueous and organic
solutions, are presented.
A HISTORICAL PERSPECTIVE OF PLASMAS
Plasmas in Elemental Analysis
The quantitative and qualitative analysis of elements is routinely performed by
One of the common ways is observing emission of electromagnetic
radiation as elements are introduced into a plasma.
A plasma is a partially ionized gas
in which a portion of the atomic or molecular species are present as ions.
examples include neon lamps, argon welding arcs, and interstellar space.'
in the most popular analytical technique is an inductively coupled plasma (ICP).
ICP uses radio frequency to generate a fluctuating magnetic field in which the ions and
Plasmas may also be formed using microwave radiation.
plasmas may be divided into categories which depend on the way energy is transferred
to the plasma.
Microwave induced plasmas (MIP) and capacitively coupled microwave
plasmas (CMP) use different means of coupling the microwave energy.
In the MIP, the
plasma is generated inside a quartz discharge tube which is held in a resonant cavity.
The microwave energy is transferred
from the generator to the cavity using a coaxial
In the CMP, a magnetron generates the microwave energy which is directed
through a waveguide to an electrode.
The plasma is formed at the top of the electrode.
In the following sections, different plasmas (CMP, MIP, and ICP) will be compared.
The history and application of CMPs as emission sources will be presented along with
the instrumentation used in this research.
In comparison to ICP, microwave plasmas offer several advantages.
low cost and widely available.
Most microwave plasmas operate at lower gas flow rates than the
ICP, thus decreasing operational costs.
There are several reviews covering MIPs.3 -7
In comparison to CMP, the MIP
has greater stability and lower background noise when operated at similar power levels.
MIPs do not have contamination or background problems associated with the electrode,
which has to be routinely replaced in CMPs.
cost system and may be operated with He or a
In comparison to ICPs, the MIP is a lower
ir, where the ICP is usually operated with
With these advantages, the MIP has been widely used as an analytical technique
to the CMP.
The MIP has been primarily used as a detector for gas
Currently, a gas
chromatograph with an MIP detector is commercially available (HP 5921 AED, Hewlett
The MIP is also a source for a mass spectrometer.11
The CMP does offer several advantages over the MIP.
Higher power levels may
cable and coupling loop with power levels above 150 W.2"
The MIP requires additional
transformers to impart continuity of the impedance between the coaxial cable and the
solid state generators. 12
to 30 mm.
A very small plasma is produced by the MIP in contrast to the
e plasma diameter in the MIP ranges from 0.5 to 4 mm with a length of 10
13,14 Dimensions of the plasma in the CMP are approximately 8-25 mm in
diameter with a length of
Since the MIP is a small plasma and must be
operated at low power levels,
there is difficulty in introducing samples as solutions.
Plasma enthalpy is insufficient in the MIP to desolvate and vaporize aerosols effectively
from directly nebulized solutions.] 3,1 5
Several research groups have introduced solutions
usually employing an
ultrasonic nebulizer or desolvation- condensation
if aerosols are
introduced over a long time or if concentrated solutions (
>500 upg mL-1) are used.16
can easily perturb
the stability and excitation
capability of the MIP.3
By operating CMPs at higher power, they are more tolerant to
molecular species and have an increased ability to desolvate, atomize, and excite species
that have been introduced.
groups have compared the performance of an ICP with the CMP.18'19
Boumans et al.20 performed the analysis of solutions by optical emission spectrometry
studied included detection
limits, matrix effects, sensitivity, and precision.
Results were corrected for instrumental
worse in CMP-OES. Th
Relative standard deviations for the 12 elements studied were
e conclusion was the ICP demonstrated an overall supremacy
as an excitation
Several different gases are employed in producing plasmas.
In ICPs, argon is
with helium and nitrogen also having been used.
The helium plasma in
an ICP, He-ICP, has problems in stability and arcing to the induction coil.
needle plasma is formed in the He-ICP which makes injection of the sample difficult.
interferences.-- Helium and argon are the preferred gases since they give no molecular
background spectra.23 Tanabe et al.24 reported the background in a helium MIP was an
order of magnitude lower than with an argon MIP.
Comparing helium and argon, helium
has a higher electrical resistivity which causes a lower power transfer efficiency in the
The thermal conductivity of helium is greater than argon.
In helium plasmas,
the heat dissipates faster toward the outer tube of the torch, increasing the chances of the
Minimization of helium "attacking" the torch is accomplished by adding
a small amount of H,
to the plasma gas.26
groups have researched
special torch designs for use with helium plasmas.14,15,27- 29 The ionization energy of He
should be more efficient as sources for emission analysis.25
Helium has the advantage
in populating the higher energy
levels of free atoms and ions.31
plasmas have been applied to the
analysis of halogens32 -34 and nonmetal
plasmas have been successfully operated using argon, helium, nitrogen,
In the past,
CMPs have not been researched
as extensively as ICP
possibly owing to the
success and commercialization of ICP and MIP systems.
the possibility that CMP has the potential to be an alternative source for OES.
This research centers on the further development of the CMP for OES.
on CMP and the techniques of MIP and ICP will only be mentioned for
instrumentation of the CMP will
be discussed in greater detail.
History of Capacitive
y Coupled Plasmas
Capacitively Coupled High
- Freauencv Plasmas
been studied for over 60 years.
They may be classified according
to the frequency of electromagnetic radiation used to produce the plasma.
radio (Rf) and microwave frequency
. Radio frequency plasmas use frequencies below
has also been
Microwave plasmas use frequencies greater than 109 Hz (GHz).
Early research centered
around hf plasmas.
, Rohde and Schwarz37 produced a "flammenbogen," or gas
first to examine the emission spectrum of this gas discharge.
They identified 02, NO,
developed a hf discharge. A high frequency oscillator output was applied to two plates,
separated vertically by up to 15 cm. The lower plate had a copper cone with a platinum
A discharge was formed at this tip by touching with an isolated conductor.
conductor caused electrons to be emitted from the resulting heat generated by the strong
sustain the discharge.
Based on this description, what they had was a plasma.
is defined as being a partially ionized gas.
determined the temperature to be 3000
In the same year, Cristescu and Grigorovici40
-3600 K using a power level of 650 W
temperature was determined by the NZ and OH bands of the spectrum.
Later, the same
authors published some of the theoretical treatments of the discharge they described in
Calculations included current
vs power characteristics of the plasma.
Badarau et a
43 used a nickel tipped electrode operating at a frequency of 43
Solutions were introduced, by a nebulization process, into the air plasma.
sensitivity of 13 elements were comparable to those obtained in an air-acetylene flame.
Dunken et al.42 employed a capacitively coupled radio-frequency plasma (50 MHz) for
the analysis of aqueous solutions.
Calibration curves were made for Ca, Sr, Ba, Cr, and
The concentration of the elements ranged from 30 to 7000 ppm.
Canada) became available which utilizes an Rf
plasma inside a graphite furnace atomic absorption spectrometer (GFAAS).44
electrode (rod) is placed in the center of the graphite tube (furnace) of the GFAAS.
27.12 MHz Rf generator is connected to the central electrode and used to generate the
The furnace acts as the counter-electrode.
This instrument is referred to as
Furnace atomization plasma emission spectrometry (FAPES) is another
Sturgeon et al.45 applied this system to the analysis of nine elements (Ag, Cd,
Detection limits (3a) were between
12% with a linear dynamic range of 2-4 orders of magnitude.
Capacitively Coupled Microwave Plasmas
Microwave plasmas were initially discovered during the development of radar
The formation of these plasmas caused problems in the design of radar
presented a new area of research.
During the 1940s, microwave plasma systems were
electron density, etc.) studied.
1960s, actual applications were reported.
1981, Dahmen47 has given an annual review on microwave plasmas.
Cobine and Wilbur48 studied plasmas produced with an
1000 MHz magnetron to produce microwaves at
Gases employed were air, nitrogen, carbon dioxide, argon, and helium.
They found the
air, N1, and CO, plasmas would melt a tungsten rod (mp =
An argon plasma
would barely ignite paper held axially in the plasma.
The explanation was argon did not
dissociate into any other species.
Future work with plasmas, using a central electrode, are based on the work on
Cobine and Wilbur.48
via a waveguide,
to the electrode.
The technique is then termed capacitively
coupled microwave plasma (CMP).
The first application of a CMP, for the analysis of
solutions or solids,
was performed by Mavrodineanu and Hughes.
Hughes49 used a 2450 MHz magnetron to produce a helium plasma.
analyzed using a graphite cup electrode.
Solid samples were
The plasma formed on the top of the electrode
determined using a pneumatic nebulizer.
The limits of detection were in the range of 10
Pb, and Zn) to 0.8 ppm (Na).
Temperature of the helium discharge was
estimated between 2900 and 3300 K by its ability to melt molybdenum but not tantalum
Jecht and Kessler50 developed a similar CMP which used a molybdenum water-
and helium. The
2370 A. This CI
They investigated plasmas produced at 2400 MHz for air, nitrogen,
, temperature was estimated at 4000 K using the NO rotational band at
MIP-OES system was then used in several applications to real samples.
Limestone and dolomit were analyzed for the concentration of Ca, Mg, Al and Fe.51
Murayama et al.53 introduced solutions by nebulization into a CMP.
This work employed a water-cooled aluminum electrode.
The detection limit, based on
two times the root-mean-square (rms) deviation of the background,
Inter-element effects of sodium on seven elements was also studied.
Work performed by various research
Kessler and coworkers,50 -52
Murayama and coworkers,53 -56 Goto et al.,
57 and Kitaqawa and Takeuchi58'59 led to the
commercialization of CMP-OES equipment by (1) Hitachi,
Tokyo, Japan, designated
Lausanne, Switzerland, and (3) Erbe-Elektromediziu,
Tuebingen, Federal Republic of
Previous CMPs utilized a solid electrode. In this design, aerosol samples were
introduced by diffusion into the outer edges of the plasma. Hwang et al.61 improved on
this design by making a tubular electrode where the sample is introduced through the
center of the plasma.
thermal vaporization device.
The CMP was applied to the analysis of several materials.
0, and Hg were determined in orchard leaves and in fish using a
The analysis of trace oxygen and hydrogen extracted from
metal was carried out using a chamber to heat the samples and allow the helium to carry
the oxygen and hydrogen to the plasma.63
During the past several years, applications and diagnostics have centered around
compounds by interfacing a
helium CMP with a gas chromatography
plasma formed on top of the cup and vaporized the sample.
analysis of solutions by discrete sampling.65
This was applied to the
Masamba and coworkers67,68 investigated
the temperature, electron number density, as well as the influence of power, observation
position, solution uptake rate and carrier gas flow on the CMP.
developed an electrode made from tungsten wire.
Ali and Winefordner69'70
Having a smaller mass, the filament
gives a high rate of sample vaporization while being inexpensive and easy to construct.
Rotational and excitation temperatures were determined along with the effect of power
and plasma gas flow rate.69
The dependence of Cu and He emission on plasma flow rate
Hueber et al.
71 developed a hydride generation system for the introduction of arsenic into
Microwave Radiation Safety
Exposure to microwaves has been related to a variety of health problems including
and disruption of
is of vital
One problem exists is the establishment of safe
In the United States, the standard safe exposure level is
at 2.450 GHz.
The standard in Russia
78 is 0.01 mW cm-3
while that in Australia7 is
The measurement of stray radiation is made at a distance of 5 cm from the
source of emission. Microwave radiation from the plasma is easily contained using wire
I I 6 i I V V arrh/^ ^ ... I B* 1* 6. r* in^ a
radiation is much more complicated.
Van Dalen et al.
78 reported that an aluminum box,
around an MIP, reduced the radiation leakage to a safe level.
The system used in the
around openings in
Microwave Corp., Hauppauge, New
Goals of This Research
The control of trace elements,
< 20 ng mL)1
, in high purity solvents is critical
to the semiconductor industry. 8-s 8
As the integration density of integrated circuits in
microchips increases, the manufacturing yields of microchips are limited by impurities
in the processing chemicals.8 Currently, the quantitation of 35 elements, at a level of
20 ng mL-1
, is required for chemicals such as amines, alcohols, and hydrocarbons.
element required is silicon.
Antifoaments, such as poly-dimethylsiloxanes, are a source
of silicon contamination in high purity solvents.
In the chemical industry, antifoaments
are added to the crude oil during the refining process.
The LOQ is found by multiplying the limit of detection (LOD) by
Multiplying the standard deviation of a blank solution by three, 3a, and dividing
by the slope of the calibration curve gives the LOD.83
Quantitation at a level of 20 ng
mL-1 requires an LOD of 2 ng mL1
The techniques used to determine silicon include inductively coupled plasma-
spectrometry (ICP-MS), and graphite furnace atomic absorption spectrometry (GFAAS).
Each of the above techniques have disadvantages that limit its ability in the analysis of
the LOD for Si is
16 ng mL-'.84
This technique does not give the
required LOD of
ICP-MS is often considered the ultimate method for trace
Typical LOD values for ICP-MS are in the parts per trillion range, pg
The determination of Si, in organic solutions, by ICP-MS i
are encountered due to spectral interference.
The isotopes and natural abundances (in
parenthesis) of Si are
(92.2%), 29 (4.
%), and 30 (3.1
Interferences at isotope
The interference at isotope 29 include 14N15N+
, while at isotope 30,
LOD for Si,
in organic solutions,
In aqueous solutions, the LOD for Si is 10 ng mL]' by ICP-MS.
89 added 37 mL min-' of xenon to the argon carrier gas.
Adding xenon gas decreased
the spectral interference of '4N4+
, by a factor of 300,
versus the background when no
xenon is present.
This technique can prove expensive because xenon is a rare gas and
costs $8-$16 per liter.
An LOD of 0.6 ng mL-U
, using isotope
is reported for Si in
The high LODs exhibited by Si, Ca, and Fe in ICP-MS are due to
the additional polyatomic peaks, formed by organic compounds, which produce spectral
Another technique often employed for Si analysis is GFAAS.
mL'1 is reported for GFAAS.8'
analysis in high purity solvents.
analyzed per sample determination
require 30 determinations. It wo
An LOD of 1 ng
This gives an LOD that meets the requirements of Si
A disadvantage of GFAAS is that only one element is
n. The quantitation of 30 elements in a sample would
uld be beneficial to have a technique that could detect
The technique of optical emission spectroscopy is frequently used for SMA.
CAPACITIVELY COUPLED MICROWAVE PLASMA INSTRUMENTATION
Microwave energy is produced in CMP using a generator;
directed down a waveguide.
the energy is then
An electrode is placed at the opposite end of the waveguide.
The electrode capacitively couples with the microwaves.
A plasma may be formed at the
tip of the electrode,
where the microwave energy is emitted from the electrode.
enough power is applied, the gas surrounding the electrode is partially ionized and auto-
ignition of the plasma occurs.
If lower power is utilized,
required using a tesla coil or by touching a piece of wire, held in an insulator, to the
Initial microwave plasma sources were medical diathermy units.5'91
of 100 to 250 W produced microwaves at 2450 MHz.
Development of microwave ovens
has decreased the cost while increasing the availability of magnetron power sources.
most common magnetrons are operated at 2450 MHz.
Waveguides, or resonant cavities, are commonly used to transmit electromagnetic
radiation of the microwave frequency.
The first idea of propagating electromagnetic
mathematics of what might occur if an electric charge was released inside a closed metal
It was not until the years 1913 to 1920 that Zahn93 and Schriever94 performed
the experimental work.
Generally, the waveguide is made of a hollow cavity that may be rectangular or
cylindrical in shape. Coaxial
comparison to coaxial cables,
cables may also be used for microwave transmission.
waveguides have a higher power handling capability, a
lower loss per unit length, and a lower cost mechanical structure.95 The coaxial cables
have a tendency to become hot and radiate heat at power levels above 100 W, causing
a loss of power.'
The reflections caused by the flanges used in connecting wavequide
sections is usually less than that associated with coaxial connectors.95
In waveguides, the
electric and magnetic fields are maintained on
the inside of the walls.
No power is
loss is also negligible,
since the guides are
normally filled with air.96
Waveguides are fabricated from highly conductive metals with dimensions similar
constructive interference pattern from superimposed microwaves multiply reflected from
Th;c nhnnrnmonnn kc ryfrrpA ,n QC Q
" .. 4:. ..n..... "97
4. Vl v., 6.1, .w /* I 1?,,
%I V.. III I I |I I J"l' l~ .-"K 1 rI. 1 1-
l lr- a 'i
This power loss is related to the skin depth (5) of the material.
Skin depth is defined as
the depth at which the current decays to 1/e (0.37) of its value at the surface.97
defines the skin depth.95
= relative permeability dimensionlesss), /.o
= permeability in free space (4r
7 Henry m-'), a = conductivity of the medium mhoss m'), and f
= frequency (Hz).
The skin depth is inversely proportional to the conductivity of the waveguide material
and frequency of the electromagnetic radiation.
will have less power lost to heat. At microwa
Materials which have small skin depths
ve frequencies, the skin depth of a metal
conductor which increases its resistive effect.
This is referred to as the
because the current is concentrated in the outer surface of the conductor.98
several metals at 2450 MHz are Ag,
1.3 pm; Cu,
1.3 pm; Au, 1.6 pm; Al,
1.6 pm and
brass, 2.7 pm.99
The metals Ag,
Cu, and Au are difficult to machine,
while Ag and Cu
also corrode easily in air.
Au is expensive.
Al and brass are the materials generally
used to manufacture waveguides.
three different ways,
described as transverse electric magnetic (TEM), transverse electric (TE), and transverse
/Zr -o o"
for the electromagnetic
two conditions must be met.95
flux lines must be
perpendicular to the waveguide walls and magnetic lines must run parallel to the surface
of the walls.95
Because of these two points, the
mode is not propagated in the
Describing the mode which is propagated, two subscripts are added to the TM and
TMmn and TEmn.
The m and n are integers that define the
quantity of half wavelengths that are in the A and B dimensions of the waveguide where
A represents the width and B the length.
in order for power currents to flow
prevents the flow of power currents.
The A dimension must satisfy the inequality
. The value where
This frequency is designated the cutoff frequency
is possible only when
f > f
The cutoff wavelength
= V, the cutoff frequency for the waveguide is
where Lr is the relative permeability dimensionlesss) and Er is the relative permittivity
The value 1r equals the magnetic permeability (or inductivity) of the
medium, k [Henry (H) m-i], divided by the permeability of vacuum or free space, po (H
Similarly, e, equals the dielectric permittivity (or capacitivity) of the medium, e
[farad (F) m-'] divided by the dielectric permittivity of vacuum or free space,
The wavelength in the guide is represented by
o (F m-').
It is defined as96
where X (= v,/f) is the wavelength in an unbounded dielectric, and
f is the operating
represented by v, and is equal to (Le)-'-
, where u and e
are the permeability (H m-1) and
permittivity (F m-')
The phase velocity in the positive z direction inside
the waveguide is
Because the cutoff frequency is a function of the modes and waveguide dimensions, the
actual size of the waveguide governs the propagation of the modes.
lowest cutoff frequency in a given guide is the dominant mode. In
The mode with the
a rectangular guide
may also be present, but only the dominant mode will propagate, and the higher modes
near the sources or discontinuities will decay very fast.96
Earlier it was noted that microwave generators are used to generate and introduce
microwaves into a waveguide.
energy from the waveguide. I
distance of mX,/4,
An electrode, or coaxial cable, is used to withdraw the
Both the generator probe and the electrode are placed at a
where m is an odd integer, from opposite ends of the waveguide.
Here the microwaves traveling to the edges return in phase with the waves going in the
This phase shift occurs since the wave shifts 900 by travelling the
Xg/4 distance, then a
shift by reflection at the wall, and another 900 by traveling
back the X./4 distance.'6
A maximum coupling efficiency
and a maximum density in
the electric fields occur when the probe and the electrode are placed a distance of mXg/4
electrode was 3 cm.
This also corresponds to X\/4.
Instrumentation Used in
Table 2 1 lists the instrumentation and manufacturers.
The materials used
to fabricate the electrodes are listed in Table 2 -2.
The dimensions of the waveguide are
given by Hwang et al.61
The torch is made from two concentric quartz tubes.
tiheP hnl d the peletrnde and cdirepet the va mnl1 vannr thrnnirh the center nf the electrnce
0 a. 0 0
ff L 3. C3O
.9= o~ cm
I: 1 .c
a, IIl 4 I
---------~~~ I--- m-^ ----m mof
0) -^- i C,
O S '' ''
: CMP Instrumentation and Manufacturers
OSMA Detector Controller
Photodiode Array Software,
HR 1000. 1 I
v, 2400 grooves
Instruments SA, Inc.,
, linear dispersion
0.5 nm mm-
High voltage d.c.
805-1A (maximum power
output of 1.6 kW)
NL 10251-2 (frequency
2.45 GHz, maximum
Electrode and Electrode Cup Materials
Corp., Grade L3810 AGKSP,
= 1.9 ppm.
Ward, Hill, MA.
= 20 ug/mg
The outer tube directs the plasma gas, in a tangential flow, around the electrode.
dimensions of the outer torch are 15.5 mm i.d. and 17.5 mm o.d.
and covering the top of the waveguide are cooling coils. Water
the coils to cool the torch.
Surrounding the torch
is recirculated through
A coaxial waveguide is place above the cooling coils.
coaxial waveguide propagates the microwaves out of the waveguide, toward the top of
This configuration increases the stability of the plasma.
A quartz chimney
( 4 cm diameter) surrounds the coaxial waveguide.
This chimney is used to keep air
magnetron to the plasma fluctuated randomly.
random fluctuations in the emission from the (
This fluctuation in current contributed to
CMP. Masamba26 did not determine the
circuit were built (D.
Hueber, unpublished results).
This system improved the stability of the CMP.
The detector used in this work is a photodiode array (PDA).
of silicon photodiodes in an integrated -circuit form.83
The PDA is made
A total of 1024 diodes (or pixels)
are arranged in a linear manner.
The PDA covers
Each diode corresponds
to 0.0198 nm (wavelength range around 300 nm). The PDA sequentially reads the signal
from each diode after a specified integration time. The integration time is the interval
the diode is allowed to collect the incident electromagnetic radiation.
from 0.033 s to greater than 2
This time ranges
Longer integration times usually lead to overexposure
integrated, the signal from each diode is added to the signal, from the same diode, from
the previous scan(s).
This allows for larger signals to be collected without overexposing
A disadvantage of the PDA includes the necessity to cool the detector in order
the dark current.
the diodes must be analyzed sequentially.
system does not allow for random access of any diode(s).
to collect and analyze data.
The PDA software was used
Additional software programs were written to improve data
allows for the average and standard deviation of a specified diode to be obtained from
a given number of spectra.
A program was also developed to display three dimensional
1.1, D. Hueber, unpublished results).
There are several ways to introduce samples into a plasma.
sample introduction technique is pneumatic nebulization.102 In pne
a capillary tube carries the nebulized sample to the spray chamber.
tube, gas flows in a concentric manner.
The most common
This causes reduced pressure at the tip of the
is produced by the Bernoulli affect.83
The aerosol passes
through a spray chamber,
which allows removal of the larger droplets.
droplets, typically less than 10 pcm in diameter, proceed to the plasma.l03
primarily used because of their ease of operation.
The disadvantages include high sample
solution consumption, low sample transport efficiency, and clogging due to particulates
as the weakest link.'04
nebulizers have been developed specifically for use
with helium gas.
have noted that nebulizers do not operate very well
Nebulizers, primarily used with ICPs, are operated with argon gas.
nebulizers, helium produces larger droplets and a nonuniform mist.
Helium is a smaller
atom than argon (1.22 A radius for He
1.91 A radius for Ar).30
have a smaller outlet orifice and operate at higher pressure (45-60 psi He vs 30-35 psi
The efficiencies of an argon nebulizer operated with helium and a helium nebulizer
in the drain.
The difference in weights,
was attributed to the
amount of vapor reaching the plasma.
The nebulizers were optimized for pressure, and
gas and solution flow rates using the signal and signal-to-noise ratio of silicon.
efficiency of the argon nebulizer,
with helium gas,
The helium nebulizer had
an efficiency of 11
Using the helium nebulizer, the LOD for Si decreased by a factor
The helium nebulizer had a higher efficiency, but lower solution flow rate (0.6
mL min-' for Ar).
visually it was observed that the helium
research in the literature on nebulizers operated with helium gas.
Another technique for introduction of samples is by thermal vaporization.
an electrode cup is used to hold a fixed amount of sample.
The plasma is
produced on top of the electrode cup.
The sample is then vaporized by the plasma.
rapidly, leading to a larger signal than by nebulization of the same amount of solution.
Another advantage of this technique is that the sample may be heated or ashed to remove
or water from
plasma is operated at low power to remove the sample matrix; a higher power level is
used to vaporize the analyte.
ectrode and electrode cup were
graphite, titanium, and tungsten.
is a stability diagram showing where three different plasma shapes
The operating stability diagram was obtained following a procedure similar
to that of Rezaaiyaan
et al. 13
The data for the diagram
obtained by igniting the discharge and allowing a stable plasma to form.
The flow rates
were 4 L min
1 helium and 150 cm3 min'1 hydrogen with a power level of 450 W
helium plasma gas was increased or decreased and visual observations performed.
experiments were repeated by keeping the helium gas flow rate constant and varying the
To determine the points where a plasma was started, an insulated wire was
touched to the electrode at each power and flow rate.
The power or gas flow was varied
until an appropriate setting was reached where the plasma formed.
Power levels of 0-
flows of 0-17 L min'
emission intensity increases
microwave power and
the plasma length
gas flow above 2 L min'1 and below 17 L min-' and power levels above 150-300 W and
the diagram are semi-quantitative and do not
define exact values.
The addition of a molecular gas (H2, N2, or 0-) or a change in the
diameter of the quartz torch alters the size and shape of each region.
by the plasma range from pinkish-blue to very bright pink.
The stability diagram has four regions:
filament plasma, spherical
plasma, and cylindrical plasma.
A photograph of the spherical plasma is given in Figure
The cylindrical plasma is shown in Figure
The filament plasma is bright
pink in col
It is a thin plasma that is approximately 2-3 mm in diameter and 10 mm
This plasma is very unstable, as it frequently moves around the surface of the
Focussing of this plasma onto the entrance slit of the spectrometer was very
Flow rates below 4 L min' typically cause the filament plasma to
attack and melt the quartz torch.
The spherical plasma generated with the CMP has previously been used to analyze
steel samples, 106 aqueous solutions,68 and arsenic71 by hydride generation.
this plasma is
The size of
1- 1.5 cm in diameter with a length of 2-3 cm depending on the power
level and flow rates of helium and hydrogen gas.
a pinkish-blue tint.
between 6 and
This plasma is very stable and exhibits
The spherical plasma changes to the cylindrical plasma at flow rates
7 L min-'
At flow rates above 6 -
, the plasma is cylindrical.
The Spherical Plasma in the He-CMP.
The Cylindrical Plasma in the He-CMP.
decreased as the depth of the electrode, inside the torch,
Lowering of the
signal to- noise ratio
for the analysis of silicon,
which will be discussed in chapter 3.
the electrode (0.6-0.8 cm). The le
This plasma has a diameter very similar to that of
ngth of the plasma ranges from 2 cm at 6 L min-1
to 4.5 cm at
17 L minl'
, with a characteristic very bright pink color.
power above 1000 W causes the emission intensity to increase and the plasma changes
from pink to almost white in color.
Like the spherical plasma, the cylindrical plasma is
very stable. The main differences include the color, size, and background due to plasma
emission. The cylindrical plasma has a background level three times higher than that of
the spherical plasma; however, the signal-to-noise for Si in the cylindrical is higher.
The cylindrical plasma will be studied in this work.
The analysis of silicon, in organic
solutions, will be performed by solution nebulization (chapter 3) and thermal vaporization
cylindrical plasma will be presented in chapter
DETERMINATION OF SILICON IN ORGANIC SOLUTION USING
Although most of the work performed with plasmas deal with aqueous solutions,
may also be analyzed.
introduction of aqueous solutions by pneumatic nebulization.61'68
organic solutions into the CMP,
The introduction of
with pneumatic nebulization, has not previously been
Introducing solutions into an MIP has proven difficult because small amounts
of sample material (3
ug) degrade the stability and extinguishes the plasma.3,'15
the MIP is operated at low powers (
200 W), the plasma does not have enough energy
to vaporize or evaporate solid or liquid samples, or to atomize the analyte species.3'79
Methods for the introduction of solutions into the MIP include heating the sample
a cooled condenser to remove the
vaporization (TV) techniques are also used for sample introduction in MIPs.23'108
technique, the sample is placed on a graphite cup,08os or metal wire,23 which is heated to
remove the solvent, followed by vaporization of the analyte into the plasma.
MIP torches and cavities are currently being utilized to increase the operating power
(40-500 W) and facilitate the introduction of aerosols. 109, 10
Researchers have used ICPs to analyze organic samples.
lubricating oils with xylene to determine 21 elements.
Brown11' diluted used
The analysis of metals in used oils
"wear metal analysis."
Blades and Hauser'1" analyzed nonmetals in xylene
using an Ar-ICP with a photodiode array detector.
Nygaard and Sotera'13 examined the
concentration of Cd, Mn, and Fe in organic solvents such as acetone, and tetrahydrofuran
using an Ar-ICP.
The research reported here will concern the analysis of Si, in organic solution,
with a He-CMI
analysis are desc
effect of power,
Development and optimization of the cylindrical He-CMP for Si
ed. Items presented will include the spatial profiles of Si, and the
plasma gas flow rate, and molecular gas addition.
The instrumentation is discussed in chapter
The organic silicon standard (5000
pg mL') was purchased
from Conostan Division of Conoco, Inc.
Kerosene from Fisher Scientific was used to dilute the organic Si standard to the desired
Slit height and width were
2 mm and 20 pm, respectively.
A UG5 filter
was employed to decrease the background emission continuum.
A pneumatic nebulizer,
designed for helium gas,
Solutions were introduced at a flow rate of 1.2 mL mini- using a peristaltic pump
Rabbit, Rainin Instrument Co.
Inc., Boston, MA).
A power level of 800 W
for the photodiode array
integration time and number of accumulations were based on S/N measurements.
use of the PDA is described in greater detail in chapter
The above conditions were
used unless stated otherwise.
The analytical signal is obtained by measuring the total signal from the
analytical sample and subtracting the signal from a blank solution.
is identical to the analytical sample except the analyte,
The blank solution
which in this research is silicon,
for the sigr
A signal due to the blank sample is called the background signal.
are the average of five measurements.
Error bars in the graphs represent
one standard deviation (la), based on five measurements.
Blank noise levels (N) were
using a curve
determined using a polynomial fit with the following exceptions.
Data for plasma gas
flow rate (Figures 3
-3) were divided into two groups according to the type of
plasma (spherical or cylindrical).
Data for signal, background, and S/N for flow rates
Signal and background data for the cylindrical plasma, 7- 11 L minl1
, was also subjected
to least squares fit.
The S/N data, representing the cylindrical plasma,
was plotted as
the average of the five data points.
to emphasize the transition which occurs with
Lines connected the data points at 6 and 7 L minr1
increasing plasma gas flow rate.
connecting the data points were employed on
the graph of S/N versus molecular gas
Silicon emission is observed at either 251.61
spherical and cylindrical plasmas were compared using an aqueous Si standard.
samples interfere with the Si
a bandhead at 281.1 nm.
16 nm line because of the OH molecular band that has
The Si line at 251.61 nm was observed in the experiment with
the 288.16 nm
When a graphite electrode was use
emission was observed at 247.9 nm.
nm line, especially at power leve
or organic samples introduced, a strong carbon
This carbon line would interfere with the Si 251.61
above 700 W.
hydrogen gas pressure were 45 psi.
Helium gas flow rates for the plasma and nebulizer
tee was installed
in the plasma gas line,
between the plasma gas flow meter and the
This allowed the introduction of molecular gases (H2, N2, or 02) to the
melting of the quartz
experiments except the molecular gas addition studies.
Cylindrical and SDherical Plasmas
In comparing the spherical and cylindrical plasmas, the helium gas flow rate was
increased from 4 L min-' to 10 L minm'
An observation height of 5 mm was employed.
This height was the optimum for the spherical plasma, based on silicon signal and signal
-to-noise ratio (S/N) measurements.
The depth of the electrode inside the quartz torch
In the experiment involving a tandem electrode, a graphite rod was employed as
the tandem electrode.
The graphite rod was mounted on an x-y translator to allow
positioning of the rod above the CMP electrode.
An insulated copper wire was attached
to the graphite rod and connected to an electrical
The distance between the
mm to 30
electrode, ranging from 4 mm to
The depth of the electrode inside the quartz torch was investigated for values
between 0 mm and 16 mm.
Positioning of the electrode in relation to the waveguide was
Alil'0 found an optimum length of the electrode inside the waveguide
should be approximately
Placing the electrode 3 cm into the waveguide reduced
the plasma acoustic noise and resulted in increased plasma excitation.
corresponds to '4 the wavelength of the microwave radiation in free space.65 The bottom
of the tungsten electrode was manufactured
to fit inside the inner tube of the quartz
This is shown in Figure
A piece of teflon tape was wrapped around the
bottom of the electrode to prevent the electrode from descending completely into the
inner tube of the torch.
Various depths could be studied by changing the position of the
The teflon tape required replacement after 15
-30 minutes, due to melting.
Determining the spatial profiles was aided by having the waveguide, magnetron
on an apparatus
the positioner provided
Measurements for the vertical profile were performed after aligning
the top of the electrode with the center of the spectrometer's entrance slit.
height is the distance the electrode was lowered below the entrance slit.
profiles were taken using two different methods.
Initially. the nlasma image was nlaced
spectrometer and moving the plasma sideways in either direction.
obtained using both methods,
plasma is reported.
Similar results were
therefore, only the result involving the centering of the
In the horizontal profile, a vertical height of 12 mm was used.
titanium, and tungsten.
gives the materials and manufacturers for the electrodes.
A graphite electrode was used
in comparing the spherical and cylindrical plasmas, examining spatial profiles, and in the
effect of a tandem electrode.
A tungsten electrode was used in examining the influence
of electrode depth, effects of power and molecular gas, and the limit of detection.
experiment compared electrodes fabricated from graphite, tungsten, and titanium.
operating the plasma at 500 W.
In comparing the electrode materials, the blank noise
level (N) was determined from the standard deviation of 10 diodes occurring between 253
nm and 254 nm.
A silicon impurity in the titanium prevented the measurement of the
background at 251.61 nm.
Limit of Detection
The limit of detection for Si was based on 3-a.
The standard deviation, (a),
calibration curve was 0.997
Results and Discussion
Cylindrical and Spherical Plasmas
This plasma gave an LOD of 17
for Si in aqueous solution.
Since this was an unacceptable LOD,
the cylindrical plasma was investigated.
-1 shows the signal obtained for Si using the spherical and cylindrical plasmas.
strongest Si emission line in the region from 240-260 nm is at 251.6 nm.
at 250.69, 251.43, 251.92, 252.41
and 252.85 nm are also observed in the spectra.
an aqueous s(
a strong ca
emission line is
Graphite electrodes require replacement after 1-2 hours of operation, due to
erosion caused by the cylindrical plasma.
If the spherical plasma is formed, the graphite
electrode required replacement after 6-8 hours.
Results obtained with
versus the spherical
superior for the analysis of Si.
Based on Figure
, the signal increased by
standard deviation of 11
measurements of a blank solution.
The background emission
level increased by
with the signal-to-noise ratio (S/N) increasing 140%.
E -Ce) --- --
\ I '
o-- _0_ 0 0- i
00-- '.0 &
(s---q ^~rq~ ps01u uoss
cylindrical plasma was formed by increasing the plasma gas flow rate from 4 L min-
10 L min'
, was added to the helium plasma gas to
prevent the spherical and cylindrical plasmas from melting the torch.
At the time of this
experiment, conditions were not optimized for the cylindrical plasma.
The Si signal, as a function of plasma gas flow rate, is shown in Figures
Plasma gas flow rates between 3.5 and 6 L min-' produced the spherical plasma,
increase in the signal and background occurred at 7 L min-'
At this plasma gas flow
the plasma changed
to cylindrical in shape.
The change from a
spherical plasma to cylindrical plasma repeatedly occurred in the region of 6-8 L min'1
Several additional observations were noted.
Background levels between 200 and 700 nm
were higher in the cylindrical plasma than in the spherical plasma.
in the cylindrical
with an increase
plasma gas flow rate.
Previous researchers have reported an increase in the silicon response,
chromatograph-microwave induced plasma (GC-MIP) system,
in a gas
with increasing plasma
gas flow rate.
This behavior is not unique to Si.
Estes et al.114 reported that 19
elements exhibited this response in a GC- MIP with a plasma gas flow rate between 40
and 800 mL mint'
The increase in Si signa
in the MIP,
was also accompanied by a
decrease in the background emission from the quartz torch and the carbon emission from
-<- ( dL
^ 5 '
r3 C -
E ` '
S ^3 -
(sl!un fj JJ!qJv) rl!suoluI uoIsstIIU
I I I
in the background signal with increasing plasma gas flow rate.
Increasing the plasma gas
flow rate decreased
the residence time of the Si atoms in the plasma.
A decrease in
residence time limited the time Si atoms have to react with 02,,
which is present in the
Oxygen may be present as a contaminant in the helium or hydrogen gases, or
from entrained air that surrounds
the plasma and
-3 gives the S/N
versus plasma gas flow rate.
The results with the spherical plasma show an increase in
the S/N with increasing plasma gas flow rate.
The cylindrical plasma gives a constant
S/N with plasma gas flow rates between
7 and 11 L min-
Future sections will discuss
the optimization of power, height, and flow rates.
A problem with the cylindrical plasma was flickering or instability
. The top of
the plasma, 5 mm above the electrode, constantly moved sideways over a 1.5 cm range.
arrangement is initially examined, followed by changes in the quartz torch configuration.
The results reported in the rest of this work will deal with the cylindrical plasma, the
spherical plasma will only be mentioned for comparison purposes.
In direct current plasmas (DCPs), initial instrument designs consisted of
producing a plasma by passing a de current
between two electrodes,
placed at a 300
The plasma appeared as an inverted "v."60
Dekker117 improved the stability,
The third electrode was placed above the two electrodes,
Chan et al.
116 positioned a
electrode above a helium-ICP (He-ICP).
This tandem electrode localized the helium
discharge and indicated a better energy transfer through capacitive coupling.116
LODs for ]
Cl, I, and S were reported for the He-ICP with and without the tandem
Power levels were 1500 W and 500 W for the He-ICP and tandem He-ICP,
y. No explanation was given why the LODs did not decrease or why higher
power was not used in the tandem system.
In the research
for this work, a graphite
electrode was positioned above the cylindrical He- CMP to investigate the effects on the
silicon signal and S/N.
Figure 3-4 gives the spectra obtained with and without the tandem electrode.
In comparing the results,
the tandem electrode increased the Si signal by 200
is the noise increased
in the S/N
The distance between the bottom electrode and the top, tandem electrode, is
to 30 mm.
observed, is varied from 4 mm to
Visually, the stability of the plasma increased
with the tandem electrode.
If the tandem electrode is moved horizontally,
off-center, the plasma remained coupled to the electrode.
The signal increased because
the coupling of the plasma
At this time, there is no explanation why the noise increased.
LODs with and without the tandem electrode are
ug mLU' and 0.8 zg mL-U
o 0 0 0 o 0 g
So o 3 3 o
~ : : ^ = : = O^-
on 0 0 -
00 N en C
(stiiuq~~ Jr q^)&iUlU ospJ
Since the addition of a tandem electrode did not benefit the LOD, it was
not investigated further.
The effect of increasing the depth of the electrode, inside the quartz torch,
of the electrode is the distance between the top of the torch
and the top of the electrode.
A depth of zero mm indicates the electrode is level with
the top of the torch.
A depth of 10 mm has the electrode
10 mm below the top of the
The results for the Si signal and background versus depth are shown in Figure
The Si signal is measured at a height of 12 mm above the electrode, corresponding
to the optimum
discussed in the following section.
Figure 3-6 gives the S/N versus depth.
show an optimum signal and S/N at a depth of 6 mm to
A decrease in signal
and S/N is observed at 13 mm and 15 mm, because the signal is transmitted through the
The transmission of the quartz is less than 100%, due to the characteristics
of the quartz and previous etching from the plasma. When the depth is greater than 12
mm, the signal is also observed 1 mm above the torch. Here, increasing the depth from
12 mm to 16 mm did not improve the signal or S/N over the previous results.
of the cylindrical He-CMP, with a depth of 10 mm, is given in Figure 2-4.
as the depth is increased to
10 mm, the diameter of the plasma decreased by
An increase in the intensity is also noticed, which appeared to indicate an increase in the
(uI 1 -
So o o 0
o 0 0
0 0 0 0
(sIUn XIinsurcuv uoSssSIiW
0n I3 n C,
Spatial profiles of the Si emission distribution are measured to locate the regions
with maximum signal and S/N.
Spatial mapping has been performed on the DCP'18 and
ICP.119'120 Blades and Horlick'12 studied the emission profiles of twelve elements, while
Kawaguchi et al.119 examined Fe, Cr, and Mn emission profiles, in the Ar-ICP.
gives the signal and S/N versus height above the electrode.
The optimum signal
and S/N are located approximately 11 mm to 14 mm above the electrode.
The curve for
S/N, Figure 3
is obtained using a polynomial fit.
Data for the S/N, in the region of
- 14 mm, is constant.
12 mm the signal decreased rapidly.
The S/N did not
decrease as rapidly as the signal, due to lower noise levels exhibited at greater heights
significant change is observed in the vertical profile of Si.
The vertical height was 12 mm above the electrode.
The value of zero mm is obtained by visually observing the plasma silhouette,
on the entrance slit of the spectrometer, and aligning the center of the plasma with the
A relatively constant Si signal is observed at +
Occasionally, the data
gives skewed results, giving a greater Si emission (20%) on one side of the electrode.
(poz!IUwJON) leug!s uon!!S
(poZIIEtUJON) It3uI!S uoZITS
Effect of Power
A plot of signal and background, versus power (W)
is found in Figure 3-9.
signal increased from 450 W to
Above 1000 W
, the signal appeared to remain
Power levels greater than
the filament plasma.
1200 W are not investigated due to the formation of
The background level increased between
10 is a graph of S/N versus power.
700 W and 1000 W
An optimum S/N is obtained in the region
of 700-975 W
. The decrease in the S/N above 1000 W is due to the signal remaining
constant while the noise level
The results are in contradiction to those of
Masamba and Winefordner.68
Masamba and Winefordner68 used the spherical He-CMP
operating at 6.5 L min~l He and 300 cm3 min-1 H,.
Their results showed the signal for
Al and Mn
power levels between 600 W
. Power levels
above 1000 W were not investigated.
The S/N for Al also increased in this power range.
The results for Mn also showed the S/N increased with power up to 900 W
. Values for
the Mn S/N at 900 W and
000 W are similar,
which suggests a plateau.
give results similar to those of Si above
Research was also performed on materials for electrode fabrication.
is formed on
top of the electrode.
One disadvantage of the CMP is the
o a 0 0
0 \ 0 0
Inv 0 In0 n 0 In
0> 0 0 0 0 0 0
a /) r o on
o 0 0 0 0 0
0 In 0 0 In
carbon emission from the graphite electrode is visible.
Electrodes were fabricated from
and tungsten (W).
Previous electrodes used in CMPs include
raphite,61,'68 Ta,64,121 and a tungsten wire.70
Graphite electrodes offer the advantages
of being cheap, easily fabricated, and able to operate at power levels up to 1200 W
emission from the He-CMP, while nebulizing kerosene, is given in Figure 3
figure shows the background being decreased with Ti and W electrodes.
A larger carbon
(247.9 nm) emission is observed with the G electrode. Using the G electrode, carbon
emission is due to both the kerosene and the graphite electrode. The G electrode only
as the electrode changed
to a rough,
A rounded surface produced a filament shaped plasma.
observed flowing through the plasma.
disadvantage of the
Ti electrode included melting above 500 W
. In Figure
Ti electrode is starting to melt and Si emission is observed; the Si is an impurity in the
In comparison to G, the
Ti decreased the background and noise by 40% and 50%,
The W electrode also produced a lower background and noise, compared to the
The background and noise decreased by 30% and 50%, respectively.
Power levels up to 1400 W are used with the tungsten electrode.
At 1400 W
emission is observed, suggesting that the electrode is being eroded by the plasma.
(sl!U li !qsv 1!suol UOss
0- 0 00
0 0 0 0-0
0 0 00 04
r~l ooo \ ^ 4
(snun~~~~~~ *-~j)^soU osiu
operation, the W electrode was used for over
150 hours at 700-1000 W
. No erosion
or pitting of the electrode was visible.
initial cost ($635.00).
The disadvantage of the W electrode is the high
However, since the electrode lasted longer than the G electrode,
it should be cost effective in the long term.
The advantages of the W electrode,
G, are a longer lifetime and decreased background and noise levels.
Molecular Gas Addition
have been used in ICPs'22"3 and CMPs68,70 for spectrochemical
The number of studies employing molecular gases (Oz, N2) in an Ar-ICP has
increased over the past several years."'
The addition of a molecular gas to an Ar-ICP
offered the advantage of greater heat transfer to analyte aerosol particles.3'
Ar-ICPs decompose refractory particles and operate with higher solvent and analyte
In a He-CMP, hydrogen gas was added to prevent the plasma from adhering
to the torch walls.68
Ali and Winefordner70 reported the addition of H2 gas at 100 mL
min1' into a He-CMP decreased the background level and noise.
In the cylindrical He-
CMP, increasing the H, gas flow rate from 0 cm3 min-' to 250 cm3 min-1 produced a
decrease in the plasma diameter and height by
14 mm to 6 mm and 20 mm to
The reduction in plasma
has been attributed to the additional absorption
of energy required in dissociating the molecular species. 25
and Hg in a He-CMP.
The addition of H, decreased the S/N for As and Hg, while the
S/N for Cr and Zn remained relatively constant."26
If organic samples are introduced into
the dominant spectral
features in the background are from CN and C,
molecular emission (see chapter 5).
The presence of 02 reduced molecular emission due
to CN and C,.
In our research,
the effects of H2, N1, and 02 on the Si signal and
S/N are investigated.
Figure 3- 12 gives the effect of the signal for various amounts of molecular gas.
Nitrogen, at a flow rate between zero cm3 mint' and 200 cm3 minT1
, slowly doubles the
The (N,)-He-CMP produced a relatively constant signal in the range from
. Both 0, and H, produced a curved response in the signal.
enhancement (180%) in the S/N. Oxygen increased the S/N by a factor greater than two,
although the signal increased over four times. This was because increasing the 02 flow
rate, from 0 cm3 min-l to 300 cm3 min-'
, doubled the noise in the background.
when organic solutions are introduced
into a plasma,
decreased as 0, was added.
The possibility exists that adding increased amounts of 02
available to produce an emission signal.
The largest increase in the S/N was observed for HI.
Upon the addition of 100
cm3 min-1 of H,, the S/N increased over five times versus the S/N without H,.
E- ) D
o .0 0
s 1 o rt
o o 0 o 00
rJ 7 o \D
0G00 0 0 0
O > N/S -: '
>^ r u ,
S / oo
/ x *
/ 0 < -
^1- ~ ~~~ CN00 \ t(
60- *- *--
250 cm3 minl'
, none or very little Si emission was observed.
The H2 makes a stronger
reducing environment, decreasing the reaction of Si with traces of O,.
as an impurity in the He plasma gas and as entrained air.
The 0, is present
Based on the increased S/N,
the addition of H, is preferred over 02 (and N,) for the analysis of Si.
Figures of Merit
The LOD for Si, in kerosene,
was measured using the spherical and cylindrical
for the spherical
cylindrical He-CMP, respectively. These values are based on three times the standard
deviation of the blank, also represented as 3o. Using the cylindrical plasma, the LOD
for Si was decreased by two orders of magnitude. In comparison, the LODs by ICP-
OES84 and GFAAS8' are 0.016 p
mL-' and 0.001
gives an LOD greater than
The linear dynamic range of the cylindrical
He-CMP is three orders of magnitude.
Relative standard deviations in the signal were
The cylindrical He-CMP was applied to the analysis of Si in organic solution.
The depth of the electrode, inside the torch
of the plasma and increased the Si signa
This decreased the flickering
Spatial profiles of the Si emission
The Si reached a maximum signal and S/N between 700-1000 W
An electrode made from tungsten exhibited lower background and noise levels,
a graphite electrode.
The tungsten electrode also has a longer lifetime than graphite or
The LODs for the He- CMP are over an order of magnitude greater
than those of ICP-OES and GFAAS.
To lower the LOD in the cylindrical He- CMP,
discussed in the next chapter.
DETERMINATION OF SILICON IN ORGANIC
SOLUTION BY THERMAL
The sample introduction technique of thermal
vaporization (TV) is often used
when lower limits of detection (LODs) are required than can be obtained by pneumatic
appropriate free species for spectrometric detection. 128
Ng and Caruso'27
electrothermal carbon cup vaporizer with a pneumatic nebulizer for the introduction of
samples into an ICP.
The LODs for the vaporizer were an order of magnitude lower
than those of the pneumatic nebulizer. 127
to nebulization, are a result of a larger
percentage of the sample being transported into the plasma.
sample reaches the plasma,
In TV, up to
while nebulizer efficiencies are usually less than
100% of the
nebulizers, the sample is diluted in a carrier gas before reaching the plasma.
are diluted less in
, which increases the atomic number density of the analyte in the
Disadvantages of TV include poorer precision, greater interference effects,
and a lower throughput of samples.83'129
In TV, a discrete amount of a sample is deposited inside or on top of an atomizer.
The atomizer may be a cup, rod,
platform, or wire.129
The atomizer is then heated in
Initially, the first step is the drying or desolvation step,
energy is applied to evaporate the solvent and leave a solid residue.
An ashing step is
where the power is increased to convert organic material to H20 and
CO,, and vaporize the volatile inorganic components.
The third step is the atomization
Here the power is increased and the sample is vaporized and atomized, producing
an atomic vapor.
The vapor is probed by a source, as in atomic absorption spectroscopy,
or carried into a plasma or flame,
where emission is observed using atomic emission
Since the atomic vapor is formed rapidly, a transient, peak-shaped signal
Additionally, a fourth "cleaning" step may be used.
Here a higher power
is employed to remove any residue from the atomizer.
Each step is optimized for power
capability of the system. '30- 3
Thermal vaporization devices have been used with CMP-OES, MIP-OES, and
Matusiewicz reviewed sample introduction techniques for MIP-OES and
is one of
Hanamura et al.62 used a
furnace vaporizer where the sample is held in a quartz crucible.
As the sample was
the volatile components were swept by a carrier gas flow into the CMP.
Detection limits for H,
for 1 g of solid sample.
O, N, C, Hg and As were in the high nanogram range
Ali et al.65'66 developed an electrode cup system.
formed on top of the sample cup.
This offered an advantage in that the sample was
plasma. 133- 136
gives a diagram of an electrode cup system.
detection (3a) were between
10 and 210 pg for nineteen elements in aqueous solution.65
This system was applied to the analysis of coal fly ash and tomato leaves.66
MIPs are also used with
The characteristics of MIP, such as low
vaporize and atomize solid or liquid samples.139
Sample atomizers include a tantalum
Heltai et al.134
combined a graphite
furnace vaporization system with argon
and 100 ng mL-'
, using 50 uL aliquots, for 13 elements.
LODs were between 0.1
The MIP had a gas circulation
that allowed venting of the solvent vapor during the drying and ashing steps,
preventing extinguishing of the plasma.
carried into the MIP.
In the atomization step, the analyte aerosol is
Problems in this type of arrangement included unstable plasma
operation during manipulation of the valves and plating of the samples onto the walls of
the transfer tubes.3'134
Operation of the TV MIP- OES system required a narrow range
of parameters for stable plasma operation.3'134
Atomizers in TV
- ICP- OES include filaments, boats, rods, and tubes made from
metal or graphite. 29
McLeod et al.
129 have reviewed TV -ICP- OES,
attention to direct sample insertion (DSI) probes and electrothermal vaporization (ETV).
use of ETV-ICP-OES
graphite furnace atomic absorption spectrometer.136 Samples are dried and vaporized into
The ICP then uses its energy more efficiently in dissociating, atomizing, and
exciting the small amount of the sample that was previously desolvated.
and is reported
to contact of the
analyte with graphite containers, 3) change in the rate of transport between the ETV and
irreproducible.'13 Even with the limitations of ETV -ICP-OES, it has been successfully
applied to the analysis of sea water,143 biological materials,135 and ceramic powders.130
TV sample introduction is using direct sample insertion-
inductively coupled plasma-optical emission
sample insertion is performed by inserting a probe, containing the sample, directly into
the ICP. 129
The probe is similar to CMP electrodes.
In DSI-ICP-OES, the plasma is
during manipulation of the probe.'29
to within a few mm of the plasma. 129
The probe is raised axially, or transversely,
Sample desolvation and ashing occurs due to the
performed by Salin and Horlick'44 and Sommer and Ohls.145
involves automating the sample introduction assembly. 146
Disadvantages of DSI-ICP-
OES included a shift in the background level following insertion of the probe into the
of the probe was required to give reproducible vaporization
and excitation conditions. 129
A difference in probe positioning by
mm produced a
change in the signal greater than 10%
automatic insertion process is about 0.5
Reproducibility with computer control of the
mm.146 DSI-ICP-OES has been applied to
the analysis of aqueous solutions, 146 aluminum oxide,148 and nickel alloys.149
The technique of TV-CMP-OES has several advantages over TV-MIP-OES
In TV-CMP-OES, the plasma is formed directly on top of the
which is holding the sample.
Analyte is not lost due to transportation
the atomizer and
Positioning of the electrode does not change
between sample determinations, negating the positioning errors that occur in DSI-ICP-
After the sample is deposited inside the cup, the plasma is ignited and used to dry,
ash, and vaporize the sample.
to TV- MIP- OES. Disadva
No manipulation of the plasma gas is required, in contrast
stages of TV CMP OES include possible contamination
due to electrode materials and changes in background levels when the microwave power
In this research, the technique of TV- CMP-OES is applied to the determination
of Si in organic solution.
A comparison is made between electrode cups fabricated from
The addition of matrix
modifiers, such as Mg(N03),, are investigated.
Finally, the LOD for Si by
TV CMP -
OES is determined and compared to those reported by GFAAS, GF-OES,
OES, and TV-ICP-OES.
The instrumentation is discussed in chapter
The electrode cup system, shown
same parameters as
reported for pneumatic nebulization, chapter
Standards were prepared in
Burdick & Jackson
used in chapter
was determined to contain silicon in the
100-500 ng mL-' range, using the technique of TV CMP- OES.
Blank MIBK samples
give silicon emission at
Electrode cups are
fabricated using the
laboratory. Thermo Electron/Tecomet (W
mington, MA) manufactured the tungsten cup.
electrode has a depression that gives a snug fit and good electrical contact with the cup.
Dimensions of the cup are given in Figure 2 -2B and are similar to those reported in the
The optimum helium and hydrogen
flow rates are
10 L min' and
These are the same as reported in chapter 3.
The vertical spatial
The optimum height is between
7 and 9 mm.
Increasing the "depth"
the electrode gave similar results to those obtained by pneumatic nebulization, as shown
The vertical height and electrode depth used in the experiments reported
in this chapter are
mm and 6 mm, respectively.
Parameters for the photodiode array
accumulation of zero.
Data points for the signals are the average of three measurements.
Background noise levels (N) are determined by measuring ten consecutive diodes at 2 nm
background noise is measured at
= 290 nm.
Due to the transient nature of the Si signal,
the background and background noise are determined using the same spectrum that gives
the maximum Si signal.
the analysis by
TV CMP OES
is added to the sample cup.
Microwave power is
increased to the level where the ashing step is performed.
ashing is performed in one step.
electrode, initiating the plasma.
In our system, the drying and
An insulated wire is briefly touched to the side of the
The ashing time starts from the point where the plasma
Before the ashing step is over, data acquisition is started.
When the ashing
step is finished, the power is manually increased to the atomization power level.
data acquisition is over, the power is raised to
1200 W for
10 s, to clean the sample
power- 135 W, atomization power-500 W, and sample size-20 pL.
are used unless otherwise noted.
Investigations into the vertical profile, integration time,
ashing time, and ashing power are performed using a titanium cup.
A tungsten cup is
determination of the LOD.
integration time is 67 ms.
In examining the temporal nature of silicon emission,
Curves drawn to show the effect of atomization power on the
submitted to a polynomial fitting function.
Determination of the Si LOD is performed
using a sample size of 30 ,L, the maximum capacity of the cup.
This involved pipetting solutions of 1000 pg mL1' of Ta or Nb into the cup,
followed by drying the solution in situ with the plasma.
cup and the analysis performed as previously described.
16 nm, except in the study involving matrix modifiers.
is employed during matrix
Silicon samples are added to the
Silicon emission is observed at
The Si line at 251.61 nm
modification studies due to interference caused by the Mg
Sensitivity is increased with the addition of modifiers.,s
In GFAAS, the elements Mg
carbide forming elements.' 15,15
In our research,
the modifier is added to the sample
cup, followed by an ashing step.
of 135 W for 15
The ashing step is performed by applying a power level
After a time of 10 s, no solution was visually observed in the cup.
Ashing times of longer than 20 s only removed the Mg modifier.
measuring the Mg 285.2 nm line.
This was observed by
Sample is then pipetted into the cup, followed by
another ashing step and finally, the atomization step.
A cleaning step is performed to
remove the matrix modifier residue. The cleaning step included the addition of 10%
HNO3, followed by igniting the plasma at 135 W. The power is then increased to 1200
in order to
remove the previous matrix modifier.
check the cleaning process. An aque'
Magnesium emission at 285.2 nm is observed to
ous solution containing 10,000 ug mL'1 Mg in
Fisher Scientific supplied the trichloroethylene.
Results and Discussion
Integration times are varied from 0.033
s to 0.33 s.
The integration time
is the time the PDA collects the signal for each scan.
Results for the signal and S/N are
presented in Fiure 4 1.
the integration time increases, the signal increases.