The applicability of the photon catalysis technique for effecting composition analysis of silicon samples was assessed. Third quarter activities were devoted to the study of impurities in silicon matrices. The evaporation process was shown to be congruent; thus, the spectral analysis of the vapor yields the composition of the bulk sample. Qualitative analysis of metal impurities in silicon was demonstrated e part per million level. Only one atomic spectral interference was noted; however, it is imperative to maintain a leak tight system due to chemical and spectral interferences caused by the presence of even minute amounts of oxygen in the active nitrogen afterglow

Galvan, L.

Heidner, R. F., III

Melzer, J.

Sutton, D. G.

English

The applicability of the photon catalysis technique for effecting composition analysis of silicon samples is assessed. In particular, the technique is to be evaluated as a detector for the impurities Al, Cr, Fe, Mn, Ti, V, Mo and Zr. During the first reporting period Al, Cr, Fe and Mn were detected with the photon catalysis method. The best fluorescence lines to monitor and determined initial sensitivities to each of these elements by atomic absorption calibration were established. In the course of these tests, vapor pressure curves for these four pure substances were mapped

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Low cost solar array project:  Composition measurements by analytical photon catalysis

The applicability of the photon catalysis technique for effecting composition analysis of silicon samples is discussed. A detector for the impurities Al, Cr, Fe, Mn, Ti, V, Mo and Zr is evaluated. During the first reporting period Al, Cr, Fe, and Mn were detected with the photon catalysis method. The best fluorescence lines to monitor and determine initial sensitivities to each of these elements by atomic absorption calibration were established. In the course of these tests vapor pressure curves for these four pure substances were also mapped. Ti and Si were detected. The best lines to monitor were catalogued and vapor pressure curves were determined. Attempts to detect vanadium were unsuccessful due to the refractory nature of this element and the limited temperature range of the evaporator

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Distribution Category UC-63
DOE/JPL-955201-78/1
LOW COST SOLAR ARRAY PROJECT:
Composition Measurements by Analytical Photon Catalysis
Second Quarterly Report
1 January 1979 - 31 March 1979
David G. Sutton, Luis Galvan, James Melzer and Raymond F. Heidner, III
The Ivan A. Getting Laboratories,
The Aerospace Corporation, E1 Segundo, California 90245
( N ASA-CP-15859E)
	 LCW CrS7 SCLAP ARPAY
	
ti79-23x13
PROJFCT: COMPCSIIION NEASUFEMENTS FY
ANALYTICAL PHCTC CATAIYSIS Cuarterly
Feport, 1 Jar. - 31 Mar. 1579 (Aerospace
	 ^^	 Jnclas
Corp., E1 Segun(?o, Calif.)
	 16 F PC A02/MF ' 1 G3/44	 25171
This work was performed for the Jet Propulsion Laboratory, California Institute of
Technology, under NASA Contract NAS7-100 for the U.S. Department of Energy.
JPL Low-Cost Silicon Solar Array Project is funded by DOE and forms part of the
Solar Photovoltaic_ Conversion Program to initiate a major effort toward the
development of low-cost Solar .Arrays.
r 9^^F s^
n O
/	 R	 ,
Technical Content Statement
This report contains information prepared by the Ivan A. Getting Laboratories of
The Aerospace Corporation under JPL subcontract. Its content is not necessarily
endorsed by the Jet Propulsion Laboratory, California Institute of Technology.
National Aeronautics and Space Administration or the U.S. Department of Energy.
3Table of Contents
Page
I. Summary	 4
II. Background
	 4
III. Progress During Reporting Period
	 5
IV. Conclusions	 8
V. Future Activity	 9
VI. References	 10
V11.	 Figures	 11
4I.	 Summary
The object of this research is to assess the applicability of the photon
catalysis technique for effecting composition analysis of silicon samples. In
particular, our technique is to be evaluated as a detector for the impurities Al, Cr,
Fe, Mn, Ti, V, Mo and Zr. During the first reporting period we detected Al, Cr, Fe
and Mn with the photon catalysis method. We established the best fluorescence
lines to monitor and determined initial sensitivities to each of these elements by
atomic absorption calibration. In the course cf these tests vapor pressure curves
for these four pure substances were also mapped.
During the second quarter we detected Ti and Si with our technique. The
best lines to monitor were catalogued and vapor pressure curves were determined.
Attempts to detect vanadium were unsuccessful due to the refractory nature of
this element and the limited temperature range of our evaporator.
I1.	 Background
Work commenced during the eek of September 11 with the assembly of our
Analytical Photon Catalysis (h1TES) device into a configuration suitable for the
study of silicon impurities. The resulting instrument and its function can best be
explained with reference to Figure 1. It consists of a vertical section of quartz
tubing 9 cm in diameter. An electrically heated metal vapor furnace is attached to
the bottom of the quartz tube, and a 60-1/sec pump, attached at the top of the
quartz tube, provides the means to evacuate the system. Argon or nitrogen
injected through the bottom of the furnace assembly entrains the flow of metal
atoms from the furnace and carries it into the quartz observation section. In this
section, the concentration of argon is much greater than the metal concentration.
A thermocouple placed in contact with the metal in the furnace allows direct
measurement of the metal atom source temperature, or an optical pyrometer may
be employed by observing the heated metal down through the observation window.
A ring-shaped quartz gas injector is inserted into the apparatus at the
junction of the furnace assembly and quartz flow tube. Through this injector,
active nitrogen is introduced into the Ar-metal atom stream. The active nitrogen,
consisting primarily of N atoms, N 2 (.A 3 2:'), and ground-state N 21 was prepared
upstream of the injector by passing N 2 gas through a 70-W microwave discharge.
The region of the quartz flow tube approximately 5 cm above the active nitrogen
injector is montored photoelectrically with an uncooled RCA IP28 phototube
attached to a one meter monochromator. Apart from neutral density filters, which
are used at higher metal concentrations to avoid high photomultiplier tube current,
no other optics are used. Hollow-cathode lamps can be placed on the opposite side
of the flow tube and at the same height as the monochromator entrance slits. They
are used as a line source for the atomic absorption measurements to calibrate the
fluorescence intensity measurements with respect to metal vapor concentration.
The Ar and N 2
 flows are adjusted to give a total pressure of 2-4 Torr. With
the microwave generator set to deliver 70 W to the N 2
 flow, a pale straw-colored
glow is observable downstream of the injector. The furnace is then set to a given
temperature, and the resulting atomic line emission intensities are measured.
iThe most intense and spectroscopically isolated lines are chosen (in ac-
cordance with Task IA of the statement of work) to be the best fluorescence lines
to monitor for the purposes of analysis. The intensity is a direct measure of the
concentration of the emitting species and can be calibrated as specified in Task 1B
at high so rce temperatures by the atomic absorption technique. If one plots the
log I vs 10 /T(K) a vapor pressure curve results as per Task IC. These three tasks
applied to Al, Cr, Fe and Mn occupied us for the first quarter. During the second
quarter we completed Tasks IABC by our examination of titanium, vanadium and
silicon.
Ill.	 Progress During Reporting Period
During the month of January we completed our testing of titanium. We
show in Figures l l and 12 the MTES spectrum and the vapor pressure curves for
this element, respectively. The titaniurn spectrum consists of groups of triplets.
The most intense individual lines occur at 365.3, 398.2, 399.0 and 399.9 mano-
meters. The two aluminum peaks that appear are due to the presence of foil that
serves as a reflector. Inadvertently it was placed too close to the heater element
and some of it was vaporized. This problem has since been eliminated by
redesigning the oven section and switching to tantalum foil. Blank experiments
done with empty crucibles now show only traces of sodium even at the highest
temperatures. The sodiurn is presurnably introduced as an impurity in the crucible.
The signal due to this element rapidly diminishes on bake-out. An example of a
blank experiment showing the spectrurn obtained with an empty crucible is given in
Figure 13.
Figure 12 summarizes our temperature dependent measurements at two
different wavelengths, 365.3 and 399.8 nm. The data at both wavelengths are fit
with straight lines calculated with a least squares fitting routine. The slopes of
both lines are so determined in this process. They agree to within 3% and 7%
with Hultgren's recommended dat:.. This excellent agreement is consistent with
the high purity quoted for our titaniurn sample and verifies the accuracy of our
calibration technique.
Attempts to calibrate Figure 12 using atomic absorption were unsuccessful.
We can estimate the lowest concentrations observed with the rate of vaporization
method outlined previously. The lowest temperature that still reVlted in a
measurable signal was IJ15 0 K. At t^is temperature titanium sublimes at a rate
of approximately 8 x 10 - grams/cm -sec. If we have identical sensitivities for Ti
and Bi then our lowest signal for titanium corresponds to a Ti concentraton of less
than 10 atoms/cm . The highest temperature our oven is capable of sustaining is
1590°K. The rate of vaporization method enab s us to ?timate our Ti co^centra-
tion at this temperature to be between 10 and 10 atoms per cm in our
observation region. These low values are consistent with the null signal we
obtained in our atomic absorption exp9riments 3 since this method is normally
insensitive to concentrations less than 10 per cm .
r	 -
6
During the month of February we attempted to detect vanadium vapor in
Our MTES apparatus. As anticipated the temperature obtainable from our oven was
insufficient to yield a signal from vanadium. We can estimate a lower limit to the
sensitivity for vanadium using the rate of evaporation technique discussed in the
first quarterly report (Dec. 1979). 5 This estimating3procedure places the lower limitof detection for vanadium at 10 atoms per cm in the gas phase. No atomic
absorption experiments were attempted, since such a low concentration is in-
compatible with detection by this classical method.
Our digital thermocouple finally arrived along with the circuit board to
provide accurate readout. Installation of this instrument will eliminate the tedious
temperature measurement routine required by use of the optical pyrometer. In
addition, no correction procedure is required, the thermocouple is inherently more
accurate and the opportunity for errors is reduced.
Since a null result was achieved with vanadium we attempted several
preliminary experiments with Si. In particular, with NBS Standard Reference
!Material #57 (SRM57). The analysis supplied by NBS for this material is reproduced
below.
SRM57
Chemical Composition (Weight Percent)
Mn P
	 S	 Si	 Cu	 Ni	 Cr	 Ti	 Al	 Zr	 Ca	 Mg	 Fe
0.034 0.008 0.005 96.8
	 0.02 0.002 0.025 0.10 	 0.67 0.025 0.73 0.01	 0.65
We can see that SRM57 is not particularly pure material. It does have the
advantage, however, of containing a large number of the impurities that concern
this investigation, and it is an NBS standard so that one may have confidence in the
supplied analysis. Since this material contains over 3% impurities, we might
expect sortie segregation and fractional distillation as the Si is melted and
vaporized. This behavior would manifest itself in vapor pressure curves that would
vary from the vapor pressure curve of the host matrix, silicon. This hypothesis will
be tested on SRM57 by measuring the vapor pressure curves of Si and as many
impurities as practical simultaneously. If all the vapor pressure curves are
identical, we have proof that the vaporization process is congruent and measure-
ment of the vapor composition will reflect the composition of the silicon sample.
7During March we analyzed the rich spectrum resulting from Si SRM57.
Shown in Figure 14 is a portion of this h1TES spectrum. In addition to the Si lines
near 250 nrn and 288 nm we can easily identify spectral lines from the following
elements:
Element
	 Wavelength of Prominent Lines
Observed from SRM57, nrn
Si 251.6, 288.2
Al 308.2, 309.2
Cr 359.4, 425.4
Mn 279.5, 403.1
Fe 239.5 M
Mg 285.2
Cu 324.7, 515.3
Na 330.2 (background impurity)
Ca 422.7, 487.8
In order to prove that the vaporization process is occuring at a rate
determined by the silicon matrix, we are constructing apparent vaporization curves
for most of these impurities, using SRM57 as the vapor source. Data is
accumulated simultaneously on the intensities of the fluorescence lines due to
impurities as well as the silicon 251.6 nm line. These data are plotted on semilog
pap?r as a function of the inverse of the source temperature. These apparent
vaporization curves are compared with the silicon curve and with the vapor
pressure curves obtained by the previous work with pure metal sources.
An example of our analysis is shown in Figure 15. The solid lines on this plot
connect data points obtained at 251.6, 422.7 and 309.2 nm and track Si, Ca, and Al,
respectively. These three curves were established simultaneous using Si SRM57 as
the, ,jurce of the vapor. For comparison the vapor pressure curves obtained earlier
for Al and Ca using the pure metal as a vapor source are plotted on the same graph
as dashed lines. One easily discerns a trend for the apparent vapor pressure curves
of the impurities (Al, Ca) to parallel the curve of the host matrix (silicon). This is
powerful evidence that the evaporation process is congruent, and thus, analysis of
tine vapor via MTES will reflect the composition of the original solid sample.
The significance of this milestone is perhaps best expressed mathematically.
If the evaporation process is congruent, the composition of the vapor will be
constant (i.e., the same as the source). Consequently, the ratio of MTES
fluorescence intensities due to an impurity, i, and silicon will be constant
1.h = C	 (1)
s
Take the log of both sides of (1) to get
7
8	
log l i - log I s = log C	 (2)
For the host Si matrix the MTES vapor pressure curve is linear in inverse
temperature.
log i s 
= T + b	 (3)
Substituting (3) into (2) gives
	
log l i
 = a + b + log C
	 (4)
T
By comparison of equations (3) and (4), we see that plots of log I . and to 1
vs. I/T would have the same slope, a. This behavior is observed in Figure 15 an5 ii
seen to be a direct consequence of congruent evaporation.
IV.	 Conclusions
Based on these results we can expand our previous table by tuo columns.
This is done below. The complete table summarizes our findings under Task IABC.
Our MTES technique appears to be very sensitive to titanium. While we can't be
precisely quantitative the sensitivity seerns comparable to Al, Cr and Mn.
element	 Al	 Cr	 Fe	 Mn	 Ti	 V	 Si
best MTES lines (nrn)	 309.2, 396.1 359.4	 344.1	 279.5	 399.8	 251.6
I'
358.1, 372.0
IG7
ancillary lines (nm)
sensitivity (cm-3)
projected
sensitivity in
Si Matrix (ppb)
308.2, 394.4 347.9, 425.4
3 x 10 5	2 x 105
30	 20
403.1	 365.3	 288.3
5 x 10 4 10 4	>105
1000	 5	 1	 >10
Our preliminary results with Si (SRM57) show that qualitative analysis is
easily accomplished with MTES. In addition, the "best lines" established with use
of the pure materials are seen to be invariant with matrix, suggesting that the
excitation process occurs independently for each species present in the vapor. If
this proves to be the case, then cooperative and matrix effects (interferences) will
be minimal.
The vapor pressure curves measured for Si, Al and Ca with Si (SRM57) as the
source suggest very strongly that our MTES measurements in the vapor phase will
be simply and directly related to the composition of the solid sample.
9V.	 Future Activity
As per the program plans we intend to investigate the NBS silicon sample
more thoroughly. In particular, we will measure additional vapor pressure curves
for impurities in SRM57. Due to the large number and high percentage of
impurities in this material it will also serve as a source for our study of
interferences, both optical and chemical. The effects of selective evaporation can
be evaluated by repeating our vapor pressure curve analysis with samples of higher
purity. We have obtained samples from SPEX Industries with measured impurities
at the ppm level. These will be evaluated in the same manner as SRM57. In
addition, we hope to begin our analysis of samples supplied by JPL near the
beginning of June. Special attention will be given during these studies to define
optimal running conditions for Al and Cu. These two elements are common to all
three samples. Thus, the beginnings of an analytical curve for these two elements
could be established by data taken under identical conditions using the three
samples (NBS, SPEX, JPL) as standards.
10
V1.	 References
1. The term Metastable Transfer Emission Spectrometry, MTES, is somewhat
more descriptive and is used synonymously with Analytical Photon
Catalysis. See Rev. Sci. Instrum. 49, 1124 (1978).
2. Ralph Hultgren, et al., "Selected Values of Thermodynamic Properties of
Metals and Alloy s;' John Wiley and Sons, Inc. (New York, 1963).
3. An. N. Nesmeyanov, "Vapor Pressures of The Elements," t, anslated and
edited by J.l. Carasso, Academic (New York, 1963).
4. C.H. Corliss and W.R. Bozman, "Experimental Transition Probabilities for
Spectral Lines of Seventy Elements," (NBS Monograph 53, 1962).
5. M. Pinta, "Detection and Determination of Trace Elements," Ann Arbor
Science Publishers, Inc. (Ann Arbor, Mich., 1962).
6. S. Dushman, "Scientific Foundations of Vacuum Technique," edited by
I.M. Lafferty, John Wiley and Sons, Inc. (New York, 1962).
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Low Cost Solar Array Project: Composition Measurements by Analytical Photon Catalysis

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