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Anal. Chem. 1992, 84, 2958-2964
Analysis of High-Purity Gallium by High-Resolution Glow Discharge Mass Spectrometry Wojciech Vieth’ and John C. Huneke Charles Evans and Associates, 301 Chesapeake Drive, Redwood City, California 94063
Methods for .lennntal analytk of hlghpurltV Ga by hlghreeokdkn g k w dbcharge mcrtr rpeotromdry are prercmted wtth apecia1 attodon to sample preparath procodurer. Evidence k presented that Ga sample8 prepared from llqukl Ga by rapld frwzkrg are free d lmpurlty sagregatknr. The methodsdescribed a k w the detwmlnatknof over 70 lmpurlty elements In Ga In a dngk analytkal m e a w m n t , wtth detectbn knlb nomlnally better than a few ppbwt and accurackr M e r than f40 %
.
INTRODUCTION The consumption of gallium for the production of 111-V semicondudor material increased rapidly over the last decade, and further increases are expected over the next decade. Depending on the application, the required purity of the gallium metal varies from 4N to 7N W E (ultrapure electronic) grade, where grade 7N corresponds to less than O.ooOOl% total impurities. High-resolution glow diecharge mass spet3”et-q (GDMS), which is rapidly replacing SSMS as an analytical tool for the direct elemental analysis of conducting solids,’+’ can also be routinely used for the qualification of high-purity gallium. This is made possible by the use of cryocooled discharge cella to cool the Ga pin and avoid melting in the GD plasma. Recent literature on SSMS and GDMS has been thoroughly reviewed by KoppenaaL’O GDMS has the advantage over SSMS in all analytical figures of merit: GDMS exhibits 1-2 orders of magnitude better detection limits, much better precision due to the excellent stability of the glow discharge ion source, and much better accuracy due to the absence of matrix effects.* The most important as well as most difficult part of Ga analysis by GDMS is sample preparation. To eliminate surface contamination from preparation and handling of a sample, plasma ‘precleaning” in the GDMS source using high sputter rate conditions is routinely applied prior to analysis. However, the low sputter rate (0.141fim/min) using the “soft”discharge conditions required to avoid melting the Ga is not sufficient for efficient sputter cleaning of the Ga surface prior to analysis. Thus the sample surface as prepared must be relativelyfree of surface contamination and must represent
* To whom correspondence should be addressed.
(1)Chu,P. K.; Huneke,J. C.;Blattner, R. J. J. Vac.Sci. Technol. 1987, A5 (3),295-301. (2) Graaserbauer, M. Mikrochim. Acta (Wien) 1987,1,291-319. (3)Jakuboweki,N.;Stuewer,D.;Vieth, W. Anal. Chem. 1987,59,18251830. (4)Jakubowski, N.; Stuewer, D.; Vieth, W. Fresenius’ 2.Anal. Chem. 1988,331,145-149. (5)Guidoboni, R.J.; Leipziger, F.D. J. Cryst. Growth 1988,89,16-20. (6)Harrison, W. W. J. Anal. A t . Spectrom. 1988,3,867-872. (7) Mykvtiuk.. M.:. Semeniuk. P.: Berman.. S. Spectrochim. Acta Reu. _ 1990,13;lr9. (8)Vieth, W.; Huneke, J. C. Spectrochim. Acta 1991,46B, 137-153. (9)Harrieon, W. W. In Inorganic Mass Spectrometry; Adams, F., Gijbels, R., Van Griken,R.,Eds.; John Wiley & Sons: New York,1988; Chapter 3,pp 85-124. (10)Koppenaal, P. W. Anal. Chem. 1992,64, 322R-327R. 0003-2700/92/0364-2958$03.00/0
bulk material, which means that the sample has to be prepared in an extremely clean environment and the elemental segregation processes often coupled with gallium crystallization have to be avoided. A special technique for Ga sample preparation for GDMS analysis by cryocooling is presented in this paper. Samples prepared from liquid Ga by cryocoolinghave a microcrystalline structure. The surface of the solid Ga sample prepared by the method described is essentially free of contamination, and for the impurity elements studied (Zn,Ge, and Sn) there is no evidence of elemental segregation. The surface regions sampled by GDMS evidently reflect the bulk composition of the Ga.
EXPERIMENTAL SECTION Inotrumentation. All measurements were carried out using the VG-9000magnetic sector glow discharge maas spectrometer (Fisom, UK)with a cryocooled discharge cell requiring a pinshaped sample (Figure 1). The sample serves as the cathode for a dc glow discharge in Ar. Atoms are sputtered from the sample surface by cathode sputtering at a rate of 0.1-1 pm/min. The sputter sampling is a nonselective process under steadystate conditions with no diffusion processes within the solid and no thermal vaporization from the surface of the solid.*I Sputtered atoms diffuse into the negative glow region of the plaema and are ionized there mostly by Penning ionization proceesas but ale0 by electron impact ionization.’* Ions passing through the cell exit aperture are accelerated into the double-focusing mass spectrometer and, after maes and energy separation, are deteded either by a Faraday cup or by a Daly electron multiplier detector. Mass resolution MIhM in the VG-9000with a 20-pm-wide entrance slit to the mass spectrometer is routinely about 5o00, which is sufficient to resolve many of the interfering ions in the mass s p e c t m from the maes peaks of the analyteions. However argides at about mass 90D and doubly-charged ions at about mass 45Dare difficult to mass resolve from analyteions. Hydride, carbide,nitride, and oxide ions, which are also veryoften difficult to maas resolve from analyte signals,are so strongly reduced by the sample and cell cryocooling system that the quality of an analysis of pure metals is relatively unaffected. GDMS Quantitation. Quantitation in GDMS for the measurement of element “x” is done using the ratio of the measured mass-separated ion currents (ZJZ,) of the element “xDand an internal standard element “s” as the analytical parameter of choice. Elemental ion currents are calculated fromthe measured isotopic ion currents by dividing the latter by the natural abundance8 of the isotopes in question. In the case of analysis of pure metals, the matrix element “m”is routinely used as an internal standard. The relative ion current (ZJZd is a linear function of the relative concentrations (cJc,) in the analyzed sample: (c,/c,) = RSF(x/m)(Z,/Z,) In the case of pure metals, c, = 1and eq 1 reduces to
(1)
c, = RSF(x/m)(Z,/l,) (2) Due to the excellent stability of the glow discharge as a source
(11)Coburn, J. W.; Harrison,W. W. Appl. Spectrosc. Rev. 1981,17, 95. (12)Vieth, W.;Huneke, J. C. Spectrochim. Acta 1990,#B, 941-949. @ 1992 A n ”
Chemlcel Society
ANALYTICAL CHEMISTRY, VOL. 64, NO. 23, DECEMBER 1, 1992 Ion Analyzer
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Table I. Comparison of RSF Values Obtained by Analysis of AI 11630 SRM (Aluminum Pechiney) Using Two Different Glow Discharge Conditions
RSF(x/Ga) element Mg
Ion Monitors Einzel Lens
Si
Ti Cr
Total Ion Monitor
Mn
Fe co Ni
Exit Slit Entrance Dalv Electrode Ion Exit Slit (Anode) Plasma Photomultiplier
cu
Faraday Cup Sample (Cathode)
\
Ion Detector Gas Inlet
Ion Source
Zn Ga Sn Pb
Th U
Flgure 1. Schematic of VG-9000 double-focusingmass spectrometer with glow discharge Ion source.
of ions, the stability of the relative ion currents (Ix/Im) is better than 5% during an analysis (1-2 h). Larger variations usually reflect inhomogeneousdistributions of measured elements. The proportionality factor RSF(x/m)used in eqs 1and 2 is the so-called "relative sensitivity factor" which relates the measured relative ion currents to the actual relative concentrations in the analyzed sample. A detailed discussion of RSFs in GDMS is presented by Vieth and Huneke.8 Accurate RSF values must be determined experimentally by the analysis of appropriate standard reference materials. Due to the nonselectivity of the sputtering process and the decouplingof the sputter atomization from the ionization processes, so-called "matrix effects" on analytical signals are not observed in GDMS.8 RSF values are matrix independent, which means that RSFs determined from the analysis of a standard comprising one matrix can be used in the analyses of other matrices. This is especially important in the case of analyses of matrices for which standards are not available, e.g. gallium. Data presented by Vieth and Huneke8 showedthat with RSF values well established by multiple analyses of numerous standards, the accuracyof elemental determination is usually better than 120% ,if standards and unknown samples are analyzedusing the sameglow dischargeconditionsand samplecell geometries. To avoid sample melting, the discharge conditions for Ga analysis (0.7 kV/1.0 mA) differ from those routinely used in our laboratory for analysis of other metals (1.0 kV/3.0 mA). However, an analysis of standard reference material (A1 11630,Aluminum Pechiney)showed that the RSFs for all certified elements are very similar under both discharge conditions (Table I). Thus it can be concluded that, by using the library RSF file established in our laboratory by multiple analysis of more than 30 standards representing different matrices, the accuracy of elemental determination in pure gallium should be only slightly worse than that obtained for other metals due to differing analytical conditions (i.e. better than 30%). Detection limits in the GDMS depend reciprocally on the magnitudeof the matrix ion current, proportionallyon RSF values for the element in question relative to the matrix element, reciprocally on the isotopic abundance of the measured isotope, and proportionally on the background signal. In the VG-9000, a Daly electron multiplier ion counting background of about 1 count/s is observed for the majority of elements. An integration time of 1-5 s/peak is routinely used for the 6-7N qualification procedure, and in the absence of interfering ions, detection limits are calculated from ion counting statistics, yielding detection limits between 0.5 and 10 ppbwt. Detection limits can be improved by additional signal integration and for some matrices can reach pptw 1e~els.l~ (13) Dunlop, J. A.; Ritala, K. E.; Gibbard, J. R.; Beauprie, R.; Pouliquen, B.; Reevera, J. H.; Huneke, J. C.; Vieth, W. J. Met. 1989, June, 18-21.
std conditions (3.0 mA/l.O kV) 0.26 f 0.01 0.31 f 0.01 0.069 f 0.003 0.37 f 0.01 0.24 f 0.01 0.16 f 0.01 0.15 f 0.01 0.22 f 0.02 1.1f 0.06 0.90 f 0.06 400 0.48 f 0.03 0.46 f 0.03 0.14 f 0.01 0.15 f 0.01
Ga analysis conditions (1.0 mA/0.7 kV) 0.32 f 0.02 0.31 f 0.02 0.075 f 0.005 0.49 f 0.03 0.31 f 0.02 0.20 f 0.01 0.17 f 0.01 0.25 f 0.01 1.1f 0.06 ~~~
~
1.21 f 0.07
d.00 0.60 f 0.04 0.68 f 0.03 0.13 f 0.02 0.16 f 0.02
Suction
Wall
Tube
Pol yeth lene
m~re\
---
I
1st Pin
---
i
2nd Pin
Flgure 2. Schematic of &.pin preparation for GDMS analysis by rapM quenching of liquid Ga in a Teflon tube.
Sample Preparation. Ga sample preparation is illustrated schematically in Figure 2. Pins suitable for GDMSanalysis were made by melting galliumin the clean polyethylenetransfer bottle and then sucking some of the material up into a clean thinwalled Teflon tube (0.d. 2.5 mm). The tube with Ga was then quenched with liquid nitrogen, with essentially instant solidification of the supercooled Ga. A Ga pin of the appropriate length (20-22 mm) was then broken from the parent rod. Two pins of each sample were prepared from the parent rod, one from the top and a second from the bottom. The analyzed area of the pin is shown in Figure 2. Data obtained from analyses of these two pins were compared to identify any problems associated with inhomogeneousdistribution of impurities within the parent rod. Possible surface contaminants were removed from the pin samples by etching the pins in cooled aqua regia (5-8 "C)for about 10 min. A microcooler has been used to assure constant and reproducible etching temperatures. The pin samples were etched immediately after solidification (less than 10-20 s) by removing the pin from Teflon tube directly into aqua regia. The etching process removes about 25% of the sample volume and develops a fine crystal structure easily observed by scanning electron microscopy (Figure 3). After chemical etching, the Ga pin was transferred immediately to cooled VLSI methanol and
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ANALYTICAL CHEMISTRY, VOL. 64, NO. 23, DECEMBER 1, 1992
were allowed to contactthe samplematerial. All tools, containers, and Teflon tubes were precleaned by etching in a HNOs-HF mixture (3:1, by volume) for more than 10 h, rinsed in deionized water and then in VLSI methanol, and dried. Although sample preparation was done in a chemical laboratory equipped only with a hood, the use of a clean room is recommended. The discharge cell and source ion optics, which are made of tantalum,were etched in aqua regia and then in a HN03-HF mixture (201),rinsed twice in deionized water and VLSI methanol using an ultrasonic bath, and dried in a vacuum oven. After the source assembly was mounted in the instrument, the inside of the discharge cell was coated with Ta by sputtering a sample of 4N pure Ta wire for 1h at 1.2 kV/6.0 mA discharge conditions. This Ta-coating procedure significantly reduces contributions from common contaminantslike C, N, 0,Na, K, Ca, etc. and has to be applied after each source and discharge cell exchange. The Ga samplewas inserted into the cryocooled discharge cell and cooled for at least 10min. After samplecooling,the discharge was started at 0.7 kV/1.0 mA discharge conditions. Data acquisition was begun almost immediately, without plasma precleaning. The matrix ion current, with an entrance slit to the mass spectrometer of 20 pm and a mass resolution M/AM = 5000, was about 0.2 nA. In the 6-7N qualification data acquisition,integrationtimes were typically 3 s/isotopeand were increased to 10 s/isotope for isotopes with natural abundance5 below 20 % . The totaltime to acquire this type of complete mass scan, which includesabout 80elements, is 25 min. An analytical sequence which included only elements detected was repeated until two consecutive analyses of the pin confirmed the result. The final result was the average of at least the last four measurements.
RESULTS AND DISCUSSION
-
81118.6616-
Flgure 3. Examples of three different crystal structures exposed on the surface of a Ga pin prepared by rapid quenching of liquid Ga and
etching in aqua regia.
rinsed in an ultrasonic bath. Finally, the sample was mounted intothe sampleholder and inserted into the cryocooled discharge cell. To avoid diffusion of impuritiestoward the gallium sample surface,as observed by Allhgre and Boudot14in gallium samples stored for a long time at room temperature, the time between sample solidification and insertion into the discharge cell was reduced to a minimum (15-20 min). Before etching, the cooled samplewas exposed to the air for only a few seconds. The longest exposureto the air (30-40 s) occurred during the mounting of the sample into the sample holder. If any impurity segregation occurred, it will only have occurred in this step, after chemical etching. Throughout the whole of the sample preparation and loading process, only precleaned tools and containers dedicated to Ga (14)Allegre, J.; Boudot, B. J. Cryst. Growth 1990,106,13s.142.
Accuracy of Trace Element Determinations in Ga. The average accuracy of elemental determination by GDMS in metal samples is better than i20% .8 The required RSFs have been established by multiple analyses of numerous metal standards and have been confirmed by frequent analyses of test samples, mostly represented by selected standards. However, the expected accuracy of elemental determinations in Ga can be worse. First, the glow discharge conditions for Ga analysis differ from those routinely used for metal analyses and for RSF determination. Even though significant variations of RSFs have not been observed between the two discharge conditions (see Table I), differences do exist and will be reflected in worse accuracy. Second, all detected impurities in pure gallium are at the sub-ppb level, much lower than those determined in certified materials. The longterm reproducibility observed in our laboratory, which was reported to range over 518% for certified standards, drops to 20-40% for ultratrace impurities (e.g. T h and U) a t ppb and lower concentration levels, due to limited counting statistics. Thus, the average accuracy expected for trace elemental determinations in Ga metal should be in the range 20-40%.
The only confiiation of accuracyfor Ga analysisby GDMS has been the analysis of two gallium samples doped with 4 ppmwt Ge. The GDMS analysis resulted in measurements of 3.4 f 0.2 and 3.6 i 0.4 ppmwt. The agreement between certified and measured values is surprisingly good, since the RSF(Ge/Ga) was obtained in a very complicated way. First, the RSF(Ge/Au) value was determined by analysis of a Ge/ Au alloy with a well-known Ge/Au ratio. Then, the RSF(Ge/Fe) value was calculated using a RSF(Au/Fe) value obtained from the analyses of Pt and Cu standards.* The RSF(Ge/Fe) obtained in this way was included as a part of the standard library RSF file. The requisite RSF(Ga/Fe) derives from the measurement of numerous Ga-doped standard Al samples. One can argue that RSF values obtained in this ”complicated”way should not be used for quantitative analysis, but the matrix independence of RSFs has been well
ANALYTICAL CHEMISTRY, VOL. 64, NO. 23, DECEMBER 1, 1992
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Table 11. Distribution of Doped Elements in Gallium Measured by GDMS (Concentration in DDmwt) ~~
sampleno. element hP bottom
1 Zn 0.063f0.016 0.057f0.007
2 Zn 0.44 f 0.02 0.48f0.02
3 Zn 0.71 f0.24 0.52 f O . l l
4 Zn 1.6f 0.3 1.2fO.l
5 Zn 2.3f 0.1 2.1 fO.5
demonstrated. This type of analytical protocol has been proven useful and gives much better results than the use of a nonstandardized technique (all RSFs equal to 1)or the use of RSF values obtained by GDMS analysis of noncertified material with secondaryvalues obtained by another technique. This is especially dangerous when the technique used for obtaining the secondary values is spark-source mass spectrometry (cf. GDMS Limitations). Analysis of Ga Samples. Six Ga samples containing adventitious Zn, two with Ge and four with Sn, were each analyzed using two pins prepared from different parts of the parent rod using procedures described above (Figure 2). Each pin was plasma sputtered in the source under normal analytical conditions for about 1.5 h and five measurements were acquired during that time. The results are presented in Table 11. The agreement between results obtained from different parts of the parent rod is very good, and no evidence for impurity segregationhas been observed to the extent that there was no increase or decrease in dopant signals. The lack of segregation is not surprising. When liquid nitrogen was poured into the polyethylene bottle containing liquid Ga at the bottom and in the Teflon tube, supercooledliquid Ga was formed. After about 2-3 s, the Ga solidified, with a soft cracking noise caused by the thermal contraction. This almost instant solidification process produced a very fine crystal structure, as shown on Figure 3. The observed crystal size is about 20 pm, and crystals are grouped in clusters or swarms with the same crystal orientation. Plasma etching for 2 h does not change the morphology of the analyzed pin (Figure 4). The crystal edges exposed on the sputtered surface are less sharp, but no preferential sputtering can be observed. Both pins of each analyzed sample were prepared at the same time. One pin was analyzed about 15 min after solidification, while the second pin was analyzed after about 2 h. The latter was stored in the interim after etching and before analysis in cooled VLSI methanol (5-8 "C). Since the analysis results from the two different pins are the same for all the pairs of analyzed samples, no diffusion of impurities toward the pin surface appears to have occurred over this period of time. Analysis of Pure Gallium. Relatively pure gallium was analyzed for selected elements using two different pins prepared from one parent rod using procedures described above (see Figure 2). The results of this analysis are presented in Table 111. Again, the agreement between results obtained from these two pins were very similar, and no impurity element segregation is indicated. The dependence of measured concentration on sputtering times for the individual analyses (a sequence of five or more acquisitions) indicates that the observed "starting" contamination originates not only from the sample surface but also from the surfaces of either the discharge cell or the sample holder or the ion optics system (Figure 5). For the first analyzed pin, signal stability was achieved after 60-80 min, while the time required to reach equilibrium for the second sample was reduced by half. This regularity was observed for each set of samples. The first sample analyzed in the cleaned and newly installed discharge cell and ion optics required the longest time to reach equilibrium, especially for elements which are well-known as common contaminants. This effect is reduced not only by chemical cleaning but by additional sputtering and coating of all new parts exposed to
6 Zn 3.2f0.1 3.5f0.6
7 Ge 0.84fO.07 0.87f0.14
8 Ge 10.5f 1.5 10.0f0.8
9 10 Sn Sn 0.17 f 0.01 3.2f0.2 0.18f0.03 2.4f0.3
11 12 Sn Sn 3.5f0.9 0.60*0.06 3.1 fO.3 0.53f0.06
Flgure 4. Examples of crystal structures on the surface of a prepared
Ga pin after 2 h exposure to plasma sputtering. The plasma sputtering has no preferential effects (compare Figure 3).
Table 111. GDMS Analysis Results of Pure Gallium (Elements Detected) (Concentration in ppbwt)
element top pin bottom pin element B 3f2 4fl Ni Na
Al Si
Fe
30f4 6fl 20f5 9fl
16f2 7f2 12f8 6f3
cu
In Te Pb
top pin
bottom pin
~
511 11f2 1.5 f 0.5 2f1 3fl
6f2 13 f 7 3f 1 4fl 2fl ~~
the GD plasma or ion beam by sputtering a tantalum sample before the analysis of the Ga samples. A second factor affecting analytical results is accidental sample tip melting, which can happen as a result of local arcing as well as a result of bad thermal contact between the sample and holder. The latter leads to catastrophic failure. In the case of sample tip melting due to brief localized heating, the tip will resolidify and the discharge will stabilize with only a smallincrease in discharge voltage (at constant current). The analytical results from a "tip-melted" sample can be very different from results obtained from an unaffected sample, indicating the segregation of impurities. A comparison of results obtained before and after a tip melting incident is shown in Table IV. Comparisonbetween SSMS and GDMS. Spark-source mass spectrometry (SSMS) has been widely used in the past
1362 ANALYTICAL CHEMISTRY, VOL. 64, NO. 23, DECEMBER 1, 1992
80
I
1
1 Nay
01
I
I
20
0
40
60
0
20
40
Analysis Time in Minutes Flgure 1. oependence of the observed concentratbns for o elements in Ga on analysis tlme.
Table IV. GDMS Analysir Results of a Pure Gallium s8lllDle (CO"Itr&On h DDbWt) ~~
element
B Al Si Fe Ni
cu
before tip melting
after tip meltmg
40 10 69
40 7 70
Cd
3
2
5 9
7 15
Te Pb
element
In
Sn
before tip meltmg
