Anal. Chem. 1988, 58, 1734-1738
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Analysis of Trimethylgallium with Inductively Coupled Plasma Spectrometry Istviin Bertenyi and Ramon M. Barnes* Department of Chemistry, University of Massachusetts, GRC Towers, Amherst, Massachusetts 01003-0035
Two methods for the analyds of trlmethylgaHlum (TMG) are described. Since TMG Is pyrophoric and volatile and the nature of Its impwtly spedes b not known, separate methods were employed for volatle and nonvolatlle Impurltles. The nonvolatlle hnpuritles (AI, Cu, Fe, Mg) were determlned by lnductlvely coupled plasma atomlc emlsslon spectrometry (ICP-AES) In an aqueous sdutbn of decomposed TMG wlth c o n v ~ m b u w z a u o n .ThevdatlleknpwltysllconhTMG also was determined by ICP-AES but wlth exponentlal dllutlon. A known quantity of TMG was placed In an exponentlal dllutbn flask, and argon swept the vapor out of the flask Into the plasma. Lhnlts of detectlon In 1 g of TMG were 2 pg of AI, 0.6 pg of Fe, 0.6 pg of Cu, and 0.08 pg of Mg. The SI detectlon llmlt was 0.6 pg/g. The analysls preclslon for practlcal samples was 10-20 %.
Metal alkyls of groups 11, 111, and IV (groups 12, 13, and 14 in 1985 notation) are used in combination with hydrides and alkyls of groups V and VI (groups 15 and 16 in 1985 notation) for the formation of semiconductor materials and alloys by means of chemical vapor deposition. The purity levels of these highly reactive organometallic compounds is of primary importance, because contaminating elements in the microgram-per-gram range may alter completely the properties of the semiconductor materials formed (1-3). The extreme reactivity of these organometallic compounds, however, makes analysis of trace impurities difficult. For example, trimethylgallium (TMG, bp 55.7 "C, vapor pressure 65.4 mmHg at 0 "C) is typical of these compounds in that it is a pyrophoric liquid that reacts instantly with air and moisture. Thus, the analysis of TMG must be performed in an inert atmosphere, or the TMG must be decomposed prior to analysis. A typical oxidation decomposition method ( 4 ) results in Ga203,which is analyzed by dc arc spectrography. These decomposition methods at best are semiquantitative and incomplete because the heat of oxide formation results in the loss of most of the volatile oxides. Materials that oxidize slowly also can be vaporized prior to conversion to oxide. We first attempted the determination of TMG impurities by direct analysis of the organometalIic dissolved in a suitable organic solvent such as xylene, followed by nebulization and analysis by inductively coupled plasma atomic emission spectrometry (ICP-AES). This approach, however, did not prove appropriate because uncertain volatilization of the organometallics occurred during nebulization (5, 6). In the present article, two methods are described for TMG analysis. One requires controlled reaction of TMG to form an aqueous solution, which is then analyzed by ICP-AES with conventional nebulization. The second technique introduces a known volume of TMG vapor to the ICP by exponential dilution. EXPERIMENTAL SECTION Apparatus. Experimental facilities were described previously (3,and the operating conditions and analysis wavelengths are indicated in Table I. 0003-2700/86/0358-1734$01.50/0
Table I. ICP-AES Operating Conditions for Determination of the Impurities of TMG" carrier
wave-
length, element Ga I A1 I Cu I Mg I1
Fe I Si I
power,
flow rate,
nm
method
kW
L/min
294.364 396.152 324.754 279.553 259.940 251.611
solution solution solution solution solution exponential dilution
0.5
0.450 0.450 0.450 0.450 0.450
0.5
0.5 0.5 0.5 0.5
0.600
Obervation height is 16 mm above induction coil; argon flows outer 15 L/min and intermediate 0.3 L/min; slit widths 50 pm; slit heieht 5 mm. Sampling. Since the TMG is pyrophoric, all sampling had to be carried out in a glovebox in flowing, dry argon or nitrogen. Solution Method. The commercially available TMG is packaged in a stainless-steel storage cylinder (Whitey cylinder with male NFT connectors,Alfa Products, Morton Thiokol). The liquid TMG transferred into an aqueous phase required specially designed glassware shown in Figure 1 consisting of a transfer adapter, an addition funnel, and a reaction flask. All glassware was purged thoroughly with dry nitrogen before transferring the TMG. To control the addition of TMG into the reaction flask, a glass-to-stainless-steel adapter was fabricated. The adapter connected the stainless-steelcontainer to the addition funnel and was purged with dry argon or nitrogen. When the storage container values were opened, about 1-1.5 mL of TMG sample from the stainleas-steelcylinder was transferred into the addition funnel with a slight positive pressure that was applied from a nitrogen tank. After transfer, the two cylinder valves were closed and the adapter was removed. A ground glass (14/35) stopper having a small opening to the glovebox atmosphere replaced the adapter in the addition funnel. The adapter must be purged and dried with nitrogen before removing it from the glovebox so as to prevent ignition of residual TMG on contact with air. Thirty milliliters of 0.5 M HC1 was added to the 100-mL reaction flask. Liquid nitrogen or acetone-dry ice was placed in a beaker under the two-neck flask to freeze the dilute acid in the flask. The open neck of the reaction flask was sealed with a ground glass (24/40) stopper. The TMG was added dropwise from the funnel into the flask, and after each addition the stopper was loosened or removed during the localized reaction of the TMG with the frozen acid to release the internal pressure. The reaction was allowed to subside after each drop was added before the next addition. This required 2-3 min/drop. After the entire quantity of TMG was added to the 0.5 M HC1, which took about 20-30 min, the frozen solution in the reaction flask was allowed to melt (about 30 min), and the reaction flask could be removed from the glovebox. The clear solution was transferred into a 1WmL beaker or volumetric flask. The reaction flask was rinsed 3 times with 10 mL of 1.5 M HC1, and the washings were combined so that the final solution was about 0.6-0.7 M HCl. The solution was diluted to volume with distilled water. To check whether the gallium was converted completely into a nonvolatile form, a 10.0-mL aliquot was taken from the reaction solution, evaporated to dryness in a 100-mLbeaker, and treated several times with a few drops of concentrated HNOBand 30% H202. The residue was dissolved with 0.5 M HCl and made up to 10 mL. The gallium concentration of the solution before and 0 1986 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 58, NO. 8, JULY 1986
Reaction flask
Flgure 1. Nitrogen-purged glass device for TMG sampling. Addition flask is 30 mm long and 30 mm 0.d. with 14/35 glass joints. The transfer connection is 22 mm 0.d. and is flushed with argon or nitrogen.
Modified serum bottle
Flgure 2. Glass container fabricated from a serum bottle with an ACsealed septum to which a 14/35 glass joht and extention have been added for holding the TMG for direct sampling. Total length Is 55 mm.
after the treatment and the carbon emission signal at the C I 193.090-nm line were compared. The aqueous solution obtained after the decomposition of TMG was analyzed by conventional ICP-AES techniques using inorganic standard solutions. Stock solutions of Al, Fe, Cu, and Mg were prepared by dissolving reagent grade salts. The quantity of TMG actually present in the aqueous phase was established by measuring the gallium concentration in the diluted aqueous TMG solution. The impurities in the TMG solution were generally determined from single-element aqueous standard solutions. The aluminum was measured with an off-line background correction method, because the high gallium concentration decreased the background around this aluminum line. Exponential Dilution Method. In order to sample the TMG liquid with a syringe (5.0-pL syringe, Hamilton 800 series with removable needle), 0.2-0.3 mL of TMG was transferred in the nitrogen-purged glovebox by means of the glass-stainless-steel adapter into a specially shaped glass container (Figure 2). A 14/35 ground glass joint and a narrow TMG reservoir were added locally to a commercial serum container with a 20-mm-0.d. aluminumsealed septum (Wheaton). Once the TMG was transferred and the container sealed with a ground glass stopper, the flask could be removed from the glovebox for storage or ICP-AES analysis. With this arrangement, aliquots of TMG could be sampled outside the glove box with a syringe as easily as any non-air-sensitive material. By insertion of the syringe needle through the septum on the top of the glass container into the TMG in the special sampling resevoir, a 3-pL aliquot was withdrawn into the syringe. The plunger was withdrawn further so that a small volume of nitrogen filled the needle. The syringe needle was flushed with argon during transfer to the exponential dilution flask.
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The analysis of TMG vapor was carried out in the vapor phase with a 270-mL glass exponential dilution flask (65 mm 0.d. and 80 mm high). The liquid sample was injected through a 21mm-diameter, rubber-lined septum located on the top of the sealed chamber. The sample vapor was mixed by a Teflon-coated magnetic stir bar with argon in the chamber, and the flask was heated on a stirring hot plate to about 40-50 OC. Two three-way valves at the entrance and exit of the exponential dilution flask allowed the flask to be isolated from the argon carrier gas while argon could flow continuously to the ICP, bypassing the flask during sample evaporation and mixing. Background ICP signal levels could be measured in this bypass position. Silicon standard solutions for the direct analysis of TMG were prepared in hexane (Fisher), because the solvent was easily vaporized and transferred quantitatively with a syringe. Tetramethylsilane (Aldrich T2400-7 NMR Grade, Me4%, bp 23 "C) and tetravinylsilane (Alfa 87668, TVS, bp 130 "C) were dissolved in redistilled, dried hexane to give stock solutions. The final concentration of the standard was to be tens of micrograms per milliliter, so the dilution of Me4%was carried out in two steps. About 40 mL of hexane was weighed in a 50-mL volumetric flask with an analytical balance and cooled to -50 to -60 OC by using a dry ice-acetone mixture. The Me4Si was added, and the flask and the solution were allowed to warm to ambient temperature, weighed again, and diluted to volume with hexane at 20 "C. This A solution contained 1-2 mg of Si/mL. For the second dilution step, about 40 mL of hexane was dispensed in another 50-mL volumetric flask cooled with dry ice-acetone. Some A solution was added; the mixture was allowed to warm to 20 "C; the flask and solution were weighed; and the solution made up to volume at 20 "C. Two different sets of silicon standards were prepared. One set was used for the determination of silicon in TMG in this experiment, and the second set was used to verify the accuracy of using Me4Si and TVS as standards. For the first set three standards containing 10,25, and 40 pg of Si/mL were prepared with TVS and Me4% With this set only one calibration function was prepared for the determination of Si. The second set of Si standards was prepared from Me4& and TVS standards at 10, 20,40, and 80 pg of Si/mL. Separate calibration functions were prepared for the Me4& and TVS standards. Since the silicon was determined in the TMG vapor, an optimum carrier gas flow rate for purging the vapor into the plasma was established. To obtain a comparatively stable organic vapor flow and concentration, a chamber containing the organic solvent was thermostated with a water-ice mixture, and an argon flow carried the vapor into the ICP. The carrier gas flow rate was varied from 0.25 to 0.80 L/min in 0.05 L/min increments. First the vapor of hexane was introduced into the plasma; then the vapor of a hexane solution that contained approximately 5 pg/mL of Me4% was sampled. The optimum carrier gas flow rate was obtained at the maximum signal-to-background ratio for a constant observation height in the ICP, and this flow was used for subsequent experiments. The Si I 251.611-nm line was measured. The high-concentration organogallium vapor was expected to cause spectral background changa during the measurement. The effects of TMG, hexane, and metallic gallium on the background were inveetigated by recording the background spectrum near each analyte wavelength (Table I). A 2% aqueous gallium solution was prepared from metallic gallium (Fisher) and nebulized into the plasma. For data acquisition the 3-pL TMG sample, transferred with a syringe into the exponential dilution flask, was allowed to evaporate completely. During an initial 10-15-s mixing period while the argon flowed through the bypass, the plasma background was recorded for 2-3 s. The computer data collection stored 25 points/s. Upon switching the bypass valves, the argon purged the vapor out of the flask into the plasma. The data were acquired for a total of 50 s and stored on a disk (DEC RLO1). The total number of data points in an experiment was 1250. When the exponential dilution flask was used, the analyte emission signal should be an exponential function of time (8-11). The concentration is given as C, = C, exp(-Vt/V) where C , = the concentration at time t , Co = the initial concen-
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ANALYTICAL CHEMISTRY, VOL. 58, NO. 8, JULY 1986
3 00
~
11
Ga I 250.02, 250.07 nm
0. 60
0.00 0
4-
20
40
60
TIME
80
1 00
Flgure 3. Exponential signal decay. Peak at 5 s corresponds to value
switching, and background is measured prior to switching. tration at t = 0, U = argon flow rate (mL/s), V = exponential dilution flask volume (mL), and t = time (s). Thus log C, is a linear function of time; its slope is -U/2.303V; and the intersection is log C,,. With a computer program developed to evaluate the data, the starting point of the dilution was located as the maximum signal (Figure 3), corresponding to t = 0. The mean of the plasma background measured for 2 s before the sample was introduced from the exponential dilution flask was subtracted from the measured analyte signal to obtain the net signal of the exponential decay. The exponential decay curve was fitted by a linear least-squares regression to the semilogarithmic plot of net signal against time. Solving the resulting linear equation at zero time gave the initial signal, which was directly proportional to the initial concentration of the silicon in the sample. The program can be extended to calculate the concentration of silicon in a sample from its exponential decay compared to a single standard (8-11).
RESULTS AND DISCUSSION Although the concentration ranges of nonvolatile metals in trimethylgallium can be estimated from the dc arc emission spectrographic analysis, volatile impurities may be lost in the sample preparation and may go undetected. Generally the forms of the volatile impurities in TMG are unknown. Matching the standards used for calibration with the impurities in TMG is impossible even when techniques are developed to ensure that the volatile compounds are retained for analysis. Dissolving TMG in an organic solvent followed by ICP-AES analysis of the solution is limited by the inability to match standard compounds in calibration standards with the metal compounds in the original TMG. Since TMG is very volatile, some of its impurities, especially silicon, may also be volatile. T o dilute the TMG in an organic solvent and to estimate the concentration of the impurities by using an organic compound in the same solvent as standard is not appropriate, because of the volatility differences of the organometallic compounds. For example, the calibration curves of the three different organosilicon compounds dissolved in dry m-xylene were obtained by using conventional nebulization with ICP-AES. The curves exhibited different slopes: tetramethylsilane, 80 (nA/pg of Si)/mL; tetravinylsilane, 3 (nA/pg of Si)/mL; and diphenylsilanediol, 0.07 (nA/pg of Si)/mL. Thus, the higher the vapor pressure of the organic silicon compound the greater the slope of its calibration curve. This change in sensitivity reflects siliconenriched vapors reaching the discharge compared to the bulk xylene solution. Other elements (e.g., Mg) in organometallic compounds exhibited similar differences with compound volatility. For this reason the final analysis of the TMG was conducted in two parts: the nonvolatile impurities were determined after the TMG was hydrolyzed, and inorganic compounds in aqueous medium were used as standards. The
0
I
2454
U
n 2510
d
2526
2542
25%
257
Wavelength, nm Flgure 4. Emission spectrum of TMG vapor and argon plasma background ranging from 249 to 257 nm. A residual Ga I 250.019-nm and 250.07 1-nm signal is apparent in the background spectrum. volatile silicon was determined in the TMG vapor. The TMG vapor was introduced into the plasma, and the vapor of volatile organosilicon compounds in hexane were used for standards. Solution Analysis. Nonvolatile metal impurities were best determined by ICP-AES analysis of a stable aqueous solution formed by the hydrolysis of TMG. A compound can be considered nonvolatile when the ICP-AES signal of its solution gives the same value as the solution collected from the drain of the spray chamber (2). After the reaction of TMG with dilute HC1, the complete conversion of the volatile gallium compound and its impurities to a nonvolatile form in the aqueous solution was evaluated so that reliable conventional sample nebulization could be used. From among the impurities determined, silicon was the first one detected in the TMG vapor (Figure 4). In subsequent studies zinc was detected, and its determination is reported separately (12). This observation indicates that either the vapor pressures of the other impurities are much lower than that of the TMG a t the sampling temperature, so their concentration in the vapor is below their detection limit, or their concentration in the liquid phase is originally undetectably low. For example, magnesium in ICP-AES is more sensitive than silicon, but it could not be detected in the vapor. The aqueous solution containing reacted TMG was examined for aluminum, copper, iron, and magnesium. The net analyte signals from the original solution and from the solution collected from the spray chamber drain were the same indicating that no selective volatilization for these elements occurred during nebulization. Complete hydrolysis of TMG was evaluated by measuring the gallium concentration and carbon signal. The Ga concentration was 14.0 mg/mL i 1.0% in the TMG solution before treatment with nitric acid-hydrogen peroxide and 14.2 mg/mL f 1.0% after treatment. The blank was e60 ng of Ga/mL. Before treatment the C I 193.090-nm net emission signal was 124 nA but only 7.3 nA after treatment. The C signal blank was 3.4 nA. The reaction of TMG was complete, and no volatile gallium compound remained in the solution. After the TMG hydrolysis the solution was saturated with gas products of the organic decomposition, which accounts for the carbon emission signal. After evaporation to dryness and treatment with HN03 and HzOz,the carbon signal in the ICP decreased to approximately the blank level. The aqueous solution resulting from the reaction of TMG in HCl was analyzed to determine the amount of TMG actually decomposed in the aqueous phase. Subsequently, impurity concentrations were reported in TMG based upon the gallium content of the solution. Although some TMG could
ANALYTICAL CHEMISTRY, VOL. 58, NO. 8, JULY 1986
Table 11. Detection Limits and Background Equivalent Concentrations
detection limits in aqueous in TMG, element soln, ng/mL pg/g A1 cu Fe Mg
32 9 9 0.8
LQD," rg/g
2
0.6 0.6 0.08
BEC," pg/mL
10
1.1
3 3
0.25 0.36
0.4
0.06
LQD is the concentration in TMG corresponding to 5 times the detection limit. bBEC is the concentration equivalent of the background. Table 111. Concentration Range of Impurities Determined in TMG
element A1 Cu Fe Mg Si
method
concn range
solution solution solution solution exponential dilution
lo% indicating that the sampling can be the major source of imprecision.
Analysis of Trimethylgallium by Exponential Dilution. The analysis of TMG by exponential dilution is based upon the introduction of a known volume of TMG into a closed exponential dilution flask. Evaporation of all of the sample occurs in the heated flask, and the vapor is transported into the plasma by argon carrier gas. Since the concentration of TMG changes exponentially in the flask, so does the Si I
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251.611-nm signal during purging. A suitable argon carrier gas flow rate was determined based upon the Si signal-to-background ratio. To obtain the background level, hexane vapor was introduced into the plasma, and the signal was detected from MelSi in hexane. The maximum signal-to-backgroundratio was obtained at 0.6 L/min. The signal-to-background ratio decreased at a gas flow rate of >0.7 L/min resulting from not only the reduction of silicon signal but also the elevation of the background. The quantity of TMG transferred into the exponential dilution flask is limited. A high organic vapor concentration would change the excitation condition in the plasma resulting in signal depression (3). Also, a t higher organic vapor concentrations the wing spectral overlap that results from the broadening of the C I 247.856-nm line increases. On the other hand, if the organic vapor concentration is low, no background elevation is to be expected at the silicon line. Since a sensitive gallium line (Ga I 250.019 and 250.071 nm) is close to the Si 251.611-nm line (Figure 4), a pronounced spectral interference was expected with TMG introduction. The background at the Si line also was elevated for a 2% aqueous gallium solution. Other Si lines were tested, but the Si 1288.158-nm, Si 1221.667-nm, and Si 1212.412-nm lines suffered from pronounced spectral interferences resulting from the high gallium concentration. In summary, both the organic and the metallic constituents of TMG at high concentration can increase the plasma background at the Si 1251.611-nm line. Considering the volume of the dilution chamber, a 3-pL sample of TMG was reasonable to carry out the analysis. No background increase from the organic vapor occurred, and the reproducibility of the sample transfer into the flask with a conventional syringe was 4.0%. Since the gallium background interference could not be eliminated, the silicon concentration determined at the Si 251.611-nm line was corrected for the gallium background measured near the Si line and expressed in terms of silicon concentration. Data Acquisition and Evaluation. The silicon emission signal as a function of time during an exponential decay is shown in Figure 3. The vapor concentration in the exponential flask for the condition used at 137 s after switching from bypass to purge decreased to 1% of its original. Therefore, a new experiment could be started every 3 min. Considering the flow rate of the diluent (0.60 L/min) and the volume of the dilution flask (270 mL) used for the experiments, the coincidence of the theoretical value of the slope of the semilogarithmiccurve (-4.56 X 104/s taking into account the data aquisition time constant of the computer) and the experimental value (-4.48 X 104/s, the average of eight replicates with a 4.0% relative standard deviation) demonstrated that the measurement corresponded to theory. The closeness of the fit was characterized by the relative percent error. The optimum fitting range was taken as the shortest decay time beyond which the relative percent error did not improve. The relative percent error is affected by the fitting region time (Figure 5). Finally, 30 s was found to be optimum; during that time sufficient decay took place to accomplish the curve fit. The signal proportional to the initial concentration was determined at zero time. For the results reported, a single linear calibration function comprising both Me4Siand TVS standards was prepared and used for the silicon determination reported, but in a subsequent experiment, separate calibration functions for Me4Si and TVS were compared at four concentration levels (10-80 Mg/mL). The overall precision of the exponential dilution method for the silicon determination was established. The initial signals of three organosilicon (two Me4Siand one TVS) standards were determined 3 times each, and a linear cali-
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ANALYTICAL CHEMISTRY, VOL. 58, NO. 8, JULY 1986
1
i-
-48 8
c a l c u l a t i o n period
16
25
33
42
58
TIME ( s e c l Figure 5. Effectof curve fitting time on the relative percent error, which is determined as the (fitted - experimentai)/fitteddata times loo%, of experimental signal decay.
bration function was fitted to the nine points. The TMG sample was measured 3 times at the Si I 251.611-nm line (e.g., 16 nA with 6.4% relative standard deviation gave 14.0 pg of Si/mL). The standard deviation of the measurement was calculated from the 95% confidence band of the calibration function. The same sample was measured 3 times at a wavelength removed from the silicon line (e.g., 3.7 nA with 3.2% relative standard deviation), and the background was expressed as the silicon concentration (e.g., 4.4 pg of Si/mL) along with its standard deviation (e.g., 0.99 pg/mL). The final silicon concentration was the difference between the values on and off the silicon wavelength (i.e., 9.6 pg of Si/mL). Their standard deviation was calculated from the variances of the on-line and off-line measurements (e.g., 1.3 pg/mL). In the 10 pg/mL Si concentration range, the relative standard deviation of the procedure was 13%. The reproducibility of multiple injections (i.e., 4-7) for each standard ranged from 13% relative standard deviation for the lowest concentration to 1% for the highest concentration. In the test of calibration accuracy with two significantly different organosilicon compounds, the TVS calibration function slope was 13% higher than the Me,Si function, which at the 99% confidence level was statistically significant. The absolute error introduced by utilizing one of the two calibration functions for the determination of the second silicon compound standard was approximately 4 pg/mL, which corresponded to a relative error of 4% and 40% at the high (80 pg/mL) and low (10 pg/mL) concentrations, respectively. The detection limit for the silicon determination in TMG was calculated. The background value measured near the silicon line was 5.68 nA f 3.2%. The net Si signal value measured on the line was 12.26 nA f 6.4%. The Si concentration of this sample was 10 pg/mL, and the detection limit of Si in TMG is equal to (0.032)(3)(10)(5.68)/(12.26) or 0.44
pg/mL. Considering the density of TMG (1.1 mL/g), the detection limit is 0.58 pg/g. A number of practical TMG samples from different test lots and cylinders were analyzed. The silicon concentration range and the mean and relative standard deviation for a typical sample are summarized in Table 111. Silicon concentrations obtained from the solution analysis method after TMG hydrolysis were lower than those obtained by the exponential dilution method. These low silicon values are believed to be incorrect, because complete decomposition of Mel% was not possible with the conditions used for TMG and recovery of organosilicon standards in the solution analysis method was not quantitative. Therefore, the determination of silicon by only exponential dilution can be recommended. The procedures described appear promising for the analysis of TMG in the hands of a skilled analyst. The analysis precision is limited to
