Plasma spectroscopic analysis of organometallic compounds

A method of analyzing a volatile, air or moisture sensitive or pyrophoric, liquid, organometallic compound for an impurity comprising inserting a sample of the compound into an exponential dilution flask, allowing substantially the entire sample to vaporize, and analyzing the vapor by plasma spectroscopy; or decomposing the sample by dropwise addition into frozen aqueous acid, diluting the decomposed sample with water, and analyzing the diluted, decomposed sample by plasma spectroscopy.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to analysis of organometallic compounds, and 
more particularly, analysis of organometallic compounds which are air or 
moisture sensitive, or pyrophoric. 
2. Description of the Prior Art 
Metal alkyls of Groups II, III and IV are used in combination with metal 
hydrides and alkyls of Groups V and VI 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 .mu.g/g range may 
completely alter the properties of the semiconductor materials formed. 
However, the extreme reactivity of these organometallic compounds makes 
analysis of trace impurities very difficult. For example, trimethylgallium 
is typical in that it is a liquid which reacts pyrophorically with air and 
moisture. Thus, analysis of such a compound must be performed under an 
inert atmosphere or the compound must be decomposed prior to analysis. 
Both of these methods have been used in the prior art with limited 
success. 
Decomposition methods are typically directed to forming oxides of the 
compound and any impurities. One procedure for decomposing 
trimethylgallium (TMG) is to add a small volume of TMG to several times 
that volume of hexane, and then decompose the TMG behind a blast shield in 
a fume hood. Distilled deionized water is added dropwise until about twice 
as much water by volume has been added as the original volume of TMG. A 
heat gun is used to evaporate the hexane. When the hexane vapor is 
replaced by water vapor, the heat gun is replaced by a torch and the 
sample is heated at a temperature of about 300-400.degree. C. until a free 
flowing oxide is generated. This oxide is then analyzed by various well 
known techniques, such as direct current arc emission spectroscopy. 
An alternative decomposition procedure is to simply ignite a small aliquot 
of TMG behind a blast shield in a fume hood. After each aliquot has ceased 
burning, an additional aliquot is treated similarly until a sufficient 
sample of oxide is obtained for analysis. 
Both of the above decomposition methods suffer from the limitation that the 
heat of oxide formation results in the loss of most of any volatile oxides 
which are generated, as well as the possibility that materials which 
oxidize slowly will be vaporized prior to conversion to oxide. These 
limitations are particularly problematical when it is considered that the 
more volatile impurities are more likely to be incorporated into the 
product during chemical vapor deposition. 
An alternative method has attempted determination of impurities by direct 
analysis of the organometallic by dissolution in a suitable organic 
solvent (e.g., methyl isobutyl ketone, xylenes, methanol/ethanol, or 
toluene), followed by nebulization and analysis by inductively coupled 
plasma-atomic emission spectroscopy (ICP-AES). 
A nebulizer is used to mix the sample to be analyzed with a suitable gas, 
e.g., argon. As the sample is ejected from the outlet of the nebulizer, 
discrete droplets are obtained which continue within the gas stream into 
the ICP unit. Only about 5-10% of the liquid which enters the nebulizer 
forms individual droplets of a size that they are carried into the ICP 
unit. It has been assumed that the remainder of the liquid simply falls 
out of the stream entering the ICP unit where it passes into a drain and 
is collected. 
This latter method suffers from the disadvantage that it must be assumed 
that the droplets entering the ICP unit are representative of the sample 
being tested, and this is not the case, especially when organic solvents 
are being used for the analysis of TMG. It must also be assumed that none 
of the solvent or analyte vapors enter the ICP except as droplets. 
Accordingly, a need existed for a method of analyzing air or moisture 
sensitive or pyrophoric organometallic compounds without the concomitant 
disadvantages of the prior art. 
SUMMARY OF THE INVENTION 
The present invention is a method of analyzing a volatile, air or moisture 
sensitive or pyrophoric, liquid, organometallic compound for impurities. 
In one embodiment, the present invention comprises inserting a sample of 
the compound into an exponential dilution flask, allowing substantially 
the entire sample to vaporize, and analyzing the vapor by plasma 
spectroscopy. This embodiment is particularly useful for determining 
volatile impurities 
In another embodiment, the sample of the organometallic compound is 
decomposed by dropwise addition into frozen aqueous acid, diluted with 
water, and then analyzed by plasma spectroscopy. This embodiment is 
especially useful for mesuring nonvolatile impurities.

DETAILED DESCRIPTION OF THE INVENTION 
In one embodiment of the present invention, a sample of the organometallic 
compound to be analyzed is inserted into an exponential dilution flask 
where substantially it is allowed to vaprize completely before being 
analyzed by plasma spectroscopy. 
As discussed above, in the prior art, a nebulizer was use to produce 
droplets for introduction of the sample into the ICP unit. That method is 
based upon the assumption that the small proportion of droplets which 
enter the ICP unit are representative of the liquid sample entering the 
nebulizer. It has now been discovered that that assumption is not true for 
at least certain impurities which are found in organometallic compounds. 
In particular, certain organosilicon compounds of high volatility appear to 
be vaporized preferentially during nebulization. This vapor is then swept 
along with the droplets by the carier gas to produce an erroneously high 
silicon impurity reading in the resulting analysis. However, this problem 
has not been previously recognized. 
The following comparative experiments illustrate the effect of volatility 
on the amount of impurity measured. In each case a standard solution of a 
silicon compound is prepared in xylene. Other suitable solvents include 
hexane, toluene, and methyl isobutyl ketone. Three compounds are chosen as 
follows: 
Comparative Sample A: Tetravinylsilane (TVS) (liquid; f.w. 136; b.p. 
130.degree. C.) 
Comparative Sample B: Tetramethylsilane (TMS) (liquid; f.w. 88; b.p. 
23.degree. C.) 
Comprative Sample C: Diphenylsilanediol (DPS) (solid; f.w. 216; m.p. 
130-150.degree. C.) 
Comprative Samples A-C are diluted in xylene to 0.5 p.p.m. (parts per 
million), 1.0 p.p.m. and 2.0 p.p.m. of silicon. Two replicates with 
Samples A and B and one replicate with Sample C are performed. Testing is 
performed on a commercially available ICP-AES unit (PlasmaTherm Generator, 
Model HFS-5000D; Minuteman Monochrometer, Model 310-SMP, 1200 l/mm 
grating) with the following signals being measured: 
______________________________________ 
Measured Signal (n Amperes) 
Concentration 
TVS TMS 
in terms of Si 
1 2 1 2 DPS 
______________________________________ 
0.5 ppm 0.14 0.14 4.7 4.5 0.032 
1.0 ppm 0.29 0.31 7.8 7.8 0.068 
2.0 ppm 0.58 0.63 16.6 16.5 0.14 
SLOPE (nA/ppm) 
0.29 0.32 8.1 8.1 0.073 
______________________________________ 
The above is a standard method for obtaining a calibration curve. However, 
instead of obtaining a single curve of a given slope, each compound 
provides a different slope, and the most volatile compound (TMS) gives the 
highest slope, while the least, essentially nonvolatile, compound (DPS) 
gives the lowest slope. The slope is a direct measure of the determination 
sensitivity. 
Further evidence for the effect of volatility on the measurement of 
impurities is seen by a comparison of the signal from nebulized original 
sample versus nebulized sample collected from the drain of the original 
sample which is then used as the pseudo-sample in a second test. The 
results are as follows: 
______________________________________ 
Silicon Signal (nAmp) 
Solution original sample 
pseudo-sample 
______________________________________ 
Xylene 2.23 2.20 
2 ppm Si (DPS) 
2.65 2.68 
2 ppm Si (TVS) 
8.35 6.0 
2 ppm Si (TMS) 
101 16.5 
______________________________________ 
The signals from the original solution and the drain solution acting as a 
pseudo-sample are significantly different except for the 
diphenylsilanediol, again indicating that the concentration of silicon in 
the drain solution is lower than in the original solution and that the 
volatility of the silicon compound is affecting the original sample from 
the nebulizer. 
To overcome this problem, the present invention employs an exponential 
dilution flask. Such flasks are known to those skilled in the art for 
purposes other than impurity determination in air- and moisture-sensitive 
and pyrophoric compounds, and any such flask can be used in practicing the 
method of the present invention. For example, a suitable flask is 
described in Inman et al, "Calibration Curve Preparation of Analytes in 
Liquid Soluion by Means of an Exponential Dilution Flask", Applied 
Specroscopy, Vol. 36, No. 2, pages 99-102 (1982), as a means of avoiing 
serial dilution for the preparation of standards. Anoter description of 
the design and use of exponential dilution flasks is Ritter et al, 
"Exponential Dilution as a Calibration Technique," Analytical Chemistry, 
Vol. 48, No. 3, pages 612-619 (1976). 
However, due to the air and moisture sensitivity or pyrophoric nature of 
the compounds to be analyzed by the present method, it is preferred to use 
an exponential dilution flask such as that in FIG. 1. Exponential dilution 
flask 10 contains a magnetic stirring bar 12 for ensuring a homogeneous 
atmosphere inside flask 10 after a liquid sample has been injected by a 
syringe through septum 14. The carrier gas, which can be any inert gas but 
is preferably argon, enters through stopcock 16 and exits through stopcock 
18 which is in the line leading to a plasma, such as an inductively 
coupled plasma. Stopcocks 16 and 18 are initially opened to purge flask 
10, and then closed for addition of the sample through septum 14. A bypass 
(not shown) is arranged such that gas flows from stopcock 16 to stopcock 
18, i.e. around flask 10, to provide a reference signal and to allow the 
sample in flask 10 to evaporate before sampling is performed. All of the 
materials coming into contact with the sample must be inert to the sample. 
Flask 10 is preferably made of glass and stopcocks 16 and 18 are 
preferably teflon. Flask 10 and the various other connections can also be 
made of teflon if desired. 
By allowing the sample substantially to vaporize completely prior to plasma 
treatment, it is assured that the sample is more closely representative of 
the vapor being used during chemical vapor deposition. Even if a small 
amount of impurity does not vaporize in the dilution flask, it is unlikely 
to affect the production of materials by chemical vapor deposition since 
its low vapor pressure would also probably result in little vaporization 
during deposition. To ensure that as much of the sample vaporizes as 
possible, the magnetic stirrer (not shown) used to spin stirring bar 12 is 
equipped with a heater. In the case of trimethylgallium, the flask is 
typically heated to between about 40.degree. and about 50.degree. C. To 
provide a sample for analysis at an appropriate concentration, about 3 
.mu.l would be injected into a flask of about 270 ml. Flask 10 can be used 
without separate, external controlled atmosphere with a conventional 
atomic plasma spectroscopy system. 
FIG. 2 illustrates a convenient container for storing a small amount of 
material during the testing procedure. Container 20 is provided with 
septum 22 for removal of samples by syringe for subsequent injection into 
flask 10 through septum 14. To provide a convenient reservoir for the 
needle of the syringe, the bottom portion 24 of container 20 is tapered 
and extended. Container 20 also is provided with ground glass opening 26 
to allow container 20 to be readily filled from a storage cylinder (not 
shown). 
FIG. 3 illustrates an adapter to facilitate transfer of material from a 
storage cylinder (not shown) to container 20. Adaptor 30 is provided with 
a fitting 32 for attachment to a storage cylinder, and ground glass 
fitting 34 for insertion into ground glass fitting 26 in container 20. 
Adaptor 30 is also fitted with tubes 36 and 38 to provide an inert 
atmosphere during transfer of material from the storage cylinder to 
container 20. A metal-to-glass seal 39 connects fitting 32 to the 
remainder of adaptor 30. 
Another embodiment of the method of analysis of the present invention is 
indirect analysis by decomposition of the sample. This embodiment would be 
more commonly used when an indication is needed of the level of impurities 
which are non-volatile. However, in contrast to the prior art, the 
indirect analysis of the present invention more closely determines the 
true level of impurities. 
Sample preparation for indirect analysis is more readily described by 
reference to FIG. 4. To reaction flask 40 is added a solution of acid. The 
choice of acid is not critical, preferred acids being hydrochloric acid 
and nitric acid. The acid solution is maintained in the frozen state by 
beaker 41 which contains liquid nitrogen or other suitable refrigerant 
such as dry ice/acetone. Reaction flask 40 is provided with two ground 
glass openings 42 and 44. Opening 42 is fitted with a stopper which can be 
removed to vent gases formed during the decomposition procedure. Reaction 
and venting are performed in a dry argon or nitrogen purged chamber. 
Opening 44 is fitted with addition funnel 46. Stopper 48 having a small 
gap to the controlled, inert gas atmosphere is removably attached to 
funnel 46. Adapter 30 can be used to transfer a sample from a storage 
cylinder (not shown) to funnel 46 by inserting adapter 30 in place of 
stopper 48. The entire assembly is flushed with an inert gas and then an 
appropriate amount of material is transferred from the storage container 
to funnel 46 with stopcock 50 being in a closed position. After addition 
to funnel 46, adapter 30 is removed and replaced by stopper 48. Stopper 52 
is placed in flask 40 after the acid solution is frozen. 
The material in funnel 46 is then added dropwise with stopper 52 being 
removed during addition. After the addition of each drop, the reaction is 
allowed to subside before another drop is added. When all the material in 
funnel 46 has been added, the solution is allowed to melt, and then 
transferred to a beaker for aqueous dilution and analysis by conventional 
nebulization techniques. 
Although the amount of material originally added to funnel 46 is not known, 
it is readily determined by comparison to a known standard gallium 
solution, which information can then be used in determining the level of 
impurities. 
Direct analysis using an exponential dilution flask differs from indirect 
analysis after decomposition, and from direct analsis using an organic 
solvent such as xylene or hexane to dissolve the organometallic compound. 
Finding standards for comparison is simple where decomposition has taken 
place, but only impurities which completely and reliably decompose can be 
accurately measured. For example, tetramethylsilane (a likely contaminant 
of TMG) is extremely difficult to decompose, even in strong acid. 
Dissolution in a solvent is sufficient provided the impurities are well 
identified so that appropriate standards can be prepared. It is only the 
use of an exponential dilution flask which allows determination of 
impurities without knowing what form they are in, e.g., organometallic or 
inorganic. This is a definite advantage. 
Although the above description has referred to inductively coupled plasma 
(ICP), the method of the present invention should also be useful with 
systems utilizing DC plasma or microwave plasma. Similarly, while atomic 
emission spectroscopy (AES) has been referred to above, the method of the 
present invention should also be useful with atomic absorption 
spectroscopy, atomic fluorescence spectroscopy, and mass spectroscopy. 
To obtain a more complete understanding of the present invention, the 
following examples are set forth. However, it should be understood that 
the invention is not limited to the specific details set forth in the 
following examples. 
EXAMPLE 1 
This example illustrates direct analysis of impurities by using an 
exponential dilution flask (EDF) such as that illustrated in FIG 1. By 
syringe, 3 .mu.l of trimethylgallium (TMC) are transferred from a 
container such as that in FIG. 2, to the EDF of about 270 ml which has 
been previously purged with argon gas. The EDF contains a magnetic 
stirring bar and is heated to about 40.degree.-50.degree. C. 
After allowing the sample to vaporize, argon gas which has been flowing 
through a bypass is directed to flow through the exponential dilution 
flask at a carrier gas velocity of about 0.6 l/min and thereby feed the 
sample to an ICP-AES unit. The silicon concentration is determined by 
comparison of the silicon line which appears at 251.6 nm of the atomic 
emission spectra. 
The exponential dilution flask results in a continually more dilute vapor 
being provided for analysis from which the concentration of the sample can 
be determined by comparison with samples of known dilution. The curve 
generated by an exponential dilution flask and how it is interpreted is 
discussed in Inman et al, referred to above. It is found that 30 seconds 
is the optimum time of decay to allow appropriate mathematical fitting to 
the dilutron cuve to provide an indication of the amount of silicon 
present in the sample being analyzed. The exponential relationship to 
concentration as a calibration technique is described by Ritter et al, 
above. 
EXAMPLE 2 
This example illustrates indirect analysis in aqueous medium. In the maner 
described above, about 1-1.5 ml of TMG is decomposed by dropwise addition 
to 30 ml of 0.5 N HCl. The HCl solution is then allowed to melt, and the 
acidity of the final solution adjusted to about 0.6-0.7 N. After complete 
dissolution of the galium comounds, the solution is transferred to a 100 
ml flask diluted with distilled water. Since the exact TMG quantity 
transferred into aqueous phase is not known, it is determined by the 
TCP-AES technique using conventional aqueous standard gallium solution. 
To make sure that the TMG decomposition is complete after its reaction with 
water, the following experiment is conducted. A known volume of aqueous 
solution containing decomposed TMG as described above is evaporated to 
dryness and treated with a few drops of concentrated HNO.sub.3 and several 
aliquots of 30% H.sub.2 O.sub.2 until the precipitate becomes white. The 
precipitate is then dissolved with 1% HNO.sub.3 and diluted to the same 
volume as before. High concentrations of acid in the solution are known to 
depress the signal and decrease the sensitivity. Accordingly, high 
concentrations of acid should be avoided, solutions of 1N or less being 
preferred. The gallium and carbon concentrations of the solutions before 
and after the treatment are compared. The results are tabulated below: 
______________________________________ 
GALLIUM CARBON 
SOLUTION CONCENTRATION SIGNAL 
______________________________________ 
distilled water 
&lt;40 ng/ml 3.4 nA 
TMG before treatment 
14.0 mg/ml 124 nA 
TMG after treatment 
14.2 mg/ml 7.3 nA 
______________________________________ 
Since the gallium concentration of the solution before and after the 
treament is essentially the same, and the carbon signal and thus content 
of the solution approaches that of the distilled water blank, this means 
that the organic gallium has been completely deomposed by reaction with 
water, and no volatile TMG remains in the aqueous phase. 
From an ICP-AES analytical point of view, the solution sample is considered 
non-volatile since the signal of the element determined before 
nebulization and after nebulization (that is, the sample recovered from 
the drain) gives the same value each time within acceptable error. 
The aqueous TMG solution is checked before and after nebulization for the 
following analytes: aluminum, magnesium, calcium, copper, iron. The net 
signals are the same indicating that no volatilization phenomenon takes 
place. The silicon, the most important contamination of TMG according to 
the direct vapor analysis of TMG, is known to be in volatile form and thus 
no attempt is made to determine it. 
The following list sets forth the detection limits of these elements from 
trimethylgallium based upon three times the detection limit of 
single-element aqueous solution and a 2% TMG solution. The detection limit 
is calculated from the standard deviation of the background signal, a 
factor of 0.03, and the net signal and sensitivity of a silicon standard 
solution. 
______________________________________ 
ELEMENT DETECTION LIMIT IN TMG 
______________________________________ 
Aluminum 5 .mu.g/g 
Iron 2 .mu.g/g 
Magnesium 0.2 .mu.g/g 
Copper 2 .mu.g/g 
Calcium 0.1 .mu.g/g 
______________________________________ 
EXAMPLE 3 
This example provides a comparison of direct versus indirect analysis for 
three TMG samples from different sources. The three samples are analyzed 
indirectly for all the listed impurities using the technique of Example 2, 
and directly for silicon using the technique of Example 1. The results are 
summarized as follows: 
______________________________________ 
Indirect Method 
Element Concentration (.mu.g/g) 
Direct Method** 
Sample 
Al Fe Cu Mg Si Si 
______________________________________ 
31 4.8* 2 5 2 8 9 
32 24 2 2 2 7 6 
33 5 11 2 1 27 43 
______________________________________ 
*The aluminum concentration for Sample 31 is by percent rather than 
.mu.g/g. 
**Measurements of silicon by the direct method are in .mu.g/ml which is 
converted to .mu.g/g by dividing by the density of TMG (1.1 g/ml). 
While the invention has been described in terms of various preferred 
embodiments, one skilled in the art will appreciate that various 
modifications, substitutions, omissions, and changes may be made without 
departing from the spirit thereof. Accordingly, it is intended that the 
scope of the present in invention be limited solely by the scope of the 
following claims.