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Materials Characterization U s i n g Elemental a n d Isotopic Analyses b y Inductively C o u p l e d P l a s m a M a s s Spectrometry B. Shushan, E. S. K. Quan, A. Boorn, D. J. Douglas, and G. Rosenblatt SCIEX, Thornhill, Ontario, Canada L3T 1P2 A new technique for elemental and isotope analysis of materials used in the semiconductor industry is presented. The technique involves the use of an inductively coupled plasma to convert trace elements to their gaseous ions followed by analysis of these ions by mass spectrometry. Examples include the quantitative analysis of trace copper by isotope dilution and the analysis of trace contaminants in boron, indium phosphide and reagent acids. Inductively Coupled Plasma Mass Spectrometry (ICP/MS) is a powerful technique for elemental and isotope analysis which has undergone extensive development over the last decade by a number of research groups (1,2,3). These research efforts have led to the recent development of commercially available ICP-MS systems which, by virtue of their novelty, are now just beginning to demonstrate capabilities well suited to the characterization of materials used in the semiconductor industry. This paper will outline the principles of ICP-MS and provide some examples of the instrumentation's analytical capabilities. Instrumentation The ICP-MS used in the present work was the ELAN 250 manufactured by SCIEX . A schematic of this instrument is shown in Figure 1. As well as being a good source for optical emission, the ICP is an excellent ion source; in fact, most elements inside the 7000°K plasma exist, to a major extent, as singly positively-charged ions (2). As in optical emission ICP systems, solutions to be analyzed are nebulized into the high temperature argon plasma. The ions produced therein are sampled through a differentially pumped interface linked to the mass spectrometer. Ions are then separated electronically by means of a quadrupole mass filter and detected using a high sensitivity pulse counting sytem. All operations exclusive of the plasma system are controlled through the instrument's computer which also provides for data manipulation and presentation. 0097-6156/ 86/0295-0284$06.00/0 © 1986 American Chemical Society
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Results & Discussion The ICP has p r o l i f e r a t e d as a method of converting chemical compounds into t h e i r elemental constituents which subsequently emit l i g h t of c h a r a c t e r i s t i c wavelengths. Accordingly, ICP has been used extensively as an emission source f o r o p t i c a l detection systems i n order to perform elemental a n a l y s i s . Since each element can emit hundreds of o p t i c a l l i n e s , the use of ICP/AES f o r multiple element a n a l y s i s , or f o r the detection of elements i n unknown or concentrated matrices, can s u f f e r from interferences due to spectr a l overlap. By contrast, ICP-MS provides inherently simpler spectral information. An example of such a spectrum i s demonstrated i n Figure 2 showing a t y p i c a l ICP-MS scan f o r a 10 ug ml" solution of mixed t r a n s i t i o n metals. The demonstrated s e n s i t i v i t y here i s 10 to 10 counts s" per ug ml" and, coupled with the nearly universal i o n i z a t i o n e f f i c i e n c y of the ICP ion source, provides t y p i c a l detection l i m i t s i n a narrow range between 0.1 to 10 ng.ml" f o r most elements. In f a c t over 90% of the elements i n the periodic table are accessible f o r such a n a l y t i c a l determinations. 1
4
5
1
1
1
The inherent s i m p l i c i t y of ICP-MS spectra implies that spectral interferences from major matrix elements can be minimal. This i s evident i n Figure 3 which shows a portion of ICP-MS scan of an indium phosphide sample. A t i n impurity i s most v i s i b l e showing peak signals a t the c h a r a c t e r i s t i c isotopes (masses 116,117,118, 119,120,122 and 124). Since indium and t i n represent a worst case scenario ( I n and S n d i f f e r by only one mass u n i t , i e . , the smallest increment) i t i s of great p r a c t i c a l importance that the signals f o r these two elements do not i n t e r f e r e with each other. The degree of separation between elements of neighbouring mass i s referred to as "abundance s e n s i t i v i t y " and i s an i n d i c a t i o n of the system's mass spectral resolving power. Abundance s e n s i t i v i t i e s f o r the ELAN " instrument are t y p i c a l l y 1 i n 10 on the low mass side of a peak, and better than 1 i n 10 on the high mass side. In the present example, the I n peak i n t e n s i t y i s estimated to be more than 10 times that of S n , and yet there i s no mass spectral overlap between those two s i g n a l s . 1 1 5
1 1 6
1
5
7
1 1 5
5
1 1 6
High abundance s e n s i t i v i t i e s help i n the accurate determination of isotope r a t i o s of elements, permitting the quantitation of these dissolved elements by isotope d i l u t i o n methods. This quantitative technique e n t a i l s adding to the sample a known amount of a stable isotope of the element one wishes to quantitate. The corresponding increase i n signal at that isotope's mass i s d i r e c t l y proportional to the amount of material added. Comparison of t h i s increase i n signal with the signal i n t e n s i t y f o r one of the undiluted native isotopes provides a method of d i r e c t quantitation w i t h i n the matrix of the sample and requires j u s t one or two measurements f o r each quantitative r e s u l t . For example, Figure 4 shows the quantitative determination of copper i n an orchard l e a f digest using the technique of isotope d i l u t i o n (4). This f i g u r e constitutes the hard-copy output from the ELAN instrument f o r t h i s a n a l y t i c a l procedure and consists of three mass spectra over the copper region
M
Figure 2.
40.0
TI
NI
FE
60.0
CO
1
Mass
CU
70.0
PS
1
80.0
i 9 iT V
AR2
Mass spectrum of a solution of elements each present at 10 ug m l " .
50.0
CR
' ' '
SR
1. 90/B
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MICROELECTRONICS PROCESSING: INORGANIC MATERIALS CHARACTERIZATION
(sample diluted) 1/1000
->(x44)
2 2 0 0 0 0 -i
Figure 3.
Mass Spectrum of an Indium Phosphide Sample (0.77% in Aqua Regia showing t i n impurity at ca. 2 ppm) In order to bring indium peaks on scale the solution was diluted 1000 times before scanning over the indium region (masses 110-115). The t i n portion of the spectrum (masses 116-135) was run undiluted.
SHUSHAN ET AL.
Elemental and Isotopic Analyses
SAMPLE
62.00
63.00
130
64.00 Mass SPIKE
65.00 66.
100
=
50-
62.00
T 63.00
SAMPLE
1
r
64.00 65.00 Mass & SPIKE
66.00
100 71.4 § 50 28.6
62.00
Figure 4.
63.00
64.00 Mass
65.00
66. i
Results for the determination of copper in an organic matric (Orchard Leaf) by isotope dilution. (TOP) Mass spectrum between 62 and 66 mass units for the unspiked sample containing 1% dissolved organic matter. The mass spectral peaks for copper were normalized and displayed in histogram format. (MIDDLE) Same as above obtained on the isotope d i l u t i o n standard which contained only Cu . (BOTTOM) Same as above obtained on the sample s o l u t i o n ; 25 ml volume spiked with 4 ug Cu . 6 5
6 5
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(mass 62 to 66) where the signals are normalized and displayed in histogram format. Figure 4 (top) i s the natural isotopic d i s t r i bution of copper in the sample showing the percent-abundance of each isotope (ie., 69.09% and 30.91% for Cu and Cu respectively). The middle spectrum i s that of the isotope-dilution standard which was, as expected, 100% Cu^S. The bottom spectrum is that of 25 ml of the orchard l e a f diqest containing 0.25gm orchard leaf material spiked with 4ug C u ^ . The ratio R of Cu to C u was measured as (71.4/28.6) = 2.50. Since the contribution of C u in the bottom spectrum is purely native, the contribution of native. Cu^S to the mass 65 intensity can be e a s i l y accounted f o r , and i s shown as the s l i g h t l y darker portion of the mass 65 histogram. The concentration of copper in the digest may now be determined d i r e c t l y : 6 3
6 5
6 5
6 3
6 3
fCu) = (ftto, - R x Bspl x Wsp Whore:
(Cul
(R Is R Is ftsp Bsp
x Bs - Rs) x Ws In ppm the r a t i o Cuf6S)/Cuf63) Is the s p i k e G J ( 6 5 ) abund. Is the s p i k e Cuf63) abund.
Wsp Is the s p i k e weight In ug Bs Is the natural Cu(63) abund. Bs Is the natural Cuf65) abund. Ws Is the sample weight
In grams
Thus [Cu] was thus calculated to be 11.3 ppm for this orchard leaf sample which was NBS c e r t i f i e d as 12 ppm. The sampling procedure required only a few seconds for each of the measurements, and only three scans or less are needed for this type of procedure. Precision is t y p i c a l l y of the order of _+0.5% and combined with the system's speed the technique is obviously suitable for multielement quantitative determinations, and those applications requiring high sample throughput. The spectral s i m p l i c i t y and s e n s i t i v i t y of ICP-MS make i t i d e a l l y suited for multi-element screening a p p l i c a t i o n s . Figure 5, for example, is a scan between 50 and 90 atomic mass units performed on a boron sample dissolved in "pure" HNO3. With this single scan i t was possible to q u a l i t a t i v e l y (and semi-quantitatively) determine the presence of chromium, germanium and iron impurities in the boron matrix (0.71% in HNO3). similar scan of the HNO3 s o l vent (not shown) revealed that those elements in parentheses were solvent impurities including vanadium, copper and gallium. Since absolute s e n s i t i v i t i e s of the technique to the various elements i s roughly similar ( i f t h e i r f i r s t ionization potentials (IP) are lower than 9.0 eV) semi-quantitative information can be e a s i l y obtained by the addition of suitable internal standards i e . , elements whose IP's are s i m i l a r to that of the analyte elements one wishes to semi-quantitate. A
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Figure 5. The ICP/MS analysis of a boron solution (0.71% in HNO3) showing chromium, germanium, and iron impurities (ca. 180, 20, and 700 ppb, respectively) between 49 and 90 mass units.
292
MICROELECTRONICS PROCESSING: INORGANIC MATERIALS CHARACTERIZATION
17.
SHUSHANETAL.
293
Elemental and Isotopic Analyses
Many q u a l i t y control applications require the rapid multi-element analysis of feed-stock samples to compare t h e i r impurity l e v e l s t o those of suitably pure standards. Figure 6 demonstrates such a procedure showing the ICP-MS analysis of two lead samples used i n battery production. Many impurities are observed i n the sample No. 1 spectrum a t substantial l e v e l s , whereas sample No. 2 exhibited fewer impurity signals and a t lower concentrations. Even though the lead i s a t a l e v e l of concentration which i s many orders of magnitude more than any elemental impurity, there i s no spectral interference from lead since i t s signals are located well above t h i s mass range. The l e v e l s and types of impurities i n samples of "pure" elements such as those evident i n the present spectra also serve as a f i n g e r p r i n t a l l u d i n g t o sample o r i g i n or method o f production. Conclusion The ICP has demonstrated i t s e l f to be an e f f i c i e n t ion source capable of i o n i z i n g most elements i n the periodic table with s i m i l a r e f f i c i e n c i e s . The combination of the ICP with the speed, f l e x i b i l i t y and spectral s i m p l i c i t y of mass spectrometry provides a powerful technique f o r the characterization and q u a l i t y control of bulk materials used i n the semiconductor industry. Literature Cited 1.
D.J. Douglas, E.S.K. Quan and R.G. Smith, Spectrochimica Acta. 38B (1983) 39
2. 3.
R.S. Houk, Y.A. Fassel and H.J. Svec, Dynamic Spectrometry6 (1981) 234, Heydon, London, ed-D/Price and J . F . J . Todd A.L. Gray and A.R. Date, Ibid P.252
4.
SCIEX
Application Note
RECEIVED August 12, 1985
Mass
