Microelectronics Processing - American Chemical Society

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A c t i v a t i o n Analysis o f Electronics Materials Richard M. Lindstrom Center for Analytical Chemistry, National Bureau of Standards, Gaithersburg, MD 20899 Neutron activation analysis is one of a small number of methods capable of multi-elemental analysis of subnanogram quantities of contaminants in semiconductors and other materials. Milligram to gram-sized samples of silicon, quartz, graphite, or organic materials are nearly ideal for the method. The physics of the processes involved is simple, and qualitative identification of components is an integral part of the quantitative analysis. Except for the need for access to a nuclear reactor, the equipment required is readily available commercially, and is comparable in cost and complexity to that used in other advanced analytical techniques. The need for elemental analysis of materials important to the electronics industry need not be belabored, since the correlation between trace element contaminants and device performance has been amply demonstrated in the literature. A systematic study of these effects requires that analytical methods be available for accurate measurement of sub-part-per-million concentrations. As technology progresses from large-scale to very-large-scale integration, the characterization of materials at sub-part-per-billion levels will be required (1_). What was true in 1970 is equally true today: "While the future is bright, the technological problems are formidable, and materials characterization must contribute in large part to their solution (2)". Matters to be considered in the selection of a method of chemical analysis include the suite of elements for which the method is useful, and the lower limit of concentration at which each element can be detected and measured, as well as the accuracy of the analysis and the cost in money and time. Few analytical methods are applicable at the concentration levels required, and none is optimum by all these criteria. This paper describes one method in common use, neutron activation analysis (NAA). The essential characteristics of NAA will

This chapter not subject to U.S. copyright. Published 1986, American Chemical Society Casper; Microelectronics Processing: Inorganic Materials Characterization ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

18.

L1NDSTROM

295

Activation Analysis of Electronics Materials

be g i v e n i n s u f f i c i e n t d e t a i l f o r the s o l i d - s t a t e p h y s i c i s t o r e n g i n e e r t o j u d g e whether t h i s method i s the one o f c h o i c e i n s o l v i n g a p a r t i c u l a r problem. Keenan and L a r r a b e e (V) have r e c e n t l y w r i t t e n a comprehensive summary o f the c h a r a c t e r i z a t i o n o f VLSI s i l i c o n , i n w h i c h the a v a i l a b l e a n a l y t i c a l methods are compared and c o n t r a s t e d ; i n a sense the p r e s e n t paper can be c o n s i d e r e d an e x p a n s i o n o f t h e i r treatment o f NAA. An e a r l i e r book, a l s o by the group a t Texas I n s t r u m e n t s (2) d i s c u s s e s NAA i n d e t a i l as the s t a t e o f t h e a r t e x i s t e d i n 1970. S i n c e t h a t time the development o f l a r g e , h i g h - r e s o l u t i o n gamma-ray d e t e c t o r s has made NAA l e s s dependent on r a d i o c h e m i s t r y and t r a n s formed i t i n t o a h i g h - p r e c i s i o n s p e c t r o s c o p i c t e c h n i q u e . There a r e many r e f e r e n c e s i n the l i t e r a t u r e t o the c h a r a c t e r i z a t i o n of e l e c t r o n i c s m a t e r i a l s by NAA and o t h e r n u c l e a r methods (3 ^ 5 6 7). Seve r a l e x c e l l e n t t e x t b o o k s on a c t i v a t i o n a n a l y s i s are a v a i l a b l e (8,9,10), t o which the r e a d e r i s r e f e r r e d f o r more complete d i s c u s s i o n of the m a t t e r s o u t l i n e d i n the p r e s e n t summary. N e u t r o n a c t i v a t i o n i s a major c o n t r i b u t e r t o modern e l e m e n t a l a n a l y s i s . I n a r e c e n t c o m p i l a t i o n o f 11,000 p u b l i s h e d a n a l y s e s o f 75 e n v i r o n m e n t a l and b i o l o g i c a l S t a n d a r d R e f e r e n c e M a t e r i a l s i s s u e d by the N a t i o n a l Bureau o f S t a n d a r d s (NBS), o v e r h a l f the a n a l y s e s r e p o r t e d were performed by NAA (1J_). A c o n s i d e r a b l e p a r t o f contemp o r a r y t r a c e - e l e m e n t g e o c h e m i s t r y i s r e l i a n t on INAA, l a r g e l y because o f the method's h i g h s e n s i t i v i t y , broad e l e m e n t a l c o v e r a g e , and the ease o f a n a l y s i s o f l a r g e numbers o f s m a l l samples w i t h a modest i n v e s t m e n t o f time (1_2). 9

9

9

9

B a s i c F e a t u r e s o f the N e u t r o n A c t i v a t i o n Method The f o r e m o s t c h a r a c t e r i s t i c o f n e u t r o n a c t i v a t i o n a n a l y s i s i s t h a t nuclear r e a c t i o n s are employed f o r chemical measurements. Reactor i r r a d i a t i o n produces n e u t r o n - r i c h i s o t o p e s which u s u a l l y decay by e m i s s i o n o f b e t a p a r t i c l e s , o f t e n accompanied by gamma r a y s as the daughter n u c l e u s d e e x c i t e s . Gamma-ray s p e c t r a are composed o f d i s c r e t e narrow l i n e s which a r e u s u a l l y w e l l r e s o l v e d by modern d e t e c t o r s . As a f a m i l i a r example, NAA i s used t o determine c o b a l t t h r o u g h the c a p t u r e o f slow n e u t r o n s by the Co-59 i s o t o p e . The r a d i o a c t i v e Co-60 product i s d e t e c t e d by measurement o f the d e l a y e d gamma r a y s e m i t t e d a t d i s c r e t e e n e r g i e s o f 1173.238 and 1332.502 keV when i t decays t o s t a b l e N i - 6 0 . I n a r e c e n t l y developed e x t e n s i o n of NAA, the gamma r a y s e m i t t e d i n s t a n t a n e o u s l y i n the n e u t r o n a b s o r p t i o n s t e p are measured; t h i s method i s most o f t e n c a l l e d n e u t r o n - c a p t u r e prompt-gamma a c t i v a t i o n a n a l y s i s , o r PGAA. The e l e m e n t a l scope o f n e u t r o n a c t i v a t i o n i s wide, w i t h as many as 51 elements h a v i n g been determined i n s t r u m e n t a l l y i n one m a t e r i a l by a c o m b i n a t i o n o f NAA and PGAA (^3). S e v e r a l t a b l e s o f d e t e c t i o n l i m i t s have been p u b l i s h e d (14,15,16,1), and a t a b l e o f e x p e r i m e n t a l l y measured d e t e c t i o n l i m i t s i n S i and S i 0 2 i s i n c l u d e d h e r e ( T a b l e I ) . A s t r i k i n g c h a r a c t e r i s t i c o f NAA i s the wide range o f d e t e c t i o n l i m i t s a c r o s s the p e r i o d i c t a b l e . Many elements are measurable i n the sub-nanogram range, but on the o t h e r hand some elements cannot be detected at a l l . A histogram o f i n t e r f e r e n c e - f r e e d e t e c t i o n l i m i t s i s g i v e n i n F i g u r e 1.

Casper; Microelectronics Processing: Inorganic Materials Characterization ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

296

MICROELECTRONICS PROCESSING: INORGANIC MATERIALS CHARACTERIZATION

T a b l e I . D e t e c t i o n L i m i t s Measured a t NBS f o r Trace Element A n a l y s i s o f S i and S i 0 by I n s t r u m e n t a l Neutron A c t i v a t i o n 2

Element

Na Mg Al CI K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn As Br Zr Ag Sb Cs Ba La Ce Sm Eu Hf Ta W Au Th U

Nuclide measured

Half-life

Na-2H Mg-27 Al-28 Cl-38 K-M2 Ca-49 Sc-46 Ti-51 V-52 Cr-51 Mn-56 Fe-59 Co-60 Ni-65 Cu-64 Zn-65 As-76 Br-82 Zr-95 Ag-110m Sb-122 Cs-134 Ba-131 La-140 Ce-141 Sm-153 Eu-152m Hf-181 Ta-182 W-187 Au-198 Pa-233 Np-239

15.0 h 9.46 m 2.2k m 37.2 m 12.4 h 8.70 m 83.8 d 5.75 m 3.75 m 27.7 d 2.58 h 44.5 d 5.27 y 2.54 h 12.7 h 244 d 26.3 h 35.3 h 65.5 d 250 d 2.70 d 2.06 y 11.8 d 40.3 h 32.5 d 46.7 h 9.32 h 42.4 d 115 d 23.9 h 2.70 d 27.4 d 2.36 d

Detection Limit (ng/g)

0.1 3000 500 5 100 2000 0.003 1000 10 1 0.1 50 0.1 2000 0.5 1 0.02 0.02 500 0.01 0.01 0.05 10 0.01 0.2 0.05 0.01 20 0.05 0.02 0.0005 0.1 0.3

1

o f 100 mg were i r r a d i a t e d f o r up t o 6 h r a t 5 x 1 0 3 and counted a t t h e maximum p o s s i b l e e f f i c i e n c y . The on l i m i t i s d e f i n e d a c c o r d i n g t o C u r r i e ( 1 7 ) .


18.

LINDSTROM


10'

12

9

10"

6

10"

10'

3

297

1

Detection Limit (g)

F i g u r e 1. H i s t o g r a m o f d e t e c t i o n l i m i t s f o r n e u t r o n a c t i v a t i o n a n a l y s i s (NAA) and n e u t r o n - c a p t u r e prompt gamma-ray a c t i v a t i o n a n a l y s i s (PGAA). Data from R e f e r e n c e 15.

N u c l e a r Consequences. The use o f n u c l e a r r a t h e r than c h e m i c a l r e a c t i o n s has s e v e r a l non-chemical consequences: 1. Both t h e a c t i v a t i n g and i n d i c a t i n g p a r t i c l e s a r e h i g h l y p e n e t r a t i n g i n g r a m - s i z e d o r s m a l l e r samples o f common mater i a l s , s o t h a t e r r o r s a r e n o t g e n e r a l l y i n c u r r e d by a n a l y t i c a l c o m p l i c a t i o n s due t o t h e n a t u r e o f t h e m a t r i x . 2. There i s l i t t l e c o r r e l a t i o n between d e t e c t a b i l i t y o f an element and i t s p o s i t i o n i n t h e p e r i o d i c t a b l e , except f o r the u s e f u l f a c t t h a t t h e major elements o f t h e e a r t h ' s s u r f a c e and atmosphere — H, C, N, 0, and S i — g e n e r a l l y have low c r o s s s e c t i o n s f o r n e u t r o n c a p t u r e . Consequently, p l a s t i c s , g r a p h i t e , q u a r t z , and s i l i c o n a r e n e a r l y i d e a l m a t e r i a l s t o i r r a d i a t e because l i t t l e o r no r a d i o a c t i v i t y i s produced from t h e major c o n s t i t u e n t s . When t h e m a t r i x cons i s t s o f any o f a number o f g e o l o g i c a l l y r a r e elements — Ga, Ge, A s , Cd, I n , Sb, and Te i n t h e p r e s e n t c o n t e x t — t h i s s e l e c t i v i t y f o r t r a c e elements i s l o s t because o f t h e l a r g e q u a n t i t y o f r a d i o a c t i v i t y produced when samples o f t h e s e elements a r e i r r a d i a t e d (Table I I ) . 3. Because c h e m i c a l m a n i p u l a t i o n s a r e o r d i n a r i l y performed o n l y a f t e r the i r r a d i a t i o n or are omitted e n t i r e l y , a c t i v a t i o n a n a l y s i s i s f r e e o f reagent blank. 4. The number o f i n t e r f e r i n g n u c l e a r r e a c t i o n s t h a t can p o s s i b l y o c c u r i s l i m i t e d by t h e c o n s e r v a t i o n o f energy, and a l l l i k e l y i n t e r f e r e n c e s can be enumerated by i n s p e c t i o n o f


298


5.

6.

the T a b l e o f N u c l i d e s (1_8) o r by c o n s u l t i n g p u b l i s h e d t a b l e s ( 1 9 ) . C o r r e c t i o n f o r t h o s e few i n t e r f e r e n c e s t h a t can o c c u r can be v e r y a c c u r a t e ( 2 0 ) . Q u a l i t a t i v e a n a l y s i s i s an i n t e g r a l p a r t o f t h e q u a n t i t a t i v e a n a l y s i s , s i n c e t h e gamma-ray spectrum and h a l f - l i f e o f a r a d i o n u c l i d e a r e h i g h l y d i a g n o s t i c f i n g e r p r i n t s o f the s p e c i e s . The s e t o f gamma-ray l i n e s e m i t t e d i n r a d i o a c t i v e decay i s w e l l c h a r a c t e r i z e d , i n t h e sense t h a t few i m p o r t a n t l i n e s a r e unknown. A l t h o u g h a r e c e n t l y p u b l i s h e d t a b l e cont a i n s 48,000 e n t r i e s ( 2 1 ) , t h e d e n s i t y i n energy space i s low compared t o t h e d e t e c t o r r e s o l u t i o n , and most r a d i o n u c l i d e s emit s e v e r a l l i n e s . Hence t h e number o f c a n d i d a t e s p e c i e s f o r each peak i n a spectrum i s s m a l l . S i n c e r a d i o a c t i v e decay f o l l o w s P o i s s o n s t a t i s t i c s , a lower l i m i t t o t h e p r e c i s i o n o f an a n a l y s i s can be o b t a i n e d by a s i n g l e measurement. I n p r a c t i c e , c o u n t i n g s t a t i s t i c s genera l l y i s the l i m i t i n g uncertainty, since chi-squared t e s t s o f t e n show t h a t the single-measurement p r e c i s i o n i s an e x c e l l e n t p r e d i c t o r o f sample-to-sample r e p e a t a b i l i t y .

K i n e t i c s o f A c t i v a t i o n A n a l y s i s . The time s c a l e o f the measurement i s determined by t h e h a l f - l i f e o f t h e r a d i o n u c l i d e s produced: i t i s o f t e n n e a r l y o p t i m a l t o i r r a d i a t e f o r one h a l f - l i f e , w a i t one h a l f l i f e , and count f o r one h a l f - l i f e . The amount o f r a d i o a c t i v i t y A(0) o f a g i v e n r a d i o n u c l i d e p r e s e n t at t h e end o f an i r r a d i a t i o n i n t e r v a l IT i s A(0) where

x

I T

= N o ( 1 - e ~ * )

N = number o f t a r g e t atoms i r r a d i a t e d o = c r o s s s e c t i o n o r r e a c t i o n p r o b a b i l i t y , cm « n e u t r o n f l u x , c n T s ~ A = In 2 / t ( 1 / 2 ) , s~ t(1/2) = h a l f l i f e of the r a d i o n u c l i d e , s IT - i r r a d i a t i o n t i m e , s. 2

(1)

2

1

1

The a c t i v i t y decays e x p o n e n t i a l l y a f t e r t h e i r r a d i a t i o n ends, t h e amount r e m a i n i n g a t a time WT a f t e r i r r a d i a t i o n b e i n g A(WT) = A(0) e "

(2)

x # W T

I f t h i s a c t i v i t y i s measured d u r i n g t h e time from WT t o WT + CT, w i t h an e f f i c i e n c y o f e counts per d i s i n t e g r a t i o n , then t h e number o f counts o b s e r v e d C i s t h e i n t e g r a l o f ( 2 ) , o r C = e A(WT)

(1-e-*

# C T

)

From (2) and (3) we have A(0)

C A e^'WT " e (1-e-*-CT)


(3)

18.

L1NDSTROM

Table I I .

Material

299


A c t i v i t i e s o f Semiconductors A f t e r Neutron Irradiation*

Radionuclide

Half-life

Si

Si-31

2.62 h

GaP

P-32 Ga-72

GaAs

Activity (mCi/g)**

Radiations

5.0

beta

14.28 d 14.10 h

0.59 170

beta b e t a , gamma

Ga-72 As-76

14.10 h 26.3 h

120 160

b e t a , gamma b e t a , gamma

Ge

Ge-71 Ge-75 Ge-77

11.2 d 82.8 m 11.30 h

4.2 130 0.99

beta b e t a , gamma b e t a , gamma

CdTe

Cd-115 Cd-117m Cd-117 Te-127 Te-129

44.6 3.4 2.6 9.4 70 m

0.92 0.26 0.61 9.6 22

b e t a , gamma gamma b e t a , gamma b e t a , gamma beta

InSb

In-1l4m Sb-122 Sb-124

49.51 d 2.72 d 60.3 d

d h h h

0.25 37 1.3

gamma,beta b e t a , gamma b e t a , gamma

1 5

*

I r r a d i a t i o n f o r 1 hour a t a t h e r m a l n e u t r o n f l u x o f 1 0 n/cm «s i s assumed. N u c l i d e s w i t h h a l f - l i v e s l e s s than one hour have been a r b i t r a r i l y o m i t t e d . These i n c l u d e i s o t o p e s o f Ga, Ge, Cd, I n , and Te. Minor n u c l i d e s and daughters a r e also omitted. ** L o c a l r e g u l a t i o n s g e n e r a l l y r e q u i r e r a d i a t i o n s h i e l d i n g and o t h e r s a f e g u a r d s f o r work w i t h 1 mCi o r more o f r a d i o a c t i v i t y t o a s s u r e t h a t t h e dose t o p e r s o n n e l i s a c c e p t a b l y l o w .

I f an unknown sample (x) and a s t a n d a r d ( s ) a r e i r r a d i a t e d t o g e t h e r and t h e i r d e c a y - c o r r e c t e d a c t i v i t i e s A(0) a r e measured w i t h t h e same e f f i c i e n c y , then by a p p l y i n g e q u a t i o n s ( 1 ) and ( 4 ) t o both sample and s t a n d a r d we have A(0,x) _ N(0,x) A(0,s) ~ N(0,s) where o, (j), I T , and e c a n c e l i n t h e r a t i o . The r e s u l t i s t h a t t h e r a t i o o f t h e number o f atoms o f t h e t a r g e t element i n t h e sample t o t h a t i n t h e s t a n d a r d i s t h e same as t h e r a t i o o f t h e c o r r e s p o n d i n g activities. I n more c h e m i c a l terms, i f [ Y ] i s t h e c o n c e n t r a t i o n o f element Y i n mass m o f m a t e r i a l , then


300


-w x -w v m(s) A(0,x) CY](x) = [ Y ] ( s ) • — — • ' . ra(x) A ( 0 , s ) r

A

(6)

I n t h i s d e m o n s t r a t i o n a h o s t o f assumptions and s i m p l i f i c a t i o n s have been made, which i n p r a c t i c e can be c o r r e c t e d f o r a c c u r a t e l y , shown t o be u n i m p o r t a n t , o r made t o c a n c e l i n t h e d e s i g n o f t h e measurements. These i n c l u d e n e u t r o n and gamma-ray a t t e n u a t i o n i n t h e sample, s p a t i a l g r a d i e n t s and t e m p o r a l changes i n n e u t r o n f l u x , d i f f e r e n c e s i n c o u n t i n g e f f i c i e n c y between sample and s t a n d a r d , m u l t i s t e p n u c l e a r r e a c t i o n s , and i n s t r u m e n t a l n o n l i n e a r i t i e s a t h i g h counting r a t e s . Making an A c t i v a t i o n - A n a l y s i s Measurement. The most prominent technique i n nuclear a n a l y t i c a l chemistry i s instrumental neutron a c t i v a t i o n (INAA), i n which t h e r m a l n e u t r o n s from a n u c l e a r r e a c t o r are used t o i r r a d i a t e t h e sample and t h e induced r a d i o n u c l i d e s a r e measured n o n d e s t r u c t i v e l y w i t h a germanium gamma-ray s p e c t r o m e t e r . S e n s i t i v i t y may be enhanced by c h e m i c a l l y s e p a r a t i n g t h e elements o f i n t e r e s t b e f o r e r a d i o n u c l i d e assay. H i g h - s e n s i t i v i t y NAA, o f c o u r s e , r e q u i r e s a c c e s s t o a r e a c t o r . I n t h e U n i t e d S t a t e s t h e r e a r e a t p r e s e n t 61 r e s e a r c h and t e s t i n g r e a c t o r s w i t h a power o f 100 KW or g r e a t e r , o p e r a t e d by u n i v e r s i t i e s , s t a t e and n a t i o n a l l a b o r a t o r i e s , and i n d u s t r i e s i n 25 s t a t e s , most o f which can be used f o r a c t i v a t i o n a n a l y s i s ( 2 2 ) . Many a n a l y s t s work o n l y w i t h l o n g - l i v e d a c t i v a t i o n p r o d u c t s and thus need not be p r e s e n t a t t h e r e a c t o r t o perform i r r a d i a t i o n s . The equipment r e q u i r e d f o r gamma-ray a s s a y ' s a d e t e c t o r and p u l s e - h e i g h t a n a l y z e r ' s i s compara b l e i n c o s t and c o m p l e x i t y w i t h t h a t needed f o r o t h e r modern a n a l y t i c a l methods. S u i t a b l e sample s i z e s f o r i r r a d i a t i o n a r e u s u a l l y a gram o r l e s s . There i s no i n h e r e n t lower l i m i t , and t h e upper l i m i t i s s e t by t h e f i n i t e t r a n s p a r e n c y o f t h i c k samples t o n e u t r o n s and by t h e p r a c t i c a l problems o f i n s e r t i n g a sample i n t o a n e u t r o n f i e l d . Very l a r g e samp l e s , such as e n t i r e s i l i c o n i n g o t s f o r n e u t r o n - t r a n s m u t a t i o n doping (23) and even o i l p a i n t i n g s (24,2£), have been s t u d i e d by NAA w i t h special f a c i l i t i e s . Step 1:

Pretreatment

o f Samples

Despite a l l p o s s i b l e care i n handling before reaching the a n a l y t i c a l l a b o r a t o r y , any sample r e c e i v e d s h o u l d be c o n s i d e r e d t o have a contaminated s u r f a c e . S i n c e t h e r e c o i l atoms produced i n t h e i r r a d i a t i o n may have e n e r g i e s o f s e v e r a l hundred eV, any a c t i v a t e d i m p u r i t y may be d r i v e n i n t o t h e s u r f a c e o f t h e sample. A judgment must be made about t h e r i s k o f a d d i n g more contaminant by i m p e r f e c t p r e c l e a n ing; g e n e r a l l y a l i g h t etch a f t e r i r r a d i a t i o n i s p r e f e r a b l e i f the n a t u r e o f t h e experiment p e r m i t s . I f t h e m a t r i x w i l l become h i g h l y r a d i o a c t i v e a f t e r neutron i r r a d i a t i o n (Table I I ) , i t may be d e s i r a b l e t o s e p a r a t e t h e elements o f i n t e r e s t b e f o r e h a n d , f o r which clean-room c h e m i s t r y and h i g h - p u r i t y r e a g e n t s may be r e q u i r e d t o keep b l a n k s w i t h i n t o l e r a b l e bounds ( 2 6 , 2 7 ) .


18.

LINDSTROM

Step 2:


301

Packaging

P r i o r t o an a c t i v a t i o n a n a l y s i s , t h e samples and s t a n d a r d s a r e packaged f o r i r r a d i a t i o n . Double containment i s u s u a l , t h e o u t e r c o n t a i n e r b e i n g d e s i g n e d f o r s a f e t y and m e c h a n i c a l l y s u i t e d t o t h e p a r t i c u l a r r e a c t o r f a c i l i t y used. Common c o n t a i n e r m a t e r i a l s a r e p o l y e t h y l e n e , p o l y p r o p y l e n e , p o l y s t y r e n e , q u a r t z , aluminum, o r g r a p h i t e . The i n n e r c o n t a i n e r must be r e a s o n a b l y r a d i a t i o n r e s i s t a n t and n o n - c o n t a m i n a t i n g t o t h e sample. S p e c i a l l y cleaned p l a s t i c s , h i g h - p u r i t y q u a r t z , r e a c t o r - o r semiconductor-grade g r a p h i t e , pure m e t a l f o i l , o r pure s i l i c o n may be used. Step 3:

Irradiation

The samples a r e i r r a d i a t e d i n the r e a c t o r f o r a t i m e between seconds and weeks, depending on t h e h a l f - l i f e o f t h e r a d i o n u c l i d e t o be meas u r e d o r t h e t o l e r a b l e w a i t i n g t i m e , whichever i s s h o r t e r . Care must be t a k e n t o a s s u r e t h a t t h e samples and s t a n d a r d s r e c e i v e e q u a l expos u r e t o n e u t r o n s , o r t h a t unequal doses be measured q u a n t i t a t i v e l y by i r r a d i a t i o n o f m o n i t o r s . The f l u x i n a r e a c t o r which o p e r a t e s a t c o n s t a n t power can be s t a b l e t o b e t t e r than 1% from day t o day, w h i l e t h a t i n a t r a i n i n g r e a c t o r may v a r y g r e a t l y on a time s c a l e o f minu t e s . The v e r y f a c t t h a t t h e r e a c t o r c o r e i s a n e u t r o n s o u r c e guarantees t h a t t h e r e w i l l be a s p a t i a l g r a d i e n t i n n e u t r o n i n t e n s i t y , which may amount t o t e n s o f p e r c e n t o r more a c r o s s a l a r g e sample container (28). S t e p 4: T r a n s f e r and Wait The i r r a d i a t i o n c o n t a i n e r i s unloaded and t h e samples and s t a n d a r d s p r e p a r e d f o r r a d i o n u c l i d e a s s a y . A f t e r i r r a d i a t i o n no c a r e whatever need be t a k e n t o a v o i d c o n t a m i n a t i o n o f t h e sample by n o n - r a d i o a c t i v e i m p u r i t i e s . Time performs t h e same f u n c t i o n as a s k i l l e d c h e m i s t i f the r a d i o n u c l i d e t o be measured has a l o n g e r h a l f - l i f e than t h e i n t e r f e r e n c e s . F o r example, t r a c e C r i s e a s i l y measured i n t h e p r e sence o f T i even though t h e n e u t r o n - c a p t u r e p r o d u c t s o f b o t h elements emit gamma r a y s o f e x a c t l y t h e same energy. Gram f o r gram, T i e m i t s 1000 t i m e s more photons p e r second than does C r i m m e d i a t e l y a f t e r a 1-minute i r r a d i a t i o n , b u t C r i s 2000 t i m e s more r a d i o a c t i v e t h a n T i two hours l a t e r . Step 5:

Chemistry

I f i n t e r f e r i n g a c t i v i t i e s a r e p r e s e n t which a r e l o n g e r - l i v e d than t h e n u c l i d e o f i n t e r e s t , i t may be n e c e s s a r y t o remove them by p o s t i r r a d i a t i o n c h e m i s t r y . A m i l d e t c h i s s u f f i c i e n t t o remove many s u r f a c e contaminants i f t h o s e c o n s t i t u t e n t s a r e washed away w i t h a f l o w i n g e t c h s o l u t i o n o r h e l d i n s o l u t i o n by complexing agents (2) t o p r e v e n t r e d e p o s i t i o n on t h e sample. F o r example, s i l i c o n i s a s u f f i c i e n t l y p o w e r f u l r e d u c i n g agent t o reduce n i t r o g e n e i g h t o x i d a t i o n numbers t o ammonium i o n d u r i n g d i s s o l u t i o n i n HF + HNO3.


302

MICROELECTRONICS PROCESSING: INORGANIC MATERIALS

CHARACTERIZATION

The advent o f e f f i c i e n t h i g h - r e s o l u t i o n gamma-ray d e t e c t o r s d u r i n g t h e p a s t decade has n e a r l y e l i m i n a t e d t h e t e d i o u s r a d i o c h e m i c a l s e p a r a t i o n o f each t r a c e element from a l l o t h e r s which was once n e c e s s a r y , a l t h o u g h group s e p a r a t i o n s a r e o f t e n a p o w e r f u l a i d t o s p e c i f i c i t y and s e n s i t i v i t y . I f t h e m a t r i x i t s e l f i s t h e major c o n t r i b u t o r t o t h e a c t i v i t y (as w i t h Ge o r most I I I - V compounds), s a f e t y p r e c a u t i o n s may be n e c e s s a r y t o p r o t e c t t h e chemist a g a i n s t r a d i a t i o n . Many procedures have been p u b l i s h e d f o r m a t r i x removal and f o r t h e s e p a r a t i o n o f t r a c e c o n s t i t u e n t s (29,1,2,30). Ga, Ge, As, and Sb may be s e p a r a t e d from o t h e r elements by d i s t i l l a t i o n , a procedure which i s w e l l s u i t e d t o h a n d s - o f f o p e r a t i o n . S o l v e n t e x t r a c t i o n and i o n exchange a r e o f t e n used t o remove both l a r g e and s m a l l amounts o f u n d e s i r a b l e components. R a d i o c h e m i s t r y r e m a i n s , however, a d i s c i p l i n e r e q u i r i n g s u b s t a n t i a l amounts o f s k i l l and time. Step 6:

R a d i o a c t i v i t y Assay

Gamma-ray s p e c t r o s c o p y owes a debt t o semiconductor t e c h n o l o g y f o r making h i g h - r e s o l u t i o n s p e c t r o s c o p y p o s s i b l e (31^). A d e t e c t o r cons i s t s o f a l a r g e , h i g h - q u a l i t y germanium c r y s t a l , 100 cm3 o r more o f n e a r - i n t r i n s i c r e s i s t i v i t y , h e l d a t l i q u i d - n i t r o g e n temperature i n vacuum d u r i n g u s e . The c r y s t a l , r e v e r s e b i a s e d t o o p e r a t e as a p - i - n j u n c t i o n d i o d e , a c t s as an i o n i z a t i o n chamber, c o n v e r t i n g t h e energy o f a 1-MeV gamma-ray photon i n t o 340,000 e l e c t r o n - h o l e p a i r s . The energy r e s o l u t i o n i n p r a c t i c e can be as good as 0.16 p e r c e n t ( f u l l w i d t h a t h a l f t h e maximum peak h e i g h t ) a t 1 MeV. Charge p u l s e s from t h e d e t e c t o r a r e a m p l i f i e d and s o r t e d a c c o r d i n g t o s i z e i n t o a h i s t o g r a m o f i n t e n s i t y as a f u n c t i o n o f gamma-ray energy. S e v e r a l f i r m s produce m u l t i c h a n n e l p u l s e - h e i g h t a n a l y z e r s which a r e c a p a b l e o f s o r t i n g p u l s e s i n t o 8000 o r even 16000 c h a n n e l s a t a r a t e o f 10^ events per second w i t h a p r o p o r t i o n a l i t y between channel number and energy o f b e t t e r than 0.1 p e r c e n t . The l a r g e number o f d a t a p o i n t s i n a spectrum makes computer-aided a n a l y s i s p r a c t i c a l l y mandatory whenever more t h a n one o r two peaks a r e t o be measured. A l t h o u g h a busy spectrum may c o n t a i n upward o f 200 peaks, each i s l o c a l l y w e l l r e p r e s e n t e d by a narrow n e a r - G a u s s i a n peak on a smooth b a s e l i n e . As a r e s u l t , t h e spectrum can be a n a l y z e d p i e c e w i s e . L a b o r a t o r y m i n i - and microcomputers a r e adequate f o r t h e s e c a l c u l a t i o n s , e s p e c i a l l y s i n c e any d i s a d v a n t a g e i n computing speed compared w i t h a mainframe computer i s o f f s e t by t h e immediacy of response w i t h a d e d i c a t e d machine. Programs have been w r i t t e n by numerous r e s e a r c h e r s and equipment m a n u f a c t u r e r s f o r t h e a n a l y s i s o f gamma-ray s p e c t r a ( 3 2 ) . The c o u n t i n g time o f t h e a c t i v a t e d sample, l i k e the i r r a d i a t i o n t i m e , i s u s u a l l y comparable t o t h e h a l f - l i f e o f t h e r a d i o n u c l i d e meas u r e d , o r as much as a day f o r s m a l l q u a n t i t i e s o f l o n g - l i v e d n u c l i d e s . Standards a r e counted under i d e n t i c a l c o n d i t i o n s o f counting e f f i c i e n c y . Step 7:

C a l c u l a t i o n of Elemental

Concentration

C o r r e c t i o n s a r e made t o t h e n e t areas o f t h e gamma-ray peaks i n t h e s p e c t r a o f sample and s t a n d a r d f o r c o n t r i b u t i o n s from u n r e s o l v e d components, r a d i o a c t i v e growth and decay, dead time and o t h e r r a t e -


18.

LINDSTROM


303

r e l a t e d c o u n t i n g e f f e c t s , and nonuniform i r r a d i a t i o n c o n d i t i o n s . Once t h e s e c o r r e c t i o n s are made, the c o n c e n t r a t i o n i s o b t a i n e d from E q u a t i o n 6. T h i s p r o c e d u r e , as d e s c r i b e d , r e q u i r e s t h a t a known q u a n t i t y o f each element t o be measured be i r r a d i a t e d under the same c o n d i t i o n s as the unknown sample. I f the r e l a t i v e p r o d u c t i o n r a t e s o f the r a d i o n u c l i d e s used are measured a c c u r a t e l y i n a p a r t i c u l a r i r r a d i a t i o n f a c i l i t y , subsequent measurements can be done w i t h l e s s l a b o r by i r r a d i a t i n g o n l y a few m o n i t o r elements t o c h a r a c t e r i z e t h e n e u t r o n spectrum. P u b l i s h e d v a l u e s o f c r o s s s e c t i o n s have been used as a measure o f t h e s e r e l a t i v e r e a c t i o n r a t e s , but the i n a c c u r a c y o f t a b u l a t e d d a t a and the p o o r l y known n e u t r o n spectrum of most r e a c t o r s makes t h i s approach u n s a t i s f a c t o r y when good a c c u r a c y i s needed. N e u t r o n - c a p t u r e prompt-gamma ray a c t i v a t i o n a n a l y s i s (PGAA) i s a r e c e n t a d d i t i o n t o the n u c l e a r a n a l y t i c a l a r s e n a l . I n t h i s t e c h n i q u e the i n s t a n t a n e o u s gamma r a y e m i s s i o n from a sample i s measured as i t i s i r r a d i a t e d i n a f l u x o f r e a c t o r n e u t r o n s (33»3ij,35). Because the sample must be s e v e r a l meters from e i t h e r the c o r e o f the r e a c t o r o r ( l e s s commonly) from the d e t e c t o r , the s e n s i t i v i t y o f t h i s t e c h n i q u e i s g e n e r a l l y p o o r e r than i n c o n v e n t i o n a l NAA. However, i t i s poss i b l e t o measure s m a l l q u a n t i t i e s o f many elements w h i c h do not g i v e r a d i o a c t i v e n e u t r o n - c a p t u r e p r o d u c t s , n o t a b l y 0.01 mg o f H, 50 ng B, and 1 mg P i n an e l e c t r o n i c s c o n t e x t . Some A p p l i c a t i o n s o f NAA i n E l e c t r o n i c M a t e r i a l s

Studies

Trace-Element C h a r a c t e r i z a t i o n of S i l i c o n . In order to define " s o l a r grade" s i l i c o n w i t h s u f f i c i e n t p r e c i s i o n t o make the optimum economic c h o i c e among p o s s i b l e p r o d u c t i o n p r o c e s s e s , D a v i s e t a l . doped a s e r i e s o f s i l i c o n i n g o t s w i t h s i n g l e t r a n s i t i o n m e t a l i m p u r i t i e s and produced c u r v e s r e l a t i n g n o r m a l i z e d p h o t o v o l t a i c e f f i c i e n c y t o the c o n c e n t r a t i o n of the contaminant (^6). E s t a b l i s h i n g the x - a x i s o f t h e s e c u r v e s was not e n t i r e l y s t r a i g h t f o r w a r d . As a c o n t r i b u t i o n t o t h i s work, NBS measured c o n c e n t r a t i o n s o f a number o f dopants i n t h e s e samples, w i t h the d e t e c t i o n l i m i t s found i n T a b l e I . An i l l u s t r a t i o n of the d i f f i c u l t y f a c e d i n t h i s work i s t h a t f o r t y p e r c e n t o f the c o n c e n t r a t i o n s determined a t NBS d i f f e r e d from the nominal c o n c e n t r a t i o n by a f a c t o r o f two or more. S p a t i a l L o c a t i o n of C o n s t i t u e n t s . D e s p i t e the f a c t t h a t NAA i s a method f o r b u l k a n a l y s i s , i t s s e n s i t i v i t y has been used t o advantage i n d i f f u s i o n s t u d i e s , by s u c c e s s i v e e t c h i n g o r m e c h a n i c a l l a p p i n g o f s u r f a c e l a y e r s (5.,.2). E s p e c i a l l y i f the r a d i o a c t i v i t y o f the l a y e r s removed i s measured d i r e c t l y r a t h e r than as the d i f f e r e n c e i n a c t i v i t y o f the s u b s t r a t e , the t e c h n i q u e can be s e n s i t i v e and a c c u r a t e . C o n t a m i n a t i o n S t u d i e s . Schmidt and Pearce (27), i n a c l a s s i c s t u d y , u s i n g NAA e v a l u a t e d s o u r c e s o f c o n t a m i n a t i o n o f s i l i c o n d u r i n g d e v i c e processing. T h e i r work demonstrated c l e a r l y t h a t h i g h - t e m p e r a t u r e o x i d a t i o n s t e p s are p a r t i c u l a r l y l i k e l y t o add t r a n s i t i o n - m e t a l c o n t a m i n a n t s . M e c h a n i c a l p o l i s h i n g compounds, b o a t s and h a n d l i n g t o o l s , and components o f ion-beam equipment were a l s o i d e n t i f i e d as being p o t e n t i a l l y troublesome.


304


When p l a t i n u m r e s i s t a n c e thermometers at NBS were found t o behave i r r e g u l a r l y , contamination of the s u r f a c e of quartz i n s u l a t o r t u b i n g was s u s p e c t e d o f b e i n g a c o n t r i b u t o r . Another cause was found a f t e r an NAA s u r v e y o f the m a t e r i a l used showed t h a t t h e t r a c e element c o n t e n t was w i t h i n the m a n u f a c t u r e r ' s s p e c i f i c a t i o n s . Boron i n S i l i c o n and G l a s s e s . As a complement t o e a r l i e r comparisons o f the r e l a t i o n between B c o n t e n t and r e s i s t i v i t y o f s i l i c o n (38), a s e t o f B-doped S i wafers was a n a l y z e d by PGAA. The c o n c e n t r a t i o n range from 3x10 ^ t o 1 x 1 0 atoms/cm^ was measured w i t h i r r a d i a t i o n times o f 10 minutes t o 3 h o u r s . B u l k boron has a l s o been determined i n a s e r i e s o f b o r o s i l i c a t e g l a s s e s as p a r t o f a s t u d y o f B concent r a t i o n near t h e g l a s s s u r f a c e (^9). The r e p r o d u c i b i l i t y among r e p l i c a t e s was 1 J , w i t h i n the c o u n t i n g s t a t i s t i c s o b t a i n e d a f t e r an i r r a d i a t i o n o f 15 t o 30 m i n u t e s . Agreement w i t h ICP a n a l y s i s o f t h e same samples was w i t h i n 2% (HO). 2 0

F i l m - T h i c k n e s s Measurements. A l t h o u g h NAA i s u s u a l l y r e g a r d e d (and most o f t e n used) as a t e c h n i q u e f o r measuring c o n s t i t u e n t s a t t r a c e l e v e l s , i t has proved v a l u a b l e f o r the a c c u r a t e measurement o f major e l e m e n t s , s u c h as the d e t e r m i n a t i o n o f t h e t h i c k n e s s of m e t a l l i c films. For example, a s e t o f g o l d f i l m s e v a p o r a t e d on s i l i c o n f o r use as i n t e r l a b o r a t o r y s t a n d a r d s f o r R u t h e r f o r d b a c k s c a t t e r i n g has been c a l i b r a t e d by INAA (M). A f t e r the a n a l y s i s was completed, comp a r i s o n w i t h g r a v i m e t r i c measurements o f the g o l d c o n t e n t showed agreement t o w i t h i n 0.3 p e r c e n t . Other u n p u b l i s h e d work a t NBS (42) has shown NAA t o have t h e s e n s i t i v i t y t o d e t e c t a 2-nm Fe f i l m on g r a p h i t e , the s e l e c t i v i t y t o determine the t h i c k n e s s o f a 10-nm film o f Pd under 5 pm o f Ag, and t h e a c c u r a c y t o determine t h e q u a n t i t y o f N i and Cr i n a m u l t i l a y e r f i l m o f 260 nm t o t a l t h i c k n e s s t o w i t h i n 0.5 p e r c e n t . Related Nuclear A n a l y t i c a l

Techniques

Thermal n e u t r o n s are c a p a b l e of i n d u c i n g a s m a l l number o f i m p o r t a n t n u c l e a r r e a c t i o n s which produce charged p a r t i c l e s . The o u t s t a n d i n g example i s o f c o u r s e U-235, whose f i s s i o n fragments can be r e g i s t e r e d by i n s u l a t i n g s o l i d s and e t c h e d i n t o o p t i c a l l y v i s i b l e t r a c k s (j£) p r o v i d e a t w o - d i m e n s i o n a l image o f the uranium d i s t r i b u t i o n on a s u r f a c e . S i m i l a r l y , n e u t r o n - i n d u c e d a l p h a p a r t i c l e s from B-10, L i - 6 , and 0-17, t r i t o n s from L i - 6 , and p r o t o n s from N-14 have been used t o v i s u a l i z e and q u a n t i f y the d i s t r i b u t i o n o f elements on s u r f a c e s or i n c u t s e c t i o n s . A l t e r n a t i v e l y , t h e energy o f t h e l i g h t charged p a r t i c l e s can be measured d u r i n g i r r a d i a t i o n . The shape o f the spectrum g i v e s g r a p h i c a l l y t h e depth d i s t r i b u t i o n o f t h e atoms w h i c h absorbed the n e u t r o n s ( M ) ; t h i s n e u t r o n depth p r o f i l i n g i s the s u b j e c t o f another paper a t t h i s symposium (}\5). R a d i o a c t i v e t r a c e r s a r e i d e a l t o o l s f o r s t u d y i n g problems o f d i f f u s i o n , phase d i s t r i b u t i o n e q u i l i b r i a , and p r o c e s s i n g contaminat i o n . Kane and L a r r a b e e s t a t e " w i t h o u t r e s e r v a t i o n , t h a t a l l r e l i able values f o r segregation c o e f f i c i e n t s f o r impurities i n s e m i c o n d u c t o r s have been d e t e r m i n e d by u s i n g r a d i o t r a c e r methods" (2). W i t h h i g h - e f f i c i e n c y r a d i a t i o n d e t e c t o r s the q u a n t i t y o f r a d i o a c t i v i t y t h a t must be handled need not be l a r g e . The method i s d e s e r v i n g o f w i d e r use. t o


18.

L1NDSTROM


305

The d i s t r i b u t i o n o f r a d i o a c t i v e atoms over the f a c e o f a c r y s t a l can be made v i s i b l e by e x p o s i n g x-ray f i l m t o the s u r f a c e . Autor a d i o g r a p h y w i t h n e u t r o n - a c t i v a t e d specimens o r those doped w i t h r a d i o t r a c e r s has been used f o r l o c a l i z a t i o n o f d e f e c t s , f o r s t u d y i n g the u n i f o r m i t y o f doping i n the c r y s t a l , and f o r measuring the uniformity of surface films (jL.A^M'JiZ). Summary T h i s paper has g i v e n i n o u t l i n e form the major c o n s i d e r a t i o n s on which a s u c c e s s f u l a p p l i c a t i o n o f neutron a c t i v a t i o n a n a l y s i s should be based. The most e f f e c t i v e a p p l i c a t i o n s o f t h i s method have i n v o l v e d c l o s e and c o n t i n u a l i n t e r a c t i o n between the p h y s i c i s t o r e n g i n e e r and the a n a l y s t from the b e g i n n i n g o f the p r o j e c t , i n order t o ensure measurements t h a t are m e a n i n g f u l and l e a d t o the s o l u t i o n of the problem b e i n g s t u d i e d . A c t i v a t i o n a n a l y s i s i s o n l y one o f many a n a l y t i c a l t e c h n i q u e s a v a i l a b l e f o r m a t e r i a l s c h a r a c t e r i z a t i o n ; i t s s e n s i t i v i t y , a c c u r a c y , s i m p l e p h y s i c s , and n o n d e s t r u c t i v e n a t u r e s h o u l d make i t an important c o n t r i b u t o r f o r y e a r s t o come.

Literature Cited 1.

2. 3. 4. 5. 6. 7. 8. 9. 10. 11.

12. 13.

14. 15. 16.

Keenan, J. A.; Larrabee, G. B. In "VLSI Electronics Microstructure Science, Vol.6: Materials and Process Characterization"; Einspruch, N. G.; Larrabee, G. B., Eds.; Academic Press: New York, 1983; p. 1. Kane, P. F.; Larrabee, G. B. "Characterization of Semiconductor Materials" McGraw-Hill: New York, 1970. Heinen, K. G.; Larrabee, G. Anal. Chem., 1966, 38, 1853. Martin, J. A. In "Semiconductor Silicon"; Haberecht, R. R., Ed.; Electrochemical Society: New York, 1969; p. 547. Kudo, K. J. Japan Soc. Appl. Phys., 1975, 44, 319. Riotte, H. G.; Herpers, U.; Weber, W. Radiochem. Radioanal. Lett., 1977, 30, 311. Bouldin, D. J. Radioanal Chem., 1982, 72, 35. Kruger, P. "Principles of Activation Analysis" WileyInterscience: New York, 1971. DeSoete, D.; Gijbels, R.; Hoste, J. "Neutron Activation Analysis" Wiley-Interscience: London, 1972. Amiel, S. "Nondestructive Activation Analysis" Elsevier: Amsterdam, 1981. Gladney, E. S.; Burns, C. E.; Perrin, D. R.; Roelandts, I.; Gills, T. E. "1982 Compilation of Elemental Concentration Data for NBS Biological, Geological, and Environmental Standard Reference Materials (NBS Spec. Pub. 260-88)" National Bureau of Standards: Washington, D.C., 1984. Laul, J. C. Atomic Energy Rev., 1979, 17, 603. Germani, M. S.; Gokmen, I.; Sigleo, A. C.; Kowalczyk, G. S.; Olmez, I.; Small, A. M.; Anderson, D. L.; Failey, M. P.; Gulovali, M. C.; Choquette, C. E.; Lepel, E. A.; Gordon, G. E.; Zoller, W. H. Anal. Chem., 1980, 52, 240. Meinke, W. W. Science, 1955, 121, 177. Vogt, J. R. (Univ. of Mo., Columbia), unpublished, 1981. Koch, O. G.; LaFleur, P. D.; Morrison, G. H. Pure and Appl. Chem., 1982, 54, 1565.


306 17. 18. 19. 20. 21. 22. 23. 24. 25.

26. 27. 28. 29. 30. 31. 32.

33. 34. 35.

36. 37. 38.

MICROELECTRONICS PROCESSING: INORGANIC MATERIALS CHARACTERIZATION Currie, L. A. Anal. Chem. 1968, 40, 586. Walker, F. W.; Kirouac, G. J.; Rourke, F. M. "Chart of the Nuclides" General Electric Company: San Jose, 1977. Erdtmann, G. "Neutron Activation Tables" Verlag Chemie: Weinheim, 1976. Lindstrom, R. M.; Fleming, R. F. In Proc. 3rd Int. Conf. Nucl. Methods in Envir. and Energy Res. (CONF-771072); Vogt, J. R., Ed.; U of Mo.: Columbia, 1977; pp. 90. Erdtmann, G.; Soyka, W. "The Gamma Rays of the Radionuclides" Verlag Chemie: Weinheim & New York, 1979. Burn, R. R.; Bilof, R. S. "Research, Training, Test, and Production Reactor Directory" American Nuclear Society: LaGrange Park, Ill, 1983. Guldberg, J., Ed. "Neutron-Transmutation-Doped Silicon" Plenum: New York, 1981. Ainsworth, M. W., Ed. "Art and Autoradiography: Insights into the Genesis of Paintings by Rembrandt, Van Dyck, and Vermeer" Metropolitan Museum of Art: New York, 1982. Carter, R. S.; Ganozcy, M.; Olin, C. H.; Schroder, I. G; Cheng, Y-T.; Olin, J. S. In "Proc. Conf. Use and Develop. of Low and Medium-Flux Research Reactors"; Harling, O., Ed.; MIT Press: Cambridge, Mass, 1982. Das, H. A. Pure & Appl. Chem., 1982, 54, 755. Murphy, T. E. in "Accuracy in Trace Analysis: Sampling, Sample Handling, Analysis" (NBS Spec. Pub. 422); LaFleur, P. D., Ed; U. S. Government Printing Office, Washington, 1976, p. 509. LaFleur, P. D.; Becker, D. A. J. Radioanal. Chem., 1974, 19, 149. Thompson, B. A.; Strause, B. M.; Leboeuf, M. B. Anal. Chem., 1958, 30, 1023. Cerofolini, G. F.; Polignano, M. L.; Picco, P.; Finetti, M.; Solmi, S.; Gallorini, M. Thin Solid Films, 1982, 87, 373. Hansen, W. L.; Haller, E. E. In "Nuclear Radiation Detector Materials", Haller, E. E.; Kraner, H. W.; Higinbotham, W. A., Eds.; North-Holland: New York 1983; p. 1. Carpenter, B. S.; D'Agostino, M. D.; Yule, H. P., Eds. "Computers in Activation Analysis and Gamma-Ray Spectroscopy"(CONF-780421), U. S. Department of Energy: Washington, DC, 1979. Failey, M. P.; Anderson, D. L . ; Zoller, W. H.; Gordon, G. E.; Lindstrom, R. M. Anal. Chem., 1979, 51, 2209. Glascock, M. D. In "Neutron-Capture Gamma-Ray Spectroscopy and Related Topics"; von Egidy, T., Gonnenwein, F., Maier, B., Eds.; Institute of Physics: London, 1982; p. 641. Anderson, D. L . ; Zoller, W. H.; Gordon, G. E.; Walters, W. B.; Lindstrom, R. M. In "Neutron-Capture Gamma-Ray Spectroscopy and Related Topics"; von Egidy, T.; Gonnenwein, F.; Maier, B., Eds.; Institute of Physics: London, 1982; p. 655. Davis, J. R.; Rohatgi, A.; Hopkins, R. H.; Blais, P. D.; Rai-Choudhury, P.; McCormick, J. R.; Mollenkopf, H. C. IEEE Trans. Elect. Dev., 1980, ED-27, 677. Schmidt, P. F.; Pearce, C. W. J. Electrochem. Soc., 1981, 128, 630. Thurber, W. R.; Carpenter, B. S. J. Electrochem. Soc., 1978, 125, 654.


18. LINDSTROM 39. 40. 41. 42. 43. 44. 45. 46. 47.


307

Riley, J. E. Jr.; Downing, R. G.; Fleming, R. F.; Lindstrom, R. M.; Vincent, D. H. J. Solid State Chem., 1984, to be published. Riley, J. E. Jr., Lindstrom, R. M. In prep, 1985. Lindstrom, R. M.; Harrison, S. H.; Harris, J. M. J. Appl. Phys., 1978, 49, 5903. Fleming, R. F. Private communication, 1984. Fleischer, R. L.; Price, P. B.; Walker, R. M. "Nuclear Tracks in Solids: Principles and Applications " University of California Press: Berkeley, 1975. Downing, R. G.; Fleming, R. F.; Langland, J . K.; Vincent, D. H. Nucl. Inst. Meth. Phys. Res., 1983, 218, 47. Downing, R. G.; Fleming, R. F.; Maki, J. T. This symposium. Szabo, E. J. Radioanal. Chem., 1974, 19, 23. Rausch, H. J. Radioanal. Chem., 1978, 44, 119.

RECEIVED June 14, 1985


Microelectronics Processing - American Chemical Society

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