Microelectronics Processing - American Chemical Society

19

Trace Element Survey Analyses

Downloaded by UNIV OF PITTSBURGH on May 3, 2016 | http://pubs.acs.org Publication Date: January 28, 1986 | doi: 10.1021/bk-1986-0295.ch019

b y S p a r k Source M a s s Spectrography Fredric D. Leipziger and Richard J. Guidoboni Northern Analytical Laboratory, Inc., Amherst, NH 03031 The role of Spark Source Mass Spectrography (SSMS) as a high sensitivity trace element analytical method is discussed. The unparalleled combination of sensitivity and complete element coverage makes SSMS especially suitable for the analysis of liquid and solid materials involved in semiconductor processing. Sample requirements are discussed. The application of SSMS to semiconductor materials, process reagents, dopants, and metals, is illustrated. Advantages and disadvantages of the technique as well as sensitivity and accuracy are discussed. Exactly twenty-three years ago this week a conference was held in Boston on ultrapurification of semiconductor materials. One third of the papers at that conference were devoted to impurity analyses (1). Spark source mass spectrography was the newest and most promising analytical technique available at the time, and I would like to compare the status of SSMS at that time to its present status. The first SSMS instrument was reported by Dempster (2) in 1946. Hannay of Bell Laboratories was responsible for the first applications to semiconductor materials (3,4) in the mid 50 s. The technique was so promising that commercial instrumentation became available in 1960. The attractive features of the technique were complete element coverage (all elements on the periodic table could be detected) and excellent sensitivity (to 1 part per billion atomic). The two major applications of SSMS at that time were semiconductor and nuclear reactor materials — both new technologies and both extremely impurity sensitive. The biggest disadvantage of the technique, although not clearly realized at the time, was lack of quantitation. The need for information regarding trace level bulk impurities in semiconductors and nuclear reactor materials was urgent and could not be satisfied by other techniques existing at the time. In the next decade well over 200 instruments were operating in industrial and government laboratories here and in Europe and Japan. During this period the lack of quantitation became apparent and a major effort was made in many laboratories to overcome this fault. f

0097-6156/86/0295-0308$06.00/0 © 1986 American Chemical Society Casper; Microelectronics Processing: Inorganic Materials Characterization ACS Symposium Series; American Chemical Society: Washington, DC, 1986.


19.

LEIPZIGER AND GUIDOBONI


309

A t t h e p r e s e n t time t h e r e a r e p r o b a b l y fewer than 80 i n s t r u m e n t s o p e r a t i n g i n t h e f r e e w o r l d , and 20 o r fewer i n t h e U n i t e d S t a t e s . The l a c k o f q u a n t i t a t i o n f o r r o u t i n e and d i v e r s e samples s t i l l e x i s t s except i n two s p e c i a l c a s e s — i s o t o p e d i l u t i o n and e l e c t r i c a l d e t e c t i o n . These two c a s e s a r e g e n e r a l l y n o t a p p l i c a b l e t o semiconductor m a t e r i a l s . However, t h e c o m b i n a t i o n o f complete element coverage and e x c e l l e n t s e n s i t i v i t y a r e s t i l l u n p a r a l l e l e d by modern a n a l y t i c a l t e c h n i q u e s devoted t o b u l k i m p u r i t y a n a l y s e s . Thus, d e s p i t e i t s i n h e r e n t s e m i - q u a n t i t a t i v e c h a r a c t e r , we have a most u s e f u l method which e f f i c i e n t l y f i l l s a need i n t h e semiconductor and h i g h p u r i t y m a t e r i a l f i e l d s . F i g u r e 1 compares s e n s i t i v i t y and element coverage of SSMS w i t h o t h e r t e c h n i q u e s . Many European l a b o r a t o r i e s u s i n g s o p h i s t i c a t e d d e n s i t o m e t r y and computer d a t a h a n d l i n g a r e p r o d u c i n g d a t a w i t h a c c u r a c i e s c l a i m e d t o b e ±5%. However, many o f t h e s e a n a l y s e s r e q u i r e two t o f i v e days p e r sample ( 5 ) . The t h r e e k e y f a c t o r s g o v e r n i n g t h e s u c c e s s f u l u s e o f SSMS today a r e knowledge o f t h e l i m i t a t i o n s of t h e t e c h n i q u e , o p e r a t o r e x p e r i e n c e , and c l e a n l i n e s s . The customer must b e made aware o f t h e semiq u a n t i t a t i v e n a t u r e o f t h e d a t a and t h e f a c t t h a t c e r t a i n elements such a s sodium, p o t a s s i u m , c a r b o n , and n i t r o g e n r e q u i r e s p e c i a l i n t e r p r e t a t i o n s . The o p e r a t o r must b e e x p e r i e n c e d i n t h e i n t e r p r e t a t i o n o f t h e d a t a — h e must b e aware o f i n t e r f e r e n c e s from compound i o n s , m u l t i p l y charged s p e c i e s and o t h e r s o u r c e s . The i n t e r p r e t a t i o n o f SSMS d a t a f a l l s i n t o two d i s t i n c t a r e a s — element i d e n t i f i c a t i o n and e s t i m a t e s o f q u a n t i t y . The c r i t e r i a u s e d in our l a b o r a t o r y f o r p o s i t i v e elemental i d e n t i f i c a t i o n a r e the p r e s e n c e o f t h e doubly i o n i z e d s p e c i e s and t h e i d e n t i f i c a t i o n o f t h e i s o t o p i c p a t t e r n when p o s s i b l e . Q u a n t i t a t i o n w i l l b e d i s c u s s e d l a t e r . L a s t , t h e i n s t r u m e n t s o u r c e must be c l e a n e d r e g u l a r l y t o a v o i d memory problems. Our approach t o t h e memory problem i s t o have a complete set o f s o u r c e p a r t s f o r each m a t r i x . A s e t o f p a r t s a r e d e d i c a t e d f o r s i l i c o n a n a l y s e s , a n o t h e r f o r g a l l i u m a r s e n i d e , e t c . These p a r t s and t h e s o u r c e i t s e l f a r e c l e a n e d on a r e g u l a r and f r e q u e n t b a s i s . When t h e s e f a c t o r s a r e under c o n t r o l , SSMS h a s proved t o b e a r e l i a b l e , r e p r o d u c i b l e technique f o r the b u l k a n a l y s i s of t r a c e impurities. An e x a m i n a t i o n o f t h e r e c e n t l i t e r a t u r e shows t h a t over 9 0 % o f t h e papers p u b l i s h e d a r e o f European o r Japanese o r i g i n . S e v e r a l e x c e l l e n t r e v i e w s (5,6) i n d i c a t e t h a t fewer than 10% o f t h e a r t i c l e s a r e concerned w i t h s e m i c o n d u c t o r s . The g r e a t e s t emphasis a p p e a r s t o be i n t h e f i e l d s o f g e o l o g y , b i o l o g y and m e t a l l u r g y . These workers u n i f o r m l y a r e a t t e m p t i n g t o produce q u a n t i t a t i v e d a t a and t h e i r approach i s t o e v a l u a t e a r e l a t i v e s e n s i t i v i t y f a c t o r (RSF) f o r each element f o r a p a r t i c u l a r m a t r i x . Then t h e RSF c a n b e a p p l i e d t o f u t u r e d e t e r m i n a t i o n s i n t h e same m a t r i x . C o o p e r a t i v e s t u d i e s show, beyond any q u e s t i o n , t h a t t h e RSF cannot b e t r a n s f e r r e d from one laboratory o r instrument t o another. Instrument

Design

The a b i l i t y t o i o n i z e a l l t h e elements o f t h e p e r i o d i c t a b l e i s t h e major s t r e n g t h o f SSMS b u t i t imposes some s e v e r e r e s t r i c t i o n s on t h e d e s i g n o f t h e mass s p e c t r o g r a p h . The h i g h energy r e q u i r e d t o produce t h e s e i o n s r e s u l t s i n a n i o n beam w i t h a l a r g e energy s p r e a d which a d v e r s e l y e f f e c t s r e s o l u t i o n . I n o r d e r t o reduce t h e energy s p r e a d

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

Casper; Microelectronics Processing: Inorganic Materials Characterization ACS Symposium Series; American Chemical Society: Washington, DC, 1986. 10

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of t h e beam, a d o u b l e f o c u s s i n g a n a l y z e r i s r e q u i r e d . A s c h e m a t i c d i a g r a m o f a t y p i c a l SSMS i s shown i n F i g u r e 2. The energy s p r e a d p r e s e n t i n a n r f spark i o n beam may b e s e v e r a l thousand v o l t s compared t o l e s s than t e n v o l t s f o r e l e c t r o n impact i o n beams. The i o n s o u r c e i s a r a d i o f r e q u e n c y vacuum d i s c h a r g e s o u r c e and i s i l l u s t r a t e d i n F i g u r e 3. The i o n i z a t i o n e f f i c i e n c y o f t h i s s o u r c e i s u n i f o r m w i t h i n a f a c t o r o f t h r e e , due t o t h e f a c t t h a t t h e i o n i z i n g energy a v a i l a b l e i s s o much g r e a t e r t h a n t h e i o n i z a t i o n p o t e n t i a l of any element. T h i s i s one o f t h e most a t t r a c t i v e f e a t u r e s o f t h e t e c h n i q u e , s i n c e i t a l l o w s u s t o a n a l y z e every element on t h e p e r i o d i c table without l a r g e v a r i a t i o n s i n s e n s i t i v i t y . The d e t e c t o r f o r most SSMS a n a l y s e s i s t h e I l f o r d Q - I I photop l a t e . T h i s d e t e c t o r i n t e g r a t e s t h e s i g n a l and a l s o p r o v i d e s a p e r manent r e c o r d o f t h e a n a l y s i s . The i n t e g r a t i n g p r o p e r t i e s o f t h e p h o t o p l a t e and i t s wide dynamic range make i t a n e x c e l l e n t d e t e c t o r f o r SSMS. These p r o p e r t i e s a l s o a l l o w u s t o d e t e c t inhomogeneities i n a sample when t h e s p e c t r a do n o t i n c r e a s e i n a r e g u l a r f a s h i o n w i t h i n c r e a s i n g charge c o l l e c t i o n . I t i s a l s o p o s s i b l e , by u s i n g a g o l d o r g r a p h i t e c o u n t e r e l e c t r o d e , t o do some probe type a n a l y s e s . I f there a r e s m a l l areas of p a r t i c u l a r i n t e r e s t those areas can be analyzed using the counter e l e c t r o d e technique. The need t o p l a c e a g e l a t i n e m u l s i o n i n t h e h i g h vacuum o f t h e a n a l y z e r s e c t i o n n e c e s s i t a t e s a prepumping chamber f o r t h e p h o t o p l a t e s . D e t a i l s of i n s t r u ment d e s i g n and s o u r c e c o n f i g u r a t i o n s a r e d i s c u s s e d b y Herzog and Franzen ( 7 8 ) . 9

Sample Requirements Samples f o r SSMS a n a l y s i s must b e c o n d u c t i n g s e l f s u p p o r t i n g s o l i d s c a p a b l e o f w i t h s t a n d i n g vacuum. Two e l e c t r o d e s a r e r e q u i r e d , i d e a l l y %" l o n g b y .050". C o n d u c t o r s and s e m i c o n d u c t o r s need o n l y t o b e c u t to s i z e and c l e a n e d b e f o r e mounting. N o n - c o n d u c t i n g powders a r e mixed w i t h h i g h p u r i t y g r a p h i t e powder and p r e s s e d i n t o t h e a p p r o p r i a t e e l e c t r o d e shape. N o n - c o n d u c t i n g s o l i d s must f i r s t b e ground b e f o r e m i x i n g w i t h g r a p h i t e and c o m p a c t i n g . L i q u i d s a r e p i p e t t e d o n t o h i g h p u r i t y g r a p h i t e powder, mixed and p r e s s e d i n t o shape a f t e r d r y i n g a t l o w h e a t . An i n c r e a s e d s e n s i t i v i t y i s p o s s i b l e w i t h l i q u i d s s i n c e t h e i m p u r i t i e s a r e c o n c e n t r a t e d on t h e g r a p h i t e . Sample p r e p a r a t i o n r e q u i r e s extreme c a r e t o a v o i d t h e i n t r o d u c t i o n o f c o n t a m i n a t i o n i n t h e c u t t i n g and g r i n d i n g o p e r a t i o n s . This i s a n a r e a where o p e r a t o r e x p e r i e n c e i s o f p r i m e i m p o r t a n c e . Extens i v e c l e a n i n g and e t c h i n g may b e r e q u i r e d t o p r o v i d e a n a n a l y s i s r e p r e s e n t a t i v e o f t h e sample c o m p o s i t i o n . A f u r t h e r c l e a n i n g step i s p r o v i d e d i n t h e i n s t r u m e n t i t s e l f b y s p a r k i n g t h e sample s u r f a c e e n e r g e t i c a l l y t o c l e a n away t h e f i r s t few a t o m i c l a y e r s . T h i s p r e spark c a n b e r e c o r d e d on t h e p h o t o p l a t e t o p r o v i d e q u a l i t a t i v e i n f o r m a t i o n about t h e c h e m i s t r y o f t h e s u r f a c e . T h i s b r i e f d e s c r i p t i o n o f sample p r e p a r a t i o n p o i n t s t o one o f t h e r e a s o n s why t h e s e n s i t i v i t y o f SSMS i s s o h i g h . We a r e w o r k i n g w i t h t h e s o l i d sample i t s e l f — t h e r e i s no d i l u t i o n f a c t o r due t o t h e n e c e s s i t y t o d i s s o l v e t h e sample. Techniques such a s AAS and ICP w i l l l o s e a f a c t o r o f 10 t o 100 i n u l t i m a t e s e n s i t i v i t y due t o d i l u t i o n o c c u r r i n g when a sample i s d i s s o l v e d . A l s o , when h i g h p u r i t y m a t e r i a l s a r e s u b j e c t e d t o d i s s o l u t i o n , t h e chances f o r c o n t a m i n a t i o n a r e h i g h , and r e a g e n t b l a n k s must b e e v a l u a t e d . I n many c a s e s t h e



POSITION OF SPARK

F i g u r e 2.

SPECTROMETER

T y p i c a l Spark Source Mass S p e c t r o g r a p h .

PHOTOGRAPHIC P L A T E MAGAZINE

FOR THE ANALYSIS OF SOLIDS

MS7 MASS



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Pulsed R F Transformer

VACC

F i g u r e 3. R a d i o Frequency Spark Source. Reproduced w i t h p e r m i s s i o n from Ref. 11 C o p y r i g h t 1965 I n t e r s c i e n c e P u b l i s h e r s .


313

MICROELECTRONICS PROCESSING: INORGANIC MATERIALS CHARACTERIZATION

314

r e a g e n t b l a n k s w i l l b e s i g n i f i c a n t l y h i g h e r than t h e e l e m e n t a l c one en t r a t i o n . Mass S p e c t r a The s p e c t r a produced b y t h e r f spark a r e q u i t e s i m p l e . Ions produced by t h e spark a r e a c c e l e r a t e d i n t o t h e a n a l y s e r r e g i o n , a r e mass s e p a r a t e d , and f a l l on t h e p h o t o p l a t e where a l a t e n t image i s g e n e r a t e d and l a t e r d e v e l o p e d . F i g u r e 4 shows a s e r i e s of mass s p e c t r a g e n e r a t e d from a s i l i c o n sample and i s chosen t o i l l u s t r a t e most o f t h e common i o n t y p e s . S i n g l y and m u l t i p l y charged s p e c i e s can b e seen a s w e l l a s compound i o n s and m o l e c u l a r i o n s . The s t r o n g e s t s e r i e s o f Unejp o^ma^ses 28^^29 and 30 a r e due t o t h e s i l c o n isotope ions Si, S i , and S i . The l i g h t e s t exposures g i v e i n f o r m a t i o n c o n c e r n i n g t h e i s o t o p i c abundance. The n e x t most abundant s e r i e s o f l i n e s ^ a r e ^ a t masses 14, 14.5, and 15. These a r e due t o t h e i o n s Si , S i & Q 2+A triply ionized seriesi s also seen a t masses 9.33, 9.67, and 10.00. These g r o u p i n g s o f m r l | i p l y i o n i z e d s p e c i e s o f t h e p a r e n t element may b e seen a s l o w a s 6 o r even l o w e r . Another f e a t u r e of s i l i c o n s p e c t r a i s molecular i o n s . Ions due t o t h e s i l i c o ^ ^ i m e r s ( S i 2 ) a r e v i s i b l e a t masses 56, 57, 58, 59 and 60. g i n c e S i i s t h e most abundant i s o t o p e t h e dimer ygns §§ 5& $ abundant. S t a r t i n g a t mass 84 ( ' ' S i ~ ) a n o t h e r s e r i e s o f m o l e c u l a r i o n s may b e seen. • p i ^ i o n r e s p o n s i b l e f o r t h e l i n e a t mass 31 i s , i n p a r t , due t o 30g^ ^ . T h i s h y d r i d e i o n i s o f g r e a t i n t e r e s t s i n c e t h e most common element p r o d u c i n g a l i n e a t mass 31 i s phosphorus.^^Hg^rever, i n t h i s c a s e , t ^ g absence o f a l i n e a t mass 15.5, due t o P indicates that S i H i s t h e cause o f t h e l i n e . T h i s f i g u r e shows a graded s e r i e s o f exposures r a n g i n g from 0.001 nanocoulombs t o 50 nanocoulombs. A t t h e l o n g e s t exposure a n o m i n a l s e n s i t i v i t y o f 0.02 p a r t s p e r m i l l i o n atomic i s o b t a i n e d f o r a m o n o i s t o p i c element. T a b l e I l i s t s a few o f t h e most common s p e c t r a l i n t e r f e r e n c e s . TJjjje d e t e r m i n a t i o n o f i r o n i n s i l i c o n i s d i f f i c u l t because o f t h e S i o d i m e r which i n t e r f e r e s w i t h t h e major i r o n i s o t o p e a t mass 56. I n t h i s c a s e i t i s p o s s i b l e t o u s e one o f t h e l e s s abundant i r o n isotopes as the a n a l y t i c a l l i n e .


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9

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3

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Some Common I n t e r f e r e n c e s i n SSMS

6 3

Cu

1 6

0

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7 9

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8 1

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The n i o b i u m - phosphorus c a s e i s one where m u l t i p l y charged



19.

LEIPZIGER A N D GUIDOBONI


315

m a t r i x ions obscure an i m p u r i t y l i n e . The o n l y s o l u t i o n i n t h i s c a s e where b o t h m a t r i x and i m p u r i t y a r e m o n o i s o t o p i c would b e i f t h e i n s t r u m e n t h a d s u f f i c i e n t r e s o l v i n g power t o s e p a r a t e t h e two l i n e s . The phosphorus l i n e i s a t m/e 30.9738 w h i l e t h e n i o b i u m l i n e i s a t m/e 30.9686. A r e s o l v i n g power o f 6000 would t h e r e f o r e b e r e q u i r e d t o a n a l y z e phosphorus i n n i o b i u m . The a n a l y s i s o f phosphorus i n s i l i c o n ^g c o m p l i c a t e d b y t h e presence of a s m a l l c o n t r i b u t i o n from t h e S i H i o n . T h i s i o n i s p r o b a b l y t h e r e s u l t o f t h e c o m b i n a t i o n o f s i l i c o n w i t h hydrogen f r o m water v a p o r i n t h e s o u r c e . S i n c e phosphorus i s m o n o i s o t o p i c no o t h e r l i n e s a r e a v a i l a b l e and t h e a n a l y s t must r e s o r t t o t h e d o u b l y i o n i z e d phosphorus l i n e a t m/e 15.5. The u s e o f d o u b l y i o n i z e d l i n e s f o j ^ a n ^ l ^ s i s i n t h i s c a s e i s p o s s i b l e s i n c e compound i o n s such a s S i H n o r m a l l y do n o t e x i s t a s doubly c h a r g e d s p e c i e s . The r e s o l v i n g power o f t h e s e s p e c t r o g r a p h s c a n b e a s h i g h a s 10000. However, u s e o f t h e s l i t s n e c e s s a r y t o a t t a i n t h i s r e s o l u t i o n r e s u l t s i n a marked i n c r e a s e i n exposure t i m e due t o l o w e r i o n transmission. Data I n t e r p r e t a t i o n S e m i q u a n t i t a t i v e e s t i m a t e s o f i m p u r i t y c o n c e n t r a t i o n s a r e made b y comparing t h e exposure n e c e s s a r y t o produce a " j u s t d e t e c t a b l e " l i n e f o r an i s o t o p e o r element o f known c o n c e n t r a t i o n w i t h t h e exposure n e c e s s a r y f o r t h e " j u s t d e t e c t a b l e " l i n e o f t h e unknown i m p u r i t y . T h i s p r o c e d u r e produces s a t i s f a c t o r y r e s u l t s w i t h i n a f a c t o r of three. Attempts t o obtain q u a n t i t a t i v e r e s u l t s e n t a i l emulsion c a l i b r a t i o n , m i c r o p h o t o m e t r y o f l i n e d e n s i t i e s , c o r r e c t i o n s f o r i o n mass and i o n energy, and computer h a n d l i n g o f t h e d a t a . D a t a i n t e r p r e t a t i o n i s d i s c u s s e d i n d e t a i l b y K e n n i c o t t (9) and Owens ( 1 0 ) . R e s u l t s o b t a i n e d b y t h i s p r o c e d u r e v a r y f r o m a c c u r a c i e s o f ± 5% t o d e v i a t i o n s o f o r d e r s o f magnitude f r o m t r u e v a l u e s . A p p l i c a t i o n t o Semiconductor

Technology

SSMS i s used s u c c e s s f u l l y f o r t h e a n a l y s i s o f m a t e r i a l s i n v o l v e d i n a l l phases o f s e m i c o n d u c t o r m a n u f a c t u r i n g . These i n c l u d e semicond u c t o r s such a s s i l i c o n and g a l l i u m a r s e n i d e , h i g h p u r i t y m e t a l s such a s g o l d and aluminum, a c i d s and e t c h a n t s such a s n i t r i c a n d h y d r o f l u o r i c a c i d s , dopants such a s phosphorus t r i c h l o r i d e and b o r o n t r i b r o m i d e , and s o l v e n t s such a s t r i c h l o r o e t h a n e . S t a r t i n g c r y s t a l growth m a t e r i a l s such a s g a l l i u m and a r s e n i c a s w e l l a s c r u c i b l e m a t e r i a l s a r e a l s o s u i t a b l e c a n d i d a t e s f o r SSMS a n a l y s e s . T h i s l i s t i n c l u d e s b o t h c o n d u c t i n g and i n s u l a t i n g s o l i d s a s w e l l a s l i q u i d s . The a n a l y s i s o f s e m i c o n d u c t o r m a t e r i a l s i s i l l u s t r a t e d i n T a b l e s I I and I I I . T a b l e I I shows comparison o f t h e i m p u r i t y c o n t e n t o f a s e r i e s o f s i l i c o n samples. These m a t e r i a l s a r e r e c e i v e d a s chunks ( i n t h e case o f s t a r t i n g m a t e r i a l s ) o r s l i c e s from a c r y s t a l . They a r e c u t t o t h e a p p r o p r i a t e s i z e and e t c h e d f o r c l e a n i n g w i t h h i g h p u r i t y a c i d . The f i g u r e i l l u s t r a t e s h i g h s e n s i t i v i t y a n a l y s i s o f t h r e e p o l y c r y s t a l l i n e s i l i c o n samples w i t h t h e -20 m a t e r i a l showing much h i g h e r aluminum c o n t e n t t h a n t h e o t h e r s . SSMS p e r f o r m s w e l l i n t h i s type of comparative a n a l y s i s .


316

MICROELECTRONICS PROCESSING: INORGANIC MATERIALS


Table I I .

B C Mg Al P CI Ca Cr Mn Fe Cu As

Comparison o f Three S i l i c o n I m p u r i t i e s i n ppm Weight

CHARACTERIZATION

Samples'

-08

-90

-20

Microelectronics Processing - American Chemical Society

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