Measurement of the Oxygen and Carbon Content ... - ACS Publications

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Measurement o f the O x y g e n a n d C a r b o n Content

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on April 5, 2016 | http://pubs.acs.org Publication Date: January 28, 1986 | doi: 10.1021/bk-1986-0295.ch012

o f Silicon Wafers by Fourier Transform I R Spectrophotometry Asian Baghdadi Semiconductor Materials and Processes Division, National Bureau of Standards, Gaithersburg, MD 20899 Fourier transform infrared (FT-IR) spectrophotometry is a rapid, nondestructive characterization technique which is being increasingly applied on a large scale to the routine measurement of the oxygen and carbon content of silicon wafers used for the fabrication of microelectronic devices. Control of the oxygen content is needed to achieve acceptable yields in modern device processing, particularly for those processes which utilize oxide precipitates to protect active regions of devices from contamination by metallic impurities during high-temperature processing. The interlaboratory reproducibility of the measurement is not adequate considering the degree of control of the oxygen that is required. This review focuses primarily on the measurement of oxygen and carbon in silicon and on methods for improving quantitative FT-IR absorption measurements on semiconductor wafers. The evolution of silicon processing technologies for producing very large-scale, high-density integrated circuits has resulted in more demanding starting material specifications, and has increased the need for nondestructive and inexpensive characterization techniques (J_) which can be applied routinely to measure the properties of s i l i con wafers in production quantities. Infrared absorption spectrophotometry, using either dispersive or Fourier transform (FT-IR) instruments, is used for the characterization of both neutral (oxygen and carbon) and electrically active impurities (e.g., group III or group V elements, as well as some oxygen complexes) in silicon. Oxygen and carbon content measurements, which can be made at room temperature without damaging the wafers, are being widely used to control material properties prior to high-temperature processing. This paper describes the configuration of oxygen and carbon impurities in silicon and techniques which can be used to improve the quality and quantitative accuracy of IR absorption spectra obtained on FT-IR instruments. It also discusses instrument-to-instrument 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.

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Oxygen and Carbon Content of Silicon Wafers

v a r i a t i o n s , as w e l l as d i f f e r e n c e s i n the s i l i c o n wafers t h e m s e l v e s , which may a f f e c t the i n t e r l a b o r a t o r y r e p r o d u c i b i l i t y of oxygen c o n t e n t measurements on a s - r e c e i v e d s i l i c o n w a f e r s .


Oxygen and Carbon i n S i l i c o n The most common use of i n f r a r e d a b s o r p t i o n f o r the c h a r a c t e r i z a t i o n of s i l i c o n wafers f o r m i c r o e l e c t r o n i c s p r o c e s s i n g i s f o r the d e t e r m i n a t i o n of the i n t e r s t i t i a l oxygen a n d , t o a much l e s s e r e x t e n t , s u b s t i t u t i o n a l carbon c o n t e n t . C l o s e to the m e l t i n g p o i n t of s i l i c o n , i t i s p o s s i b l e to d i s s o l v e as much as 30 p a r t s p e r m i l l i o n a t o m i c (ppma) oxygen i n s i l i c o n (2)• W e l l below the m e l t i n g p o i n t , howe v e r , the e q u i l i b r i u m s o l i d s o l u b i l i t y i s c o n s i d e r a b l y lower so t h a t , d u r i n g h i g h - t e m p e r a t u r e p r o c e s s i n g , the s u p e r s a t u r a t e d oxygen can condense i n t o a v a r i e t y of complexes and p r e c i p i t a t e s , as shown i n T a b l e I. S i n c e most s e m i c o n d u c t o r s i l i c o n c r y s t a l s a r e p u l l e d from c r u c i b l e s l i n e d w i t h f u s e d s i l i c a u s i n g the C z o c h r a l s k i c r y s t a l growth method, the s i l i c o n melt c o n t a i n s s i g n i f i c a n t amounts of d i s solved oxygen. M o r e o v e r , the d i s t r i b u t i o n of oxygen i n the m e l t i s not u n i f o r m , s i n c e i t depends upon a number of c r y s t a l growth paramet e r s i n c l u d i n g the r o t a t i o n r a t e s of the c r u c i b l e and the c r y s t a l , the melt a s p e c t r a t i o ( i . e . , the r a t i o of the m e l t h e i g h t to the c r u c i b l e d i a m e t e r ) , the p u l l r a t e of the c r y s t a l , temperature f l u c t u a t i o n s i n the m e l t , the c r u c i b l e d i s s o l u t i o n r a t e and the e v a p o r a t i o n of s i l i c o n monoxide from the exposed m e l t s u r f a c e ( 3 0 . F o r t u n a t e l y , the oxygen i n c o r p o r a t i o n has been e m p i r i c a l l y found to be f a i r l y r e p r o d u c i b l e f o r a g i v e n a p p a r a t u s and s e t of growth c o n d i t i o n s , s o t h a t s i l i c o n s u p p l i e r s have been a b l e to produce c r y s t a l s w i t h s p e c i f i e d oxygen c o n t e n t s (over a s u b s t a n t i a l p o r t i o n of the i n g o t ) to meet s p e c i f i c p r o c e s s r e q u i r e m e n t s ( 2 , 4 ) • Typical Czochralski cryst a l s c o n t a i n 10 t o 20 ppma of oxygen, w i t h b o t h r a d i a l and a x i a l macroscopic v a r i a t i o n s . There are a l s o v a r i a t i o n s on a s u b m i l l i m e t e r s c a l e caused by changes i n the i n s t a n t a n e o u s c r y s t a l growth r a t e (5-7)• These m i c r o s c o p i c v a r i a t i o n s i n the oxygen c o n c e n t r a t i o n can i n t r o d u c e s y s t e m a t i c e r r o r s i n the measurement of the oxygen d i s t r i b u t i o n a c r o s s a s i l i c o n wafer ( 8 , 9 ) . C z o c h r a l s k i c r y s t a l s grown u s i n g a d o u b l e - c r u c i b l e t e c h n i q u e (3_), magnetic f i e l d s ( 1 0 ) , o r n i t r i d e - l i n e d c r u c i b l e s (11) have lower and more u n i f o r m oxygen c o n c e n t r a t i o n s , a l t h o u g h these t e c h n i q u e s have n o t y e t been adopted on a commercial s c a l e . Oxygen o c c u p i e s i n t e r s t i t i a l s i t e s i n the s i l i c o n l a t t i c e . This was demonstrated by showing t h a t the l a t t i c e parameter of s i l i c o n c o n t a i n i n g h i g h c o n c e n t r a t i o n s of oxygen was g r e a t e r than t h a t of low-oxygen s i l i c o n (\2_) • S i n c e i s o l a t e d oxygen i n s i l i c o n i s e l e c t r i c a l l y n e u t r a l , i t was proposed t h a t the oxygen atom forms bonds w i t h two n e a r e s t - n e i g h b o r s i l i c o n atoms which g i v e up t h e i r o r i g i n a l bonds w i t h each o t h e r ( 1 3 - 1 5 ) • A t room temperature two a b s o r p t i o n bands a r e o b s e r v e d , a s t r o n g , broad band a t 1107 cm"* and a much weaker band a t 515 cm""* • The 1107 c m " band has been a t t r i b u t e d to an a n t i - s y m m e t r i c s t r e t c h i n g mode of the t r i a t o m i c S i - O - S i d e f e c t " m o l e c u l e " (15), and the 515 c m " mode has been r e c e n t l y p r o p o s e d as a symmetric s t r e t c h i n g m o t i o n , s i m i l a r t o v i b r a t i o n s o c c u r r i n g i n d i s i l o x a n e (16). Near l i q u i d n i t r o g e n t e m p e r a t u r e s , a n o t h e r band appears a t 1205 c m " ( 1 7 ) , as w e l l as some f a r - i n f r a r e d b a n d s . At l i q u i d h e l i u m t e m p e r a t u r e s , the broad band a t 1107 c m " s p l i t s i n t o a 1

1

1

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

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MICROELECTRONICS PROCESSING: INORGANIC MATERIALS CHARACTERIZATION


Table I .

Examples o f t h e E f f e c t s o f A n n e a l i n g on t h e C o n f i g u r a t i o n o f Oxygen i n S i l i c o n

Anneal Time & Temperature

Initial Oxygen Configuration

R e s u l t i n g Oxygen Configuration & i t s Electrical Activity

450°C, 1-10 h r s

Interstitial oxygen

S i O complexes, double donors known as t h e r m a l donors ( T D s )

27

Electrically inactive o x y g e n - s i l i c o n complexes or very small microprecipitates

27

Reference

x

f

650°C, 30 mins

SiO complexes formed a t 450°C

700-800°C up t o 100 h r s

Nucleation centers, s u c h as S i O complexes o r carbon-oxygen complexes

x

x

73 100"A - 1000 A, donors (ND»s)

1000-1150°C, 2-10 h r s

Microprecipitates or other nucleation centers

S i 0 p r e c i p i t a t e s , ~1 Mm, no e l e c t r i c a l a c t i v i t y

74

1350°C, hrs

Any of the above d e f e c t s

I n t e r s t i t i a l oxygen no electrical activity

20

2


12.

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number of s h a r p l i n e s a t 1128 t o 1137 c m " . These l i n e s c o u l d be s u b d i v i d e d i n t o s e t s o f l i n e s whose i n t e n s i t i e s were i n the r a t i o o f the r e l a t i v e abundance of Si-0- Si, Si-0- Si, Si-O- Si, Si-0- Si, S i - O - S i , and S i - O - S i . T h i s r e d u c e s the number of observed l i n e s f o r a s i n g l e s i l i c o n i s o t o p e to three or p o s s i b l y f o u r (}&)• The r e m a i n i n g l i n e s have been e x p l a i n e d as b e i n g due t o a d d i t i o n a l l e v e l s i n the f a r - i n f r a r e d , due t o o s c i l l a t i o n s o f the oxygen atom as i t t u n n e l s between i t s two n e i g b o r i n g s i l i c o n atoms t o e q u i v a l e n t p o s i t i o n s around the a x i s (17)* Figure 1 i s a schem a t i c e n e r g y - l e v e l d i a g r a m , showing t h e a l l o w e d i n f r a r e d o p t i c a l t r a n s i t i o n s o f a model o f the S i - 0 - S i complex, which r e s u l t s i n the m u l t i p l e - l i n e spectrum ( 1 7 ) . 2 8

2 9

2 9

2 9

3 0

2 8

3 0


2 8

2 8

2 9

2 8

3 0

3 0

2 8

E l e c t r i c a l l y a c t i v e d e f e c t s can a l s o e x h i b i t a b s o r p t i o n i n the i n f r a r e d due t o changes i n t h e i r e l e c t r o n i c s t a t e s . F o r example, a c c e p t o r o r donor i m p u r i t i e s , a t c r y o g e n i c t e m p e r a t u r e s , can have a b s o r p t i o n l i n e s due t o t r a n s i t i o n s from t h e ground s t a t e o f t h e n e u t r a l impurity to shallow h y d r o g e n - l i k e or h e l i u m - l i k e l e v e l s l y i n g j u s t below t h e i r r e s p e c t i v e band edges ( 1 9 ) . Thus IR a b s o r p t i o n c a n be used t o d e t e c t the p r e s e n c e o f e l e c t r i c a l l y a c t i v e s i l i c o n - o x y g e n complexes which may form d u r i n g h i g h - t e m p e r a t u r e p r o c e s s i n g ( 2 0 ) . S i l i c o n - o x y g e n complexes can be formed by a n n e a l i n g the wafers a t temperatures such as 4 5 0 ° C (20), as was shown i n T a b l e I. The c o n f i g u r a t i o n o f these complexes has n o t been c l e a r l y d e t e r m i n e d , b u t from t h e k i n e t i c s o f t h e i r f o r m a t i o n they i n v o l v e a r e l a t i v e l y s m a l l number of oxygen atoms, p o s s i b l y t h r e e t o f i v e atoms, f o r m i n g the s i l i c o n - o x y g e n complex. The 4 5 0 ° C complex i s e l e c t r i c a l l y a c t i v e , and i t has been shown t o form donor (21) l e v e l s j u s t below t h e c o n d u c t i o n band e d g e . At c r y o g e n i c temperatures, the c a r r i e r s " f r e e z e o u t " onto the o x i d e complex, so t h a t an IR spectrum of the specimen e x h i b i t s a s e r i e s o f s h a r p i n t e n s e bands (22)• The a b s o r p t i o n p r o c e s s i s shown s c h e m a t i c a l l y i n F i g u r e 2 . The d e t a i l s of the a c t u a l f o r m a t i o n and c o n f i g u r a t i o n o f these d e f e c t s a r e s t i l l t h e s u b j e c t o f a c t i v e debate ( 2 3 - 2 6 ) , b u t t h e i r IR s p e c t r a have been i d e n t i f i e d as b e i n g due t o n e u t r a l h y d r o g e n - l i k e o r s i n g l y - i o n i z e d h e l i u m - l i k e donors (22)• The s p e c t r a have n o t a l l been a s s i g n e d t o a s i n g l e o x y g e n - s i l i c o n complex, b u t r a t h e r t o a s e r i e s o f complexes which come i n t o e x i s t e n c e one a f t e r the o t h e r , and which e x h i b i t d i f f e r e n t a n n e a l i n g b e h a v i o r s (22^. These d e f e c t complexes which a r e known f o r h i s t o r i c a l r e a s o n s as t h e r m a l donors (TD) c a n be d e s t r o y e d by a r e l a t i v e l y s h o r t (~30 minutes) h e a t t r e a t m e n t a t a b o u t 6 5 0 ° C ( 2 7 ) . F u r t h e r l o n g - t e r m h e a t t r e a t m e n t s , a t temperatures r a n g i n g from 550 t o 9 5 0 ° C , c r e a t e a new c l a s s o f d o n o r s , which have been termed new donors (ND) ( 2 8 ) . These donors a r e a l s o n o t f u l l y u n d e r s t o o d , b u t they a r e presumably a type o f o x i d e m i c r o p r e c i p i t a t e . Oxide m i c r o p r e c i p i t a t e s r a n g i n g i n s i z e from 100 A t o 1000 A have been d e t e c t e d i n s i l i c o n by t r a n s m i s s i o n e l e c t r o n m i c r o s c o p y (29,30) f o l l o w i n g h e a t t r e a t m e n t s i n t h a t temperature r a n g e . Long-term a n n e a l i n g a t temperatures above 1 0 0 0 ° C promotes the f o r m a t i o n o f l a r g e r (>1 um) o x i d e p r e c i p i t a t e s . The growth o f the o x i d e p r e c i p i t a t e s r e s u l t s i n a r e d u c t i o n i n t h e i n t e n s i t y o f the i n t e r s t i t i a l oxygen band a t 1107 c m " , and i n the growth of broad new b a n d s , the most prominent of which i s c e n t e r e d a t about 1225-1230 c m " ( 3 1 - 3 3 ) . The s i z e and shape of these p r e c i p i t a t e s , as w e l l as t h e i r c o m p o s i t i o n and c r y s t a l l o g r a p h y , c a n have i m p o r t a n t e f f e c t s on t h e i r IR s p e c t r a ( 3 1 ) . F o r m a t i o n of the p r e c i p i t a t e s g e n e r a l l y a l s o r e s u l t s i n the c r e a t i o n 1

1


212



cm

1

| n, i>

I 2,0> | 2, ± 2>

1203-0 1189-0

|1,0> |0, ± 2>

| 1, ± 1>

1157-2

|0, ± 1>

| 0, 0>

1136-0

|0,0>

v< 2

9 [xm bands

517-0

1, ± 1> 0, ± 3>

I 3, ± 1> | 3, ± 3>

120-4 110-4

I 2, 0> | 2, ± 2>

783 671

I 1,0> I 0, ± 2>

| 1, ± 1>

29-3

| 0, ± 1>

0

| 0, 0>

V

2

I 0, 0>

central force labelling

harm. osc. labelling Figure

1.

E n e r g y - l e v e l diagram f o r i n t e r s t i t i a l oxygen i n s i l i c o n , showing o b s e r v e d v i b r a t i o n a l t r a n s i t i o n s i n t h e n e a r and f a r i n f r a r e d f o r a S i - 0 - S i complex. (Reproduced w i t h p e r m i s s i o n from Ref. 17. C o p y r i g h t 1970 The R o y a l S o c i e t y . ) 2 8

2 8


12.

BAGHDADI

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of networks o f d i s l o c a t i o n s around the p r e c i p i t a t e s . These d i s l o c a t i o n s can serve to t r a p f a s t - d i f f u s i n g m e t a l l i c i m p u r i t i e s . This i s the b a s i c mechanism which i s used i n the i n t r i n s i c g e t t e r i n g (IG) o f u n d e s i r a b l e i m p u r i t i e s t o improve the y i e l d o f s e m i c o n d u c t o r f a b r i c a t i o n p r o c e s s e s (21)• Carbon i n s i l i c o n i s a much s i m p l e r and b e t t e r u n d e r s t o o d d e fect. S i n c e , a t room t e m p e r a t u r e , i t o c c u p i e s s u b s t i t u t i o n a l s i t e s and s i n c e i t i s i s o e l e c t r o n i c w i t h the s i l i c o n atoms i n the h o s t l a t t i c e , i s o l a t e d c a r b o n atoms have o n l y s u b t l e o r i n d i r e c t e f f e c t s on the p r o p e r t i e s of the s i l i c o n . I t s i n f r a r e d spectrum i s s t r a i g h t f o r w a r d , c o n s i s t i n g of a s i n g l e l o c a l v i b r a t i o n a l mode f o r each i s o tope. The C absorbance band i s c o m p l e t e l y dominated by a t w o - p h o non s i l i c o n l a t t i c e band a t 610 c m " (see I n f r a r e d A b s o r p t i o n Measurements )• The i n t e n s i t y o f the two-phonon band i s v e r y s t r o n g l y dependent on t e m p e r a t u r e , as shown i n F i g u r e 3. The open t r i a n g l e s r e p r e s e n t the i n t e n s i t y of the s i l i c o n l a t t i c e band a t 610 c m " , w h i l e the c l o s e d t r i a n g l e r e p r e s e n t s the a b s o r p t i o n c o e f f i c i e n t due t o s u b s t i t u t i o n a l carbon a t 300 K f o r a nominal c a r b o n c o n c e n t r a t i o n of 1 ppma. Thus the temperature o f the r e f e r e n c e and specimen w a f e r s must be k e p t w i t h i n 1 ° C o f each o t h e r i n o r d e r t o m a i n t a i n a good match between t h e specimen and r e f e r e n c e s p e c t r a . T h i s degree o f temperature c o n t r o l i s n o t n e c e s s a r y f o r the measurement o f the i n t e r s t i t i a l oxygen peak h e i g h t , s i n c e t h e a b s o r p t i o n due t o t h e s i l i con l a t t i c e band a t 1120 c m " (shown as the open c i r c l e s i n F i g u r e 3) i s much s m a l l e r than the a b s o r p t i o n due t o i n t e r s t i t i a l oxygen f o r a wafer c o n t a i n i n g 20 ppma oxygen (shown as the c l o s e d c i r c l e i n F i g u r e 3). However, when the r a t i o o f the a b s o r p t i v i t y o f the oxygen band t o the a b s o r p t i v i t y of the 610 c m " s i l i c o n band (3j4) i s used f o r the c o m p u t a t i o n o f the oxygen c o n t e n t , the temperature dependence o f t h e s i l i c o n l a t t i c e band must be taken i n t o a c c o u n t . 1

2


1

1

1

1

The g r e a t r e d u c t i o n i n the i n t e n s i t y o f t h e i n t e r f e r i n g 610 c m " s i l i c o n l a t t i c e band a t low temperatures might be e x p e c t e d t o l e a d t o i n c r e a s e d s e n s i t i v i t y and a c c u r a c y i n c a r b o n d e t e r m i n a t i o n s based on c r y o g e n i c IR measurements. However, the r e f l e c t i o n l o s s e s a t each of the c r y o s t a t windows a l m o s t outweigh t h i s advantage (35)• The r e m a i n i n g advantage i s u s u a l l y t o o s l i g h t t o j u s t i f y the a d d i t i o n a l time f o r c o o l i n g t h e sample and f o r p r e p a r i n g s p e c i a l s m a l l specimens n e c e s s a r y f o r most l o w - t e m p e r a t u r e measurements. The d i s t r i b u t i o n of c a r b o n i n s i n g l e - c r y s t a l s i l i c o n i s n o t uniform (36). S i n c e the carbon atom i s s m a l l e r than the s i l i c o n atom i t r e p l a c e s , h i g h l o c a l c o n c e n t r a t i o n s o f c a r b o n c a n produce a l o c a l i z e d s t r a i n i n the l a t t i c e , which can s e r v e as n u c l e a t i o n c e n t e r s f o r the f o r m a t i o n o f s t r u c t u r a l d e f e c t s such as " s w i r l " p a t t e r n s ( 3 6 ) . For t h i s r e a s o n , f l o a t - z o n e d s i l i c o n i n t e n d e d f o r the f a b r i c a t i o n o f s e m i c o n d u c t o r power d e v i c e s i s o f t e n s p e c i f i e d t o c o n t a i n low c a r b o n c o n c e n t r a t i o n , e . g . , l e s s than 1 ppma. Carbon has been r e p o r t e d t o p l a y a r o l e i n t h e heterogeneous n u c l e a t i o n o f o x i d e p r e c i p i t a t e s (37-40). In p r i n c i p l e , c a r b o n n u c l e a t i o n o f o x i d e m i c r o p r e c i p i t a t e s c o u l d be used t o advantage i n d e s i g n i n g IC p r o c e s s e s . In p r a c t i c e , however, the n o n u n i f o r m i t y o f the carbon d i s t r i b u t i o n r e n d e r s t h i s an i m p r a c t i c a l approach. Thus i n o r d e r t o a v o i d u n c o n t r o l l e d v a r i a t i o n s i n the oxide p r e c i p i t a t i o n , l o w - c a r b o n s i l i c o n i s o f t e n s p e c i f i e d f o r p r o c e s s e s which use i n t r i n s i c g e t t e r i n g . Low-carbon s i l i c o n must a l s o be used f o r the f a b r i c a t i o n of e x t r i n s i c i n f r a r e d d e t e c t o r s t o p r e v e n t the f o r m a t i o n o f c a r b o n -


1

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Conduction Band

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Bound States

'//////////////////////A (b)T ~ 1 5 ° K , No Illumination

V///////////////////// (c) T ~ 15 °K, IR Illumination: Characteristic Absorption Lines

F i g u r e 2. E n e r g y - l e v e l diagram showing t h e a b s o r p t i o n o f r a d i a t i o n i n n-type s i l i c o n under v a r i o u s c o n d i t i o n s : (a) room temp e r a t u r e , ( b ) same specimen a t c r y o g e n i c t e m p e r a t u r e s , w i t h o u t any i l l u m i n a t i o n , and ( c ) same specimen a t c r y o g e n i c t e m p e r a t u r e s , w i t h sub-bandgap i l l u m i n a t i o n , showing c h a r a c t e r i s t i c a b s o r p t i o n o f t h e IR photons from t h e ground s t a t e o f t h e donor t o bound s t a t e s j u s t below t h e c o n d u c t i o n band.

14

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12

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where I ( v ) i s the i n t e n s i t y of the t r a n s m i t t e d l i g h t a t wavenumber v r e a c h i n g the d e t e c t o r , x i s the t h i c k n e s s of the p a r a l l e l p l a t e , T and R are the n o n s c a t t e r e d t r a n s m i t t a n c e and the n o n s c a t t e r e d ( s p e c u l a r ) r e f l e c t a n c e , r e s p e c t i v e l y , w i t h the s u b s c r i p t s f and b s p e c i f y i n g the f r o n t and back s u r f a c e s of the s p e c i m e n , I ( v ) i s the i n t e n s i t y of the i n c i d e n t r a d i a t i o n and a ( v ) i s the a b s o r p t i o n c o e f f i c i e n t of the specimen a t wavenumber v . E q u a t i o n 1 has been r e w r i t t e n from i t s u s u a l form to a c c o u n t f o r the e f f e c t s o f u n p o l i s h e d f r o n t o r back s u r f a c e s which may s c a t t e r the r a d i a t i o n i n a d d i t i o n to t r a n s m i t t i n g or r e f l e c t i n g i t . F o r the case o f a s i l i c o n wafer w i t h b o t h f r o n t and back s u r f a c e s p o l i s h e d , T = 0.7 and R = 0 . 3 , so t h a t a l m o s t 10% o f the t r a n s m i t t e d r a d i a t i o n i n t r a n s p a r e n t r e g i o n s o f the s p e c trum p a s s e s through the specimen more than o n c e . A l t h o u g h specimens p r e p a r e d e s p e c i a l l y f o r q u a n t i t a t i v e IR measurements would n o r m a l l y have b o t h s u r f a c e s p o l i s h e d , t y p i c a l p r o d u c t i o n s i l i c o n w a f e r s commonly have o n l y a s i n g l e p o l i s h e d s u r f a c e . The o p p o s i t e s u r f a c e i s g e n e r a l l y lapped and e t c h e d , e i t h e r w i t h an a c i d o r a l k a l i n e e t c h ( 5 4 ) , o r sometimes d e l i b e r a t e l y damaged, e i t h e r m e c h a n i c a l l y or with a high-power l a s e r . The d e l i b e r a t e l y damaged back s u r f a c e s a r e used to c r e a t e d i s l o c a t i o n networks i n the s i l i c o n w a f e r , away from the a c t i v e r e g i o n s of the d e v i c e s , which can t r a p or " g e t t e r " h e a v y - m e t a l i m p u r i t i e s , and t h e r e b y improve the o v e r a l l y i e l d o f the f a b r i c a t i o n p r o c e s s (55)• F i g u r e 6 i s the a b s o r p t i o n spectrum of a s i l i c o n wafer w i t h a rough back s u r f a c e . The s t r o n g l y s l o p i n g b a s e l i n e i s due to the wavenumber dependence o f the s c a t t e r i n g a t the back s u r f a c e . In t h e s e c a s e s , the a b s o r p t i o n c o e f f i c i e n t due to an i m p u r i t y , i n a r e g i o n w i t h o u t s i l i c o n l a t t i c e b a n d s , i s g i v e n by (53):


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The c o r r e c t i o n f a c t o r S i s c l o s e t o z e r o f o r a wafer w i t h a v e r y rough back s u r f a c e , such as the one whose spectrum i s shown i n F i g u r e 6, and i s 0.09 f o r a l i g h t l y - d o p e d ( e . g . , r e s i s t i v i t y 5 ft cm o r g r e a t e r ) wafer w i t h both s u r f a c e s p o l i s h e d . The back s u r f a c e s of most w a f e r s , however, f a l l between t h e s e two e x t r e m e s , so t h a t S must be d e t e r m i n e d i n o r d e r to measure a c c u r a t e l y the c o n c e n t r a t i o n o f an #



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impurity. S e v e r a l methods have been p r o p o s e d t o o b t a i n the m u l t i p l e r e f l e c t i o n correction factor S (53,56-58). I f t h e wafer has a u n i form t h i c k n e s s , the r a t i o o f the a m p l i t u d e s o f the s e c o n d a r y t o p r i mary i n t e r f e r o g r a m s c a n be used t o o b t a i n S (53^) • A l t e r n a t i v e l y , t h e v a l u e o f the b a s e l i n e i n a t r a n s p a r e n t r e g i o n o f the spectrum can a l s o be used t o d e t e r m i n e the v a l u e o f S ( 5 6 ) • This requires that the d e t e c t o r system l i n e a r i t y be w e l l - e s t a b l i s h e d , the r e f l e c t i o n s between the specimen s u r f a c e s and the s p e c t r o m e t e r components be u n d e r s t o o d q u a n t i t a t i v e l y , and t h e r e be no o t h e r p r o b l e m s , such as a s h i f t o f t h e p o s i t i o n o f the r a d i a t i o n a t the d e t e c t o r when the b a c k ground i n t e r f e r o g r a m i s b e i n g o b t a i n e d . A t h i r d method uses the magnitude o f t h e a p p a r e n t absorbance o b t a i n e d f o r a m u l t i p h o n o n s i l i con l a t t i c e absorbance band o f known v a l u e as a c a l i b r a t i o n t o c a l c u l a t e the t r u e v a l u e o f the absorbance due t o the i m p u r i t y (51) • This method, however, c a n be q u a n t i t a t i v e o n l y when the s c a t t e r i n g i s n o t dependent upon t h e wavelength o f the r a d i a t i o n . This i s not g e n e r a l l y the c a s e , as i s i l l u s t r a t e d i n F i g u r e 6 . Once the v a l u e o f S has been d e t e r m i n e d by one o f t h e s e methods, E q u a t i o n 2 c a n be used t o o b t a i n the a b s o r p t i o n c o e f f i c i e n t o f the s p e c i m e n . Measurements o f t h e Oxygen C o n t e n t o f S i l i c o n Wafers The s h o r t - t e r m r e p r o d u c i b i l i t y o f i n f r a r e d a b s o r p t i o n s p e c t r o s c o p y i n the d e t e r m i n a t i o n of the oxygen c o n t e n t o f a t y p i c a l p r o d u c t i o n s i l i con wafer c a n be g o o d , as shown i n F i g u r e 7. These d a t a r e p r e s e n t t w e n t y - f o u r c o n s e c u t i v e measurements o f a s i n g l e s p o t on a s i l i c o n wafer w i t h an e t c h e d back s u r f a c e . The d a t a were o b t a i n e d a t 4 c m " r e s o l u t i o n on an F T - I R s p e c t r o p h o t o m e t e r equipped w i t h a mercury cadmium t e l l u r i d e d e t e c t o r , a KBr/Ge b e a m s p l i t t e r and a G l o b a r (59) IR s o u r c e . A 1 mm a p e r t u r e was p l a c e d i n f r o n t o f the s i l i c o n w a f e r , and 200 scans were taken f o r each d a t a p o i n t . Unfortunately, this degree o f r e p r o d u c i b i l i t y o n l y demonstrates the p o t e n t i a l p r e c i s i o n of F T - I R measurements. In f a c t , v a r i a t i o n s o f up t o 20% have been r e p o r t e d (60-62)• These v a r i a t i o n s can o c c u r i n the measurement o f the same specimen by d i f f e r e n t l a b o r a t o r i e s o r i n t h e a n a l y s i s of a g i v e n specimen b e f o r e and a f t e r i t has undergone d i f f e r e n t b a c k surface treatments. E x p e r i e n c e s of t h i s k i n d have caused a l a c k o f c o n f i d e n c e i n the p r e c i s i o n o f the IR method f o r the d e t e r m i n a t i o n o f the oxygen c o n t e n t o f p r o d u c t i o n s i l i c o n w a f e r s . T h i s problem has been a s c r i b e d t o d i f f e r e n c e s among the wide v a r i e t y o f i n s t r u m e n t models and computer s o f t w a r e packages used i n the measurements (610 • V a r i a t i o n s i n the specimen c o n d i t i o n , i . e . , dopant l e v e l , b a c k s u r f a c e t r e a t m e n t o r oxygen d i s t r i b u t i o n have a l s o been a s o u r c e o f poor r e p r o d u c i b i l i t y ( 8 , 9 , 5 2 , 5 3 , 6 3 ) . 1

S h i v e and S c h u l t e (61) have d e s c r i b e d a r e f e r e n c e m a t e r i a l s a p p r o a c h t o improve the p r e c i s o n o f the IR a n a l y s i s method. The r e f e r e n c e s i l i c o n wafers were g i v e n one o f the s t a n d a r d b a c k - s u r f a c e treatments: a c i d e t c h e d ( A E ) , b a c k - s u r f a c e damaged (BSD), o r e n hanced g e t t e r e d ( E G ) . The EG wafers were a c i d - e t c h e d w a f e r s w i t h a p o l y c r y s t a l l i n e s i l i c o n l a y e r d e p o s i t e d on t h e i r rough s i d e . Each wafer was q u a r t e r e d and then one of t h e q u a r t e r s e c t i o n s was p o l i s h e d on the back s u r f a c e . An oxygen c o n t e n t was a s s i g n e d t o each wafer by a n a l y z i n g t h i s d o u b l e - s i d e p o l i s h e d (DSP) s e c t i o n . The oxygen c o n t e n t s were determined a u t o m a t i c a l l y from the t r a n s f o r m e d s p e c t r a u s i n g computer programs w r i t t e n by the i n s t r u m e n t ' s m a n u f a c t u r e r .



222


500

650

800

950

1100

1250 1400

WAVENUMBERS (cm' ) 1

Figure 6.

Spectrum o f a s i l i c o n w a f e r w i t h a back s u r f a c e w h i c h was m e c h a n i c a l l y damaged t o p r o v i d e g e t t e r i n g s i t e s .

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£ 8 0.0

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J_ 8.0

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I -L 16.0 20.0 24.0

28.0

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Measurement Number

F i g u r e 7.

R e p r o d u c i b i l i t y o f c o n s e c u t i v e measurements o f t h e same spot on a C z o c h r a l s k i s i l i c o n wafer c o n t a i n i n g a p p r o x i m a t e l y 7.3 x i o i n t e r s t i t i a l oxygen atoms/cm^ 1 7



12.

BAGHDADI

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The b i a s of the d a t a , which was d e f i n e d as the d i f f e r e n c e between the oxygen c o n t e n t d e t e r m i n e d f o r the DSP s e c t i o n and the oxygen c o n t e n t d e t e r m i n e d f o r the s i n g l e - s i d e p o l i s h e d (SSP) s e c t i o n s , was d e t e r mined f o r each type o f back surface* c o n d i t i o n . I t s h o u l d be n o t e d t h a t the oxygen c o n c e n t r a t i o n s quoted i n S h i v e and S h u l t e ' s paper were based upon the " o l d " ASTM c a l i b r a t i o n c o e f f i c i e n t ( 6 4 ) , and have been c o n v e r t e d here t o v a l u e s u s i n g the most c u r r e n t ASTM c o e f f i c i e n t (48). T h i s method t u r n e d o u t t o be s u c c e s s f u l w i t h the BSD w a f e r s , b u t n o t w i t h the AE o r EG wafers (61)• The r e a s o n f o r the f a i l u r e w i t h t h e AE and EG back s u r f a c e c o n d i t i o n s was a t t r i b u t e d t o the d r a m a t i c v a r i a b i l i t y o f the s c a t t e r i n g o f these w a f e r s . In these c a s e s , a method f o r o b t a i n i n g the m u l t i p l e r e f l e c t i o n c o r r e c t i o n d i r e c t l y from the specimen i n t e r f e r o g r a m o r s p e c t r u m , such as one o f the methods d e s c r i b e d i n the s e c t i o n on I n f r a r e d A b s o r p t i o n M e a s u r e ments, s h o u l d be u s e d . The c a l i b r a t i o n method a l s o worked w e l l w i t h DSP w a f e r s . Thus t h e c a l i b r a t i o n method was s u c c e s s f u l e i t h e r when the f u l l m u l t i p l e r e f l e c t i o n c o r r e c t i o n c o u l d be a p p l i e d , i . e . , i n DSP w a f e r s , o r when t h e back s u r f a c e was so rough t h a t the magnitude o f the m u l t i p l e - r e f l e c t i o n c o r r e c t i o n was v e r y s m a l l , f o r example, i n the c a s e f o r BSD w a f e r s . Huber and S t a l l h o f e r r e a c h e d s u b s t a n t i a l l y s i m i l a r conclusions i n a recent a r t i c l e (65). High c o n c e n t r a t i o n s o f f r e e c a r r i e r s have i m p o r t a n t e f f e c t s on the p r e c i s i o n o f the oxygen c o n t e n t d e t e r m i n a t i o n i n s i l i c o n wafers by IR s p e c t r o p h o t o m e t r y (63). A b s o r p t i o n o f t h e IR beam by the f r e e c a r r i e r s (see F i g u r e 8) can reduce the s i g n a l - t o - n o i s e r a t i o o f the measurement u n t i l , f o r low r e s i s t i v i t y w a f e r s , m e a n i n g f u l m e a s u r e ments can no l o n g e r be made. The f r e e c a r r i e r s a l s o a f f e c t the r e f l e c t a n c e o f t h e s i l i c o n , thus c h a n g i n g the magnitude o f the m u l t i ple-reflection coefficient. However, s i n c e wafers w i t h r e s i s t i v i t i e s much below 0.1 ft*cm c a n n o t be measured, the e r r o r due t o t h i s e f f e c t i s l e s s than 1%. F i g u r e 9 shows the c o n t r i b u t i o n o f the f r e e c a r r i e r s t o t h e b a s e l i n e f o r the case o f n - t y p e s i l i c o n , as c a l c u l a t e d by Weeks ( 6 3 ) . These c a l c u l a t e d b a s e l i n e s proved t o be an e x c e l l e n t s i m u l a t i o n o f the b a s e l i n e s o f t r a n s m i s s i o n s p e c t r a o b t a i n e d f o r a s e r i e s o f n - t y p e s i l i c o n samples w i t h f r e e c a r r i e r c o n c e n t r a t i o n s r a n g i n g from 5 x 1 0 donors/cm to 5 x 1 0 d o n o r s / c m . The e f f e c t of t h i s c u r v i n g b a s e l i n e must be taken i n t o a c c o u n t i f the q u a n t i t a t i v e a n a l y s i s o f the spectrum r e l i e s on the r a t i o o f the oxygen peak a t 1107 c m " t o one o f the s t r o n g s i l i c o n l a t t i c e absorbance bands a t lower wavenumbers. 1 3

3

1 8

3

1

Some of the l a c k o f i n t e r l a b o r a t o r y r e p r o d u c i b i l i t y may be caused by t h e use o f d i f f e r e n t i n s t r u m e n t s . V a r i a t i o n s i n the oxygen c o n t e n t d e t e r m i n a t i o n c a n be due t o the use of d i f f e r e n t a p o d i z a t i o n f u n c t i o n s i n d i f f e r e n t FT-IR instruments, to d i f f e r e n t designs of t h e i r o p t i c a l benches or s i m p l y t o d i f f e r e n c e s i n the performance of t h e i r components. A p o d i z a t i o n f u n c t i o n s a r e used t o reduce the amp l i t u d e o f a r t i f a c t s i n the t r a n s f o r m e d s p e c t r a , caused by the t r u n c a t i o n o f t h e i n t e r f e r o g r a m s by t h e i n s t r u m e n t . Four o f t h e most commonly used a p o d i z a t i o n f u n c t i o n s a r e b o x c a r , c o s i n e , H a p p - G e n z e l , and t r i a n g u l a r ( 6 6 ) . The e f f e c t o f t h e s e d i f f e r e n t a p o d i z a t i o n f u n c t i o n s on the magnitude of the oxygen absorbance i s shown i n F i g u r e 10 (66)• Note t h a t t h e s e d a t a a r e n o t i n t e n d e d t o be used t o compare the peak h e i g h t a t one r e s o l u t i o n t o the peak h e i g h t a t a n o t h e r r e s o lution. They s h o u l d o n l y be used a t a g i v e n r e s o l u t i o n , t o compare the e f f e c t s of d i f f e r e n t a p o d i z a t i o n f u n c t i o n s a t t h a t r e s o l u t i o n .


224


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16 -

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8

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3

10

4

10

5

Resistivity (ohm-cm) F i g u r e 8.

E r r o r , based upon two s t a n d a r d d e v i a t i o n s o f a s e t o f 10 measurements, i n the d e t e r m i n a t i o n of the i n t e r s t i t i a l oxygen c o n t e n t of s i l i c o n a s a f u n c t i o n o f r e s i s t i v i t y f o r b o t h n - and p - t y p e s i l i c o n , e x p r e s s e d as a p e r c e n t a g e of the oxygen c o n c e n t r a t i o n . (Reproduced w i t h p e r m i s s i o n from R e f . 62. C o p y r i g h t 1983 ASTM.)



12.

BAGHDADI

Figure

9.

225


C a l c u l a t e d a b s o r p t i o n d u e t o f r e e - c a r r i e r s i n 500-um t h i c k n-type s i l i c o n c o n t a i n i n g 5 x 1 0 t o 1 x io donors/cm ( R e p r o d u c e d w i t h p e r m i s s i o n f r o m R e f . 62. Copyright 1983 ASTM.) 1

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4

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Resolution (Wavenumbers) Figure

10. T h e e f f e c t o f t h e a p o d i z a t i o n f u n c t i o n o n t h e r e l a t i v e m a g n i t u d e o f t h e a b s o r b a n c e b a n d a t 1107 c m due t o i n t e r s t i t i a l oxygen i n s i l i c o n . These d a t a were o b t a i n e d o n a 0.87-mm t h i c k s i l i c o n w a f e r p o l i s h e d o n b o t h s i d e s . The e r r o r bars p l o t t e d on the t r i a n g u l a r apodization curve c a n a l s o be a p p l i e d t o t h e o t h e r c u r v e s . They r e p r e s e n t t h e greatest standard d e v i a t i o n f o r any o f the data sets ( c o s i n e , Happ-Genzel, o r t r i a n g u l a r ) obtained a t t h a t r e s o l u t i o n (65). - 1


226


The oxygen determination is most often carried out at a resolution of 4 cm" ; at this resolution, the use of triangular apodization results in a reduction of the peak height by about 4%, compared to the peak height obtained by the use of boxcar apodization. The effects of beam geometry and polarization have also been calculated (66), and found not to be significant, except in the most extreme cases. For example, the change in the reflectance of a silicon wafer for a vertical, columnar IR beam with a maximum angle of incidence at the sample of 10 deg, polarized horizontally, is only 0.6%, compared to the case of an unpolarized beam. On the other hand, detector system nonlinearity in one instrument was found to be in part responsible for the lack of precision in the oxygen content determination (66)• Therefore, the lack of interlaboratory reproducibility is apparently not due to different optical bench designs in the different instruments, but rather is attributed to differences in the software or to the performance of the detector systems. The American Society for Testing and Materials (ASTM) recommends obtaining the interstitial oxygen concentration in atoms/cm by multiplying the peak absorptivity at 1107 cm" by 2.45 x 10 (48); the same conversion coefficient is also used by the German standards organization, Deutsches Institut fttr Normung (DIN) (48,67). This value was obtained by calibrating the infrared absorptivity using vacuum fusion analysis. However, the Japan Electronics Development Association (JEIDA), using charged-particle activation analysis, has obtained a significantly different value, 3.0 x 10 (68). The ASTM and DIN conversion coefficient for obtaining the substitutional carbon content from the peak absorptivity at 605 cm" is 1.0 x 10 (69) • This coefficient was determined using deuteron activation analysis as the absolute analytical method (70)•


1

3

1

17

17

1

17

Conclusions Fourier transform infrared spectrophotometry is used widely in the semiconductor industry for the routine determination of the interstitial oxygen content of production silicon wafers. However, the lack of interlaboratory reproducibility in this method has forced the use of ad-hoc calibration methods. The sources of this lack of reproducibility are just beginning to be understood. As investigation of this problem continues and wider acceptance is gained for improved experimental and analytical techniques, a greater degree of reproducibility should be achieved. Furthermore, new standard test methods and standard reference materials being developed by the ASTM (TV), DIN (72^), JEIDA (Jl) r Bureau Centrale de Materiaux References (73) should also significantly improve the precision of the impurity content determination. a n d

t n e

Literature Cited 1. 2.

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Measurement of the Oxygen and Carbon Content ... - ACS Publications

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