Microelectronics Processing - American Chemical Society

Downloaded by NANYANG TECHNOLOGICAL UNIV on June 8, 2016 | http://pubs.acs.org Publication Date: January 28, 1986 | doi: 10.1021/bk-1986-0295.ch023

23 Effects o f Various Chemistries o n Silicon-Wafer Cleaning D. Scott Becker, William R. Schmidt, Charlie A. Peterson, and Don C. Burkman FSI Corporation, Chaska, MN 55318

This paper reviews the current knowledge on aqueous cleaning of s i l i c o n wafers. Some new information regarding the chemical action of the most common cleaning solutions is revealed. Information on individual solutions as well as the sequential use of more than one solution is presented and discussed. Through the use of Secondary Ion Mass Spectrometry and Laser Defect Scanning, the best overall cleaning was accomplished through the sequential use of the four cleaning solutions (mixed in parts per volume) 4H SO (96%):1H O (31%), 100H O:1HF(99%), 5H O:1H O (31%) :lNH OH( 29%), and 5H O:1H O (31%) :1HCl(3%) in that order. It was also found that reversing the order of use of the hydrofluoric acid and ammonium hydroxide hydrogen peroxide solutions yielded a wafer surface lower in metal contamination yet higher in particle contamination. 2

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The performance of semiconductor devices may be altered by the presence of contamination before, during and after device fabrication. Because of t h i s , i t is necessary to achieve the highest degree of cleanliness possible during semiconductor wafer processing. There are many cleaning procedures and the most desirable process w i l l depend on which step in device fabrication i t follows and which step i t precedes. This paper reviews the current 0097-6156/ 86/0295-0366$06.00/0 © 1986 American Chemical Society Casper; Microelectronics Processing: Inorganic Materials Characterization ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

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u n d e r s t a n d i n g o f t h e m o s t common s i l i c o n w a f e r c l e a n i n g p r o c e d u r e s a n d r e v e a l s new i n f o r m a t i o n o n t h e i r c h e m i c a l behavior. Most p r o c e s s e s i n v o l v e s e q u e n t i a l u s e o f several cleaning solutions. The s e q u e n c i n g i so f g r e a t s i g n i f i c a n c e and w i l l be d i s c u s s e d i n d e t a i l .


Contaminants C o n t a m i n a n t s o n a s u r f a c e may b e c l a s s i f i e d a s m o l e c u l a r , ionic o r atomic. The m o l e c u l a r c o n t a m i n a t i o n , p r i m a r i l y o r g a n i c , c o u l d be o i l s , waxes ( f r o m m o u n t i n g , slicing, e t c . ) , p h o t o r e s i s t , p a r t i c l e s from wafer c a r r i e r s and boxes and a i r b o r n e hydrocarbons. Complications created by t h e s e c o n t a m i n a n t s w i l l depend on t h e p r o c e s s t h a t would f o l l o w . These contaminants c o u l d r e s u l t i n poor adhesion o f d e p o s i t e d m a t e r i a l s , changes i n o x i d a t i o n and E P I d e p o s i t i o n r a t e s as w e l l as l e a d t o t h e f o r m a t i o n of c r y s t a l l i n e d e f e c t s . I n a d d i t i o n , the hydrophobic n a t u r e o f some t y p e s o f o r g a n i c m a t e r i a l i n h i b i t s t h e i n t e r a c t i o n o f aqueous c l e a n i n g s o l u t i o n s w i t h the wafer s u r f a c e , t h e r e f o r e , l i m i t i n g removal o f i o n i c and atomic contaminants. F o rthese reasons, t h eremoval o f o r g a n i c c o n t a m i n a t i o n i s u s u a l l y t h ef i r s t s t e p o f a c l e a n i n g process. The presence o f an o x i d e l a y e r on a s i l i c o n wafer i s i n e v i t a b l e i f the wafer has been exposed t oan oxygen o r water atmosphere. O x i d i z i n g agents such as H2O2 w i l l also oxidize a s i l i c o n wafer. I t might be n e c e s s a r y t o r e m o v e some o f t h i s o x i d e i n o r d e r t o e a s e the removal o f entrapped contaminants. Presence o f cont a m i n a t i o n c a n be d e t r i m e n t a l t o a p r o c e s s i n g s t e p . An example would be t h e i n t e r f e r e n c e t h a t p a r t i c l e s cause d u r i n g F.PI d e p o s i t i o n ( 1 , 1 2 , 1 3 ) « I o n i c a n d a t o m i c c o n t a m i n a t i o n may come f r o m t h e environment o r r e s u l t from t h e use o f p r o c e s s i n g chemicals. The a d s o r p t i o n o f h a l i d e i o n s ( 2 , 3 ) and m e t a l c a t i o n s (3.) f r o m common p r o c e s s i n g r e a g e n t s h a v e b e e n investigated. I t w a s f o u n d t h a t i o n s may b e a d s o r b e d i n c o n c e n t r a t i o n s as g r e a t as 1 0 ? ions/cm . Oxidized w a f e r s a d s o r b e d m o r e Na+ t h a n d i d b a r e s i l i c o n w a f e r s (2). A d s o r p t i o n o f some m e t a l s s u c h a s g o l d a n d s i l v e r were more d r a m a t i c i n HF. B e c a u s e a w a f e r w i t h o u t a n o x i d i z e d s u r f a c e i s i ngood e l e c t r i c a l c o n t a c t w i t h an aqueous s o l u t i o n i t i s capable o f e l e c t r o c h e m i c a l reduct i o n o f some m e t a l i o n s . This reverse plating results i n a t o m i c m e t a l c o n t a m i n a t i o n . T h i s i d e a was r e i n f o r c e d when i t was f o u n d t h a t t h e a d s o r p t i o n o f g o l d o n s i l i c o n was t h r e e t o f o u r o r d e r s o f m a g n i t u d e g r e a t e r t h a n o n silica (3). Ions and a t o m i c m e t a l s c o n t a m i n a t i o n c a n be d e t r i mental t o device performance f o r a v a r i e t y o f reasons. S m a l l i o n s may m i g r a t e u n d e r t h e i n f l u e n c e o f a n e l e c t r i c f i e l d o r a t high temperature. Metals w i l l also diffuse 1

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at high temperatures. Ionic contamination can result i n d r i f t c u r r e n t s and s u r f a c e p o t e n t i a l d r i f t i n g (4,5). Heavy m e t a l s c a n a f f e c t m i n o r i t y c a r r i e r l i f e t i m e s and s u r f a c e r e c o m b i n a t i o n v e l o c i t i e s (6,7)• Both types o f contamination can a f f e c t t h eformation o f accumulation and d e p l e t i o n l a y e r s .


Cleaning

Chemistries

P e d e r a n d K o o n t z (8) a n d P e t e r s a n d D e c k e r t (9.) h a v e e v a l u a t e d v a r i o u s p h o t o r e s i s t s t r i p p i n g methods. Their eval u a t i o n s were based on a b i l i t y t o remove p h o t o r e s i s t as w e l l as t h ea b i l i t y t o leave a c r i t i c a l l y c l e a n s u r f a c e . Some o f t h e m e t h o d s e v a l u a t e d w e r e 1) o x i d i z i n g a g e n t s s u c h a s H 0 2 , HC1, H2SO4, HNO3, HCI-H2O2, H2SO4-H2O2; 2) c a u s t i c s s u c h a s NH4OH, H2O2-NH4OH; 3) s o l v e n t s y s t e m s s u c h a s C C I 4 , J - 1 0 0 , A - 2 0 , CH3COCH3, CH3COC2H5 a n d 4) ashing ( a i r ) and plasma s t r i p p i n g . Both reports found t h a t some s y s t e m s w e r e g o o d f o r p h o t o r e s i s t s t r i p p i n g a n d o t h e r s were good f o r c r i t i c a l c l e a n i n g . The o r g a n i c s o l v e n t s y s t e m s w e r e g o o d f o r s t r i p p i n g p o s i t i v e r e s i s t but d i dnot leave a clean surface. Ashing and plasma s t r i p p i n g were v e r y e f f e c t i v e on negat i v e r e s i s t b u t they t o od i d n o t y i e l d a c l e a n surface after positive resist stripping. The aqueous c l e a n i n g methods y i e l d e d v a r i o u s r e s u l t s o f which o n l y two were f o u n d t o be e f f e c t i v e . T h e C r O Q - H ^ S O l j a n d H2SO4-H2O2 were v e r y e f f e c t i v e i n p h o t o r e s i s t s t r i p p i n g b u t d i d n o t leave a clean surface. A problem from using chromic a c i d s o l u t i o n s i s t h a t i tc a n l e a v e chromium i o n s on a s i l i c o n wafer surface. T h i s s h o u l d n o t be s u r p r i s i n g s i n c e i t i s w e l l known t o c h e m i s t s t h a t a f t e r c h r o m i c a c i d h a s been used t o c l e a n glassware i t i sextremely d i f f i c u l t t o e x t r a c t a l l o f t h e chromium i o n s o u t o f t h eg l a s s . As m e n t i o n e d e a r l i e r , t h e p r e s e n c e o f t h e s e i o n s c a n be detrimental t o device performance. The o n l y s o l u t i o n w h i c h was v e r y e f f e c t i v e i n c l e a n i n g a s t r i p p e d s u r f a c e w a s t h e NH4OH-H2O2 s o l u t i o n , e v e n t h o u g h t h i s s o l u t i o n was n o t e f f e c t i v e f o r s t r i p p i n g photoresist. B e c a u s e s t r i p p i n g methods do n o t l e a v e a good c l e a n s u r f a c e and r e q u i r e f u r t h e r c l e a n i n g i n an aqueous s o l u t i o n i t m i g h t be e a s i e r t o s t r i p t h e r e s i s t and c l e a n t h e s u r f a c e i n aqueous s o l u t i o n s . B a s e d o n t h e d i s c u s s i o n s above, t h ebest organic c l e a n i n g procedure w o u l d b e t o u s e H2SO4-H2O2 t o r e m o v e a n y h e a v y o r g a n i c c o n t a m i n a t i o n a n d f o l l o w w i t h a NH4OH-H2O2 c l e a n i n g s t e p . An o x i d e l a y e r o n a s i l i c o n w a f e r i s m o s t c o m m o n l y r e m o v e d w i t h HP o r NH4P-HP ( B O E ) s o l u t i o n s . T h e r e a c t i o n r e s u l t s i nformation o f s i l i c o n hexafluoride which i s w a t e r s o l u b l e (10). T h e b u f f e r e d HP i s u s e d w h e n a constant oxide etch rate i s required. C o m p l i c a t i o n due 2



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t o NH4P p r e c i p i t a t i o n c a n o c c u r . B e c a u s e o f NH4F p r e c i p i t a t i o n a n d e c o n o m i c c o n s i d e r a t i o n s , HP i s t h e p r e f e r r e d reagent f o r oxide removal as part o f a c l e a n i n g procedure. Two p r o b l e m s c a n a r i s e f r o m t h e HP t r e a t m e n t o f s i l i con w a f e r s . The r e v e r s e p l a t i n g o f m e t a l s such as g o l d has a l r e a d y been mentioned. The o t h e r problem i s t h a t HP-treated s i l i c o n i s very susceptible t o hydrocarbon contamination. H e n d e r s o n (1^1) f o u n d t h a t w h e n H P - t r e a t e d w a f e r s were p l a c e d i n t o a n oven a t h i g h t e m p e r a t u r e s , carbon c o n t a i n i n g adsorbants on the s i l i c o n s u r f a c e decomposed t o form 3-SiC p a r t i c l e s . These p a r t i c l e s a r e known t o c r e a t e p r o b l e m s s u c h a s n u c l e a t e p o l y c r y s t a l l i n e regions d u r i n g the growth o f e p i t a x i a l s i l i c o n (12,13)* Because o f t h e s e two p r o b l e m s i t i s recommended t h a t a f t e r HP t h e w a f e r s s h o u l d b e c l e a n e d t o r e m o v e t h e h e a v y metals and carbon c o n t a i n i n g adsorbants. C a t i o n i c c o n t a m i n a t i o n may b e l o w e r e d b y r i n s i n g i n water, acids, bases, a c i d i c peroxide, basic peroxide o r s o l u t i o n w i t h c h e l a t i n g agents (2,3*14,15)• Water r i n s i n g o f s o d i u m c o n t a m i n a t e d s i l i c o n w a f e r s was e f f e c tive. A 6N HC1 s o l u t i o n w a s f o u n d t o b e m o r e e f f e c t i v e than the water r i n s i n g . I f t h e s o d i u m was c h e m i c a l l y a d s o r b e d t h e a c t i o n o f HC1 c a n b e v i e w e d a s a s i m p l e c a t i o n exchange r e a c t i o n . I n g e n e r a l , HC1 s o l u t i o n s w e r e v e r y e f f e c t i v e i nremoving metals due t o t h i s as w e l l as the complexing a b i l i t y o f c h l o r i d e ions. I t was a l s o r e p o r t e d t h a t sodium c o n t a m i n a t e d w a f e r s t h a t were s t o r e d i n room a i r h a d l o w e r d e s o r p t i o n r a t e s i n b o t h w a t e r a n d HC1 t h a n d i d t h e f r e s h s a m p l e s . U s e o f a n NH4OH s o l u t i o n c a n b e e f f e c t i v e i f t h e contaminant h a s a tendency t o form amino complexes, such as Cu (NH3)i| . H o w e v e r , some c a t i o n s s u c h a s M g 2 , A l 3 and P e 3 w i l l f o r m i n s o l u b l e h y d r o x i d e complexes i n b a s i c solutions. Because o f t h i s , m e t a l s need t o be removed by acidic solutions. Chelating agents are capable o f f o r m i n g w a t e r s o l u b l e c o m p l e x e s w i t h many m e t a l i o n s . Y e t , c h e l a t i n g agents were i n e f f e c t i v e i n p r e v e n t i n g m e t a l c o n t a m i n a t i o n i n e t c h a n t s o l u t i o n (2)* Metals that r e v e r s e p l a t e d onto the s u r f a c e were i n e f f e c t i v e l y removed by most s i n g l e component c l e a n i n g s o l u t i o n s . I t was f o u n d t h a t i t w a s n e c e s s a r y t o o x i d i z e t h e s e m e t a l s b e f o r e they c o u l d be removed. Prom t h e i n v e s t i g a t i o n s o f K e r n (140 i t was r e v e a l e d t h a t t h e m o s t e f f e c t i v e c l e a n i n g w a s a c c o m p l i s h e d w i t h H2O2 b a s e d s o l u t i o n s c o n t a i n i n g HC1 o r NH4OH. B a s i c p e r o x i d e s o l u t i o n s , t y p i c a l l y 5:1:1 p p v i n H 0 : H 2 0 2 ( 3 1 % ) :NHi|OH( 29%), a r e e f f e c t i v e i nremoving o r g a n i c c o n t a m i n a t i o n as w e l l as group I B and group I I B m e t a l s . Preliminary reports indic a t e t h a t t h i s b a s i c p e r o x i d e m i x t u r e may b e e f f e c t i v e i n removing g o l d as w e l l . The a c i d i c hydrogen p e r o x i d e s o l u t i o n s , t y p i c a l l y 5:1:1 p p v i n H 2 O : H 2 0 2 ( 3 1 % ) : H C 1 ( 3 8 % ) , were v e r y e f f e c t i v e i n removing a l l o f the m e t a l s t h a t were i n v e s t i g a t e d . + 2

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One d r a w b a c k o f h y d r o g e n p e r o x i d e s o l u t i o n s i s t h a t they r e a d i l y decompose. Decomposition i s greater at h i g h e r p H s and t h e p r e s e n c e o f m e t a l s w i l l a c c e l e r a t e the decomposition. A n o t h e r p o t e n t i a l p r o b l e m i s when t h e peroxide c o n c e n t r a t i o n gets too low, i n a b a s i c s o l u t i o n , t h e s o l u t i o n w i l l e t c h a s i l i c o n s u r f a c e (1JO . Solution decomposition a n d c o n c e n t r a t i o n v a r i a t i o n may b e e l i m i n a t e d by use o f c e n t r i f u g a l s p r a y p r o c e s s i n g s y s t e m s (15.)., I n t h e s e s y s t e m s t h e c l e a n i n g s o l u t i o n s a r e m i x e d o n - l i n e , i n c o n t r o l l e d r a t i o s immediately before being sprayed onto the wafer s u r f a c e . Downloaded by NANYANG TECHNOLOGICAL UNIV on June 8, 2016 | http://pubs.acs.org Publication Date: January 28, 1986 | doi: 10.1021/bk-1986-0295.ch023

T

Chemical

Sequencing

Most c l e a n i n g p r o c e d u r e s r e q u i r e t h e s e q u e n t i a l use o f t w o o r m o r e c l e a n i n g s o l u t i o n s . T h i s may be necessary when m o s t c l e a n i n g p r o c e d u r e s r e q u i r e t h e s e q u e n t i a l use o f two o r more c l e a n i n g s o l u t i o n s . W a f e r s a r e c o n t a m i n a t e d w i t h more t h a n one t y p e o f contamination. A l s o , a c l e a n i n g s o l u t i o n c a n r e m o v e some c o n t a m i n a n t s w h i l e l e a v i n g a d i f f e r e n t c o n t a m i n a n t on t h e w a f e r s u r f a c e ( e . g . HP). I t h a s a l s o b e e n m e n t i o n e d t h a t one type of contaminant could i n t e r f e r e w i t h the removal of another type. C h e m i c a l s e q u e n c i n g has been d i s c u s s e d i n great d e t a i l . A m i c k (If) and Burkman, e t a l (16) s u g g e s t e d t h a t a c l e a n i n g s e q u e n c e s h o u l d b e l7~~removal o f o r g a n i c m a t e r i a l , 2) r e m o v a l o f o x i d e l a y e r s a n d 3) r e m o v a l o f m e t a l l i c and i o n i c c o n t a m i n a n t s . Prom t h e d a t a a v a i l a b l e o n s i l i c o n w a f e r c l e a n i n g , s t e p 1) c o u l d b e a c c o m p l i s h e d w i t h SPM a n d / o r APM, s t e p 2) c o u l d be a c c o m p l i s h e d w i t h a n HP s o l u t i o n , a n d s t e p 3) w i t h HPM. SPM r e f e r s t o a 4:1 p p v m i x t u r e o f H 2 S 0 i | ( 9 6 % ) : H 2 0 2 ( 3 1 % ) . APM r e f e r s t o a 5:1:1 ppv m i x t u r e of H 0:H202(31%):NH40H(29%). HPM i s t y p i c a l l y a 5:1:1 Ppv mixture of H 0:H202(31%):HC1(38%). 2

2

Experimental

Procedures

The m e t a l l i c and i o n i c c l e a n i n g p r o c e d u r e s s t u d i e d by K e r n w e r e e v a l u a t e d by r a d i o t r a c e r m e t h o d s . This techn i q u e r e q u i r e s the use o f r a d i o n u c l i d e e n r i c h e d reagents. R e c e n t l y , a p r o c e d u r e u t i l i z i n g s e c o n d a r y i o n mass spectrometry (SIMS) has been d e v e l o p e d f o r s t u d y i n g v e r y l o w l e v e l s o f c o n t a m i n a t i o n on s i l i c o n s u r f a c e s ( 1 8 ) . T h i s t e c h n i q u e does not r e q u i r e r a d i o n u c l i d e e n r i c h e d r e a g e n t s s o n o r m a l r e a g e n t s may be u s e d . The SIMS t e c h n i q u e a l s o has t h e a d v a n t a g e t h a t i t i s s e n s i t i v e enough t o d e t e c t c o n t a m i n a t i o n on a w a f e r a f t e r c l e a n i n g w i t h the best c l e a n i n g procedures. The h i g h s e n s i t i v i t y o f S I M S c a n be s e e n i n F i g u r e 1. The A u g e r E l e c t r o n S p e c t r u m (AES) o f t h e s i l i c o n w a f e r o n l y shows S i and 0. A SIMS s p e c t r u m o f t h e same s a m p l e s h o w s t h e p r e s e n c e o f s e v e r a l c o n t a m i n a n t s n o t d e t e c t e d i n t h e AES spectrum. T h e r e a r e , h o w e v e r , some l i m i t a t i o n s i n t h e S I M S t e c h -


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MES

-UPVET


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98/26/85 EXftB

V/F

KINETIC ENERGY, EV

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68/26/85 EXSB

• Slj.

fiTOMIC MOSS UNITS F i g u r e 1. AES a n d SIMS s p e c t r a s p o t on a c o n t a m i n a t e d w a f e r .

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t h e same


372


nique. The t e c h n i q u e i s n o t s u i t a b l e f o r d e t e c t i n g i r o n o r heavy m e t a l s on s i l i c o n . The most i n t e n s e i r o n peaks F e 2 and F e o c c u r where t h e s i l i c o n peaks due t o S i and Si 2 occur. Heavy m e t a l s such as g o l d have l o w i o n y i e l d s w h i c h g i v e s them a h i g h d e t e c t i o n l i m i t . A l l o f t h e c l e a n s t o be d i s c u s s e d were e v a l u a t e d f o r m e t a l i o n c o n t a m i n a t i o n b y u s i n g t h e SIMS p r o c e d u r e p r e v i o u s l y d e s c r i b e d b y P h i l l i p s , e t a l (^18). P a r t i c l e cont a m i n a t i o n was d e t e r m i n e d u s i n g a T e n c o r S u r f s c a n s e t a t maximum s e n s i t i v i t y ( l y m ) w i t h a n e d g e e x c l u s i o n s e t t i n g o f 7« A l l o f t h e samples were p r e p a r e d and c l e a n e d as p r e v i o u s l y d e s c r i b e d (1_9) u n l e s s s t a t e d o t h e r w i s e . +

+

+


+

Results

and D i s c u s s i o n

An e x p e r i m e n t t h a t e v a l u a t e d t h e n e c e s s i t y o f o x i d e r e m o v a l a s a s t e p i n c l e a n i n g was p e r f o r m e d ( 1 9 ) • Contaminated w a f e r s were c l e a n e d u s i n g t h e two sequence o f S P M , H F , APM, HPM o r S P M , APM, HPM. Table I gives the SIMS r e s u l t s t h a t were o b t a i n e d . Table

I.

The I n f l u e n c e

o f Oxide Removal on

Cleaning

Contaminant/Silicon Cleaning Process SPM, H F , APM, HPM SPM, APM, HPM

Peak x 10-

Na(23)

K(39)

Cu(63)

26 90

156 246

23 66

These r e s u l t s i n d i c a t e d t h a t removal o f t h e n a t i v e oxide w i t h HF a f f o r d s a c l e a n e r s u r f a c e . T h e same c o n c l u s i o n was a s c e r t a i n e d b y B e y e r ( 1 0 ) . F u r t h e r e v i d e n c e t h a t t h e HF s t e p i s g o o d f o r r e m o v i n g some m e t a l i o n c o n t a m i n a t i o n c a n b e s e e n f r o m t h e e x p e r i m e n t t h a t c o m p a r e d w a f e r s c l e a n e d b y APM a n d APM, H F . T a b l e I I s h o w s t h e r e s u l t s o f t h e S I M S a n a l y sis. T a b l e I I . R e m o v a l o f M e t a l I o n s W i t h HF

Cleaning

tit

APM APM,

C o n t a m i n a n t P e a k / S i l i c o n ( 2 8 ) P e a k x 10"P Process~Na(23) Mg(24) A l ( 2 7 ) K(39) Ca(40l

HF

13

7

18807 97

*T9 30

73 48

I t c a n b e s e e n t h a t o m i s s i o n o f t h e HF s t e p r e s u l t s i n a surface with greater metal i o n contamination. The most s i g n i f i c a n t o b s e r v a t i o n was t h a t t h e a l u m i n u m c o n t e n t was o v e r two o r d e r s o f m a g n i t u d e g r e a t e r on t h e sample w i t h o u t a n HF s t e p . T h e HF s t e p w a s e f f e c t i v e i n l o w e r i n g t h e l e v e l s o f t h o s e m e t a l s shown above i n T a b l e I I . A d i f f e r e n t e x p e r i m e n t was p e r f o r m e d i n o r d e r t o e v a l u a t e t h e p a r t i c l e c o n t a m i n a t i o n d u e t o HF t r e a t m e n t . I n Table I I Ii t c a n b e s e e n t h a t t r e a t m e n t o f a w a f e r w i t h SPM t h e n HF changes t h e p a r t i c l e l e v e l on f o u r - i n c h w a f e r s from t h r e e


23.

BECKER ET AL.

373


t o 80. When t h e HP s t e p w a s f o l l o w e d b y APM t h e n HPM t h e p a r t i c l e l e v e l s a f t e r t h e c l e a n were a p p r o x i m a t e l y t h e same a s b e f o r e t h e c l e a n . Table

III.

T o t a l P o i n t Defects on Pour-inch Before and A f t e r Cleaning SPM, Before


3

HP After

80

Wafers

SPM, H P , APM, HPM After Before

4

5

P r o m t h i s e x p e r i m e n t i t w a s c o n c l u d e d t h a t t h e A P M , HPM c y c l e s removed t h e p a r t i c u l a t e c o n t a m i n a t i o n that r e s u l t e d f r o m t h e HP s t e p . I t was p o i n t e d o u t e a r l i e r t h a t t h e b e s t c l e a n f o r hydrocarbon contamination w a s SPM f o l l o w e d b y A P M . I t was a l s o s h o w n t h a t a f t e r a n HP s t e p a n o t h e r s t e p i s r e q u i r e d t o remove r e s i d u a l p a r t i c u l a t e c o n t a m i n a t i o n , s u g g e s t i n g t h a t t h e APM s t e p s h o u l d f o l l o w t h e HP s t e p . An e x p e r i m e n t w a s p e r f o r m e d i n w h i c h h i g h l y c o n t a m i n a t e d w a f e r s were c l e a n e d b y t h e t w o s e q u e n c e s o f SPM, APM, HP, HPM a n d S P M , H P , A P M , HPM (19.). I n T a b l e I V t h e r e s u l t s o f t h a t e x p e r i m e n t a r e shown. Table

IV.

T h e E f f e c t o f APM a n d HP S e q u e n c i n g

C o n t a m i n a n t P e a k / S i l i c o n Peak X 10"° Cleaning Cu Na K Al Ca Mg Cr Process (23) (39) (40) (24) (52) (63) (27) SPM,APM, ND ND 70 ND ND 27 32 HP,HPM SPM,HP, ND 131 ND 30 25 134 645 APM,HPM ND = n o n e d e t e c t e d

Total Point Defects

The HP, was

236 86

r e s u l t s c l e a r l y show t h a t t h e s e q u e n c e o f SPM, APM, HPM w a s t h e m o r e e f f e c t i v e i n r e m o v i n g m e t a l s , b u t i t t h e l e s s e f f e c t i v e f o rp a r t i c l e r e d u c t i o n . The p a r t i c l e l e v e l s c a n b e e x p l a i n e d o n t h e b a s i s t h a t HP l e a v e s a s u r f a c e r i d d e n w i t h p a r t i c l e s t h a t c a n b e r e m o v e d m o r e e f f i c i e n t l y b y A P M , HPM t h a n b y HPM alone. The d a t a i n T a b l e I I showed t h a t t h e m e t a l s o f i n t e r e s t w e r e p r e s e n t i n o n l y t r a c e a m o u n t s a f t e r a n HP step. So t h e d i f f e r e n c e i n m e t a l c o n t a m i n a t i o n between the two c l e a n i n g p r o c e s s e s i n T a b l e I V must r e f l e c t one o f t h e d i f f e r e n c e s b e t w e e n t h e u s e o f A P M , HPM a n d o n l y HPM a f t e r a n HP s t e p . The r e s u l t s suggest i t would be a d v a n t a g e o u s t o u s e APM l a s t i n a c l e a n i n g s e q u e n c e t o yield a particulate clean surface. B e c a u s e o f t h i s we evaluated the metal contamination on wafers cleaned by HP, A P M , HPM a n d H P , HPM, A P M . F i g u r e 2 s h o w s t h e S I M S spectra o f these cleans. I t c a nbe seen f r o m t h e s p e c t r a that metal i o ncontamination r e s u l t e d f r o m u s i n g t h e APM step l a s t as evidenced by t h e presence o f a l a r g e alumi-




HF, HPM

25

36

APM,

HF, APM, HPM

35

25

38

35

ATOMIC MASS UNITS

F i g u r e 2 . SIMS s p e c t r a t a k e n f r o m s i l i c o n w a f e r s p r o c e s s e d b y t h e c l e a n i n g s e q u e n c e s o f H F , APM, HPM, a n d H F , HPM, APM.


23.

BECKER ET AL.

375


num peak. This i n d i c a t e d that metal ion contamination ( A l , Ca, and Mg) a c t u a l l y increased from the use of APM. Since aluminum i s i n s o l u b l e i n N H 4 O H the most probable source i n the APM s o l u t i o n would be the H 2 0 2 « There are a v a r i e t y of c l e a n i n g processes used i n the m i c r o e l e c t r o n i c s i n d u s t r y . The r e s u l t s of an e x p e r i ment which compared the c l e a n i n g sequence of SPM,APM,HP,HPM to some of the more common c l e a n i n g sequences are l i s t e d i n Table V (19)*


Table V.

Chemical Sequence Comparison by SIMS

Cleaning Process SPM,APM,HP,HPM SPM,HP HC1/HN0 ,HP,HC1/HN03 3

Fuming N i t r i c

Contaminant P e a k / S i l i c o n Peak X Na Mg Al K Ca Cr ( 2 3 ) (24) ( 2 7 ) ( 3 9 ) (40) (52) 15 57 ND T 0 3 270 50 30 30 ND 2 1 0 3 6 0 8 0 38

179

160 171

93

ND

380 1763

440

57

10— Cu (63) T5 20

340

5 6 0 1 6 0 246

I t can be seen that the lowest l e v e l s of m e t a l l i c contamination r e s u l t e d from c l e a n i n g with SPM, APM, HF, HPM, and SPM, HF. However, t h i s SIMS data d i d not r e v e a l the problems of heavy metal and p a r t i c l e contamination a f t e r using HF. Hence, of these four cleans, the best o v e r a l l c l e a n i n g was accomplished using the SPM, APM, HF, HPM sequence. Conclusion I t was revealed that some metal and p a r t i c l e contamination may r e s u l t from the use of c e r t a i n c l e a n i n g solutions. In t u r n , these contaminants may be removed by other c l e a n i n g s o l u t i o n s . Because of t h i s , as the p u r i t y of processing chemicals change so w i l l the most e f f e c t i v e c l e a n i n g process. The most e f f e c t i v e c l e a n i n g process w i l l a l s o depend on what contaminants are present and what contaminants must be removed. Based on the r e s u l t s that were obtained the best c l e a n i n g sequence f o r m e t a l l i c contamination was SPM, APM, HF, HPM. Reversing the order of the APM and HF steps was more e f f e c t i v e f o r p a r t i c l e reduction and s l i g h t l y l e s s e f f e c t i v e f o r metal i o n contamination.

Literature Cited 1. 2. 3. 4. 5.

Batsford, K. O.; Thomas, D. D. Elect. Comm. 1963, 38(3), 354. Kern, W. RCA Review 1970, 31, 207. Kern, W. RCA Review 1970, 31, 234. Hofstein, S. R. IEEE Trans. on Elect. Dev. 1967, ED-14, 749. Snow, E . H . ; Grove, A. S.; Deal, B. E.; Sah, C. T. J. Appl.Phys. 1965, 36, 1664.


376


6.

Goetzberger, A . ; Shockley, W. J . Appl. Phys. 1960, 31, 1821. B u l l i s , W. M. Solid-State Elect. 1966, 9, 143. Feder, D. O.; Koontz, D. E . Symposium on Cleaning of Electronic Device Components 1959, ASTM STP No. 246, 40. Peters, D. A . ; Deckert, C. A. J . Electrochem. Soc. 1979, 126(5), 883. Beyer, K. D.; Kastl, R. H. J . Electrochem. Soc. 1982, 129(5), 1027. Henderson, R. C. J . Electrochem. Soc. 1972, 119(6), 772. Joyce, B. A . ; Neave, J . H . ; Watts, B. E . Surf. S c i . 1969, 15, 1. Thomas, D. J. D. Phys. Status Solidi 1966, 13, 359. Kern, W.; Poutinen, D. A. RCA Review 1970, 31, 187. Burkman, D. C . ; Peterson, C. A . ; Schmidt, W. R. in "Treatise on Clean Surface Technology"; M i t t a l , , E d . ; Plenum Press: New York, to be published. Burkman, D. C. Semiconductor International 1981, 4(7), 103. Amick, J . A . Solid State Technol. 1976, 19(11), 47. P h i l l i p s , B. P.; Burkman, D. C . ; Schmidt, W. R.; Peterson, C. A. J . Vac. S c i . Technol. 1983, A1(2), 646. Burkman, D. C . ; Schmidt, W. R.; Peterson, C. A . ; P h i l l i p s , B. P . , Proc. Semiconductor 83 International, Birmhingham, England Sept. 1983. Also available from the authors as FSI TR217.

7. 8. 9. 10.


11. 12. 13. 14. 15. 16. 17. 18. 19.

RECEIVED September 13, 1985


Microelectronics Processing - American Chemical Society

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