An in situ Study of Aqueous HF Treatment of Silicon by Contact Angle Measurement and Ellipsometry

An in situ Study of Aqueous HF Treatment of Silicon by Contact Angle Measurement and Ellipsometry

G. Gou ld * and E. A. I r e n e * *

Department of Chemistry, University of North Carolina, Chapel Hill, North Carolina 2 7514

A B S T R A C T

T h e app l i c a t i on of in situ e l l i p s om e t r y a n d in situ c o n t a c t ang le m e a s u r e m e n t to s e m i c o n d u c t o r s u r f a c e s is in t roduced . I t s h o u l d b e r ecogn ized t h a t t he t e c h n i q u e s d i s c u s s e d are app l i cab l e to all s e m i c o n d u c t o r su r faces in any a m b i e n t l i qu id phase . E x p e r i m e n t a l a n d i n s t r u m e n t a l c o n s i d e r a t i o n s for the use of t h e s e t e c h n i q u e s are d i s c u s s e d as wel l as exper - i m e n t a l r e su l t s for ana lys i s of s i l i con a n d s i l icon d i ox ide surfaces . T h e s e resu l t s p e r t a i n to Si su r faces in a m b i e n t a q u e o u s H F a n d y ie ld e v i d e n c e of a f l u o r o c a r b o n f ihn p r e s e n t o n t h e Si su r face fo l lowing e t c h of a SiO2 film.

The o p e r a t i o n of t he MOSFET, one of t he m o s t c o m m o n m i c r o e l e c t r o n i c devices , d e p e n d s on t he app l i ca t i on of an e lec t r ic field ac ross a t h i n i n s u l a t i n g film (gate) of SiOa to a Si s u b s t r a t e b e n e a t h . High qua l i ty p e r f o r m a n c e of t he se dev ices r equ i r e s c l ean ing a n d pa s s i va t i on of t he Si surface. T h e u se of hydro f luo r i c acid as a w e t c h e m i c a l e t ch for re- m o v a l of SiO2 f rom Si is c o m m o n in t he f ab r i c a t i on of M O S F E T devices . Hydrof luor i c acid is k n o w n to rap id ly e t c h SiO2 b u t to a t t ack Si on ly mi ld ly (1). This b e h a v i o r m a k e s H F a use fu l a g e n t for r e m o v a l of na t ive SiO2 f rom Si, be fo re t h e r m a l ox ida t i on to fo rm t he gate insu la tor , for p u r p o s e s of de l i nea t i on a n d for o p e n i n g con t ac t ho les t h r o u g h t he SiOa to the Si subs t ra te . S ince t he o p e r a t i o n of M O S F E T dev ices is d e p e n d e n t on t he qua l i ty of t he S i - S i Q in ter face , i t is i m p o r t a n t to u n d e r s t a n d the ef fec t of H F t r e a t m e n t s on b o t h S i Q a n d Si. In th i s p a p e r t he resu l t s of e x p e r i m e n t s u s i n g e l l i p somet r i c a n d c o n t a c t ang le m e a s u r e m e n t s app l i ed in situ in a m b i e n t H F so lu t ion to s t u d y t h e c h a n g e s c a u s e d by H F t r e a t m e n t of Si a n d SiO~ su r faces is desc r ibed .

T h e ef fec t of c h e m i c a l t r e a t m e n t s c o n t a i n i n g H F on Si a n d SiO2 su r faces and the ef fec t of s u c h H F t r e a t m e n t s on t he k i n e t i c s of Si o x i d a t i o n ha s b e e n s t ud i ed by a n u m b e r of inves t iga to r s . These inves t iga to r s h a v e e m p l o y e d X P S (2-5), A E S (6-8), R B S (9), S IMS (10), m u l t i p l e i n t e rna l re- f lec t ion (5), e l l i p s o m e t r y (2, 11-13), a n d r ad ioac t ive t r ace r s (14-16). M a n y of t h e s e s tud ies i nd i ca t e an ef fec t of the H F t r e a t m e n t , b u t t he on ly t e c h n i q u e t h a t can rou t i ne ly de t ec t t h e p r e s e n c e of f luor ide spec ies on t he Si or S i Q sur face fo l lowing H F t r e a t m e n t is t h e r ad ioac t ive t r ace r ana lys i s (14-16). Th i s ana lys i s d e t e r m i n e s sur face f luor ide concen - t r a t i o n b y m e a s u r e m e n t of 18F decay fo l lowing e x p o s u r e of a s a m p l e su r face to 18F l abe led hydro f luo r i c acid.

T h e p r e v i o u s l y n o t e d ana ly t i ca l m e t h o d s are all ex situ ana ly se s of Si or SiO2 surfaces . P e r h a p s of e v e n g rea t e r im- p o r t a n c e for t he u n d e r s t a n d i n g of the i n t e r ac t i on of Si a n d SiO2 w i t h H F is an in situ app roach , e n a b l i n g sur face anal- yus i s to b e p e r f o r m e d d u r i n g i m m e r s i o n . T he resu l t s of b o t h in situ e l l i p somet r i c and in situ con t ac t ang le meas - u r e m e n t s , w h e n c o n s i d e r e d jo int ly , y ie ld e v i d e n c e of a re- s idua l fi lm at t h e Si su r face fo l lowing SiO2 e tch by d i lu te HF.

Experimental Ellipsometry.--For m e a s u r e m e n t s of t h i n f i lms on sol id

surfaces , e l l i p s o m e t r y has b e e n wide ly u s e d b e c a u s e of i ts c o n v e n i e n c e , accuracy , a n d e x t r e m e l y h i g h sens i t iv i ty w h i c h e n a b l e s d e t e c t i o n of s u b m o n o l a y e r cove rage of surfaces b y a d s o r b e d spec ies (17-19). In re f lec t ion ellip- some t ry , t h e m e a s u r e d p a r a m e t e r s ' t ' ( a m p l i t u d e ratio) a n d A ( p h a s e change ) are r e l a t ed to t h e s a m p l e a n d a m b i e n t op- t ical p r o p e r t i e s b y t h e f u n d a m e n t a l e q u a t i o n of ellip- s o m e t r y

t a n �9 e x p (ih) = f(na, nF, Us, k, Ls, ~)

w h e r e nA and nF are t h e a m b i e n t a n d film re f rac t ive in- dexes , respect ive ly , /~s is t he s u b s t r a t e re f rac t ive index , X is t h e w a v e l e n g t h of fight, LF is the film th ickness , and $ is the

�9 Electrochemical Society Student Member. �9 * Electrochemical Society Active Member.

ang l e of i n c i d e n c e w i th w h i c h t he l igh t i m p i n g e s on the sample . T h e expl ic i t fo rm of f a b o v e is g iven in Azzam a n d B a s h a r a (17). I f hA, X, a n d ~ are k n o w n , t he t h i c k n e s s of a fi lm on a s u b s t r a t e (Lr) or the c o m p l e x s u b s t r a t e re f rac t ive i n d e x (ns), a s s u m i n g no film is p resen t , can be ca lcu la ted f rom an e l l i p somet r i c m e a s u r e m e n t .

T h e sens i t iv i ty of the e l l i p some te r m a k e s it des i r ab le for t he s t u d y u n d e r t a k e n ; however , app l i c a t i on to m e a s u r e - m e n t s p e r f o r m e d in a m b i e n t l iqu id p h a s e s leads to al tera- t ion of p a r a m e t e r s nA a n d ~ f rom typ ica l values. S i n c e t he s a m p l e is to be i m m e r s e d in solut ion, t he a m b i e n t refrac- t ive i n d e x (hA) will be d i f fe ren t f rom t h a t of air. A fused silica s ample cell, s h o w n schemat ica l ly in Fig. 1, had to be de- s i g n e d in o rde r to fix t he s a m p l e s in solut ion. T h e s a m p l e is he ld in t he ver t ica l p l ane s u c h t h a t t he laser l igh t passes t h r o u g h one w indow, reflects f rom the sample , a n d pas ses ou t t he o t h e r w indow. Fo r typ ica l e l l i p somet r i e measu re - m e n t s p e r f o r m e d in air, t h e i n s t r u m e n t is set to an ang le of i n c i d e n c e of 70.00 ~ b e c a u s e a h i g h degree of sens i t iv i ty is o b t a i n e d nea r th i s angle. Ideal ly, t he s a m p l e cell wou ld be bu i l t s u c h t h a t each a rm w o u l d be 70.00 ~ f rom the n o r m a l to t h e sample , b u t c o n s t r u c t i o n of a cell w i th s u c h h igh ac- c u r a c y in t he a r m ang les p roves qu i t e difficult . I t is im- po r t an t , h o w e v e r to k n o w the exac t ang le of t he a r m s in t h e cell b e c a u s e n o r m a l i n c i d e n c e of t he l igh t on t he win- d o w s m u s t b e ensu red . Un le s s the l igh t passes t h r o u g h the w i n d o w s at exac t ly n o r m a l inc idence , t he ang le at w h i c h t h e l igh t s t r ikes t he s a m p l e will be d i f fe ren t f rom the ang le se t on t he i n s t r u m e n t due to r e f r ac t ion of l igh t as it passes t h r o u g h the w i n d o w into t he l iqu id m e d i u m . This refrac- t i on leads to a d i f fe ren t ang le ~ f rom t h a t set on t he ins t ru - m e n t , a n d any e r ror i n t r o d u c e d in 4~ leads to e r ror in the el- l i p s o m e t r i c m e a s u r e m e n t .

T h r o u g h an a l i g n m e n t p r o c e d u r e t he ang le of i n c i d e n c e of th i s pa r t i cu l a r s a m p l e cell is d e t e r m i n e d . Th i s proce- d u r e ut i l izes t he par t ia l re f lec t ion of t he i n c i d e n t laser b e a m f rom the sur face of the cell w indow. The s a m p l e cell is a d j u s t e d s u c h t h a t t he b e a m ref lec ted f rom the first win- d o w is d i r ec t ed exac t ly b a c k in to t he i n c i d e n t beam. This e n s u r e s t h a t t he i n c i d e n t b e a m s t r ikes the first w i n d o w at n o r m a l inc idence . With t he s a m p l e cell f ixed at th i s posi- t i on t he ana lyzer a rm of the e l l i p some te r is ro t a t ed un t i l t h e b e a m ref lec ted f rom the sample , a f te r pa s s ing t h r o u g h t he s e c o n d w indow, pas ses t h r o u g h a p i n h o l e p laced at the

SAMPLE ~ FLOW INLET

FLOW OUTLET HOLDERS

Fig. 1. Fused silica sample cell used for in situ ellipsometric meas- urements. Fused silica windows are sealed on the end of each arm.

1535 ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 128.118.88.48Downloaded on 2014-11-08 to IP

http://ecsdl.org/site/terms_use

1536 J. Electrochem. Soc.: S O L I D - S T A T E S C I E N C E A N D T E C H N O L O G Y June 1988

e n d of t he ana lyzer arm. The ang le b e t w e e n t he a r m s is t h e n r ead f rom a ca l ib ra t ed scale on t he e l l i p s om e t e r ba se w i t h an accu racy of 0.01 ~ The accu racy of 4) was ver i f ied by f ind ing less t h a n 10% va r i a t ion b e t w e e n m e a s u r e m e n t s t a k e n w i t h o u t t he cell and in situ wi th a m b i e n t wa te r for SiO2 fi lms r a n g i n g f rom 23 to l l00/k.

All t h i c k n e s s e s r e c o r d e d are t he r e su l t of m a n u a l two- zone nu l l e l l i p somet r i c m e a s u r e m e n t s u s i n g a n a l igned a n d ca l i b r a t ed r e sea rch qua l i ty e l l i p some te r w i th a 6328A He-Ne lase r l igh t source . A m b i e n t re f rac t ive i n d e x e s are m e a s u r e d w i t h a n A b b e R e f r a c t o m e t e r to _+0.0001 u n i t s a n d (b = 70.76 ~ as f o u n d by t he a l i g n m e n t p r o c e d u r e de- s c r i bed above . The s am p l e s u sed were 1 in. d i a m p- type s ing le -c rys ta l s i l icon wafers of (100) o r i en t a t i on w i th 2 ~ c m n o m i n a l res i s t iv i ty w h i c h were RCA c l eaned (20), H F d ipped , a n d t h e r m a l l y oxidized. T he ox ida t i on c o n d i t i o n s we re 700~ for 48h in d ry o x y g e n (<5 p p m H20 as meas- u r e d at t he fu rnace exhaus t ) , w h i c h yie lds a n e l l ipsomet r i - cal ly m e a s u r e d SiO2 film t h i c k n e s s of a b o u t 250A. T h e s e s a m p l e s are t h e n i m m e r s e d in t he a m b i e n t e t ch so lu t ion (450:1 by v o l u m e de ion ized water /49% MOS g rade HF) in t he s a m p l e cell w h i c h is f ixed at t he focus of t he ellip- somete r . E l l i p some t r i c m e a s u r e m e n t s are t a k e n eve ry 10 m i n in i t ia l ly a n d t h e n eve ry 5 ra in as t he S i Q film th ick- ne s s a p p r o a c h e d zero.

Contact angle measurement.--The con t ac t ang le meas- u r e m e n t is a su r face sens i t ive t e c h n i q u e t h a t has b e e n pre- v ious ly s h o w n to be ab le to de tec t c h a n g e s on Si a n d S i Q sur faces (21, 22). The c o n t a c t ang le is de f ined b y t he equi- l i b r i u m of t he t h r e e sur face t e n s i o n vec to r s at a solid- l i q u i d - v a p o r in t e r face a n d is re la ted to t h e s e b y the Y o u n g e q u a t i o n (23). The we t t i ng b e h a v i o r of sol id sur faces is di- r ec t ly r e l a t ed to t he con tac t angle. In general , if a l iqu id ha s a lower sur face t e n s i o n t h a n t he sur face w h i c h it contactS, t he l iqu id will s p r e a d on the surface, w h i c h is ter- m e d a l ipophi l i c s i t ua t ion and will yield smal l con t ac t angles. I f t h e l iqu id has a h i g h e r sur face t e n s i o n t h a n t he surface, t h e l iqu id will no t sp read on the sur face a n d t he sol id su r face is said to b e l i p o p h o b i c a n d large con t ac t ang les resul t . Th i s we t t i ng b e h a v i o r can be u n d e r s t o o d b a s e d on t h e t h e r m o d y n a m i c t e n d e n c y to m i n i m i z e t he e n e r g y of a sys t em, name ly , the sur face e n e r g y of t he solid. Wi th re- ga rd to sur face energy, two classes of solid sur faces exist . H i g h e n e r g y sur faces have sur face ene rg ies f rom 500 to 5000 d y n / c m a n d cons i s t of all me ta l s a n d m e t a l oxides, wh i l e low e n e r g y sur faces h a v e sur face t e n s i o n s of less t h a n 100 d y n / c m a n d cons i s t m a i n l y of w a x e s a n d poly- mers . Wi th t he e x c e p t i o n of t he l iqu id meta ls , all l iqu ids h a v e su r face t e n s i o n s of less t h a n 100 dyn/cm. U s i n g t he m i n i m i z a t i o n of sur face e n e r g y c o n c e p t it is e x p e c t e d t h a t all l iqu ids ( excep t l iqu id meta ls ) will we t all h i g h e n e r g y surfaces . Fo r t he case of low ene rgy surfaces , howeve r , s o m e l iqu ids will we t a sur face whi l e o the r s will not , de- p e n d i n g on t he sur face t e n s i o n s of the specif ic sur face and l i qu ids involved . E x p e r i m e n t a l l y , i t is e x p e c t e d t h a t liq- u ids w i t h sur face t e n s i o n s lower t h a n t h a t of the sol id surface will y ie ld l ipophi l ic in te rac t ions , wh i l e l iqu ids of h i g h e r sur face t e n s i o n t h a n t he sur face will y ie ld l ipopho- bic i n t e rac t ions . Z i s m a n has s h o w n (23) t h a t a r e l a t i onsh ip ex i s t s b e t w e e n the cos ine of the con t ac t ang le a n d t he liq- u id sur face t e n s i o n (TLV) for lOW e n e r g y surfaces , we t by l iqu ids w i t h a va r i e ty of su r face t ens ions . This r e l a t i onsh ip e n a b l e s e x t r a p o l a t i o n of a q u a n t i t y t e r m e d t he cr i t ical surface t e n s i o n (7c) of t he low e n e r g y surface. Z i s m a n has s h o w n t h a t 7c is c losely re la ted to the sur face s t r u c t u r e a n d c o m p o s i t i o n of a low e n e r g y surface.

In o rde r to p e r f o r m t h e con tac t ang le m e a s u r e m e n t in a m b i e n t l iqu id phases , t he i n v e r t e d b u b b l e t e c h n i q u e (24) was used. The s a m p l e cell is s h o w n in Fig. 2 a n d is con- s t r u c t e d en t i r e ly of fused silica w i th a po l i shed f ron t face to e n a b l e v iewing . In th i s cell, t he Si s a m p l e is c o m p l e t e l y i m m e r s e d a n d he ld s u c h t h a t the sur face to be i nves t i ga t ed is inve r t ed . A s ingle n i t r o g e n gas b u b b l e ( -0 .5 m m diam) can be r e l ea sed f rom t he capi l lary and will float to the s a m p l e sur face a n d e s t ab l i sh the t h r e e - p h a s e e q u i l i b r i u m s h o w n in the inse t box of Fig. 2. Note t h a t th i s t h r e e - p h a s e e q u i l i b r i u m cons i s t s of a v a p o r b u b b l e w h i c h con tac t s a sol id sur face s u r r o u n d e d by l iquid, w h i c h is t he i nve r se of

r ; $

,

v////A s / / ' / / / / 2

I L i

I i I

\ / ~-" ~ _ _ / FLOW INLET

HOLDERS ~ CAPILLARY

LOW OU.TL. T I I 1 GAS FLOW

Fig. 2. Fused silica sample cell for in situ contact angle measurement. The three-phase equilibrium and the resulting contat angle, O, for the inverted bubble are shown in the inset box where S, L, and V stand for solid, liquid, and vapor, respectively.

t h e typ ica l c o n c e p t i o n of t he con t ac t angle. The ang le (0) m e a s u r e d , howeve r , is still t he ang le at w h i c h t he l iqu id c o n t a c t s t he sol id surface. The key fea tu re of th i s s a m p l e cell is t h a t m a x i m u m e x p o s u r e of the s a m p l e to the de- s i red so lu t ion is ma in t a ined .

I n o rde r to m e a s u r e t he con t ac t angle, s o m e as soc ia t ed i n s t r u m e n t a t i o n is needed . The s a m p l e cell is p l aced in the focus of a ho r i zon ta l ly m o u n t e d s te reo m i c r o s c o p e t h a t is c o n t i n u o u s l y va r i ab le f rom 20 to 100X. A sepcia l ly de- s i g n e d h o l d e r is u sed to ad jus t the s a m p l e cell s u c h t h a t b u b b l e s will r e m a i n s t a t iona ry on t he i nve r t ed s a m p l e surface, r e s i s t ing f lo ta t ion to a m o r e e l eva ted posi t ion . Trans - l a t ion s tages e n a b l e pos i t i on ing of the b u b b l e in t he objec- t ive of the m i c r o s c o p e for magn i f i ca t ion of t he t h r e e - p h a s e e q u i l i b r i u m . The cell i s . i l l u m i n a t e d f rom the rear, and a po la ro id c a m e r a is c l a m p e d to one of t he m i c r o s c o p e eye- p ieces for p h o t o g r a p h y . The con t ac t ang le is m e a s u r e d f rom the p h o t o g r a p h w i th an accu racy of -+ 3 ~ T h e s amp le s u s e d for t h e in situ con t ac t ang le m e a s u r e m e n t are ident i - cal to t h o s e u sed in t he e l l i p somet r i c analysis . T h e s e s a m p l e s are first i m m e r s e d in de ion ized water , a n d t h e n a 500:1 (by vo lume) so lu t ion of H 2 0 : c o n c e n t r a t e d (49%) H F is i n t r o d u c e d . S ince the so lu t ions u sed are aqueous , t he t e r m s h y d r o p h o b i c and h y d r o p h i l i c to d e n o t e w e t t e d and n o n w e t t e d sur faces will be u sed hence fo r th . P h o t o g r a p h s are t a k e n be fo re t he e t ch so lu t ion is i n t r o d u c e d and t h r o u g h o u t the e t ch p rocess to o b t a i n t he con t ac t angles .

Results and Discussion The film t h i c k n e s s vs. e tch t ime da ta for t he in situ ellip-

s o m e t r i c e x p e r i m e n t is s h o w n in Fig. 3. I t is e v i d e n t f rom the l inea r i ty of t he p lo t t h a t the e t ch ra te is c o n s t a n t a t 3.1 A / m i n to a t h i c k n e s s of j u s t less t h a n 50A (70 rain). B e y o n d 70 ra in (be low 40A S i Q film th ickness ) , t he e t ch ra te de- c reases s ign i f ican t ly a n d at 90 m i n t he SiO2 film t h i c k n e s s r e a c h e s a m i n i m u m of 20/~. Af ter 90 rain, t he film th ick- ne s s r e m a i n s v i r tua l ly c o n s t a n t for t he d u r a t i o n of th i s ex- p e r i m e n t . The c o n s t a n t e t ch ra te in the in i t ia l s tages of im- m e r s i o n in e t c h a n t is e x p e c t e d b e c a u s e hydrof luor i c acid is k n o w n to e t ch S i Q (1). The u n e x p e c t e d re su l t is t h a t the fi lm t h i c k n e s s n e v e r r eaches a zero value.

Ca lcu la t ion shows t h a t over 30,000A of S i Q cou ld be e t c h e d f rom the s a m p l e cell wal ls a n d the Si s a m p l e be fo re t h e H F w o u l d be en t i r e ly dep le ted , so t he r e s idua l film is n o t a r e su l t of d e p l e t i o n of t he H F in the e t ch solut ion. In o rde r to ru le ou t the poss ib i l i ty of sy s t ema t i c e x p e r i m e n t a l error , ca lcu la t ions of t he c h a n g e in t h i c k n e s s o f a 20A SiO2 film fo l lowing sys t ema t i c va r i a t ion of 4), nA, a n d nslo2 are p e r f o r m e d . Er rors of as m u c h as -+ 5 ~ in 4) lead to c h a n g e s of on ly 4A in fi lm t h i c k n e s s a n d er rors of -+ 0.001 in nA ( ten t i m e s t he i n s t r u m e n t a l u n c e r t a i n t y of t he r e f r ac tome te r ) l ead to c h a n g e s of less t h a n 1A in SiO2 t h i c k n e s s . The S i Q

) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 128.118.88.48Downloaded on 2014-11-08 to IP


Vol. 135, No. 6 A Q U E O U S H F T R E A T M E N T O F S I L I C O N 1537

OXIDE THICKNESS v s . ETCH TINE P S I v s . DELTA FOR HF ETCH EXPERIMENTS

250

200

r.n 150 GB W Z b~

~ 100 I. .-

50

0

0

0

0 I I I 0 20 40

0

0

I

60

R e s i d u e !

0 0 0 0 0 0 0

I I r

80 :tOO 120

TIME (MINUTES)

OC

i 4 0

Fig. 3. Plot of etch time vs. Si02 film thickness for an Si02 film on a Si substrate immersed in 450:1 H20:HF solution.

re f rac t ive i n d e x u sed is 1.465, a n d it is a s s u m e d to be uni- f o rm t h r o u g h o u t the SiO2 film. Taft (25) shows e v i d e n c e t h a t dens i f i ca t ion of SiO2 occurs nea r t he Si-SiO~ in t e r f ace l e a d i n g to a h i g h e r SiO2 re f rac t ive i n d e x a n d p r o p o s e s t he e x i s t e n c e of an in t e r l aye r at t he S i - S i Q in te r face w i t h re- f rac t ive i n d e x of 2.9. U s i n g th i s index , a t h i c k n e s s of ove r 8A w o u l d sti l l be ca lcu la ted as a r e s idua l fo l lowing H F etch. I f ns,o2 is ac tua l ly lower in m a g n i t u d e t h a n t he va lue used , t h e n c h a n g e s in t h i c k n e s s of la rger m a g n i t u d e resul t , b u t t h e s e a lways lead to g rea te r fi lm t h i c k n e s s t h a n t h a t re- por ted , so t he r e s idua l wou ld be larger . The fact t ha t the c h a n g e s in t h i c k n e s s i n t r o d u c e d by large sys t ema t i c varia- t ion of t he e x p e r i m e n t a l p a r a m e t e r s are re la t ive ly smal l a n d ver i f i ca t ion of the r e s idua l by r epea t ed e x p e r i m e n t s o n s imi la r s a m p l e s en s u r e s t h a t the r e su l t is no t spur ious .

The da ta of Fig. 3 h a s b e e n ana lyzed u n d e r the a s s u m p - t ion t h a t a n SiO2 film is p resen t . I t is also poss ib le to ana- lyze t h e e l l i p some t r i c p a r a m e t e r s A a n d 1I to ob ta in the s u b s t r a t e op t ica l cons t an t s , a s s u m i n g no film is p r e s e n t on t h e subs t r a t e . A ca l cu la t ion of th i s t ype yields a va lue of 3.866-0.074i for the c o m p l e x i n d e x of the s a m p l e surface. Th i s is a n ave rage va lue of all m e a s u r e m e n t s t a k e n af ter 90 m i n in Fig. 3 w i th a dev i a t i on of _+ 0.002 in t he real par t and + 0.005 in t h e e x t i n c t i o n coefficient . This va lue can be com- p a r e d to a c c e p t e d va lues for t he opt ica l c o n s t a n t s deter- m i n e d for a Si s u b s t r a t e (26, 27) w h i c h are 3.865-0.018i. T h e r e is qu i t e close a g r e e m e n t in the real pa r t b u t less t h a n sa t i s fac to ry a g r e e m e n t in t he i m a g i n a r y part .

T h e e x i s t e n c e of t h e r e s idua l in the e t ch e x p e r i m e n t in- d ica tes t h a t a ba re Si s u b s t r a t e does no t r e su l t f r om e tch b y HF, so t he ca lcu la t ion of t he s u b s t r a t e opt ica l c o n s t a n t s is n o t e x p e c t e d to agree w i th t he a c c e p t e d values , bu t i t is u se fu l to c o m p a r e the ca lcu la ted and a c c e p t e d values . The be s t e v i d e n c e t h a t a r e s idua l fi lm ex i s t s on t he H F e t c h e d Si su r face is s een in a p lo t of t he r aw A, ~ data, Fig. 4, for s imi la r e t ch e x p e r i m e n t s . T he o p e n c i rc les r e p r e s e n t i n g t h e e x p e r i m e n t a l SiO2 e tch da ta fall c lose to the theo re t i ca l d a s h e d l ine for an SiO2 film on Si, b u t once t he m i n i m u m t h i c k n e s s has b e e n r e a c h e d (near h - 155~ t he po in t s fall o n a d i f f e ren t c u r v e as i l l u s t r a t ed by the t r iangles . T he fact t h a t t he A, �9 po in t s n e v e r r e a c h the ca lcu la ted zero ox ide t h i c k n e s s and the fact t h a t the va lues o b t a i n e d af ter t he m i n i m u m e x p e r i m e n t a l t h i c k n e s s fall on a d i f f e ren t cu rve leads to t he c o n c l u s i o n t h a t a film or layer o the r t h a n S i Q is f o r m e d on t he Si su r face in H F so lu t ion fo l lowing e tch- ing. S i n c e t he n e w film c o m p o s i t i o n is u n k n o w n , t he optical c o n s t a n t s of the film are u n k n o w n , so no ac tua l fi lm t h i c k n e s s can be ob ta ined . It is i n t e r e s t i ng to note, however , t h a t the reg ion in w h i c h t h e s e e x p e r i m e n t a l po in t s fall is i n d i c a t i v e of a fi lm t h a t is a b s o r b i n g at 6328A. The

I I I [ I I I I

CALCULATED ZERO o ETCH BACK OXIDE THICKNESS z~ LAYER GROWTH

-- - - THEORETICAL 180 L ~ CURVE

160 I tu ENTAL tu AY={T R rr MINIMUM OXIDE

RO H w THICKNESS: 2nm 2 :t40

~" ~ INITIAL OXIOE _J 0

oLU 120 ~o~k THICKNESS:85nm

ETCHIN iO0 ~ ~ " ~ ~ , : b ~ ~ ~:~d:'~~

I I ] r I I I I

0 2 4 6 8 :tO ~-2 ~4 ~.6

PS I (DEGREES)

Fig. 4. Plot of ellipsometric parameters ! ' vs. A for several in situ etch studies. Open circles are before minimum thickness is reached and triangles are after the minimum thickness. The filled circle represents the calculated A, �9 values for zero oxide thickness. The dashed line represents the theoretical ~ , & relationship for Si02 on Si in water am- bient.

on ly pos i t ive conc lu s ions f rom the ins i tu e l l i p somet r i c ana lys i s are t h a t e t c h i n g of S i Q in d i lu te a q u e o u s H F pro- ceeds f rom 230A d o w n to - 4 0 A film t h i c k n e s s and t h a t be- y o n d th i s po in t an a b s o r b i n g film t h a t is r e s i s t an t to and /o r f o r m s in H F r e m a i n s on the Si subs t ra t e , bu t t he film iden- t i ty a n d t h i c k n e s s are u n k n o w n . I t is t he in situ c o n t a c t ang le m e a s u r e m e n t t h a t is able to p rov ide i n f o r m a t i o n re- g a r d i n g t he i den t i t y of t he film.

S t a r t i n g w i th an e l l ipsomet r i ca l ly m e a s u r e d film th ick- ne s s of 230A of S i Q on Si, a con t ac t ang le of 39 ~ is meas- u r e d in de ion ized wa te r be fore any e t ch so lu t ion is in t roduced . W h e n t he e t ch so lu t ion is i n t roduced , t he con t ac t ang le c h a n g e s qu ick ly f rom the ini t ia l va lue of 39~ ~ a n d s tays at th i s va lue t h r o u g h o u t t he e t ch process . Af te r a b o u t 90 rain, suf f ic ient t i m e for e t ch of all t h e SiO2, t he con t ac t ang le c h a n g e s ve ry rap id ly to a va lue of 78 ~ a n d th i s va lue pe r s i s t s for as long as t he e t ch so lu t ion is p resen t .

The in i t ia l c h a n g e f rom 39 ~ to 8 ~ is, at th i s t ime, no t ye t ful ly e x p l a i n e d b u t obv ious ly m u s t be a t t r i b u t e d to the e t ch so lu t ion . The con t ac t ang le is d e p e n d e n t on the l iqu id su r face t e n s i o n (~/LV), a n d ~LV for d i lu te H F wou ld be differ- en t t h a n ~L'V for p u r e water . However , t he e t ch so lu t ion is so d i lu te t h a t a s ign i f ican t c h a n g e in (~LV) wou ld no t be ex- pec ted . The ini t ia l con t ac t ang le c h a n g e m u s t be a t t r i b u t e d to s o m e o t h e r effect of t he H F solut ion. S ince the two obvi- ous cha rac t e r i s t i c s of t he e t ch so lu t ion are ac id i ty a n d t he p r e s e n c e of f luor ide ions, the c h a n g e in c o n t a c t ang le can m o s t l ikely b e a t t r i b u t e d to one or t he o t h e r or b o t h of t h e s e e l ement s . W h e n a s a m p l e of 100A of SiO2 on Si is p l a c e d in 500:1 H20:HC1, a con t ac t ang le of 37 ~ is measured , a n d it r e m a i n s u n c h a n g e d for t he l h d u r a t i o n of t he e x p e r i m e n t . W h e n a s imi la r s a m p l e is p l aced in 400:1 H20:NH4F, the con t ac t ang le c h a n g e s to 8 ~ af te r a b o u t 30 rain. E l l i p some t r i c m e a s u r e m e n t of SiO2 t h i c k n e s s for an S i Q film on an Si s u b s t r a t e before a n d af ter i m m e r s i o n for 16h in 250:1 H20:NH4F ind ica t e s t h a t NH4F does no t e t ch SiO2. We t h e r e f o r e a sc r ibe the in i t ia l con t ac t ang le c h a n g e to s o m e effect of f luor ide ion on the ox ide surface.

T h e final con t ac t ang le change , f rom 8 ~ to 78 ~ , is m o s t sig- n i f i can t as far as co r re l a t ion to t he e l l i p somet r i c resul ts . F r o m the ear l ier d i scuss ion , it is e v i d e n t t h a t t he 8 ~ con t ac t ang le is i nd i ca t i ve of a hyd roph i l i c sur face as a n t i c i p a t e d for a m e t a l ox ide (h igh e n e r g y surface), wh i l e the 78 ~ ang le is i nd i ca t i ve of a h y d r o p h o b i c surface. The dras t i c c h a n g e in t he w e t t i n g b e h a v i o r i nd ica t e s t h a t the s a m p l e sur face



1538 J. E l e c t r o c h e m . Soc.: S O L I D - S T A T E S C I E N C E A N D T E C H N O L O G Y J u n e 1988

COS CONTACT ANGLE VS.

I I I I

SOLUTION SURFACE TENSION

I I I I I I

Table I. Surface constitution of various low energy films adsorbed on metal surfaces and the measured critical surface tension of the film

(after Ref. (23))

~.0

0 .8

<~ p-

~ 0 . 6 p -

u~ o

o _ C r i t i c 0 . 4 T e n s i o n = 2 7 d y n e s / c m ~ o

0 . 2

0 . 0 I I I I ( I I I I I 20 25 30 35 40 45 50 55 60 65 70 75

LV SURF. TENS. ( dynes / cm)

Fig. 5. Plot of cos 0 vs. 7bY for methanol:water solutions in contact with Si substrate from which the Si02 has been etched by HF. The inter- section of the plotted points with the cos 0 = 1 line gives a 7c of 27 dyn/cm.

changes significantly as the SiO2 is etched completely away. The sample surface is hydrophilic throughout the etch of SiO2 but becomes hydrophobic, apparently simul- taneous with the appearance of the residual film on the Si as seen using ellipsometry.

Since this residual surface is hydrophobic, it must have a low surface energy. With total exposure of the sample surface to solvent prior to gas bubble introduction, solvent molecules could adhere to the sample even after the gas bubble has floated to the sample surface. The influence of such adsorbed molecules is termed the spreading pressure and the variable ~r is used in a modified version of the Young equation (28) to account for alteration of the contact angle equilibrium.. Fowkes, McCarthy, and Mostafa (29) have shown that this ~ term is 0 for liquids of high surface tension, such as water, in contact with low energy surfaces, which means that the hydrophobic surface following HF etch in this experiment-is not a result of solvent-surface interactions. Determinations of the surface tension of the (100) plane of Si by measurment of the minimum energy required to propagate a crack yield values of over 2000 dyn/em (30). This categorizes Si as a high energy solid and it must be concluded that the hydrophobic surface is not Si because Si should be hydrophilic. The probable explanation for the low surface energy of the etched Si is adsorption of a surface film. Adsorbed surface films are known to drastically alter the wetting behavior (and there- fore the surface energy) of a solid, and the effect of these films on the wetting behavior is virtually independent of the solid on which they are adsorbed (23). According to this explanation, the wetting behavior of the Si surface indicates coverage of a layer of material impervious to the etch solution that is of lower surface tension than the am- bient etch solution.

Since the residual film on the etched Si has a low energy surface , a p lo t of cos 0 vs. 7LV can be der ived . By inc reas ing t he p r o p o r t i o n of m e t h a n o l in a m e t h a n o l / w a t e r solut ion, t he su r face t e n s i o n of t he so lu t ion can be va r i ed con t inu - ous ly f rom 72 d y n / c m (pure water) to 23 d y n / c m (pure m e t h a n o l ) (31). F ive d i f fe ren t m e t h a n o l / w a t e r so lu t ions are p r e p a r e d and, in each of the so lu t ions , 1% c o n c e n t r a t e d H F (by vo lume) is m a i n t a i n e d to i n su re t h a t a na t ive SiO2 layer does no t fo rm on t he Si surface. S a m p l e s of Si s u b s t r a t e s t h a t h a d b e e n e t c h e d in c o n c e n t r a t e d (49%) H F are im- m e r s e d in each of the m e t h a n o l / w a t e r so lu t ions a n d the c o n t a c t ang le is measu red . The r e su l t i ng cos 0 vs. 7LV plot is s h o w n in Fig. 5. Th e n o n l i n e a r i t y of the p lo t is obv ious a n d it has b e e n s h o w n to be due to h y d r o g e n b o n d i n g be-

Surface 7c constitution dyn/cm at 20~

Fluorocarbon surfaces --CF3 6 --CF2H 15 CF3 and --CF2-- 17

--CF2-- 18 --CH~CF~ 20 --CF2--CFH-- 22 --CF2--CHr-- 25 --CFH--CH2-- 28

Hydrocarbon surfaces --CH3 (crystal) 22 --CH3 (monolayer) 24

CH2-- 31 --CH~- and --CH-- 33 --CH-- (phenyl ring edge) 35

tween the liquid and the solid surface (23). The point of in- tersection between the experimental line and the cos 0 = 1 line gives 7c = 27 dyn/em. Table I shows possible surface structures and corresponding values of critical surface tension for films adsorbed on metal surfaces (23). Only hydrocarbon and fluorocarbon structures are found to have critical surface tensions close to 27 dyn/cm. This agrees with the indication of the absorbing nature of the film as determined by ellipsometric analysis, since many organic materials absorb in the infrared region. It appears as though the purely hydrocarbon structure may be ruled out due to the evidence of hydrogen bonding between the solid and liquid. Carbon-hydrogen bonds are not polar enough to hydrogen bond, but fluorine-carbon bonds are polar enough to allow hydrogen bonding between the solid and liquid. The source of fluorine is obviously the HF, and hydrocarbons are present in the form of organic residues from ion exchange resins used for water purifica- tion. At this time a specific fluorocarbon structure cannot be identified with the silicon surface, but it appears that the presence of a fluorocarbon species at the Si surface is the most probable explanation of a significant lowering of the solid surface tension by a species that can hydrogen bond with the liquid.

C o n c l u s i o n s The process of etching of SiO2 films on Si substrates by

dilute aqueous HF is studied by using two in situ tech- niques~ellipsometry and contact angle measurement. The in situ ellipsometrie analysis is used to track the etch of SiO2 by HF. It is found that the etch rate drops from 3.1 /~]min to 0 before a SiO2 film thickness of 0 is reached and while a significant HF concentration remains. Calculation of substrate optical constants, assuming no film is present on the etched sample surface, show an expected less than satisfactory agreement with the actual optical constants for a bare Si surface. A plot of the ellipsometric parameters

vs. A shows that a film that is not SiO2 forms on the sample surface following HF exposure. The optical constants of this film are unknown, so no calculation of thickness can be made, but the film does appear to be infrared absorbing. The in situ contact angle measurement shows that a Si substrate with a thermally grown SiO2 film on it is hydrophilic in the presence of dilute aqueous HF throughout etching and, following removal of the SiO2, the substrate surface is hydrophobic. Silicon dioxide and silicon surfaces are high energy and so the hydrophobic behavior of the surface following HF etch in this study indicates the existence of an adsorbed low energy film. Clues as to iden- tification of the film present on a Si substrate following aqueous HF etch are provided by analysis of the critical surface tension of a Si substrate from which the SiO2 film has been etched by HF. The only possibilities for satisfy- ing the ~c value obtained are films containing some type of hydrocarbon or fluorocarbon species, and this identifica- tion agrees well with the absorbing nature of the film. Due to the evidence of hydrogen bonding between surface and liquid, a fluorocarbon film seems most likely.



Vol. 135, No. 6 AQUEOUS HF TREATMENT OF SILICON 1539

Acknowledgment This work was sponsored in part by the Office of Naval

Research (ONR).

Manuscript submitted June 10, 1987; revised manuscript received Jan. 20, 1988.

.REFERENCES 1. S.M. Hu and D. R. Kerr, This Journal, 114, 414 (1967). 2. S. I. Raider, R. Flitsch, and M. J. Palmer, ibid., 122, 413

(1975). 3. F. J. Grunthaner and J. Maserjian, IEEE Trans. Nucl.

Sic., NS-24, 2108 (1977). 4. A. Licciardello, O. Puglisi, and S. Pignataro, Appl.

Phys. Lett., 48, 41 (1986). 5. E. Yablonovitch, D. L. Allara, C. C. Chang, T. Gmitter,

and T. B. Bright, Phys. Rev. Lett., 57, 249 (1986). 6. J. M. Charig and D. K. Skinner, Surf. Sci., 15, 277

(1969). 7. C. C. Chang, ibid., 23, 283 (1970). 8. R. C. Henderson, This Journal, 119, 772 (1972). 9. R. L. Meek, T. M. Buck, and C. F. Gibbon, ibid., 12@,

1241 (1973). 10. B.F. Philips, D. C. Burkman, W. R. Schmidt, and C. A.

Peterson, J. Vac. Sci. Technol., A1, 646 (1983). 11. R. J. Archer, This Journal, 104, 619 (1957). 12. F. P. Fehlner, ibid., 122, 1745 (1975). 13. G. Gould and E. A. Irene, ibid., 134, 1031 (1987). 14. W. Kern, RCA Rev., 31, 207 (1970). 15. K. D. Beyer and R. H. Kastl, This Journal, 129, 1027

(1982).

16. G. B. Larrabee, K. G. Heinen, and S. A. Harrell, ibid., 114, 867 (1967).

17. R. M. A. Azzam and N. M. Bashara, "Ellipsometry and Polarized Light," North-Holland Publishing Co., New York (1977).

18. F. L. McCrackin, E. Passaglia, R. R. Stromberg, and H.L. Steinberg, J. Res. Nat. Bur. Stand., 67A, 363 (1963).

19. G. A. Bootsma and F. Meyer, Surf. Sci., 14, 52 (1969). 20. W. Kern and D. A. Puotinen, RCA Rev., 31, 187 (1970). 21. R. G. Freiser, This Journal, 121, 669 (1974). 22. R. Williams and A. M. Goodman, Appl. Phys. Lett., 25,

531 (1974). 23. W. A. Zisman, in "Contact Angle: Wettability and Ad-

hesion," F. M. Fowkes, Editor, Chap. 1, Advances in Chemistry Series, Vol. 43, American Chemical Soci- ety, Washington, DC (1964).

24. D. McLachlan, Jr., and H. M. Cox, Rev. Sci. Instrum., 46, 80 (1975).

25. E. Taft and L. Cordes, This Journal, 126, 131 (1979). 26. D. E. Aspnes and J. Theeten, ibid., 127, 1359 (1980). 27. H. R. Philipp, J. Appl. Phys., 43, 2835 (1972). 28. H. W. Fox and W. A. Zisman, J. Colloid Sci., 5, 514

(1950). 29. F. M. Fowkes, D. C. McCarthy, and M. A. Mostafa, J.

Colloid Interface Sci., 78, 200 (1980). 30. R. J. Jaccodine, This Journal, 110, 524 (1963). 31. R. C. Weast, "CRC Handbook of Chemistry and Phys-

ics," p. F-35, CRC Press, Inc., Boca Raton, Florida (1981).

Growth and Etching of Germanium Films by Chemical Vapor Deposition in a GeCI4-H2 Gas System

Hiromu Ishii and Yasuo Takahashi

NTT Electrical Communications Laboratories, 3-1, Morinosato Wakamiya, Atsugi-shi, Kanagawa 243-01, Japan

ABSTRACT

The etching and growth of germanium films are investigated using a GeC14-H2 gas system in the temperature range of 490~176 At relatively low GeC14.partial pressures less than 2 x 10 .3 torl:, epitaxial growth of Ge is observed on Ge (100) surfaces, whereas at GeCI4 partial pressures higher than 2 • 10 _3 torr, etching of the Ge film is found to occur. In the experiments utilizing patterned substrates, where the surface consists of defined areas of Ge and SiQ, Ge is found to deposit selectively only on the exposed Ge regions. The growth reactions of Ge epitaxial films proceed through the Langmuir- Hinshelwood mechanism: the surface reaction takes place between two hydrogen atoms dissociatively adsorbed and a surface-adsorbed GeC12 molecule. GeC12 molecules adsorbed on the surface are formed directly from GeC14 molecules, not through gas-phase reduction by hydrogen. On the other hand, the etching reaction of Ge films is proved to be a reverse disproportionation reaction: GeC14 + Ge ~ 2GeCI2. Based on the analyses of growth and etching reactions, the equation of the Ge epitaxial film's growth rate is derived as a function of GeC14 partial pressure, hydrogen partial pressure, and growth temperature.

Recently, major interest has been shown in the applica- tion of Ge epitaxial film as an interlayer between a GaAs layer and a Si substrate (1, 2) or an ohmic contact layer in GaAs FET's (3). Chemical vapor deposition (CVD) and mo- lecular beam epitaxy (MBE) have been used in order to form Ge epitaxial films. Compared with the MBE tech- nique, one a d v a n t a g e of the CVD method is that ultrahigh vacuum cleaning of the substrate surface is unnecessary before Ge growth. Furthermore, another advantage of CVD is that selective growth applied for self-aligned LSI processes is possible since its film growth mechanism is based on surface chemical reactions (3, 10). For these rea- sons, the thermal decomposition of GeH4 has been exten- sively studied (4-10). The crystalline quality and electrical properties of grown Ge films have been investigated and the growth mechanism has been clarified (10). Compared with GeH4 system, the GeC14-H~ gas system has not been examined in great detail. Cave et al. tried to apply Ge epitaxial films formed using this gas system for semicon- ductor d e v i c e p r o c e s s e s (11). Miller et al. reported that b o t h g r o w t h and etching reactions of Ge film in the gas s y s t e m occur depending on GeClJI-I2 mole ratio at a con- stant temperature of 880~ (12). However, the Ge film for-

mation mechanisms necessary to improve crystalline quality and thickness controllability have not yet been clarified.

The film growth mechanism in halide-hydrogen gas system of SiC14-H2 was offered assuming chemical equilibrium in the gas phase (13). This assumption, however, is not reasonable since actual film growth conditions in CVD is in a nonequil ibr ium state (12). Analyses based on surface chemical reactions have successfully explained the film growth mechanisms in thermal decomposition of Sill4 (14) and GeH4 (10). The purposes of this work are, from the viewpoint of surface chemical reaction, to clarify the Ge epitaxial film growth and etching mechanisms in a GeC14- H2 gas system and to derive the equation of the Ge epitaxial film growth rate as a function of GeC14 partial pressure, hydrogen partial pressure, and growth temperature.

Experimental A low pressure CVD apparatus with a lamp-heated hori-

zontal reactor was used. The experiments were performed in the temperature range of 490~176 under a total pressure of 6.5 torr. The temperature was monitored by an optical pyrometer and the pressure was measured with a ca-



Documents

An in situ Study of Aqueous HF Treatment of Silicon by Contact Angle Measurement and Ellipsometry