- Email: [email protected]
Characterization of real surfaces of vitreous silica by ellipsometry
Mat. Res. Bull. Vol. 9, pp. 1503-1510, 1974. Printed in the United States.
P e r g a m o n P r e s s , Inc.
CHARACTERIZATION OF REAL SURFACES OF VITREOUS SILICA BY ELLIPSOMETRY* K. Vedam** and M. Malin Materials R e s e a r c h Laboratory The Pennsylvania State UniversRy University Park, Pennsylvania 16802 (Received September 17, 1974; Communicated by R. C. DeVries) ABSTRACT It is shown that e l l t p s o m e t r y can be successfully used to c h a r a c t e r i z e the contaminant surface film as well as the damaged s u r face l a y e r s o n optically t r a n s p a r e n t m a t e r i a l s . The r e s u l t s of e l l i p s o m e t r i c m e a s u r e m e n t s on vitreous silica s p e c i m e n s subjected to various surface t r e a t m e n t s such as mechanical polishing, c h e m i c a l etching and s p u t t e r - c l e a n i n g a r e p r e s e n t e d and discussed. Introduction It is well known that all m a t e r i a l s exposed to the n o r m a l a t m o s p h e r e a r e c o v e r e d with contaminant f i l m s of oxide or nitride or sulfide, etc. on the surface. F u r t h e r , the surface l a y e r s of the m a t e r i a l s t h e m s e l v e s a r e usually damaged o r under strain, depending on t h e i r preparative history. Recently it has b e e n shown (1) that e l l i p s o m e t r y can be used to c h a r a c t e r i z e nond e s t r u c t i v e l y both t h e s e contaminant films as well as the "damaged" surface l a y e r s on optically opaque m a t e r i a l s like silicon. E llipsometr~ of T r a n s p a r e n t M a t e r i a l s T h e o r e t i c a l calculations using exact equations of e l l i p s o m e t r y r e v e a l that the sensitivity for detectin~ and c h a r a c t e r i z i n ~ the contamLnant films on g l a s s e s and other t r a n s p a r e n t m a t e r i a l s by ellLpsometry is very high if the an~le of incidence is close to the B r e w s t e r ' s (or principal) an~le ~B. Howe v e r , p r e l i m i n a r y m e a s u r e m e n t s on vitreous silica (optical ~rade Suprasil Type I) r e v e a l e d that as the angle of incidence approaches ~bB, the e x p e r i mental u n c e r t a i n t y also i n c r e a s e s . In other words, w h e r e t h e o r e t i c a l l y the m e a s u r e m e n t should be most sensitive, e x p e r i m e n t a l uncertainty is also very high. This b e c o m e s c l e a r on examination of Fig. 1, which shows the v a r i a * R e s e a r c h supported by the Office of Naval Research, Metallurgy P r o g r a m . **Also affiliated with the D e p a r t m e n t of Physics. 1503
1504
SILICA-SURFACE S T U D Y B Y E L L I P S O M E T R Y
I &
/~
~
I11
I
~
Vol. 9, No. II
le~ J WTREOUSS/Lea
7 : ' ~o.
r nl = 1.40
330~ i ~iI" ~!.J O¢0 ¢0=~.7. ="R"~' 0 6",
I~0
----n1=1"52
~
[
2C
r
~ ,I.61
FIG. 2 1.0
R.O
v/ (degrees)
FIG.
3,0
(4,'~) curves for vitreous silica for varLous c o n t a m i n a n t ftlrns on it.
1
tton of the ellLpticity p a r a m e t e r s A and ~ with f i l m t h i c k n e s s at f o u r d i f f e r e n t v a l u e s of the a n g l e of i n c i d e n c e on v i t r e o u s s i l i c a . It is s e e n that a s the a n g l e of i n c i d e n c e Ls r e d u c e d f r o m 56.6 ° to 5 5 . 7 °, f o r an i n c r e a s e in f i l m t h i c k n e s s of 50,~, A i n c r e a s e s f r o m about 10 ° to 60 °. As the a n g l e of Lncidence is f u r t h e r r e d u c e d to 55.60 ° (note: CB = 55.59 °) it c a n be s h o w n that the c h a n g e in A b e c o m e s a l m o s t 360°; i. e . , w h e n ¢ = ~B, ~ b e c o m e s e x p e r i m e n t a l l y i n d e t e r m i n a t e e v e n thouqh t h e o r e t i c a l l y it is m o s t s e n sitive. C o n s e q u e n t l y , we h a v e to a r r L v e at a c o m p r o m i s e to obtain f a i r l y high sensLtivity and yet r e a s o n a b l y low d e g r e e of u n c e r t a i n t y in the r e s u l t s . A f t e r a few t r i a l m e a s u r e m e n t s , it w a s decLded to fix the an~le of i n c i d e n c e at 58 °, L. e . , about 2 . 4 ° m o r e t h a n ~B f o r v i t r e o u s s i l i c a . V a r i a t i o n of ~ and ~ in d e g r e e s with f i l m t h i c k n e s s in ,~, f o r light i n c i d e n t f r o m a i r onto v a r i o u s f i l m s of r e f r a c t i v e LndLces n l on a t r a n s p a r e n t s u b s t r a t e of r e f r a c t i v e index n 2.
SLnce the r e f r a c t i v e index of v i t r e o u s s i l i c a is known p r e c i s e l y , with the f i x a t i o n of the a n g l e of i n c i d e n c e ~o a s 58 °, it is an e a s y m a t t e r to c o m pute the v a l u e s of (Zl,~.) f o r v a r i o u s v a l u e s of r e f r a c t i v e i n d i c e s and t h i c k n e s s e s of film, m a k i n ~ u s e of the e x a c t e q u a t i o n s of e l l i p s o m e t r y . Such c u r v e s a r e s h o w n Ln Fig. 2. F i e u r e 3 r e p r e s e n t s the e n l a r g e d versLon of F i g . 2 at low f i l m t h i c k n e s s e s . It Ls s e e n that f o r the Ldeal f i l m f r e e s u r f a c e of v i t r e o u s s i l i c a 4 o = 0 a n d ~ o = 3 . 7 9 ° ; and a s the f i l m t h i c k n e s s L n c r e a s e s both and ~ Lncrease i~ the r e f r a c t i v e index of the f i l m n 1 Ls s m a l l e r t h a n that (n2) of v i t r e o u s s i l i c a . But Lf n l is g r e a t e r t h a n n2, both A and " ~ d e c r e a s e as f i l m t h i c k n e s s i n c r e a s e s . H a s h m a r k s a r e Lndicated at e v e r y 100~ i n c r e a s e in t h i c k n e s s of fLlm. H e r e it is w o r t h w h i l e to point out that (4, ~) c u r v e s for o t h e r t r a n s p a r e n t ~ l a s s e s of d i f f e r e n t r e f r a c t i v e i n d i c e s w i l l be v e r y s i m i l a r , e x c e p t that the value of ~b d e c r e a s e s a s the r e f r a c t i v e index of the s u b s t r a t e
Vol. 9, No. 11
S I L I C A - S U R F A C E STUDY BY E L L I P S O M E T R Y
1505
~lass increases and vice versa. t t31
F r o m a (A, ~) measurement on a specimen of vitreous silica, we can immediately fix the refractive index n I and iooZ thickness of the film d I on the vitreous silica with the help of Fi~. 2 provided, of course the vitreous silica is free from o d a m a g e d surface layers. Consider on the other hand the case where the sur4.5 35 37 3.9 4.1 4.3 face layers are d a m a g e d (say, densified) ~1 ( ° ) <~ such that the refractive index of the d a m a g e d l a y e r is h i ~ h e r t h a n 1. 460, the r e f r a c t i v e index of v i t r e o u s s i l i c a . It is obvious that f o r s u c h a s y s t e m "~o the value of '÷' c o r r e s p o n d [ n ~ to z e r o f i l m ' VITREOUS SILICA 345 t h i c k n e s s will be l e s s t h a n 3 . 7 9 ° c o r r e s r n2 ,T 1 . 4 ~ ponding to the i n c r e a s e d " e f f e c t i v e " value ¢ ,~ 5 8 " of the r e f r a c t i v e index of the s u b s t r a t e . ooJ ~ = 5,=s/~, %= 1.61 If a f i l m (say, of w a t e r ) is to f o r m on the 335 s u r f a c e of s u c h a v i t r e o u s s i l i c a with d a m a g e d s u r f a c e l a y e r s the (z~,'2) c u r v e FIG. 3 would c o r r e s p o n d i n g l y be shifted to the E n l a r g e d v e r s i o n of FLg. 2 at low left in FiB. 2. H e n c e if by a c a r e f u l film thicknesses. s e r i e s of m e a s u r e m e n t s with f i l m f r e e s u r f a c e of s u c h a s y s t e m ~1o can be d i r e c t l y m e a s u r e d , t h e n we c a n i m m e d i a t e l y c h a r a c t e r i z e the d a m a g e d l a y e r by its e f f e c t i v e r e f r a c t i v e index and t h i c k n e s s . In o t h e r w o r d s I( 7~ -'~o)1 Ls a m e a s u r e of the d e ~ r e e of p e r f e c t i o n of the s u b s t r a t e .
~
F u r t h e r , it is s e e n f r o m Fig. 3 that when the t h i c k n e s s of the f i l m is l e s s t h a n about 50/~, ~' is e s s e n t i a l l y c o n s t a n t i r r e s p e c t i v e of the r e f r a c t i v e index of the film, just like the c a s e of s i l i c o n (1). H e n c e , we m a y c o n c l u d e that at low f i l m t h i c k n e s s e s I ( % - A)I and I( 9o - ',~') i can r e s p e c t i v e l y be c o n s i d e r e d a m e a s u r e of the f i l m t h i c k n e s s and the d e g r e e of p e r f e c t i o n of the subs t r a t e . T h i s s t a t e m e n t a p p e a r s to be valid f o r both opaque a s w e l l a s t r a n s parent materials. Havin~ thus e s t a b l i s h e d the feasibLltty of c h a r a c t e r i z a t i o n of r e a l s u r f a c e s of t r a n s p a r e n t m a t e r i a l s by e l l i p s o m e t r y , the r e s u l t s of e l l t p s o m e t r i c m e a s u r e m e n t s on v i t r e o u s s i l i c a s p e c i m e n s s u b j e c t e d to v a r i o u s s u r f a c e t r e a t m e n t s s u c h a s m e c h a n i c a l polishing, c h e m i c a l e t c h i n g and s p u t t e r c l e a n i n ~ by d i f f e r e n t ions, a r e p r e s e n t e d and d i s c u s s e d below. R e s u l t s and D i s c u s s i o n T a b l e 1 l i s t s the e l l i p s o m e t r t c p a r a m e t e r s m e a s u r e d on a s a m p l e of v i t r e o u s s i l i c a , w h i c h w a s m e c h a n i c a l l y p o l i s h e d a c c o r d i n g t o the u s u a l p r o c e d u r e of u s i n ~ d i a m o n d p a s t e s with p r o g r e s s i v e l y d e c r e a s i n g g r a i n size. A f t e r p o l i s h i n g as w e l l a s f u r t h e r s u r f a c e t r e a t m e n t s to be d i s c u s s e d below, the s p e c i m e n w a s t h o r o u g h l y w a s h e d and r i n s e d with AR quality a c e t o n e and e t h e r in s u c c e s s i o n and d r i e d b e f o r e e l l t p s o m e t r i c m e a s u r e m e n t s w e r e c a r r i e d out.
1506
SILICA-SURFACE
STUDY
TABLE
BY ELLI PSOMETRY
1
E H t p s o m e t r t c p a r a m e t e r s m e a s u r e d on v i t r e o u s s i l i c a polished with diamond paste. n 2 = 1.460; Z = 5461°; ~b = 58 ° . D a m a g e d laye v
Surface History
~i
f
A
in d e g r e e s
ideal c a s e
3.79
360.00
0. 52
290. 00 334. 00
2.55 3.93 4.16 4.04 4.05 4.05 3.99
4. 3. 1. 0. 0. 0.
-
00 04 90 96 96 71
II
R is seen that the value of i~ m e a s u r e d on the mechanically polished sample is considerably different f r o m 3.79 ° , the value corresponding to the ideal d a m a g e f r e e and f i l m - f r e e
mechanically polished & uRrasonLc cleanLng in
acetone 2 rain. etch in 5 % HF 4 rain. " " " " 6 min. " " " "
Vol. 9, No.
1.530 1. 502 1.448 1.452 1.455 1. 455 1.455 1.456
967 627 334 760 790 97O 970 975
case. Reference to Fig. 2 indicates that the m e a s u r e d set of (A,~) values can arise due to the presence of a film or d a m a g e d layer of
r e f r a c t i v e index of about I. 52 on the 8 min. " " " " s u b s t r a t e of vitreous 12 mLn. " 'f " " silica. Exact c a l c u 60min. " " " " lations wLth the help 900 min. " " " " of a c o m p u t e r yielded the values 1. 530 and 967)~ as the refractLve index n 2' and t hick n es s d 2' of the s u r f a c e l a y e r . This i n c r e a s e d value of the r e f r a c t i v e index of the su rface l a y e r indicates that the s u r f a c e l a y e r s of the v i t r e o u s sLlica s p e c i m e n have b e e n p e r m a n e n t l y densified during the polishing stage. S i m i l a r o b s e r v a t i o n s of the p r e s e n c e of densified l a y e r on v a r i o u s g l a s s e s have been made by o t h e r w o r k e r s (2, 3). In the c a s e of silicon (1) w h e r e also such o b s e r v a t i o n s of the p r e s e n c e of d a m a g e d s u r f a c e l a y e r s on m e c h a n i c a l l y polished s p e c i m e n s w e r e noticed, it was found that c h e m i c a l polishing techniques can s u c c e s s f u l l y r e m o v e the d a m a g e d s u r f a c e l a y e r s . Hence, the vitreous s i l i c a sj~ecLmens w e r e also subjected to a s e r i e s of c h e m i c a l polishing t r e a t m e n t in 5 % H F solution and the r e s u l t s of such chemLcal polishing a r e also ~iven in Table 1. It is s e e n that in the e a r l y stages the HF solution does r e d u c e the t h i c k n e s s of d a m a g e d s u r f a c e l a y e r s - - as s e e n f r o m the i n c r e a s e of ~ and a l s o f r o m the values of n 2' and d 2 computed f r o m t h e s e (A, ~,) values. However, it is s e e n that as d is s o lu tio n of v i t r e o u s s i l i c a in HF p r o c e e d s by continued etch in g in HF at r o o m t e m p e r a tu re , the o u t e r m o s t l a y e r s a r e found to have distinctly d ifferen t effective r e f r a c t i v e index in such s p e c i m e n s subjected to diamond polishing in the e a r l y stages. T h e s e studies c l e a r l y Lndicate that c h e m i c a l polishing of v i t r e o u s sLlica which have been m e c h a n i c a l l y polished with diamond paste e a r l i e r does not yield a s t r a i n - or d e f e c t - f r e e s u r f a c e . Anot he r a p p r o a c h to this p r o b l e m of r e m o v a l of the densified s u r f a c e l a y e r s is by s p u t t e r - c l e a n i n g the s u r f a c e , u sin g low e n e r g y inert gas tons to bombard the s u r f a c e Ln high vacuum. In p a r t i c u l a r , it was felt the u s e of v e r y low e n e r g y ions - - L e . , ions with just sufficient e n e r g y to knock loose the s u r f a c e a t o m s , e s p e c i a l l y in the last stages of the s p u t t e r - c l e a n i n g
Vol. 9, No. 11
SILICA-SURFACE STUDY BY ELLIPSOMETRY
1507
p r o c e s s might TABLE 2 yield a clean Ellipsometric parameters measured on vitreous silica surface withpolLshed with diamond taste. out caus ing any damage to n2 d2 the substrate. Surface history (o) (o) Table 2 lists the values of A 0. 00 ideal case 3.79 and ~ m e a s u r e d before and 1.560 mechanically polished 1.38 2 1 2 . 4 3 925 after sputter* +Ar ions sputtered 4.03 354.36 + O ions sputtered 4.00 360.02 1. 456** 1150"* cleaning, vitreous silica s p e c i m e n with a r g o n ions. *solution with t r a n s p a r e n t film condition not possLble. The sputterLn~ * * s e e text. p r o c e d u r e adopted was to use 500 ev a r g o n ions at RF power level of 50 watts for an initial p e r iod of 20 minutes, followed by low e n e r g y a r g o n ions 50 ev (RF power ~-5 watts) for 10 minutes. It may be mentioned that the RF power level of 5 watts was found to be the minLmum level at which the plasma could be sustained in our e x p e r i m e n t a l s y s t e m (MRC sputtering unit). It was found that the vitreous sLlica s p e c i m e n exhibited a faint brownish tint on the surface when subjected to s p u t t e r - c l e a n i n g with a r g o n ions - - possibly due to oxygen deficiency in the surface l a y e r s of the sputtered specimen. Hence, the s p e c i m e n was subjected to another cycle of sputtering but with oxygen ions, in the hope that some of the s p u t t e r i n g oxygen ions may get trapped in the surface. However, f r o m Table 2 it is s e e n that the value of ~ is still far f r o m 3.79 ° c o r r e s p o n d i n g to the s t r a i n free surface. In other words though a clean surface can be produced by m e c h a n i c a l polishing using diamond paste followed by s p u t t e r cleaning, the surface so produced is far from the desLred ideal surface. !
T
,ll
E r n s b e r g e r (4) has shown by ion-exchange technique the p r e s e n c e of n u m e r o u s Grtffith (5) c r a c k s on the s u r f a c e s of plate glass and has d e m o n s t r a t e d that m e c h a n i c a l d a m a g e to g l a s s s u r f a c e s would result in flaws which can and often do s u r vive the polishing operation. V e r y r e c e n t l y H o m e r and Crawford (6) have d i r e c t l y o b s e r v e d with s c a n ning e l e c t r o n m i c r o s c o p e shallow s a u c e r shaped etch pits (Fig. 4) on the s u r f a c e s of a n u m b e r of glasse~ which w e r e lightly abraded and polished (or fire polished) before etching in HF solutions for varying perLods of FIG. 4 time. In other words vitreous Schematic d i a g r a m of damaged surface silica s p e c i m e n s polished with l a y e r produced d u r i n g polishing with diamond paste g e n e r a l l y exhLbit diamond paste, before and a f t e r etchLng rough s u r f a c e morphology, in HF. though on a m i c r o s c o p i c scale,
1508
SILICA-SURFACE S T U D Y B Y ELLI P S O M E T R Y
Vol. 9, No. 11
on etching with HF. It is r e a s o n a b l e to expect a s i m i l a r behavior if the s p e c i mens were s p u t t e r - c l e a n e d instead of etched in HF. The influence of such a rough morphology on the e l l i p s o m e t r i c p a r a m e t e r s should be considered. However, the t h e o r e t i c a l analysis (7-9) of this problem, taking into account the s t a t i s t i c a l nature of the rough surface is yet to be worked out so that meanLngful c o m p a r i s o n with the e x p e r i m e n t a l r e s u l t s can be made. At the s a m e time it will be e x t r e m e l y useful to explore this problem e x p e r i m e n t a l l y , by making use of scanning e l e c t r o n m i c r o s c o p e and the CESEMI (Computer EvaluatLon of Scanning E l e c t r o n Microscope Images) technique developed by White et al. (10) o r the FECO (Fringes of Equal Chromatic Order) i n t e r f e r o m e t r i c technique developed by Bennett et al. (11) to c h a r a c t e r i z e the roughness of the surface and m e a s u r e the e l l i p s o m e t r i c par a m e t e r s on the s a m e sample. Table 2 brings out another important feature worth mentioning. In the case of a r g o n ion s p u t t e r - c l e a n e d s p e c i m e n s it was mentioned e a r l i e r that the surface exhLbited faLnt b r o w n i s h tint. Efforts to obtaLn n2' and d2' with the a s sumption that the film is non-absorbing was not successful, i n d i c ~ t L n g that the surface film i s optically absorbLng. Durin~ the course of t h e s e e x p e r i m e n t s , it was found that vitVitreous silica poILshed wRh CeO2. reous silica s p e c i m e n s polished with c e r i u m oxide yielded "~'and A A values quite close to ~ o and Ao. Possibly the r e a s o n for the s u c c e s s Surface history (o~ (°) of CeO2 i~ polishLng without leaving behind a sLgniftcant damaged layer ideal case 3.79 360. 00 should be attributed to its h a r d n e s s mechanically polished 3.79 359.64 (5 Mohs) being close to that of vit2 rain. e t c h i n 5 % H F 3.77 359, 93 reous silica (4.9 Mohs). F r o m 360.29 6 rain. " " " " 3.80 Table 3 it is seen that the d i s s o l u 20mtn. " " " " 3.80 360.43 tion of vitreous sL1Lca polished with 720rain. " " " " 3.81 360. 80 CeO2 Ln dilute HF is also quite dLff e r e n t f r o m that given in Table 1 for vitreous silica polished with diamond paste. In o t h e r words, t h e r e appears to be no evidence for the p r e s e n c e of the thin l a y e r with effective r e f r a c tive index 1.455 and thickness of TABLE 4 about 1000~ on the surface of vitVitreous silica polished with CeO2. reous silica polished with CeO 2 and subsequently etched in HF.
TABLE 3
i
,
. . . . .
m
i
i
i
r
|
i
i
Surface h i s t o r y ideal case mechanically polished + A r ions sputtered +O ions sputtered
(°) L3" 79
;_;;3.77 3.77
(°) 0.00 , - - - - - - - - - - - -
358.57
358.79
Table 4 lists the e l l i p s o m e t rLc p a r a m e t e r s of vitreous sLlica s p e c i m e n s polished with c e r i u m oxide, on sputtering with argon and oxygen ions. It is seen that "~is e s s e n t i a l l y constant within e x p e r i mental e r r o r . The r e c e n t field ion m i c r o s c o p i c studies (12) on s u r faces bombarded with various ions
Vol. 9, No. 11
SILICA-SURFACE STUDY BY ELLIPSOMETRY
1509
r e v e a l that in ~eneral the c r y s t a l l i n e o r d e r of the outermost l a y e r s are totally destroyed even when the ener~3T of the bombarding ions is f a i r l y low (--50 eV). Since the vitreous s i l i c a is a l r e a d y a n o n c r y s t a l l i n e m a t e r i a l the d i s o r d e r in its outermost l a y e r s cannot be f u r t h e r i n c r e a s e d by s p u t t e r - c l e a n i n g p r o c e s s and hence it is not s u r p r i s i n ~ to find the value of y observed on such s p u t t e r cleaned vitreous silica to be a l m o s t the same a s ~ o corresponding to the ideal case.
Refe r e n c e s 1. K. Vedam and S. S. So, Surface ScL 299, 379 (1972). 2. F. M. E r n s b e r C e r , Ann. Rev. Mat. Sci. 2_, 529 (1972). 3. H. Yokota, H. Sakata, M. Nishibori and K. Kinosita, Surface Sci. 1~6, 265 (1969). 4. F. M. E r n s b e r ~ r , P r o c e s s in C e r a m i c Science, Vol. 3, p. 58. P e r a'amon P r e s s , New York (1963); Advances in Glass Technology, p. 511. Plenum P r e s s , New York (1962). 5. A. A. Griffith, Phil. T r a n s . Roy. Soc. 221A, 163 (1920). 6. P. N. Homer and B. J. Crawford, Glass Tech. 11, 10 (1970). 7. C. A. F e n s t e r m a k e r and F. L. McCrackin, Surface Sci. 16, 85 (1969). 8. I. Ohlidal and F. Lukes, Opt. Acta 1_99, 817 (1972). 9. R. S. Sirohi, Optics Communs. 1, 304 (1970). 10. E. W. White, H. A. McKinstry and A. Diness, The Science of C e r a m i c Machining and Surface F i n i s h i n g , p. 309. NSF Spec. Publ. #348 (May 1972). 11. H. E. Bennett, J. M. Bennett, J. L. Stanford and P. C. Archibald, Proc. Third Conf. on High Power Infrared L a s e r Window M a t e r i a l s , p. 1127. A F C R L - T R - 7 4 - 0 0 8 5 (HI), Special Report No. 174 (1974). 12. J. M. Walls, E. Braun and H. N. Southworth, Proc. Sixth Int. Vac. C o n g r e s s 1974, Jap. J. Appl. Phys. (1974)(to be published); J. M. Waals, Ph.D. T h e s i s , University of Aston, B i r m i n g h a m , England (1973).
P e r g a m o n P r e s s , Inc.
CHARACTERIZATION OF REAL SURFACES OF VITREOUS SILICA BY ELLIPSOMETRY* K. Vedam** and M. Malin Materials R e s e a r c h Laboratory The Pennsylvania State UniversRy University Park, Pennsylvania 16802 (Received September 17, 1974; Communicated by R. C. DeVries) ABSTRACT It is shown that e l l t p s o m e t r y can be successfully used to c h a r a c t e r i z e the contaminant surface film as well as the damaged s u r face l a y e r s o n optically t r a n s p a r e n t m a t e r i a l s . The r e s u l t s of e l l i p s o m e t r i c m e a s u r e m e n t s on vitreous silica s p e c i m e n s subjected to various surface t r e a t m e n t s such as mechanical polishing, c h e m i c a l etching and s p u t t e r - c l e a n i n g a r e p r e s e n t e d and discussed. Introduction It is well known that all m a t e r i a l s exposed to the n o r m a l a t m o s p h e r e a r e c o v e r e d with contaminant f i l m s of oxide or nitride or sulfide, etc. on the surface. F u r t h e r , the surface l a y e r s of the m a t e r i a l s t h e m s e l v e s a r e usually damaged o r under strain, depending on t h e i r preparative history. Recently it has b e e n shown (1) that e l l i p s o m e t r y can be used to c h a r a c t e r i z e nond e s t r u c t i v e l y both t h e s e contaminant films as well as the "damaged" surface l a y e r s on optically opaque m a t e r i a l s like silicon. E llipsometr~ of T r a n s p a r e n t M a t e r i a l s T h e o r e t i c a l calculations using exact equations of e l l i p s o m e t r y r e v e a l that the sensitivity for detectin~ and c h a r a c t e r i z i n ~ the contamLnant films on g l a s s e s and other t r a n s p a r e n t m a t e r i a l s by ellLpsometry is very high if the an~le of incidence is close to the B r e w s t e r ' s (or principal) an~le ~B. Howe v e r , p r e l i m i n a r y m e a s u r e m e n t s on vitreous silica (optical ~rade Suprasil Type I) r e v e a l e d that as the angle of incidence approaches ~bB, the e x p e r i mental u n c e r t a i n t y also i n c r e a s e s . In other words, w h e r e t h e o r e t i c a l l y the m e a s u r e m e n t should be most sensitive, e x p e r i m e n t a l uncertainty is also very high. This b e c o m e s c l e a r on examination of Fig. 1, which shows the v a r i a * R e s e a r c h supported by the Office of Naval Research, Metallurgy P r o g r a m . **Also affiliated with the D e p a r t m e n t of Physics. 1503
1504
SILICA-SURFACE S T U D Y B Y E L L I P S O M E T R Y
I &
/~
~
I11
I
~
Vol. 9, No. II
le~ J WTREOUSS/Lea
7 : ' ~o.
r nl = 1.40
330~ i ~iI" ~!.J O¢0 ¢0=~.7. ="R"~' 0 6",
I~0
----n1=1"52
~
[
2C
r
~ ,I.61
FIG. 2 1.0
R.O
v/ (degrees)
FIG.
3,0
(4,'~) curves for vitreous silica for varLous c o n t a m i n a n t ftlrns on it.
1
tton of the ellLpticity p a r a m e t e r s A and ~ with f i l m t h i c k n e s s at f o u r d i f f e r e n t v a l u e s of the a n g l e of i n c i d e n c e on v i t r e o u s s i l i c a . It is s e e n that a s the a n g l e of i n c i d e n c e Ls r e d u c e d f r o m 56.6 ° to 5 5 . 7 °, f o r an i n c r e a s e in f i l m t h i c k n e s s of 50,~, A i n c r e a s e s f r o m about 10 ° to 60 °. As the a n g l e of Lncidence is f u r t h e r r e d u c e d to 55.60 ° (note: CB = 55.59 °) it c a n be s h o w n that the c h a n g e in A b e c o m e s a l m o s t 360°; i. e . , w h e n ¢ = ~B, ~ b e c o m e s e x p e r i m e n t a l l y i n d e t e r m i n a t e e v e n thouqh t h e o r e t i c a l l y it is m o s t s e n sitive. C o n s e q u e n t l y , we h a v e to a r r L v e at a c o m p r o m i s e to obtain f a i r l y high sensLtivity and yet r e a s o n a b l y low d e g r e e of u n c e r t a i n t y in the r e s u l t s . A f t e r a few t r i a l m e a s u r e m e n t s , it w a s decLded to fix the an~le of i n c i d e n c e at 58 °, L. e . , about 2 . 4 ° m o r e t h a n ~B f o r v i t r e o u s s i l i c a . V a r i a t i o n of ~ and ~ in d e g r e e s with f i l m t h i c k n e s s in ,~, f o r light i n c i d e n t f r o m a i r onto v a r i o u s f i l m s of r e f r a c t i v e LndLces n l on a t r a n s p a r e n t s u b s t r a t e of r e f r a c t i v e index n 2.
SLnce the r e f r a c t i v e index of v i t r e o u s s i l i c a is known p r e c i s e l y , with the f i x a t i o n of the a n g l e of i n c i d e n c e ~o a s 58 °, it is an e a s y m a t t e r to c o m pute the v a l u e s of (Zl,~.) f o r v a r i o u s v a l u e s of r e f r a c t i v e i n d i c e s and t h i c k n e s s e s of film, m a k i n ~ u s e of the e x a c t e q u a t i o n s of e l l i p s o m e t r y . Such c u r v e s a r e s h o w n Ln Fig. 2. F i e u r e 3 r e p r e s e n t s the e n l a r g e d versLon of F i g . 2 at low f i l m t h i c k n e s s e s . It Ls s e e n that f o r the Ldeal f i l m f r e e s u r f a c e of v i t r e o u s s i l i c a 4 o = 0 a n d ~ o = 3 . 7 9 ° ; and a s the f i l m t h i c k n e s s L n c r e a s e s both and ~ Lncrease i~ the r e f r a c t i v e index of the f i l m n 1 Ls s m a l l e r t h a n that (n2) of v i t r e o u s s i l i c a . But Lf n l is g r e a t e r t h a n n2, both A and " ~ d e c r e a s e as f i l m t h i c k n e s s i n c r e a s e s . H a s h m a r k s a r e Lndicated at e v e r y 100~ i n c r e a s e in t h i c k n e s s of fLlm. H e r e it is w o r t h w h i l e to point out that (4, ~) c u r v e s for o t h e r t r a n s p a r e n t ~ l a s s e s of d i f f e r e n t r e f r a c t i v e i n d i c e s w i l l be v e r y s i m i l a r , e x c e p t that the value of ~b d e c r e a s e s a s the r e f r a c t i v e index of the s u b s t r a t e
Vol. 9, No. 11
S I L I C A - S U R F A C E STUDY BY E L L I P S O M E T R Y
1505
~lass increases and vice versa. t t31
F r o m a (A, ~) measurement on a specimen of vitreous silica, we can immediately fix the refractive index n I and iooZ thickness of the film d I on the vitreous silica with the help of Fi~. 2 provided, of course the vitreous silica is free from o d a m a g e d surface layers. Consider on the other hand the case where the sur4.5 35 37 3.9 4.1 4.3 face layers are d a m a g e d (say, densified) ~1 ( ° ) <~ such that the refractive index of the d a m a g e d l a y e r is h i ~ h e r t h a n 1. 460, the r e f r a c t i v e index of v i t r e o u s s i l i c a . It is obvious that f o r s u c h a s y s t e m "~o the value of '÷' c o r r e s p o n d [ n ~ to z e r o f i l m ' VITREOUS SILICA 345 t h i c k n e s s will be l e s s t h a n 3 . 7 9 ° c o r r e s r n2 ,T 1 . 4 ~ ponding to the i n c r e a s e d " e f f e c t i v e " value ¢ ,~ 5 8 " of the r e f r a c t i v e index of the s u b s t r a t e . ooJ ~ = 5,=s/~, %= 1.61 If a f i l m (say, of w a t e r ) is to f o r m on the 335 s u r f a c e of s u c h a v i t r e o u s s i l i c a with d a m a g e d s u r f a c e l a y e r s the (z~,'2) c u r v e FIG. 3 would c o r r e s p o n d i n g l y be shifted to the E n l a r g e d v e r s i o n of FLg. 2 at low left in FiB. 2. H e n c e if by a c a r e f u l film thicknesses. s e r i e s of m e a s u r e m e n t s with f i l m f r e e s u r f a c e of s u c h a s y s t e m ~1o can be d i r e c t l y m e a s u r e d , t h e n we c a n i m m e d i a t e l y c h a r a c t e r i z e the d a m a g e d l a y e r by its e f f e c t i v e r e f r a c t i v e index and t h i c k n e s s . In o t h e r w o r d s I( 7~ -'~o)1 Ls a m e a s u r e of the d e ~ r e e of p e r f e c t i o n of the s u b s t r a t e .
~
F u r t h e r , it is s e e n f r o m Fig. 3 that when the t h i c k n e s s of the f i l m is l e s s t h a n about 50/~, ~' is e s s e n t i a l l y c o n s t a n t i r r e s p e c t i v e of the r e f r a c t i v e index of the film, just like the c a s e of s i l i c o n (1). H e n c e , we m a y c o n c l u d e that at low f i l m t h i c k n e s s e s I ( % - A)I and I( 9o - ',~') i can r e s p e c t i v e l y be c o n s i d e r e d a m e a s u r e of the f i l m t h i c k n e s s and the d e g r e e of p e r f e c t i o n of the subs t r a t e . T h i s s t a t e m e n t a p p e a r s to be valid f o r both opaque a s w e l l a s t r a n s parent materials. Havin~ thus e s t a b l i s h e d the feasibLltty of c h a r a c t e r i z a t i o n of r e a l s u r f a c e s of t r a n s p a r e n t m a t e r i a l s by e l l i p s o m e t r y , the r e s u l t s of e l l t p s o m e t r i c m e a s u r e m e n t s on v i t r e o u s s i l i c a s p e c i m e n s s u b j e c t e d to v a r i o u s s u r f a c e t r e a t m e n t s s u c h a s m e c h a n i c a l polishing, c h e m i c a l e t c h i n g and s p u t t e r c l e a n i n ~ by d i f f e r e n t ions, a r e p r e s e n t e d and d i s c u s s e d below. R e s u l t s and D i s c u s s i o n T a b l e 1 l i s t s the e l l i p s o m e t r t c p a r a m e t e r s m e a s u r e d on a s a m p l e of v i t r e o u s s i l i c a , w h i c h w a s m e c h a n i c a l l y p o l i s h e d a c c o r d i n g t o the u s u a l p r o c e d u r e of u s i n ~ d i a m o n d p a s t e s with p r o g r e s s i v e l y d e c r e a s i n g g r a i n size. A f t e r p o l i s h i n g as w e l l a s f u r t h e r s u r f a c e t r e a t m e n t s to be d i s c u s s e d below, the s p e c i m e n w a s t h o r o u g h l y w a s h e d and r i n s e d with AR quality a c e t o n e and e t h e r in s u c c e s s i o n and d r i e d b e f o r e e l l t p s o m e t r i c m e a s u r e m e n t s w e r e c a r r i e d out.
1506
SILICA-SURFACE
STUDY
TABLE
BY ELLI PSOMETRY
1
E H t p s o m e t r t c p a r a m e t e r s m e a s u r e d on v i t r e o u s s i l i c a polished with diamond paste. n 2 = 1.460; Z = 5461°; ~b = 58 ° . D a m a g e d laye v
Surface History
~i
f
A
in d e g r e e s
ideal c a s e
3.79
360.00
0. 52
290. 00 334. 00
2.55 3.93 4.16 4.04 4.05 4.05 3.99
4. 3. 1. 0. 0. 0.
-
00 04 90 96 96 71
II
R is seen that the value of i~ m e a s u r e d on the mechanically polished sample is considerably different f r o m 3.79 ° , the value corresponding to the ideal d a m a g e f r e e and f i l m - f r e e
mechanically polished & uRrasonLc cleanLng in
acetone 2 rain. etch in 5 % HF 4 rain. " " " " 6 min. " " " "
Vol. 9, No.
1.530 1. 502 1.448 1.452 1.455 1. 455 1.455 1.456
967 627 334 760 790 97O 970 975
case. Reference to Fig. 2 indicates that the m e a s u r e d set of (A,~) values can arise due to the presence of a film or d a m a g e d layer of
r e f r a c t i v e index of about I. 52 on the 8 min. " " " " s u b s t r a t e of vitreous 12 mLn. " 'f " " silica. Exact c a l c u 60min. " " " " lations wLth the help 900 min. " " " " of a c o m p u t e r yielded the values 1. 530 and 967)~ as the refractLve index n 2' and t hick n es s d 2' of the s u r f a c e l a y e r . This i n c r e a s e d value of the r e f r a c t i v e index of the su rface l a y e r indicates that the s u r f a c e l a y e r s of the v i t r e o u s sLlica s p e c i m e n have b e e n p e r m a n e n t l y densified during the polishing stage. S i m i l a r o b s e r v a t i o n s of the p r e s e n c e of densified l a y e r on v a r i o u s g l a s s e s have been made by o t h e r w o r k e r s (2, 3). In the c a s e of silicon (1) w h e r e also such o b s e r v a t i o n s of the p r e s e n c e of d a m a g e d s u r f a c e l a y e r s on m e c h a n i c a l l y polished s p e c i m e n s w e r e noticed, it was found that c h e m i c a l polishing techniques can s u c c e s s f u l l y r e m o v e the d a m a g e d s u r f a c e l a y e r s . Hence, the vitreous s i l i c a sj~ecLmens w e r e also subjected to a s e r i e s of c h e m i c a l polishing t r e a t m e n t in 5 % H F solution and the r e s u l t s of such chemLcal polishing a r e also ~iven in Table 1. It is s e e n that in the e a r l y stages the HF solution does r e d u c e the t h i c k n e s s of d a m a g e d s u r f a c e l a y e r s - - as s e e n f r o m the i n c r e a s e of ~ and a l s o f r o m the values of n 2' and d 2 computed f r o m t h e s e (A, ~,) values. However, it is s e e n that as d is s o lu tio n of v i t r e o u s s i l i c a in HF p r o c e e d s by continued etch in g in HF at r o o m t e m p e r a tu re , the o u t e r m o s t l a y e r s a r e found to have distinctly d ifferen t effective r e f r a c t i v e index in such s p e c i m e n s subjected to diamond polishing in the e a r l y stages. T h e s e studies c l e a r l y Lndicate that c h e m i c a l polishing of v i t r e o u s sLlica which have been m e c h a n i c a l l y polished with diamond paste e a r l i e r does not yield a s t r a i n - or d e f e c t - f r e e s u r f a c e . Anot he r a p p r o a c h to this p r o b l e m of r e m o v a l of the densified s u r f a c e l a y e r s is by s p u t t e r - c l e a n i n g the s u r f a c e , u sin g low e n e r g y inert gas tons to bombard the s u r f a c e Ln high vacuum. In p a r t i c u l a r , it was felt the u s e of v e r y low e n e r g y ions - - L e . , ions with just sufficient e n e r g y to knock loose the s u r f a c e a t o m s , e s p e c i a l l y in the last stages of the s p u t t e r - c l e a n i n g
Vol. 9, No. 11
SILICA-SURFACE STUDY BY ELLIPSOMETRY
1507
p r o c e s s might TABLE 2 yield a clean Ellipsometric parameters measured on vitreous silica surface withpolLshed with diamond taste. out caus ing any damage to n2 d2 the substrate. Surface history (o) (o) Table 2 lists the values of A 0. 00 ideal case 3.79 and ~ m e a s u r e d before and 1.560 mechanically polished 1.38 2 1 2 . 4 3 925 after sputter* +Ar ions sputtered 4.03 354.36 + O ions sputtered 4.00 360.02 1. 456** 1150"* cleaning, vitreous silica s p e c i m e n with a r g o n ions. *solution with t r a n s p a r e n t film condition not possLble. The sputterLn~ * * s e e text. p r o c e d u r e adopted was to use 500 ev a r g o n ions at RF power level of 50 watts for an initial p e r iod of 20 minutes, followed by low e n e r g y a r g o n ions 50 ev (RF power ~-5 watts) for 10 minutes. It may be mentioned that the RF power level of 5 watts was found to be the minLmum level at which the plasma could be sustained in our e x p e r i m e n t a l s y s t e m (MRC sputtering unit). It was found that the vitreous sLlica s p e c i m e n exhibited a faint brownish tint on the surface when subjected to s p u t t e r - c l e a n i n g with a r g o n ions - - possibly due to oxygen deficiency in the surface l a y e r s of the sputtered specimen. Hence, the s p e c i m e n was subjected to another cycle of sputtering but with oxygen ions, in the hope that some of the s p u t t e r i n g oxygen ions may get trapped in the surface. However, f r o m Table 2 it is s e e n that the value of ~ is still far f r o m 3.79 ° c o r r e s p o n d i n g to the s t r a i n free surface. In other words though a clean surface can be produced by m e c h a n i c a l polishing using diamond paste followed by s p u t t e r cleaning, the surface so produced is far from the desLred ideal surface. !
T
,ll
E r n s b e r g e r (4) has shown by ion-exchange technique the p r e s e n c e of n u m e r o u s Grtffith (5) c r a c k s on the s u r f a c e s of plate glass and has d e m o n s t r a t e d that m e c h a n i c a l d a m a g e to g l a s s s u r f a c e s would result in flaws which can and often do s u r vive the polishing operation. V e r y r e c e n t l y H o m e r and Crawford (6) have d i r e c t l y o b s e r v e d with s c a n ning e l e c t r o n m i c r o s c o p e shallow s a u c e r shaped etch pits (Fig. 4) on the s u r f a c e s of a n u m b e r of glasse~ which w e r e lightly abraded and polished (or fire polished) before etching in HF solutions for varying perLods of FIG. 4 time. In other words vitreous Schematic d i a g r a m of damaged surface silica s p e c i m e n s polished with l a y e r produced d u r i n g polishing with diamond paste g e n e r a l l y exhLbit diamond paste, before and a f t e r etchLng rough s u r f a c e morphology, in HF. though on a m i c r o s c o p i c scale,
1508
SILICA-SURFACE S T U D Y B Y ELLI P S O M E T R Y
Vol. 9, No. 11
on etching with HF. It is r e a s o n a b l e to expect a s i m i l a r behavior if the s p e c i mens were s p u t t e r - c l e a n e d instead of etched in HF. The influence of such a rough morphology on the e l l i p s o m e t r i c p a r a m e t e r s should be considered. However, the t h e o r e t i c a l analysis (7-9) of this problem, taking into account the s t a t i s t i c a l nature of the rough surface is yet to be worked out so that meanLngful c o m p a r i s o n with the e x p e r i m e n t a l r e s u l t s can be made. At the s a m e time it will be e x t r e m e l y useful to explore this problem e x p e r i m e n t a l l y , by making use of scanning e l e c t r o n m i c r o s c o p e and the CESEMI (Computer EvaluatLon of Scanning E l e c t r o n Microscope Images) technique developed by White et al. (10) o r the FECO (Fringes of Equal Chromatic Order) i n t e r f e r o m e t r i c technique developed by Bennett et al. (11) to c h a r a c t e r i z e the roughness of the surface and m e a s u r e the e l l i p s o m e t r i c par a m e t e r s on the s a m e sample. Table 2 brings out another important feature worth mentioning. In the case of a r g o n ion s p u t t e r - c l e a n e d s p e c i m e n s it was mentioned e a r l i e r that the surface exhLbited faLnt b r o w n i s h tint. Efforts to obtaLn n2' and d2' with the a s sumption that the film is non-absorbing was not successful, i n d i c ~ t L n g that the surface film i s optically absorbLng. Durin~ the course of t h e s e e x p e r i m e n t s , it was found that vitVitreous silica poILshed wRh CeO2. reous silica s p e c i m e n s polished with c e r i u m oxide yielded "~'and A A values quite close to ~ o and Ao. Possibly the r e a s o n for the s u c c e s s Surface history (o~ (°) of CeO2 i~ polishLng without leaving behind a sLgniftcant damaged layer ideal case 3.79 360. 00 should be attributed to its h a r d n e s s mechanically polished 3.79 359.64 (5 Mohs) being close to that of vit2 rain. e t c h i n 5 % H F 3.77 359, 93 reous silica (4.9 Mohs). F r o m 360.29 6 rain. " " " " 3.80 Table 3 it is seen that the d i s s o l u 20mtn. " " " " 3.80 360.43 tion of vitreous sL1Lca polished with 720rain. " " " " 3.81 360. 80 CeO2 Ln dilute HF is also quite dLff e r e n t f r o m that given in Table 1 for vitreous silica polished with diamond paste. In o t h e r words, t h e r e appears to be no evidence for the p r e s e n c e of the thin l a y e r with effective r e f r a c tive index 1.455 and thickness of TABLE 4 about 1000~ on the surface of vitVitreous silica polished with CeO2. reous silica polished with CeO 2 and subsequently etched in HF.
TABLE 3
i
,
. . . . .
m
i
i
i
r
|
i
i
Surface h i s t o r y ideal case mechanically polished + A r ions sputtered +O ions sputtered
(°) L3" 79
;_;;3.77 3.77
(°) 0.00 , - - - - - - - - - - - -
358.57
358.79
Table 4 lists the e l l i p s o m e t rLc p a r a m e t e r s of vitreous sLlica s p e c i m e n s polished with c e r i u m oxide, on sputtering with argon and oxygen ions. It is seen that "~is e s s e n t i a l l y constant within e x p e r i mental e r r o r . The r e c e n t field ion m i c r o s c o p i c studies (12) on s u r faces bombarded with various ions
Vol. 9, No. 11
SILICA-SURFACE STUDY BY ELLIPSOMETRY
1509
r e v e a l that in ~eneral the c r y s t a l l i n e o r d e r of the outermost l a y e r s are totally destroyed even when the ener~3T of the bombarding ions is f a i r l y low (--50 eV). Since the vitreous s i l i c a is a l r e a d y a n o n c r y s t a l l i n e m a t e r i a l the d i s o r d e r in its outermost l a y e r s cannot be f u r t h e r i n c r e a s e d by s p u t t e r - c l e a n i n g p r o c e s s and hence it is not s u r p r i s i n ~ to find the value of y observed on such s p u t t e r cleaned vitreous silica to be a l m o s t the same a s ~ o corresponding to the ideal case.
Refe r e n c e s 1. K. Vedam and S. S. So, Surface ScL 299, 379 (1972). 2. F. M. E r n s b e r C e r , Ann. Rev. Mat. Sci. 2_, 529 (1972). 3. H. Yokota, H. Sakata, M. Nishibori and K. Kinosita, Surface Sci. 1~6, 265 (1969). 4. F. M. E r n s b e r ~ r , P r o c e s s in C e r a m i c Science, Vol. 3, p. 58. P e r a'amon P r e s s , New York (1963); Advances in Glass Technology, p. 511. Plenum P r e s s , New York (1962). 5. A. A. Griffith, Phil. T r a n s . Roy. Soc. 221A, 163 (1920). 6. P. N. Homer and B. J. Crawford, Glass Tech. 11, 10 (1970). 7. C. A. F e n s t e r m a k e r and F. L. McCrackin, Surface Sci. 16, 85 (1969). 8. I. Ohlidal and F. Lukes, Opt. Acta 1_99, 817 (1972). 9. R. S. Sirohi, Optics Communs. 1, 304 (1970). 10. E. W. White, H. A. McKinstry and A. Diness, The Science of C e r a m i c Machining and Surface F i n i s h i n g , p. 309. NSF Spec. Publ. #348 (May 1972). 11. H. E. Bennett, J. M. Bennett, J. L. Stanford and P. C. Archibald, Proc. Third Conf. on High Power Infrared L a s e r Window M a t e r i a l s , p. 1127. A F C R L - T R - 7 4 - 0 0 8 5 (HI), Special Report No. 174 (1974). 12. J. M. Walls, E. Braun and H. N. Southworth, Proc. Sixth Int. Vac. C o n g r e s s 1974, Jap. J. Appl. Phys. (1974)(to be published); J. M. Waals, Ph.D. T h e s i s , University of Aston, B i r m i n g h a m , England (1973).