Silicon Oxidation - ACS Symposium Series (ACS Publications)

3 Silicon Oxidation A Process Step for the Manufacture of Integrated Circuits Eugene A. Irene

Downloaded by SUFFOLK UNIV on January 19, 2018 | http://pubs.acs.org Publication Date: October 2, 1985 | doi: 10.1021/bk-1985-0290.ch003

Department of Chemistry, University of North Carolina, Chapel Hill, NC 27514

The manufacture of integrated circuits is essentially microfabrication in two dimensions as it is presently practiced. The microfabrication aspect arises simply from the marketing requirement for a high density of high performance devices on a manufactured chip. The manufacturable device density has risen dramatically in the last ten years with a equally dramatic decrease in the cost per device. An idea of the magnitude of this progress is obtained when we consider the fact that very powerful personal computers are available to consumers at easily affordable costs. Some presently available personal computers approximate the performance of main frames of the 1960's. The packing density of present day devices is primarily determined by the lateral dimensions that can be reliably produced by lithography. Of course some electronic effects arise when the devices get smaller and closer viz. latch-up and hot electron effects, but clever engineering tactics have thus far prevented these effects from retarding further progress. Processing in a direction normal to the substrate surface is relatively easier to control. In this direction one must consider the doping depth and sharpness of impurity profiles in the substrate, and above the substrate the film thicknesses and quality are crucial. The film thicknesses, and in many devices, the depths of dopants in the substrate are usually small relative to the lateral device dimension, hence arises the two dimensional aspect of the manufacturing process. Moreover, present manufacturing practice is limited to one device per area of Si, however, vertical integration schemes are probably not far off and are receiving considerable research effort. The review to follow is aimed at addressing one aspect of the integrated circuit manufacturing process, v i z . , the preparation of a multipurpose silicon dioxide film on the surface of silicon. The present discussion is focused on silicon oxidation, which is at present the best way to produce a thin film of silicon dioxide, SiO . In integrated circuit processing schemes this step is repeated many times. Each repetition addresses a different manufacturing issue in 2

0097-6156/ 85/ 0290-0031 $06.00/0 © 1985 American Chemical Society

Stroeve; Integrated Circuits: Chemical and Physical Processing ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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the p r o c e s s . In the d i s c u s s i o n t h a t f o l l o w s , each of these i s s u e s w i l l be a d d r e s s e d . Each r e p e t i t i o n r e q u i r e s an a l t e r a t i o n of the b a s i c o x i d a t i o n p r o c e s s , but always w i t h the same end, v i z . the p r e p a r a t i o n of a f i l m of S i C ^ . We w i l l d i s c u s s these p r o c e s s e s as w e l l as o t h e r techniques ( o t h e r than thermal o x i d a t i o n ) to produce f i l m s of SiO^. I t i s c r u c i a l to s u c c e s s f u l m a n u f a c t u r i n g that the s i l i c o n o x i d a t i o n p r o c e s s be w e l l u n d e r s t o o d . To t h i s end there i s ongoing r e s e a r c h i n the f i e l d and we w i l l d i s c u s s t h i s r e s e a r c h . F i n a l l y , the f u t u r e needs and r e q u i r e m e n t s f o r t h i n f i l m s i n the manufacture of i n t e g r a t e d c i r c u i t s w i l l be a d d r e s s e d .


Why

Silicon

Oxidation?

I n o r d e r to become o r i e n t e d to the importance of the s i l i c o n o x i d a t i o n process s t e p , i t i s necesary to c o n s i d e r the fundamental p r i n c i p l e s of e l e c t r o n i c d e v i c e o p e r a t i o n . The great r e v o l u t i o n i n i n t e g r a t e d c i r c u i t r y took p l a c e as i t was d i s c o v e r e d t h a t d e v i c e s c o u l d be c o n s t r u c t e d on the s u r f a c e of a semiconductor. T h i s d i s c o v e r y of how to u t i l i z e a semiconductor s u r f a c e r a t h e r than the b u l k i s , i n t h i s a u t h o r ' s o p i n i o n , the s i n g l e most important reason f o r the advancement of e l e c t r o n i c s technology and the s i m u l t a n e o u s emergence of S i as the premier semiconductor. Indeed, S i i s mediocre as a semiconductor i n terms of b u l k s e m i c o n d u c t i n g p r o p e r t i e s such as c a r r i e r m o b i l i t y . However, f o r S i , i t was found t h a t the s u r f a c e e l e c t r o n i c s t a t e s r e s u l t i n g from the t e r m i n a t i o n of the c r y s t a l l a t t i c e c o u l d be reduced to t o l e r a b l e l e v e l s by exposure of the s u r f a c e to o x i d a t i o n c o n d i t i o n s ( 1 ) . F i g . l i l l u s t r a t e s that the s u r f a c e of S i has a l a r g e number of u n s a t i s f i e d bonds, s o - c a l l e d " d a n g l i n g bonds", as a r e s u l t of the t e r m i n a t i o n of the l a t t i c e . The number of such bonds i s of ^ o r d e r of the number of s u r f a c e atoms which i s a p p r o x i m a t e l y 10 cm 2. I t was p r e d i c t e d t h a t these d a n g l i n g bonds would produce an equal number of e l e c t r o n i c s t a t e s i n the band gap of s i l i c o n ( 2 ) . T h i s number of s t a t e s i s of the same o r d e r or g r e a t e r than the number of c u r r e n t c a r r i e r s on the s u r f a c e of a semiconductor. T h e r e f o r e , i t was c o r r e c t l y reasoned t h a t u n l e s s these s t a t e s were d r a s t i c l y reduced, the s u r f a c e e l e c t r o n i c p r o p e r t i e s of the semiconductor c o u l d not be used f o r d e v i c e c o n s t r u c t i o n . The p r e d i c t e d number of s t a t e s on S i was indeed v e r i f i e d , and more i m p o r t a n t l y , i t was d i s c o v e r e d t h a t the number c o u l d be reduced s e v e r a l o r d e r s of magnitude by s i m p l y e x p o s i n g the s u r f a c e to the atmosphere and t h e r e f o r e p e r m i t t i n g a n a t i v e s u r f a c e o x i d e to f o r m ( l ) . Of even g r e a t e r importance was the f i n d i n g t h a t the p u r p o s e f u l f o r m a t i o n of a d v e n t i t i o u s o x i d e by the exposure of the S i to an o x i d i z i n g ambient at h i g h temperatures would reduce the number of s u r f a c e s t a t e s by f i v e o r d e r s of magnitude. T h i s p r o c e s s of r e d u c i n g the number of s u r f a c e e l e c t r o n i c s t a t e s to t o l e r a b l e l e v e l s i s i n the e l e c t r o n i c s p a r l a n c e termed " p a s s i v a t i o n " as c o n t r a s t e d w i t h the meaning w i t h i n the f i e l d of e l e c t r o c h e m i s t r y . A l t h o u g h t h e r e e x i s t s c o n s i d e r a b l e r e s e a r c h on the s u b j e c t of e l e c t r o n i c p a s s i v a t i o n , o n l y f o r S i has the p a s s i v a t i o n been adequate, and t h e r e f o r e o n l y f o r S i has the i n t e g r a t e d c i r c u i t t e c h n o l o g y developed to such an advanced s-tate. In f a c t , even w i t h i n S i technology o t h e r f i l m d e p o s i t i o n methods have been attempted such as p h y s i c a l and chemical d e p o s i t i o n techniques and t n e


3.

33

Silicon Oxidation

IRENE

these have been shown to be u s e f u l f o r some purposes, but none has yet s u p p l a n t e d thermal o x i d a t i o n of S i . The approximate comparative r e s u l t s from o t h e r t e c h n i q u e s as w e l l as f o r S i o x i d a t i o n are shown i n Table I . The c o n c l u s i o n i s t h a t the o x i d a t i o n p r o c e s s i s fundamental to modern i n t e g r a t e d c i r c u i t t e c h n o l o g y and we have t h e r e f o r e answered the q u e s t i o n "Why" posed above.

Table I .

Comparison of S u r f a c e S t a t e s R e s u l t i n g from D i f f e r e n t SiC>2 P r e p a r a t i o n Methods


SiO^ P r e p a r a t i o n Technique

Approximate Number of Surface E l e c t r o n i c S t a t e s (per a r e a )

Thermal O x i d a t i o n

10

Chemical

Vapor D e p o s i t i o n (CVD)

10 - 10

P h y s i c a l Vapor D e p o s i t i o n (PVD)

10 - 10

No S i 0

10

2

on S i

1 0

1 1

1 2

1 1

1 3

1 5

For the case of i n t e g r a t e d c i r c u i t t e c h n o l o g y t h e r e are o t h e r v e r y important uses f o r p r e p a r i n g SiO^ on S i . I t i s c l e a r t h a t the e l e c t r o n i c p a s s i v a t i o n i s the most important and fundamental use f o r SiO^ and t h a t the o t h e r uses to be d i s c u s s e d are bonuses, and i n many i n s t a n c e s c o u l d be accomplished e q u a l l y w e l l and sometimes b e t t e r w i t h o t h e r m a t e r i a l s and o t h e r processes than Si02 and oxidation. Other Uses For S i l i c o n D i o x i d e on

Silicon

I f we c o n s i d e r the p h y s i c a l p r o p e r t i e s of SiO^ i t s e l f , then s e v e r a l of the o t h e r uses become o b v i o u s . SiO^ i s a wide band gap i n s u l a t o r (about 9eV). As such, the e l e c t r i c a l i n f l u e n c e of SiO^ on the c o n d u c t i o n process on the S i s u r f a c e i s n i l except f o r the r e d u c t i o n of the d a n g l i n g bonds as d i s c u s s e d above. Thus one o b t a i n s the e l e c t r o n i c p a s s i v a t i o n w i t h o u t any troublesome i n t e r f e r e n c e . The SiO^ f i l m , b e i n g a good i n s u l a t o r , w i l l ^ b e a b l e to support a r a t h e r l a r g e e l e c t r i c f i e l d ( g r e a t e r than 10 V/cm). Such an e l e c t r i c f i e l d a p p l i e d a c r o s s the o x i d e f i l m w i l l a l t e r the S i s u r f a c e p o t e n t i a l and t h e r e b y modulate the c o n d u c t i o n of c a r r i e r s i n c o n d u c t i v e channels c r e a t e d at the S i s u r f a c e . T h i s e f f e c t i s the o p e r a t i o n a l p r i n c i p l e f o r the f i e l d e f f e c t t r a n s i s t o r , FET, which when c o n s t r u c t e d from a metal c o n t a c t to an o x i d e f i l m on a semiconductor s u r f a c e i s then c a l l e d a MOSFET as shown i n F i g . 2 . A l s o , when one wishes to prepare a h i g h d e n s i t y of d e v i c e s on a semiconductor, the i s o l a t i o n of each d e v i c e from the a d j a c e n t d e v i c e i s i m p o r t a n t . I t i s i n t e r e s t i n g to c o n s i d e r t h a t the molar


34

C H E M I C A L A N D PHYSICAL PROCESSING O F INTEGRATED CIRCUITS

dangling bonds


surface

^

s i l i c o n (100) surface F i g u r e 1.

Side View o f (100) S i S i n g l e c r y s t a l , showing d a n g l i n g bonds a t the s u r f a c e .

METAL LINE

,,M,nw,n,,h^.!,) ll

ISOLATION.. * OXIDE ^SOURCE n

4.

GATE OXIDE

F i g u r e 2.

p-TYPE SILICON

M e t a l - Oxide - S i l i c o n f i e l d e f f e c t MOSFET.

transistor,


3.

IRENE

35

Silicon Oxidation

volume of S i 0 i s 120% l a r g e r than the molar volume of S i . T h i s f a c t can be used to p r o v i d e one method of i s o l a t i o n . C o n s i d e r the a d j a c e n t MOSFET d e v i c e s shown i n F i g . 2 . I f one can s e l e c t i v e l y o x i d i z e the r e g i o n s between the d e v i c e s , the t h i c k n e s s of o x i d e formed w i l l r i s e above the u n o x i d i z e d r e g i o n s by more than 100%. The s e l e c t i v e o x i d a t i o n can be a c c o m p l i s h e d w i t h the use of an o x i d a t i o n mask. T y p i c a l of such a masking m a t e r i a l i s a f i l m of silicon nitride. I n order to d e l i n e a t e d e v i c e r e g i o n s f o r the purpose of d o p i n g , s t r u c t u r a l e t c h i n g of the s u b s t r a t e to f a b r i c a t e mesa or r e c e s s e d s t r u c t u r e s and to d e f i n e c o n d u c t i n g l i n e s , a masking f i l m i s r e q u i r e d . Si0„ f i l m s are o f t e n used f o r a l l these masking o p e r a t i o n s except f o r o x i d a t i o n masking. S i ^ N ^ i s o f t e n used f o r t h i s purpose. However, S i ^ N ^ r e q u i r e s a d i f f e r e n t method of p r e p a r a t i o n c a l l e d c h e m i c a l vapor d e p o s i t i o n , to be d i s c u s s e d l a t e r and i t i s known to cause damage to the S i s u b s t r a t e as a r e s u l t of a large t e n s i l e i n t r i n s i c s t r e s s ( 3 ) . The e x i s t e n c e of a f i l m of a hard s t a b l e SiO^ f i l m on S i s e r v e s to p r o t e c t the S i from damage and i m p u r i t i e s . I n summary, over and above the p r i m a r y use of SiO^ as an e l e c t r i c a l p a s s i v a t i n g f i l m , f o u r a d d i t i o n a l uses of SiO^ have been i d e n t i f i e d . These uses are: e l e c t r i c a l o p e r a t i o n , i s o l a t i o n , m a s k i n g , and p r o t e c t i o n of the S i .


2

How

i s SiO^

Prepared?

As i n d i c a t e d by the t i t l e , the method of thermal o x i d a t i o n of S i i s p r e s e n t l y the most important method to produce S i 0 f i l m s , e s p e c i a l l y f o r c r i t i c a l a p p l i c a t i o n s where the number of s u r f a c e e l e c t r o n i c s t a t e s and s u r f a c e charge i s i m p o r t a n t . Before f o c u s i n g on the d e t a i l s of t h a t p r o c e s s , i t i s u s e f u l to c o n s i d e r a l t e r n a t e methods to produce u s e f u l f i l m s f o r m i c r o e l e c t r o n i c s a p p l i c a t i o n s . Other than thermal o x i d a t i o n of S i , the most important method to p r e p a r e p a s s i v a t i n g f i l m s on semiconductors i s c h e m i c a l vapor d e p o s i t i o n , CVD. CVD u t i l i z e s v o l a t i l e s p e c i e s t h a t e i t h e r alone or by r e a c t i o n produce the d e s i r e d m a t e r i a l i n t h i n f i l m form on a s u b s t r a t e . For example c o n s i d e r the p r o d u c t i o n of SiO^ f i l m s by CVD. S i H ^ i s o f t e n used as the v o l a t i l e source of S i . The S i H ^ i s r e a c t e d w i t h a source of oxygen. T y p i c a l c h o i c e s are (X^* ^ 0 , 0^ and HO, a l t h o u g h the l a t t e r two are q u i t e v i r u l e n t and t h e r f o r e u s u a l l y a v o i d e d . The v o l a t i l e r e a c t a n t s are mixed and p o s s i b l y d i l u t e d i n a f u r n a c e above 300°C and which a l s o c o n t a i n s the S i s u b s t r a t e s to be c o a t e d . With the a b i l i t y to c o n t r o l the r e a c t a n t c o n c e n t r a t i o n s , t e m p e r a t u r e , and t o t a l f l o w r a t e s , u n i f o r m t h i n f i l m s w i t h r e a s o n a b l y good p r o p e r t i e s can be prepared f o r a l l but the c r i t i c a l i n t e r f a c i a l a p p l i c a t i o n s where very low s u r f a c e s t a t e d e n s i t i e s are r e q u i r e d . Such CVD f i l m s are q u i t e u s e f u l f o r masking o p e r a t i o n s of a l l d e s c r i p t i o n s , f o r i s o l a t i o n of a d j a c e n t d e v i c e s and f o r p r o t e c t i n g d e v i c e s from exposure to c o n t a m i n a t i o n . However, w i t h o u t o t h e r t r e a t m e n t s , the CVD p r o c e s s cannot produce the low s u r f a c e s t a t e l e v e l s r e q u i r e d i n a c t i v e d e v i c e r e g i o n s such as the gate r e g i o n of a MOSFET. Yet a wide v a r i e t y of f i l m s can be p r e p a r e d f o r an e q u a l l y wide v a r i e t y of a p p l i c a t i o n s by CVD. O f t e n t i m e s , the b a s i c CVD p r o c e s s i s m o d i f i e d to u t i l i z e such energy sources as plasmas, r a d i o frequency i n d u c t i o n h e a t i n g , or 9



36

CHEMICAL AND

PHYSICAL PROCESSING OF INTEGRATED CIRCUITS

photon s o u r c e s . I n a d d i t i o n to c h e m i c a l vapor methods are the p h y s i c a l vapor d e p o s i t i o n , PVD, t e c h n i q u e s . E v a p o r a t i o n and s p u t t e r i n g are the b a s i c PVD t e c h n i q u e s . E v a p o r a t i o n simply u t i l i z e s a source of d e s i r e d m a t e r i a l p l u s a source of energy to v a p o r i z e the d e s i r e d m a t e r i a l . S u b s t r a t e s are p o s i t i o n e d so as to r e c e i v e the recondensed m a t e r i a l . S p u t t e r i n g u t i l i z e s a c c e l e r a t e d i o n s to e j e c t s p e c i e s from a source m a t e r i a l , which can then recondense on a s u b s t r a t e . Both PVD t e c h n i q u e s r e q u i r e vacuum c o n d i t i o n s f o r s u c c e s s f u l a p p l i c a t i o n . Many v a r i a n c e s of the b a s i c PVD t e c h n i q u e s are found i n a c t u a l p r o c e s s e s . For e v a p o r a t i o n , the source of energy i n c l u d e s r e s i s t a n c e , i n d u c t i o n and e l e c t r o n beam h e a t i n g of the s o u r c e , w h i l e plasmas and r e a c i v e ambients are commonly employed i n s p u t t e r i n g . PVD t e c h n i q u e s can be used f o r m e t a l s , s e m i c o n d u c t o r s , and d i e l e c t r i c s i n a p p l i c a t i o n s where low s u r f a c e s t a t e and i n t e r f a c e charge d e n s i t i e s are not r e q u i r e d as was the case f o r CVD. With t h i s i n f o r m a t i o n about the v a r i o u s o t h e r methods used to prepare t h i n f i l m s f o r m i c r o e l e c t r o n i c s , we are ready to c o n s i d e r the d e t a i l s of the thermal o x i d a t i o n step and are a b l e to i d e n t i f y the e s s e n t i a l d i f f e r e n c e s among the p r o c e s s s t e p s . The

Thermal O x i d a t i o n

Process

With the a i d of F i g . 3 the o p e r a t i o n a l f e a t u r e s of the S i o x d i a t i o n p r o c e s s are u n d e r s t o o d . A source of thermal energy i s r e q u i r e d , and t h i s i s u s u a l l y a r e s i s t a n c e heated t u b u l a r f u r n a c e . Since the p r o c e s s i s t h e r m a l l y a c t i v a t e d and l a r g e batches of s u b s t r a t e s are u s u a l l y p r o c e s s e d at a t i m e , r e s i s t a n c e h e a t i n g i s p r e f e r r e d , because i t p r o v i d e s a l a r g e and l e v e l temperature zone. Cleaned S i s l i c e s are p l a c e d on cleaned fused s i l i c a c a r r i e r s and pushed i n t o the hot zone of the f u r n a c e . The furnace i s l i n e d w i t h a h i g h p u r i t y fused s i l i c a tube. Pure 0« or HÔ or a m i x t u r e are the u s u a l o x i d a t i o n ambients. The d e s i r e d S i 0 2 f i l m t h i c k n e s s i s grown by c o n t r o l l i n g the t e m p e r a t u r e , time and o x i d a t i o n ambient. Once o x i d a t i o n i s complete a s h o r t h i g h temperature anneal i s r e q u i r e d to reduce i n t e r f a c e charge to d e s i r e d l e v e l s . The s i m p l i c i t y of t h i s p r o c e s s and the emphasis on c l e a n l i n e s s are important c h a r a c t e r i s t i c s which are a p p r e c i a t e d from a c o n s i d e r a t i o n of the model f o r S i o x i d a t i o n . A double w a l l f u r n a c e l i n e r tube i s p r e f e r r e d i n o r d e r to m i n i m i z e i m p u r i t y d i f f u s i o n through the tube at h i g h t e m p e r a t u r e s . The

S i l i c o n Oxidation

Model

F i r s t l y , i t i s important to r e a l i z e t h a t the S i o x i d a t i o n model i s a s u b j e c t of a c t i v e r e s e a r c h at many l a b o r a t o r i e s around the w o r l d . The i n f o r m a t i o n to be d i s c u s s e d about the model i s i n t h i s a u t h o r s o p i n i o n , commonly a c c e p t e d , however t h e r e e x i s t s d i f f e r e n c e s w i t h the d e t a i l s , as i s commonly the case i n any a c t i v e r e s e a r c h a r e a . Such d e t a i l and c o n t r o v e r s y w i l l be a v o i d e d i n t h i s r e v i e w . There i s u n i v e r s a l agreement t h a t the r a t e of thermal o x i d a t i o n of s i l i c o n d e c r e a s e s as the S i 0 f i l m grows(4-6). I t has been e s t a b l i s h e d t h a t the o x i d a t i o n r e a c t i o n t h a t produces SiO^ o c c u r s at the S i - S i 0 i n t e r f a c e ( 7 - 9 ) . From a l l the a v a i l a b l e i n f o r m a t i o n , the S i 0 formed i n an i m p u r i t y f r e e environment at 2

2

9



i

Gases Out wOut

Push Rod

End Cap /

R

u

h

G

a

s | n p u t

Furnace Windings

.Wafer Boat

Double Wall Fused Silica Tube

F i g u r e 3. A B a s i c S i l i c o n O x i d a t i o n System.

s


•Gases Input

a

1

0

m

73

38

CHEMICAL AND PHYSICAL PROCESSING OF INTEGRATED CIRCUITS

temperatures below 1200 C i s amorphous, conformal and adherent to the S i s u r f a c e r e g a r d l e s s of the f i l m t h i c k n e s s . These o b s e r v a b l e s have l e d the e a r l y workers i n t h i s f i e l d (see f o r example r e f s 4-6) to c o n s i d e r o x i d a t i o n by a l i n e a r - p a r a b o l i c process ( 1 0 ) . T h i s l i n e a r - p a r a b o l i c , L-P, model i s understood u s i n g F i g . 4 . In t h i s f i g u r e one sees two predominant f l u x e s . The f i r s t , F^, i s the F i c k i a n d i f f u s i o n of o x i d a n t a c r o s s the SiO^ f i l m to the S i - S i O ^ i n t e r f a c e , where the r e a c t i o n w i t h S i produces a f l u x of growing SiO^ , F~. The t r a n s p o r t of o x i d a n t to the o u t e r S i 0 s u r f a c e through the gas phase and the d i s s o l u t i o n of o x i d a n t i n SiO^ has been observed to be r a p i d under u s u a l o x i d a t i o n c o n d i t i o n s , and t h e r e f o r e these f l u x e s can be s a f e l y i g n o r e d . The f l u x e s , F^ and F^ must be i n a steady s t a t e , s i n c e one f l u x c o n t r o l s the o t h e r as e i t h e r the s u p p l y or s i n k . The d i f f u s i o n f l u x can be r e p r e s e n t e d by F i c k ' s f i r s t law as: Downloaded by SUFFOLK UNIV on January 19, 2018 | http://pubs.acs.org Publication Date: October 2, 1985 | doi: 10.1021/bk-1985-0290.ch003

2

where D i s the p o s i t i o n a l l y independent d i f f u s i v i t y of o x i d a n t , and C and L are the c o n c e n t r a t i o n of o x i d a n t and f i l m t h i c k n e s s r e s p e c t i v e l y . The r e a c t i o n at the i n t e r f a c e can be c o n s i d e r e d as a f i r s t o r d e r c h e m i c a l r e a c t i o n i n o x i d a n t and i s w r i t t e n as:

where k i s the f i r s t o r d e r r a t e c o n s t a n t and i s the c o n c e n t r a t i o n of o x i d a n t at the S i - S i 0 i n t e r f a c e . From a steady s t a t e a p p r o x i m a t i o n where the f l u x e s a d j u s t to each o t h e r we o b t a i n : ?

The

r a t e of f i l m f o r m a t i o n , d L / d t , i s g i v e n as: dL/dt = F / ( l

where (% i s the number of o x i d a n t m o l e c u l e s i n c o r p o r a t e d i n t o a u n i t volume of S i 0 . The i n t e g r a t i o n of the above r a t e e q u a t i o n y i e l d s a l i n e a r - p a r a b o l i c e q u a t i o n of the form: 2

t = AL + BL

+ const.

There have been s e v e r a l s i m i l a r methods to e v a l u a t e the i n t e g r a t i o n c o n s t a n t ( 4 , 1 1 ) . Since i t i s commonly accepted t h a t t h e r e e x i s t s an i n i t i a l regime of o x i d a t i o n i n dry 0 which does not conform to the L-P model, t h i s author used the boundary c o n d i t i o n t h a t the L-P regime commences at t L . With t h i s c o n d i t i o n the f i n a l L-P equation i s ( l l ) : 9

t-t :

0

= A(L-L

2

0

2

) + B(L -L ) o

The c o n s t a n t s A and B have dimensions of the r e c i p r o c a l s of the l i n e a r and p a r a b o l i c r a t e c o n s t a n t s , r e s p e c t i v e l y as:


3.

IRENE

39

Silicon Oxidation A = 1/^

; B = l/k

p

From t h i s a n a l y s i s i t i s c l e a r t h a t i n o r d e r to p r e d i c t the s p e c i f i c time to produce a d e s i r e d SiO^ f i l m t h i c k n e s s , f o u r parameters need to be known v i z . t , L , and k . Such i n f o r m a t i o n i s n e c e s s a r y i n o r d e r to c h a r a c t e r i z e d p r o c e s s . The c o n s t a n t s can be o b t a i n e d w i t h r e a s o n a b l e a c c u r a c y from dense data s e t s through the use of curve f i t t i n g p r o c e d u r e s . Two s t u d i e s ( 1 1 , 1 2 ) show t h a t the dense data s e t s are c o n v e n i e n t l y o b t a i n e d u s i n g i n - s i t u e l l i p s o m e t r y which observes the o x i d e growth i n the f u r n a c e . The t h i c k n e s s - t i m e data are then f i t t e d to a l i n e a r i z e d e x p r e s s i o n o b t a i n e d by d i v i d i n g the r e s u l t a n t L - P e q u a t i o n above by L - L ( 1 1 ) . Values f o r t , L are o b t a i n e d from the data set i t s e l f using°an i t e r a t i v e procedure? For the f i r s t i t e r a t i o n , a l l the data i n an L - t set i s f i t t e d to the model and t , L i s assumed to be z e r o . Values f o r A and B are o b t a i n e d as w e l l as a s t a t i s t i c a l g o o d n e s s - o f - f i t parameter. For the next i t e r a t i o n , the s m a l l e s t L , t p o i n t i s dropped from the set f o r f i t t i n g purposes, but t h i s data p o i n t i s taken to be t , L f o r the i t e r a t i o n . A g a i n , the q u a l i t y of the f i t i s r e c o r d e d . $his° procedure i s r e p e a t e d through the data s e t . The q u a l i t y of f i t parameter i s seen to improve as the c o r r e c t v a l u e o f t , L i s approached and then remain l e v e l beyond the best v a l u e . T h e r e f o r e from t h i s procedure the p o i n t i n L , t space f o r the onset of the L - P model i s o b t a i n e d a l o n g w i t h the r a t e c o n s t a n t s which best d e s c r i b e the o x i d a t i o n p r o c e s s . I n o r d e r to r e p r o d u c i b l y prepare SiO„ of the d e s i r e d t h i c k n e s s f o r a g i v e n r e q u i r e m e n t , we must be a b l e to a s s e s s the q u a l i t y of the o x i d a t i o n model i t s e l f . C o n s i d e r a b l e r e s e a r c h has been focused on t h i s problem and t h i s work w i l l be b r i e f l y summarized.


Q

Q

How

q

Good Is The Model?

F i r s t , we must c o n s i d e r t h a t t h e r e i s the i n i t i a l regime which does not conform to L - P k i n e t i c s . By the f i t t i n g procedures o u t l i n e d above, t h i s regime was found to extend to 40nm at o x i d a t i o n temperatures around 1100°C to 5nm at 800°C f o r o x i d a t i o n i n dry 0^ and to e s s e n t i a l l y zero f o r o x i d a t i o n u s i n g H 0 as the o x i d a n t ( 1 1 , 1 3 ) . These r e s u l t s are i n r e a s o n a b l e agreement w i t h o t h e r s t u d i e s t h a t used l e s s dense data s e t s and g r a p h i c a l f i t t i n g p r o c e d u r e s ( 4 ) . At the p r e s e n t time t h e r e i s no s a t i s f a c t o r y model f o r the i n i t i a l regime. For the data t h i c k e r than L , we can use the l i n e a r and p a r a b o l i c r a t e c o n s t a n t s , k^ and k , to monitor the e f f e c t s of changes on the s p e c i f i c p r o c e s s in** the L - P model v i z . i n t e r f a c e and/or t r a n s p o r t phenomena. The main p e r t u r b a t i o n s to the r a t e c o n s t a n t s a r i s e from the use of d i f f e r e n t ambients such as HÔ or dry 0 , and d i f f e r e n t o x i d a t i o n temperatures. The e f f e c t s of ^ 0 on o x i d a t i o n k i n e t i c s are w e l l documented(5,6,13,14). A s y s t e m a t i c study of HÔ e f f e c t s u s i n g dense data s e t s and the curve f i t t i n g procedures o u t l i n e d above have shown a r a t h e r sharp i n c r e a s e i n k w i t h EÔ c o n c e n t r a t i o n and a smoother i n c r e a s e i n k..(13) as shown ?n F i g . 5 . Even t r a c e amounts 9

9


40


Linear - P a r a b o l i c Model

0 GAS


2

F i g u r e 4.

Si0 SOLID 2

Si CRYSTAL

Oxidant - S i l i c o n D i o x i d e F i l m - S i l i c o n System w i t h D i f f u s i v e F l u x , F^, and R e a c t i o n F l u x , F^.

[H 0] (ppm) 2

F i g u r e 5.

Linear, k and P a r a b o l i c , k , Rate Constants as a F u n c t i o n o f the Water in* the O x i d a t i o n Ambient.


3.

Silicon Oxidation

IRENE

41

of HÔ were shown to have profound e f f e c t s on the k i n e t i c s ( 1 4 ) . For example 25 ppm HÔ i n 0^ has been found to i n c r e a s e the o v e r a l l r a t e of o x i d a t i o n by about 20%. The e f f e c t s have been a t t r i b u t e d to two o p e r a t i v e phenomena. One i s the e f f e c t of the H 0 i t s e l f on the r e a c t i o n at the S i - S i O ^ i n t e r f a c e . HÔ i s known to 5e a more v i r u l e n t o x i d a n t than 62, and t h e r e f o r e we a n t i c i p a t e a r a t e enhancement due to ^ 0 , and i n d e e d , an enhancement of k^. has been o b s e r v e d . The o t h e r e f f e c t i s r e l a t e d to the f a c t t h a t fÔ r e a c t s w i t h S i O and breaks up the S i - O - S i network forming h y d r o x y l groups. The o v e r a l l e f f e c t i s a l o o s e n i n g of the network from which a d i f f u s i v i t y enhancement should be o b s e r v e d . Indeed, a sharp i n c r e a s e i n k i s observed w i t h o n l y s m a l l amounts of 1^0 i n the o x i d a t i o n ambEent as i s expected f o r a s t r u c t u r a l a l t e r a t i o n . Other i m p u r i t i e s such as Na and HC1 have a l s o been s t u d i e d ( 6 , 1 5 - 1 7 ) w i t h r e s p e c t to k i n e t i c e f f e c t s . While the r e s u l t s are not as c l e a r to r e l a t e to the s p e c i f i c r a t e c o n s t a n t s as f o r 1^0, both of these i m p u r i t i e s i n c r e a s e the o v e r a l l r a t e of o x i d a t i o n . Na i s j u d i c i o u s l y avoided d u r i n g o x i d a t i o n as i t produces m o b i l e p o s i t i v e charges i n the o x i d e t h a t a l t e r the S i s u r f a c e p o t e n t i a l and t h e r e b y d i m i n i s h d e v i c e performance. HC1, on the o t h e r hand, i s p u r p o s e l y added to the o x i d a t i o n ambient to remove any Na t h a t may have entered the p r o c e s s i n g environment. Hence, the k i n e t i c e f f e c t s f o r t h i s m a t e r i a l need to be known even though complete understanding i s l a c k i n g . A v a i l a b l e studies y i e l d considerable i n f o r m a t i o n that i s u s e f u l f o r processing(5,15-17). The e f f e c t of o x i d a t i o n temperature on the r a t e c o n s t a n t s i n the L-P regime i s shown i n F i g . 6 ( 1 8 ) . I t i s seen t h a t there i s c o n s i d e r a b l e c u r v a t u r e i n the A r r h e n i u s p l o t s . T h i s c u r v a t u r e i n d i c a t e s t h a t n e i t h e r of the steps y i e l d i n g the f l u x e s , F. and F2, r e p r e s e n t k i n e t i c a l l y simple r a t e s t e p s . T h i s f i n d i n g should not r e a l l y be s u r p r i s i n g , as the L-P model i s q u i t e s i m p l i s t i c . However, i t should be remembered t h a t the L, t data f i t the L-P e q u a t i o n to b e t t e r than 10% e r r o r ( l l ) , which i s q u i t e r e s p e c t a b l e f o r k i n e t i c s models, and the model i s p h y s i c a l l y s a t i s f y i n g .The c o n c l u s i o n c o n c e r n i n g the L-P model i s t h a t the model very a d e q u a t e l y r e p r e s e n t s the o x i d a t i o n k i n e t i c s f o r the L-P regime. The r a t e c o n s t a n t s are used i n d u s t r i a l l y to d e f i n e p r o c e s s e s and as s t a r t i n g parameters f o r p r o c e s s d e s i g n . I t should be c l e a r t h a t there are r e a l c o n c e p t u a l problems w i t h the L-P model n o t w i t h s t a n d i n g i t s p r a c t i c a l u t i l i t y . The e x i s t e n c e of a regime of o x i d e t h i c k n e s s e s t h a t does not conform to the model p l u s the evidence f o r more complex steps has opened new r e s e a r c h avenues and the quest f o r an even more s a t i s f a c t o r y model. 9


?

A Revised

O x i d a t i o n Model

I n a d d i t i o n to the evidence above t h a t demonstrates the inadequacy of the L-P model, t h e r e e x i s t s f u r t h e r i n f o r m a t i o n which w i t h the above evidence r e q u i r e s some r e v i s i o n f o r the L-P model. Before the evidence i s p r e s e n t e d , i t i s u s e f u l to c o n s i d e r the m e c h a n i c a l c i r c u m s t a n c e s t h a t e x i s t at the S i - S i 0 i n t e r f a c e d u r i n g o x i d a t i o n . The molar volume of S i 0 i s 120% l a r g e r than t h a t of S i . Thus we can e n v i s i o n t h a t when S i c o n v e r t s to Si02» volume must be found to permit the r e a c t i o n to proceed. For normal chemical r e a c t i o n s t a k i n g p l a c e w i t h f i n e l y d i s p e r s e d m a t e r i a l s i n the gas 2

2


42



F i g u r e 6.

A r r h e n i u s p l o t s o f t h e l i n e a r (a) and p a r a b o l i c (b) r a t e constants. (Reproduced w i t h p e r m i s s i o n from R e f . 18. C o p y r i g h t 1978 The E l e c t r o c h e m i c a l S o c i e t y . )



3.

IRENE

43

Silicon Oxidation

or s o l u t i o n phases t h i s s i t u a t i o n u s u a l l y p r e s e n t s no d i f f i c u l t y However, f o r the case of a r i g i d t h i n f i l m t h a t s t r o n g l y adheres to the r i g i d s u b s t r a t e , m e c h a n i c a l consequences a r i s e . For the p a r t i c u l a r case of S i w i t h a growing h i g h molar volume SiO^ f i l m , we would a n t i c i p a t e the a c c u m u l a t i o n of an i n t r i n s i c compressive f i l m s t r e s s ( t e n s i l e i n the S i ) . When t h i s s t r e s s i s l a r g e enough, i t c o u l d a c t u a l l y oppose the foward c h e m i c a l r e a c t i o n . However, p r i o r to the c e s s a t i o n of the r e a c t i o n , the s t r e s s c o u l d cause d e f e c t f o r m a t i o n i n the S i and/or d e l a m i n a t i o n of the o x i d e . The f a s c i n a t i n g o b s e r v a t i o n i s t h a t f o r o x i d a t i o n above about 1000 C, we observe no i n t r i n s i c s t r e s s , no damage to S i and no d e l a m i n a t i o n . Below about 900°C, an i n t r i n s i c s t r e s s i s observed t h a t i n c r e a s e s w i t h d e c r e a s i n g temperature, an i n c r e a s e i n o x i d e d e n s i t y , o x i d e f i x e d i n t e r f a c e charge, and i n c r e a s e d s u r f a c e s t a t e s a l l of which i n c r e a s e w i t h d e c r e a s i n g o x i d a t i o n t e m p e r a t u r e ( 1 9 - 2 2 ) . Of course as d i s c u s s e d above, there i s a l s o a change i n the o x i d a t i o n k i n e t i c s below 1000 C. A l l of these phenomena can be q u a l i t a t i v e l y e x p l a i n e d u s i n g the concept of v i s c o u s f l o w i n the oxide. The o b s e r v a t i o n of v i s c o u s f l o w i n Si02 f i l m s was f i r s t r e p o r t e d by E e r N i s s e ( 2 3 , 2 4 ) . E s s e n t i a l l y , a compressive i n t r i n s i c s t r e s s was found to e x i s t i n SiO^ f i l m s grown below 1000 C and t h i s s t r e s s was r e l i e v e d at h i g h e r t e m p e r a t u r e s . The d e n s i f i c a t i o n of S i 0 f i l m s was r e p o r t e d ( 2 0 , 2 1 ) and a u n i f i e d model t h a t e x p l a i n s both the o c c u r r e n c e of s t r e s s i n S i 0 and the h i g h e r d e n s i t y was p u b l i s h e d ( 2 5 ) . T h i s model u t i l i z e d the concept of v i s c o u s r e l a x a t i o n i n a Maxwell s o l i d . The main i d e a i s d e p i c t e d i n F i g . 7 where the molar volume change i s seen to cause the s t r e s s and d e n s i t y i n c r e a s e which are both r e l i e v e d v i a v i s c o u s f l o w at s u f f i c i e n t l y h i g h t e m p e r a t u r e s . These i d e a s were r e c e n t l y i n c o r p o r a t e d i n t o a r e v i s e d o x i d a t i o n model(26). P a r t of t h i s r e v i s i o n m o d i f i e s the o x i d a t i o n e x p r e s s i o n to i n c l u d e the s t r e s s d r i v e n v i s c o u s r e l a x a t i o n . From a c o n s i d e r a t i o n of SiO^ as a simple Maxwell s o l i d the e x p r e s s i o n f o r F, becomes: 9

9

where o- and ]^ are the o x i d e s t r e s s and v i s c o s i t y . A new r a t e c o n s t a n t i s d e t i n e d which does not i n c l u d e the o r i e n t a t i o n dependence t h a t i s now made e x p l i c i t through Cg^. This new way of e x p l a i n i n g the low temperature k i n e t i c s may have great t e c h n o l o g i c a l r e l e v a n c e , s i n c e p r o c e s s i n g demands r e q u i r e lower p r o c e s s i n g temperatures and t h i n n e r SiO^ and t h e r e f o r e i t i s c r u c i a l t h a t we understand the m e c h a n i c a l e f f e c t s of the o x i d a t i o n p r o c e s s . Alternative Oxidation

Technologies

Because of m a n u f a c t u r i n g demands f o r lower process t e m p e r a t u r e s , i n t e r e s t i n two a l t e r n a t i v e o x i d a t i o n techniques has been r e k i n d l e d . One technique i s h i g h p r e s s u r e o x i d a t i o n . From the L-P model we can see that an i n c r e a s e d o x i d a n t p r e s s u r e w i l l a c c e l e r a t e t r a n s p o r t a c r o s s the growing f i l m and thereby i n c r e a s e the r a t e . T h i s technique was i d e n t i f i e d i n the 1960's (27) but was ahead of



44


COMPRESSIVE STRESS

F i g u r e 7.

A v i s c o u s f l o w model f o r s i l i c o n d i o x i d e f i l m f o r m a t i o n . (Reproduced w i t h p e r m i s s i o n from Ref. 25. C o p y r i g h t 1982 The E l e c t r o c h e m i c a l S o c i e t y . )


3. IRENE

Silicon Oxidation

45

i t s time then. I n the middle 1970's i t was r e i n t r o d u c e d ( 2 8 ) and p r o p e r t i e s o f the r e s u l t i n g o x i d e s s t u d i e d ( 2 1 ) . Commercial equipment i s p r e s e n t l y i n use a t many major semiconductor p r o d u c i n g companies, and the b e n e f i t s o f t h i s t e c h n i q u e have been d i s c u s s e d . The b a s i c k i n e t i c s and f i l m f o r m a t i o n mechanism seems t o be s i m i l a r to normal 1 atm S i 0 growth(21,28,29) and t h e f i l m p r o p e r t i e s a l s o appear s i m i l a r to 1 atm o x i d e s grown a t the same low temperature as the h i g h p r e s s u r e o x i d e s . However, s u b s t a n t i a l data p r o v i n g these a s s e r t i o n s i s s t i l l l a c k i n g . The o t h e r n o v e l technique i s plasma o x i d a t i o n . An oxygen plasma i s brought i n c o n t a c t w i t h the s i l i c o n s u r f a c e . The plasma c o n t a i n s many s m a l l and h i g h l y e n e r g e t i c s p e c i e s t h a t a p p a r e n t l y g r e a t l y enhance the r a t e o f o x i d a t i o n . T h i s t e c h n i q u e was a l s o i n t r o d u c e d f o r S i o x i d a t i o n i n the 1 9 6 0 s ( 3 0 ) , and a l s o ahead o f i t s time then. Newer s t u d i e s a r e not i n agreement as t o the mechanism(31,32). High o x i d a t i o n r a t e s a r e a c h i e v a b l e e s p e c i a l l y when a n o d i z a t i o n o c c u r s by p l a c i n g a p o t e n t i a l on the S i . I t appears t h a t some form o f h i g h temperature treatment i s r e q u i r e d t o b r i n g the i n t e r f a c i a l p r o p e r t i e s i n l i n e w i t h thermal o x i d e . Research i s a c t i v e i n both these a r e a s . Of l e s s importance p r e s e n t l y , but perhaps o f f u t u r e s i g n i f i c a n c e , i s the use o f h i g h i n t e n s i t y l i g h t from l a s e r s to enhance the o x i d a t i o n r a t e ( 3 3 - 3 5 ) . Even l e s s i s known about t h i s process than the o t h e r two. Much r e s e a r c h i s p r e s e n t l y t a k i n g p l a c e on these t e c h n i q u e s and t h e f u t u r e m a n u f a c t u r i n g requirements w i l l undoubtedly s t i m u l a t e the search f o r y e t more and b e t t e r p r o c e s s i n g methods. 2


f

Conclusions In the p r e c e e d i n g d i s c u s s i o n , the thermal o x i d a t i o n o f S i was shown to be a v a l i d and i n t e g r a l process step i n the manufacture o f i n t e g r a t e d c i r c u i t s . Only t h i s process produces the h i g h e s t q u a l i t y SiO^ f o r c r i t i c a l d e v i c e o p e r a t i o n . Other methods t h a t a l s o produce S i O i n t h i n f i l m form, and t h a t a r e a l s o v a l i d p r o c e s s steps were c o n t r a s t e d w i t h thermal o x i d a t i o n and the u t i l i t y o f the steps was d i s c c u s s e d . The o x i d a t i o n model was shown which y i e l d s c o n s i d e r a b l e p r e d i c t a b i l i t y t o process e n g i n e e r s . Problems w i t h the model were p o i n t e d o u t , v i z . low temperature o x i d a t i o n and t h i n f i l m growth. These growth regimes a r e becoming t e c h n o l o g i c a l l y v e r y i m p o r t a n t . In o r d e r t o p r o v i d e a more s u i t a b l e model r e s e a r c h i s underway a t s e v e r a l l a b o r a t o r i e s as w e l l as the e x p l o r a t i o n o f s e v e r a l n o v e l f i l m growth t e c h n i q u e s . I n t e g r a t e d c i r c u i t m a n u f a c t u r i n g demands w i l l c o n t i n u e t o d r i v e r e s e a r c h a t improving the thermal o x i d a t i o n process s t e p . ?

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

Atalla, M.M.; Tannenbaum, E . ; Scheibner, E . J . Bell System Tech. J., 1959, 38, 749. Shockley, W. Phys. Rev., 56, 1939, 317. Irene, E.A. J. Electron. Mater., 1976, 5, 287. Deal, B . E . ; Grove, A.S. J . Appl. Phys., 1965, 36, 3370 Pliskin, W.A. IBM J. Res. and Develop., 1966, 10, 198.


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6. 7. 8. 9. 10. 11. 12.


13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35.

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RECEIVED March 12, 1985


Silicon Oxidation - ACS Symposium Series (ACS Publications)

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