PROCESS CHARACTERISATION FOR LPCVD DEPOSITION OF SiO2 FILMS FROM TEOS LIQUID SOURCE

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PROCESS CHARACTERISATION FOR LPCVD DEPOSITION OF SiO2 FILMS FROM TEOS LIQUID

SOURCE

S. Rojas, P. Serra, W. Wu, F. Santarelli, G. Sarti, F. Minni

To cite this version:

S. Rojas, P. Serra, W. Wu, F. Santarelli, G. Sarti, et al.. PROCESS CHARACTERISATION FOR

LPCVD DEPOSITION OF SiO2 FILMS FROM TEOS LIQUID SOURCE. Journal de Physique

Colloques, 1989, 50 (C5), pp.C5-83-C5-90. �10.1051/jphyscol:1989513�. �jpa-00229536�

JOURNAL DE PHYSIQUE

Colloque

C5,

suppl4rnent au

n05,

Tome

50, mai

1989

PROCESS CHARACTERISATION FOR LPCVD DEPOSITION OF SiO, FILMS FROM TEOS LIQUID SOURCE

S. ROJAS, P. SERRA, W.S.

WU, F.

SANTARELLI*\I)G.C.

SARTI* and F. MINNI*

S . G . S .

Thomson Microelectronics, 1-20041 Agrate Brianza, Italy '~ip. Ingegneria Chimica e di Processo-Universitb di Bologna. Viale Risorgimento

2,

1-40136 Bologna, Italy

RBsumk Les caractkristiques du prockdb de dkp6t de films de SiOZ h partir de TEOS liquide ont btB Btablies dans

un

rbacteur industriel sous basse pression,,6quip6 de

4

plaquettes. L'influence des variables expkrimentales les plus importantes

(T, P,

flux de TEOS et de 02) sur la vitesse de d6p6t et l'uniforrnitk du film a Btk dkterminbe. Les rksultats ont ktk analyses en fonction de conclusions ktablies

A

partir de considbrations prbliminaires de rnod6lisation. Les valeurs optimales des param&tres ont 6t6 identifibes et utiliskes pour dkvelopper industriellement les prockd6s

A

faible et haute vitesse de dbp6t.

Abstracc - Process characterisation of SiOn films deposition from te- traethylorthosilicate (TEOS) liquid source has been performed by using a LPCVD industrial system, handling

4 "

wafers.

The influence of

the

most significant manipulated variables

(such as

the process temperature and pressure and the TEOS and

0 2

flow rate) on the deposition rate and the film uniformity was investigated.

Results were analyzed on the basis of the conclusions drawn from pre- liminary modelling considerations.

Optimal sets of parameters were then identified and used to develop industrial processes at low and high deposition rate.

1

- INTRODUCTION

In the fabrication of semiconductor devices, deposited silicon dioxide films (undoped and doped) are used for a variety of purposes: insulation between conductive layers, final passivation, protective masks and, recently, SiO2 films obtained from liquid surces are also used as interpoly oxide in CMOS processes.

Different techniques can be used to produce these films: atmospheric or low pressure deposition, plasma enhanced deposition, etc.

Insulation between conductive layers is usually obtained by chemical vapor deposition of silicon dioxide in atmospheric or low pressure systems by oxi- dizing silane with oxygen in a surface-catalyzed reaction.

Recently films of silicate glasses have been deposited in low pressur-e sys- tems at temperature in the range 700-750

" C

by decomposing compounds such as TEOS

( t e t r a e t h y l o r t h o s i l i c a t e ) ,

which is liquid under normal conditions.

These films have been found to exhibit low defectivity, high thickness uni- formity, good electrical properties and good step coverage

/ 1 , 2 / .

Besides, TEOS is a source material of easier handling and higher safety than silane

/ 3 / .

Various works have been published dealing both with film characterisation and film properties for SiOz films deposited from TEOS sources in different reac- tor geometries under different operating conditions/4-8/.

This paper deals with the process characterization of the TEOS-SiO2 deposi-

( 1 )

Work partially supported by C.N.R./P.F."Materiali e Dispositivi per 1'61ettro- nica a Stqto Solido"

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1989513

C5-84 JOURNAL

DE

PHYSIQUE

t i o n i n a LPCVD r e a c t o r where t h e r e a c t a n t s a r e f e d i n a d i s t r i b u t e d way t o t h e r e a c t o r : t h e r o l e o f manipulated process v a r i a b l e s as temperature, t o t a l pressure and TEOS gas f l o w r a t e on t h e d e p o s i t i o n r a t e and on t h e t h i c k n e s s u n i f o r m i t y o f t h e f i l m has been i n v e s t i g a t e d .

2- EXPERIMENTAL

The d e p o s i t i o n s were performed i n a standard h o r i z o n t a l h o t w a l l LPCVD system w i t h a t h r e e zones temperature c o n t r o l .

A schematic o f t h e system i s d e p i c t e d i n f i g . 1 .

T E O S F U R N A C E

c

^{G L S R I N G L I D} I N S E R T r

I I I

^{T C}

⁼

T E M P E R A T U R E CONTROL

i 1 I

PS Y POWER SUPPLY

1 I 1

^G G A S FEED S Y S T E M

F i g . l The experimental l a y o u t G

The wafers were placed v e r t i c a l l y back t o back w i t h a spacing o f 9 . 5 mm i n q u a r t z boats having v e r t i c a l s l o t s i n t h e bottom and closed covers. The boats were p o s i t i o n e d i n a f i x e d zone o f t h e tube on t h e c a n t i l e v e r l o a d i n g system.

The gases a r e f e d t o t h e r e a c t o r through two d i s t r i b u t i o n m a n i f o l d s (one f o r t h e oxygen, one f o r TEOS o r n i t r o g e n ) l o c a t e d beneath t h e wafers, c l o s e t o t h e arms o f t h e c a n t i l e v e r l o a d i n g system. Each m a n i f o l d c o n s i s t e d o f a l o n g q u a r t z tube c l o s e d a t one end, w i t h a s e r i e s o f h o l e s placed i n such a way t o guarantee t h e same feed t o each r e g i o n between two f a c i n g wafers.

Oxygen was used i n o r d e r t o achieve b e t t e r o v e r a l l o p e r a t i n g c o n d i t i o n s as i t w i l l be discussed i n t h e f o l l o w i n g .

The f l o w o f oxygen i n t r o d u c e d i n t o t h e tube was c o n t r o l l e d by a MFC whereas t h e amount o f TEOS vapor was a u t o m a t i c a l l y c o n t r o l l e d by a t h r o t t l e v a l v e and a pressure t r a n s d u c e r .

The TEOS bubbler was k e p t a t a c o n s t a n t temperature o f 50°C and t h e l i n e from t h e bubbler t o t h e tube was k e p t a t 6OSC, i n o r d e r t o a v o i d TEOS condensation i n t h e l i n e .

The pressure i n t h e tube was a u t o m a t i c a l l y k e p t c o n s t a n t a t t h e s e l e c t e d value by means o f a t h r o t t l e v a l v e .

100 mm s i l i c o n wafers, p-type w i t h 7-10 ohm-cm r e s i s t i v i t y , were used f o r c h a r a c t e r i s a t i o n .

F i l m t h i c k n e s s was measured i n f i v e p o i n t s per wafer by u s i n g a Nanospec system and t h e w i t h i n - w a f e r u n i f o r m i t y was evaluated as p e r c e n t v a r i a t i o n E o f t h e f i l m t h i c k n e s s d, defined as (dmax

-

dmin)/(dnax

+

d m i n ) ~ 1 0 0 .

3- PRELIMINARY ANALYSIS

The TEOS decomposition occurs w i t h i n t h e r e a c t o r through an heterogeneous r e a c t i o n which t a k e s p l a c e a t t h e wafer s u r f a c e and can be represented re- s o r t i n g t o a commonly accepted o v e r a l l s t e c h i o m e t r i c e q u a t i o n such as

S Y S T E M C O M P U T E R

Homogeneous decomposition can be s a f e l y n e g l e c t e d a t t h e v e r y low pressure under which t h e r e a c t o r i s operated: an i n d i r e c t b u t s i g n i f i c a n t s u p p o r t t o

P P Z VACUUM PUMP

t h i s statement i s given by t h e Auger E l e c t r o n i c Spectroscopy which shows no evidence o f carbon compounds contamination which, on t h e con,trary, i s appar- e n t i n f i l m s obtained i n r e a c t o r s operated a t atmospheric pressure /8,9/.

The o v e r a l l process can t h e r e f o r e be considered as a sequence o f p h y s i c a l and chemical steps: d i f f u s i o n o f TEOS toward t h e wafer surface, a d s o r p t i o n and decomposition r e a c t i o n a t t h e surface, d e s o r p t i o n and d i f f u s i o n o f t h e gase- ous decomposition products from t h e s u r f a c e t o t h e gas body.

T h i s s i t u a t i o n i s t y p i c a l o f LPCVD processes: i t has been widely i n v e s t i g a t e d and models have been developed both from a q u i t e general p o i n t o f view /10- 13/ and w i t h s p e c i f i c regard t o TEOS decomposition /4,14/.

However, these r e s u l t s cannot be d . i r e c t l y a p p l i e d here s i n c e a t t e n t i o n has been t h e r e focused on t h e popular m u l t i p l e - d i s k - i n - t u b e r e a c t o r where t h e main process stream f l o w s from one end t o t h e opposite one o f the r e a c t o r and t h e f l o w v a r i e s c o n t i n u o u s l y along t h e tube as t h e r e s u l t o f a balance between t h e d e p l e t i o n o f t h e r e a c t a n t and t h e adduction o f t h e gaseous reac- t i o n products. Indeed, i n t h i s type o f r e a c t o r , convection i s confined i n t h e o u t e r r e g i o n o f t h e tube, i .e. between t h e edges o f t h e wafers and t h e wal l o f t h e r e a c t o r w h i l e i n each space o r c e l l between two adjacent wafers i t i s d i f f u s i o n which s u p p l i e s r e a c t a n t s t o t h e surfaces where t h e r e a c t i o n takes place. The c o n c e n t r a t i o n p r e v a i l i n g w i t h i n each c e l l i s t h e r e f o r e s i g n i - f i c a n t l y a f f e c t e d by t h e e v o l u t i o n which occurs i n t h e process stream along t h e p a t h from t h e i n l e t t o t h e c e l l i t s e l f and t o o b t a i n a s a t i s f a c t o r y i n t e r w a f e r u n i f o r m i t y o f t h e f i l m t h i c k n e s s may be a c r i t i c a l t a s k .

A d i f f e r e n t s i t u a t i o n occurs here s i n c e t h e p e c u l i a r arrangement adopted f o r t h e r e a c t a n t s d i s t r i b u t i o n p o t e n t i a l l y guarantees a u n i f o r m feed: t o each c e l l : t h e same phenomena should occur i n each c e l l between two f a c i n g wafers and t h e r e f o r e a s i n g l e c e l l must be considered t o model t h e process.

As t h e r e s u l t o f t h e i n j e c t i o n o f t h e r e a c t a n t s beneath t h e wafers a v e l o c i t y f i e l d develops i n t h e c e l l , t h e v e l o c i t y being d i r e c t e d along t h e wafers' surface. A t t h e same time, s i n c e t h e clearance S between two wafers i s much s m a l l e r than t h e diameter (2.Rn) o f t h e wafers, i t can be s a f e l y assumed t h a t c o n c e n t r a t i o n i s u n i f o r m i n any plane normal t o t h e wafers surface.

Assuming t h a t i ) t h e temperature i s uniform w i t h i n t h e c e l l , i i ) t h e wafer surfaces can be considered as square p l a t e s 2.Rw wide i i i ) t h e s u r f a c e reac- t i o n i s n - t h order w i t h respect t o t h e TEOS c o n c e n t r a t i o n and t h a t i v ) t h e d i s t r i b u t i o n o f t h e c o n c e n t r a t i o n c o f t h i s r e a c t a n t can i s one-dimension,al v a r y i n g from t h e bottom t o t h e t o p o f t h e p l a t e , c i s governed by t h e f o l - lowing equation:

w i t h t h e r e l e v a n t boundary c o n d i t i o n s . I n eqn. ( 1 ) z i s a l i n e a r coordinate along an upward v e r t i c a l d i r e c t i o n w i t h t h e o r i g i n a t t h e lower edge o f t h e p1 a t e .

The v e l o c i t y v i s a c t u a l l y v a r y i n g (decreasing) along t h e wafer as t h e r e s u l t o f t h e SiOz d e p o s i t i o n a t t h e surface. The e f f e c t s due t o non-equimolarity o f t h e s u r f a c e r e a c t i o n have been considered i n /14/ i n an otherwise p u r e l y d i f f u s i o n a l s i t u a t i o n and r e s u l t e d i n a reduced u n i f o r m i t y o f t h e f i l m t h i c k - ness. I n t h i s s i t u a t i o n , however, a convective f l o w due t o e x t e r n a l f o r c e s occurs and t h e r e f o r e t h e v a r i a t i o n s o f the.mass center v e l o c i t y v, due t o t h e n e t mass f l u x a t t h e surface, w i l l be l e s s important and w i l l be neglected.

Equation ( 1 ) can be r e w r i t t e n i n dimensionless form as

d y 1 d2y

----

⁼

--- ---

_{- Da- y n}

d < P e d 5 2 where

I n eqn. ( 2 ) t h e o r i g i n a l v a r i a b l e s a r e made dimensionless through vo, a c h a r a c t e r i s t i c s c a l e f o r t h e v e l o c i t y , C O , t h e i n l e t TEOS c o n c e n t r a t i o n a t z=O, R n , t h e wafer r a d i u s : t h e r e s u l t i n g dimensionless parameters, Pe and Da,

JOURNAL DE PHYSIQUE

a r e t h e P e c l e t and t h e Damkohler numbers r e s p e c t i v e l y .

As i t i s w e l l known, Pe can be i n t e r p r e t e d as t h e r a t i o o f t h e c h a r a c t e r i s t i c s c a l e s o f c o n v e c t i v e and d i f f u s i o n a l t r a n s p o r t , w h i l e Da i s t h e r a t i o o f t h e c h a r a c t e r i s t i c s c a l e s o f t h e s u r f a c e r e a c t i o n and t h e c o n v e c t i v e t r a n s p o r t . The v a l u e o f each o f these two parameters w i l l t h e r e f o r e a f f e c t t h e p e r f o r - mance o f t h e apparatus. I n p a r t i c u l a r t h e value o f Pe i s very i m p o r t a n t t o

i d e n t i f y t h e p r e v a i l i n g t r a n s p o r t mechanism and t o determine t h e r e s u l t i n g u n i f o r m i t y o f t h e f i l m t h i c k n e s s : should, f o r instance, Pe be l a r g e , convec- t i o n would be t h e e f f e c t i v e t r a n s p o r t mechanism and a n o n - u n i f o r m i t y would appear i n t h e f i l m t h i c k n e s s .

I n t h e s i t u a t i o n a t hand, however, T i s q u i t e l a r g e and, above a l l , Pt i s so small t h a t D e i s expected t o be very l a r g e : then Pe w i l l be very small and convection w i l l be n e g l i g i b l e . Under these c o n d i t i o n s t h e r e f o r e t h e c e l l approaches a p e r f e c t l y mixed system and t h e r e a c t a n t c o n c e n t r a t i o n can be considered almost u n i f o r m w i t h i n t h e gas space: r e s u l t s can t h e r e f o r e be i n t e r p r e t e d i n terms o n l y o f t h e competing r o l e s o f d i f f u s i o n w i t h i n t h e b u l k o f t h e gas and o f chemical r e a c t i o n a t t h e surface.

Then t h e product o f Pe and Da must be considered and no longer t h e i r i n d i v i d - u a l values: t h u s t h e o n l y s i g n i f i c a n t parameter t u r n s o u t t o be

which can be i n t e r p r e t e d as t h e r a t i o between t h e c h a r a c t e r i s t i c s c a l e s o f s u r f a c e r e a c t i o n and d i f f u s i o n a l t r a n s p o r t toward t h e r e a c t i o n s u r f a c e and i s t h e r e f o r e e q u i v a l e n t t o t h e T h i e l e modulus f o r a r e a c t i o n w i t h i n a porous c a t a l y s t .

I t may be t h e r e f o r e worth reminding some very general t r e n d s which r e s u l t f o r a r e a c t i o n o c c u r r i n g w i t h i n a porous c a t a l y s t .

As a m a t t e r o f f a c t , when r e a c t i o n i s t h e c o n t r o l l i n g s t e p t h e r e a c t a n t c o n c e n t r a t i o n i s almost u n i f o r m a11 over t h e r e a c t i o n volume which i s t h e n e x p l o i t e d u n i f o r m l y and e f f e c t i v e l y . When, on t h e c o n t r a r y , d i f f u s i o n i s t h e c o n t r o l l i n g s t e p t h e d i s t r i b u t i o n o f t h e r e a c t a n t c o n c e n t r a t i o n e x h i b i t s a l a r g e n o n - u n i f o r m i t y and t h e r e a c t i o n volume i s consequently e x p l o i t e d i n a l e s s u n i f o r m and e f f e c t i ' v e way.

These c o n s i d e r a t i o n s can be e a s i l y extended t o t h e s i t u a t i o n a t hand i n o r d e r t o o b t a i n some e f f e c t i v e g u i d e l i n e s f o r a q u a l i t a t i v e a n a l y s i s o f t h e e x p e r i - mental r e s u l t s , w i t h s p e c i a l regards t o t h e v a r i a t i o n o f t h e f i l m t h i c k n e s s . I t may, i n p a r t i c u l a r , be i n f e r r e d t h a t t h e t h i c k n e s s percent v a r i a t i o n E can be r e l a t e d t o t h e v a l u e o f 92 and increases when 9 2 increases.

The way 9 and t h e n E a r e a f f e c t e d by changes i n t h e temperature and t h e pressure o f t h e process w i l l be apparent i n t h e f o l l o w i n g paragraph.

4- DISCUSSION OF THE RESULTS

Process c h a r a c t e r i s a t i o n was accomplished by examining t h e e f f e c t s due t o v a r i a t i o n s i n t h e process temperature, t h e tube pressure, t h e TEOS l i n e pressure and t h e oxygen f l o w r a t e . Each parameter has been v a r i e d one a t a t i m e w h i l e keeping t h e o t h e r s c o n s t a n t .

The v a r i a t i o n s o f t h e parameters were chosen i n a range compatible w i t h t h e system c o n t r o l and t h e d e p o s i t i o n f e a s i b i l i t y .

The chosen ranges were:

-temperature 650 - 800 " C -tube pressure 200

-

800 m t o r r -TEOS p r e s s u r e f

-

^{5 t o r r}

-02 f l o w r a t e 0

-

200 sccm

R e s u l t s can be summarized as f o l l o w s : I n b h e n c e & .the prroce-64 ;eem~a&tuae

The d e p o s i t i o n r a t e ( D . R . ) was found t o i n c r e a s e when t h e process temperature was increased. The r e s u l t i n g t r e n d i s g i v e n i n f i g . 2 and i s q u i t e s i m i l a r t o those g i v e n i n /4,5,8/ where t h e e x p l o r e d range o f t h e process temperatures was very c l o s e t o t h e one considered here.

However t h e apparent a c t i v a t i o n energy extimated from a l i n e a r i n t e r p o l a t i o n over t h e whole i n v e s t i g a t e d range t u r n s o u t t o be Eactz23 kcal/mol which i s much s m a l l e r t h a n t h e values (240 k c a l / m o l ) r e p o r t e d i n t h e l i t e r a t u r e /4,5,8/. On t h e o t h e r s i d e these values appear t o be e x t i m a t e d o n l y on t h e

b a s i s o f t h e D.R. values obtained f o r T< 72OSC, no matter how l a r g e t h e explored T range was.

It i s nevertheless apparent t h a t a l i n e a r i n t e r p o l a t i o n over t h e whole inves- t i g a t e d range may be questionable s i n c e a f l a t t e n i n g o f t h e curve occurs a t t h e h i g h e s t temperatures. T h i s suggests t h a t when t h e process temperature i s increased beyond a given value ( 2 720°C) the o v e r a l l process remains an a c t i - vated one b u t some changes occur i n t h e r e l a t i v e weight o f the concurring elementary steps: an i n c r e a s i n g r o l e o f d i f f u s i o n i n t h e c o n t r o l o f t h e over- a l l process can be conceived, s i n c e d i f f u s i o n i s an a c t i v a t e d process w i t h an a c t i v a t i o n energy smaller than t h a t o f a chemical r e a c t i o n .

T h i s i n t e r p r e t a t i o n i s f u r t h e r supported by t h e a n a l y s i s o f t h e concomitant t r e n d i n t h e percent v a r i a t i o n E o f t h e f i l m thickness: a very good u n i f o r m i - t y ( c o l % ) i s obtained f o r T< 700°C w h i l e E increases more and more f o r higher values o f T a t which, furthermore, t h e l a r g e s t values o f t h e f i l m thickness

13..

'''..c ,... 9

1 , I I : I I I I

0 . 8 0 0 . 9 0 1 . 0 0 1 . 1 0 1 . 2 0

1000/T ( K )

-+-- deposition rate .-.--D.-.- thickness % variation

F i g . 2 Deposition r a t e and thickness percent v a r i a t i o n as a f u n c t i o n o f t h e process temperature ( P t

=

400mtorr, P T E O S

=

3 t o r r )

occur i n the lower p a r t o f t h e wafers, i . e . c l o s e t o t h e r e a c t a n t i n l e t p o i n t s .

I t i s worth n o t i n g t h a t these arguments a r e c o n s i s t e n t w i t h t h e r e c a l l e d general statement t h a t an i n c r e a s i n g r o l e o f d i f f u s i o n causes a l e s s uniform e x p l o i t a t i o n o f t h e r e a c t i o n space.

InUuence & lehe p a o ~ e d - ~ t o e paefrawre(Pt)

When Pt was v a r i e d i n t h e range 200-800 m t o r r t h e d e p o s i t i o n r a t e was found t o be o n l y s l i g h t l y a f f e c t e d by t h e value o f P t , a t l e a s t i n t h e i n v e s t i g a t e d range ( f i g . 3 ) .

As a matter o f f a c t an increase i n Pt acted on D.R. i n two opposite ways, i . e . p o s i t i v e l y through an increase o f TEOS p a r t i a l pressure and n e g a t i v e l y through a r e d u c t i o n o f t h e value o f t h e e f f e c t i v e d i f f u s i v i t y which r e s u l t s i n an i n c r e a s i n g c o n t r o l r o l e o f d i f f u s i o n .

From f i g . 3 i t i s apparent t h a t t h e f i r s t o f t h e two e f f e c t s was t h e p r e v a i l - i n g one.

On t h e o t h e r s i d e t h e f i l m u n i f o r m i t y was found very good and p r a c t i c a l l y i n s e n s i t i v e t o Pt even i f a t t h e lowest values o f Pt a s l i g h t increase o f E

was found.

This t r e n d o f E w i t h Pt i s q u i t e d i f f e r e n t from t h e one given i n /8/ where E

i s reported t o increase s i g n i f i c a n t l y w i t h Pt i n a r e a c t o r where process c o n d i t i o n s were q u i t e c l o s e t o t h e ones considered here, b u t t h e distance between two f a c i n g wafers was 4 . 7 mm, i . e . smaller than t h e one used here.

This discrepancy can be e a s i l y j u s t i f i e d i f one remembers t h e occurrence o f t h e geometric f a c t o r (2-Rw)/S i n t h e d e f i n i t i o n o f $ 2 and i s , furthermore, c o n s i s t e n t w i t h t h e r e s u l t s given i n t h e same reference / 8 / which show t h a t an increase i n S a f f e c t s f i l m u n i f o r m i t y p o s i t i v e l y .

The l i n e a r dependence o f D.R. on Pt and t h e almost constant values o f E a r e s u r e l y due t o t h e l i m i t e d Pt range explored.

As a matter o f f a c t , as f a r as D.R. i s concerned, t h e r e i s no doubt t h a t t h e curve o f f i g . 3 must be f o r c e d t o pass through t h e o r i g i n when Pt goes t o zero. A c t u a l l y a pronounced n o n - l i n e a r i t y i s apparent i n t h e l i m i t o f Pt

-->

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0 from t h e data r e p o r t e d i n / 1 , 5 / . Furthermore, i n / 5 / t h i s n o n - l i n e a r i t y i s accounted f o r by a proposed r a t e equation which i m p l i e s t h a t i ) adsorption on t h e wafer s u r f a c e i n v o l v e s two a c t i v e s i t e s ; i i ) t h e r e a c t i o n i s o f order n = 0 . 5 i n t h e l i m i t o f P t

-->

0 .

These c o n s i d e r a t i o n s a l l o w a l s o a reasonable e x t r a p o l a t i o n of t h e present r e s u l t s t o f i g u r e o u t t h e E t r e n d which can be expected a t very low pres- sures: i n t h i s P t range d i f f u s i o n occurs as Knudsen d i f f u s i o n w i t h a c o e f f i - c ~ e n t D K which i s independent o f P t and, assuming n = 0 . 5 , i t f o l l o w s t h a t

where p0 i s a c h a r a c t e r i s t i c TEOS p a r t i a l pressure i n t h e r e a c t o r .

I n view o f t h e general c o n s i d e r a t i o n on t h e i n f l u e n c e o f 0 2 on E i t i s reaso- nable t o conclude t h a t when P t

-->

0 , s i n c e $ 2 increases, a pooreer u n i f o r m i - t y o f t h e f i l m w i l l r e s u l t , as i t i s a l s o suggested by our data a t P t = 2 0 0 mtorr.

process total pressure (mTorr)

- +

- deposition rate

...I I d.--- thickness % variation

F i g . 3 Deposition r a t e and t h i c k n e s s percent v a r i a t i o n as a f u n c t i o n o f t h e process t o t a l pressure P t ( T = 7 0 0 ' C ; P T E O S

=

3 t o r r )

Iniluence TEOS U n e p a ~ w r e ( P ~ ~ o s )

Since no r e l i a b l e flowmeter was a v a i l a b l e f o r t h i s gas, t h e pressure i n t h e m a i n l i n e , upstream t h e r e a c t o r , was changed and assumed t o be s i g n i f i c a n t o f t h e gas f l o w r a t e f e d t o t h e space between two f a c i n g wafers.

The r e s u l t i n g changes i n D.R.and E are given i n f i g . 4 .

i TEOS pressure (Torr)

+

deposition rate ... 1 , .... thickness % variation

Fig.4 Deposition r a t e and thickness percent v a r i a t i o n as a f u n c t i o n o f t h e pressure i n t h e TEOS feed l i n e (T

=

7 0 0 ° C ; P t

=

4 0 0 m t o r r )

The same t y p e o f c o n s i d e r a t i o n s made on t h e e f f e c t s o f P t on D.R. can be repeated here t o o and no f u r t h e r comment i s done.

Inh&ence & .the oxvcren &Low na;te

Oxygep i s n o t r e q u i r e d f o r t h e TEOS d e c o m p o s i t i o n and i s known t o be u n i n f l u - e n t i a l on t h e d e p o s i t i o n p r o c e s s , a t l e a s t i n t h e i n v e s t i g a t e d temperature range: a c t u a l l y i t was found t o be u n e f f e c t i v e on t h e d e p o s i t i o n r a t e and t h i c k n e s s p e r c e n t v a r i a t i o n , as shown i n f i g . 5 .

N e v e r t h e l e s s i t s use was f o u n d t o a f f e c t t h e o p e r a t i n g c o n d i t i o n s b e n e f i c i a l - l y r e d u c i n g t h e w a l l f o u l i n g i n t h e r e a c t o r downstream l i n e s .

o x y g e n f l o w sccm --t d e p o s i t i o n rate . . . . . , 3 , . . . . t h i c k n e s s % variation

F i g . 5 D e p o s i t i o n r a t e and t h i c k n e s s p e r c e n t v a r i a t i o n as a f u n c t i o n o f t h e oxygen f l o w r a t e ( T

=

700 ' C ; P t = 400 m t o r r ; P T E O S = 3 t o r r )

I n t e t w a h m unido.wnh5.y

The i n t e r w a f e r u n i f o r m i t y was n o t so good as expected: a c t u a l l y , a t u n i f o r m t e m p e r a t u r e o p e r a t i o n , f i l m t h i c k n e s s was found t o i n c r e a s e ( f u l l l i n e curves i n f i g s . 6 ) f r o m t h e door end t o t h e e x h a u s t one o f t h e r e a c t o r , t h e v a r i a -

A 7 6 0 t f

0 1 2 3 4 5 6 7

d o o r e n d e x h a u s t e n d

boat position

+

flat t e m p e r a t u r e profile T = 700 C tilted temperature profile

A 5 0 0 0 0 1 2 3 4 5 6 7

!

d o o r end e x h a u s t e n d

boat position

+

flat temperature profile T = 720 C tilted temperature profile

F i g . 6 The f i l m t h i c k n e s s as a f u n c t i o n o f t h e wafer p o s i t i o n a l o n g t h e r e a c t o r ( P t = 700mtorr ; P T E O S = 3 t o r r )

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t i o n s b e i n g w i t h i n

+

4% w i t h r e s p e c t t o t h e b u l k average t h i c k n e s s . T h i s i n c o n v e n i e n c e can be a t t r i b u t e d t o a m a l d i s t r i b u t i o n i n t h e r e a c t a n t f e e d and can be e a s i l y removed ( s e e dashed l i n e c u r v e s i n f i g s . 6 ) r e s o r t i n g t o a v e r y s m a l l g r a d i e n t o f t e m p e r a t u r e a l o n g t h e r e a c t o r .

5-CONCLUSIQNS

A p a r a m e t r i c c h a r a c t e r i s a t i o n was p e r f o r m e d f o r SiOz d e p o s i t i o n f r o m a TEOS l i q u i d s o u r c e , i n t h e t h i c k n e s s range 400

+

⁶⁰⁰⁰ A.

The i n f l u e n c e o f t h e most s i g n i f i c a n t m a n i p u l a t e d v a r i a b l e s such as t h e p r o c - e s s t e m p e r a t u r e and p r e s s u r e , t h e TEOS and oxygen f l o w r a t e was i n t e r p r e t e d

i n t e r m s o f t h e competing r o l e s o f t h e TEOS d i f f u s i o n towards t h e w a f e r s and o f t h e d e c o m p o s i t i o n r e a c t i o n a t t h e s u r f a c e .

Temperature was f o u n d t o be t h e most e f f e c t i v e o p e r a t i n g parameter s i n c e t h e p r o c e s s e x h i b i t e d a h i g h s e n s i t i v i t y t o i t s v a l u e .

R e s u l t s were used t o i d e n t i f y p r o p e r s e t s o f o p e r a t i n g c o n d i t i o n s f o r i n d u s - t r i a l p r o c e s s a t low ( 6 0 A/min) and h i g h (115 A/min) d e p o s i t i o n r a t e .

The f o r m e r p r o c e s s i s o p e r a t e d a t T

=

700°C and P t = 700 m t o r r t o d e p o s i t Si02 f i l m s , 700 A t h i c k , which can be used as i n t e r p o l y d i e l e c t r i c f o r h i g h q u a l i t y c a p a c i t o r s i n AD c o n v e r t e r d e v i c e s / l 5 / .

The l a t t e r one i s o p e r a t e d a t T = 720'C and P t = 700 m t o r r t o d e p o s i t f i l m s t h i c k e r t h a n 4000 A t o be used as LDD s p a c e r s and t r e n c h f i l l i n g o x i d e .

REFERENCES

/ l / R.M.Levin, K . E v a n s - L u t t e r o d t , J.Vac.Sci.Technol.B1,(1),54(1983) / 2 / F.S. Becker et d.,J.Vac.Sci.Technol.B4,(3),732(1986)

/3/ J.C.Schumacher C o . , N e w s l e t t e r N.32

/4/ H.Huppertz, IEE Trans.Electron.Devices,~,658(1979)

/ 5 / A.C.Adams, C.D.Capio, J . E l e c t r o c h e m . S o c . , m ( 6 ) , 1 0 4 2 ( 1 9 7 9 ) / 6 / R.M.Levin, A.C.Adams, J.Electrochem. S o c . , m , 1 5 8 8 ( 1 9 8 2 ) /7/ F.S.Becker, S.Roh1, J.Electrochem. S o c . , B , 2 9 2 3 ( 1 9 8 7 ) /8/ F.S.Becker et d., J.Vac.Sci.Technol.B5(6),1555(1987) /9/ E.L.Jordan, J . E l e c t r o c h e m . S o c . , ~ ( 5 ) , 4 7 8 ( 1 9 6 1 )

/10/ A.E.T. K u i p e r et d., J..Electrochem. S o c . , m ( 1 0 ) , 2 2 8 8 ( 1 9 8 2 ) / 1 1 / K.F.Jensen, D.B.Graves, J.Electrochem. S o c . , m ( 9 ) , 1 9 5 0 ( 1 9 8 8 ) /12/ K.F.Roenigk, K.F.Jensen, J.Electrochem. Soc.,=(2),448(1985) /13/ A.Yecke1, S.Middleman, J . E l e c t r o c h e m . Soc.,134(5),1275(1987) / 1 4 / W.Velander, D.White j r , J.Electrochem. Soc.,134,951(1987) /15/ W.S.Wu et d., P r o c e e d i n g s ESSDERC 88,(C4),397(1988)