RECRYSTALLIZATION OF Si ON INSULATING SUBSTRATES BY USING INCOHERENT LIGHT SOURCES

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RECRYSTALLIZATION OF Si ON INSULATING SUBSTRATES BY USING INCOHERENT LIGHT

SOURCES

Marjorie Haond

To cite this version:

Marjorie Haond. RECRYSTALLIZATION OF Si ON INSULATING SUBSTRATES BY USING INCOHERENT LIGHT SOURCES. Journal de Physique Colloques, 1983, 44 (C5), pp.C5-327-C5- 336. �10.1051/jphyscol:1983549�. �jpa-00223135�

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Colloque C5, supplement au nO1O, T o m e 44, octobre 1983 page C5-327

RECRYSTALLIZATION OF S i ON INSULATING SUBSTRATES BY USING INCOHERENT LIGHT SOURCES

M. Haond

Centre National! drEtude des MZ&cornrnunications, BP 9 8 , 38243 Meyl!an Cedez, France

RBsumB:

Cet article passe en r e v u e les differents resultats o b t e n u s d a n s l a realisation d e f i l m s m i n c e s d e Silicium d e p o s e s sur un substrat isolant et recristallises a l'aide de faisceaux d e r a d i a t i o n s incoherentes. C e s s y s t h m e s de recuits utilisent s o i t d e s rubans de graphite c h a u f f e s , s o i t d e s l a m p e s B halogene ou 31 arc. D a n s t o u s l e s c a s , on obtient de grandes s u r f a c e s monocristal- l i n e s d e silicium o r i e n t g e s < l o o >

.

Abstract:

T h i s paper reviews the main results obtained to date in the recrystallization o f thin S i l i c o n O n Insulator f i l m s by m e a n s of incoherent light sources. D i f f e r e n t s o u r c e s s u c h a s g r a p h i t e heaters,halogen tungsten filament l a m p s and mercury arc lamps have been investigated. Large area monocrystalline <loo> S i f i l m s have been obtained using t h e s e various means. T h e defects remaining in the f i l m s a r e d i s c u s s e d , i.e. g r a i n and/or s u b g r a i n boundaries, precipitates and strain. Electrical measurements a r e a l s o reported. Current research i s devoted f i r s t t o the design of appropriate set-ups and shaping of the energy beams a n d , s e c o n d , to film patterning i n an aim to reject grain boundaries out of the a c t i v e a r e a s of devices.

INTRODUCTION

Crystalline f i l m s of Si on insulating s u b s t r a t e s (501) are of g r e a t interest f o r n u m e r o u s applications. They a r e of s p e c i a l interest in the field of VLSI circuitry w h i c h demands high-performance single-crystal f i l m s on insulating s u s t r a t e s

.

In t h i s paper, w e will f o c u s on the more recent developments i n r e s e a r c h i n t h i s field of application. A s compared to bulk silicon VLSI, SO1 material is expected to lead to an increase in both the speed of MOS transistors and the density of integration in circuits. The speed of M O S F E T s i s mainly affected by parasitic c a p a c i t a n c e s resulting from i n t e r c o n n e c t i o n s on field o x i d e s whereas higher density is hindered by the necessity o f isolating transistors from one another. Silicon-on-sapphire (50s) first seemed a means of overcoming these d r a w b a c k s , owing to i t s insulating f e a t u r e s 1 Nevertheless, i t h a s not- had the expected commercial d e v e l o p m e n t , because of the high cost of s a p p h i r e s u b s t r a t e and indeed the poor Si/insulator interface.

A s a result of lattice mismatch and a difference in thermal

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1983549

C5-328 J O U R N A L DE PHYSIQUE

expansion c o e f f i c i e n t s , S O S d e v i c e s characteristically have low m o b i l i t i e s and high leakage currents. Replacing t h e poor Si/sapphire interface by the long-used and studied Si/Si02 interface should provide an adequate s u b s t r a t e if defect-free single-crystal S i f i l m s c a n be grown on Si02. A w i d e variety o f t e c h n i q u e s have therefore been investigated /2-5/. These c a n be divided into two groups: first, those involving crystallization in a solid phase; s e c o n d , those in which s i l i c o n i s recrystalli- z e d f r o m t h e melt. Melting i s obtained via heating a deposited polysilicon film with an energy-beam (electrons or photons).

After a r e v i e w of t h e main t e c h n i q u e s currently being investigated, w e will present the r e s u l t s and problems encountered i n zone-melting recrystallization o f S i o n insulating s u b s t r a t e s by using incoherent light from lamp- or carbon strip-heaters.

I. SO1 FROM THE SOLID PHASE

Most industrial laboratories working on L S I s would like to be a b l e t o obtain device-worthy S O 1 i n a way a s c o m p a t i b l e a s possible with their usual semiconductor processes. They have, accordingly, investigated different t e c h n i q u e s for processing w a f e r s without going through t h e liquid phase. M o r e o v e r , the solid phase s e e m s more promising f o r 3-D integration o f c i r c u i t s , thanks to reduced thermal s t r e s s and a low temperature more compatible with t h e other processing steps.

1. The Epitaxial L a t e r a l Overgrowth (ELO) technique /6/

u t i l i z e s a classical C V D p r o c e s s f o r growing s i l i c o n by epitaxy through o p e n i n g s etched in the S i 0 2 l a y e r of an oxidized silicon wafer. A silicon film i s deposited a t 1000-1150°C in a s e r i e s of growth steps, each o n e being followed by an etching s t e p (in a m i x t u r e of HCL and H2) to prevent the random nucleation occuring at the e d g e s of the openings and on the S i 0 2 surface. The silicon epilayer g r o w s up through the c h a n n e l o p e n i n g s and then proceeds to expand laterally over the S i 0 2 mask.

2. It is a l s o p o s s i b l e t o obtain a buried oxide l a y e r by Implantation of Oxygen into a Si-wafer (SIMOX) / 7 , 8 / . T h i s dielectric isola'tion provides an abrupt Si/SiO2 interface.

Subsequent thermal annealing w i l l i m p r o v e the Si/Si02 i n t e r f a c e quality. S o f a r , t h i s t e c h n i q u e h a s b e e n limited by t h e time required for processing industrial wafers and the need for high f l u x implanters.

3. An electrochemical technique also s e e m s promising for obtaining S i i s l a n d s o n Si02. By anodically etching s i l i c o n in a hydrofluoric acid (HF) electrolyte, o n e g e t s a porous s i l i c o n (FIPOS) /9,10/. S u b s e q u e n t oxidation, properly carried o u t , results in a complete dielectric isolation of predetermined monocrystalline s i l i c o n regions. T h e s i z e of t h e islands c a n be controlled by a d i f f e r e n c e in t h e anodization and oxidation r a t e s between differently doped materials.

11. RECRYSTALLIZATION FROM THE MELT

A large amount of work has been devoted to t h i s m e a n s of crystallization because of i t s similarity with the zone-melting technique widely used in growing and zone-refining single-crystal s i l i c o n r i b b o n s 1 1 H o w e v e r , a s H u r l e h a s s a i d /12/: " T h e growth o f good quality s i n g l e c r y s t a l s from the melt i s s t i l l an art a s w e l l a s a s c i e n c e

".

S i n c e G a t et al. 1 3 showed that a scanned c w laser beam c o u l d significantly increase the g r a i n s i z e o f polysilicon through local melting of the f i l m , beam crystallization of SO1 from the melt h a s been the s u b j e c t of n u m e r o u s investigations.

Fig. 1 s h o w s the r a n g e of time d u r a t i o n s accessible with the presently a v a i l a b l e energy beams. They correspond to different molten s t a t e durations related to the dwelling time of the s c a n n e d beams. O n e c a n n o t i c e that they differ by many o r d e r s of magnitude from l a s e r s to lamp-heaters, leading to l a r g e d i f f e r e n c e s in quenching rates. T h e influence of both the diffusion o f impurities and the temperature gradients will therefore be f a r different from o n e technique t o another.

I e-beams I I e-beams 1 I heaters I

Fig.1: schematic showing the range of dwelling times a c c e s s i b l e with d i f f e r e n t heat sources.

P u l s e d beams h a v e been used to grow grains of silicon on low c o s t transparent s u b s t r a t e s owing t o a rapid quenching w h i c h prevents heat from being transferred to the underlying substrate.

The fast s c a n n i n g ( m / s ) o f a c w l a s e r h a s a l s o b e e n used i n t h i s type of application /14/

.

T h e coherent nature of the energy b e a m s h a s - n o t proved to be a n a b s o l u t e neccessity for achieving local zone-melting. The advantage to using coherent radiation i s that it becomes possible t o adjust the energy t o fit the absorptivity of the material being treated, in order to melt it d o w n to a desired thickness, without significantly affecting t h e s u r r o u n d i n g or underlying material. T h i s technique therefore s h o w s potential for eventual multilayered s t r u c t u r e s in 3-5 i n t e g r a t e d c i r c u i t applications.

I n c a s e s where the underlying layers or s u b s t r a t e s s u p p o r t being heated u p ( %1400°C), the s a m p l e w i l l be preheated, thereby increasing the absorption coefficient and broadening the absorption spectrum. Whatever the heat s o u r c e u s e d , the silicon film i s molten within a narrow z o n e or band which is scanned a c r o s s the sample.

Temperature g r a d i e n t s following the moving m o l t e n z o n e bring about growth of s i l i c o n g r a i n s a t t h e liquid-solid interface after a competition between random nuclei. I f the beam i s c i r c u l a r , and if the growth r a t e along t h e interface i s c o n s t a n t , g r a i n s propagate at right angles to the interface, resulting in

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t h e f a m o u s " c h e v r o n - l i k e " s t r u c t u r e / 1 3 / . C o n s e q u e n t l y , beam s h a p i n g t o p r o d u c e a c o n c a v e t r a i l i n g e d g e w i l l r e s u l t i n g r a i n s g r o w i n g o u t w a r d t o w a r d s t h e s c a n n e d l i n e b o u n d a r i e s / 1 5 / .

P r o b l e m s a r i s e when t h e m e l t i n g o f a z o n e o c c u r s i n a c o n t i n u o u s f i l m : t h e l o c a l c h a n g e i n d e n s i t y a s s o c i a t e d w i t h m e l t i n g i n d u c e s s u r f a c e - t e n s i o n g r a d i e n t s a n d m a s s - t r a n s p o r t , r e s u l t i n g i n a r i p p l i n g o f t h e t h i n f i l m s u r f a c e / 1 6 / . M o r e o v e r , l i q u i d s i l i c o n d o e s n o t w e t S i 0 2 w e l l . I t t h e r e f o r e t e n d s t o a g g l o m e r a t e i n t o d r o p l e t s when t h e s u r f a c e t e n s i o n s a r e t o o g r e a t f o r i n s t a n c e when t h e m o l t e n z o n e b e c o m e s o v e r l y w i d e . K a m i n s / 1 7 / h a s s h o w n t h a t i n t h e c a s e o f a s c a n n e d c w * l a s e r w h e r e t h e d w e l l i n g t i m e i s on t h e o r d e r o f 1 msec., a 6 0 A S i 3 N 4 l a y e r o n t o p o f t h e p o l y - s i l i c o n i s s u f f i c i e n t t o m a i n t a i n s a t i s f a c t o r y s m o o t h n e s s o f t h e r e c r y s t a l l i z e d f i l m . I n t h e c a s e o f i n c o h e r e n t r a d i a t i o n s , t h e d w e l l i n g t i m e i s o n t h e o r d e r o f 1 s e c . a n d t h e m o l t e n z o n e i s f a i r l y b r o a d ; a m o r e s u b s t a n t i a l c a p l a y e r i s t h e r e f o r e r e q u i r e d . I t h a s b e e n s h o w n / 1 8 / t h a t a 1 - 2 pm t h i c k S i 0 2 e n c a p s u l a n t l a y e r i s n e c e s s a r y f o r a c h i e v i n g s u r f a c e f l a t n e s s a n d s o m e t i m e s a 3 0 0 1 t h i c k S i 3 N 4 l a y e r o n t o p o f t h a t p r o v i d e s b e t t e r w e t t i n g o f l i q u i d s i l i c o n o n t h e u n d e r l y i n g S i 0 2 1 F i g . 2 ( u p p e r l e f t ) s h o w s t h e f i n a l m u l t i l a y e r e d s t r u c t u r e c o m m o n l y u s e d t o r e c r y s t a l l i z e t h e p o l y - S i f i l m s .

SAMPLE DISPLACEMENT A k*,,, :

F i g . 2 : U p p e r l e f t : s c h e m a t i c c r o s s - s e c t i o n n a l d i a g r a m s h o w i n g t h e s t r u c t u r e o f t h e e n c a p s u l a t e d s a m p l e . L o w e r r i g h t : s c h e m a t i c s h o w i n g t h e m o l t e n z o n e i n t h e s a m p l e i n z o n e - m e l t i n g u s i n g o u r l a m p - h e a t e r .

111. INCOHERENT RADIATION SYSTEMS

S i n c e z o n e m e l t i n g was f i r s t d e s c r i b e d / 2 0 / , n u m e r o u s h e a t i n g t e c h n i q u e s , s u c h a s r e s i s t a n c e a n d i n d u c t i v e h e a t e r s , h a v e b e e n e x p e r i m e n t e d w i t h , i n o r d e r t o a c h i e v e z o n e m e l t i n g . T h e i r d e v e l o p m e n t f o r t h e g r o w t h o f s i n g l e - c r y s t a l g e r m a n i u m o r s i l i c o n r i b b o n s h a s b e e n e x t e n s i v e . I n 1 9 6 3 , M a s e r j i a n / 2 1 / u s e d a s c a n n e d e l e c t r o n - b e a m t o r e c r y s t a l l i z e a G e - f i l m o n a n i n s u l a t i n g s u b s t r a t e . R e c e n t l y , F a n e t a l . / 2 2 / h a v e a p p l i e d t h e same b a s i c t e c h n i q u e t o t h e c r y s t a l l i z a t i o n o f t h i n f i l m s o f S O I . The c o n f i g u r a t i o n o f t h e i r s y s t e m i s s h o w n i n F i g . 3. The s a m p l e i s p l a c e d o n a s t a t i o n a r y g r a p h i t e s h e e t w i t h t h e p o l y - S i f i l m f a c i n g u p . The s h e e t i s r e s i s t i v e l y h e a t e d u p t o 1 1 0 0 - 1 3 0 0 ° C a n d p r e h e a t s t h e s a m p l e . The a d d i t i o n a l a m o u n t o f e n e r g y n e c e s s a r y t o m e l t t h e p o l y - S i f i l m i n a n a r r o w z o n e i s p r o v i d e d b y a m o v a b l e g r a p h i t e h e a t e r ( 1 mm w i d e ) p o s i t i o n n e d a b o v e t h e s a m p l e .

MOVABLE UPPER

STRIP-HEATER LOWER

STRIP -HEATER

\ /

I'

ZONE

Fig. 3 : S c h e m a t i c diagram of the graphite strip-heater oven used at MIT f o r zone-melting recrystallization of Si-films on Si02.

W e have applied the s a m e principle but used halogen l a m p s instead o f graphite h e a t e r s /24/. They offer s e v e r a l advantages.

T h e P i r s t is t h a t the h e a t e r s a r e not in c l o s e contact with the s a m p l e s which eliminates the risk of contamination. It a l l o w s a reduction in volume of the processing chamber and more freedom with t h e ambiant atmosphere,since the s a m p l e s only are placed in it. Moreover, given the o p t i c a l n a t u r e of the energy beam, the s p o t c a n be h a n d l e d with optical d e v i c e s and t h u s any desired s h a p e can be easily induced in the molten z o n e (see Fig.2). Fig.4 s h o w s the configuration described i n Ref. 2 5 , t h a t w e h a v e used t o recrystallize SOI. For thermal i n s u l a t i o n , the s a m p l e i s placed horizontally on three c e r a m i c pins. W e have used 1 5 0 W halogen lamps having their filament at one f o c u s of a built-in ellipsoidal reflector. Their emission i s i n the r a n g e of the solar s p e c t r u m , with a peak around 0.8 pm. One l a m p preheats the s a m p l e u p t o 1000°C from below and another lamp i s focused on the upper face of the s a m p l e c a u s i n g melting of the Si-film

Fig. 4 : Schematic configuration of our lamp-heater. We have used 1 5 0 W halogen lamps. Either the lamps or the s a m D l e a r e scanned.

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on a 6*4 mm elliptical spot. By partial masking of t h e beam, w e get the convex-to-the-liquid molten z o n e s h a p e ( s e e Fig. 3).

B e c a u s e of the low power available in our experiment, w e were not a b l e t o recrystallize over e n t i r e 4-in. wafers. W e h a v e therefore used only 6 - 1 0 mm wide strips c u t out of 4-in. wafers. By moving t h e upper l a m p at 0.7 mm/sec.,we have grown single-crystal

< l o o > S i films. Another incoherent r a d i a t i o n s t u d y h a s b e e n published /26/ which u s e s t h e beam o f a short Hg arc-lamp (instead of a halogen lamp) focused on the s a m p l e by means o f a n elliptical reflector. I n t h i s c a s e , the s a m p l e i s preheated from the back by a s i l i c o n c a r b i d e c o a t e d graphite heater.

Lam e t al. /27/ have s h o w n that S i c c o a t i n g s on graphite h e a t e r s a r e necessary t o avoid precipitation of S i c in the s a m p l e after recrystallization.

IV. CHARACTERIZATION OF RECRYSTALLIZED F I L M S

A s already mentioned and t o be noted first i s t h e (100) t e x t u r e generally observed. W e h a v e obtained s i n g l e c r y s t a l a r e a s 6 cm long and 4-5 mm w i d e with a <loo> direction normal to the s u r f a c e of the film and a < l o o > a x i s parallel to the s c a n d i r e c t i o n /28/. T h e in-plane misorientation over the s a m p l e i s g e n e r a l l y l e s s than 1 degree except along the e d g e s of the recrystallized line w h e r e random nucleation l e a d s t o the f o r m a t i o n of short "chevrons"

.

Fig. 5 : O p t i c a l micrograph of a lamp-recrystallized area. The s a m p l e has been chemically decorated.

T h i s area i s monocrystalline. T h e l i n e s a r e m a d e o f dislocation a r r a y s constituting s u b g r a i n boundaries.( Scale: 1 cm = 1 0 0 pm ) .

defects. T h e l i n e s observed are made up of s u b g r a i n boundaries w h i c h consist o f dislocation a r r a y s generated to a c c o m o d a t e t w o l o w a n g l e misoriented c r y s t a l l i n e r e g i o n s /28/. O n e can s e e i n Fig. 5 t h a t after a competition in the transition r e g i o n , the s u b g r a i n s grow wider (right part of the photograph) a n d then tend to an equilibrium width of about 1 0 0 - 2 0 0 p m in the speed range used (1-0.7 mm/s). It is worth noting that w e did not f i n d any c r y s t a l l i n e d e f e c t s i n s i d e the s u b g r a i n s

t h e m s e l v e s (no twins nor stacking faults). P r e c i p i t a t e s can b e found along the s u b g r a i n boundaries, resulting from segrega- tion of impurities. W e have published /30/ extensive crystalline c h a r a c t e r i z a t i o n of these f i l m s and g i v e n s o m e e x p l a n a t i o n s a s to the origin of these remaining defects which we believe have to be related t o a cellular growth /12,31/.

We s t i l l d o not understand, however, why the Si-film is recrystallized i n a (100) texture. T h e r e i s no "memory effect"

with respect to the original (100) substrate. G e i s et al. /32/

h a v e reported t h e recrystallization of poly-silicon over S i 0 2 i n which parallel o p e n i n g s were provided down to a (111) oriented S i wafer. A s soon a s the melt coming from the unseeded region and solidifying in a (100) texture came in contact with the (111) seeded g r o w t h , t h e latter w a s occluded and a (100) texture remained. It h a s been advanced /33/ that the encapsulant layer plays a n important r o l e i n the o r i e n t a t i o n of t h e f i l m , because o f minimized interfacial f r e e energy between S i and S i 0 2 f o r the (100) planes. T h e MIT group /34/, w h o h a s recrystallized l a r g e s a m p l e s , have a l s o observed a (100) texture but the in-plane

< l o o > - a x i s w a s s o m e t i m e s m o r e than 2 0 d e g r e e s o f f from the s c a n direction. T h i s large-angle misorientation cannot be accomodated by the generation of dislocations but r e s u l t s in the f o r m a t i o n o f g r a i n s s e p a r a t e d b y g r a i n b o u n d a r i e s about 1 mm apart. A s g r a i n b o u n d a r i e s a r e detrimental t o e l e c t r o n i c devices, researchers have tried to avoid them. A s they a r i s e from local large-angle misorientations, i t h a s b e e n tried to e n s u r e a <loo>-orientation by seeding the growing c r y s t a l film from a (100) single-crystal. T h i s i s achieved by etching o p e n i n g s in the insulating layer before deposition of t h e poly-silicon film s u c h that the latter c o m e s i n t o c o n t a c t w i t h the o r i g i n a l (100) Si-substrate

.

T h e thermal profile induced inside the constriction will f o r c e the grain- or e v e n the subgrain-boundaries outward t o w a r d s the e d g e s of the molten line, in a way similar to the beam-shaping already described.

V. ELECTRICAL MEASUREMENTS I N RECRYSTALLIZED SO1 F I L M S

S o m e preliminary electrical measurements h a v e been d o n e on our lamp-recrystallized samples. D L T S spectra and C-V c u r v e s of MOS c a p a c i t o r s /36/ s h o w the high quality of t h e material: 1.Ell traps/cm3 and 1 .El0 interface states/cm2. eV at the upper interface. The MOS c a p a c i t o r s a r e 3 0 0 pm in d i a m e t e r , i.e. larger than the s u b g r a i n boundary spacings. S o m e transistors have been made, s h o w i n g m o b i l i t i e s a s h i g h a s 6 5 0 cm2/V.s, which i s on the order of the s u r f a c e mobility in n-channel t r a n s i s t o r s made in bulk Si. T h i s doping l e v e l i s attributed t o phosphorous present in t h e LPCVD r e a c t o r used f o r the deposition of o u r encapsulating S i 0 2 layer. Many l a b o r a t o r i e s h a v e published m e a s u r e m e n t s on d e v i c e s m a d e f o r t h e study of t h e respective electrical influence o f grain- and subgrain-boundaries. The s e l e c t i v e annealing technique, developped by C o l i n g e et al. /37/

i n o u r laboratory with a c w Ar+ laser p r o v i d e s a good way o f studying the influence of grain boundaries. T h i s technique uses the antireflecting property of S i 3 N 4 s t r i p e s on poly-Si to induce

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Scan direction

4

Fig. 6 : Optical micrograph s h o w i n g the localization of grain boundaries by s e l e c t i v e annealing. I n t h i s c a s e a c w Ar+ laser i s used and the power i s increasing from right t o left w h e n the laser beam i s scanned.

a per.iodic variation in the crystallization front. When the t h i c k n e s s o f t h e n i t r i d e i s adapted to the wavelength of the coherent radiation used, the r e g i o n s absorbing the most energy a r e the l a s t t o f r e e z e , t h u s localizing the d e f e c t s and grain boundaries a s s h o w n in Fig.6. T h i s localization makes it possible t o study the influence of a grain boundary on the behaviour o f a MOSFET. More detailed r e s u l t s have been presented at this C o n f e r e n c e by C o l i n g e e t al. /38/. T h e y s h o w that w h e n a grain boundary i s perpendicular to the channel of a t r a n s i s t o r , the threshold voltage i s increased by the potential barrier induced by t h e trapping of c a r r i e r s at the boundary. On the contrary, w h e n t h e grain boundary i s parallel to t h e channel, a high l e a k a g e current flows through the transistor /39/. Tsaur et al.

/40/ have analyzed the effects o f s u b g r a i n boundaries on the majority c a r r i e r transport in M O S F E T s and on test c i r c u i t s such a s r i n g o s c i l l a t o r s /41/. They h a v e s h o w n t h a t the m o b i l i t i e s and delay times respectively are not significantly affected by the p r e s e n c e o f s u b g r a i n boundaries either parallel or perpendicular t o t h e c h a n n e l s of the transistors used. The influence of s u b g r a i n b o u n d a r i e s on the aging o f the d e v i c e s has not yet b e e n studied. S u b g r a i n boundaries d o , however, seem to c a u s e the f o r m a t i o n o f s h a l l o w g r o o v e s in s u b s e q u e n t epitaxial growth of S i l a y e r s on the recrystallized f i l m s / 4 2 / . T h i s could affect the f a b r i c a t i o n of bipolar transistors.

CONCLUSIONS

Microzone-melting has proved to be a satisfactory means of r e c r y s t a l l i z i n g thin f i l m s o f s i l i c o n deposited o n an insulating a m o r p h o u s substrate. Incoherent radiations c a n be used both for preheating the s a m p l e s and local melting of the thin film. Most of the results published s o far have been obtained on s a m p l e s that a r e relatively s m a l l (in comparison t o industrial wafers), and s o m e important problems have to be solved before this

t e c h n i q u e c a n b e a p p l i e d t o i n d u s t r i a l I C p r o c e s s i n g . I f i t seems p o s s i b l e t o a v o i d o r l o c a l i z e t h e g r a i n b o u n d a r i e s , i t w i l l b e m o r e d i f f i c u l t t o g e t r i d o f t h e s u b g r a i n b o u n d a r i e s w h i c h may a f f e c t f u r t h e r e p i t a x i a l g r o w t h n e c e s s a r y f o r m a k i n g b i p o l a r t r a n s i s t o r s .

A n o t h e r p r o b l e m t o b e s o l v e d i s t h e m e c h a n i c a l b e h a v i o u r o f t h e w a f e r s u n d e r t h e r m a l s t r e s s , n a m e l y w a r p a g e a n d s l i p l i n e s . T h i s p o i n t i s b e c o m i n g m o r e a n d m o r e c r i t i c a l a s t h e e r a o f s u b m i c r o n i c t e c h n o l o g y a p p r o a c h e s .

The SO1 g r o u p a t CNET M e y l a n i s a c k n o w l e d g e d , n a m e l y R . C a r r e a n d D. B e n s a h e l f o r t h e i r h e l p i n t h e p r e p a r a t i o n o f t h i s p a p e r . G r a t e f u l t h a n k s a r e d u e t o J . D a r g e n t f o r c r i t i c a l r e a d i n g o f t h e m a n u s c r i p t .

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