Microelectronics Processing - ACS Publications - American Chemical

5 Chemical Vapor Deposition

Downloaded by PRINCETON UNIV on November 11, 2014 | http://pubs.acs.org Publication Date: May 5, 1989 | doi: 10.1021/ba-1989-0221.ch005

Klavs F. Jensen Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, M N 55455

Chemical vapor deposition (CVD) of thin solid films from gaseous reactants is reviewed. General process considerations such as film thickness, uniformity, and structure are discussed, along with chemical vapor deposition reactor systems. Fundamental issues related to nucleation, thermodynamics, gas-phase chemistry, and surface chemistry are reviewed. Transport phenomena in low-pressure and atmospheric-pressure chemical vapor deposition systems are described and compared with those in other chemically reacting systems. Finally, modeling approaches to the different types of chemical vapor deposition reactors are outlined and illustrated with examples.

C J H E M I C A L VAPOR D E P O S I T I O N IS A K E Y P R O C E S S i n m i c r o e l e c t r o n i c s fab-

rication for the d e p o s i t i o n of t h i n films of metals, semiconductors, a n d i n sulators o n s o l i d substrates. A s the n a m e indicates, c h e m i c a l l y r e a c t i n g gases are u s e d to synthesize the t h i n s o l i d films. T h e use of gases distinguishes c h e m i c a l vapor d e p o s i t i o n ( C V D ) f r o m p h y s i c a l d e p o s i t i o n processes s u c h as s p u t t e r i n g a n d evaporation a n d imparts versatility to the d e p o s i t i o n t e c h nique. T h e reactions u n d e r l y i n g C V D t y p i c a l l y occur b o t h i n the gas phase a n d o n the surface of the substrate. T h e energy r e q u i r e d to d r i v e the reactions is u s u a l l y s u p p l i e d t h e r m a l l y b y h e a t i n g t h e substrate or, i n a f e w instances, b y h e a t i n g the gas. A l t e r n a t i v e l y , photons from an u l t r a v i o l e t ( U V ) l i g h t source or from a laser, as w e l l as energetic electrons i n plasmas, are u s e d to d r i v e l o w - t e m p e r a t u r e d e p o s i t i o n processes. Various reaction schemes, i n c l u d i n g p y r o l y s i s , r e d u c t i o n , o x i d a t i o n , d i s p r o p o r t i o n a t i o n , a n d h y d r o l y s i s o f the reactants, have b e e n u s e d to p r o d u c e a large v a r i e t y o f t h i n films relevant to m i c r o e l e c t r o n i c s processing. T a b l e I 0065-2393/89/0221-0199$15.75/0 © 1989 American Chemical Society

In Microelectronics Processing; Hess, D., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1989.

MICROELECTRONICS PROCESSING: C H E M I C A L ENGINEERING ASPECTS

200

Table I. C V D Films for Microelectronic Application Application

Type

Insulator

Oxide Nitride Oxynitride Element Group I I - V I compound Group I I I - V compound


Semiconductor

Conductor

Oxide Metal Silicide

Superconductor

Examples Si0 , A1 0 , Ti0 , Ta 0 , B O , P O Si N , Si.N^H, BN S i O , N , ΑΙ,Ο,Ν Si, Ge 2

3

2

3

2

2

5

2

s

2

s

4

t

ZnS, ZnSe, CdTe, CdHgTe GaAs, AlGaAs, A l G a N , InP, GalnAs, GalnAsP, InSb, GaSb S n 0 , S n 0 , ZnO W, M o , C r , A l WSi , MoSi , TiSi Nb Sn, N b N , YBa Cu O 2

2

3

2

3

2

2

2

3

x

lists examples of these films, a n d T a b l e II gives examples of C V D processes. G e n e r a l reviews of C V D a n d r e l a t e d t h i n - f i l m deposition processes are a v a i l able i n a n u m b e r o f books a n d survey papers (1-6). T h i s chapter i n c l u d e s o n l y a short o v e r v i e w of the m a i n processes a n d concentrates o n the f u n damental physicochemical phenomena underlying C V D .

CVD Processes General Process Considerations. T o be useful, a C V D process m u s t p r o d u c e t h i n films w i t h r e p r o d u c i b l e a n d controllable properties i n c l u d i n g p u r i t y , c o m p o s i t i o n , film thickness, adhesion, crystalline s t r u c t u r e , a n d surface m o r p h o l o g y . T h e g r o w t h rates must be reasonable, a n d the deposition m u s t not have significant i m p a c t o n the microstructures already f o r m e d i n the substrate. T h e d e p o s i t i o n t i m e must be sufficiently short, a n d the t e m p e r a t u r e has to b e l o w e n o u g h so that dopant solid-state diffusion does not smear the results of previous processing steps. T h e acceptable l i m i t s o n the properties of the films vary w i t h the a p p l i c a t i o n , b u t stringent d e m a n d s are characteristic i n the processing of m a terials for electronic applications. T h e demands increase w i t h the l e v e l of integration, the decrease i n device size, a n d the c o m p l e x i t y of the device. S e m i c o n d u c t o r films for h i g h - p e r f o r m a n c e d i g i t a l d e v i c e s ( e . g . , A l G a A s - G a A s ) have to be perfectly single crystalline w i t h i m p u r i t i e s i n the l o w p a r t - p e r - b i l l i o n range. F i l m thickness u n i f o r m i t y is generally c r i t i c a l i n m a i n t a i n i n g the same device characteristics (e.g. t h r e s h o l d voltages) across each substrate a n d f r o m substrate to substrate. F u r t h e r m o r e , applications for h e t e r o j u n c t i o n digital a n d o p t i c a l devices r e q u i r e that the interface c o n centration b e t w e e n successive layers of semiconductors changes o v e r a few monolayers or is graded i n a c o n t r o l l e d m a n n e r . T h e p r o d u c t i o n of films w i t h r e p r o d u c i b l e and controllable e l e c t r i c a l , optical, a n d m e c h a n i c a l properties requires p u r e C V D reagents that do not


In Microelectronics Processing; Hess, D., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1989. 3

4

2

Overall Reaction

Si Epitaxy Low-pressure C V D (LPCVD)

2

4

3

4

x

2

4

2

4

2

2

x

2

3

3

3

3

3

6

2

3

4

4

2

2

3

3

3

3

In(CH ) + P ( C H ) ^ I n P

3

3

6

2

2

2

3

4

4

SiHU-xCL + H -> Si + xHCl, χ = 0, 2, 3, or 4 S i H - ^ Si + 2 H SiH + N 0 - > Si0 + N SiH Cl + NH -» Si N WF + H -» W + HF S i H ^ Si + 2 H Very Low Pressure C V D 2HC1 + 2 G a - » 2GaCl + H Vapor-phase Epitaxy 12GaCl + 4AsH + 2As + As (VPE) ±* 12GaAs + 12HC1 Ga(CH ) + A s H —» GaAs + C H Metallorganic C V D Ga(CH ) + A1 (CH ) + A s H (MOCVD) - » Al Gai_ As + C H S i H ^ a:SiH Plasma-enhanced C V D S i H + N H —> Si,N,:H (PECVD) SiH + N 0 - j | SiO, Photon-assisted C V D

System

Table II. C V D Systems

800- -1000 300- -600 400- -700 300- -600 600- -700

101 kPa 50 50 50-500 60-500

800- -1100 800- -900

11- -13 26- -29 26- -29 40 35

9 11- -13

7, ί*, 16 20, 21 20 167 66, 71 22, 23

1050- -1450 850- -950 900- -1000 1000- -1100 500- -700 850- -1150

101 kPa 50 50 50 50 1

101 kPa 101 kPa

Representative Reference

Temperature (K)

Pressure (Pascal)

Deposition


202


p r o d u c e b y p r o d u c t s that incorporate i n t o the g r o w i n g film a n d that do not interact w i t h gas-handling a n d reactor construction materials. T h e substrate has to b e p r o p e r l y c l e a n e d a n d p r e p a r e d for the deposition to a v o i d r e s i d u a l surface i m p u r i t i e s that can create defects i n the g r o w i n g film.


T h e C V D reactor m u s t be d e s i g n e d a n d operated i n such a m a n n e r that changes i n film thickness, crystal structure, surface m o r p h o l o g y , a n d i n t e r face c o m p o s i t i o n c a n b e accurately c o n t r o l l e d . T h e o v e r a l l process p e r f o r m ance d e p e n d s o n the reactor design a n d process variables such as reactant concentrations, flow rates, e n e r g y i n p u t , pressure, a n d substrate conditions. A m o d e l d e s c r i b i n g the relationship b e t w e e n the performance a n d the process variables allows the p r e d i c t i o n o f process results a n d the o p t i m i z a t i o n of process variables for a specific a p p l i c a t i o n . H o w e v e r , the interactions a m o n g d e p o s i t i o n c h e m i s t r y , transport processes, a n d g r o w t h m o d e s are c o m p l e x a n d , c o n s e q u e n t l y , p o o r l y u n d e r s t o o d . T h e r e f o r e , C V D process d e v e l o p m e n t has progressed t h r o u g h extensive one-parameter-at-a-time exp e r i m e n t a t i o n a n d e m p i r i c a l design rules. S e v e r a l C V D processes have e v o l v e d to accommodate the applications of C V D films i n microelectronics processing. T h e various processes are t y p ically c h a r a c t e r i z e d i n terms of the o p e r a t i n g pressure a n d t e m p e r a t u r e , as w e l l as the means of e n e r g y i n p u t . T a b l e I I gives examples of t y p i c a l C V D processes a n d o p e r a t i n g conditions.

CVD

Processes

at

Atmospheric

and

Reduced

Pressures.

A t m o s p h e r i c to slightly r e d u c e d pressures (~ 1 0 0 - 1 0 kPa) are u s e d p r i m a r i l y to grow epitaxial (i.e., single-crystalline) films of S i a n d c o m p o u n d s e m i c o n ductors such as G a A s , I n P , a n d H g C d T e . T h e s e processes generally i n v o l v e h i g h g r o w t h temperatures ( > 8 5 0 °C for S i a n d 4 0 0 - 8 0 0 °C for most c o m p o u n d semiconductors), a l t h o u g h the reactor walls are c o o l e d to m i n i m i z e i m p u r i t y generation. S i C l is the classical reactant for the epitaxial g r o w t h of S i , b u t it has b e e n r e p l a c e d b y S i H C l , S i H C l , a n d S i H to decrease the d e p o s i t i o n t e m p e r a t u r e a n d to m i n i m i z e solid-state difiusion out of the substrate i n t o the g r o w i n g film. I n g e n e r a l , the d e p o s i t i o n t e m p e r a t u r e for epitaxial g r o w t h decreases w i t h the C I content of the reactant from 1150 ° C for S i C l to 850 °C for S i H (7, 8). G o o d single-crystalline films are easier to p r e p a r e w i t h C l - c o n t a i n i n g c o m p o u n d s t h a n w i t h S i H , because the reverse e t c h i n g r e action b y H C 1 p r e f e r e n t i a l l y occurs at defect sites. 4

3

2

2

4

4

4

4

Vapor-phase epitaxy ( V P E ) is a w e l l - d e v e l o p e d t e c h n i q u e for g r o w i n g group I I I - V c o m p o u n d semiconductors, specifically G a A s a n d G a l n A s P , from the c o r r e s p o n d i n g h y d r i d e s a n d halides of the i n d i v i d u a l components (9). T h e d e p o s i t i o n process essentially relies o n the t e m p e r a t u r e d e p e n d e n c e of the e q u i l i b r i u m d i s t r i b u t i o n of the d e s i r e d film m a t e r i a l (e.g., G a A s ) a n d the gas-phase species (e.g., G a C l , A s , a n d A s ) . B y i m p o s i n g a t e m p e r a t u r e gradient o n the reactor, the gas-phase species is f o r m e d i n a hot r e g i o n a n d 2

4


5.

JENSEN

Chemical Vapor

203

Deposition

the e q u i l i b r i u m is shifted towards the d e s i r e d s o l i d (e.g., G a A s ) i n a slightly colder r e g i o n . T h i s process has b e e n u s e d successfully to grow G a A s a n d G a l n A s P . H o w e v e r , the h i g h reactivity o f A l C l a n d t h e r m o d y n a m i c l i m i t a t i o n s o f A l h a l i d e c o m p o u n d s make it difficult to deposit A l - c o n t a i n i n g films b y V P E (I). T h i s l i m i t a t i o n is a serious constraint because m a n y devices use A l G a A s - G a A s structures. A n alternative C V D process, metallorganic C V D ( M O C V D ) , also c a l l e d organometallic vapor-phase epitaxy ( O M V P E ) , has attracted considerable Downloaded by PRINCETON UNIV on November 11, 2014 | http://pubs.acs.org Publication Date: May 5, 1989 | doi: 10.1021/ba-1989-0221.ch005

attention, because i t allows greater flexibility t h a n V P E does i n the synthesis of t h i n , h i g h - p u r i t y , epitaxial films o f c o m p o u n d semiconductors

(10-13).

T h i s t e c h n i q u e entails the transport o f at least one of the film constituents as an organometallic c o m p o u n d . F o r example, A l G a A s can b e

deposited

from organometallic sources o f G a a n d A l (e.g., G a ( C H ) a n d A l ( C H ) , 3

3

2

3

6

respectively) a n d A s H . A l t e r n a t i v e l y , an organometallic source of A s c o u l d 3

b e u s e d (14, 15). T h i s t e c h n i q u e has b e e n u s e d to g r o w n u m e r o u s g r o u p I I - V I a n d group I I I - V c o m p o u n d semiconductors, i n c l u d i n g G a A s , A l G a A s , G a l n A s P , G a S b , I n S b , Z n S e , a n d C d H g T e , for optoelectronic a n d h i g h speed electronic devices (JO, 12). T h e h o r i z o n t a l reactor ( F i g u r e l a ) is a classical configuration for g r o w t h at atmospheric or r e d u c e d pressure. T h i s reactor is n o w p r i m a r i l y u s e d for research a n d for the epitaxial g r o w t h of c o m p o u n d semiconductors, along w i t h the v e r t i c a l reactor ( F i g u r e l b ) . T h e b a r r e l reactor ( F i g u r e l c ) is the p r i m a r y reactor for S i epitaxy (16, 17), a n d small barrels are b e g i n n i n g to be u s e d i n G a A s technology (18, 19). So-called " p a n c a k e " reactors also find use i n S i technology. T h e reactor walls are t y p i c a l l y cooled, except for V P E applications, to m i n i m i z e particulate a n d i m p u r i t y p r o b l e m s caused b y d e p o s i t i o n o n the walls. H o w e v e r , this c o o l i n g also creates large t h e r m a l gradients that i n d u c e c o m p l e x , b u o y a n c y - d r i v e n secondary flows. I n h o r i z o n t a l a n d b a r r e l reactors, the susceptor is t i l t e d relative to the m a i n flow d i r e c t i o n to i m p r o v e film u n i f o r m i t y along the l e n g t h of the susceptor. U n i f o r m i t y is f u r t h e r c o n t r o l l e d i n the b a r r e l reactor b y adjusting the i n l e t gas n o z z l e a n d s p i n n i n g the b a r r e l . I n v e r t i c a l reactors, the susceptor is often rotated to r e d u c e film thickness a n d c o m p o s i t i o n variations, b u t the rotation s p e e d ( 5 - 4 0 r p m ) is generally m u c h l o w e r than that n e e d e d to generate an i d e a l rotating-disk flow (500-2000 r p m ) . Low-Pressure C V D Processes. Low-pressure C V D ( L P C V D ) (~101 Pa) is the m a i n tool for the p r o d u c t i o n of polycrystalline S i d i e l e c t r i c a n d passivation films u s e d i n S i I C (integrated-circuit) manufacture (1, 20, 21). T h e m a i n advantage of L P C V D is the large n u m b e r o f wafers that can be coated simultaneously w i t h o u t d e t r i m e n t a l effects to film u n i f o r m i t y . T h i s capability is a result of the large diffusion coefficient at l o w pressures, w h i c h


204


allows the g r o w t h rate to be l i m i t e d b y the rate of surface reactions rather than b y the rate of mass transfer to the substrate. T y p i c a l l y , reactants can be u s e d w i t h no d i l u t i o n , a n d therefore g r o w t h rates are o n l y an o r d e r of m a g n i t u d e less t h a n those possible at atmospheric conditions, i n w h i c h h i g h d i l u t i o n ratios are u s e d to a v o i d gas-phase n u c l e a t i o n . V e r y l o w pressure processes (—1.3 Pa) have also b e e n u s e d for the g r o w t h


of single-crystalline S i at r e l a t i v e l y l o w temperatures (22, 23). L o w - p r e s s u r e o p e r a t i o n is also advantageous for the g r o w t h o f c o m p o u n d - s e m i c o n d u c t o r superlattices b y r e d u c i n g flow recirculations a n d i m p r o v i n g interface a b ruptness (24). F i g u r e s l e a n d I f illustrate two t y p i c a l L P C V D reactor configurations. T h e s e reactors operate at —50 P a , a n d w a l l temperatures are a p p r o x i m a t e l y

Induction coil

Silicon wafers

^*c^» ooo ooo /ooo ooo

ooo ooo ooo ooo ooo

Horizontal CVD Reactor (a)

Vertical CVD Reactor (b)

j_Gas Inlel^ Radiant heaters -Quartz bell jar ^Exhaust Barrel CVD Reactor

Pancake CVD Reactor

(c)

(d) 3-zone temperature control

Gas injector

- Exhaust

inlet Gas flow Vertical LPCVD Reactor

(β)

Horizontal LPCVD Reactor (f)

Figure 1. Typical CVD reactor configurations, (a) Horizontal reactor, (b) ver tical reactor, (c) barrel reactor, (d) pancake reactor, (e) cross-flow LPCVD reactor, and (J) conventional multiple-wafer-in-tube LPCVD reactor.


5.

JENSEN

Chemical Vapor

205

Deposition

e q u a l to those of t h e deposition surfaces. T h u s , deposition also takes place o n t h e w a l l s , a situation that raises p o t e n t i a l particulate p r o b l e m s . T h e h o r izontal m u l t i p l e - w a f e r - i n - t u b e L P C V D reactor ( F i g u r e l e ) is t h e d o m i n a n t configuration for S i I C manufacture. T h e vertical-flow reactor ( F i g u r e If) gives better u n i f o r m i t y a n d l o w e r particulate counts than t h e h o r i z o n t a l geometry does b u t at t h e expense of l o w reactant utilization (25). Plasma-Enhanced C V D .

P l a s m a - e n h a n c e d C V D ( P E C V D ) has r e -

c e i v e d considerable attention i n microelectronics processing because o f its Downloaded by PRINCETON UNIV on November 11, 2014 | http://pubs.acs.org Publication Date: May 5, 1989 | doi: 10.1021/ba-1989-0221.ch005

ability to grow films at r e l a t i v e l y l o w temperatures a n d to i m p a r t special m a t e r i a l properties that cannot b e r e a l i z e d w i t h c o n v e n t i o n a l t h e r m a l p r o c esses (26-29). Plasmas u s e d i n microelectronics processing are w e a k l y i o n i z e d gases c o m p o s e d o f electrons, ions, a n d n e u t r a l species. T h e c h a r g e d species concentrations range from 1 0 to 1 0 / c m , a n d t h e ratio o f c h a r g e d 9

1 2

3

species to n e u t r a l species ranges from 1 0 " to 1 0 " . 6

4

T h e s e plasmas, also c a l l e d glow discharges, are generated b y a p p l y i n g an external electric field to t h e process gas at l o w pressures ( 0 . 1 - 5 0 Pa). T h e result is a m i x t u r e o f h i g h - e n e r g y , " h o t " electrons ( 1 - 1 0 e V ; 1 0 - 1 0 4

5

K) a n d " c o l d " ions a n d n e u t r a l species (300 K ) . T h i s h i g h electron energy relative to t h e l o w t e m p e r a t u r e o f n e u t r a l species makes discharges useful i n d r i v i n g C V D reactions. Inelastic collisions b e t w e e n t h e h i g h - e n e r g y electrons a n d n e u t r a l m o l ecules result i n , a m o n g other processes, electron-impact i o n i z a t i o n a n d m o l e c u l a r d i s s o c i a t i o n . E l e c t r o n - i m p a c t i o n i z a t i o n helps to sustain t h e discharge, a n d m o l e c u l a r dissociation creates free radicals that c o n t r i b u t e to the d e p o s i t i o n processes. T h e created ions, electrons, a n d n e u t r a l fragments participate i n c o m p l e x surface reactions that form t h e basis of the film g r o w t h . P o s i t i v e - i o n b o m b a r d m e n t of surfaces i n contact w i t h the p l a s m a plays a k e y role b y m o d i f y i n g material properties d u r i n g d e p o s i t i o n . A d i r e c t - c u r r e n t (dc) bias p o t e n t i a l may b e a p p l i e d to t h e excitation electrode to increase t h e i o n e n e r g y a n d enhance t h e d e s i r e d effects o f i o n b o m b a r d m e n t (30). F o r radiation-sensitive substrates such as c o m p o u n d

semiconductors,

afterglow d e p o s i t i o n systems have b e e n d e v e l o p e d (31). I n these processes, the radicals are f o r m e d i n t h e glow discharge a n d t h e n transported o u t o f the discharge r e g i o n to a d o w n s t r e a m d e p o s i t i o n zone. T h i s p l a s m a configuration eliminates i o n b o m b a r d m e n t a n d allows t h e selective activation o f reactants b y r e g u l a t i n g the species that flow t h r o u g h or bypass t h e discharge. A d d i t i o n a l g r o w t h considerations, as w e l l as process m o d e l i n g a n d plasma diagnostics u n d e r l y i n g plasma-enhanced C V D are f u r t h e r discussed b y H e s s a n d G r a v e s i n C h a p t e r 8, w h i c h is specifically d e v o t e d to p l a s m a processing. Photoassisted C V D .

I n a d d i t i o n to t h e r m a l energy a n d e l e c t r o n -

i m p a c t reactions, photons (e.g., U V light) can also d r i v e C V D reactions In Microelectronics Processing; Hess, D., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1989.

206



(32-37). T h i s process has t h e advantage o f l o w - t e m p e r a t u r e d e p o s i t i o n , w h i c h m a y b e n e e d e d for t h e g r o w t h o f t h i n films o n t e m p e r a t u r e - s e n s i t i v e substrates s u c h as c o m p o u n d semiconductors a n d p o l y m e r s . I f a laser is u s e d as t h e light source, fine lines o f materials c a n b e w r i t t e n d i r e c t l y , a n d possibly, l i t h o g r a p h i c steps c a n b e a v o i d e d a n d damaged lines can b e r e p a i r e d . Photons c a n p r o m o t e C V D reactions b y different routes (34). O n e or m o r e photons m a y l e a d to t h e gas-phase photolysis o f a reactant a n d t h e formation o f reactive fragments. T h e l i g h t c a n also b e absorbed b y adsorbed surface species that t e n d to u n d e r g o reactions l e a d i n g to t h i n - f i l m formation. A l t e r n a t i v e l y , t h e photons c a n alter t h e electronic states o f t h e substrate surface a n d t h e r e b y p r o m o t e film reactions. F i n a l l y , t h e light can b e transf o r m e d into heat i n t h e t o p surface l a y e r a n d t h e r m a l l y d r i v e t h e d e p o s i t i o n process. T h i s c o n v e r s i o n to t h e r m a l energy is essentially e q u i v a l e n t to t h e r m a l C V D , b u t i f a laser is u s e d , t h e process has t h e advantages o f increased e n e r g y flux, r a p i d h e a t i n g , a n d a spatially w e l l - d e f i n e d d e p o s i t i o n area. Because o f t h e i r p o t e n t i a l d i r e c t - l i n e - w r i t i n g applications, p h o t o l y t i c - a n d pyrolytic-laser-assisted C V D processes are areas o f active research (30-39), a n d most reactor systems are s m a l l special-purpose laboratory reactors. Photosensitization is u s e d for large-area p h o t o c h e m i c a l l y s t i m u l a t e d C V D , because t h e generation o f a sufficient p h o t o n flux o v e r a large area to d r i v e t h e c h e m i s t r y d i r e c t l y is difficult. U s u a l l y , H g excited b y a n external H g l a m p is u s e d as a sensitizer. T h e energy i n t h e excited H g is t h e n transferred to other gas-phase species that decompose a n d react to f o r m a t h i n film. T h e process is u s e d i n h o r i z o n t a l reactors for t h e d e p o s i t i o n o f SiO a n d SiN H from S i H , N 0 , a n d N H (40-42) a n d to assist t h e d e p osition o f C d H g T e , i n w h i c h H g is a natural gas-phase constituent (43). x

y

z

4

2

3

Other C V D Processes. C V D also finds extensive use i n t h e p r o d u c t i o n of p r o t e c t i v e coatings (44,45) a n d i n t h e manufacture of o p t i c a l fibers (46-48). W h e r e a s t h e i m p o r t a n t q u e s t i o n i n t h e d e p o s i t i o n o f p r o t e c t i v e coatings is analogous to that i n microelectronics (i.e., t h e deposition o f a coherent, u n i f o r m film), t h e fabrication o f optical fibers b y C V D is f u n d a m e n t a l l y different. T h i s process involves gas-phase n u c l e a t i o n a n d transport of t h e aerosol particles to t h e fiber surface b y thermophoresis (49, 50). H e a t i n g t h e d e p o s i t e d p a r t i c l e layer consolidates i t i n t o t h e fiber structure. O f t e n , a t h e r m a l p l a s m a is u s e d to enhance t h e t h e r m o p h o r e t i c transport o f the particles to t h e fiber walls (48, 51). T h e gas-phase n u c l e a t i o n is d e t r i m e n t a l to other C V D processes i n w h i c h t h i n , u n i f o r m s o l i d films are d e s i r e d .

CVD Fundamentals C h e m i c a l vapor d e p o s i t i o n o f t h i n films involves gas-phase a n d surface r e actions c o m b i n e d w i t h transport processes. F i g u r e 2 gives a schematic r e p -


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207

Deposition

Main Gas Flow Region

0

Gas Phase Reactions

?

I

τRedesorption of

Transport to Surface

| f


Desorption of Volatile Surface Reaction Products

F i , m

Precursor

Surface Diffusion

Adsorption of Film Precursor

Nucleation and Island Growth

S t e

P

G

Figure 2. Schematic of transport and reaction processes underlying

r

o

w

t

n

CVD.

resentation of the different e l e m e n t s of the g r o w t h process, w h i c h can b e 1

s u m m a r i z e d i n the f o l l o w i n g steps: 1. mass transport i n the b u l k gas flow r e g i o n from the reactor inlet to the d e p o s i t i o n zone, 2. gas-phase reactions l e a d i n g to the formation of f i l m precursors and b y p r o d u c t s , 3. mass transport of f i l m precursors to the g r o w t h surface, 4. adsorption of film precursors o n the g r o w t h surface, 5. surface diffusion of film precursors to the g r o w t h sites, 6. i n c o r p o r a t i o n of film constituents i n t o the g r o w i n g film (is land), 7. desorption o f b y p r o d u c t s o f the surface reactions, a n d 8. mass transport of b y p r o d u c t s i n the b u l k gas flow r e g i o n away f r o m the d e p o s i t i o n zone towards the reactor exit. S i m i l a r reaction sequences have b e e n i d e n t i f i e d i n other c h e m i c a l l y reacting systems, specifically catalytic c o m b u s t i o n (52, 53), solid-fuel c o m b u s t i o n (54), transport a n d reaction i n h i g h - t e m p e r a t u r e incandescent lamps (55), a n d heterogeneous catalysis (56 a n d references w i t h i n ) . T h e e l e m e n t a r y reactions i n h y d r o c a r b o n c o m b u s t i o n are better u n d e r s t o o d than most C V D gas-phase reactions are. S i m i l a r l y , the surface reaction mechanisms u n d e r l y i n g h y d r o c a r b o n catalysis are better k n o w n than C V D surface reactions. T h e d e p o s i t i o n of S i b y the r e d u c t i o n of S i H is a c o n v e n i e n t example of a C V D process that clearly displays the reaction steps j u s t l i s t e d . S i H 4

4


208

MICROELECTRONICS PROCESSING: C H E M I C A L E N G I N E E R I N G ASPECTS

diluted in H

2

is the starting m a t e r i a l . A s the S i H

4

is transported into the

hot gas phase adjacent to the substrate, it pyrolyzes to f o r m S i H

2

and H

2

according to the f o l l o w i n g reaction: SiH

4

±5 S i H

+ H

2

(1)

2

T h e silylene m o l e c u l e , S i H , is v e r y reactive a n d r a p i d l y inserts itself into 2

H , silane, a n d h i g h e r silanes w i t h almost no activation energy (57-59), as 2


s h o w n b y the f o l l o w i n g reactions: SiH

4

+ SiH

2

±¥ S i H

6

(2)

Si H

6

+ SiH

2

±> S i H

8

(3)

2

2

3

T h e h i g h e r silanes m a y lose h y d r o g e n b y a reaction s i m i l a r to that g i v e n i n e q u a t i o n 1: Si H 2

6

±5 S i H 2

4

+ H

(4)

2

A l l the s i l i c o n h y d r i d e s are adsorbed o n the S i surface. T h e unsaturated species a n d h i g h e r silanes are adsorbed m o r e r e a d i l y than silane (60). T h e adsorbed silicon species difiuse o n the surface to g r o w t h sites, w h e r e the S i is i n c o r p o r a t e d i n the g r o w i n g film a n d the b y p r o d u c t , h y d r o g e n , is released and e v e n t u a l l y desorbs as h y d r o g e n molecules. T h e relative rates of surface diffusion, n u c l e a t i o n , a n d adsorption g o v e r n the crystalline m o r p h o l o g y of the g r o w i n g film. T h i s a n d other f u n d a m e n t a l issues are discussed i n subsequent sections, along w i t h f u r t h e r information o n specific C V D systems, i n c l u d i n g systems for S i a n d G a A s deposition.

Nucleation and Growth Modes. T h e three p r i m a r y g r o w t h modes for t h i n films are i l l u s t r a t e d i n F i g u r e 3 (61). I n t h r e e - d i m e n s i o n a l i s l a n d g r o w t h , r e f e r r e d to as V o l m e r - W e b e r g r o w t h , small clusters are n u c l e a t e d d i r e c t l y o n the substrate surface. T h e clusters g r o w into islands of the film m a t e r i a l that e v e n t u a l l y coalesce to f o r m a continuous film ( F i g u r e 3a). T h i s g r o w t h m o d e takes place w h e n the film atoms are m o r e strongly b o u n d to each other than to the substrate. T h i s g r o w t h m o d e applies to silicon g r o w t h o n insulators (e.g., S i o n S i 0 , S i N , or A l 0 ) (8, 62-64) a n d is also a c o m m o n g r o w t h m o d e for metals o n insulators. A l u m i n u m C V D is an ext r e m e example of a C V D process i n w h i c h a catalyst such as T i C l is n e e d e d to nucleate the clusters (65, 66). 2

3

4

2

3

4

T w o - d i m e n s i o n a l l a y e r - b y - l a y e r g r o w t h ( F i g u r e 3c), also c a l l e d F r a n c k - v a n d e r M e r w e g r o w t h , occurs w h e n the film atoms are e q u a l l y or less strongly b o n d e d to each other than to the substrate. T h i s g r o w t h m o d e applies to homoepitaxy o n clean substrates (e.g., S i o n Si). T h e p r e s e n c e of


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Deposition

Surface Coverage

θ

(191). I n this m a n n e r , t h e e x p e r i m e n t a l data can b e c o r r e l a t e d , b u t t h e m o d e l has l i m i t e d capability for p r e d i c t i n g behavior b e y o n d t h e p a r t i c u l a r set o f e x p e r i m e n t s u s e d to fit the m o d e l . I n fact, because o f t h e l o w values o f t h e R e y n o l d s n u m b e r ( H g *

(41)

T h e energy is subsequently transferred from H g * to other gas-phase species b y collisions. M o d e l i n g studies of this process are few, b u t the concepts of p h o t o c h e m i c a l reaction e n g i n e e r i n g (236,237) can be adapted to this system.

Conclusion C h e m i c a l vapor d e p o s i t i o n is a k e y process for the g r o w t h of electronic materials for a large variety of devices essential to m o d e r n technology. Its flexibility a n d r e l a t i v e l y l o w deposition temperatures make C V D attractive for future device applications i n S i and c o m p o u n d - s e m i c o n d u c t o r t e c h n o l ogies. T h e process involves gas-phase a n d surface reactions that m u s t be c o n t r o l l e d to achieve d e s i r e d m a t e r i a l a n d electronic properties. E x c e p t for silane c h e m i s t r y , C V D c h e m i c a l mechanisms a n d kinetics are p o o r l y characterized. C u r r e n t l y , the interest i n u n d e r s t a n d i n g C V D c h e m i s t r y is g r o w i n g , a n d the results w i l l be essential to the future d e v e l o p m e n t of the process. C V D reactors i n v o l v e transport p h e n o m e n a that are analogous to those f o u n d i n o t h e r c h e m i c a l l y reacting systems, specifically heterogeneous catalytic reactors a n d c o m b u s t i o n systems. L P C V D reactor m o d e l i n g involves m a n y of the same issues of m u l t i c o m p o n e n t difiusion reactions that have b e e n s t u d i e d i n the past decade i n connection w i t h heterogeneous catalysis. C o m p l e x fluid-flow p h e n o m e n a strongly affect the performance of atmospheric-pressure C V D reactors. T w o d i m e n s i o n a l a n d some t h r e e - d i m e n s i o n a l flow structures i n the classical h o r izontal a n d v e r t i c a l C V D reactors have b e e n e x p l o r e d t h r o u g h flow v i s u a l -


5.

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Deposition

255

ization a n d d e t a i l e d transport computations, a n d an increased u n d e r s t a n d i n g of the role o f gas expansions, b u o y a n c y effects, a n d reactor enclosure design in the o v e r a l l transport p h e n o m e n a is e m e r g i n g . H o w e v e r , fully t h r e e - d i m e n s i o n a l structures r e m a i n to be c o m p l e t e l y u n d e r s t o o d . T h e applications of n o n c o n v e n t i o n a l C V D processes, such as laser a n d p l a s m a processing, are l i k e l y to expand, a n d these applications r e q u i r e an increased f u n d a m e n t a l u n d e r s t a n d i n g of the u n d e r l y i n g c h e m i s t r y a n d trans port processes.


Abbreviations and Symbols «AC

lattice constant of c o m p o u n d A C

c

concentration b u l k concentration i n p u t concentration heat capacity

Co c D P

molecular difusivity b i n a r y m o l e c u l a r diffusivity m u l t i c o m p o n e n t diffusivity of trace i i n m i x t u r e m

£>,„,

refers to gaseous state w h e n u s e d as a superscript

g H,

m o l a r e n t h a l p y of c o m p o n e n t i m o d i f i e d B e s s e l function of the first k i n d , of o r d e r zero

h

m o d i f i e d B e s s e l f u n c t i o n of the second k i n d , of o r d e r one flux of c o m p o u n d i relative to mass average v e l o c i t y thermal conductivity surface reaction rate constant

J, k K k, fcj, fc k

2

x

L

n

o u t w a r d n o r m a l to the deposition surface

n

g

n u m b e r of gas-phase reactions

s

n u m b e r of surface reactions

n

n,

film

Ni M 0 Ρ ΡΑ, P M

Pi Peq r r

rate constants rate constant at high-pressure l i m i t l e n g t h of d e p o s i t i o n zone i n L P C V D reactor

*

n u m b e r of film atoms i n species i flux of c o m p o n e n t i average m o l e c u l a r w e i g h t refers to the standard state w h e n u s e d as a superscript pressure partial pressures of A a n d M , respectively partial pressure o f c o m p o n e n t i e q u i l i b r i u m pressure radial coordinate critical n u c l e a t i o n radius

R

gas constant

R/

j t h gas-phase reaction


256

ft/

s

j t h surface reaction L P C V D tube radius S i wafer radius refers to the surface w h e n used as a superscript

S

n u m b e r of species

Γ

temperature

t

time l i n e a r velocity

H,

fi

w

υ V Downloaded by PRINCETON UNIV on November 11, 2014 | http://pubs.acs.org Publication Date: May 5, 1989 | doi: 10.1021/ba-1989-0221.ch005


average l i n e a r v e l o c i t y atomic v o l u m e m o l a r v o l u m e of film m o l e fraction c o m p o n e n t i d i m e n s i o n a l concentration

y ζ

axial coordinate

Zfc α

axial position of fcth wafer

α?

t h e r m a l difiusivity ratio

Ί

surface tension activity coefficient o f c o m p o n e n t i

7, δ δ Δ

0

ζ η μ ν, ξ Ρ

Φ Ω

boat area relative to t u b e area

film thickness film thickness at l e a d i n g edge of substrate wafer spacing dimensionless r a d i a l p o s i t i o n effectiveness factor viscosity c h e m i c a l p o t e n t i a l of c o m p o n e n t i stoichiometric coefficient stoichiometric coefficient for surface reactions dimensionless radial position density Thiele modulus interaction p a r a m e t e r (see e q u a t i o n 11)

Acknowledgments I am grateful for support f r o m the N a t i o n a l Science F o u n d a t i o n , the C a m i l l e a n d H e n r y D r e y f u s F o u n d a t i o n , the G u g g e n h e i m F o u n d a t i o n , a n d the M i n nesota S u p e r c o m p u t e r Institute. I a m also grateful to the f o l l o w i n g colleagues w h o have c o n t r i b u t e d to this p a p e r t h r o u g h research a n d discussion w i t h m e : L . D a - C h e n g , E . O . E i n s e t , D . 1. F o t i a d i s , K . G i a p i s , S. K i e d a , D . W . K i s k e r , T. F . K u e c h , D . R . M c K e n n a , B . S. M e y e r s o n , H . K . Moffat, T. J . M o u n t z i a r i s , P. E . P r i c e , J r . , K . F. R o e n i g k , D . S k o u b y , a n d W . R i c h t e r .


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RECEIVED for review December 30, 1987. ACCEPTED revised manuscript November 16, 1988.


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