Physical Vapor Deposition Reactors - Advances in Chemistry (ACS

Downloaded by UNIV OF CALIFORNIA SAN FRANCISCO on February 17, 2015 | http://pubs.acs.org Publication Date: May 5, 1989 | doi: 10.1021/ba-1989-0221.ch004

4 Physical Vapor Deposition Reactors T. W. Fraser Russell, Bill N. Baron, Scott C. Jackson, and Richard E. Rocheleau 1

2

Institute of Energy Conversion and the Department of Chemical Engineering, University of Delaware, Newark, D E 19716

Physical vapor deposition (PVD) is used for the deposition of semi -conductor,insulator, and metal layers in the fabrication of a variety of electronic devices. Reactors for PVD are characterized by direct line-of-sight transport of molecular species from the gas phase to the desired substrate, where they react to form solid films with the desired properties. A reactor-and-reaction analysis of PVD quantitatively examines the generation of gas-phase species, spatial distribution of species arriving on the substrate, and surface reactions leading to film growth.

T H E S E M I C O N D U C T O R , I N S U L A T O R , o r conductor layers i n microscale o r larger scale electronic devices such as a photovoltaic c e l l are created i n a reactor. T h e reactor needs to b e d e s i g n e d a n d operated to p r o d u c e materials that have the d e s i r e d optical a n d electronic properties. T h e design of reactors is a n o n t r i v i a l research a n d design p r o b l e m . I n this chapter, some o f the theoretical a n d e x p e r i m e n t a l framework for this research a n d for more-ef fective designs o f physical-vapor-deposition-type reactors w i l l b e d e v e l o p e d . A w i d e v a r i e t y of names are used i n t h e electronic i n d u s t r y for various types o f reactors, b u t two b r o a d classifications of reactors based o n the means b y w h i c h t h e m o l e c u l a r species are d e l i v e r e d to the substrate are useful: direct line-of-sight i m p i n g e m e n t a n d d i f i u s i v e - c o n v e c t i v e mass transfer. T h e t e r m " c h e m i c a l vapor d e p o s i t i o n " ( C V D ) has b e e n u s e d generally to describe Current addresses: 'Engineering Department, Ε. I. du Pont de Nemours and Company, Wilmington, D E 19898 Hawaii Natural Energy Institute, University of Hawaii, Honolulu, HI 96822 2

0065-2393/89/0221-0171$08.00/0 © 1989 American Chemical Society

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

172

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

the diffusive-conveetive mass transfer. T h e critical issues i n C V D are the following: • c h e m i c a l reaction i n fluid phase • transport to substrate ( m o m e n t u m , mass transfer, a n d heat transfer) Downloaded by UNIV OF CALIFORNIA SAN FRANCISCO on February 17, 2015 | http://pubs.acs.org Publication Date: May 5, 1989 | doi: 10.1021/ba-1989-0221.ch004

• film g r o w t h T h e t e r m " p h y s i c a l vapor d e p o s i t i o n " ( P V D ) has b e e n u s e d for line-of-sight transport. T h e c r i t i c a l issues i n P V D are the following: • film g r o w t h • spatial d i s t r i b u t i o n o n substrate • m o l e c u l a r b e a m f r o m source G r o w t h o f t h e film is a p r i m a r y c o n c e r n for b o t h reactor types, b u t the transport p h e n o m e n a i n a C V D reactor are m o r e difficult to analyze. K n o w l edge o f t h e fluid mechanics a n d heat a n d mass transfers, often for a v e r y c o m p l e x g e o m e t r y , is r e q u i r e d . I n a line-of-sight P V D reactor, the transport of m o l e c u l a r species to the substrate can b e a n a l y z e d m o r e easily. A P V D - t y p e reactor can b e one i n w h i c h molecules reach the surface d i r e c t l y i n a m o l e c u l a r b e a m from some source or sources i n w h i c h raw materials are v a p o r i z e d . A t the pressures c o m m o n l y used (0

c h a r g e at

Figure 2. Typical source bottle for PVD-type reactor. (Reproduced with permission from reference 5. Copyright 1982 American Institute of Chemical Engineers.) F o r the evaporation o f C d S p o w d e r , dp/dt is constant at about 0.17 X 10~ g / c m - s (5). N e g l e c t i n g the d e n s i t y change i n this example w o u l d i n t r o d u c e an e r r o r o f o n l y about 1 0 % i n the calculated effusion rates. 3

3

T h e v o l u m e t r i c flow rate f r o m the source depends u p o n the pressure difference across the orifice, P - P T h e vapor pressure i n the c h a m b e r c o n t a i n i n g the r a w material, P , is calculated f r o m the charge t e m p e r a t u r e . T h e charge t e m p e r a t u r e , T is d e t e r m i n e d from an e n e r g y balance that considers the heat transfer from the w a l l o f the source to the charge a n d the loss d u e to the heat of vaporization o f the source material. I f the heat transfer is b y radiation, as is the case for a s o l i d source, a n d i f the charge t e m p e r a t u r e T is u n i f o r m , t h e n the energy balance takes the f o l l o w i n g f o r m : c

v

c

h

x

VCJTJdt

=

-p AH g

R

+ F F o(T v

e

2

4

-

DTMs


(3)

176


i n w h i c h C is the heat capacity of the source material, àH is the latent heat o f v a p o r i z a t i o n of the source m a t e r i a l , F is the v i e w factor, F is the effective e m i s s i v i t y , σ is the S t e f a n - B o l t z m a n n constant, T is the t e m p e r ature o f the charge, T is the t e m p e r a t u r e o f the bottle w a l l , a n d A is the surface area o f the source m a t e r i a l charge. p

R

v

e

l

2

s

U s u a l l y , the w a l l t e m p e r a t u r e , T , can b e m e a s u r e d a n d u s e d to c o n t r o l


2

p o w e r to the source. A m o r e c o m p l e t e treatment that i n c l u d e s the energy balance for the source bottle was g i v e n b y R o c h e l e a u (6). E q u a t i o n s 1 a n d 3 are c o u p l e d t h r o u g h the mass flow, p q, w h i c h d e p e n d s i n a c o m p l i c a t e d m a n n e r o n the charge t e m p e r a t u r e a n d the v a p o r pressure o f the source m a t e r i a l . T h e flow t h r o u g h the exit orifice can range from free m o l e c u l a r flow to viscous flow. I f the K n u d s e n n u m b e r (ratio o f the m e a n free p a t h , X , to the orifice d i a m e t e r , D) is greater t h a n 1 (i.e., XJD > 1), t h e n the flow is free m o l e c u l a r . I f XJD < 0.01, the flow is viscous. A t r a n s i t i o n r e g i o n exists b e t w e e n these l i m i t s . g

m

T h e m e a n free p a t h X is r e l a t e d to the pressure b y s i m p l e k i n e t i c m o l e c u l a r t h e o r y a c c o r d i n g to e q u a t i o n 4 m

Κ in which p

m

(4)

= (itRT /2Mj ^/ /

1

Pm

is the pressure at the p o i n t o f interest (usually taken as the

average o f the pressures u p s t r e a m a n d d o w n s t r e a m o f the restriction), R is the i d e a l gas constant, M

is the m o l e c u l a r w e i g h t of the m a t e r i a l

w

from the source, a n d μ is the gas viscosity, μ is evaluated at p

m

flowing

and T . x

F o r a c o m p o u n d A B that evaporates c o n g r u e n t l y a c c o r d i n g to the f o l l o w i n g reaction (AB), ±* A

g

+ l/n(B ) n

(5)

g

or for the evaporation of an alloy, the properties u s e d to d e t e r m i n e the gasphase properties are those of the m i x t u r e . F o r the b i n a r y dissociative e v a p oration of c o m p o u n d A B , the average m o l e c u l a r w e i g h t is g i v e n b y M

w

= [nM

A B

/(M / ) + (nM B

/ 2

A

1 / 2

)]

(6)

2

i n w h i c h η is the s t o i c h i o m e t r i c coefficient. T h e e q u i l i b r i u m vapor pressure is g i v e n b y

(7) P

E

= Κρ < >[(1 + η/

η+1

n)n"

n / ( 1 + n )

]


(8)

4.

RUSSELL et al.

in w h i c h K

Physical Vapor Deposition

177

Reactors

is the e q u i l i b r i u m constant for dissociative evaporation a n d P

p

is the e q u i l i b r i u m vapor pressure. K

is a function o n l y of t e m p e r a t u r e

p

e

Τ

γ

and can be found i n appropriate handbooks or the literature. E q u a t i o n s 1 a n d 3 are solved at the same t i m e b y u s i n g an appropriate n u m e r i c a l a l g o r i t h m for simultaneous first-order differential equations. C a l culation of the t e r m p q g

m a y r e q u i r e an iteration w i t h i n the integration


r o u t i n e , d e p e n d i n g o n the n u m b e r o f flow restrictions i n series a n d the flow r e g i m e that is e n c o u n t e r e d . F o r two restrictions i n series, p q depends u p o n g

the pressures P , P c

i 5

and P . 0

W h e n the area of the charge is sufficiently large relative to the area of the orifice (10 times or greater), the c h a m b e r pressure, P , can be set e q u a l c

to the e q u i l i b r i u m pressure, P ,

at T

e

is the pressure i n the v a c u u m

P

v

0

c h a m b e r a n d is usually orders of m a g n i t u d e b e l o w P . T h e r e f o r e , little e r r o r c

is made i f P

= 0 is assumed. A n iteration for systems w i t h an orifice b u t

Q

no nozzle proceeds as follows: 1. k

m

is c o m p u t e d b y u s i n g Ap l

e

for p

m

(equation 4).

2. T h e K n u d s e n n u m b e r , X / D , a n d the aspect ratio, L / Γ , are m

o b t a i n e d b y u s i n g the m e a n free path ( X ) , orifice radius, Γ, m

and

orifice l e n g t h , L . LIT is fixed for a g i v e n geometry, b u t is a function of pressure.

X /D m

3. W h e n X / D is greater than 1, free m o l e c u l a r flow exists, a n d m

a factor, K , is u s e d to m u l t i p l y the e q u a t i o n for an orifice for free m o l e c u l a r flow (Table I). Κ aids i n d e a l i n g w i t h the t r a n sitions b e t w e e n orifice a n d p i p e flows (5). F o r an i d e a l orifice, Κ =

1, a n d for a l o n g p i p e , Κ =

π/2[(2// -

i n t e r m e d i a t e situations i n w h i c h LIT >

ÏJT/L. F o r

1.5

Κ = (1 + 0 . 4 L / r ) / [ l + 0 . 9 5 L / r + 0 . 1 5 ( L / r ) ]

(9)

2

Table I. Constitutive F l o w Equations Flow Regime

Pipe

Orifice

Free molecular (KID

> 1)

pq %

=

T (TTMJ2RT) 2

x

1/2

P s i ?

(Pi ~ ρύ

L

Viscous (KID

-16^LRT

< 0.01)

pq g

=

ν

/

X [2p ( g

0

Pl

-

p )] 2

M

~ 16μΧΖΐΓ

1/2

NOTE: The variables are defined in the text and at the end of the chapter.


τ] A

J

Pi)

TTT C 2

vi)

( P l

(Ρι

178


4. F o r X / D b e t w e e n 1 a n d 0.01, a fairly c o m m o n c o n d i t i o n , m

the e q u a t i o n for free m o l e c u l a r orifice flow is m o d i f i e d b y m u l t i p l y i n g w i t h a constant C . T h e total pressure d r o p t h r o u g h the orifice is o b t a i n e d b y u s i n g the f o l l o w i n g equations

Physical Vapor Deposition Reactors - Advances in Chemistry (ACS

Recommend Documents