Colloids and Surfaces in Reprographic Technology - American

7

Monolayer

Particle

Arrays

Formed

by

Vapor

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Deposition

1

DAVID ROBERTSON and ARNOLD L. PUNDSACK Xerox Research Centre of Canada, Mississauga, Ontario, Canada L5L1J9

The process by which vapor-deposited m a t e r i a l forms two-dimensional arrays of s p h e r i c a l p a r t i c l e s j u s t beneath the s u b s t r a t e surface has been s t u d i e d i n detail. A model has been proposed i n which the numbers and s i z e s o f particles are determined by the coupled processes of particle growth (by capture o f diffusing molecules) together with particle coalescence. Expressions have been derived f o r particle s i z e and number d e n s i t y as functions of d e p o s i t i o n parameters. Experimental evidence i s presented i n support of the model f o r the case of selenium p h y s i c a l l y vapor-deposited onto a heated thermoplastic s u b s t r a t e . F i n a l l y , the t e c h n o l o g i c a l a p p l i c a t i o n of the deposit morphology as dry m i c r o f i l m i s reviewed.

In conventional p h y s i c a l vapor d e p o s i t i o n , the m a t e r i a l to be deposited i s emitted i n atomic or molecular form from a heated source and i s allowed to impinge on a s o l i d s u b s t r a t e . The process i s c a r r i e d out i n a vacuum chamber i n order to avoid contamination of the deposit and a l s o to avoid the p o s s i b i l i t y that the emitted vapor may condense i n the gas phase. The r a t e of evaporation from the source i s c o n t r o l l e d v i a the source temperature which i s normally much higher than the s u b s t r a t e temperature. Atoms or molecules impinging on the s u b s t r a t e may be more or less e l a s t i c a l l y r e f l e c t e d back i n t o the gas phase o r , more commonly, they s t i c k to the s u b s t r a t e where they are h e l d by a t t r a c t i v e forces. I f the energy o f a t t r a c t i o n i s very high r e l a t i v e to the thermal energy of the s u b s t r a t e , the deposited molecule i s e f f e c t i v e l y bonded or chemisorbed to the s u b s t r a t e . 1

Current address: St. Regis Technical Center, West Nyack, NY 10994. 0097-6156/82/0200-0123$06.00/0 © 1982 American Chemical Society Hair and Croucher; Colloids and Surfaces in Reprographic Technology ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

124

REPROGRAPHIC TECHNOLOGY

For l e s s s t r o n g b i n d i n g , we have a s t a t e of p h y s i c a l adsorption i n which the adsorbed species i s mobile — able t o migrate p a r a l l e l to the surface by means of t h e r m a l l y a c t i v a t e d hops. Adatoms (or adsorbed molecules as the case may be) may a l s o r e c e i v e enough thermal energy from the s u b s t r a t e to desorb and r e t u r n to the vapor phase. The p r o b a b i l i t y of d e s o r p t i o n per u n i t time depends on the temperature and b i n d i n g energy to the substrate through an Arrhenius-type r a t e expression kT

a = v exp (-AGdes/ )

(1)

where V i s a frequency the same order of magnitude as the s u b s t r a t e ' s Debye frequency, that i s , t y p i c a l l y 10 to 10 Hz, k i s Boltzmann's constant and AG^es *a c t i v a t i o n free energy f o r desorption which, f o r systems of i n t e r e s t to us, takes values the order of 0.1 to 0.3 eV. While adatoms are m i g r a t i n g over the s u b s t r a t e and o c c a s i o n a l l y r e - e v a p o r a t i n g , they continue to impinge at a r a t e B. For high s u b s t r a t e temperatures, when d e s o r p t i o n i s frequent, or f o r low r a t e s of impingement, i t i s p o s s i b l e that an e q u i l i b r i u m adpopulation n w i l l be e s t a b l i s h e d so that the r a t e of impingement 3 i s balanced by the r a t e of d e s o r p t i o n Otn. This e q u i l i b r i u m w i l l be s t a b l e provided that the adpopulation 3/Ct i s s u f f i c i e n t l y small so that encounters between m i g r a t i n g adatoms are infrequent compared to the r a t e of d i s s o c i a t i o n of the c l u s t e r s which form when adatoms meet and bind w i t h each other. Under those c o n d i t i o n s of high temperature and low r a t e of impingement, no macroscopic deposit w i l l be formed. On the other hand, i f the adpopulation becomes large enough, c l u s t e r formation and growth i s favored. Once a c l u s t e r grows i n excess of a c r i t i c a l s i z e (depending on temperature and d e p o s i t i o n r a t e ) , the p r o b a b i l i t y of capture of an adatom exceeds the p r o b a b i l i t y of decay and the c l u s t e r then grows spontaneously. E v e n t u a l l y , growing c l u s t e r s merge to form a continuous f i l m . It i s c l e a r that whether we deposit a f i l m or not depends on the impingement r a t e and s u b s t r a t e temperature f o r a given combination of m a t e r i a l s . We can, i n f a c t , map out the c o n d i t i o n s f o r f i l m d e p o s i t i o n , u s u a l l y on a p l o t of l o g (impingement r a t e ) versus r e c i p r o c a l temperature as shown i n Figure 1. At high temperatures and low rates of impingement, there i s no deposit aside from the e q u i l i b r i u m adpopulation while f o r low temperatures (when b i n d i n g to the s u b s t r a t e i s i n f l u e n t i a l ) and r a p i d d e p o s i t i o n r a t e s , we observe f i l m formation. This i s a h i g h l y s i m p l i f i e d p i c t u r e of f i l m d e p o s i t i o n i n the conventional sense. In the present work, we w i l l r e l a x the unstated assumption that the s u b s t r a t e i s impermeable to s

t n e

Hair and Croucher; Colloids and Surfaces in Reprographic Technology ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

7.

ROBERTSON AND PUNDSACK

Vapor

Deposition

RECIPROCAL TEMPERATURE Figure 1. Schematic plot of impingement rate vs. reciprocal absolute temperature showing conditions for film growth in conventional physical vapor deposition.


125


126

the deposited m a t e r i a l and observe c o n t r a s t s and s i m i l a r i t i e s to c o n v e n t i o n a l vapor d e p o s i t i o n . F i r s t , however, i t i s appropriate to present some background on the discovery o f subsurface p a r t i c u l a t e d e p o s i t s . Normally, s u b s t r a t e s are impermeable and deposits are h e l d out on the s u r f a c e . However, some years ago, workers Q ) at Xerox observed, while d e p o s i t i n g selenium onto f l e x i b l e p l a s t i c s u b s t r a t e s , that "good", continuous f i l m s were grown except under c e r t a i n circumstances when the deposit took the form of p a r t i c l e s . Examination of these deposits revealed that the p a r t i c l e s were s p h e r i c a l , f a i r l y uniform i n s i z e and, most remarkable, were organized i n t o a monolayer array j u s t beneath the surface of the s u b s t r a t e . F i g u r e 2 shows a top view of a t y p i c a l deposit as observed using the t r a n s m i s s i o n e l e c t r o n microscope. There are u s u a l l y the order of 10 p a r t i c l e s per cm of d e p o s i t . These measure 100 to 200 nm i n diameter and r e s i d e at a depth of a few tens of nm. The s i z e s and numbers of p a r t i c l e s depend on d e p o s i t i o n c o n d i t i o n s as w i l l be shown i n the next section. Figure 3 i l l u s t r a t e s the r e s t r i c t e d range of c o n d i t i o n s i n which p a r t i c u l a t e deposits were observed f o r a p a r t i c u l a r thermoplastic s u b s t r a t e . As i n Figure 1, we observe no deposit f o r s u f f i c i e n t l y low rates of impingement and/or high s u b s t r a t e temperatures. Under those c o n d i t i o n s , frequent d e s o r p t i o n insures that s u p e r c r i t i c a l aggregates do not form. The " a c t i v a t i o n energy" a s s o c i a t e d with the slope of that boundary corresponds to the d e s o r p t i o n process. For r a p i d impingement and/or low temperatures, macroscopic deposits form but there i s a t r a n s i t i o n between continuous f i l m s and subsurface p a r t i c l e s . The slope of the l i n e forming the p a r t i c l e / f i l m boundary corresponds to the a c t i v a t i o n energy f o r the f l u i d i t y of the s u b s t r a t e as a f u n c t i o n of temperature. This suggests that at low temperatures, the s u b s t r a t e behaves as a s o l i d , impermeable to the deposited m a t e r i a l which i s h e l d out on the surface i n the form of a f i l m . For high temperatures, the s u b s t r a t e behaves l i k e a viscous f l u i d which i s capable of wetting and e n g u l f i n g p a r t i c l e s by c a p i l l a r i t y forces and i n t o which m a t e r i a l can d i f f u s e . I f the rate of impingement i s very h i g h , p a r t i c l e s formed on the surface grow very r a p i d l y and coalesce i n t o a s t a b l e f i l m before they are able to migrate below the s u r f a c e . With t h i s s e m i - q u a n t i t a t i v e e x p l a n a t i o n as a s t a r t i n g p o i n t , w e ' l l now proceed to develop a q u a n t i t a t i v e model of p a r t i c l e growth and coalescence i n terms of d e p o s i t i o n c o n d i t i o n s which can be used i n a process c o n t r o l s t r a t e g y . 9

2



Vapor

1

Deposition

Figure 2. Plan view transmission electron micrograph of spherical selenium particles grown in a thermoplastic substrate. (Reproduced, with permission, from Ref. 2. Copyright 1981, American Institute of Physics.)

z

CM

i

E o

3

5

0 As'

10' 1017

x

3 d

I0

1 6

-

z LU LU O 2

1015 . 14

z or

7.


Vapor

Deposition

where V = Bft i s the rate of d e p o s i t i o n expressed as the rate of t h i c k e n i n g o f a uniform f i l m of e q u i v a l e n t volume. It i s p o s s i b l e to d e r i v e the mass balance analogous to (8) for the general case when re-evaporation i s s i g n i f i c a n t but the s p e c i a l case above, c a l l e d "complete condensation" i s very common and t e c h n o l o g i c a l l y very important. If we now combine our expressions (4) and (6) f o r the i s o l a t e d p a r t i c l e growth r a t e , (7) f o r the coalescence r a t e and ( 8 ) , the mass balance and i n t e g r a t e from some i n i t i a l time t when N p a r t i c l e s are present, we a r r i v e at the analytical results: Q

Q

Comparison w i t h Experiment In a vacuum chamber h e l d a t about 2 x 10 T o r r , selenium was evaporated from a source a t 250°C and allowed to impinge, through an a d j u s t a b l e s l i t , onto a t r a v e l l i n g web of thermoplastic-coated p o l y e s t e r f i l m maintained at 105°C. The time of exposure of the s u b s t r a t e t o the vapor source i s simply the s l i t width d i v i d e d by the web speed. Deposits were made f o r a s e r i e s of s l i t widths ( i . e . exposure times) and the r e s u l t i n g samples were examined i n the t r a n s m i s s i o n e l e c t r o n microscope. From image a n a l y s e r s t u d i e s o f the micrographs, average p a r t i c l e s i z e and number d e n s i t y were determined. These data appear i n Table I together with some q u a n t i t i e s derived from the date. I f the model i s c o r r e c t and Equations (9) and (10) apply, we would expect the q u o t i e n t s Q

. = =

[ N

- l / 3 . - l / 3 ] / [ 2/3_ 2/3] N

t

t

(

u

(

1

)

2

| | 4 ir V | l / 3

2

)

and 3

13

Q" = NR /t = 3V/4TT to be constant.

< > (1*) These have been c a l c u l a t e d from the data and


132


appear i n Table I . They appear to be reasonably constant w i t h the p o s s i b l e exception of data f o r the longest exposure time. The model a l s o p r e d i c t s that the "coverage" or 6=TTR N should tend to a constant value of 3/2C This i s observed. A l l these data are c o n s i s t e n t with values: C=2 and V=4.72xl0~ cm s " or 2.8 Pm m i n " . The l a t t e r agrees with what i s expected under the experimental c o n d i t i o n s employed. P a r t i c l e s i z e s and numbers are shown i n Figure 5 p l o t t e d as experimental data and from Equations (9) and (10). E r r o r bars are estimated from the s t a t i s t i c s of measurements from the TEM frames. Agreement i s very good. Another p r e d i c t i o n of the model i s that the s i z e and number of p a r t i c l e s are uniquely determined by the t o t a l amount of m a t e r i a l deposited, i r r e s p e c t i v e of c o n d i t i o n s , provided complete condensation i s o p e r a t i v e . This can be r e a d i l y appreciated by n o t i n g t h a t , i n Equations (9) and (10), time appears only i n c o n j u n c t i o n with r a t e so that Vt (the q u a n t i t y deposited) i s the e f f e c t i v e independent v a r i a b l e . This p r e d i c t i o n has been v e r i f i e d through experiments i n which r a t e and temperature were v a r i e d f o r a given specimen. The deposit morphology depended only on how much m a t e r i a l was deposited and was i n s e n s i t i v e to the way i n which i t was done. These, and o t h e r , observations confirm the v a l i d i t y of the model at l e a s t under c o n d i t i o n s of complete condensation. 6

1

1

Table I . Experimental data f o r number d e n s i t y of subsurface p a r t i c l e s and t h e i r average radius at d i f f e r e n t d e p o s i t i o n times. Q', Q" and 8 are defined i n t e x t . t(s)

N(cm" )

1.50

3.05xl0

1.67

2.69

0.085

3.033xl0"

1.88

2.13

0.095

2.14

1.78

2.50

2

9

R(cm)

Q

Q'

0.080x10"4

0

n

1.041xl0~

6

0.613

0.989

0.610

4.118

0.921

0.604

0.110

3.871

1.107

0.677

1.43

0.125

3.725

1.117

0.802

3.00

1.02

0.160

3.950

1.393

0.820

3.75

0.64

0.195

4.268

1.265

0.765

5.00

0.29

0.305

5.089

1.646

0.848

4


7.


Vapor

Deposition


133

134


Imaging Technology based on Monolayer P a r t i c l e

Array

The geometry of the deposit has led to the development of a novel imaging system. As described by Goffe (I) and Pundsack ( 4 ) , images are formed due to e l e c t r o p h o r e t i c m i g r a t i o n of l i g h t - s t r u c k p a r t i c l e s . The m a t e r i a l s package i s sketched i n Figure 6. The f i l m c o n s i s t s of an aluminized p o l y e s t e r base overcoated with a 1.5 Pm l a y e r of t h e r m o p l a s t i c . A monolayer of selenium p a r t i c l e s i s formed j u s t beneath the upper surface of the thermoplastic l a y e r by vapor d e p o s i t i o n as described above. Images are formed by: f i r s t , s e n s i t i z i n g the f i l m with a p o s i t i v e or negative corona charge; second, imagewise exposure and f i n a l l y , development i n which the thermoplastic l a y e r i s softened e i t h e r by heat or exposure to a s w e l l i n g s o l v e n t , a l l o w i n g l i g h t - s t r u c k p a r t i c l e s (which had acquired a charge i n the exposure step) to migrate to the opposite surface of the l a y e r under the i n f l u e n c e of e l e c t r o p h o r e t i c f o r c e s . Contrast i s achieved by removing un-migrated p a r t i c l e s by f l u s h i n g them away with s o l v e n t , f o r example. The f i l m ' s mid-exposure s e n s i t i v i t y i s about 1 erg cm at 400 nm. The image shows a maximum o p t i c a l d e n s i t y of about 2 and a background d e n s i t y of 0.2. Gamma l i e s i n the range 1 to 3 and r e s o l u t i o n i s b e t t e r than 250 l i n e s per mm. Pundsack (4) showed that the f i l m ' s imaging c h a r a c t e r i s t i c s depend on the p a r t i c l e s i z e d i s t r i b u t i o n : o p t i c a l d e n s i t y i s found to be p r o p o r t i o n a l to the t o t a l volume of selenium migrated per u n i t area and that i s equal to the cumulative volume of a l l p a r t i c l e s which have received more than some minimum exposure. But since the minimum exposure v a r i e s i n v e r s e l y as the square of the p a r t i c l e s i z e , i t follows that a p l o t of cumulative volume of o v e r s i z e p a r t i c l e s versus the inverse square of p a r t i c l e s i z e has the same form as the p l o t of o p t i c a l d e n s i t y versus exposure. This r e l a t i o n s h i p between imaging p r o p e r t i e s and deposit morphology, together with the a b i l i t y of the model to p r e d i c t p a r t i c l e s i z e s and numbers as functions of d e p o s i t i o n time and c o n d i t i o n s , makes i t p o s s i b l e to prepare f i l m s with predetermined f u n c t i o n a l characteristics.



Vapor

Deposition

135

Selenium Spheres, 0.3//m Thermoplastic Layer, 1.5//m Semitransparent Conductive Layer, 10-30 nm Transparent Base, 75//m

Figure 6. Cross-sectional view of particle migration film structure. (Reproduced, with permission, from Ref. 2. Copyright J981, American Institute of Physics.)



136 Literature Cited 1. 2. 3. 4.

Goffe, W. L. Photogr. S c i . Eng. 1971, 15, 304. Robertson, D.; Pundsack, A. L. J. Appl. Phys. 1981, 52, 455. S t o w e l l , M. L. Thin Films 1968, 1, 55. Pundsack, A. L. Photogr. S c i . Eng. 1974, 18, 642.

RECEIVED April 30,

1982


Colloids and Surfaces in Reprographic Technology - American

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