thin polymer films

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Thin Solid Films, 18(1973) 157-172 El sev ie r Sequo ia S .A . , Lausan ne - - -P r in t ed in Sw i t ze r land 157

Invi ted Review

THIN POLYMER FILMS

M . W H I T E

Brighton Polytechnic, Brighton ( Gt. Britain)(Received Jun e 14, 1973)

CONTENTS

1. Introduction2. Thermal evaporation

2.1. Polymer degradation2.2. Film properties

3. Pyrolysis4. Radio frequency sputtering

4.1. Pressure effects4.2. Substrate bombardment

4.3. Sputtered PTFEAcknowledgementsReferences

s U M M A RY

The paper reviews work in two main fields relating to the product ion of thinfilms of polymeric materials, that of thermal evaporation of bulk polymer andthat of radio frequency sputtering. Work in the field of thermal evaporationhas been almost entirely concentrated on films produced from polyethylene,although some work is reported on PTFE and PAN. Radio frequency sputtering,still in its infancy as far as polymers are concerned, has been carried out on PTFE.The paper reviews progress so far, and compares results. The production offilms by pyrolysis to form Parylenes is shown to be perhaps the most promisingof the techniques discussed.

l. INTRODUCTION

Thin films of polymeric material are finding increasing importance asencapsulants and dielectrics in the microelectronics industry. Many polymershave excellent electrical and mechanical properties when used as bulk material,but these properties are usually modified and often degraded when the materialis subjected to thin film deposition processes.


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158 M. WHITE

The production of thin films from polymers can be carried out by a numberof techniques, which can be summarised as follows.

(1) Deposition by polymerization of a monomer by means of a glow dis-

charge, or by substrate bombardment by electrons.These techniques have attracted by far the greatest interest from research

workers and from industry. They have been reviewed in this jour nal by Mearns 1(2) Deposition from solution.Some polymers, e.g. polyvi nylfor mal (formvar), can be made into thin film

form by casting from solution onto the surface of a substrate, or onto water.Others are "' grown "" by immersing a s ubst rate in to a solut ion o f polymer. Thetechnique is often used to form single crystals of polymers for structural deter-minations, but is not generally used for manufacturing purposes.

(3) Deposition from bulk polymer by conventional thermal evaporationtechniques.

(4) Deposition from bulk polymer by radio frequency sputtering.(5) Deposition and subsequent polymerization of monom ers produced by

vacuum pyrolysis of suitable dimer material.

Films of polymers have been prepared by a number of other methods,generally suitable for a limited number or range of polymeric materials. Onesuch of interest is the production of polystyrene and polymethylmethacrylatefilms by spont aneou s po lymerization onto the surface o f freshly cleaved mica z.Films as thick as 400 A can be grown af ter exposure for abo ut 10 min to monom ersat pressures of 15-20 torr. Such films, however, are not continuous.

This review will be concerned with the production of polymer films frombulk material using the methods of thermal evaporation and radio frequencysputtering, and also with the technique of vacuum pyrolysis for the productionof Parylene films.

2 . THERMAL EVAPORATION

The production of thin films from polyethylene by direct thermal evaporationof the bulk material has attracted more attention than most other polymericmaterials. The first report of polyethylene evaporation is attributed to Madors ky 3who pyrolysed a number of solid polymers at pressures o f 10 -6 torr within thetemperature range 380-475 C. The pyrolysis was reported to lead to a wax-likedeposit on a substrate, together with gaseous fractions and a solid residue. Coo peret al . ~ attempted to produce single crystals of polyethylene by a thermal evapora-tion process and ended up with a deposit described as "po lye thy len e wax'"White s evaporated polyethylene from an open boat source at 290 ~C and 10 -4torr and produced wax-like deposits which had solidified droplets of solid poly-ethylene imbedded in them. As the temperature of the evaporation source wasraised the diameter of the droplets was found to increase: some with diametersas large as 0.5 mm were reported.

Contamination of the deposit by solid matter when evaporating poly-


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T H I N P O LY M E R F IL M S 1 5 9

e t h y le n e is d u e t o t h e v i g o r o u s b o i li n g a c ti o n o f th e m o l t e n p o l y m e r, t o g e t h e rw i t h t h e r a p i d e v o lu t i o n o f b r e a k d o w n p r o d u c t s . U n c o n t a m i n a t e d f il m s h a v eb e e n o b t a i n e d b y d e li b e ra t e ly c h o o s i n g a l o w e v a p o r a t i o n t e m p e r a t u r e a n d t h u sa s l o w r a te o f d e p o s i t i o n 6 , b u t s p e ci a ll y d e si g n ed s o u r c e s o f t h e " p e p p e r- p o t "

type 7 and o the r s us ing a com bina t ion o f in t e rnal ba ffl e s and f la sh evapo ra t ion sh a v e e n a b l e d d e p o s i t io n r a t e s a s h ig h a s 1 0 0 0 / ~ m i n - 1 t o b e a tt a in e d . Tw o s u c hsources a re i l l u s t ra t ed in F ig . 1 . A s imple tubu la r so urce su rm oun ted by hea teds ta in le s s - s t ee l meshes th rough which the vapour s t r eam passes on i t s pa th to thes u b s t r a t e h a s e n a b l e d d e p o s i t i o n r a te s a s h i gh a s 2 0 0 0 A m i n - ~ t o b e o b t a i n e d o n asubs t r a t e 15 cm ve r t ica l ly abo ve the source 9. Sa t i s f ac to ry th i ck (5 lam) f ilms haveb e e n p r o d u c e d w i th t h is s o u r c e , a l t h o u g h o t h e r w o r k e r s h a v e r e p o rt e d t h a t v e r yth ick (0~1 ram) f ilms o f e vap ora ted po lye thy len e a re unsa t i s f ac to ry 1.

B . . . . I I I l l l

I I

A - S t r ip h a t e r I IP - S t a i n l e s s s t e e l ~ - - =

body "" "~'"C - T h e r m o c o u p le

a p e r t u r e - - CA ~

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i1~1 N I - B B - R a d i a t io ns h i e l dC - M o ly b c l. e n u m h e a t e r

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Fig . 1 , Evapora t ion sources fo r so l id po lymers : ( a ) "peppe r-po t " sou rce ; (b ) ba ff l ed sou rce wi thho ppe r f eed a r r ang em en t fo r supp ly o f po lym er pe l le t s.

2.1. Polymer degradation

T h e e v a p o r a t i o n o f p o l y m e r i c m a t e r ia l is b a s ic a l ly t h e e v a p o r a t i o n o fthe rm al de grada t ion p ro duc t s . A sec to r fie ld mass spec t rom ete r a t tached to ane v a p o r a t i o n c h a m b e r h a s i n d i c a t e d t h e c o n s i d e r a b l e d e g r a d a t i o n t h a t o c c u r sd u r i n g e v a p o r a t i o n o f p o l y e th y l e n e 7. O b s e r v a t i o n o f th e m a s s s p e c t r u m o b t a i n e d

~ fro m P T F E p y r o l y s e d w i th i n t h e i o n s o u r c e o f a s im i l a r i n s tr u m e n t h a s s h o w n ad i ff e re n t p a t t e r n o f d e g r a d a t io n , b u t a t h o r o u g h o n e n o n e t h e l e s s 11. M u r a k a m iand Sh in tan i 12 have r ecen t ly r epor t ed obse rv a t ions on the degrad a t ion o fTe f l o n - F E P a t 4 0 0 C , w h i c h p r o d u c e d a m a s s s p e c t r u m e s s en t ia l ly s i m i l ar t ot h a t f r o m P T F E . F i g u r e 2 il lu s tr a te s t h e m a s s s p e c t r a o b t a i n e d b y t h e rm a ld e g r a d a t i o n o f p o l y e t h y le n e a n d P T F E . T h e d e g r a d a t i o n o f p o l y v i n y l c h l o r id ehas b een desc r ibed b y Gu p ta and S t . P ie r re a3.

D e t a i le d e x p e r im e n t a l e x a m i n a t i o n o f t h e d e c o m p o s i t i o n o f P T F E a th i gh p r e s s u re s h a s b e e n c a r ri e d o u t b y K u p e let al. 14 a n d a t n o r m a l e v a p o r a t i o npressu res by V arhegy i and Szeke ly ~5. The l a t t e r w orke r s , u s ing Tef lon , con-s ide red tha t the d eco m pos i t ion i s e s sen t ia l ly the r eve r se o f the po lym er iza t ion


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THIN POLYMER FILMS 161

process, i . e . it involves the format ion of free radicals due to rando m chain scission.Apart from the monomer and about 2.5~ of C3F6 they found practically noother product present during decomposition. The radical concentra tion was foundto vary during the decomposition process, and to depend s trongly on the previous

thermal history of the sample.It is important to note that these experimental results were obtained by

thermo-gravimetric methods, and were therefore not subject to the results of thefurther degradation processes that may occur in mass spectrometry.

The kinetics of the thermal degradation of polymers have been describedby Friedman 16, and a method has been introduced for evaluating kinetic para-meters from thermal analysis data a v. The kinetic equation is written in the form

--=-dC e_em r (1 - C)"dT fl

where C is the degree of conversion ( i .e . degradation), T the temperature in K,A a pre-exponential factor in s- a, fl the linear heating rate in K rain- 1, E theactivat ion energy in cal too l- t, R the gas constant and n the kinetic order. F or areaction that follows this equation throughout its course, Fr iedman uses the form

A ( d C / d T ) ,= e-e/Rr' (1-C)"

where the subscript i refers to a specific point on a thermogram. By means of three

points it is possible to evaluate n, E and A from a single thermogram. Friedmanalso shows that i f one of the points is that at which the rate of conversion is amaximum, then only one other point is required on the same thermogram for anevaluat ion of all three parameters. It is also pointed out tha t da ta f rom differentialthermal analysis (DTA), mass spectrometric thermal analysis (MTA) and thermalvolatilization analysis (TVA) may be treated in similar fashion to yield kineticparameters.

The relationship between the degradation o fpolyethylene and the evaporatingtemperature has been described by Luff and White 18 who observed an activated

process with an activation energy of 85 kcal mol - 1 at evaporat ion temperaturesabove 350 C. Amelin et a l . a9 compared the activation energy for thermal andmechanical degradation of polyethylene, polymethylmethacrylate and poly-styrene. In each case they found a two-stage degradation process, illustrated bythe type of curve shown in Fig. 3. The first, low temperature stage, with activationenergy typically 25 kcal tool- 1 for polyethylene, was attributed to the rupture ofweak bonds in the solid polymer, and possibly to the removal of side chains fromthe main polymer molecule. The second stage, the onset o f which for polyethylenewas at a temperature o f 390 C, exhibited an activation energy of 70 kcal tool - tand was attributed to the rupture of primary bonds in the linear molecule.

The energy required to rupture a polymer cross-link bond has been calculatedby Beuche to be as low as 10 eV 20. Shulman 2 and Collins e t a l . a t have pointedout that further f ragmentation 9f mass species degraded thermally in the evapora-tion source is likely to occur in the ion source o f some mass spectrometers, whereelectron energies of up to 70 eV are commonly employed. The mass spectra


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162 M. W HITE

'7 m

0

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g

p.

l O

\1.0

01 \\ " ~ O ..o ~ .

1'50 1 '55 ~.'60 (65 1'70 175 ~.'eo1 (oK 1) x 1 0 3

--f

Fig. 3. Ev apo rat ion rate of polyethylene vs. source tempera ture s howing a two-stage degrad ationprocess 1 s.

o b t a i n e d b y t h e s e m e a n s a r e t h e r e f o r e u n l i k e ly t o r e p r e s e n t a c c u r a t e l y t h e a c t u a ld e g r a d a t i o n p r o d u c e d p u r e l y b y t h e rm a l m e a n s .

M u r a k a m i a n d S h i n ta n i 12 r e p o r t e d n o s u c c e s s i n t h e e v a p o r a t i o n o f P T F E ,b u t t h i n f il m s h a v e b e e n d e p o s i t e d b y t h is m e t h o d 2 z' 23 a n d s o m e o f t h e i r p h y s i c a lp r o p e r t i e s h a v e b e e n d e t e rm i n e d . S o m e p r o p e r t ie s o f e v a p o r a t e d p o l y e t h y l e n ea n d P T F E f il m s a r e c o m p a r e d in Ta b l e I. U n l i k e t h e e v a p o r a t i o n o f p o l y e t h y le n e ,w h i c h p r o c e e d s t o c o m p l e t i o n a n d l e av e s n o r e s i d u e s , b o t h P T F E 22 a n d Te f lo n -F E P 12 d e g r a d e t o l e a v e a r e s id f ie o f c a r b o n i n th e e v a p o r a t i o n s o u r c e . T h e p r e s e n c eo f f re e c a r b o n is i n d i ca t e d i n t he m a s s s p e c t r u m o f e v a p o r a t e d P T F E i n F i g . 2.

TABLE IC O M PA R I S O N O F F I L M A N D B U L K P R O P E R T I E S F O R E VA P O R AT E D P O LY E T H Y L E N E A N D PTFE

P o l y e t h y l e n e P T F E

Fil m 2 6 Bu Ik Fi hn ~ 2 B u lk

Dielectric con st ant 2.2 2.3 2.0 3.6 2.1Ta n 6 (1 kH z) 0.002 0.0002 0.02 0.0001St re ng th (V rn 1) 10 8 4 x 10 7 lO T

2.2. Film propertiesT h e s u b s t r a t e w h i c h r e c ei v e s a d e p o s i t d u r i n g t h e r m a l e v a p o r a t i o n o f a

p o l y m e r h a s i n c id e n t u p o n i t a l a rg e n u m b e r o f d e g r a d a t i o n p r o d u c t s , t h e p r o -p e r ti e s o f w h i c h a r e u n l i k e ly t o h a v e m u c h s i m i la r it y w i t h t h o s e o f t h e o ri g in a l


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p o l y m e r f r o m w h i c h t he y c am e . P o s t - d e p o s it io n t r e a t m e n t a n d e v e n t r e a tm e n td u r i n g d e p o s i t i o n h a v e b e e n c a r r ie d o u t i n a t t e m p t s t o c a u s e c r os s - li n k in g b e t w e e nspec ies a r r iv ing a t t he subs t r a t e , i n o rde r to p rod uce a so r t o f po lym er ic m a te r i a lw h o s e p r o p e r t i e s a r e a s n e a r l y a s p o s s ib l e t h e s a m e , o r h o p e f u l l y b e t te r, t h a n t h o s e

o f the o r ig ina l m a te r i a l. W hi te and Lu ff~4 have desc r ibed a n u l t r av io le t i r r ad ia t ionp r o c e s s i n w h i ch a m e d i u m p r es s u re m e r c u ry d i sc h a rg e l a m p w a s m o u n t e di n si d e th e d e p o s i t i o n c h a m b e r. U l t r a v i o l e t i r ra d i a t io n o f th e s u b s t r a t e d u r i n g a n da f t e r d e p o s i t i o n f r o m p o l y e t h y l e n e c a u s e d c r o s s - l i n k i n g t o o c c u r, r e d u c e dcrys taUin ity and im prov ed the h igh t em pera tu re p roper t i e s o f the films . P ro -l o n g e d e x p o s u r e ( o f t h e o r d e r o f a n h o u r ) t o u l t ra v i o l e t r a d i a t io n a f t e r d e p o s i t io np r o d u c e d f il m s w h i c h w e r e i n s o lu b l e i n m o s t c o m m o n s o l ve n t s, e v e n t h o u g ht a k e n t o t e m p e r a t u r e s o f u p t o 9 0 C .

C r y s t a ll in i ty o f p o l y e t h y l e n e fi lm s c a n b e i m p r o v e d b y p o s t - d e p o s i t i o n a n -

nea l ing a t a t em pera tu re o f 150 C , w i tho u t p rev ious i r rad ia t ion . In f ra red abso rp -t io n s p e c t r a o f b o t h i r r a d i a te d a n d a n n e a l e d s p e c i m e n s h a v e s h o w n r e m a r k a b l es imi la ri ty to the sp ec t rum f rom the o r ig ina l bu lk p o lym er 18.

S a t o u e t a l . 2 s h a v e d e s c r ib e d a m e t h o d o f p r o d u c i n g s in g le c ry s t a ls o f p o l y -e t h y l en e b y h e a t t r e a t m e n t o f v a c u u m - d e p o s i t e d f i lm s i n x y le n e . E l e c tr o n m i c r o -s c o p y o f th e r e s u lt in g c r y s ta l s h a s s h o w n t h e m t o b e m o r p h o l o g i c a l l y s i m i la r t op o l y e t h y l e n e c r y s t a l s g r o w n f r o m s o l u t i o n . M e a s u r e m e n t s m a d e b y g e l - p e r m e a -t io n c h r o m a t o g r a p h y a n d o n v i s c o s it y s h o w e d t h a t t h e m o l e c u l a r w e i g h t o f th ed e p o s i t e d m a t e r i a l w a s a b o u t 1 4 0 0( i . e . e a c h m o l e c u l e c o n t a i n e d a b o u t 1 00

m ethy lene un i ts ) , wherea s tha t o f the s t a r t ing m a te r i a l w as o f the o rde r o f 10 000 .T h e t e m p e r a t u r e o f th e e v a p o r a t i o n s o u r c e f o r t h e se e x p e ri m e n t s w a s o f th e o r d e ro f 300C , wh ich i s low er than tha t no rm al ly used 7 ' 8, 18 and w hich has beens h o w n t o l e a d t o l o w m o l e c u l a r w e i g h t d e p o s i t s 18. T h e i n f r a re d a b s o r p t i o n s p ec -t r u m o f t h e s e s in g le c r y s ta l s h a s b e e n i n t e r p r e t e d b y S a t o ue t a l . t o s h o w e v i d e n c eo f d o u b l e b o n d c o n t e n t a n d o f c r y st a ll in i ty.

A n a t t e m p t t o g r o w e v a p o r a t e d p o l y e t h y l e n e e p i t a x i a l l y o n N a C 1 h a s b e e nr e p o r t e d b y F i s c h e r 26 , w h o f o u n d t h a t c r y st a ll in e p o l y e t h y l e n e c o u l d i n f a c tbe p roduc ed , g row ing wi th it s (110) p lane pa ra l l e l t o the NaC1 < 110 > d i r ec t ion .

The e l ec t r i ca l p rope r t i e s o f po lye thy lene f i lms have been the sub jec t o fs tudy by re la t ive ly few w orke r s . Lu ff27 repor t ed va lues o f pe rmi t t iv i ty c lose tothe va lue fo r bu lk p o lye thy lene , bu t the va lues fo r lo s s tang en t were typ ica l ly ano r d e r o f m a g n i t u d e g r e at e r, a s s h o w n i n Ta b l e I . M i y o s h i a n d C h i n o 6 c a r r ie d o u tm e a s u r e m e n t s o n e v a p o r a t e d f il m s a n d s o l u t i o n - g r o w n c r y s ta l s, a n d r e p o r t e d f o rthe i r f i lms a region o f nega t ive res is tance .

L i t tl e h a s b e e n r e p o r t e d e i t h e r o n t h e m e c h a n i c a l p r o p e r t i e s o f e v a p o r a t e df ilms o r on the i r g row th b ehav iou r. Lu ff and W hi te 18 de te rm ined the s t r e s s inpo lye thy lene f i lms dur ing and a f t e r depos i t ion , and Luff27 has o bse rv ed change si n s u rf a c e c o n d i t i o n s o f f il m s d u r i n g t h e c o u r s e o f a n n e a li n g . M u r a k a m i a n dS h i n ta n i 12 h a v e o b s e r v e d t h e g r o w t h b e h a v i o u r o f Te f l o n - F E P f il m s, a n d h a v er e p o r t e d t h a t s m o o t h f i l m s w e r e o b t a i n e d o n l y w h e n t h e s u b s t r a t e t e m p e r a t u r ewas abov e 300 C .

T h e e v a p o r a t i o n o f p o l y a c r y lo n i t ri le ( PA N ) to f o r m s e m i c o n d u c t i n g p o l y m e rf i l m s h a s b e e n d e s c r i b e d b y S u z u k ie t a l . 2 8 P A N p o w d e r w a s h e a t ed i n a ir a t


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164 M. WHITE

200 ~'C and then evapora ted a t 450 C at a pressure of 10-5 tor r on to fused quar tzsubstrates. Films became semiconducting after heat treatment in nitrogen atatmospheric pressure. The values of resistivity and activation energy obtained atvarious treatment temperatures are shown in Table II.

TABLE I1RELATIONSHIP BETWEEN HEAT TREATMENT TEMPERATURE AND ELECTRICAL PROPERTIES OF PAN FILMS(FROM SUZUKI et al. 2s)

Heat treatment temperature Resisticitv Activation encrg.vCon due t ion A h.w n?~ ioH(' C) (f~ m)(eV) (eV)

400 10 ~ o450 10 ~500 105 0.4 1.4

3. PYROLYSIS

The discovery by Szwarc 29 tha t the vacuum pyrolysis of xylene could lead,on condensation, to the p roduction of poly-p-xylylene initiated a considerableinterest in the process, mai nly because the resulting films were strong enough foruse as windows in particle counting tubes. The early work by Szwarc producedfilms which were contamina ted by low molecular weight by-products o f the reac-tion, and the process was no more than 25 % efficient. The improved method des-cribed by Go rh am 3 uses di-p-xylylene as the sta rting material, and this is pyrolysedin vacuum at 600 C. This process forms two molecules ofp-xylylene which spon-taneously polymerize when incident on substrates maintained at a temperatureof less than 30 C. The poly mer so formed is a high molecular weight linearpoly-p-xylylene. The process is shown schematically in Fig. 4.

600"C 30"C~Olt0rr 0 lt0rr

Fig. 4. Pyrolysis ofp-x ylylen e and poly meriza tion to form poly-p-xylylene.

Gorham describes the process as being suitable for a wide range of sub-stituted poly-p-xylylenes, and gives full details of the preparation of films fromseven different starting materials, which are summarised in Table III. The familyof materials form ed by this process have come to be described by the generic term"Parylene". All Parylene films are formed as linear polymers, and are trans-parent, eolourless and tough.

Spivack and Fer ran te 31 have used films of poly(chloro-p-xylylene), known asParylene C, as an encapsulating medium, and have determined the permeabilityro water vapour a nd the cont inui ty of the films within the thickness range 292 Ato 7.3 pro. Lee 3z has used Parylene films as encapsulating agents for various


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T H I N P O L Y M E R F IL M S 1 6 5

TABLE I l lPYROLYSIS AND CONDENSATION COND ITIONS FOR A NU MB I~ qF MONOMERS USED TO FORM PARYLENE FILMS(FROM GORHAM 30)

Monomer Pyrolysis temperature Condensation Polymer code

( C ) Temperature Pressure(C) (tort)

p-xylylene 600 30 1.0 N2-m ethyl p-xy lylene 660 60 0.1 M2-ethyl p-xylylene 660 90 0.12-chloro p-xylylen e 600 90 0.12-cya no p-xylyle ne 660 130 0.12-brom o p-xy lylene 660 130 0.5D ichloro p-xylylene - 130 0.1 C

s e m i c o n d u c t i n g d e v ic e s , a n d h a s s h o w n t h a t f i l m s o f t he o r d e r o f 3 p m t h ic kp r o v i d e c o m p l e t e p r o t e c t i o n a g a i n s t w a t e r v a p o u r , w i ll w i t h s t a n d o p e r a t i n gt e m p e r a t u r e s o f u p t o 175 C , a n d d o n o t a f f e c t t h e e l e c t ri c a l p r o p e r t i e s o f t h ee n c a p s u l a t e d d e v ic e .

L o e b 33 h a s d e s c r ib e d f o u r m e t h o d s o f u s i n g p o l y m e r i c m a t e r i a l f o r e n -c a p s u l a t i o n , a n d h a s g iv e n d e t a il s o f e x p e r i m e n t s o n t h e p r e p a r a t i o n a n d p r o p e r t i e so f P a r y l e n e C . E a r l i e r a p p l ic a t i o n s d e s c r i b e d b y L o e b i n cl u d e t h e c o a t i n g o f

a l u m i n i u m f oil w i t h P a r y l e n e N i n t h e l a rg e s ca le m a n u f a c t u r e o f c a p a c i t o rd i el ec t ri cs 34. S p i v a c k 35 h a s d e t e r m i n e d t h e m e c h a n i c a l p r o p e r t i e s o f a n u m b e ro f P a r y l e n e s u n d e r c o n d i t io n s o f b ia x i a l a n d u n i a x i a l s tr es s. F i g u r e 5 s h o w s t h er e l a ti o n s h i p h e o b t a i n e d b e t w e e n r u p t u r e p r e s s u r e a n d f i lm th i c k n e s s f o r P a r y l e n eC , t h e s t ra i g h t lin e in d i c a t in g t h a t t h e m e c h a n i c a l p r o p e r t i e s a r e in d e p e n d e n t o f

1 2 0

1oo

" I -

~ 8o

~ 6oo.

rl-2O

0 I I Io 1 0 2 0 3 0

Thickr~ess ()am)

Fig. 5. Rupture pressure rs. film thickness for a spec imen of Parylene C. The film was unsu pportedover an aperture of diameter 0.2 cm 35 (after Spivack).


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11/16


p r e s s u r e s o f 1 0 - 1 t o r r ( m f p 0 .5 m m ) t h e p a s s a g e o f s p u t t e r e d m a t e r i a l t o t h es u b s t r a t e i s e s se n t ia l ly o n e o f d if f u si o n t h r o u g h t h e p l a s m a . L o s s o f e n e rg y o f th e 'spu t t e red m a te r i a l l ead ing to an inc reased poss ib i l i t y o f be ing invo lved in co l li s ionr e a c t io n s m a y s o m o d i f y i t t h a t t h e p r o p e r t ie s o f t h e d e p o s i t e d fi lm m a y b e

con s ide ra b ly d i ff e ren t f rom those o f the t a rge t m a te r i a l, a s i l l u s tr a t ed in F ig . 6 .

ITarget particle

(~) Gas =,tornG Electron

LPolycrystelline d u estructure etch J to low yield componentpa ttern J (e.g. contarninat on)

p=lO'2torr~r~ ETtypicially ( ~ p= lO 3 to rrr n n f p ~ rnrn

Diffusion E ~ " -O.leV ET=IOeV" Direct transp ort

@ @@1

Targetprocesses

Gas (plasma)processes

Ta rget- subst atedistancetypically 50turn

S u b s t r a t ep r o c e s s e s

Fig. 6 . Target and substrate phen om ena during r. f. sput ter ing (af ter Ho lland and P riest land3s, bypermission ofVacuum).

C h a p m a n 39 h a s d e s c r i b e d i n t e re s t in g " f o c u s s i n g " e f fe c ts o b s e r v e d i n s p u t t e ri n gf r o m m i x e d t a rg e t s, a n d h e o b t a i n e d w h a t a p p e a r t o b e s t ra i g h t l in e t ra j e c to r i es o fspu t t e red m a te r i a l even a t p res su res a s h igh as 14 to r r, when the mea n f ree pa th wasa p p r o x i m a t e l y o n e - t e n th o f h is t a rg e t - s u b s t r a t e s e p a r a t io n . A l t h o u g h n o c o m p l e t ee x p l a n a t i o n o f t h e p h e n o m e n o n h a s b e e n g i ve n , it is t h o u g h t l ik e ly t h a t i t is d u et o n o n - u n i f o r m s u b s t r a t e b o m b a r d m e n t b y h ig h e n e rg y e l ec t ro n s , d u e t o t h ed i ff e ren t seconda ry e l ec t ron em iss ion coe ffi c ien t s o f the com pos i t e t a rge t m a te r ia l s .

Pr ies t land an d I -Iersee 4, wo rkin g w i th a d io de r.f . spu t te r ing sys tem , hav eo b s e r v e d t h a t d e p o s i t i o n r a te s o f s o m e m a t e r ia l s a c t u a ll y i n c r e as e w i t h r is e ing a s p r e s s u r e in t h e c h a m b e r- - t h e r e v e rs e o f r e su l ts o b t a i n e d w i t h d .c . d i o d espu t t e r ing sys tems . At a p res su re o f 10 -1 to r r i t has been es t ima ted tha t in thed .c . d i o d e c a s e o n l y 10 ~o o f a t o m s s p u t t e r e d f r o m t h e t a rg e t e v e r t r a v e l b e y o n dt h e c a t h o d e d a r k s p a c e 41 . S p u t te r i n g o f m e t a l s i n a n r.f . d i o d e s y s t e m s h o w s t h a tthe pe rcen tage inc rease in depos i t ion r a t e wi th r i s ing p ressu re inc reases a s thea tom ic w e igh t o f the spu t t e re d m a te r i a l i nc reases 4, an d th i s i s exp la ined byc o n s i d e r i n g t h e g a s c o l l i s i o n c r o s s s e c t i o n , w h i c h d e p e n d s o n t h e d i a m e t e r a n de n e rg y o f t h e s p u t t e re d a t o m s . T h e g r e a t e r t h e r e l a ti v e m a s s o f th e s p u t t e r e da t o m c o m p a r e d w i t h t h a t o f th e g a s a t o m , t h e g r e a t e r t h e p e r s is t e n c e o f i ts i ni ti ald i r e c ti o n o f tr a ve l . F o r p o l y m e r i c m a t e ri a l, h o w e v e r, in w h i c h t h e i n d i v i d u ala t o m s ( a n d o f t e n p a r t s o f p o l y m e r c h a i n s) h a v e m a s s c o n s i d e r a b l y le s s t h a n m o s tm e t a l s , a n d o f t e n le ss t h a n t h a t o f t h e g a s a t o m , h i g h p r e s s u r e s p u t t e r in g m u s t b egove rned a lm os t en t i r e ly by d iffusion .


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T H I N POLYMER FILMS 16 9

by the k ine t i c ene rgy o f the spu t t e red pa r t ic l e s , ac cou n t ing fo r ab ou t 20 9/o o f thei n c i d e n t f l u x , a n d e l e c t r o n a n d i o n b o m b a r d m e n t , w h i c h t o g e t h e r a c c o u n t f o rthe r emain ing 80% . I t was ca lcu la t ed tha t on ly abo u t 5 % of the app l i ed r.f. po we ra c t u a l l y r e a c h e s t h e s u b s t r a t e a s a n i n c i d e n t f l u x ; t h e m a i n c o o l i n g m e c h a n i s m

f o r fi lm s d e p o s i t e d o n t o i n s u la t in g s u b s t r a t e s w a s f o u n d t o b e b y r a d ia t io n .6OO

500 ~ 2kWc-A.4OOcm

1.0KW -A,4.0cm4 o c

~ 3 0 0 '

2 0 0 w :6 5 cm

t~3 0.1 kWc-A,65m10(3

Ae PressureSm toreC ~ I J t

o 5 10 15 20 25Sput t~zr incj t im e ( ra in)

F i g . 8 . S u b s t r a t e t e m p e r a t u r e a s a f u n c t i o n o f s p u t t e r i n g p o w e r a n d d u r a t i o n f o r g o l d f i lm s ~3.

Pr ies t l and 5 has sh ow n tha t a m e ta l g r id a t s ubs t r a t e po ten t i a l p l ace d c loseto the subs t r a t e su r face co l lec t s m os t o f the e l ec t rons , r e su lt ing in a s ign i fi can tr e d u c t i o n i n s u b s t r a t e t e m p e r a t u r e s . Te m p e r a t u r e v a l u e s m e a s u r e d a t t w o p o w e rlevel s and show ing the e ffec t o f the g r id a re g iven in T ab le V.

TABLE V

EFFECT ON SUBSTRATE TEMPERATUR E D URING SPUTTERING OF A METAL GRID PLACED CLOSE TO THE SUB-STRATE AND AT SUBSTRATE POTENTIAL (FROM PRIESTLAND4 5)

R f . anode cu r ren t Spu t t e r ing t ime M esh cove r ing Temp. r eached by subs t r a t e(A) (min) (C)

0.2 20 No 215Yes 87

0.3 20 N o 390Yes 205

4 .3 . Spu t te red P TF E

L i t tl e w o r k h a s b e e n r e p o r t e d o n t h e p r o d u c t i o n a n d p r o p e r t i e s o f r.f .s p u t t e r e d p o l y m e r f i l m s ; w h a t h a s b e e n n o t e d h a s a l l t a k e n p l a c e u s i n g P T F E a st h e ta rg e t m a t e r i a l. T h e f ir st r e p o r t o f r.f , s p u t t e r e d P T F E , b y J a c k s o n a n d H a r r o p ,a p p e a r e d i n 1 96 7 s4 a n d w a s f o l lo w e d b y a d e t a i le d d e s c r i p t io n o f t h e d e t e r m i n a t i o no f t h e f r ic t io n o f s p u t t e r e d P T F E f il m s s s . I n b o t h c a s e s s p u t t e r in g w a s i n it ia t e db y t h e i n t r o d u c t i o n o f a rg o n t o t h e c h a m b e r, b u t a s s o o n a s t h e r.f. d i s c h a rg e


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170 M. WHITE

became self-sustaining the argon supply was cut off. Sputtering was then con-tinued by means of CzF~ liberated from the target.

Mathia s a nd Miller 56 have reported on the decomposition of PTF E in aglow discharge. Using vapour phase chromatography techniques they have

shown that at a total pressure of 11.5 torr (of which the carrier gas exerted apressure of 4 torr) the principle decomposition products, in order of increasingconcentra tion, consisted o f CF+, C3Fs, C3F~,, C:F(, and C2F a. C2F 4 accountedtor over 85>o of the total decomposition products. Mathias and Miller alsodetected free carbon and undissociated polymer. The initial colour of theirdischarge was pink, and this changed rapidly to blue as the decomposition pro-ceeded. They noted that a considerable amount of solid yellow polymer wasformed, together with free carbon deposits, when helium was used as a carriergas, but that when oxygen was present no carbon was detected and the polymerdeposit was white in colour.

The decomposition is described in terms of the formation of free-radicalfragments which occurs by ra ndo m cleavage of the polymer chain. In referring tothis description of decomposition initiation Morr ison and Robertson sv appearto have understood the term "random chain cleavage" to imply cleavage atrandom points on a PTFE target surface, instead of at random points alonga given PTFE polymer chain. They argue that scanning electron micrographs ofthe target surface after sputtering show that preferential removal of material hastaken place due to the non-random cleavage of polymer chains caused by theinheren t erystallinity of the target.

The detailed mechanism of the pr oducti on of C F 2 radicals leading to C2F 4,described by Math ias and Miller 56 and quoted by Mor ris on and Robertson 57,includes the production of free fluorine. Recent experiments by Buckton inwhich PT FE is sputtered by argon at a pressure o f 2 10 -3 torr directly into theion source of a mass spectrometer have shown no evidence for the presence offluorine, but have confirmed the presence of all other mass species. SputteredPT FE films have also been examined by Pratt and Laus man ss. They used a triodesputtering system in which an initial discharge was excited in argon at a pressureof 2 x 10-3 torr. The applicat ion of an r.f. voltage to the PTF E target prod uced achange in colour of the discharge from pink to blue, associated with the presenceof C2F4. Like the previous workers TM 55, sv Prat t and Lausman s8 found tha tthe deposited material was chemically inert, an d exhibited an infrared absorptionspectrum very similar to that from bulk PTFE. They determined the electricalproperties of their films by forming planar capacitors between aluminium elec-trodes. The properties of sputtered PTFE films are compared with those of bulkPTFE in Table VI. which also includes details of the sputtering process used by

each experimental group.Although three groups of workers have reported results on r.f. sputteredPTFE it is difficult to compare their results exactly because all used differentsputtering techniques. In two cases ss' sv argon was used only to initiate thedischarge, while in the third s8 it was used thr oug hou t the sputtering process.The ch amb er pressure used by Mor ris on and Robertson s7 was 10 -2 torr and,at a target-substrate separation of greater than 30 mm, virtually no sputtered


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T H I N P O LY M E R F I LM S

T A B L E V ICOMPARISON OF PROPERTIES OF R.F. SPUTTERED P T F E

171

Harrop and Morrison and Pratt and BulkHarropS s Rober tsonS 7 Lausman s s

C o l o u r Ye l lo w Ye l l o w Ye l l o w W h i t eT a n 6 ( 1 k H z ) 0 . 00 1 0 . 0 01 0 . 0 0 6 0 . 0 00 1S t r e n g t h ( V m - l ) _ _ 4 x 1 0 s 1 08Re s is t iv i ty (~ m) - 3 x 1014 7 1016 1017-1019D i e l e c tr i c c o n s t a n t - 1 . 5 - 3 1 .7 2 .2H a r d n e s s ( V. P. N . ) 1 2 .5 1 2 .5 - 5M a x . t e m p e r a t u r e 4 5 0 - 4 5 0 3 0 0A r g o n u s e d I n it i al ly I n i ti a l ly C o n t i n u o u s l yP r e s s u r e ( t o r r ) 1 0 - 3 1 0 - 2 2 1 0 - ~M e a n f r ee p a t h ( m m ) 5 0 5 2 5A p p a r a t u s D i o d e D i o d e, w i th T r i od e

m e s h a n o d e

mate r i a l r eached the subs t r a t e wi thou t undergo ing numerous co l l i s ions wi th int h e p l a sm a , m a k i n g t h e l ik e l ih o o d o f f u r t h e r f r a g m e n t a t i o n e x t re m e l y h ig h . P r a t ta n d L a u s m a n 58 u s e d a t o t a l p r e s s u r e o f 2 x 1 0 - a t o r r ( m e a n f re e p a t h 2 5 m m )b u t d o n o t m e n t i o n t h e i r t a rg e t - s u b s t r a t e s e p a r a t io n . A l t h o u g h t h e y u s e d a rg o nt h r o u g h o u t t h e ir e x p e r i m e n t a l d e p o s i t io n , t h e r e m o v a l o f t h e r.f . v o l ta g e a l s oresu l t ed in the d i sapp ea ran ce o f the o r ig ina l a rgon g low. I t i s t he re fo re o f con-

s ide rab le in t e res t t ha t t he p rop er t i e s o f the f ilms ob ta ine d by th ree d i ff e ren tme thods a re so s imi la r.R . f . spu t t e r ing o f p l a s t ic s is s ti ll ve ry m uch in i ts i n fancy. D egra da t ion ,

s u b s t r a t e b o m b a r d m e n t , p r o d u c i n g v e r y h i g h d e g re e s o f c r os s -l in k in g , a n d i o nc o n t a m i n a t i o n a ll t e n d t o p r o d u c e f i lm s w i th p r o p e r t i e s i n f e r i o r to t h o s e o f t h eb u l k m a t e r i al . T h e y a r e , h o w e v e r, s u p e r i o r to t h o s e f il m s p r o d u c e d b y th e r m a le v a p o r a t i o n .

A ( ' K N O W L E D G E M E N T S

A c k n o w l e d g e m e n t s a r e m a d e t o D r. N . P e n t l a n d f o r p e r m i s s i o n t o p u b l i s ht h is r e vi ew, a n d t o M r. M . R . B u c k t o n f o r p e rm i s s i o n t o q u o t e f r o m h is r e su l ts ,w h i c h a r e a s y e t u n p u b l i s h e d a n d w h i c h a r e p a r t o f a p r o j e c t w h i c h i s t h e s u b j e c to f a n S R C C A P S a w a rd .

A c k n o w l e d g e m e n t i s a l s o m a d e f o r p e r m i s s io n t o i n c l u d e fi g ur e s a n d m a t e r i a lf o r ta b l e s f r o m p u b l i s h e d w o r k .

R E F E R E N C E S

l A . M . M e a r n s ,Thin Solid Films, 3 (1969) 201 .2 V . I . T r o f i m o v , A . E . C h a l y k h a n d E . I . E v k o ,Soviet Phys.--Solid State, 13 (1971) 512 .3 S . M a d o r s k y ,J. Polym. Sci., 9 (1951) 133.4 A . C . C o o p e r , A . K e l l e r a n d J. R . S . W a r i n g ,J. Polym. Sei., 11 (1953) 215 .5 M . W h i t e , Vacuum, 15 (1965) 449 .6 I . M i y o s h i a n d K . C h i n o ,Japan. J. Appl. Phys., 6 (1967) 181.7 P. P. L u f f a n d M . W h i t e ,Vacuum, 18 (1968) 437 .


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1 7 2 M . W H I T E

S A. F. Perveev and G. A. Muranova. P r i h o r y 1 Te k h n i k a E k , v ~ e ri m e n t a .. ~ .4 (1969) 200.9 M. White , to be published.

10 U.K. P a ten t 991 ,840 .11 R. D. Collins, D. Fiveash and L. Holland, Va c u u m , 1 9(1969) 113.12 Y. Murakami and T. Shintani, Thin So l id F i lms , 9(19723 301.

13 V. V. Gup ta and L. E. St. Pierre, J . Polym. Sci . , 8(19703 37.14 R, E. Kupel , M. Nolan, R. G. Keegan, M. Hite and L. D. Scheel, Anal . Chem. , 36(19643 386.15 G. Varhegyi and T. Szekely. ,4eta C him . Aca d, S ci. (Hun~,,ary,' , 73 (19723179.16 11. L. Fri cdmarl. .l ..l fu~ rom o/. 5,~-i .,I-15(It367357I 7 H.L . Friedman, Potym. Lettcr,~,7 (1969) 4 I.18 P. P. Luff and M. White, T h i n S o l i d F i lm s ,6 ( 19703 175.19 A. V. Amelin, O. F. Pozdn yakov , V. R. Regel and T. P. Sanfirova, S o v i e t P h y s . - - S o l i d S t a t e , 1 2

(19713 2034.20 A. M. Beuche, J . Po lym. Sc i . , 19(1956) 297.21 G. P. Shulrnan, J . Polym. Sci . , B3(19653 911.22 J. A. Lane, Br igh ton P o ly techn ic P ro~co t Repor t No . 156.1967.23 Proc . Seven th Conf . Sac . Vacuum Coa te r s , 1964 ,p. 78.24 M. White and P. P. Luff, U.K. Pa ten t App l . 393 /69 ,1969.25 M. Satou, Y. Wat anabe and H. Hayashi , J . Po lym. Sc i . , 10(1972) 835.26 E. W. Fischer, K o l l o i d Z . , 1 5 9(19583 108.27 P .P . Luff, Ph ,D. Thes i s ,Brighton Polytechnic, 1969.28 M. Suzuki, Y. Tana ka and S. Ito, Japan. J . Appl . Phys . , 10(197]) 817.29 M. Szwarc, Disc ' . Faraday Sac . , 2(19473 46.30 W. F. Gorham, J . Polym. Sci . , 4(1966) 3027.31 M. A. Spivack and G. Ferrant e, J . E tec t rochem. Sac . , 116(19693 1592.32 S. M. Lee, N A S A C o n t r ac t N o . N A S 1 2 -2 0 11 ,1969.33 W.E. L o e b , S P E J . , 2 7 ( 1 9 7 1 3 4 6 .34 F. Cariou, W. E. Loeb and D. J. Valley, Proc . Elect ronic" Co m pon ents Con/ i , W ash ington D.C, ,

1965, p. 54.35 M. A. Spivack, Ret'. Sei. Instr.. 43(1972) 985.36 W. D. Niegisch, J. A p p l . P h y s . , 3 8 ( 1 9 6 7 ) 4 11 0 .37 G. N. Jackson, Thin S ol id Fi lms, 5 (19703209.38 L. Holl and and C. R. D. Prie stla nd, Va c u u m , 2 2(19723 133.3~, B. N. (-'hapman. Th in Sc l id Fi/m~ ~' (Iq7 3}R41.40 C.R . I). Pricslland alld S. 11crsee. J~l~tmm.2-~41~723 103.41 A. M. Toxen, Phys . Re~ . , 123(1961)442 .42 I. Brodie, L. T. Lamont and D. O. Myers, J. V~e. Sci . TechnoL, 6(t9693 124.

43 G. N. Jackson, Vacuum, 19(t9693 493.44 L. Hol land and C. R. D. Prie stland, Va c u u m , 2 2(1972) 143.45 C. R. D. Priestland, M. Phi l . Thes is ,Brighton Polytechnic, 1972.46 J. W. Coburn, Rev. Sci . Ins t r. , 41(19703 1219.47 J. W. Coburn and E. Kay, Appl . Phys . Le t t e r s , 18(1971) 435.48 J. W. Cobu rn and E. Kay, J . App l . P hys . . 43 (19723 4965.49 R. E. Jones, C. L. Standley and L. I. Maissel, . I . Appl. Phys. , 38(19673 4662.50 P. A. B Toombs..I. Phv< D , 1 f196~) 662.51 It. R. Koenig Z.llltt L. I. Maissel, IB.1l J . Ru.< De ce/op. , f4(19711) 16,~.52 D. G. Muth, J. Vac. Sci. Teehnot..~h'(1971399.53 S. S. Lau, R. H. Mills and D. G. Muth, J. Vae. Sei . Tec hno l. , 9 (197231196,

54 N. F . Jackson and P. J. Harro p, U.K. P ray. Pa t . 29059 /67 ,1967.55 R. Har rop and P. J. Harro p, Thin So l id F i lms , 3(19693 109.56 E. Mat hias and G. H. Miller, J . Phys . Chem. , 71(1967) 2671.57 D. T. Mor rison and T. Roberts on, Thin So l id F i lms , 15 (19733 87.58 I. H. Prat t and T. C. Lausman, Thin So l id F i lms , 10(1972) 151.

thin polymer films

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