Correlation between thickness distribution of evaporated thin film and emission characteristic of a vapour source

Vacuum

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of e v a p o r a t i n g the film material in ultrahigh v a c u u m onto substrate surfaces prepared inside the vacuum chamber. The a d v a n t a g e that this t e c h n i q u e has over the o t h e r m e t h o d s of film p r e p a r a t i o n is t h a t the c o n t a m i n a t i o n of the substrate a n d deposited film can be kept very low. Some of the films m a d e u n d e r these clean c o n d i t i o n s grow by the generation and coalescence of t h r e e - d i m e n s i o n a l nuclei. O t h e r s exhibit m o n o layer-like growth. Films whose g r o w t h begins with the formation of nuclei usually contain a b o u t 10 TM dislocation lines per cm'-' a n d may also c o n t a i n stacking faults a n d microtwins. Most of these defects are generated when n u c l e i - - w h o s e lattices are either displaced or rotated relative to one a n o t h e r coalesce to form a c o n t i n u o u s layer. The films which exhibit monolayer-like growth are particularly interesting if the b o n d i n g across the irtterface between o v e r g r o w t h a n d substrate is strong, a n d the misfit between the two lattices is small (less t h a n a b o u t 5 per cent). U n d e r these c o n d i t i o n s the initial layers of deposit are elastically strained to give a c o h e r e n t interface between o v e r g r o w t h a n d substrate. T h e defect c o n t e n t of films whose growth begins with the f o r m a t i o n of a c o h e r e n t interface seems to be controlled by the defects present in the substrate. It has been f o u n d t h a t films which contain few dislocations, s t a c k i n g faults a n d twins can be g r o w n on carefully p r e p a r e d substrates. It is also possible to prepare films which have the crystal structure of the s u b s t r a t e rather t h a n the n o r m a l s t r u c t u r e of the o v e r g r o w t h material. Single-crystal films of fcc c h r o m i u m , ?-iron a n d fi-cobalt can be grown at r o o m t e m p e r a t u r e by depositing the metals in ultrahigh v a c u u m o n t o c o p p e r or nickel surfaces prepared inside the v a c u u m chamber.

Correlation between thickness distribution of evaporated thin film and emission characteristic of a vapour source

Boguslaw Stepien, Instytut Tele 1, Radiotechniczny, Warsaw 4, Poland D e t e r m i n a t i o n of the emission characteristic of a v a p o u r source can be d o n e from the measurernents of the thickness distribution of the deposit on a flat "'receiving" surface. However, for an axially symmetrical source there is often some deviation of its axis from a n o r m a l to the "'receiving" surface. In such a case thickness d i s t r i b u t i o n is a s y m m e t r i c and so the lines of equal thickness of an e v a p o r a t e d film are not f o r m i n g circles but ellipses. This p r o b l e m is of great i m p o r t a n c e for the source with s t r o n g directional characteristic, eg " c h i m n e y " source r e p o r t e d by D r u m h e l l e r . The influence of the a b o v e - m e n t i o n e d deviation on the thickness distribution for two emission characteristics of the v a p o u r source, viz f/fl/_~ cos 20 and f / 0 / ~ cos t), has been given. C o m p a r i s o n between theoretical thickness distribution calculated for a given emission characteristic a n d experimental thickness d i s t r i b u t i o n can only be done if the latter is symmetric, so " s y m m e t r i z a t i o n ' " of experimental asymmetric thickness d i s t r i b u t i o n has to be done. Two modes of s y m m e t r i z a t i o n has been discussed. T h e first m o d e consisted m calculating mean thickness from two m e a s u r e m e n t s performed at the same distance from the centre of the "'receiving" surface but on its opposite sides. The second m o d e consisted in rotating "receiving" surface d u r i n g e v a p o r a t i o n . It has been f o u n d that for emission characteristics f/0/"cos 20 only the first m o d e of s y m m e t r i z a t i o n - provided that 146

m e a s u r e m e n t s are done only for points lying on the long axis of the ellipse of equal thickness give experimental thickness distribution in agreement with calculated one. For the surface source f / 0 / > cos 0 the mode of symmetrization does not influence the agreement between theoretical and experinlental thickness distributions. In both cases the resulls are the same a n d in a g r e e m e n t with theoretical predictions. Based on the a f o r e - m e n t i o n e d results the thickness distribution of the film e v a p o r a t e d from the D r u m h e l l e r source has been investigated. The asymmetric results has been symmetrized according to the first mode. A c o m p a r i s o n was made bet~veen experimental thickness distribution and different theoretical ones.

Programmed control of vacuum deposited multilayers

J English, T Putner and L Holland, ('entral Research Laboratory, Edwards HL~'h Uacttttm International Limite~L Manor Royal. Crawh'y, Sussex, L)t,~,ktnd In the p r o d u c t i o n of multilayer thin film devices, made on a process cycling basis, the coating must exhibit a consistency in (a) o p e r a t i o n a l characteristics, (b) p e r f o r m a n c e and It) quality. Thin fihn studies have shown that these factors are d e p e n d e n t on the c o n d i t i o n s prevailing d u r i n g the growth of the films in the coating. A range of i n s t r u m e n t s have been developed for m o n i t o r i n g those factors which have greatest influence on the properties of a depositing film. For example (i) crystal oscillators are used to measure rnass deposited and this q u a n t i t y can be translated into deposition rate (ii) optical p h o t o m e t e r s are used to measure thickness in terms of the film's optical characteristics (iii) partial pressure m e a s u r e m e n t and gas analysis of the residual gas a t m o s p h e r e can be made using mass spectrometers and (iv) the t e m p e r a t u r e of the substrate can be m o n i t o r e d by a t h e r m o c o u p l e a n d may be controlled from this. Experience has s h o w n that differences in film characteristics from cycle to cycle are introduced by the o p e r a t o r due to subjective j u d g m e n t . With the aid of the i n s t r u m e n t s referred to above, it is now possible to eliminate effectively the o p e r a t o r by an a u t o m a t i c system which can be p r o g r a m m e d to control the growth rate, substrate temperature, chemical c o m p o s i t i o n a n d thickness of the films in a coating. T h e a u t o m a t i c coating plant to be described is used to deposit multi-layer optical interference films. For this purpose, the thickness of each layer is determined by p h o t o m e t r i c m e a s u r e m e n t of reflectance or transmittance, a u t o m a t i c a l l y t e r m i n a t i n g deposition either at a precise reading or at a m a x i m u m or m i n i m u m , d e p e n d i n g on the coating design. Films are prepared by a reactive e v a p o r a t i o n process and in this case it is i m p o r t a n t to provide the correct growth rate and active gas a t m o s p h e r e conditions. Rate of deposition is maintained c o n s t a n t using a quartz crystal mass m o n i t o r a n d rate meter with feedback control. The gas c o m p o s i t i o n is controlled on open loop by a d m i t t i n g a suitable gas mixture t h r o u g h a preset control valve. Investigations of the influence of the gas c o m p o s i t i o n on the film characteristics have been made using a partial pressure gauge. A p r o g r a m m e d sequence controller based on transistor logic was developed to control the process cycle of a coating, using up to four sources for different materials, each with its own preheat a n d e v a p o r a t i o n conditions. Each layer may be terminated at a particular photo-