Page 30 - Spring 2025 Electron

Institution of Electronics News

FROM THE ARCHIVES
very different characteristics from those of the bulk have most effect on the structure and properties of an
material. The effect of gas contamination can be re- evaporated layer are:-
duced, however. Firstly the rate of evaporation can
be made much higher than the example given for (i) Beam intensity or rate of evaporation. The grain
aluminium, and secondly, there is a very strong getter- size diminishes with increasing beam intensity,
ing action by some metals, which reduces the gas due to the presence of greater numbers of
pressure to a very low level. It has also been suggested nuclei 8, 9 .
Vacuum Deposition of Thin Films and their Uses that at high beam intensities, the gas molecules are (ii) Angle of vapour incidence. Increasing the angle
swept out of the path of the vapour beam, many being
of incidence favours the formation of large grains
in the Electronics Industry trapped at the chamber walls and as an initial layer with orientation in the direction of incidence .
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on the substrate.
by G. SIDDALL* Assuming that we have all the conditions required (iii) Substrate temperature. The mobility of the
(A lecture demonstration delivered at the Twelfth Annual Electronics Exhibition and Convention of for evaporation to take place, let us consider the various atoms on the surface is increased with substrate
Atoms arriving at the surface
temperature .
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the Northern Division of the Institution of Electronics, of July, 1957; and also given to our London ways of producing thermal evaporation inside the therefore have more opportunity of forming a
Division on October 17th, 1957) chamber. preferred structure.
INTRODUCTION other methods of deposition in common use which re- If the material is conducting, it can be exploded from (iv) Residual gas pressure. At high residual gas
Perhaps it is not generally realised to what extent quire low pressures. Cathodic Sputtering for example a thin wire by discharging a capacitor through it. This pressure the degree of contamination is in-
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the process of vacuum deposition is in use today. is a much older process, which has taken on a new lease method was first used by Faraday in inert atmospheres. creased , which can influence both the structure
Before 1939 it was a technique of somewhat limited of life due to the development of so-called “Reactive An evaporant may be heated indirectly using a resist- and purity of the deposit.
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laboratory use, which was given a sudden impetus Sputtering.” There is also the method known as gas- ance heated refractory metal or an induction heated
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during the early stages of the war by a demand for plating in which a metal is introduced into the vacuum crucible or susceptor . The crucible or the evaporant
chamber in combination, for example as a carbonyl
high efficiency optical systems. The deposition of which is gaseous at normal temperatures. The gas is can be heated by means of electron bombardment. The
magnesium fluoride for anti-reflection coating or lens broken down in the chamber by heat and the liberated method adopted in a particular case must be chosen on
blooming therefore became well-known, and after the its merits. Chemical reactions may take place between
war some manufacturers were left with an expensive metal is deposited on surfaces inside. This is known the source material and the evaporant, and the “wet-
piece of equipment on their hands which stimulated the also as the Pyrolitic process. ting” properties of the molten charge must also be
search for new applications. Undoubtedly some of the It will be interesting from the point of view of com- considered when choosing a suitable source material.
greatest manufacturing outlets have been concerned parison to describe these methods in more detail.
with the decorative finishing of metals, plastics and So far we have examined the vacuum conditions
papers by the deposition of highly reflecting films of Thermal Evaporation necessary for deposition and the ways of producing the
aluminium. In this process a material is heated in vacuum so that vapour. The distribution from a vapour source is also
important, and if one neglects the interaction with the
With the development of better pumping systems, the the vapour atoms emitted may leave the source unim- residual gas in the chamber, an exact parallel exists
potentialities of Vacuum Coating were considerably peded. Under these conditions the vapour pressure of with the distribution of light from a source . Basically
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increased until today very many articles are finished by the heated material will normally be greater than the there are two types of source, the point source, with
this process. One can go into any department store pressure of the residual gases in the chamber. The radial distribution, and the surface source with a dis-
and buy half a dozen articles which have been finished emitted vapour atoms will travel outwards from the tribution according to Knudsen’s cosine law. This is
by vacuum coating, for example plastic toys, buttons, source and condense on the relatively cooler parts of the analogy in the Kinetic Theory to Lambert’s law of
sequins, mirrors, jewellery, indoor fittings and a host of the chamber. The atoms will be propagated along radiation. The practical vapour source is usually
others. Most new cars have aluminised headlamp re- straight paths if the average distance an atom can travel directed to some extent and has characteristics which
flectors, and several other car fittings are finished with before collision with a gas molecule is comparable with lie between the two theoretical possibilities. A know-
decorative aluminium coatings. Practically every piece the physical dimensions of the chamber. The distance ledge of illumination engineering is extremely useful in
of optical equipment one can buy has a bloomed optical an atom can travel without colliding with a gas mole- determining the distribution of evaporated material
system, with considerable improvement in light trans- cule is known in the Kinetic Theory as the mean free arriving at a surface from different arrangements of
mission where there is a large number of air-to-glass path of the vapour atoms in the residual gas and is vapour sources. Reference to illumination theory 7
interfaces. Thus it is a process which is becoming related to the size of the atoms and to the residual gas will show that the distribution to be expected from
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increasingly important and this applies particularly to density. Dushman quotes the following figures for the simple light sources has already been calculated.
the technical applications, since the decorative aspects mean free path of oxygen molecules in ogygen at
are now being thoroughly exploited. The physics of 25°C.:-
vacuum deposition are not generally well known, there- 5.09 cm. at 10 -3 mm. Hg. Of the work that has been published on thin films, Figure 1.
fore I shall outline the fundamentals of the technique, 50.9 cm. at 10 -4 mm. Hg. much has been directed towards a study of film proper- A view of the apparatus inside the chamber of a
and then describe some uses which are already well ties and their relationship to the conditions of forma- modern vacuum coating unit.
established in the electronics industry, together with The figures for water vapour and nitrogen are of a tion. A film can exist in a stable state on a surface
some new and interesting developments. similar order. A working pressure of 10 mm. Hg. either as a monolayer or as a system of agglomerates The techniques of thermal evaporation have been
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depending upon the mobility of the atoms on the sur-
For more detailed information reference should be would therefore satisfy the mean free path condition for face. It has been shown that the size and orientation advancing over the past ten years, and the stage has
plant of normal size. One must also consider the effect
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made to a complete work on the subject . of gas molecules striking the substrate whilst the film is of the crystals can be roughly related to the melting been reached where an adequate pumping system can
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condensing. It can be calculated that at 10 mm., the point of the material concerned, so that films formed be devised to meet most requirements. The technical
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DEPOSITION PROCESSES rate at which oxygen atoms strike a surface at room from high melting point metals generally have small problems which remain are connected with the design
When we speak of Vacuum Deposition we usually temperature, is such that a monomolecular layer could crystallites with no particular orientation whilst low of vapour sources and special jigs for production and
refer to the process of evaporation of a material in high be formed in less than one second. This corresponds melting point metals give large crystallites with orienta- experimental work. Fig. 1 is a photograph of the de-
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vacuum and its subsequent deposition or condensation to about 5 x 10 atoms per sq. cm. striking the surface tion influenced by the substrate. The factors which tail inside a modern evaporation chamber of advanced
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on a preferred base. There are, however, other ways per second. Holland quotes the following practical
of producing a metal vapour under conditions of re- example. If aluminium is evaporated at a pressure of 35
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duced gas pressure, apart from the method of 10 mm. to form a film 500Å thick in 1 minute then it
direct evaporation by heating. In fact there are two can be calculated that over 1,000 gas molecules strike 31
the surface for every aluminium atom arriving there. Become a member: https://membermojo.co.uk/ie
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*Vacuum Coating Laboratory, Edwards High Vacuum Ltd. It is hardly surprising then that thin films often show Become a subscriber: https://membermojo.co.uk/subscriber
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