Page 37 - Spring 2025 Electron

and their structure is usually more uniform. For ex- (iv) Conducting transparent films of gold and of
cadmium oxide are in use for electrically heated
are thicker for a given order of resistance value. They ample, a nickel chromium film of 100 ohms/sq. would windscreens in cars and aeroplanes , for pre-
be only about 100Å units thick compared with a
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are also usually deposited under conditions which cadmium oxide film of 1000Å having the same resist- venting the formation of ice or mist. These films
ensure that they are partly oxidised during formation, ance value. are capable of operating up to about 180°C., the
thus any further oxidation in the atmosphere is unlikely best may have optical transparencies of 80% for
unless the films are again raised to the high temperature The choice of substrate for the deposition of conduct- a film resistance of 8-10 ohms/sq. Conducting
of their formation, which is usually in excess of 300°C. ing films for resistance coatings by evaporation or by glass is being produced commercially in Britain
One can almost say that they are already protected by sputtering is limited to certain types of base material. and in the U.S.A.
their own oxide layer. Films of these alloys on glass In the first place the base must either be non-volatile,
can be made stable to within ±½% for values up to or the volatiles must be removable without the base
about 300-400 ohms per square and deposited with losing its mechanical properties. It must also not con- Electroluminescence
about the same order of accuracy. Because of the high tain so much volatile material that the vacuum pumps One very important application of conducting highly
temperature of deposition, the range of base materials cannot achieve low pressures. In spite of this limita- transparent films is now of interest to electronic
is restricted to glass, quartz, mica and other crystalline tion many organic and inorganic materials are suitable. engineers.
bases. If one requires the deposition of a stable conducting
Reference has already been made to the use of thin layer, however, the base must also withstand higher
gold films deposited on bismuth oxide in the section temperatures. Stable evaporated films are limited
dealing with cathode sputtering, but there are many mainly to chromium alloys for which the base must be
more combinations which are of use. Gold, platinum raised to at least 300°C. during deposition, similarly
CONSTANT CHARGE and silver have all been deposited between different during cathodic sputtering of oxides and metals the
base is subjected to heavy ion bombardment with con-
ELECTROSTATIC LOUDSPEAKER sandwich layers on glass with good results. No figures sequent increase in temperature. Glass, quartz, and
are yet available on stability, but it seems likely that
Figure 6d. the upper limit of resistivity for stability is lower than certain ceramics, have been used preferentially, but
Basic arrangement of an electrostatic loudspeaker for for the chromium alloys, probably being of the order some success has also been achieved with laminated
high fidelity reproduction of sound. 150 ohms per square. glass cloth, bonded with special resins such as silicone,
charge characteristics, which are necessary to avoid Higher orders of resistivity may be obtained by the epoxy, polyester, and melamine.
non-linearity distortion. It is interesting to note that evaporation of carbon films or certain semi-conducting Tests on the stability of films deposited on various
the high resistance may be incorporated in the conduct- oxides such as iron oxide or chromium oxide, but the organic substrates as opposed to glass are being carried
ing coating itself. Authorities on the subject 31, 32 have most promising technique for semi-conducting oxide out in our laboratories. An upward drift is apparent
pointed out that this would ease the problem of match- and nitride production is Reactive Sputtering. The in many of the films, but some are showing promise of
ing the output to a capacitive load by providing importance of this technique lies in the fact that the stability. Nickel-chromium alloy deposited on glass
distributed resistance and capacity, thus giving some conductivity of a particular oxide film can be varied laminates is already on the market in the United States
characteristics of a transmission line with much more repeatedly over quite wide limits by control of the and high stability is claimed. It has the advantage of
constant impedance. amount of oxygen in the otherwise inert atmosphere of flexibility, and can also be punched and stamped out
e 0
An apparatus has been made to demonstrate the the glow discharge . For example, when sputtering into odd shapes. Colloidal silver contacts are provided,
and the film is claimed to be protected. It is produced
simple construction of an electrostatic speaker for cadmium oxide, if pure oxygen is used in the discharge, by Filmohm Corporation, New York. ELECTROLUMINESCENT PAN EL
the resultant film is almost completely non-conducting,
treble response; it consisted of aluminised " Melinex " but if a mixture of 5% oxygen 95% argon is used, films
stretched across a perforated grid. The speaker was of 100 ohms per square may be readily produced, and Some of the better known applications of resistive Figure 7.
energised by a small transistor oscillator delivering 1.5 by varying the concentration of oxygen, the resistance coatings which have been produced by vacuum deposi- Schematic diagram of the construction of an electro-
watts from three pocket torch batteries. The output of a film of the same order of thickness, i.e., about tion are given below:- luminescent panel with improved dielectric properties.
was of the order 300V r.m.s. at 1.5 kc/s and the sound 1000Å, may be progressively increased up to the value Waveguide attenuator vanes have been in pro-
was sufficient to fill a medium sized lecture hall. The of several megohms. Similar effects are observed with (i) Phosphors have been produced which emit light
demonstration only serves to illustrate the basic prin- the oxides of tin and indium, but the values are not so duction since about 1949; automatic plant is in under the influence of an alternating field of the order
ciple of the electrostatic loudspeaker, yet it is certain repeatable. Unfortunately cadmium oxide films have use in the U.S.A. for the evaporation of nickel 50,000 volts/cm. These are known as electrolumines-
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that now plant is available for the continuous aluminis- to be protected against acid atmospheres, but this may chromium alloys on to glass. cent phosphors . The construction of a light source
ing of " Melinex " then the electrostatic speaker in its be done quite easily by means of another evaporated or (ii) Anti-static coatings have been used on meter takes the form of a parallel plate capacitor with one
most refined form becomes a commercial probability. sputtered dielectric layer or by a protective layer of a glasses and cases, and in certain types of par- electrode optically transparent, the phosphor being sus-
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pended in the dielectric. The schematic diagram Fig. 7
resin based lacquer. ticle accelerators to prevent the build-up of shows the construction of an improved electrolumines-
Resistive Coatings Preliminary experiments on the stability of unpro- charge on non-conducting surfaces . High cent light source. Sputtered cadmium oxide is used for
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resistance films of metals and oxides are used for
Before discussing specific examples of resistive coat- tected cadmium oxide films show that there is an initial transparent coatings, lower resistance coatings the transparent conductor because of its high trans-
ings, let us first consider the basic limitation. ageing effect in which the resistance decreases by 8% may be almost opaque. parency and good current carrying capacity, and
evaporated aluminium is used for the back electrode.
Metal films having a useful order of resistance are during the first week, thereafter there is a gradual in- (iii) Thin film resistors are being used at present only This also makes an effective reflector to increase the
usually so thin that they are completely unstable, partly crease in resistance of up to 5% after a period of 3 for specialised applications, for example, as efficiency of the lamp, and has self-healing properties
due to atmospheric oxidation and partly due to their months. Data cannot be found for other oxides but bolometers , proximity gauges, and for certain in the event of electrical breakdown. The phosphor
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inherent granular structure. Some exceptions to this there is every reason to suppose that they are at least types of potentiometer. Large area films have itself is sprayed on, in suspension with a melamine
are special high resistance alloys containing nickel and as stable as cadmium oxide. The given figures for also been used for potential plotting. The possi- resin in solvents, and to improve the electrical charac-
chromium and some noble metals when sandwiched cadmium oxide refer to unprotected films. Cadmium bility of using resistance films on flexible plastic teristics a further dielectric layer is included. This may
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between protective layers. oxide is known to be subject to atmospheric deteriora- base materials which can be cut to required consist of a separate melamine resin film, or a laminated
tion so that protection might be expected to give better
If
The alloys containing nickel and chromium already results. Oxide films give more promise of stability dimensions is only now being realised. sheet of thin “ M e l i n e x . ” the “Melinex” is used,
have a higher bulk resistivity, and therefore the films than metal films because they are very much thicker
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