the erratic behaviour of thin metal films is well known and is the subject of an extensive literature , but as shown by the foregoing , a better understanding of film properties is beginning . summarizing the introduction , it can be stated that the anomalous electrical properties of films are principally due to their structural imperfections and to the thermodynamic instability produced when metal vapour is abruptly condensed to the solid phase . the changes of resistivity and temperature coefficient of resistance , which occur upon heating or ageing a film , arise from the re-ordering of the structure , the relief of high internal stresses and the further oxidation or gas absorption of the film . these changes are parallel to those occurring in fine resistance wires upon annealing after cold-working . however , the gas absorption and higher degree of lattice imperfection in vacuum-deposited films cause much greater variation of properties . there is an increasing amount of evidence that high stability resistance films can be obtained by correct annealing treatment and suitable protection . it is the purpose of this paper to describe a method of making reasonably stable resistance elements by the vacuum deposition of nickel-chromium alloy on glass and to discuss their properties in terms of the processing conditions . 2 . practical requirements . substrate . the surface of the supporting substrate should be smooth and uniform , and both chemically and mechanically stable at temperatures up to about 350 &amp;deg; C in atmosphere and vacuum . any variation of surface smoothness gives a corresponding variation of film resistance value , because the film is thin enough to be greatly affected by the state of the surface . for example , a film of resistance as low as 10 O &amp;sol; square on a polished glass surface may be discontinuous when deposited under identical conditions on a finely ground glass surface . it is a characteristic of a film deposited from the vapour that the grains tend to grow on surface prominences , which trap the atoms first arriving there and act as centres for nucleation . films as thick as 1000 A&amp;ring; may be discontinuous when deposited on a coarse surface because large grains are formed which do not touch , and the thickness must be increased before conductivity is observed . such films tend to be unstable because their conductivity depends upon contacts between large grains . the most suitable substrate materials are found amongst glasses and ceramics . good results have already been obtained using glasses of high silica content such as Pyrex or Vycor . these are two of the few glasses unaffected by water vapour . many other glasses , including some borosilicates , devitrify in contact with water , and their surfaces become powdery because small crystals of metal silicates are formed . soda-lime glasses are not used because their surfaces are also chemically unstable . during flame polishing , when the glass is bombarded by thermally produced gas ions or ionic bombardment in a glow discharge , free sodium ions are active at the surface of the glass . they combine with water vapour from the gas atmosphere to form sodium hydroxide and deliquescent sodium silicate by reaction with silica in the glass . some further reaction with the deposited film must be expected . ceramics possessing good chemical and mechanical properties are available ; however , their surface smoothness is often variable because of the sintering process used in their manufacture . glazing is not always a satisfactory solution to this problem because standard glazes are often based upon some of the unsuitable glasses already described . very careful examination of surface smoothness is needed when choosing a ceramic material for the support of vacuum-deposited films . the temperature coefficient of linear expansion of metals is usually an order higher than that of glass or ceramics , and this factor partly contributes to the high internal stresses which have been observed in thin films . however , once the films have been annealed , the effect of the expansion of the base on the resistance of the film is very small , compared with the average temperature coefficient of resistance of films , and is insignificant when compared with that of bulk metals . the resistance alloy . in the early stages of deposition of a metal film , aggregates of metal atoms ( nuclei ) are formed on the substrate . the number of nuclei is dependent upon the physical and chemical properties of the metal and substrate , and upon the rate of deposition . as the nuclei increase in size they grow together , eventually to form a continuous film . the second stage of growth is marked by the onset of electrical conductivity , and the rate of change of resistance with film thickness is very high . unfortunately , the most useful resistance values coincide with this unstable region of thickness for many metallic conductors . the most successful high-resistance films have been made by depositing chromium and alloys of chromium with nickel , silicon , titanium , etc . for example , nickel-chromium alloys have a high bulk resistivity ( 80-130 mO cm ) and therefore films of this alloy are much thicker than films of the pure metals for the same resistance value . films of resistance 400 O &amp;sol; square are at least 80 A&amp;ring; thick , more or less continuous and are outside the very unstable region of thickness . nickel-chromium alloys also have a low temperature coefficient of resistance in bulk , and are very resistant to chemical attack because of the compact protective oxide layer which forms in contact with an oxidizing atmosphere . the formation of double oxides having a spinel structure has been shown on nickel-chromium alloys under examination by electron diffraction , and this reason has been given to account for their high chemical stability . evaporation conditions . the lowest values of temperature coefficient of resistance and the best stability are achieved in films deposited under conditions favouring oxidation . during deposition the substrate is heated to relieve the internal stress in the film , but this treatment can also increase the rate of oxidation . the residual gas atmosphere in the chamber of a kinetic vacuum system is highly oxidizing due to the high proportion of water vapour at the normal working pressure ( 10-4 mm Hg ) . assuming that the partial pressure of water vapour is only 10-5 mm Hg , then it is calculated approximately that 5 x 1015 molecules cm-2 s-1 strike the substrate surface . if the rate of deposition of chromium metal is about 3 A&amp;ring; s-1 , then ten water vapour molecules strike the surface for every chromium incident atom . thus there is sufficient oxygen ( in the form of water vapour ) available at the source for the film to be highly oxidized at normal rates of deposition . oxidation also occurs after the chromium atoms have left the vapour source , giving rise to the familiar gettering effect . the fall in pressure can readily be observed on the vacuum gauge . thus the first few atomic layers deposited during the gettering period are highly oxidized , and when the chamber has been cleaned up the deposit is more metallic . after evaporation ceases , the deposited film remains open to oxidation . thus the deposited film is inhomogeneous and approximates to a sandwich layer of oxide &amp;sol; metal &amp;sol; oxide , in which the two outer layers are more highly oxidized than the inner layer . the exact state of oxidation of the deposited film is unknown and a further effect of oxidation can be observed upon baking in air . the final resistance change upon annealing may then be positive or negative , because the decrease attributed by Vand to lattice transformation may be greater or less than the increase due to further oxidation . heat treatment and protection . heat treatment carried out during or after deposition serves three purposes : ( 1 ) high internal stresses in the film are relieved ; ( 2 ) some defects in the crystal lattice are removed , thus improving the heat-stability ; ( 3 ) a protective oxide layer is completed , making the film less subject to external atmospheric attack . in practice , it has been found advisable to heat the substrate in vacuum before deposition to a temperature of at least 300 &amp;deg; C . a further heating period in air for 30 min at 300 &amp;deg; C completes the annealing of the film . the electrical properties of resistance tapes and wires are stabilized by annealing and by cyclic baking in air or hydrogen . this treatment reduces the strains and dislocations set up during the drawing of the wire . thus the treatment required by a vacuum-deposited film is similar . baking during and after deposition re-orders the crystal lattice , and improves the resistance stability with time , also forming a compact oxide surface layer . several fast baking cycles carried out in air hasten the changes of resistance up to 300 &amp;deg; C , which become smaller with each successive cycle . 3 . experimental work . evaporation technique . the preparation of nickel-chromium resistance films was carried out in a vacuum deposition plant having a 12 in diameter chamber equipped with pumps capable of reducing the residual gas pressure in the vacuum chamber below 10-4 mm Hg in 5 minutes . provision was made for two h.t lead-through electrodes ( for cleaning by positive ion bombardment ) , three electrodes for the evaporation source and several smaller electrodes for connecting the radiant heater , thermocouple , and resistance monitor . the evaporation source was heated by electron bombardment ( fig 1 ) . this source consisted of a stainless steel supporting block ( forming the anode ) on which was mounted a 1/4 in diameter special ceramic hearth 1/4 in high . nickel-chromium wire ( 22 s.w.g ) was fed through a stainless steel guide tube to the centre of the hearth . the feed mechanism was mounted at the side of the hearth , and allowed the wire to be fed either continuously or to be intermittently operated by a handwheel outside the vacuum chamber . the nickel-chromium wire was bombarded by electrons emitted from a small hot filament of 0.020 in diameter tungsten wire ( forming the cathode ) , supported 1/8 in above the top of the hearth . the cathode heater supply was obtained from a 8 v , 100 A transformer with secondary winding insulated from earth and primary for 15 kv . the anode and cathode were connected across a suitable h.t supply , having a maximum power of 1.5 kw at 3 kv ; the anode was held at earth potential and the cathode at negative 3 kv . the top of the hearth was hollowed to enable the wire to melt and form a bead from which evaporation could take place . substrates and workholders . special jigs were made to hold flat specimen plates of Pyrex , soda glass and ceramic . during each evaporation , the resistance of one plate was monitored by connecting the end terminals to an external circuit for resistance measurement . a simple ohmmeter was used for monitoring the resistance value during evaporation . the accuracy of the measurements was only of the order &amp;plusmn; 2 % , but the results were used only to indicate the approximate value of the resistance during evaporation . a bridge method of measurement was used to determine accurately the resistances of the slides , and is described more fully later . the workholder consisted of a simple jig constructed so that only 1/8 in at each end of the slides was masked by the clamp , and these were placed next to the monitor plate . the workholder was supported 4 in above the evaporation source by means of a tripod . a radiant heater , dissipating 750 w at 110 v , was mounted above the workholder to raise the temperature of the substrate to 300 &amp;deg; C before evaporation . the temperature of the substrate was measured by means of a chromel-alumel thermocouple placed inside the vacuum chamber with its junction resting on the top face of the workholder . the thermocouple was connected to an external meter calibrated to read degrees centigrade , covering a temperature range from 0 to 500 in divisions of 10 degrees . a special chamber assembly was constructed for deposition of films on cylindrical formers and , for monitoring their resistance , a static flat glass slide was used . by experiment a simple relationship between the resistance of the static plate and the resistance of the cylindrical formers was obtained , thus enabling the evaporation to be roughly monitored . contacts . the method of making contact to the deposited film influenced both the accuracy with which the film could be measured and the ultimate stability . 