Substrate temperature in thin film deposition

Short communications

Substrate temperature in thin film deposition

X Solid line. Upper face x Dashed line. Lower face o Solid line. Upper face o Dashed line, Lower face

Measurements are described on a substrate heater which allows easy setting o f the Io wer substrate temperature. Large temperature differences across the substrate are noted.

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In the vacuum deposition of thin films a knowledge of the substrate temperature is important because it is one of the critical parameters in film growth, influencing several factors such as the sticking coefficient, nucleation conditions and the degree of epitaxy obtained on single crystal substratesL Magnetic film parameters are also determined in part by the substrate temperature. Conflicting reports on orientation ~,3, and on Hall mobility~'% have been reported for supposedly similar substrate temperatures. Most authors report the substrate temperature without mention of where it is measured 6. Only a few give detailsL The commonest method of heating the substrate is by radiation onto the side of the substrate away from the evaporation source a. An elegant method, depending on conduction, has recently been reported 9. The method reported here also depends on conduction. However the main purpose of this paper is to show the difference which can exist between upper and lower faces of the substrate. These appear larger than the values (20°C and 90°(2) quoted a,a. A simple, cheap, substrate heater was made for thin film deposition in an Edwards 12E6 vacuum coating unit. The copper plates are each 10 cm by 7 cm by 0.2 cm thick and the heater is mounted between two of these. The third plate is recessed to take a glass microscope slide which is 0.13 cm thick. The exposed area of the glass is 2.1 cm by 6.2 cm. This third plate can easily be detached from the other two so that the heater is not disturbed. The electrical supply to the heater was from an auto-transformer with an Avometer to monitor the current as in Figure 1. The temperatures of the upper and lower

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Figure2. Upper and lower substrate face temperatures versus heating current for two different pressures.

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faces of the substrate were measured using 1 mm dia iron/constantan thermocouples obtained from Messrs Spembley Technical Products Ltd. The thermal emf's were measured on a Muirhead potentiometer type D-972-A. The temperature difference was recorded on a Servoscribe RESI 1 chart recorder. The results are given in Figures 2 and 3. For the method of heating used very large temperature differences occur. This difference increases with increase in temperature of the upper surface of the substrate. Using Figure 2 it is possible to select any desired temperature for the lower surface of the substrate by setting the temperature of the upper surface. One obvious difficulty in this calibration

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Figure 3. Temperature difference across substrate versus heating current for two different pressures. was being certain of good thermal contact between the lower thermocouple and the substrate. Light contact with a sharpened point was felt to be too uncertain. Fixing with silver dispersion or an araldite/ copper-dust glue was tried but the final method was clamping under

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Short communications a small area pad of Sindanyo, which is an electrical insulating asbestos. Thin film thermocouplesz° would have given certainty but were not adopted due to their limited use in practice to special problems ~t. Several experimental runs gave substantially the same results, showing a considerable substrate temperature difference exists between the heater and source surfaces when using a contact conduction-type heater. The front surface substrat¢ temperature can be set by a previous calibration experiment. References 1 D B Holt, J Mater Sci, 1 (3), 1966, 280. 2 R Zuleeg, Sol-State Elect, 6, 1963, 193. a j F Hall, J Opt Soc Amer, 45, 1955, 717. .a j Dressner and F V Shallcross, J Appl Phys, 34 (8), 1963, 2390. R S Muller and B G Watkins, Proc IRE, 52 (4), 1964, 425. 6 D M Hughes and G Carter, Vacuum, 17 (10), 1967, 555. 7 A Catlin and R R Humphris, Basic Problems in Thia Fihn Physics p 175. Vandenhoeck and Ruprecht, Gottingen (1966). s M M Hanson, P E Oberg and C H Tolman, J Vac Sci Tech, 3 (5), 1966, 277. 0 C J Robbie and C T H Stoddart, J Sci h~strum Set 2, vol I, 1968, 56. lo R Marshall, L Atlas and T Pumer, J Sci hlstrum, 43, 1966, 144. ii M V Belous and C M Wayman, J Appl Phys, 38 (13), 1967, 5119. W R Oliver Robert Gordon's Institute o f Technology School o f Physics Schoolhill Aberdeen Scotland

Barium carbonate deposits on valve components The processing of an electron tube starts with pumping and beating in order to degas all components thoroughly. For economical reasons, especially in the mass production of radio valves, these treatments should be completed within a short time. By heating the cathode sleeve to about 1400°K the alkaline earth carbonates decompose quickly, which causes a rise in the carbon dioxide pressure. According to Lander ~ the equilibrium pressure of carbon dioxide over barium carbonate amounts to about l0 tort at 1400°K. Part of the carbon dioxide is reduced to carbon monoxide by the binder residue and other reducing agents. The gas mixture evolved is in general exhausted by the pump through "the stem", a thin and relatively long tubular connection. From our experiments the conclusion was reached that the aerodynamic resistance of the stem usually hampers a quick removal of the gas. During the pumping process a pressure of about I torr was observed, which delays the decomposition of some of the carbonates. At the end of the pumping process there may still be a pressure of about 2 >: l0 -2 tort carbon dioxide. Due to the high temperature used during the decomposition of the carbonates some barium oxide evaporates. The oxide is deposited at the grids, the anode and the mica spacers which are at a lower temperature. These deposits, while forming, are exposed to a carbon dioxide pressure which exceeds and gradually approaches the equilibrium pressure of barium carbonate at the lower temperature (according to Lander 2 × l0 -2 torr at 1070°K). It is likely that the oxide deposits will at least partially be converted into barium carbonate. From these considerations it may be concluded that the low carbon dioxide pressure often observed during life originates from those barium carbonate deposits which where formed during pumping and escaped from decomposition during screening. By extrapolation according to Lander the equilibrium pressure of carbon dioxide over barium carbonate amounts to 6.10 -1° torr at 673°K. The carbonate deposits will slowly decompose during functioning of the valve on life. Reference I j j L'ander, J Amer Chem Soc, 73, 1951, 5794. J van den Berg and H J R Perdijk N V Philips' Gloeilampenfabrieken Elcoma Central Physico-Chernical Laboratory Eindhoven The Netherlands

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