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Research on Vacuum Evaporated and Cathode Sputtered Thin Films

Report Number: WADD TR 60-381 Part III
Author: Belser, Richard B.
Author: Woolf, William E.
Corporate Author: Engineering Experiment Station, Georgia Institute of Technology
Laboratory: AF Avionics Laboratory
Date of Publication: 1963-10
Pages: 112
Contract: AF 33(616)-6379
Project: 4150
Task: 415003
AD Number: AD0422916


Some 1,000 metal films have been deposited by vacuum evaporation or sputtering on glass or monocrystals of NaCl, MgO or LiF and the structures of 628 were analyzed by electron or X-ray diffraction. A yield of 24 per cent parallelly oriented (PO) films for the total effort was increased to 49.5 per cent during the last 10 months; a yield > 80% could be expected by utilizing the best techniques developed.

For films of gold and silver deposited on NaCl by evaporation a closed oven was found conducive to successful PO film growth; temperatures and rates employed were, respectively, 210°-300°C and 200-300 A/min for gold and 130°-220°C and 120-800 A/min for silver. PO gold (35) and silver (23) films were deposited on NaCl by sputtering with yields greater than 60% at temperatures of 110°-310°C for gold and 25°-130° for silver. No PO gold films were ever obtained on MgO (34) by either the evaporation or sputtering techniques.

During the first phase of the work PO copper films (146) were deposited readily on NaCl or MgO with 51.5% yield at temperatures in the range of 180°-700°C using a hot plate or carbon induction furnace heater; in the current phase PO iron (7) and nickel (13) films were deposited on NaCl, MgO or LiF with yelds of 100% and 54% respectively. MgO and LiF, in general, were better as substrates for these metals than NaCl. Temperatures were in the range 185° to 450° C and rates from 150 to 2500 A/sec. It is evident that PO films of copper, iron and nickel can be grown under much less critical conditions than PO films of gold or silver.

Plots of electrical resistivities versus substrate temperature at deposition temperature ranges in which PO growth was normally obtained. However, minimum resistivities of copper and silver on NaCl were high, > 2 ρb, compared with gold on NaCl or copper on MgO ~ 1.1 ρb.

Films of the rare earth metals dysprosium (7) and thulium (27) were prepared by evaporation onto glass and the electrical resistance of the films were monitored with changes in time, temperature and atmosphere.  Initial resistivities (in vacuo) and final resistivities (in air) were shown to be dependent on thickness, deposition rate and substrate temperature at the time of deposition.  Lowest resistivities (1.8 ρb) were noted for annealed films, thicker films, and films deposited at higher rates.  A temperature coefficient of resistance of 0.0030/°C was found for one thulium film.  This is much higher than the bulk reported value (.00195/° C), which appears to be wrong by comparison with other metals.  The films were strongly adherent to glass and of normal metallic appearance.

The deposition of 79 InSb films on NACl, 33 during the final phase, did not produce any parallely oriented films. Sputteriing produced nearly stoichiometric InSb films but evaporation did not. Resistivities in the range of 221 to 0.25 ohm-cm were recorded for films for which thicknesses were measured. The best films (3 exhibiting some PO material) were deposited by sputtering at temperatures of 375° C. Resistivities were in the range of 0.2 to 2 ohm-cm. The sticking coefficient of the deposit atoms is very low at this temperature, about 20%. A higher pressure than is provided by either normal sputtering or evaporation in vacuo appear as worthy of further investigation. Employment of the sputtering of InSb on other substrates than NaCl should be undertaken.

In conclusion, the conditions required for successful epitaxial growth of metal films on single crystal substrates are particular to the metal-substrate pair. Whereas for noble metals the conditions for successful parallelly oriented growth appear restricted to rather limited substrate choices, substrate temperature ranges, deposition rates and heating methods, and other very favorable conditions (the closed oven or the sputtering plasma); more active metals such as copper, nickel and iron can tolerate wider and less well controlled ranges of these same conditions. This contrast in behavior suggests that the presence of a strong tendency toward chemical bonding between the metal and an element of the substrate or the presence of a favorable environment for chemical reaction such as the sputtering plasma greatly enhance the probability of achieving parallelly oriented metal film growth upon a selected substrate.

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