Ion-Assisted Deposition of Protective Overlayers for Magneto-Optic Alloys
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ION-ASSISTEDFOR DEPOSITION OF PROTECTIVE OVERLAYERS MAGNETO-OPTIC ALLOYS. Kenneth D. Cornett, Ursula 3. Gibson, Anthony Taylor, Optical Sciences Center. University of Arizona, Tucson, AZ 85721.
Abstract Rare-Earth Transition-Metal alloys such as Tb-Fe-Co are being studied and used as magneto-optic data storage materials. These materials are susceptible to oxidation by either oxygen or water vapor, particularly the rare earth component. Pitting corrosion is also a problem when protective overlayers have pinholes or a porous microstructure. Both degradation mechanisms are significant for application of this material to optical data storage. We have used ion assisted deposition (lAD) to produce protective overlayers of refractory oxides, such as A1202 and ZrO 2. These layers were deposited both with and without IAD onto iron films and exposed to environments with controlled temperature and humidity. A scanning micro-reflectometer capable of detecting micrometer-sized pinholes was used to monitor the degradation of the iron layer with exposure time. Introduction lAD is known to alter the microstructure of evaporated films[ 1-3] and affect the corrosion resistance of bilayer structures[4]. We chose AI202 and ZrO 2 since they have received considerable study with lAD, but have not been as widely studied as protective overlayers for TbFeCo. They also have different preferred microstructures and behave somewhat differently under lAD. We also chose to use evaporated Fe films as a degradable media rather than TbFeCo. This is both practical and purposeful, since the investment required to deposit TbFeCo is large. TbFeCo samples received from other sources must have some kind of interim protective overlayer which would first have to be removed, and may not be completely effective. Immediately overcoating an iron film with the desired overlayer without breaking vacuum was a preferable first step. Furthermore, pinhole formation studies on Fe/overlayer systems should be a good test of overlayer integrity. Once the parameters to produce the best overlayers on iron were determined, optimimized layers could be applied to TbFeCo for further evaluation. Deposition Procedure All films were deposited in a vacuum system equipped with a liquid nitrogen trapped diffusion pump, resistive and electron beam evaporation sources, and a sample introduction system on a stainless steel belljar. Base pressures measured by an ion gauge were in the range of 2-4x 10-• torr. For most films, a residual gas analyzer (RGA) was available to measure the partial pressures of several gases. A Kaufman-type ion source was used to ion sputter the substrates prior to deposition as well provide ion assistance during the deposition of the protective overlayers. Argon gas was used in the ion mill at all times. No changes in the partial pressures of residual H2, H20, N2, 02, or CO2 were observed when Ar was flowed through the ion source. The total system pressure measured by the ion gauge was approximately lxl0-4 torr during operation of the ion source. Iron was res
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