Ibad Diffusion Barriers Between YBaCuO and Si
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IBAD DIFFUSION BARRIERS BETWEEN YBaCuO AND Si ANTON GREENWALD,* JAMES HIRVONEN,* NARENDRA JAGGI, # and JOHN ALLEN" * Spire Corporation, Patriots Park, Bedford, MA 01730 # Dept. of Physics, Northeastern Univ., Boston, MA 02115 ** CVC Products Inc., 525 Lee Road, Rochester, NY 14603
ABSTRACT Different materials were investigated for use as diffusion barriers between silicon and yttrium barium copper oxide (YBCO). Ion beam assisted deposition (IBAD) of 300nm zirconia films at room temperature prevented interdiffusion at sintering temperatures over 800°C. Similar MgO and indium-tin oxide films did not prevent degradation at elevated temperatures. All films deposited on top of YBCO reduced the effects of extended atmospheric exposure. INTRODUCTION The overall objective of this research is to combine superconducting and semiconducting circuitry on the same substrate. Though thermal expansion coefficients are not well-matched, silicon and sapphire can be used as substrates if there is a diffusion barrier between YBCO (yttrium-barium-copper-oxide) and the substrate which prevents chemical interactions during processing [1,2]. Materials which have been investigated as buffer layers include Zr 2 O (possibly stabilized with yttria), MgO, and MgA120 4 . The use of high temperature heat treatments, typically over 900°C, to densify or to crystallize these films was found to be necessary [2,31. Lower temperature deposition of this barrier is desirable as it avoids interactions of the barrier layer with the underlying substrate material [4] and would allow the fabrication of semiconductor circuitry before deposition of superconducting material. It is also possible to deposit semiconducting materials after the superconducting film is fabricated. In this case, a diffusion barrier must be placed on top of the YBCO film to prevent interactions from later processing. Also, because YBCO is moisture sensitive, this film must act as a diffusion barrier to water vapor and oxygen. Several materials have been tried to seal the upper surface of YBCO films, including polymers, [5] CaF 2 , [6] etc. This research tested the use of ion beam assisted deposition (IBAD) for fabrication of dense layers at room temperature of zirconia, magnesia, and indium-tin oxide (ITO) for use as diffusion barriers against moisture attack and between YBCO films and silicon. Previous data shows that ZrO 2 can be deposited with theoretical maximum density by IBAD at 20°C [7]. ITO was tested for use as a possible transparent electrical contact material in optical devices. IBAD of ITO films has also been demonstrated [8]. SAMPLE PREPARATION The apparatus used for IBAD deposition of the films was previously described. [9] It combined evaporation (of oxide charges) with simultaneous ion bombardment. Typical evaporation rates were 0.12 nm/sec. Part of the sample was shielded from the ion beam during all depositions so that the effect of ion beam bombardment could be determined separately from all other parameters. Substrates were bombarded by argon ions, at 600 eV and a
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