Sputter Deposition and Thermally Induced Phase Transformation of Non-BCC Chromium Thin Films
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Sputter Deposition and Thermally Induced Phase Transformation of Non-BCC Chromium Thin Films M. J. OKeefe*, S. Horiuchi*, J. P. Chu.. and J. M. Rigsbee*** * Solid State Electronics Directorate, Wright-Patterson AFB, OH 45433-6543 ** Nippon
Mining Co. LTD, Tokyo 105 Japan ***University of Illinois, Dept. of Materials Science and Engineering, Urbana, IL 61801 ABSTRACT The crystal structure of sputter deposited chromium thin films on Coming 7059 glass, polytetrafluoroethylene, and cold rolled (110) oriented low carbon steel a-Fe substrates was investigated as a function of 0 and C incorporation into the growing Cr film. The as-deposited crystal structure of the films was found by X-ray diffraction to be either highly oriented (110) BCC a-Cr or (200) oriented A-15 8-Cr. Chemical analysis of the films by Auger electron spectroscopy determined that the 8-Cr phase formed when the combined 0 and C impurity concentration in the film was -15-30 at.%. At total impurity concentrations above -30 at.% or below -10 at.% standard BCC a-Cr formed. The crystal structure of the films was not influenced by the substrate material. X-ray photoelectron spectroscopy of the Cr 2pl/ 2-2p3 /2 orbitals indicated that the dominate binding state of both the BCC a-Cr and A-15 8-Cr films was characteristic of elemental Cr. Vacuum annealing of the A- 15 8-Cr films at 500WC for one hour transformed the crystal structure into BCC a-Cr without a measurable change in chemical composition. The incorporation of 0 and C into the growing Cr film is believed to impurity stabilize the A-15 structure and favor its formation over the BCC structure. I. INTRODUCTION Physical vapor deposition (PVD) of metallic thin films is used in a wide variety of industrial applications. Vacuum techniques such as evaporation, sputter deposition, and ion plating are an
integral part in the fabrication of surface coatings and metallic layers, especially in the
microelectronics industry. Deposition of Cr by PVD methods has been used in commercial
applications ranging from diffusion barriers[l] to corrosion protection[2] to interfacial adhesion
promoters in polymeric composites[3]. Chromium forms a body centered cubic (BCC) crystal
structure (a-Cr) under equilibrium processing conditions. However, it has been reported[4,5]
that an A-15 Cr crystal structure, designated 8-Cr[4], can be formed using PVD techniques. It
has also been proposed that the A-15 structure is not an allotrope of Cr but an entirely different
phase of suboxide, Cr3 0[6]. Published results on the processing conditions favorable for formation of the A- 15 structure have often conflicted. For instance, the role of the underlying substrate material in promoting the 8-Cr phase has been both proposed[7] and rejected[8]. Analysis of 20-40 nm thick A-15 films by Rutherford backscattering lead to the conclusion that
the A-15 phase was not stabilized by impurities[9] while more recent work using Auger
spectroscopy[6] demonstrated the influence of oxygen on the formation of the A- 15 structure. There is a gene
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