Oxidation of AU-SI Alloy Films
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C. A. HEWETT*, K. E. BOHLIN**, M. W. RANDOPLH* AND S.S. LAU* * University of California at San Diego, Department of Electrical Engineering and Computer Science, La Jolla, CA 92093 ** Permanent Address: Institute of Technology, Uppsala University, Sweden ABSTRACT We have investigated silicon oxide formation on co-deposited Au-Si alloy films at low temperatures. Alloy compositions spanning the Au-Si binary phase diagram have been examined. It was found that alloys with compositions above 40 at. % Au were most efficient in promoting oxide formation. These results are compared with the Au on Si sample configuration where Si is transported through the Au before oxidizing. The possibility of using alloys to form Au lines with a self passivating oxide coating after patterning and oxidation is discussed. INTRODUCTION As integrated circuit device dimensions decrease, there has been an associated decrease in the interconnect linewidth. This decrease in linewidth, however, has led to an increased resistance contribution to RC time delay. This in turn has stimulated an investigation for process-compatible materials with resistivities lower than that of doped poly-Si for use in interconnect applications. Thus, transition metal silicides with resistivities an order of magnitude lower than that of poly-Si have been utilized in interconnect applicationsill. Recently it has been proposed that metal silicides may be patterned to form interconnect lines, then oxidized to form lines consisting of the pure metal with a protective Si0 2 coating1 21. In a previous paper1 31 we have suggested that the Au-Si system may be an appropriate choice for such an interconnect scheme. This system has the advantage that oxidation of Au-Si may be carried out at low ( < 200'C) temperatures, thereby minimizing the disturbance of shallow dopant profiles. Pure Au also possesses a very low resistivity. Furthermore, as interconnects
are deposited on insulators, it is important to note that the presence of Si at the Au/Si0 2 or Au/Si 3 N4 interface greatly enhances the film adhesion. Finally, we expect the presence of Si in Au to enhance the ease of dry processing of such films, thereby allowing easy patterning. It has been shown that for an Au film on Si, annealing in air results in an oxide film which saturates in thickness, with the saturated thickness proportional to the initial thickness of the Au 1 41 film . -It has also been shown that when Si has been diluted with sufficient Au, Si bonds take on a metallic character15 ,6,7 1. The minimum concentration of Au necessary to induce a metallic Si bond is 70 at. %. Below this concentration Si bonds regain their (normal) covalent nature. When Au is deposited on a clean Si surface a diffuse interfacial layer 60-130A in thickness is formed between the 1 71 bulk Au film and the bulk Si substrate . In this diffuse layer Au and Si are intermixed, with the Au concentration high enough that the Si atoms are in a metallic state. It has been speculated, therefore, that the role of the Au film during oxidation is to
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