Detection of Al and Mg contamination in sputtered Pt films by Auger electron spectroscopy and secondary ion mass spectrometry

Abstract

Traces of metallic contaminants, codeposited during the sputtering of Pt in Si integrated circuit fabrication, can cause deleterious metallurgical reactions during subsequent processing. Anomalous lateral diffusion and evidence of the formation of a liquid phase during sintering have been observed. Wafers exhibiting these symptoms have been found to undergo gross PtSi attack during the subsequent Pt etch in aqua regia. Silicide damage is then followed by a high probability of AuSi eutectic alloy formation which causes resistive paths that shunt many emitter-base junctions and Schottky diodes. The principal metallic contamination observed in this study originated from the wafer trays and other parts of the sputtering station, which were constructed from standard commercial grades of Al alloys containing Al, Mg and Cu. Within sputtered films of Pt 500Åthick, Al has been identified and profiled by means of Auger electron spectroscopy (AES) and secondary ion mass spectrometry. The degree of metallic contamination could be varied by modifying the sputtering procedure; this was confirmed by AES and successfully correlated with subsequent PtSi attack. In the series of experiments described in this paper, any contamination of Pt layers by Al and Mg was virtually eliminated by coating the wafer trays and adjacent parts of the sputtering station with approximately 1 μm of Pt before deposition of the 500Ålayer on the device slices. The Auger signal from Al in Pt films deposited on device wafers, after the above-described precaution, dropped to a level below noise but still detectable. At these contamination levels, PtSi attack by aqua regia was eliminated. Close monitoring of the matallic contamination should therfore be maintained in all sputtering facilities used for Si integrated circuit manufacture.