Sputtering of metal alloys containing second-phase precipitates

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Abstract
Glow‐discharge optical spectroscopy has been used as a real time monitor of the transient sputtering characteristics of alloy samples containing various amounts of second‐phase precipitates. All alloys investigated reached a steady‐state sputtering condition in which the chemical composition of the sputtered target material was equal to that of the bulk target. The transient sputtering time required to reach steady state was directly related to the time required to attain a steady‐state rate of target surface cone formation and annihilation. Scanning Auger and electron microprobe analyses were used to show that cone formation in these alloys was due to masking of the matrix by slow sputtering second‐phase precipitates and surface agglomerates. At steady state, the cone number density was found to be a function of bulk‐alloy precipitate density, Ar sputtering pressure, and the sample sputtering rate. Increasing the Ar pressure resulted in increased scattering of sputtered atoms back to the target surface, while changes in overall target sputtering rate affected the surface migration and, hence, surface distribution of low‐yield species. The average cone base diameter for Inconel 718, Incoloy 800, and Monel K‐500 alloys was approximately 8 μm. The average depths removed during transient dc sputtering at an Ar pressure of 40 mTorr (5.3 Pa) and 2 kV were 1.5 μm for Inconel 718, 1.3 μm for Incoloy 800, and 0.1 μm for Monel K‐500. For comparison, high‐purity GaAs single‐crystal samples sputtered under the same conditions reached steady‐state sputtering after the removal of less than 100 Å. In all cases, films deposited on glass substrates at ambient temperature during steady‐state sputtering had the same chemical composition as the bulk target. Inconel 718 and Incoloy 800 films also exhibited second‐phase precipitates similar to those in the bulk targets.