Investigation of Sputtered β-Tantalum Thin Films

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Abstract
Sputtered β-tantalum thin films were produced in a two-chamber vacuum system (entrance chamber and sputtering chamber) with typical background pressure of 1×10−7 Torr. Both the resistivity and sheet-resistance uniformity were almost entirely dependent on cathode voltage; a sheet resistance uniformity of ±1% over a 3 in. by 4 in. substrate was achieved. The application of negative-anode bias voltage was found to control β-tantalum film resistivity; a 53% reduction in resistivity was caused by a −200-V anode bias voltage applied during a 4-kV cathode sputtering run. A relationship between resistivity and plasma potential was found, which unifies both cathode voltage and anode-bias voltage effects. The electrostatic environment of the substrate appears to control the resulting β-tantalum film properties. Continuous sputtering over a period of several days was required to initially achieve equilibrium conditions in the sputtering chamber. The application of direct-substrate negative bias voltage during cleanup reduced the β-tantalum resistivity and, in some cases, produced a mixed structure of both bcc and β tantalum.
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