Stabilizing inductively coupled plasma source impedance and plasma uniformity using a Faraday shield

Abstract

Standing waves unavoidably develop on inductively coupled plasma sources because they are mismatched transmission line systems. In addition, the electrical properties of the induction coil (or plasma source) are coupled to those of the plasma, since the coil–plasma system behaves similarly to a transformer. As a result, the input impedance, output-to-input current ratio (OICR), and electromagnetic fields symmetry of the coil are affected by the plasma conditions, which in turn influence the uniformity of plasma generation and ion flux to the wafer surface. In this article we examine the effect of plasma loading on the coil’s impedance and OICR with and without a Faraday shield. Measurements of the coil’s electrical properties without a Faraday shield showed a significant increase in the OICR with plasma conditions. Similar measurements with a Faraday shield showed that coupling between the coil and the shield becomes dominant over coupling between the coil and the plasma. Langmuir probe measurements showed that the radial profiles of ion density and electron temperature remained azimuthally symmetric for different plasma conditions with the shield.