Spatial structure of plasma potential oscillation and ion saturation current in VHF multi-tile electrode plasma source

Abstract

Increasing RF frequency in the VHF range in the PECVD of Silicon for photovoltaic applications enables a higher rate of deposition while maintaining film quality. However, plasma uniformity on large substrates is compromised as wavelength effects are encountered at high frequencies. Using a segmented electrode consisting an array of individual tiles each 180° out of phase with adjacent tiles, a scalable VHF source that is not affected by wavelength effects is achieved. Due to the 180° phase shift between tiles, a spatial structure is imposed on the plasma. In this paper the spatial structure of the plasma potential oscillation and the ion saturation current density are examined. A capacitive probe measures the oscillation of the plasma potential. Spatial profiles of the plasma potential oscillation exhibit peaks on tile centers at odd harmonics (1F and 3F). The measured even harmonic (2F) is small in amplitude, as expected for the differential power feed, and peaks at tile boundaries. Spatial profiles of ion saturation current show peaks in plasma density over the tile-to-tile boundaries, with non-uniformity increasing with RF power. A discussion of the measured profiles at a variety of powers and pressures is presented.