Sheath Measurements for a Trapezoidal Wafer Bias Waveform using LIF*

Nonsinusoidal bias waveforms are being used on plasma etch tools to positive effect. This work reports on a particular version consisting of a short positive voltage spike to draw electrons, and a longer negative-going ramp to provide approximately constant current for the ion etch species.  Investigations of the sheath during the various phases of the applied bias waveform are performed in a low pressure Argon plasma using Laser Induced Fluorescence (LIF). Additional diagnostics employing hairpin and Langmuir probes mounted on a 3D probe drive system will be used to see changes in the bulk plasma immediately above the RF sheath.  We expect that the approximately linear dV/dt of the ramp can be adjusted to create an approximately constant sheath. Forthcoming results will be reported.