Remediation of Charging During Pulsed Plasma Etching of High Aspect Ratio Features in Dielectric Materials: Applications to Cryogenic Etching

Charging of high aspect ratio (HAR) features in dielectric materials during plasma etching can divert the trajectories of incoming ions resulting in defects and feature distortion. Positive potentials of several hundred volts to a few kV are produced as a result of the incident high energy ions having narrow angular distribution whereas incident electrons typically have lower energies and broader angular distributions. A remedy for feature charging is pulsed substrate biases. During the power-on portion of the pulse, high energy ions produce net positive charge in features. During the power-off portion of the pulse electrons are accelerated into the feature to neutralize positive charge. In this paper we discuss results from a computational investigation of charging dynamics during pulsed plasma etching of HAR dielectric structures using a 3D voxel-based model, the Monte Carlo Feature Profile Model (MCFPM), which receives energy and angle resolved fluxes of incoming species toward the wafer from the Hybrid Plasma Equipment Model. MCFPM follows the trajectories of charged and neutral species into the feature, and predicts evolution of the feature due to etch and deposition processes. Electric potentials inside the feature are obtained from solution of Poisson's equation. The system is a multi-frequency capactively coupled plasma of pressures of tens of mTorr. Charging dynamics during pulsed plasma etching of HAR features will be discussed for fluorocarbon and cyrogenic etching of SiO₂ and ONO stacks. For conditions when a continuous bias produced an etch stop due to excessive positive charging, using pulsed biasing enabled full etching of the feature. For conditions where a continuous bias produced a fully etched feature, pulsed biases with half the average power produced a fully etched feature with similar rate and signficantly less positive charging.