Extraction of highly directional electron beam in capacitively coupled plasma with fast-rising positive pulse bias

High-aspect-ratio (HAR) etching remains vulnerable to profile distortion driven by ion-induced charge accumulation, because electron transport to HAR features is weak in conventional capacitively coupled plasmas. Here, we investigate a positive-pulse biasing method in which a single fast-rising positive pulse is superposed on a driving RF waveform to enhance electron extraction. Using two-dimensional, three-velocity component particle-in-cell/Monte Carlo collision simulations, we examine how pulse timing and pulse waveforms affect electron extraction and the subsequent plasma response. The extracted electron energy is governed by the pulse rise time through the field reversal strength; a shorter rise time strengthens the field reversal and thereby increases the directionality of the extracted electrons. The 1 ns rise time provides the most directional electron extraction in this study, but the accompanying plasma perturbation indicates a trade-off between enhanced directional extraction and reduced perturbation of the subsequent plasma state. In contrast, the pulse on-time has only a minor effect on the extracted electron directionality. Overly large pulse amplitudes, however, lead to excessive extraction of weakly directional electrons with limited contribution to charge neutralization, producing a large post-pulse voltage offset. This offset slowly recovers toward its pre-pulse state, causing a substantial deviation of the ion energy distribution. These results indicate that rise time and timing are key control parameters for directional electron extraction, while plasma perturbation and ion energy distortion must also be considered in selecting the operating conditions.