Evaluation of charge density at hole bottom of capillary plates in a pulsed dual-frequency capacitively coupled plasma

In recent years, high-aspect-ratio hole etching by reactive ion etching (RIE) using pulsed dual-frequency capacitively coupled plasma (CCP) has become difficult in the fabrication of 3D NAND memory devices. In particular, the anomalous profile caused by ion orbit deflection due to positive charge-up inside the hole pattern has become a major problem. In order to solve the problem, it is important to understand the behavior of ions inside the hole and the mechanism of charge-up. In this study, the absolute density of the positive charge accumulated at the hole bottom is evaluated by measuring the high-frequency voltages around the hole pattern on a CCP cathode electrode using high-voltage probes. As a model of the hole pattern, lead-glass capillary plates with electrodes at the bottom are placed on the cathode through an insulated alumina plate. The charge density is evaluated from a one-dimensional equivalent circuit model using the measured voltage, plasma potential, and plate capacitance. The aspect ratio of the hole strongly affects the amount of positive and negative charged particles that reach the hole bottom. It is also confirmed that more negative charges reach the hole bottom by applying a low-frequency bias i.e. driving at dual frequency.