Positive Pulse Voltage Superposition to an RF Electrode in a Dual-Frequency Capacitively Coupled Plasma for Relaxation of Positive Charging at Bottom of High Aspect Ratio Holes

Reactive ion etching has become important in the fabrication of 3-D NAND flash memory. One of the major challenges in the fabrication process is the high-aspect-ratio (HAR) holes etching, where positive charge accumulation at the bottom is known as the issue. To mitigate the charging problem, it is essential to understand the mechanisms of charge accumulation and relaxation processes. For this purpose, we have developed a method to estimate the charge density at the hole bottom, using a capillary plate (CP) as a model of the HAR holes. Charge density was evaluated from measurements of voltages at the top and the bottom of the CP, as well as from careful measurement of the capacitance of the CP. In this study, time-resolved measurements of the charge density at the bottom of a CP (aspect ratio: 100) were conducted in a power-modulated 40/2 MHz dual-frequency capacitively coupled plasma at a modulation frequency of 1 kHz. When the 2 MHz power was increased, gradual decrease in the bottom charge density from positive to negative was observed. Furthermore, when a positive pulse voltage (+1 kV, pulse length: 50 ms) was superposed to the RF electrode, synchronized with a high-power phase of the 2 MHz bias power, negative charging of the bottom was drastically enhanced. This behavior suggests a significant influx of electrons into the HAR hole during the positive pulse application.