Energy-resolved angular-distribution measurement of Ar ions impinging on RF electrode in a dual frequency capacitively coupled plasma

In the recent trend of high-aspect hole etching with aspect ratios of 100 or higher, it has become important to understand the angular distribution of ions impinging on the RF electrode. For ion angular distribution (IAD) measurements, we have developed a specially designed capacitively coupled plasma (CCP) source. In this device, ions from the plasma are extracted through a sampling orifice into a drift chamber directly connected to the RF electrode, and after passing through the drift chamber, are detected by a micro-channel plate (MCP) assembly with a phosphor screen. To guarantee a field-free space in the drift chamber, the RF electrodes and the drift chamber are at the same potential, and all electrical equipment connected to the drift chamber is electrically floating with RF bias voltage. Using this experimental setup, the angular distributions of ions and energetic charge-exchange neutrals has been measured in single-frequency (13.56 MHz) Ar CCP and 40 MHz/2 MHz dual-frequency Ar CCP. This apparatus can also realize energy-resolved IAD measurement by applying time-of-flight method. By combining deflector plates with pulse-modulated voltage and an ICCD camera, time-resolved ion profile on the MCP is obtained and, from numerical analysis of the profile, energy-resolved IAD is obtained. The IAD has main Gaussian profile originated from ion thermal motion with small intensity tail component. From pressure dependence measurement of the IAD, increase of the tail component with increasing the pressure is observed and small angle scattering of Ar ion in the sheath is suggested as the origin of the tail component.