Paper PS2-WeA10
Time Resolved Characterization of Pulsed ICP CH₄ – H₂ Based Plasmas

Wednesday, October 31, 2012, 5:00 pm, Room 25

Pulsed rf plasmas are increasingly being employed for plasma etching or deposition. In the case of etching, as the semiconductor feature sizes decreases and feature density increases, the industry is facing the increasing challenge of finding new plasma processes for the requirement of the next devices generation. CH₄-H₂ based plasmas are widely used for dry etching of II-VI and III-V semiconductors. Pulsed plasmas combined to time resolved measurements are known to be powerful tools to study species kinetics. In this study, pulsed CH₄-H₂ plasmas created in a low pressure inductively coupled rf plasma (ICP) are analyzed by time resolved optical emission spectroscopy (TR-OES), mass spectrometry (TR-MS) and electrostatic probe measurements. Plasma conditions are the following: 800 or 1000 W RF power, 20 mTorr pressure, 5 to 50 ms period and 10 to 50 %duty cycle. A cylindrical Langmuir probe and a planar probe are used to measure the electron density (n_e), electron temperature (T_e) and ion flux density. The species creation kinetics is investigated by TR-OES. Since no emission occurs during the off time (n_e and T_e decreases drastically), the double pulse technique is used with a probing pulse time of 0.75 ms. This communication is focused on pseudo-actinometry for the kinetics of H atoms and radicals (CH..). Mass spectrometry is carried out with a time resolved and energy-resolved mass spectrometer on CH₃. For TR-MS, the transit times in the apparatus are calculated for each ion.

During the post-discharge, the decay characteristic time is below 1 ms for H atom density. The time evolution of ne over one period reveals that ne needs 1 ms to reach a stationary state and decreases with a characteristic time of about 0.1 ms. However, the decrease time of Te seems to be shorter than 0.1 ms. Although the H atom kinetics is very fast, and characterized by a decay time below 1 ms, it is slower than the time evolution of n_e and T_e. However, considering the diffusion coefficient of H atoms in the various gas mixtures, different rise and decrease characteristic times could indicate different reaction probabilities on the walls, in relation with the gas mixture. Characteristic times and associated reaction probabilities, using Chantry’s formalism are determined to be equal to 0.52 ms (on time) 0.80 ms (off time) which corresponds to H loss probability γ_on=0.3 and γ_off=0.1. Similar mass spectrometry measurement on CH3 plasma species will be presented. Influence of ion bombardment on the H loss rate at the chamber walls is discussed and this seems to be the key point for chamber conditioning in the pulsed mode.