The modelling of radio frequency hydrogen plasmas in the reactive ion etching of GaAs

Abstract

Radio frequency hydrogen plasma etching of GaAs wafers can lead to large-scale stoichiometric changes in the GaAs substrate which vary with the process pressure and power and can affect device performance. These changes are thought to be due to the penetration depth of hydrogen species and the probability of an ion or neutral species either stopping in the surface oxide layer or penetrating the bulk substrate. A computer model has been devised which follows ion trajectories across the sheath and predicts ion and neutral species energy distributions (IEDs) at the sample surface. These IEDs are modulated by collision processes in the sheath and the potential drop between plasma and sample. Predictions for average ion energy and therefore penetration depth follow the same trend as that for the damage induced in the sample which has been quantified using angle-resolved photo-electron spectroscopy.

A model of capacitively coupled radio-frequency methane/hydrogen plasmas for III-V semiconductor etching applications

“…The potentials at the surface of the quartz were therefore measured using a metallic plate at the same potential as the quartz surface, connected by a ceramic-covered wire to a high-voltage probe and oscilloscope. The resulting DC component of the measured potentials are 0.3-0.4 times the DC bias [31].…”

“…Between the sheaths, the bulk of the plasma is field free. Recent modelling of argon and hydrogen ion energies in the current plasma cell have indicated that these approximations are acceptable and give good agreement with ion energies at the anode [31].…”

“…The potential distribution across the sheath was derived from the Child-Langmuir collisionless model. Previous workers have used expressions for the electric field across the sheath derived from the assumption of linear fields [17], the Child-Langmuir potential and ion mobility limited approximations [18], though it has been found [31,32] that the differences in the IEDs from the different expressions is negligible. Using the Child-Langmuir relationship for a space-charge limited current, the extent of the sheath at a time t is given by…”

“…At low pressures, the ions arrive at the cathode with average energies given by the average sheath potential and at higher pressures scattering leads to somewhat lower energies [31].…”