Investigation of modulated radio frequency plasma etching of GaAs using Langmuir probes

Abstract

Radio frequency ClCH3/H2 plasma etching of GaAs is examined in the 10–140 mTorr pressure range using square-wave modulation of the excitation source to control the etching. A Langmuir probe is used to measure time-resolved electron density, characteristic temperature, and floating potential during the plasma afterglow period. The ClCH3/H2 plasma electron energy is found to be 1.2±0.3 eV. The near afterglow plasma density decay has a time constant in the order of τ=30 μs at 140 mTorr for 10%–20% ClCH3 in H2 and τ=100 μs for H2. The floating potential continues to decay into the far afterglow, with a characteristic time of the order of milliseconds. The Langmuir probe measurements indicate that in ClCH3 plasmas the near afterglow is dominated by electron attachment, whereas the far afterglow is dominated by ambipolar diffusion. The GaAs etch rate experiments show that surface reactions continue into the far afterglow, dominating the behavior of the time average etch rate.