Surface chemistry and damage in the high density plasma etching of gallium arsenide

Abstract

Anisotropic pattern transfer with low damage in compound semiconductor dry etching requires an in depth understanding of the chemical processes that occur at the plasma/semiconductor interface that promote the removal of volatile product species. In situ mass spectrometry has been used to study product evolution during high density plasma etching of GaAs in a Cl2/Ar chemistry. Through definitive surface temperature control and configuration of the mass spectrometer to sample through the substrate platen, a comprehensive picture of the etch process was obtained. Evolution of etch products of GaAs (AsClx and GaClx) was monitored as neutral flux (pressure), ion flux (microwave power) and ion energy (substrate bias) are varied to identify conditions where ion-driven surface chemistry is dominant. Observations show that fluxes of atomic chlorine neutrals and ions are required at the substrate to maximize etch product formation. These conditions are optimally met at low microwave powers (⩽300 W) and pressures (⩽1.0 mTorr) in our system. The ion energy dependence of product formation shows regions of thermal/chemical etching for energies less than 50 eV, ion-assisted chemical etching for energies between 50 and 200 eV, and sputtering for energies greater than 200 eV. Ideal processing conditions are ascertained from the trade off between maximum ion-assisted etch rate and minimal electronic damage, as measured by photoreflectance spectrometry. The effectiveness of post-etch chlorine treatments on returning of the surface Fermi level to its pre-etched value was also investigated.