Laser projection-patterned etching of (100) GaAs by gaseous HCl and CH₃Cl

Laser projection-patterned etching of GaAs in a HCl and CH₃Cl atmosphere performed using a pulsed KrF-excimer laser (λ=248 nm, τ=15 ns) and deep-UV projection optics (resolution 2 μm) is reported. The etching process carried out in a vacuum system having a base pressure of 10^-6 mbar is shown to result from a purely thermochemical reaction. Etching takes place in two steps: (i) between the laser pulses, the etchant gas reacts with the GaAs surface-atomic layer to form chlorination products (mainly As and Ga monochlorides), (ii) local laser surface heating results in the desorption of these products allowing further reaction of the gas with the surface. The influence of the etching parameters (laser energy density, gas pressure and pulse repetition rate) on the etch rate and the morphology of the etched features was studied. Etch rates up to 0.15 nm per pulse, corresponding to the removal of 0.5 GaAs molecular layer, are achieved. The spatial resolution of the etching process is shown to be controlled by the heat spread in the semiconductor and by the nonlinear dependence of the etch rate on the surface temperature. As a result, etched features smaller or larger than the projected features of the photomask are achieved depending on the laser energy density. Etched lines having a width of 1.3 μm were obtained at low fluences by the projection of 2 μm wide lines onto the GaAs surface.