LASER-PROJECTION-PATTERNED ETCHING OF GAAS IN A CHLORINE ATMOSPHERE

Laser-projection-patterned etching of gallium arsenide in a chlorine atmosphere performed with a pulsed KrF excimer laser (lambda = 248 nm, tau = 15 ns) and deep uv projection optics (resolution: 2-mu-m) 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. Local laser surface heating results in the desorption of the chlorination products (AsCl(n) and GaCl(n); n = 1-3) formed between laser pulses. The results of a thermal model that gives the temporal evolution of the surface temperature during the laser pulse are reported: They have been used to calculate the activation energy of the etching process namely, a DELTA-E = 0.35 eV. The influence of the etching parameters (laser energy density, gas pressure, and pulse repetition rate) on the etch rate was investigated. The effect of the laser irradiation in vacuum, Ar, and Cl2 ambient on the surface morphology is also reported. The spatial resolution of the etching process is shown to be controlled by the heat spread in the semiconductor surface and by the nonlinear dependence of the etch rate on the surface temperature. As a result, etched features smaller or larger than the photomask features are achieved depending on the laser energy density. Resolution down to 4-mu-m with a system having an optical limit of 2-mu-m was achieved at low fluences.