Progress in dry etching processes [reactive ion etching (RIE) and electron cyclotron resonance etching, (ECRE)] is described, concerning etch geometry and fidelity of pattern transfer by using advanced techniques to produce masks of photoresist and Al2O3 or SiO2. In contrast to capacitively coupled hydrogen/methane discharges, in which the etch rate of GaAs is significantly lower than that of InP, in ECR discharges by the simple variation of the gas composition, the etch rate of GaAs can be driven to values comparable with InP. The different on-line monitoring techniques are compared. It is shown that optical emission spectroscopy can be applied successfully even with sample areas of about 2 cm(2) at etch rates of 50 nm/min.