The BCl3/(Ar,He) etching process for AlGaAs layers is described and analyzed. It was thoroughly investigated whether the mechanism of plasma etching could be evaluated using the loading effect. Very smooth sidewalls of the structures were obtained with using an optimized process to treat the photoresist after its development. It is shown that various end point detection methods can complement each other and enable etch depth monitoring with a depth resolution of significantly better than 50 nm at a total depth of more than 2.5 mu m.