Deep in situ dry-etch monitoring of III-V multilayer structures using laser reflectometry and reflectivity modeling

Deep reactive ion etching of III-V multilayer structures is an important issue for long wavelength vertical cavity surface emitting laser (VCSELs) where full laser structures are usually very thick. Test etchings were performed on $GaAs/AlxGa1−xAs$ Bragg mirror structures and monitored using laser reflectometry at 651.4 nm. In order to perform very deep etching, up to 9 μm, we designed and fabricated a special two-level mask made up of a thick nitride layer and a thin nickel layer. The etching rate is a complex function of many parameters and may change from run to run for similar structures. Therefore, it is important to have a method to control accurately the process in situ by continuously matching, experimental curves with the results of the reflectivity modeling. Here, we present a model, based on the Abelès matrix method, of the normal incidence reflectivity of a multilayer stack as a function of etch depth. Comparison between the model and the observed reflectivity variation during etching makes it possible to distinguish individual layers with very high precision.