The fabrication of QWIPs and various grating structures for detectivity improvements requires the ability to selectively or non-selectively or non-selectively etch GaAs/AlGaAs, GaAs/InGaP, AlInAs/InGaAs and other heterostructure systems with high resolution. For selective etching of GaAs over ALGaAs, plasma chemistries involving BCl3 or SiCl4 with additions of SF6 or CF4 can provide selectivities over 600:1 when the ion energies and fluxes in the plasma are relatively low. The selectivity is severely reduced when the ion energy exceeds approximately 150 eV, because of sputter-enhanced removal of the AlF3 which provides the etch-stop reaction. Fluorine-free, chlorine- based chemistries provide equi-rate etching of AlxGa1-xAs across the entire composition range as long as water vapor is excluded from the reactor chamber. This is readily achievable in load-locked systems, and BCl3 is an attractive choice as the plasma chemistry because of its ability to get water vapor and to attack the native oxide on III-V materials. The CH4/H2 plasma chemistry is selective for InGaAs over AlInAs at low ion energies, and also provides smooth controlled etching of II-VI compounds such as HgCdTe and related materials. ECR plasma sources have a number of advantages over conventional RIE systems, including higher etch rates, but typically display lower etch selectivities because of the very high ion fluxes. We also describe new etch processes developed for the III-N materials such as GaN, AlN and InN. Their high bond energies relative to the more common compound semiconductors have generally led to reports of low etch rates and the need for very high dc self-biases, but we show it is possible to use new plasma chemistries such as ICl under ECR conditions which produce efficient etching of these materials.