We have developed an unconventional approach to down‐stream plasma etching: only a small area of the substrate is exposed to the low pressure, reactive gaseous environment. The remainder of the substrate is outside the miniature plasma chamber, providing physical access for probing apparatus. Etch rates of 6 μm/h were obtained. The process can be especially useful wheninsitumonitoring of the effects of etching is required. Using this process, we improved the responsivity of a semiconductor optical detector deposited on top of an optical waveguide. This was accomplished by monitoring the transmitted intensity of light in an integrated optical waveguide while etching a thin semiconductor film covering a small region of the waveguide.