Etching structures for state-of-the-art electronic and optoelectronic devices such as heterojunction bipolar transistors, optical waveguide modulators, gratings, and vertical-cavity surface-emitting lasers often requires nonselective etching with depth accuracy on the order of +/-8 nm. We disclose the application of in situ optical reflectance monitoring during chlorine reactive-ion-beam etching of III-V compound heterostructure devices for real-time determination of etch depth to +/-8 nm independent of total etch depth. High-vertical-resolution etching of thick, layered structures is achieved through use of a resonant periodic set of reflective interfaces, greatly enhancing the reflected amplitude oscillations without detrimental effects on device performance. This method demonstrates that slight modifications of material structure to optimize monitor response greatly enhance the accuracy of nonselective dry etching.