The authors report on the development and characterization of a plasma etching method that utilizes process steps common to both the well-known Bosch and the cryogenic deep reactive ion etching methods for silicon. This hybrid process uses cyclical etch steps that alternate between etching and passivating chemistries as in the Bosch process, while still maintaining sample temperatures at -100 degrees C on a cryogenically cooled stage. The advantages of this process are superior control of wall profiles for isolated features, minimization of grass formation, and the elimination of an expensive gas, c-C4F8, required in the Bosch passivation step. The authors show examples of x-ray optic elements deep etched to 100 mu m depth with the cyclic cryogenic process. (c) 2008 American Vacuum Society.