In a reactive ion etching with a mixture of methane and hydrogen, the authors observed selectivity in an etched InP surface or polymeric surface in a wafer, on which SiO2 mask was patterned into a 2-μm-wide grating, when the hydrogen flow rate was 5 SCCM (SCCM denotes cubic centimeter per minute at STP) at a methane flow rate of 40 SCCM. The polymers deposited on the InP surface were increased when the width of the grating pattern was increased. The InP surface configuration, such as etching or polymer deposition selectivity in unpatterned or patterned surfaces, was found to vary with hydrogen flow rate. They consider this to result from hydrogen radicals diffusing from the mask to the window in the direction across the grating pattern. The window is then loaded with the hydrogen radicals to a concentration sufficient to etch InP in the patterned surface.