Abstract Nanomold is the most important part for the nanoimprint technique which determines the obtained feature size. We prepared relievo nanomolds by selectively etching of the a-Si/ SiN x multilayer thin-film deposited by a PECVD system. SEM results showed that the mold feature sizes were controllable on the nanometer scale and the structures depended on the conditions of the film preparation and the following etching process. Ultrasonic processing and short etching time are necessary to get the desired patterns of good quality by the chemically selective etching process, especially for strips thinner than 20 nm. The substrate surface morphology also affects the mold structures greatly.