The scanning tunneling microscope has been employed to study the morphology, atomic surface structures, and grain interface of hydrogenated nanocrystalline silicon (nc-Si:H) before and after hydrofluoric acid (HF) etching. It was found: (1) The nc-Si:H films were composed of many different sizes of grains and these grains were composed of many finer grains. (2) There were line structures on the surface of the fine grains and loop structures at the grain boundaries without HF treatment. After etching, two more structures were observed: loop structures on the surface of fine grains and spider bonding structures besides the interface of fine grains. (3) The loop structures found at the grain boundaries was larger and more irregular than those on the grain surfaces. Line structures were similar to crystal silicon, but the distance between lines was enlarged. Considering the experimental results, a discussion was made about the formation mechanism of these atomic structures. © 1997 American Vacuum Society.