Effect of ion beam etching on the surface roughness of bare and silicon covered beryllium films

Abstract

Due to low density, high mechanical rigidity and thermal conductivity, ease of machining beryllium is one of the most promising materials for space mirrors, including for studies of the solar corona in the wavelength range of 13–30.4nm. An obstacle to the widespread use of this material is its large surface roughness after mechanical polishing. In this paper, using samples of 200nm thick beryllium films deposited on silicon substrates and polished bulk beryllium, we studied the main aspects of using ion-beam etching for finish polishing of beryllium. We present the results of investigation pertaining to the influence of the neon ion energy and angle of incidence on the beryllium films surface roughness. We measured the etching rates depending on the angle of incidence and energy of neon ions. We found that 400eV is the optimal energy for neon ion etching ensuring slight surface roughness smoothing in the range of incidence angles of ±40°. The deposition of 200nm amorphous silicon films onto beryllium and their subsequent etching with the 800eV argon ions improve the effective surface roughness integrated across the range of the spatial frequencies of 0.025–60μm−1, from σeff=1.37nm down to σeff=0.29nm. The effectiveness of the smoothing technology for X-ray applications has been confirmed by the results of the study of the reflective properties of the Mo/Si mirrors deposited on the substrate. The reflectivity at a wavelength of 13.5nm increased from 2% for the substrates with the surface roughness of σeff=2.3nm (the roughness value corresponds to the as-prepared bulk Be substrates and is taken from the literature) up to 67.5% after the smoothing technology.