Radio frequency magnetron sputtering of thick film amorphous beryllium

Abstract

Thick film coatings of beryllium, needed for the low-Z ablator layer in proposed laser fusion targets, have been prepared using high rate magnetron rf sputtering. The requirements for these Be coatings include thicknesses from 5 to 50 μm, complete freedom from surface defects, and an average surface roughness of 100 nm or less. These specifications are difficult to achieve if crystalline growth occurs during sputtering, since the surface roughness of thick grainy films can be on the order of micrometers. We have sputtered very smooth, dense Be films up to 25 μm thick with surface roughness less than 100 nm. X-ray diffraction analysis of impurity doped films indicates an amorphous-like structure. Impurity stabilized amorphous Be with smooth surfaces is reported on both 15°C cooled copper and nominal 200°C glass substrates. The sputtering parameters (substrate temperature, deposition rate, argon pressure, and impurity gas levels) affecting surface roughness and film structure are discussed in terms of SEM, AES, and x-ray diffraction results.