Sputtering yield measurements at glancing incidence using a quartz crystal microbalance

Low energy sputtering yields at grazing incidence have been investigated experimentally using a quartz crystal microbalance (QCM) technique. This method involved precoating the QCM with a thin film of the desired target material and relating the resonance frequency shift directly to mass loss during ion bombardment. A highly focused, low divergence ion beam provided a well defined incidence angle. Focusing most of the ion current on the center of the target allowed for higher sensitivity by taking into account the radial mass sensitivity of the QCM. Measurements of Mo, Cu, and W sputtering yields were taken for low energy (80–1000 eV) Xe+ and Ar+ to validate this experimental method. The target films ranged from 3.5 to 8.0 µm in thickness and were deposited so that their crystal structure and density would match those of the bulk material as closely as possible. These properties were characterized using a combination of scanning electron microscope imagery, profilometry, and x-ray diffraction. At normal incidence, the sputtering yields demonstrated satisfactory agreement with previously published work. At angles of incidence up to 40° off normal, the data agreed well with predictions from existing theoretical models. Sputtering yields were found to increase by a factor of 1.6 over this range. The optimum angle for sputtering occurred at 55°, after which the yields rapidly decreased. Measurements were taken up to 80° from the surface normal