The Scanning Force Microscope as a Tool for Studying Optical Surfaces

Abstract

Traditional instruments that have been used to study optical surfaces include the Nomarski microscope, various types of optical and mechanical profilers, scanning electron microscope, and transmission electron microscope. Although most of these instruments are sensitive to very small height variations, they have limited lateral resolution. Hence, features such as tiny scratches, closely spaced structure of metal and dielectric films, tiny laser damage craters, and impact craters from space debris cannot be seen by the traditional surface characterization instruments. With the advent of the scanning tunneling microscope in 1982 followed by a variety of scanning probe microscopes, new characterization tools have become available for studying optical surfaces. The scanning tunneling microscope has limited usefulness since it requires that the sample surface be conducting. Most optical samples and their multilayer film coatings are insulating. Even metals such as aluminum, copper, silicon, beryllium, and molybdenum, all of which have optical applications, are covered with layers of native oxide, making them nonconductive. Fortunately the scanning force microscope (SFM), formerly called the atomic force microscope (AFM), is ideally suited for studying optical surfaces.1 It uses a pyramidal silicon nitride probe with a tip radius ~400 Å, which contacts the surface with very light loading, approximately 1000 times smaller than the loading used with the highest performance commercial mechanical profilers. Under suitable conditions, the SFM can show structure on high quality optical surfaces that has rms heights down to the subangstrom level and lateral dimensions of a few tens of angstroms. The SFM can resolve lateral dimensions ~1000 times smaller than those resolved with most optical profilers and ~100 times smaller than is possible with the best mechanical profilers.