Symmetry descriptors for Si wafer characterisation for Scanning Helium atomic beam microscopy mirror

摘要

The Scanning Helium Atom Microscope is a new technique currently under development which has potential to become a powerful tool in life science, material engineering and other fields of science. One of the most important components of the microscope is a specially shaped mirror that focuses the helium atom beam onto a sample's surface. The mirror quality affects the diameter of the focused beam and consequently the microscope resolution. Thus, the mirror surface roughness and its shape must be controlled accurately. The mirror is formed from a very thin Si crystal membrane that is deformed under a precise electric field. The Si membrane production process is a complex issue and it is very difficult to obtain membranes of uniform thickness: some remaining thickness variations are always present. These variations affect the mirror shape generated by the electrostatic field and prevent optimal focusing of the helium beam. Here, our aim is to characterize the typical thickness variations observed in membranes. We find that whilst the perfectly symmetric membrane is very difficult to produce, it is possible to define criteria for the selection of the best subset of membranes from a larger production run. Our characterization and selection via "symmetry descriptors" will ultimately dimmish aberrations in He focusing. In the paper, the "symmetry descriptors" will be defined, rationalised and discussed in detail.