Soft X-Ray Contact Microscopy of Biological Specimens: Aluminum-Coated Silicon Nitride Windows as XUV Filters

The optimum wavelengths for soft x-ray contact microscopy of biological specimens are between 2.2 and 4.4 nm as, relative to water, this gives maximum contrast between carbon- and oxygen-containing constituents. Irradiation outside of this so-called "water window" interferes with either the resolution and/or the contrast that can be obtained. The previous belief that silicon nitride windows act as an effective filter for the longer wavelength, XUV radiation generated from laser-produced plasmas is shown to be suspect. However, in this study the use of aluminum-coated windows, which effectively exclude these longer wavelengths, still permits successful imaging of biological specimens. The added strength imparted to coated windows also ensures their survival during the imaging process, thereby demonstrating that the tissues can remain hydrated at the time that the image is formed in the photosensitive resist material used in this technique.