Application of Nomarski interference contrast microscopy as a thickness monitor in the preparation of transparent, SiC-based, cross-sectional TEM samples

Reflected light optical microscopy using a Nomarski prism and a differential interference contrast filter have been employed in concert to achieve a technique that provides an accurate color reference for thickness during the dimpling and ion milling of transparent transmission electron microscopy samples of 6H-SiC(0 0 0 1) wafers. The samples had thin films of AlN, GaN, and Au deposited on the SiC substrate. A sequence of variously colored primary and secondary interference bands was observed when the SiC was thinner than 20 μm using an optical microscope. The color bands were correlated with the TEM sample thickness as measured via scanning electron microscopy. The interference contrast was used to provide an indication of the dimpling rate, the ion milling rate, and also the most probable location of perforation, which are useful to reduce sample breakage. The application of pressure during the initial cross-sectional preparation reduced the separation of the two halves of the sample sandwich and resulted in increased shielding of the film surface from ion milling damage.