CHROMIUM COATING FOR HIGH RESOLUTION CRYO SEM

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ffigh quality imaging of macromolecular structure of biological samples can be obtained when combining high resolution SEM with advanced thin metal film coating techniques. Because the yield of secondary electron (SE) signal from biological samples is low, a thin layer of fine grain metal film is often necessary to enrich the SE-1 signal yield and enhance the contrast of features of interest. Surface topographic contrast depends on the ratio of SE-USE-11I collection, and the SE-II signal yield depends on the amount and interaction volume of the backscattered electron (BSE) signal (Peters, 1982). Therefore the metals with low BSE yield should be chosen for high resolution SEM coating. Chromium, which has the features of low atomic weight (Z--24) and sufficient SE'signal yield, is an appropriate choice and it was first used by Peters (1982) for high resolution SEM imaging. To create a reproducible pure chromium film, a clean vacuum is the most crucial requirement because Chromium will be oxidized very quickly whenever oxygen exists. Oxidized chromium compound is a totally different metal and is low in the SE yield and low in mass. So, in this project, we will attempt to solve the leaking and oxidation problem so that we can get consistent and clean chromium coating.