Electron Microscopic Determination of Surface Elevations and Orientations

Abstract

The methods of determining object thickness in electron microscopy are briefly reviewed. Uncertainties in interpretation of surface replicas from a consideration of intensities alone are discussed and three different replicas of etch figures in pure aluminum are presented. The stereoscopic method is analyzed and an equation is derived relating the parallax of image points in stereo micrographs to the elevations in an object. This equation is the parallax equation of aerial photography but written in the form f=(cscσ/2M)×103 microns/mm,where σ is the stereo angle and M the total magnification. The equation is experimentally verified utilizing the cubic etch figures of aluminum. Elevations in the range 0.1–2μ can be determined to within 10 percent and frequently with greater accuracy. A new cartridge for obtaining stereo micrographs at an angle of 10° is described and thicknesses as small as 150±50A are measured using this cartridge. Examples of the use of stereo micrographs are given illustrating different orientations of pearlite in steel and the scratches on a polished steel surface. Elevation changes of 200A and less can be measured in the silica replica of the polished surface. The theoretical resolution of the stereo method is given by xmin=ω/2 sin σ,where ω is the resolution normal to the optic axis and σ is the stereo angle.