RESOLUTION AND CONTRAST IN THE CONVENTIONAL AND THE SCANNING HIGH RESOLUTION TRANSMISSION ELECTRON MICROSCOPES.

The Conventional Electron Microscope (CEM) and the high resolution Scanning Transmission Electron Microscope (STEM) are analyzed and compared using a simplified theoretical model and a geometrical optical treatment. The contrast and resolution are numerically evaluated for a variety of bright and dark field operating conditions using test objects consisting of a single carbon or gold atom. Inelastic scattering effects are not evaluated. Particular attention is paid to the nature of the illumination source in the CEM, and to the detector size in the STEM. The general conclusion is that when bright field phase contrast is to be used the CEM is superior, but in dark field the STEM is superior as long as the resolution distance is larger than 1A. When the STEM detector size is changed, the resolution distance varies by up to almost 30%, but in order to utilise any improvement a significant loss of signal must be suffered. In both microscopes the dark field resolution is always better than the bright field resolution.