IMAGE SHARPNESS MEASUREMENT IN SCANNING ELECTRON-MICROSCOPY - PART II

A method for qualitative and quantitative analysis of scanning electro n microscope (SEM) images for the determination of sharpness is presen ted in this paper. Described is a procedure for qualitative analysis b ased on a software program called SEM Monitor that can be applied to r esearch or industrial SEMs for day-to-day performance monitoring. The idea is based on the fact that, as the electron beam scans the sample, the low-frequency changes in the video signal show information about the larger features and the high-frequency changes give data on finer details. The image contains information about the primary electron bea m and about all the parts contributing to the signal formation in the SEM. If everything else is kept unchanged, with a suitable sample, the geometric parameters of the primary electron beam can be mathematical ly determined. An image of a sample, which has fine details at a given magnification, is sharper if there are more high frequency changes in it. In the SEM, a better focused electron beam yields a sharper image , and this sharpness can be measured. The method described is based on calculations in the frequency domain and can also be used to check an d optimize two basic parameters of the primary electron beam, the focu s, and the astigmatism.