Image Noise Reduction Using Vibration Mode Shaping for Scanning Electron Microscope

Abstract

This paper addresses Scanning Electron Microscope (SEM). There is the quality problem that the image noise caused by the acoustic noise disturbs observation for correct shape of the specimen. Thus, SEM be less influenced by the acoustic noise is expected.This paper approaches to this image noise problem from the mode shapes of the base plate, and the transmission path of the acoustic noise and vibration. Firstly, the mode shapes of the base plate with conventional rib are compared to those of the plain base plate, and no remarkable increase in natural frequencies and no change in mode shapes are shown. Secondary, new base plate, which has two twisting mode as 1st and 2nd mode by arranging position of the rib, is developed. In those two twisting modes, vibration amplitude around the center of the base plate is smaller than that in bending modes. Thus, less vibration is transmitted to the specimen chamber and to the specimen in twisting modes than bending ones. Finally, the developed base plate is assembled to SEM, and remarkable reduction of the image noise under the condition of the acoustic noise below 250Hz is shown. This base plate has already applied to the products on the market.