Abstract
This paper addresses a countermeasure against the transmission of the floor vibration for electron microscopy. According to the rocking mode formed with the frame and the rubber on the floor, the vibration over 40Hz was increased horizontally at the vibration isolators on top of the frame. In addition, the vertical vibration over 40Hz on the floor showed different phase at each foot of the frames that cause the inclination of the frame. This inclination also caused the horizontal vibration at the isolators even if the frame had been rigidly fixed to the floor. In this paper, we used these two causes to offset the horizontal vibration each other. Thin layers of the viscoelastic material that have high damping coefficient were set under the frame, which restrict the inclination of the frame at the eigenmode. By tuning the natural frequency of this mode slightly lower than the frequency of the inclined floor vibration, the horizontal vibration at the top of the frame was offset. Moreover, the tuned thin layer structure increased the damping coefficient of the twisting mode of the frame and decreased the vibrations in much higher frequency over 400Hz excited by acoustic noise. These tuned thin layer structure helped to avoid the amplification of the vibration above 40Hz and to decrease the fluctuation on STEM image. This tuned thin layer structure has already applied to the electron microscopes on the market.
