Development of computer-assisted minimal-dose system with beam blanker for TEM.

A computer-assisted minimal-dose system has been developed for the high-resolution observation of radiation-sensitive samples using a transmission electron microscope (TEM). This system consists of a CCD camera, a beam blanker and a control computer (PC) that also controls the TEM. A sample is illuminated by an electron beam only when the CCD camera takes images; otherwise, the beam blanker cuts off the electron beam. Emulated images, which are calculated from the images taken and the variable parameters of the TEM, such as magnification and sample stage position, are displayed on the control PC display. After a few times of repetition of exposures and emulations, the sample is positioned to final observation area. Subsequently to select the observation area, the system automatically adjusts the focus position from two different illuminating angle images and the TEM conditions are appropriate for taking a final image. The total electron dose before the final image is taken can be markedly decreased because the sample is irradiated by an electron beam only when images are taken. For the final image, a three-dimensional Fourier filtering method (3DFFM), which corrects the spherical aberration for the through-focus image series and enables the selection of the optimum focus condition later, is also included in our system.