AN AUTOFOCUS METHOD FOR A TEM

Autofocus and correction of astigmatism of a transmission electron microscope (TEM) based on measuring a beam-tilt-induced image displacement is proposed and its theoretical limitations are studied. Tilting the illumination beam displaces the specimen image on screen when the TEM is out of focus. This displacement has a known relationship with the defocus. Autofocusing is possible by tilting the beam, measuring the image displacement, calculating the defocus and correcting it. Correction of astigmatism is possible by measuring the defocus in different directions. The method is fast because it calculates and corrects the defocus in one step. It works with many types of specimens because it utilizes both the amplitude and phase contrast of a bright field image. The precision of this method depends on the precision of the image displacement estimation. The shifted and unshifted images differ because of shot noise, instrumentation noise, and aberrations caused by the beam tilt. An expression is derived, containing parameters of the TEM and measuring system, for the achievable precision in estimating the displacement. This expression is a tool for optimizing the automatic focussing procedure and the measuring system. It does not depend on any particular estimation method with which the displacement is calculated. Computer simulations for a TEM equipped with a Vidicon videocamera have been carried out. They show that at Scherzer defocus (86 nm) the minimum measuring time required for focussing the TEM with a precision of 5 nm is about 50 ms. The precision is less satisfactory (>30 nm) when, with the same measuring time, the TEM is far out of focus or very near focus. The precision improves proportionally to the square root of the measuring time. © 1987.