CORRECTION OF 3-FOLD ASTIGMATISM FOR ULTRA-HIGH-RESOLUTION TEM

Due to the influence of three-fold astigmatism, the spatial resolution obtained on many modern high-resolution. transmission electron micros copes is often not better than 0.2 nm. Although beam tilt can mask the effect of three-fold astigmatism when imaging periodic specimens, the correction of three-fold astigmatism is required for obtaining isotro pic ultra-high resolution (less than or equal to 0.15 nm). Three-fold astigmatism also hampers the correct alignment of the microscope, resu lting in additional beam tilt and two-fold astigmatism, and consequent ly in a further loss of resolution. The amount of three-fold astigmati sm of an instrument is found to be stable for periods well in excess o f a year. After hardware correction through excitation of a magnetic h exapole in the objective lens stigmator unit of our Philips CM300ST FE G, we reduced the magnitude of the three-fold astigmatism from a value of 1.1 mu m, for this particular instrument, to less than 0.1 mu m, a value at which it allows at least 0.1-nm spatial resolution. Addition ally, we show that the microscope can now be properly aligned for ultr a-high-resolution experiments using the standard manual alignment proc edures.