Correction of three-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 microscopes 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 isotropic ultra-high resolution (less than or equal to 0.15 nm). Three-fold astigmatism also hampers the correct alignment of the microscope, resulting in additional beam tilt and two-fold astigmatism, and consequently in a further loss of resolution. The amount of three-fold astigmatism of an instrument is found to be stable for periods well in excess of a year. After hardware correction through excitation of a magnetic hexapole in the objective lens stigmator unit of our Philips CM300ST FEG, 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. Additionally, we show that the microscope can now be properly aligned for ultra-high-resolution experiments using the standard manual alignment procedures.