Fast Aberration Measurement in Multi-Dimensional STEM

The main resolution limits in high-performance (S)TEM are due to the imperfections or ‘aberrations’ of the electron lenses. Correction of these aberrations requires the ability to quickly and accurately measure lens aberrations to align or ‘tune’ an aberration-corrector. The conventional method to measure aberrations in the TEM is to record a series of bright field (BF) images in a Zemlin tableau. However, the reciprocity-equivalent method in STEM of recording a series of tilted BF images is inherently inefficient if only a single BF detector is available. Aberration measurement methods based on recording and analyzing the whole scattering distribution (Ronchigram) provide an efficient use of the signal available in the STEM, however, normally only a small number of probe positions is used [3, 4]. We have recently demonstrated a method based on acquiring Ronchigrams at a multiplicity of probe positions [5]. This procedure results in 4-dimensional dataset, which can be transformed to give an array of real-space images of a sample. Cross-correlating the resulting images gives the gradient of the aberration function, from which the aberrations are determined by a least-squares fit [5]. This procedure provides rapid, accurate aberration measurements and can in principle be extended to arbitrary-order measurements. Figure 1 shows an example aberration measurement based on this method. A series of Ronchigrams was recorded with a nominal 1 nm step between probe positions. A typical Ronchigram is shown, along with measured and fitted shifts, which are used to find the aberration function.