Method for adjusting a stem equipped with an aberration corrector
The invention relates to a method for adjusting a C s corrector in a STEM using a crystalline sample. The method comprises recording a through-focus series, converting the obtained images to Fourier space, thus forming a set of images alike diffraction images. By then determining the symmetry of the Fourier images, the corrector can be tuned for better symmetry, and the transfer limit can be determined by determining the maximum distance of the spots from the center. By repeatedly performing these steps, the corrector can be tuned to its optimum performance.
1. A method for adjusting a Scanning Transmission Electron Microscope (STEM), said STEM equipped with an objective lens and an aberration corrector for correcting aberrations of the objective lens, the aberration corrector excited with an excitation, and the objective lens focused to a focal value F, the method comprising the repeated steps of:
a) recording a through-focus series of high-resolution STEM images I 1 . . . I n of a crystalline or poly-crystalline part of a sample, the through-focus series covering a defocus range,
b) transforming said images to representations R 1 . . . R n in reciprocal space, the representations showing a plurality of spots arranged in a pattern, each of the spots showing an intensity, each of the spots indicating a periodicity occurring in the image,
c) determining for each of the representations R 1 . . . R n an asymmetry in intensity distribution of at least part of the spots,
d) changing the excitation of the aberration corrector and the focal value F based on an optical model of the aberration corrector and the determined asymmetries,
said repetition of steps aborted when:
the representation in reciprocal space of an optionally recorded in-focus image shows a symmetry better than a predetermined value and shows a maximum transfer limit higher than a predetermined transfer limit, and/or
the changes in excitation of the aberration corrector and objective lens are below predetermined values, and/or
the number of repetitions exceeds a predetermined maximum number of repetitions, the predetermined maximum number of repetitions being greater than one.
2. The method of claim 1 in which the excitation of the aberration corrector is changed to minimize the combined effect of two-fold astigmatism (A 1 ), coma (B 2 ) and three-fold astigmatism (A 2 ).
3. The method of claim 1 in which the reciprocal images R 1 . . . R n are formed using a Discrete Cosine Transform, a Discrete Fourier Transform, a Discrete Hardaman Transforms, or a Power Spectrum Density.
4. The method of claim 1 in which the method is performed on a sample to be analyzed.
5. The method of claim 1 in which the method is performed on a sample mounted on a sample carrier together with another sample, the other sample to be analyzed.
6. The method of claim 1 in which the aberration corrector is a corrector of spherical aberration and/or a corrector of astigmatism.
7. The method of claim 1 in which the images are recorded of different parts of the sample so as to avoid contamination.
8. The method of claim 1 , further comprising the repeated step of changing the defocus range.
9. The method of claim 1 , further comprising the repeated step of recording a high-resolution STEM image using the altered values of excitation and/or focus, transform said image to a representation in reciprocal space and determine the symmetry of said representation as well as the distance from the centre of the pattern at which spots can be observed, said distance giving an indication of the maximum transfer limit.