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The aberration measurements using Ronchigram in STEM are mainly based on focus change [6], autocorrelations from amorphous specimens [7], or Fourier transforms of crystalline specimens [8]. The accuracy of the latter two methods depends on the accuracy of the initial calibration of the defocus.
Lin et al. [1] proposed a method to determine the aberrations of probeforming lenses by using several focal sets of the Ronchigram and Sawada et al. [2] applied a method called SRAM (segmental Ronchigram autocorrelation function matrix) to correct aberrations. A Ronchigram was also applied to calculate the aberration parameters, and was further used in the autotuning procedure of the probeforming lens system by Krivanek et al. [3,4].
Sawada et al. [5] had used segmental Ronchigram autocorrelation function matrix (SRAM) method to measure the aberration coefficients before and after aberration corrections. Figure 2745 shows the central parts of the Ronchigram patterns in a focal series. The Ronchigrams from the amorphous film were divided into 7 × 7 segments in order to calculate the aberration coefficients (up to fifthorder) and to perform autocorrelation functions. The absolute values of each aberration coefficient were calibrated by referring to the difference between the defocus values of the two Ronchigrams [1]. Furthermore, the residual aberration can also be obtained by comparing the experimental and calculated simulations of Ronchigrams.
Figure 2745. The focal series of Ronchigrams: (a) Underfocus of 128 nm and (b) Overfocus of 128 nm. (c) and (d) Grided images of (a) and (b), respectively. (e) and (f) Autocorrelation functions from (a) and (b), respectively.
Adapted from [5]
[1] J. A. Lin, J. M. Cowley, Ultramicroscopy 19 (1986) 31.
[2] H. Sawada, T. Sannomiya, F. Hosokawa, T. Kaneyama, Y. Kondo, Y. Tanishiro,
K. Takayanagi, Proc. IMC 16 (2006) 632.
[3] O.L. Krivanek, N. Dellby, A.R. Lupini, Ultramicroscopy 78 (1999) 1.
[4] N. Dellby, O.L. Krivanek, P.D. Nellist, P.E. Batson, A.R. Lupini, J. Electron
Microsc. 50 (2001) 177.
[5] Sawada H, Sannomiya T, Hosokawa F, Nakamichi T, Kaneyama T,
Tomita T, Kondo Y, Tanaka T, Oshima Y, Tanishiro Y, and Takayanagi
K (2008) Measurement method of aberration from Ronchigram by
autocorrelation function. Ultramicroscopy 108: 1467–1475.
[6] Q. M. Ramasse, A. L. Bleloch, Ultramicroscopy 106 (2005) 37.
[7] H. Sawada, et al., Ultramicroscopy 108 (2008) 1467.
[8] A. R. Lupini, S. J. Pennycook, Journal of Electron Microscopy (Tokyo) 57 (2008)
195.
