A systematic comparison of on-axis and off-axis transmission Kikuchi diffraction

Abstract

The capabilities of the novel on-axis transmission Kikuchi diffraction (TKD) technique were explored in a systematic comparison with conventional off-axis TKD. The effect of experimental parameters on the appearance of on-axis and off-axis Kikuchi patterns was measured and discussed. In contrast to off-axis TKD, on-axis TKD is more sensitive to changes in beam current and beam energy and less sensitive to changes in working distance and detector distance. Moreover, on-axis TKD has a distinct advantage over off-axis TKD due to enhanced pattern intensity, which allows reduction of the beam current or an increase in the acquisition rate. The physical and effective spatial resolution were measured with detector-typical parameters. Even though the spatial resolution of both configurations did not differ significantly under test conditions, on-axis TKD enables measurement over large areas with the determined resolution, whereas off-axis TKD is more sensitive to beam drift. Band detection by the Hough-transform led to indexing of, on average, one additional Kikuchi band when measuring with on-axis TKD compared to off-axis TKD and operated more stable on on-axis patterns.

Spatial Resolutions of On-Axis and Off-Axis Transmission Kikuchi Diffraction Methods

“…On the other hand, less scattering events lead to a smaller electron beam broadening, and thus to a lower spatial The spatial resolution of the on-axis TKD was better than that of the off-axis TKD at the same accelerating voltage and thickness; the largest gap was 8 nm. The only difference between the two configurations was the intersection angle between the phosphor screen and incident beam (α) [5]. In the off-axis TKD, the intersection angle had a magnitude of only a few degrees, which led to a high gnomonic distortion and low diffraction intensity.…”

“…At a given depth, the path length through the sample is smaller for the on-axis TKD, which implies that fewer scattering events may occur before the electrons reach the detector. Therefore, the on-axis TKD has a thicker source region of detected intensity, which is beneficial to the lateral spatial resolution, but detrimental to the depth spatial resolution [5,22].…”

“…Compared to the off-axis TKD, the new configuration provides higher signal intensity and lower distortion. Furthermore, either the electron dose or acquisition time can be reduced 20 times to yield an equivalent pattern quality as that in the off-axis TKD method [4][5][6].…”

“…The PSR corresponds to the smallest distance from a large-angle grain boundary where the overlapping pattern of both crystals appears, while the ESR indicates the smallest distance across a boundary where the patterns are still indexable by a software algorithm [13,14]. Niessen et al compared the on-axis and off-axis TKD methods in many aspects, including the spatial resolution [5]. They investigated the effect of working distance in spatial resolution at detector-typical microscope parameters and demonstrated that the PSR of the on-axis TKD was slightly improved compared to that of the off-axis TKD.…”

On the depth resolution of transmission Kikuchi diffraction (TKD) analysis

“…A better understanding of the spatial resolution, angular resolution and Kikuchi pattern formation mechanisms in TKD are still required to understand the potential and the limitations of the technique and so define how it can best be applied to addressing real materials questions at the nano-scale. A number of authors [5,7,[10][11][12] have studied the spatial resolution of TKD analysis; especially lateral resolution where it has been shown that combining high accelerating voltages and low sample thicknesses give the best results [5]. By comparison the depth resolution (dd) has been much less investigated.…”