Automated STEM Distortion Correction for Arbitrary Scan Patterns

C55-10b-270600The electromagnetic coils in a scanning transmission electron microscope (STEM) suffer from hysteresis and have a finite rise time that slows their response to desired changes in scan position. As 2D and 4D STEM detectors and scan generators continue to increase in speed, distortions in the acquired data have become increasingly evident due to these adverse characteristics of scan coils. One potential solution is to substitute these electromagnetic coils with alternatives with improved high-speed characteristics. While such a solution may be beneficial for new STEM instruments, it will likely be costly and disruptive—if not altogether impractical—to modify the large number of existing STEM instruments with new types of beam deflectors.We propose developing a new distortion-corrected scan generator based on our new ultra-fast digital scan controller called DE-FreeScan, which differs from most other scan controllers in that it enables the use of unconventional/arbitrary scan patterns and has fine control over the output voltages sent to the electron microscopes scan coils. This system will measure distortions present in the scan coils based on STEM acquisitions of calibration specimens. A model will be developed to account for distortions present in any arbitrary probe movement. Then, the output voltages from the scan generator for each scan point will be perturbed to compensate for the measured distortions during STEM acquisitions. During Phase I, Direct Electron will acquire STEM images of calibration specimens using DE-FreeScan with a variety of dwell times and three different scan patterns: raster, serpentine, and rectangular spiral. The distortion field for each image will be calculated using several methods, including optical flow analysis and the sampling moiré method. Data acquired at different dwell times will be used to model the time constants of the microscope lenses. The distortion field model will be verified by post-processing ultra-fast STEM images. Finally, for the three specific scan patterns tested in Phase I, the scan generator output voltages will be perturbed according to the calculated model, allowing correction of scan distortions in hardware during acquisition instead of by software in post-processing. The resulting distortion-corrected STEM scan generator will enable low-distortion ultra-fast conventional and 4D STEM, while giving users the freedom to use any desired scan pattern. Critically, such a system would be compatible with nearly all existing STEM microscopes, without requiring expensive and disruptive changes to the microscope column.