Tecnai on-line help manual -- Options - UT Southwestern

<strong>Tecnai</strong> on-line help Options 120Options Software version 29.1.5 The HM-STEM magnification calibration procedureThe calibrations procedure consists of a calibration for selected magnifications (unlike the TEMmagnifications, which are made from discrete lens settings with magnification-dependent random errors,the STEM magnifications are controlled by the amplitude of the scan which quite linear, so only a fewmagnifications suffice). The following procedure is used:• Direct cross-grating calibration of a lower HM-STEM magnification (not necessarily the lowermostbecause this may be too low to resolve the grating squares effectively). If the image distortion is toohigh, the software will automatically adjust the distortions in an iterative procedure until the distortionsare less than 0.1% (vector lengths of the grating square dimensions) and 0.1° (angle betweenvectors).• Calibration of the image shift at the same HM-STEM magnification (this does not calibrate amagnification, but the image shift itself, used later).• Calibration of several additional magnifications. The software detects the magnification at which thecross-grating method does not work anymore and automatically switches to the image shift method.The STEM calibration procedure will be run for a total STEM rotation angle of 0°. This means that thefree (user) STEM rotation is set to 0°, and the aligned value (which corrects for the mismatch betweenstage axes and STEM image axes) is also temporarily set to 0° (the latter value is reset at the end of theprocedure to the starting value). Effectively this means that the smallest distortions and most accuratemagnifications are achieved at the same STEM rotation angle. For this purpose it is necessary for usersto set the free rotation to the compensating value of the alignment value (which is included in thecalibration report).Note: The STEM magnifications are strongly dependent on the pivot points, perpendicular correctionsand distortion adjustment. For STEM magnifications as accurate as possible, the same STEM settingsshould be used as during the calibration. It is therefore advised to adjust the pivot points andperpendicular corrections carefully before the calibration and store these settings in an alignment file(after the calibration, because that may have included adjustment of the STEM distortion) and havingusers always use those alignments. If the STEM distortions initially were a long way off, stop theprocedure after the distortion adjustment and check the pivot points and perpendicular corrections, andthen restart the procedure.9.1.6 The Microprobe STEM magnification calibration procedureMicroprobe STEM is the STEM mode in which the illumination system is in Microprobe (theminicondenser lens is set to positive). This mode is used for special applications in which the size of thebeam is not decisive, but rather the illumination angle (which is smaller in Microprobe), which in turngives smaller diffraction disks. Microprobe STEM can be reached from normal STEM by switching fromNanoprobe to Microprobe. Because TIA is not aware of normal (Nanoprobe) or Microprobe STEM, itdoes not have separate magnification tables. For Microprobe STEM we therefore have to make sure thatthe internal calibration in the <strong>Tecnai</strong> software is defined such that the same nominal magnification innormal and in Microprobe STEM gives the same actual magnification. This is done by the MicroprobeSTEM calibration procedure.