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

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<strong>Tecnai</strong> on-line help Alignments 62<strong>Tecnai</strong> 12 Software version 2Notes:• It is useful to activate the Scope function on the STEM Imaging Control Panel. This function displaysthe detector signal levels on a fixed scale and thereby makes it easier to judge the alignment (theSTEM image has its contrast and brightness re-adjusted after an image has been collected, whichmakes it difficult to judge the signal levels from the detector).• The near-axis position (where the STEM Bright-Field/Dark-Field detector assembly is located) is at~30 mm from the screen center to the ENE (slightly away from the screen center to the top right).This physical position is of course the same for all camera lengths but the actual diffraction shiftsrequired to get there depend on the camera length (which magnifies the diffraction shift), so thesettings are dependent on the camera length.• Because hysteresis can have considerable effect on the diffraction-pattern position, you should usethe normalization facility whenever the optical conditions have changed (the automaticnormalizations take care of this, so it is advised to keep them enabled).• Some of the lowermost camera lengths are too small to reach the near-axis detector (typically thecamera length must be at least 90 mm). There is no point in trying to align these. Simply go on to thenext camera length.• Some of the large camera lengths will overlap the bright-field disk onto the dark-field detector and aretherefore of little use. You can skip these camera lengths as well.• When the camera length changes, the signal on the detectors will change, so it will be necessary toadjust contrast and brightness. For this purpose, the alignment procedure makes it easy to switch todetector control by pressing R2 (toggle between detector alignment and BF detector control).• Since the size of the Condenser aperture determines the beam convergence angle, it alsodetermines the size of the diffraction disks and thereby what the effective camera lengths are forBF/DF imaging (if the central beam is too large, it will fall on the DF detector). To determine if thebeam doesn't have long tails due to the combined effects of spherical aberration and largeconvergence angles (and thus poorly defined spots), you can observe the focused beam innanoprobe mode (for the particular condenser aperture). Normally the two smallest condenserapertures are the ones that are suitable for STEM.• Some camera lengths are difficult to use at low STEM magnifications because the sphericalaberration of the diffraction lens makes it impossible to have the diffraction remain sufficientstationary to keep it on the detectors (the pattern described by the central-beam disk looks like a birdflying, with its wings flapping up and down). Either use higher magnifications or other camera lengthsin those cases.12.8 HM-STEM Distortion adjustmentPurpose: Adjust the AC deflection coils settings so that the resulting STEM image has no distortions.Importance: ESSENTIAL for proper STEM imaging.Method: Collect an image of a cross-grating and adjust the relative strength of X and Y coils, and theirperpendicularity.Important:• The specimen must be at the eucentric height. To do this properly, go to the Nanoprobe mode andpress the Eucentric Focus button. The objective lens current is now set to the proper value forNanoprobe (provided the SA objective preset has been aligned properly). Then bring the specimeninto focus with the Z height of the specimen stage. (By doing it this way you will attain a morereproducible setting than by using the alpha wobbler.)• During STEM alignment TIA (Tem Imaging & Analysis) must be running.
<strong>Tecnai</strong> on-line help Alignments 63<strong>Tecnai</strong> 12 Software version 2ProcedureThe alignment procedure consists of five steps:• The first step is a preparation step to center and focus the beam.• In the second step a STEM image must be made of a cross-grating specimen.• In the third step the scan rotation is set so the horizontal lines of the cross-grating are horizontal inthe STEM image. The rotation required should be less than ~15° (see note below).• In the fourth step the distortions are adjusted. Multifunction X controls the trapezoid distortion (thevertical lines of the grating are not perpendicular to the horizontal lines), while the Multifunction Yadjusts the relative strengths of the horizontal and vertical scans (rectangular distortion).• A check on the previous alignment with the image rotated by 90°.If necessary, iterate steps three and four (see also notes below).Notes:• It is essential that the pivot point alignment has been done properly before the distortion adjustmentis done.• The cross-grating used must be inserted into the microscope in such a way that the horizontal andvertical lines in the grating are roughly parallel to the horizontal and vertical in the image when thescan rotation (and the default scan rotation) is close to 0° (within ~15°), otherwise the alignment isvery difficult (requires many iterations). Judge whether the orientation of the cross-grating is suitablein the second or third step of the procedure. If the orientation is further away from 0°, remove thespecimen holder from the microscope and rotate the specimen until its orientation is suitable for thisalignment.• The distortions of the image may be judged more easily by drawing a square in the image. Click onthe Image Selection Tool toolbar button in TIA , click in the top left quadrant of the image and dragthe cursor to bottom right while keeping the Shift key on the keyboard pressed down (the latter forcesthe image selection drawn to remain square). It is then easy to count the number of cross-gratingsquares in the horizontal and vertical dimensions and making sure that the numbers are the same.• Description• The deflection coils used in the microscope are not all equal and exactly the same from onemicroscope to another. There are therefore alignment procedures that will result in adjustments thatcompensate for the variation in the coils. For the deflection coils used in TEM operation, the twoadjustable parameters are the pivot points and the perpendicular correction. The pivot adjusts for thedifference in strength between upper and lower coil (e.g the X coil). The perpendicular correctionadjusts for a rotation between the upper and lower coils (e.g. the upper and lower X coils) by addinga small deflection to the other lower coil (in that case the lower Y coil). For use in TEM this issufficient.• In STEM, however, additional adjustments are necessary, otherwise image distortions (stretch of onedirection relative to the other and angular distortion) appear. These adjustments correct for thedifference in strength between the X and Y coils and for any deviation from 90° between the X and Ycoils. In principle this can be done both for the upper and lower coils separately but in practice this istoo difficult to align and a single adjustment is sufficient.12.9 HM-STEM Default rotationPurpose: Define the rotation correction for STEM imaging.Importance: CONVENIENCE for having the STEM image (at 0° scan rotation) in the same orientationas the TEM image (on the viewing screen).Method: Observe the scan frame on the viewing screen and rotate it until it runs as described below.
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Tecnai on-line help manual -- Alignments - UT Southwestern

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