MICROSCOPE ALIGNMENT
- Check the high voltage rack:
- All the lenses should be in remote operation.
- The e-gun filament current should be at 1.75 A.
- The Wehnelt voltage must be -460 V or less (i.e. more in absolute value, e.g. -470 etc) so that emission current is zero
- The HV must be zero after the sample transfer. You will ramp HV up when you are closer to the objective.
Figure.1 High voltage rack.
- Remove all apertures from the optical path in the SPELEEM. Namely, illumination aperture, field limiting aperture,
contrast aperture are fully retracted (0 mm); the analyser exit slit is aligned with the red mark at about 4 mm.
- Move the sample towards the objective, always looking from the top viewport.
Approach upto the point where the objective is still visible, ~ 5 mm from the objective. The same distance shold separate the handle from the brass stopper.
- Ramp up the HV, always looking at the MCH pressure. Arcs (i.e. HV disharges) are always anticipated by an increase of the pressure. If you notice that the pressure rises, just stop ramping up the HV and wait! Arcs MUST be avoided because they can severly damage your sample and the objective. In order to ramp UP HV do the following:
- Set the leakage current maximum to 1.9 (in the knob units). This prevents arcs to be too much distructive!
- Quickly increase HV up tp 12 kV; here the risk of arcs is minimal.
- Increase HV to 18000 kV with great care, checking continiously the main chamber ion gauge. If pressure rises, stop and wait (it might take several minutes) the pressure goes down. If necessary, decrease the HV. In this way you will condition the sample and slowly burn out all sharp features acting as field emitters.
If you need more information on ramping up the HV, please refer to the relevant section in the frequently asked questions page.
- The next steps is to find the e-beam with sample far away from the objective.
- Start UMeasure and turn the grabber to preview on mode with exposure of 0.02 s. As the Wehnelt is still closed, there must be no electrons on the MCP. The CCD camera shows only noise.
- Select a field of view of 10 microns and set the start voltage to 0 eV (mirror mode).
- Keep watching the camera grabber in Uview and slowly turn up the emission current to 0.02 microamps (and not more!) by increasing the Wenhelt voltage. If the CCD detects electrons, please avoid that image intensity rises more than 10% of the full camera range.
- Otherwise, you have to look for the e-beam! As the sample is supposed to be far from the ideal focus position (1590 mA), you need to decrease the objective exitation current until the edge of the e-beam appears on the screen. You can start from OBJ current = 1590 mA and continue until you reach 1100 mA.
- If you find the e-beam at IOBJ less than 1590 mA you are safe . This meanse that the sample is far from the objective.
- If you find cannot find the beam at the e-beam at IOBJ less than 1590 mA but instead at more than 1590 mA, you need to retract the manipulator because you are too close to the objective!
- If you do not find the e-beam [:(], please stop what you are doing and ask for help [:)]
- Now you need to bring the sample close to the objective, to the ideal focus position.
Keep the preview on in Uview and set the objective current bak to 1590 mA. By doing this you will loose the e-beam. Don't worry, it's normal.
- Slowly and carefully approach the sample towards the objective, until the e-beam (re-) appears on the detector. Watch the MCH pressure! if it rises, stop and wait until the sample is conditioned.
- Continue approching the objective until you see features on the surface and the image intensity reaches a maximum. If necessary, you can increase the emission current but please avoid that image intensity rises more than 10% of the full camera range.
- If you cannot tell where the focus is (when the sample is still far away), try finding a feature on the sample surface by moving the X and Y knobs.
- Now you have to adjust the sample tilt, so that the LEEM e-beam becomes perpendicular to the sample surface.
- increase the start voltage so that you find the transition mirror mode - LEEM (the image becomes dark). It will be easier to find objects on the surface.
Choose one and bring it to the centre of the imaged field of view.
- Toggle the objective using LEEM2000. Start with ~0.1-0.2% toggle amplitude) and adjust the vertical and horizontal tilt knobs in order to minimize the movement of the object. If you are effectively correcting the tilt, the image will become darker. Slightly decrease the start voltage to find the MEM-LEEM transition and continue.
- You must set the manipulator tilt and the e-beam ILLUMINATION DEFLECTORS to minimise the start energy at which the transition MEM-LEEM occurs. Iterate the procedure until you convergence is reached (two or three iterations should suffice). If the alignemt proceeds well, the object on the surface has become sharper. As you toggle the objective current the image breathes homogeneouly and simmetrically.
- Now you need to correct the astigmatism, by using the OBJECT STIGMATORS.
- Find a defect (a spot with more or less circular shape).
- Defocus the objective, until you see a bright border around the spot.
- Change the OBJ stimagtors A and B until this border appears uniform and symmetric around the object.
- If necessary correct the illumination using the deflectors. Few iterations might be needed
- Align the imaging column. The best trajectory of the e-beam is the one which is perturbed less by the variation of focus of each lens. In practice, you use deflectors (aligners) which are capable of deflecting the e-beam and bring it as close as possible to the optical axis of the lens. Here, the image qality is best and aberration minimal. To find the axis of the lens, you toggle the lens excitation (apply a sinusoidal perturbation to the lens current), and you bring the refernce object used for aliging the sample tilt to the position where it does not move during toggling.
The smallest field limiting aperture (FLA), inserted in the image plane at the beam splitter, can be consodered a "virtual object". Aligning using the smallest FLA as reference provides the best way to align the microscope for LEEM, LEED and dispersive planve operation modes.
- Insert the smallest Field-Limiting aperture toggling the
Tranfer Lens. Try to find the position giving the most symmetric movement. If the circle still is not breathing about a fixed point, then adjust with IMAGE EQUALIZER X and Y.
- Toggle the Field Lens, adjust its aligners to get a symmetric breathing / minimise displacement of the image.
- Toggle the Intermediate Lens, adjust its aligners to get a symmetric breathing / minimise displacement of the image.
- Toggle P1, adjust its aligners to get a symmetric breathing / minimise displacement of the image.
- Toggle Retarding Lens, adjust its aligners to get a symmetric breathing / minimise displacement of the image.
- Do not need to align INNER LENS and ACC LENS. Inner lens does not have aligners. Ass Lens aligners are already optimsed for less abberrations in dispersive plance operattion.
- ADJUST ANALYSER SEL to get best image quality. It is very sensitive!
Appropriate values are between 71.78-72.85.
- toggle ANALYSER SEL and optimise ACC LENS in order to minimise image motion during toggling. This focusses ACC lens on the right image plane!
- Toggle P2, adjust its aligners to get a symmetric breathing / minimise displacement of the image.
- Toggle P3, adjust its aligners to get a symmetric breathing / minimise displacement of the image.
If the alignment is correct, the FLA circle should be close to the center of the detector. The microscope is aligned. Note that if one moves to a completely different position with the sample manipulator (e.g. few hundred microns away), one may need to readjust the tilt (without touching the rest of the alignment). You can save the LEEM2000 settings using the button in the upper left corner with an appropriately indexed file name. Later on, you can recover these settings with the "Load" button.
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