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====== Warning: experimental procedure ======

Several problems have been encountered while trying to create a glass mask. These problems may prevent you from succesfully creating a mask.

  - The glass was too thin, combined with being slightly smaller than 50*50mm. This resulted in movement of the glass during exposure. The glass needs to be atleast 1.56mm thick, but preferably not much thicker than this.
  - Some dust came onto my glass plate during spinning or baking. After removing the resist with NMP, also the MoGe got damaged during the cleaning process. This resulted in charging effects. Never recycle a dirty piece of glass, use a new one instead. It may be usefull to sputter a conducting layer on several at once.
  -  Another piece of glass had damaged spots, which were not immediately visible. I only noticed 'comets' in the resist while spinning/baking. The damaged spots were easily visible under the microscope, even without resist. It's recommended to use high quality (polished) glass, which may have to be ordered elsewhere, and it's also recommended to examine the glass under the microscope before sputtering, to reduce time loss.
  -  For an unknown reason, one of the lines on the sample got an interuption in it after exposure. Always examine your mask after development, but before sputtering. This has been fixed by locally exposing this spot again and developing again. Possible ways to prevent this is make sure that you're sample is absolutely clean, or by writing 2 lines with half the width next to each other (so if one gets interupted, the other one will still make contact). Also a slightly higher dose may help.

====== Introduction =======

Optical masks can be created using ebeam lithography. This guide will explain the differences between making a normal structure and making an optical mask. It is important that you read the manual for regular lithography, you can find it [[Electron beam lithography#manual|here.]]

====== Structure ======

  - Before designing a large structure, check the max stage travel given below in 'UV alignment'
  - It may be required to write multiple writefields. In this case you will need to stitch them together. This is best done by having your contact lines in each writefield end with a T shape.
  - The exosting masks have a line width of 20 um, it's recommended to have atleast this width, but wider lines can be used. Typically you want lines which are wider than the size of a dust particle.


====== Sample preparation ======

  - Use a piece of glass with dimensions 50*50 mm, with a thickness of 1.56 mm. You can get this at the LIS. Quarts glass is best, but regular glass will be fine. The 50*50mm is an upper boundary, larger peices will not fit into the sample holder. The 1.56mm is a lower boundary, if your glass is too thin, the clamps won't work and your glass will move during the process.
  - Carefully clean the glass, avoid scratches.
  - Use the [[Z-400#manual|Z400]] to sputter 10-20 seconds (no longer!!!) of MoGe. This will prevent charge building up in the ebeam.
  - Due to oxidation of the MoGe, it is important that you do not keep the glass exposed to air for longer periods of time. Also cleaning between sputtering and spinning has showed negative effects on the MoGe.
  - Spin MAA, recipe 2, bake 30 min at 160 degrees.
  - Spin PMMA, recipe 2, bake 30 min at 140 degrees.

====== Loading the sample ======

  - Insert the mask sample holder in bronze 'sample holder' holder, such that the undercut is exactly where the transfer stick will be. Try to avoid rotations.
  - Put the glass piece on the sample holder. One screw is slightly smaller than the other, allowing you to mount the glass without completely unscrewing the screws.
  - Screw the holder to the transfer stick, it is possible to clamp the the X-ray blockling glass to the holder if you rotate the clamp.
  - Load the sample holder as usual. Be very carefull, the holder has only a few milimeters space in the loadlock.
  - Keep the working distance at the 4x4 marker. Some non-linear effects have been measured with the WD at short (20-25 mm) working distances. It's recommended to work at a distance of atleast 30 mm (4x4 marker is approximately 34 mm on this holder), to minimize non linear behaviour.



====== Focussing ======

  - Move the stage to the coordinates x=10.598, y=41.416. If you want to use this procedure more often, it may be convenient to save these coordinates in the stage control screen.
  - Turn the SEM on as usual.
  - You should now be above a rectangle with dimensions of 3.04mm x 2.50mm. A few milimeters to the right is another rectangle with approximately the same dimensions.
  - Focus on one of the rectangles, at this point, it does not have to be exact.
  - PCD in.
  - Try to maximize the current, if it's more than 15-20 nA max it is good. Use aperture 1 for larger currents. Don't use 0, it's a hole.
  - If this can not be achieved, you will need to align the ebeam gun, use the x and y shift and tilt to maximize the current very carefully. A slight misalignment can significantly reduce the maximum current. Ask for help if you don't know how to do this.
  - Reduce the current and focus again on one of the rectangles, this time exact.

====== Writefield alignment ======

  - Go to microscope control, select 4000 mm writefield and press set.
  - Move the stage to the centre of the left rectangle.
  - Open the position list align4000.
  - Edit the coordinates. Both the U,V coordinates and result have to be edited. This is 4 number per marker. Change the values (currenly +/- 1.5mm) into u=+/-1.52mm and v=+/-1.25mm.
  - Save this position list under another name, for example 'align optical mask'.
  - Align the writefield on the corners of the left rectangle, using a zoom of 15x.
  - Set the shift to 0.

====== UV alignment ======

  - Move the stage to the bottom right of the left rectangle, this is exactly the centre of the upper edge of the piece of glass and set this as U=0, V=0
  - Correct the angle along the piece of glass or the rectangles. To get the best stitching result, it is important that the UV and the writefield are exactly paralel to eachother.
  - If you move now to the coordinate U=0, V=-25 (PCD in!!!) you should be at the centre of the glass, it may be convenient to make this the new UV origin. If the holder and UV coordinates are not rotated the X and Y coordinates should be approximately X=12.893, Y=15.587.
  - The maximum range of the stage travel is to X=-3.585 to 31.486 and Y=-5.172 to 79.000. This gives you a range of U=-16.478 to 20.759 and V=-17.593 to 63.413 without rotation, relative to the centre of your piece of glass. Take this in mind when designing your structure.

====== Exposure ======

  - A current of 10 nA is recommended to prevent excesive writing times, it can be higher if needed.
  - Use a dose of 160-200.
  - When you write many writefields, it may be safe to write only a few at once, and check the max possiblecurrent between the writing sessions. If this has dropped too low, you may have to re-align the gun.
  - Expose as usual.
  - After unloading your sample, develop 60 seconds in the PMMA developer (MIBK+3*IPA), followed by the PMMA post developer (IPA).

====== Sputtering ======

  - Sputter 5 minutes (80 nm) to have a mask that does not oxidize, but easy to damage
  - Or sputter MoGE for a mask that's harder to damage, but will oxidize.
  - Lift off using NMP or acetone.