Lithography - MJB4

Adopted from Process Flow for Tunnel Junctions and TLM Modified by NTS 01/23/13. Modified by NTS 140602, 150115, 150928.

Note: Sample must be cleaved and in sample holders to begin.

Prep

• Setup hotplate and thermometers

• Take photoresist out of the refrigerator because it needs to be at room temp

  • Crack PR fridge to let fumes get vacuumed up, then open.

  • AZ 5214E-IR

• Check MJB4 lamp power

  • Check the power unit is working and on CH1/365

  • Insert empty mask holder, raise the stage, and expose for 10-20 seconds

  • Look at the output power listed on the power unit, it should be ~7.5 mW/cm^2

  • If it is not, you need to email Mertech to calibrate the lamp.

  • Power less than 6.5 can ruin your litho. 6.5-7.5 is not ideal but OK for process dummies.

Solvent Clean (ACE, IPA, DI: 2 min)

• Create a clean workstation before beginning.

  • Spray bench with IPA if needed, then wipe clean

  • Do this by getting clean wipes from the dispenser and lay them down anywhere you plan to work.

  • For Regrowth, layer wipes from back to front, and right to left, and go up to/over lip of bench

• Setup solvent beakers and dipper basket

  • Always fill each subsequent beaker higher than the previous

Note: The dipper baskets are labeled. Only use those that are appropriate for your immediate task. Otherwise, keep them in the bin and close the bin to prevent any contamination. Note: Gentle agitation during cleaning process is recommended.

• Acetone: 2 min

• 2-propanol: 2 min

  • Go get UHV foil for boat

  • Scrap UHV foil is ideal, never trust other person’s boat

• DI water cascade rinse: 2 min

• Make foil boat

• N2 blow dry

• Put samples in boat

Dehydration bake

• 150C oven: 5 min

• Let Samples cool in boat

• Setup hotplate for prebake with thermometers

  • Set hotplate to prebake temperature

  • Thermometers are setup up in a particular way. Short silver is at NE position, tall silver is NW, tall black is SW, short black is SE. Thermometers should be on flat foil and positioned so that the edges of the hotplate are tangential to the thermometer. All thermometers should be rotated so that the dial is in normal, easy reading position.

• Clean up acetone and IPA beakers

Spin PR

Spinner Prep

• Make sure spinner chuck is clean

  • Our own 0.5” chuck should be clean

  • Spray wipe with acetone and wipe chuck off, swap gloves, repeat

    • Don’t get solvent on inside

  • Spray wipe with IPA and wipe chuck off

  • N2 dry chuck

  • If chuck is not at spinner you want it at, assume it’s dirty

• Make sure you have pipettes for PR

Note: Set spinner to almost max time, always use your timer!

• Check the spinner by placing a junk wafer piece or non-critical sample on it. Make sure it spins correctly and at the correct RPM (4k) before placing on a critical sample or PR.

Spin on resist (AZ 5214E-IR, 4k rpm, 30 sec)

• Check Sample 1 and N2 blow

• Put Sample 1 on chuck and center

• Check centering and RPM

Note: When putting PR onto the wafer, try not to put on so much that it leaks off the sides because this can get into the vacuum and mess things up. Also, make sure that there is an adequate amount, as to completely coat the wafer with an even amount. If the integer number of drops you must apply is too little (too much) for the sample, wait for it to spread a little before spinning (spin immediately after application)

• Put PR (AZ 5214E-IR) in pipette carefully and drop a few drops into the PR container (to eliminate air)

• Drop on PR, no bubbles, and Spin

  • 4k rpm

  • 30 sec

• Check wafer when done. No streaks.

• Repeat as needed

• When done, empty pipette into PR container and return to fridge

• Clean chuck as above

• Clean spinner bench

Pre-Bake

(evaporate solvents before lithography)

• 100C hotplate: 1 min

• Let samples cool in boat while you prep aligner

• Check that mask is clean

  • Set hotplate to IR bake temperature

Exposures

First exposure (Soft contact, 11 sec)

• Choose stage for your sample

• Prepare height of aligner stage to be close to (but lower than) what you will want with the sample.

• Put Mask onto holder, ~1 mm from edge, turn on vacuum, slide into aligner, and tighten screws. Only touch the mask corners and edges.

Note: When placing the mask onto the holder, place it so that the chrome side of the mask faces away from the holder. (Brown side faces sample after placement.) Keep your hand under the mask when moving the loaded mask holder

• Load sample onto stage and lower stage a few turns

• While looking from the front of the aligner at the sample (between the mask and stage), rotate contact lever, and then raise the stage til it looks like the sample and its reflection on the mask are about to touch.

• Align features, then raise stage til contact.

  • The best way to tell contact is by looking (through the microscope) for rings to appear at a corner of the sample or an edge

  • Can also look for a brightening of the mask metal, meaning there’s pressure on the mask

  • Whichever corner or edge displays contact first on your first sample will probably be the corner/edge to display contact first for the other samples.

Note: Will feel a tiny increase in resistance of the knob.

• Parameters

  • Align + expose

  • Soft contact (we do this now to avoid sample movement)

  • 11 sec on MJB4 contact aligner

Note: Do not face the aligner when the wafer is being exposed. Also, make sure to move your other samples away from the aligner, as not to ruin them from scattered UV light. Usually this means putting a toolbox or bin between them and the aligner.

• Lower the stage using the front knob about four numbers (knob number increases)

  • Center the stage rotation knob then Rotate contact lever

  • Unload sample and Repeat as needed

IR bake

(Image Reversal Bake)

• 115 C hotplate: 115 sec (1 min 55 sec)

Note: Very important step, do not over/under bake

• Remove mask from aligner

Second exposure

(Flood exposure = w/o mask)

• 60 sec flood exposure on MJB4 aligner

• Do not use hard contact (can’t select for flood exposure)

Note: Only put one wafer on at a time so as not to ruin wafers with scattered UV light.

Develop and Inspect

• 30 sec in MF-26A followed immediately by DI cascade rinse: 2min

Note: Gentle agitation the whole time. Note: Does not stop developing after you take the wafer dipper out of the developer solution, make sure to immediately dunk into DI water and rinse.

• N2 blow dry

• Put stuff away and clean up bench

• Inspect Pattern

The key parameter for this process is the IR bake. If the pattern looks like features are way too big (overdeveloped), then the sample was under baked. If the features look too small (underdeveloped), then it was over baked. Also, too long of an Exposure 1 can lead to underdev, and too short can lead to overdev.