Initial Process Flow for Tunnel Junctions and TLM (For the complete beginner)

Written by Adam Crook July 1, 2010

Modified by Thien-An Nguyen August 10, 2010

Modified by Rodolfo Salas 9/15/11

Top Contact Lithography:

• Solvent Clean

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

  • Note: Create a clean workstation before beginning. Do this by getting clean wipes from the dispenser and lay them down anywhere you plan to work. Also, keep in mind that the wafer baskets are labeled. Only use those that are appropriate for your immediate task. Otherwise, keep them in the basket and close the basket to prevent any contamination.

  • Note: Gentle agitation during cleaning process is acceptable

  • Acetone‚ 2 min

  • 2-propanol‚ 2 min

  • DI water rinse‚ 2 min

  • Note: After placing wafer basket in the DI water, put under running DI water

  • N2 blow dry

• Dehydration bake

  • 150C oven, 5 min

• Spin on resist

  • Note: Check the spinner by placing a junk wafer piece on it. Make sure the it spins correctly and at the correct RPM before placing on your actual wafer. When putting PR onto the wafer try to not 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 so as to completely coat the wafer with an even amount.

  • AZ 5214E-IR

  • 4k rpm

  • 40 sec

  • Note: Check wafer when done. No streaks.

• Pre-Bake (evaporate solvents before lithography)

  • 90C oven, 10 min.

• First exposure (TLM mask 1)

  • Note: Remember to sign on/off machine

  • Note: When placing mask onto the holder, place it so that the chrome side of the mask faces away from the holder. (Copper side faces sample after placement)

  • Turn the knob in front to ~18 after putting mask into the aligner

  • Align features, then turn knob to ~13 or whenever contact is made

  • Note: Will feel a noticeable change in resistance of the knob

  • 13 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 so as to not ruin them from scattered UV light.

• IR bake (Image Reversal Bake)

  • 110C blue oven 2 min 15 sec

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

• Second exposure (Flood exposure = w/o mask)

  • 30 sec on MJB4 aligner

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

• Develop with AZ 726 mif

  • 60 sec followed immediately by

  • Note: Gentle agitation the whole time.

  • Note: Does not stop developing after you take the wafer basket out of the developer solution, make sure to immediately dunk into DI water and rinse

  • DI Rinse

  • N2 blow dry

The key parameter for this process is the bake. If the pattern looks like its features are way to big then the sample was under baked. If the features look too small then it was over baked.

Top Contact Metalization:

• Start vent of CHA#1 (takes about 12 min)

• Go to acid hood and prepare surface cleaning solution

  • HCL:DI (1:10)

• Once bell jar lifts on CHA #1

  • Dip samples for 15 sec in solution (precision is not needed here)

  • Rinse for 30 sec in DI

  • Blow dry with N2

• Take samples immediately to CHA #1

  • Vacuum and clean sealing surface

  • Load metals (vacuum hearth if dirty)

  • Load samples

  • Pump down

• Deposit contact metal

  • pGaAs or pGaSb, Ti/Pd/Au (100/200/1000 A), Ti/Pt/Au (100/300/1000 A)

  • nGaAs, Au/Ge/Ni/Au (100/100/100/1000 A)

  • nGaSb, Au/Sn/Au/Ni/Au (50/100/100/100/1000 A)

• lift off metal in Acetone or Kwik Strip

  • 20-40 min in Acetone or 40-60 min 90C Kwik Strip

  • Gentle spray with Acetone or Kwik Strip if possible

  • 2-propanol

  • DI rinse

  • Blow dry with N2

Up until now the TLM and tunnel junction processes are identical

Tunnel Junction process:

Requires etching through tunnel junction and back contact. For wet etching do the etch prior to back metallization for dry etching do the metallization first.

Mesa Etch

• Dektak metal stack and record height

• Prepare etch solution

  • GaAs, H3PO4:H2O2:H2O 1:1:25

  • GaSb - ??

• Etch for 20 sec

• Dektak to determine etch rate

• Etch 110% of desired etch depth

• DI rinse 2 min

• Blow dry with N2

Back Contact

• See top contact and use appropriate metallization

TLM process

Requires a second lithography to define mesa area and etching Mesa etch lithography

• Solvent clean consistent with top contact lithography

• Dehydration bake

• Spin on resist AZ 5209 (positive resist)

  • 3k rpm

  • 30 sec

• Pre-exposure bake

  • 90C oven, 5min

• Align and expose

  • 20 sec on MJB4 contact aligner

Etch Mesa

• Same as tunnel junction process

• Strip resist in Acetone for 5 min

• 2-propanol

• DI rinse

• Blow dry with N2