Procedure for Stamping on SiO2 Wafers
Hamza Aziz
The stamping procedure can be divided into four sub-procedures:
- Wafer Cleaning
- Metal Deposition
- Stamping
- Etching
A. Wafer Cleaning?
- Take Silicon dioxide wafers and put them in isopropanol solution and ultrasonicate for 5 minutes.
- After ultrasonication, take wafers out of the solution and wash them with nanopure water then isopropanol.? Dry the wafer with argon, or compressed air can.
(Optional)
- Place the ultrasonicated wafers in a clean, dry vial and put 1 part Conc. hydrogen peroxide then 2 parts Conc. H2SO4 (this solution is called Pirahna solution).
- Place a micro stirring bar inside the vial and put the vial in a water beaker on the hot plate.
- Heat the water up to boiling temperature while stirring the Pirahna solution.? Let this go on for about 10-15 minutes.
- Drain the Pirahna into the acid waste bottle.
- Take the wafers out and wash them with nanopure water and then dry them.?Rinse the vial thoroughly with water before throwing it away.
B. Metal Evaporation
- Take wafers and load them onto the holder in the evaporator
- Take the vacuum down to about 1x10-5 and deposit 3 nm of titanium on the wafer.
- After depositing titanium, deposit 50-100nm of Ag at the same vacuum.
C. Stamping?
- Prepare a .5mM alkanethiol solution, using octadecanethiol and ethanol.? Solutions of lesser concentrations can also be used, i.e .25mM.? The solution has to be stirred for a while before the alkanethiol dissolves.
- Hold the stamp from the sides and clean the surface with ethanol or isopropanol using a swab.? Try not to touch the surface while cleaning.? Dry the stamp.??
- Gently coat the stamp surface with alkanethiol using the swab.??
- Dry the stamp for approx. 1 min. in gas.
- Place wafer on a level surface.? The best thing is to put the wafer on top of another wafer.? This would guarantee a flat surface and prevent the PDMS from touching the table.
- Do not press down upon the PDMS because it would distort the features and could possibly coat the whole surface.
- Let the stamp sit for 10-15 sec., then take it off and let the wafer dry.
D. Etching
- Prepare a 20mL Silver etching solution (.3806g of K2S2O3, 0.0660g of K3Fe(CN)6, and 0.0084g of K4Fe(CN)6 and pour nanopure water to make 20mL solution.
- Pour solution in a 20mL vial and cover the vial with aluminum foil.
- Uncover the vial before use. Put a stirring bar in the solution and the put the setting for 300rpm.
- Pick up the wafer and dip it in the etchant for 16 seconds (for a 50nm thick Ag layer) then take the wafer out and wash with water and dry with argon.
- Then wash the wafer in HF for 15 seconds
- Then again wash it in the Ag etchant until all of the silver is gone.