E-beam Resist Handling Procedure:
Revision 2.3 October 28, 2003
James W. Conway
Electron Beam Lithography Resist often are more moisture sensitive and reactive than the Novalak based resist systems employed in optical lithography. They will require more care and diligence in their handling to achieve a quality process result, minimize contamination, and to achieve uniform film thickness across your wafers.
Our standard E-beam resist we use here at SNF is 5% 495K Molecular Weight PMMA, Poly Methyl Methacrylate, but these guidelines are applicable to all E-beam resist handled here in the Lab. The 2% 950 K MW PMMA is our high resolution resist.
Please review the Optical Lithography manual coating processes for additional info.
Cleanliness of process benches, application equipment, wafer chucks and
-Always wipe down the process bench surfaces with IPA or water using lint-free cloth. This aids in removing contamination sources and particulate debris from the deck surfaces. Be sure to maintain the proper chemical handling methods, as some benches do not allow solvents to be used in them. Ask a staff member to help you.
-Deck and Spinner interior surfaces must be covered with Aluminum Foil to minimize particles and traces of materials from previous processing operations.
-Solvent clean your syringes and wafer chuck in Acetone and IPA at the Solvent Bench, blow off these items with the N2 Blow off gun, and allow to dry an addition minute or two on lint-free before moving back to the spinner area.
- All wafers you cover with resist should have been cleaned using standard wafer cleaning methods, depending on your contamination level dependency. Clean and semi-clean wafers should undergo the Std. Pre-diffusion Wafer Cleaning process.
Deposition of Resist Material:
The Headway Spinner in the Photolithography area is our normal PMMA spinning tool. It has very reproducible spin speed control and will achieve as good as 1% thickness uniformity across a 100 mm Si wafer.
- Completely cover the interior of the bowl with aluminum Foil. NO solvents or resist material should ever be introduced to the drain of this bench whatsoever.
- You should run several dummy wafers before starting in order to set and confirm the spin speed and set time accurately to the process specifications.
- It is important to maintain cleanliness and minimize exposure to air of both the resist in the storage bottles and the material to be dispensed onto your wafers.
To reduce cost and minimize waste of resist material you should use either clean BD Type II Glass, or Plastic PE/PP syringes, and PTFE filters to transfer and dispense your resist material on to your wafers. I recommend the 10 ml size syringes. Plastic syringes and filters are available in the SNF stock room. BD Type II syringes are available at Bio-stores. Small chips coating can employ the 2.5 ml syringes.
- Direct from a wide mouth bottle using the syringe without the Luhr lock filter, fill the syringe to the fill line. Invert the syringe end up, push to plunger bleed off any extra air bubbles and place the filter onto the tip securely. Prime the filter to remove bubbles, and allow a very small volume of resist (~1ml) to flow out of the syringe. This can go directly into the foil in the base of the spinner. (But never allow any material to go down the drain, as it should all be covered with Al foil.)
Spin Deposition of Resist Material:
- Carefully place and center your wafer onto the Wafer Chuck, and confirm the vacuum chuck securely holds your wafer. Close the cover of the Spinner if open.
-Blow off your wafer with N2 Gun to displace particles.
-Dispense 2 4 ml of resist material covering approximately 1/3 of the wafer surface and without delay immediately activate the spinner by pressing gently on the pedal on the floor. Allow the spinning operation to complete. Stand back! If a wafer flies off the spinner simply press the rear section of the pedal to stop the spinning operation.
- Carefully remove the wafer with tweezers, grasping the wafer perpendicular to the cleavage planes adjacent the flat, and place on either the hotplate or into a quartz wafer boat for the oven soft bake.
-Repeat with your next wafer(s) until you are finished.
PLEASE: Clean up the Headway Spinner area after each use:
Remove all resist and Al Foil from the spinner bowl and wipe down the edges and deck surfaces removing all traces of resist material. This waste is placed into a polyethylene bag and label with a hazardous waste label. Place into the waste receptacle. Replace the foil covering all surfaces of the spinner bowl.
Wipe down the deck leaving a clean area ready for the next Lab members use.
Soft Baking of Resist Materials:
Soft Bake to remove most of the solvents from the resist and to form a continuous polymer layer of resist across your wafers. You must fully cure your films before exposure in order to not contaminate the UHV vacuum system of the EBL systems. This is very important!
Some of our PMMA processes can be performed on a Hotplate, other processes are better soft baked in a N2 purged Oven. The Blue M Oven using Process No. 1 at 170 degrees Celsius for 2 hours is the standard bake process for Std. 5% 495K MW PMMA. Hot plate bakes can also be employed to soft bake your wafers.
Carefully remove the wafers from the Quartz oven boat and place them immediately into a clean Polypropylene wafer carrier box. Be sure the new box is labeled with your Name, CORL login, Process Lot Number, and Process Name and Date.
Optical Microscope Inspection:
Many defects and artifacts of the spin process can be observed by eye such as Comets and Spin irregularities. But you should also examine the films using an optical microscope at both 5 10 X and at 200 500 X magnification. Seek out and count the number of particles and defects you find in 20 fields of view and note the counts in your lab notebook. This takes only a minute or two but yields good information.
This will help you later to identify the best wafers for your work and to monitor the cleanliness of the wafers you are processing.
You did inspect your wafers after the cleaning process right!!
Store your wafers before exposure in a clean dry location preferably
w/ N2 purge.
If you run into problems any SNF staff member is happy to assist you! -- JWC
Stanford Nanofabrication Facility
Last Modified 10/28/2003