Negative Photoresist Photolithography Process
Lithography consists of six basic steps: Wafer Preparation, applying the photoresist coat,
softbaking, exposing, post-exposure baking, and development.
Notes: Carry the wafers being processed in a quartz wafer carrier during the lithography process.
Prepare Wafer
- Cleaning
- Cleaning - Use Simple Clean with Acetone and IPA
- Dehydration Bake
- Set the Clean Oven to 120° C.
- Transfer wafers to a metal wafer carrier.
- Bake the wafers for 10 minutes in the oven.
- Remove the wafer carrier from the oven using the metal carrier handle.
- Transfer wafers back to quartz carrier
Apply Photoresist
- Adhesion Promotion
- Adhesion Promotion is not a necessary step but can be used to get better adhesion of photoresist to substrate. Adhesion promotion is especially useful when dealing with very small features < 1 micron.
- HMDS
- Mount the wafer on the Laurell Spinner.
- Apply 4 drops of HMDS to the wafer
- Spin the wafer at 3000 rpm with an acceleration of 500 rpm for 10 seconds.
- SurPass3000> (Wafers do not need dehydration bake)
For best results, use the spin coat application. Immersion clean can also be used.
- Dispense/Spin Coat Application
- To DI water wetted substrate, dispense 1 mL/seconds for 30-100 seconds, 100-500 rpm. Do not allow substrate to dry prio to final risnse.
- Final rinse with DI water, 30-60 seconds.
- Spin Dry
- Immersion Clean
- Soak wafer for 30 seconds in SurPass 3000 container.
- Rinse 30-60 seconds in DI water.
- Blow dry with N2 gun.
- Photoresist Coating
- Mount the wafer on the Laurell Spinner (Wafer should already be mounted if HMDS was used).
- Apply a quarter sized puddle of resist to wafer.
- Determine the required spin speed from the spin curves or Process Parameters Tables.
- Set the spinner to spin at that speed with an acceleration equal to the speed/second (no ramping). This might need to be modified (resist dependent).
- Spin
- Clean the spinner when done.
Softbake
- Determine the softbake temperature from the Process Parameters Tables.
- Heat the hotplate next to the Laurell Spinner to the required temperature.
- Place the wafer on the hotplate, start timing.
- When the time is up, remove the wafer.
Expose
- Clean Mask
- Place the mask in the mask cleaning holder.
- Apply Acetone to the chrome surface. Do not allow the mask to dry with Acetone on it.
- Scrub the chrome surface with a swab.
- Rinse the mask with IPA.
- Expose
- Determine the exposure dose from the swing curve for the photoresist (or from the Process Parameters Tables).
- Measure the light intensity of the Karl Suss aligner (~10mW/second).
- Calculate the exposure time (Exposure dose / Measured Intensity)
- Expose the wafer.
- If soft or hard contact was used, clean the mask.
- Create an entry in the Karl Suss aligner log book when done.
Post Exposure Bake
- Determine the bake temperature from the Process Parameters Tables.
- Set a hotplate to the required temperature.
- Place the wafer on the hotplate, start timing.
- When the time is up, remove the wafer.
Develop
- Place the wafer on a dry wafer holder.
- Determine the required development time from the Process Parameters Tables and the set the wet bench timer.
- Start the timer. Place the wafer in the developer bath (using the wafer holder).
- Quickly remove the wafer from the developer bath and rinse it in the water bath for 30 seconds (using the wafer holder).
- Place the wafer on a clean room wipe. Blow dry with N2 gun.
- Dry the wafer holder.