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

Clean the wafer, 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 prior to final rinse. 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

This step is generally only for negative resists. 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.

Hard Bake

This step is generally only for positive resists. 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.