Experiments in Photolithography with SU-8 Photoresist

Apr 2017

Overview

Keywords: SU-8 Photoresist, SU-8 Developer, Spin coating, Photolithography, Photoresist, Cleanroom, Darkroom, Darkfield Mask, Mask Aligner, Reactive Ion Etcher (RIE), Microtechnology, Nanotechnology, MEMS, NEMS, NUFAB, Negative Photoresist

Procedure

  1. Clean a 3-inch silicon wafer and create a hydrophilic layer of silicon dioxide with the RIE using 100 SSCM oxygen at 13.3Pa for five minutes. The hydrophilic layer of silicon dioxide allows photoresist to stick and spread evenly during the coating process.
  2. Use the SU-8 spin coater to spin 3ml of SU-8 onto the wafer by spinning the plate at 100rpm/s for five seconds and then 300rpm/s for 5 seconds. The method of spin coating used creates an even layer of photresist of about 25 micrometers thick.
  3. The resulting wafer now needs to be soft baked with a hotplate. First it should bake for 65 Celsius for 3 minutes and then 95 Celsius for 7 minutes. This step solidifies the photoresist prior to the exposure which improves line fidelity by limiting the fluidity of the particles.
  4. Now the wafer is placed in the UV Flood System with a mask placed over it. An exposure dose of 200 mJ/cm2 for 22 seconds at an intensity of 8 W/cm2 should be performed.
  5. After exposure a post-exposure bake should be performed. Bake at 65 Celsius for 1 minutes and then 95 Celsius for 3 minutes on a hotplate.
  6. The wafer is now placed and soaked in a developer solution for about one minute. The developer will remove the photoresist that had been exposed to UV light. Then rinse the wafer with isopropyl alcohol. If white residue is present on the surface of the wafer, return the wafer to the developer solution. All the white residue should be removed before continuing. Dry the wafer with nitrogen gas.
  7. Examine the wafer to determine structural accuracy with a microscope.
  8. Bake the wafer at 200 Celsius for 6 minutes to improve cross-linking.

Technologies Used

  • Reactive Ion Etcher (RIE)
  • Positive Photoresist
  • Darkfield Mask
  • SU-8 Spincoater
  • Mask Aligner
  • Optical Microscope
  • Hotplate