Copolymer and PMMA for lift-off

Introduction

Lift-off is a particular technique to pattern a material layer on a substrate with no etching step, just by using lithography techniques. In this case, we use a copolymer to create an undercut in the pattern. This undercut is mandatory to obtain a good quality lift-off. Indeed, with the combination of undercut and metal evaporation, a shadowing effect will occur during deposition creating a discontinuity in the metal thin film thus easing the lift-off process.

Associated documents and references

Equipment Needed

Preliminaries

Copolymer EL11 is kept in the fridge. To use EL11, grap the bottle in the fridge and allow it to reach room temperature: about 1 hour. Then pour some of the resist into one of the amber container. Label the container and keep it at room temperature. Put the PMMA bottle back into the fridge.

Warning: Cold EL11 (or any other resist) bottle MUST NOT be opened as long as it doesn't reach room temperature as water will condensate in the bottle
 and deteriorate the resist quality and thus your process.

Process Steps

  1. Set one hotplate at 150 °C and another one at 180 °C
  2. Clean your sample using solvent or acid clean procedures
  3. Dehydrate the sample on hotplate at 180 °C for 5 min.
  4. Cool down the smaple on the chill plate
  5. Spin coat EL resist with one of the Laurell programms between E and H, according to the curve from PMMA datasheet
  6. Bake sample on hotplate at 150°C for 90s
  7. Cool down the sample on the chill plate
  8. Process PMMA as usual
  9. Spin coat PMMA according to the table at the end of this page. Look-up the PMMA datasheet for more detailed information: PMMA datasheet
  10. Bake sample on hotplate at 180°C for 90 s
  11. Exposition dosage needs to be tuned according to your patterns and resist thicknesses
  12. Develop in MIBK:IPA 1:3 or IPA:H2O 9:1. The development time depends on EL and PMMA thicknesses, and the size of the patterns
  13. Rinse in IPA after MIBK:IPA, or in water after IPA:H2O for 20s
  14. Blow dry with N2