SPR 955 is an I-line series of photoresists developed by microchem, it's similar to SPR 220 but is formulated for thinner viscosities and higher resolution. We've been able to consistently get 500nm gratings in 0.97µm of SPR 955 in the stepper, on silicon. The 0.97µm thickness was chosen because it's one of the peaks listed in the datasheets swing curve.
SPR 955 (0.97µm)
- Vapor prime wafer with HMDS
- Spin 0.97µm in ACS, CEE 200X #1 or CEE 100CB
- Bake at 100°C for 90 sec
- Verify thickness if possible, for thin resists small thickness variations drastically change the necessary exposure dose.
|Silicon||0.30 sec||5.5 sec|
- Bake at 110°C for 90 sec
Notes: When photoresist is this thin the swing curve will have a larger effect; small thickness variations will have a larger change in the necessary exposure. Also at the end of the process you will have closer to .9-.94µm of resist.
These gratings are on silicon and exposed via the stepper:
This plot shows the ideal exposure time for several develops in the ACS. The 0.3 sec exposure with a AZ 300 25 sec develop in the ACS seems to be a good point.
The next plot shows the CD loss vs the amount of defocus in the stepper. ±900nm is about the limit before it's no longer going to work. Larger features will of course have a wider focus range.