Lift-Off with I-Line Imaging Resist + LOL2000 Underlayer
This process uses LOL2000 as the underlayer, underneath an SPR955 imaging resist layer.
The LOL2000 dissolves in developer regardless of exposure conditions, so as soon as the Imaging resist has been fully the developed, the LOL2000 will start to develop out and undercut.
The SPR955 Imaging resist can be exposed on any I-Line exposure tool. You can find starting exposure parameters for SPR955CM-0.8 on the recipe pages for the exposure tool you are using:
MA6 Contact / GCA Stepper #1 / GCA Stepper #2 / MLA150
You can also replace the SPR955 Imaging PR with another I-Line PR (for example, negative PR), assuming it uses a similar developer - just change only the Imaging PR bake/exposure steps below.
For Steppers and the MLA, you may want to run a focus-exposure matrix to determine the best dose/focus, since focus is a significant variable that depends on the underlying substrate's optical properties as well.
Tips
Please see our Bi-Layer Lift-Off Tutorial to understand the limitations and requirements for good lift-off.
- Remember that the PR protecting the gaps between adjacent metal traces can lift-off during the develop! You can reduce the develop time to mitigate this.
- Need underlayer thickness approx. 2x the desired metal thickness.
- The underlayer will develop laterally from both sides which can lift-off the imaging resist, so:
- Minimum gap between adjacent metals should then be: greater than 0.5 * underlayer thickness
- Want at least 30-50nm of underlayer width left to support the imaging resist, so the PR doesn't fall over/collapse.
- Make sure to use a vertical line-of-sight evaporation such as EBeam4 or EBeam1. EBeam3 is not vertical unless special care is taken.
Suggested Process for Liftoff
| PEII Asher: O2 300mT/100W, 1 min | This is equivalent to Dehydration Bake, eg. 135°C, 1min bake. |
| LOL 2000, Spin @ 2500 rpm, 40 sec | LOL 2000 is stored right on the spinner bench (not in refrigerator). Dispense into a bottle for your group.
See datasheet for develop rate vs. bake temp, and spin curves. |
| Bake at 180°C, 5 min | Use one of the bench-top adjustable hotplates. Bake temp determines dissolution rate. |
| SPR955CM-0.9 spin @ 3krpm (Spin Recipe #5) | This is the imaging resist. You can replace this with another I-Line PR, just make sure to also change the bake and exposure params. |
| Bake at 95°C, 90 sec | Use built-in hotplate in Bay 6 spinner bench. |
| Expose on I-Line tool with appropriate Dose/Focus | Get starting params (Dose/Focus) for SPR955CM-0.9 from the Recipes section of your favorite I-Line exposure tool:
MA6 Contact / GCA Stepper #1 / GCA Stepper #2 / MLA150 - For steppers and MLA, you may want to run a focus-exposure matrix to determine the best dose/focus. - You can overexpose to ensure full exposure if feature dimension is tolerant. |
| PEB at 110°C, 90 sec | |
| Develop in AZ300MIF,
90 sec |
Overdevelop ~30sec to undercut the LOL2000, after Imaging PR is developed through. |
| DI Rinse and N2 Dry | For very narrow/small PR ridges remaining (eg. <2µm), very gentle DI rinse + dry might be necessary to prevent PR collapsing. |
| Microscope inspection | May be able to see a 2nd line behind the Imaging PR edge, which is the undercut into the LOL underlayer. |
| PE-II Asher, O2, 300mT/100W, 30sec | Improves metal adhesion to substrate. May also widen features by few hundred nm. |
| Metal Evaporation:
Adhesion Layer (Ti, Cr): 5nm Metal Layer (any metal): 100nm |
Many common metals (Au, Pt) require the use of an adhesion layer like Ti or Cr, which create an interface between oxide and metal interfaces. For some metals you can omit this, for example Aluminum. |
| Liftoff:
Soak in AZ NMP rinse at 80 C (hot-water bath or hotplate) overnight (>>2hrs) |
Observe every few hours to see if metal has fully lifted off.
If you see metal still attached in large (>1cm) areas with photoresist, you need to wait longer.
- Soaking for 1hr in NMP, then - Transfer to Fresh NMP @ 70°C Ultrasonic bath for 5 min, then, - ISO beaker in Ultrasonic bath for 1 min, then - DI rinse and N2 Dry |
| Solvent clean:
ISO soak 1min, ISO soak (clean beaker) 1min N2 dry on napkin |
Only move to this step if metal liftoff has completed (PR has fully dissolved everywhere). |
| Microscope Inspection | Check for PR residue (use DIC/Nomarski mode), and clean metal liftoff. |
| Optional:
PE-II Asher, O2, 300mT/100W, 30sec |
If you see PR residues in the microscope, this can be used to etch away those residues. |
Written by Demis D. John, May 2022