SU-8 Process Notes

Original Document Compiled by SeungBum? & AaronParness.

Contents:

Pereparation

• Items Prepared Personally: Wafer, Taflon Tweezers, Wipes, Blank Glass + Mask (or Cr Patterned Glass), SU-8, Developer, SU-8 Datasheet (download from Microchem site)
• Item Usually Stored in MERL Clean Shop: UV Filter (Optional), Aceton, Methanol, Isopropanol, Distilled Water

Equipment Description

• Spin Coating Equipment: Used to coat SU-8 on the wafer. Programmable.
• Hot Plate: Used to bake SU-8 coated wafer.
• Exposure Machines: Used to expose UV light through mask onto coated wafer.

SU-8 Process

Wafer Preparation

It doesn't need to clean wafer if it is new. In this case, just singe wafer on hot plate at 100C for 10 min or at 200C for 5 min.

1. Cleaning: A. 5 min acetone soak B. 5 min methanol soak C. 5 min IPA soak D. Rinsing with distilled water E. dry with air gun (optional)
2. Singe wafer at 100C for 10 min or at 200C for 5 min
3. Wafer cooling down in Room Temperature about 5 min (Optional)

Spin Coating

1. Program Spin Coating Equipment based on SU-8 datasheet. Still need little adjustment. (* for example, Ramp to 500 rpm at 100 rpm/second for 25 second, then Ramp to 1400 rpm at 300 rpm/second for 34 second.)
2. Put wafer on the center of spin coating equipment holder. Test rotating. If it looks like out of center, adjust it.
3. Pouring SU-8 on the center of wafer, from just above surface of wafer in order to reduce air bubble intrusion.
4. Start spin coating

Soft Bake

1. Pre-bake at 65C, time is written on the datasheet
2. Soft-bake at 95C, time is written on the datasheet
3. Ramp down on the hot plate at 65C for 5min
4. Take off wafer and let it cool for 10 min at RT
5. After cooling, check wafer by metal tweezers SU-8 surface is sticky or rigid. If it is sticky and soft, it should be something wrong. Raise temperature next time.

Exposure

Expose SU-8 under exposure machine. Dose time is set based on the datasheet. Energy of MERL exposure machine is 20mJ/sec.

Post Exposure Bake

1. Pre-bake at 65C, time is written on the datasheet
2. Post-bake at 95C, time is written on the datasheet
3. Ramp down on the hot plate at 65C for 5min
4. Take off wafer and let it cool for 10 min at RT

Development and Rinse

1. Soak in developer, development time is on the datasheet.
2. Rinse with IPA. If you see something white film appeared on the surface, then soak into developer again.

Hard Bake (Optional)

To dry and remove development residual, you may need hard bake.

1. Pre-bake at 65C for 5 min.
2. Hard-bake at 150C for 15 min.
3. Ramp down at 65C for 5 min.
4. Take off wafer and let it cool down for 10 min at RT

Process Tip

1. Always prepare zip lock bag to put in garbage from process. The bag is usually on the cabinet at MERL clean shop. After process, seal it and throw it into garbage bin at MERL clean shop.
2. Use 2 hot plates. Set one hot plate at 65C and another at 95C. This makes process faster. You just move wafer from one to another after pre-baked.
3. Always put aluminum foil on the hot plate surface for protecting hot plate equipment from SU-8 residual. This foil decreases surface temperature about 3C. So, you must set temperature add 3C~5C to value written on the datasheet. For example, set pre-bake temperature 68C or 70C.
4. Even after cooling down, touching by tweezers makes trench on the surface. To avoid it, use metal plate for carrying purpose. This metal plate looks like book end. You can find it near hood at MERL clean shop.
5. When exposure, using UV filter can make more clear vertical walls.
6. When exposure, if you need long exposure times, multiple exposure may be helpful to get good features. (for example, instead of 30 second one time, 15 sec twice with 10 second rest between each exposure.)