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SU-8 Information Page
SU-8 comes in many varieties with different densities and viscosities, allowing for the fabrication of structures with a wide range of thicknesses. The correct SU-8 variety and spin speed must be used to achieve a specific thickness. Other fabrication procedures also change due to thickness and the type of SU-8.
The table below shows thickness, spin speed, pre-bake times, post-exposure bake times, and development times based off of data available in MicroChem's datasheets. Some recipes developed at BYU are also in the table. Also below are notes on SU-8 processing and helpful links and datasheets.
SU-8 |
Thickness (in µm) |
Spin Speed (in rpm) |
Minutes to Pre-Bake @65° |
Minutes to Softbake @95° |
Minutes to PEB @65° |
Minutes to PEB @95° |
Development Time |
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*All values approximate
**Indicates a BYU exclusive recipe - see SU-8 processing notes for more detail
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SU-8 Basics
- SU-8 is a negative, epoxy based, near-UV photoresist designed for MEMS and other microelectronic applications. It was originally developed and patented by IBM. SU-8 is processed using standard lithography techniques. When SU-8 is exposed to UV light its molecular chains cross-link, causing the SU-8 to solidify. SU-8 is highly transparent in the ultraviolet range. This allows for the fabrication of relatively thick (hundreds of micrometers) structures with nearly vertical side walls.
- Two companies have licenses from IBM to sell SU-8: MicroChem and Gersteltec.
Modifications to SU8 Recipes when using the South Aligner
- The south aligner in the BYU Cleanroom now has new filters. The following are changes that should be made to compensate for this new filter.
Photoresist / Application | Thickness (um) | Original Exposure Time (s) | New Optimal Exposure Time (s) | Percent Difference (%) | SU8-10 / Core | 5 | 15 | 21 | +40 | SU8-10 / Pedestal Core | 11 | 16 | 23 | +40 | SU8-10 / Ridge | 10 | 17 | 24 | +40 | SU8-10 / Pedestal Ridge | 18 | 16 | 23 | +40 | SU8-3005 / Micropore | 5 | 15 | 21 | +40 |
SU-8 Processing
- 1. Substrate Pretreat
- Clean wafer/substrate
- Dehydration Bake - 200° C for at least 5 minutes
- **BYU Dehydration Bake - 150° C for 15 minutes
- (Optional) Apply Omnicoat
- 2. Spin Coat
- Put wafer in spinner and set spin speed for desired thickness - see table for more information
- Microchem recommends: ramp to 500 rpm at 100 rpm/second then ramp to the final speed at 300 rpm/second and hold final speed for 30 seconds
- **BYU 5 µm SU-8 10 core: 500 rpm at 100 rpm/sec (6 sec) then 4400 rpm at 1200 rpm/sec (60 sec) and 6000 rpm at 6000 rpm/sec (2 sec) to remove edge beads
- Apply approximately 1ml of SU-8 per inch of substrate diameter to the center of wafer
- Spin to spread out the SU-8
- After applying the SU-8, avoid grasping the wafer with tweezers, as this will push up ridges in the SU-8.
- 3. Softbake
- Pre-bake the wafer at 65° C
- Ramp to 95° C and bake for more time
- A hot plate is the recommended baking method
- **BYU tip: cool back down to 65° C on the hotplate then to room temperature on a level nonmetal surface
- For bake times see the table
- 4. Exposure
- SU-8 is optimized for near UV (350-400nm) exposure
- For best results energy below 350nm should be filtered out
- Expose the wafer to UV light on an aligner
- If the time is too short, the features will come off during development
- Overexposure will increase the width of features
- Thicker SU-8 requires more exposure energy
- For more information on exposure energy check the MicroChem datasheets in the "Links and Datasheets" section
- 5. Post Exposure Bake (PEB)
- Bake the wafer at 65° C
- Slowly ramp to 95° C and bake for more time
- A hot plate is the recommended baking method
- Do not rapidly cool the wafer after the PEB
- **BYU tip: cool back down to 65° C on the hotplate then to room temperature on a level nonmetal surface
- For bake times see the table
- 6. Development
- Pour some SU-8 developer into a glass dish
- Place the wafer in the developer and gently agitate the developer the whole time
- Strong agitation is recommended for structures with a high aspect ratio or large thickness
- Development rates vary widely with agitation, temperature, and other processing parameters
- Pour the SU-8 developer into the waste container and rinse the dish with IPA when finished
- Typical development times are in the table
- 7. Rinse and Dry
- Rinse the wafer with IPA
- If a white film is visible while rinsing then more development is needed
- Dry with a nitrogen gun
- 8. Hard Bake/Cure(optional)
- A hard bake is recommended if the SU-8 is to be left on as part of the final device or if there will be further thermal processing
- A hard bake may also help to anneal any surface cracks after development
- Typical hard bake temperatures are in the range of 150 °C to 250 °C
- Typical times are between 5 and 30 minutes
- Better results seem to be achieved by ramping from room temperature to the bake temperature and then ramping back down
- **BYU 5 µm core process:
- **Hard bake 1 - ramp on hotplate from RT to 200°C, bake at 200°C 10 min, ramp down to 95°C
- **Descum - PE2, 90 sec 50 W, 100 sccm Oxygen
- **Hard bake 2 - ramp on hotplate from RT to 250°C, bake at 250°C 5 min, ramp down to 95°C
- 9. SU-8 Process Diagram from MicroChem
SU-8 Curing Images - Images of Cured SU8 from experiments done at BYU
SU-8 Links and Datasheets
- Table of Properties for SU-8 2000 and 3000 from MicroChem
- SU-8 2-25 Datasheet from MicroChem
- SU-8 50-100 Datasheet from MicroChem
- SU-8 2000-2015 Datasheet from MicroChem
- SU-8 2025-2075 Datasheet from MicroChem
- SU-8 2100 and 2150 Datasheet from MicroChem
- SU-8 3000 Datasheet from MicroChem
- MicroChem SU-8 page - An SU-8 supplier with useful SU-8 data sheets and FAQ
- Gersteltec SU-8 page - Another SU-8 supplier with data sheets
- MEMScyclopedia - Webpage dedicated to collecting all known data on SU-8
- SU-8 Forum - A forum created by Sporian Microsystems for discussing SU-8
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