SU-8 lithography

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SU-8 is a negative-tone, epoxy-based photoresist that is commonly used for creating PDMS molds for soft lithography. It has also been used as a masking material for metal electroplating and as a structural material for MEMS devices.

Equipment

SU-8 is restricted to processing the ROBIN laboratory except for the exposure step.

Spinning Equipment

Users can spin SU-8 on the Laurell Spinner on Solvent Bench 94 and on the Spinner CEE Apogee ROBIN. Hotplates on Solvent 94 are available for baking.

Exposure Equipment

Users can use the MJB 45S in the ROBIN laboratory or one of the contact aligners in the clean room to expose their SU-8 films.

A 360 nm low-pass filter (PL-360LP, Omega Optical) is available in the ROBIN laboratory. Using the filter is recommended for improved sidewall verticality.

Etching Equipment

SU-8 is epoxy-based and somewhat difficult to remove from substrates.

There is a heated tank of Remover PG on Solvent 94 that can be used to swell and delaminate SU-8 from your wafer. If you used OmniCoat with your SU-8 film, the Remover PG will dissolve the OmniCoat and release the SU-8.

O2 plasma can also be used to remove SU-8, especially thin films. SU-8 is an allowed material on the Glen 1000P and the YES Plasma Cleaner.

Piranha solution can also be used to remove SU-8, but is not as effective as it is on positive-tone photoresists. SU-8 etching with piranha is allowed on Acid Bench 92.

Characterization Equipment

The thickness of SU-8 can be chacaterized using the Dektak 6M in the ROBIN laboratory if the film is <200 µm thick or using the Nanospec 6100 if the thickness is <11 µm. For films thicker than 200 µm, please use one of the height profilometers in the dicing saw room.

Optical microscopy is essential for checking the development of SU-8 films.

Applications

SU-8 is most commmonly used in soft lithography to define molds for PDMS.

It can also be used as a structural material for MEMS devices and a masking material for electroplating.

Processing Parameters for SU-8

The SU-8 2000-series datasheets provide guidelines for preparing SU-8.

Recommendations

While the SU-8 datasheets are excellent starting points, our users have found several tips for processing SU-8.

  • For films at least 50 µm thick, it is recommended to allow the spun film to relax overnight while covered by a glass petri dish before doing the soft bake as recommended. This prevents bubbles and wrinkles in the film.
  • Soft bake for twice the recommnded time at 95°C to ensure that the solvent is completely baked off.
  • For thick films of SU-8, ramp the temperature directly on the hotplate and cool the hotplate to ~40°C after baking to minimize thermal shock.
  • Use 1.5x the recomended exposure dose to ensure the SU-8 is fully exposed and sticks to the substrate. Reduce or increase exposure dose depending on your features.
  • When using the long-pass filter, double your exposure time.
  • Wait 10 minutes after exposure for optimal Lewis acid formation before post baking.

External References