SU-8

SU-8 is a commonly used epoxy-based negative photoresist. Negative refers to a photoresist whereby the parts exposed to UV become cross-linked, while the remainder of the film remains soluble and can be washed away during development.

As shown in the structural diagram, SU-8 derives its name from the presence of 8 epoxy groups. This is a statistical average per moiety. It is these epoxies that cross-link to give the final structure.

It can be made into a viscous polymer that can be spun or spread over a thickness ranging from below 1 micrometer up to above 300 micrometers, or Thick Film Dry Sheets (TFDS) for lamination up to above 1 millimetre thick. Up to 500µm the resist can be processed with standard contact lithography. Above 500µm absorption leads to increasing sidewall undercuts and poor curing at the substrate interface. It can be used to pattern high aspect ratio structures. An aspect ratio of (> 20) has been achieved with the solution formulation and (> 40) has been demonstrated from the dry resist. Its maximum absorption is for ultraviolet light with a wavelength of the i-line: 365 nm (it is not practical to expose SU-8 with g-line ultraviolet light). When exposed, SU-8's long molecular chains cross-link causing the polymerisation of the material. SU-8 series photoresists use gamma-butyrolactone or cyclopentanone as the primary solvent.

SU-8 was originally developed as a photoresist for the microelectronics industry, to provide a high-resolution mask for fabrication of semiconductor devices.

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