SU-8 photoresist

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 500 μm, the resist can be processed with standard contact lithography. Above 500 μm, 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 at IBM in the 1980s as a negative-tone, epoxy-based photoresist for microelectronics applications. Commercial production was established in the 1990s by MicroChem Corp., which licensed the technology and became the primary supplier to the MEMS and microfabrication markets. In 2017, MicroChem was acquired by Nippon Kayaku Co., Ltd. of Japan and now operates as Kayaku Advanced Materials, Inc., an American subsidary of Nippon Kayaku Co., Ltd, which remains the main commercial supplier of SU-8 resists.

SU-8 is now mainly used in the fabrication of microfluidics (mainly via soft lithography, but also with other imprinting techniques such as nanoimprint lithography) and microelectromechanical systems parts. It is also one of the most biocompatible materials known and is often used in bio-MEMS for life science applications.

SU-8 is composed of Bisphenol A Novolac epoxy that is dissolved in an organic solvent (gamma-butyrolactone GBL or cyclopentanone, depending on the formulation) and up to 10 wt% of mixed Triarylsulfonium/hexafluoroantimonate salt as the photoacid generator.

SU-8 absorbs light in the UV region, allowing fabrication of relatively thick (hundreds of micrometers) structures with nearly vertical side walls. The fact that a single photon can trigger multiple polymerizations makes the SU-8 a chemically amplified resist which is polymerized by photoacid generation. The light irradiated on the resist interacts with the salt in the solution, creating hexafluoroantimonic acid that then protonates the epoxides groups in the resin monomers. The monomer are thus activated but the polymerization will not proceed significantly until the temperature is raised as part of the post-expose bake. It is at this stage that the epoxy groups in the resin cross-link to form the cured structure. When fully cured, the high crosslinking degree gives to the resist its excellent mechanical properties.

The processing of SU-8 is similar to other negative resists with particular attention on the control of the temperature in the baking steps. The baking times depend on the SU-8 layer thickness; the thicker the layer, the longer the baking time. The temperature is controlled during the baking in order to reduce stress formation in the thick layer (leading to cracks) as the solvent evaporates.