SU-8 Photoresist

SU-8 Photoresist

SU-8 is a commonly used epoxy-based negative photoresist. It is a very viscous polymer that can be spun or spread over a thickness ranging from <1 micrometer up to >300 micrometer and still be processed with standard contact lithography. It can be used to pattern high aspect ratio (>20) structures. Its maximum absorption is for ultraviolet light with a wavelength of 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 solidification of the material. SU-8 series photoresists use gamma butyrolactone 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. It 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.

SU-8 is highly transparent in the ultraviolet region, allowing fabrication of relatively thick (hundreds of micrometers) structures with nearly vertical side walls. After exposition and developing, its highly cross-linked structure gives it high stability to chemicals and radiation damage. Cured cross-linked SU-8 shows very low levels of outgassing in a vacuum. However it is very difficult to remove, and tends to outgas in an unexposed state.

The main developer for SU-8 is 1-Methoxy-2-propanol acetate.