SU-8 photoresist

SU-8 photoresist is a high-contrast, epoxy-based photoresist that was originally developed in the late 1980s. It is primarily used in the field of microfabrication and nanotechnology for creating microstructures and MEMS (MicroElectroMechanical Systems) devices. SU-8's main advantage lies in its ability to form thick, high aspect ratio structures with excellent structural integrity, making it a popular choice for various applications in semiconductor manufacturing, biotechnology, and microfluidics.

Properties and Composition

SU-8 consists of three main components: an epoxy resin base, a photoactive compound (PAC), and a solvent. The epoxy resin is responsible for the structural integrity of the final product, while the PAC enables the resin to cross-link when exposed to ultraviolet (UV) light, a process known as photopolymerization. The solvent, typically gamma-butyrolactone (GBL), is used to adjust the viscosity of the photoresist for coating purposes.

One of the defining characteristics of SU-8 is its high sensitivity to UV light, which allows for the creation of very thick layers (up to several hundred micrometers) in a single coating step. This is in contrast to other photoresists that may require multiple coating and exposure steps to achieve similar thicknesses.

Applications

SU-8 photoresist has found widespread use in various fields due to its unique properties. In semiconductor manufacturing, it is used for creating high aspect ratio structures, such as waveguides and microelectromechanical systems (MEMS). In biotechnology, SU-8 is utilized for fabricating microfluidic devices, which are essential for lab-on-a-chip applications, and for creating molds for soft lithography. Additionally, its biocompatibility makes it suitable for certain biomedical applications.

Processing

The processing of SU-8 involves several steps, including pre-baking, exposure to UV light, post-exposure baking, and development. Pre-baking is necessary to remove the solvent and prepare the film for exposure. During exposure, the areas of SU-8 that are exposed to UV light undergo cross-linking, becoming insoluble in the developer. Post-exposure baking enhances the cross-linking process, improving the structural integrity of the developed features. Finally, the undeveloped areas are removed using a developer, revealing the microstructures.

Challenges and Limitations

While SU-8 photoresist offers many advantages, it also presents certain challenges. One of the main issues is the difficulty in removing the photoresist after processing, especially for thick layers, due to its high cross-link density. Additionally, the exposure and baking steps require precise control to achieve the desired results, and any deviations can lead to defects in the final structures.

Conclusion

SU-8 photoresist has become a cornerstone material in the fields of microfabrication and nanotechnology, enabling the creation of complex microstructures with high precision. Its ability to form thick, high aspect ratio structures in a single step, combined with its structural integrity and biocompatibility, make it a versatile tool for researchers and engineers alike.