SU-8 Resists: FAQs

What is the difference between the SU-8 and SU-8 2000 resist products?
SU-8 resists were first introduced commercially by MicroChem in 1996. These resists are formulated in gamma butyrolactone (GBL) solvent. Recently we introduced the SU-8 2000 series, which are formulated in cyclopentanone solvent and have improved coating and adhesion properties. In addition they are compatible with conventional edge bead removal processes directly after spin coating.

How thick can I coat SU-8 or SU-8 2000 in a single spin coat process?
The SU-8 and SU-8 2000 resin/solvent systems have unique molecular properties, which enables us to build very high solids formulations with relatively low viscosity. This allows us to offer a broad range of products with a wider range of film thicknesses than any other commercially available resist. Our products are designed to produce uniform resist films from <1um to >300um with conventional single spin coat processes.

I often observe bubbles in my resist film after coating or during the prebake process. How can I eliminate the bubbles?
If bubbles are observed in the bottle, we recommend heating the resist bottle in an upright position (50-60 C) for 30 minutes, which will reduce the viscosity and allow the bubbles to rise out of solution. Once the bubbles have been removed, allow the resist to reach room temperature prior to use.

Bubbles are commonly observed in the resist film when dispensed with a syringe. To prevent bubble formation during dispense, increase the size of the orifice and remove the air from the syringe prior to dispensing. We are unaware of bubbles associated with auto dispense processes when dispensing from bubble-free bottles.

How do I produce SU-8 structures with near vertical sidewalls in very thick films?
SU-8 and SU-8 2000 resists are optically transparent and highly functional, which permits high aspect ratio structures in very thick films to be imaged with optimized lithographic processes. Optimization techniques include; spectral shaping of the exposure bandwidth- remove shorter wavelengths that are absorbed in the upper portion of the resist film and result in negatively sloped sidewalls; fine tuning the exposure dose and post exposure bake (PEB) process in order to obtain uniform cross-link density throughout the resist film and by optimizing the prebake and develop process. For more information about suggested baseline processes, please refer to our SU-8 and SU-8 2000 technical data sheets or your SU-8 technical sales representative.

What types of developer are compatible with SU-8 and SU-8 2000 resists?
SU-8 and SU-8 2000 resists are solvent developable. A number of safer solvent systems, including SU-8 developer, ethyl lactate and diacetone alcohol work well with SU-8. A two stage immersion develop process can be used to increase throughput, especially for very thick films or cyclopentanone (SU-8 2000 thinner), followed by a second bath of SU-8 developer.

Can thick film SU-8 structures be developed in spray mode?
Yes, they can. In fact we recommend spray develop processes and use them routinely in our labs for thin as well as thick film develop applications. Optimized spray develop processes are fast, clean, compatible with high aspect ratio structures and reduce the risk of resist adhesion failure commonly associated with lengthy immersion/sonic develop processes.

How do I strip SU-8?

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SU-8 is a highly functional epoxy and therefore extremely difficult to strip. Conventional stripper solvents, including MicroChem's Remover PG will swell and lift off partially cross-linked SU-8, but will not remove hard baked (cured) SU-8. However, dozens of SU-8 users have successfully developed stripping processes. Techniques include RIE plasma ashing, laser ablation, molten salt baths, CO2 crystal and water jets and pyrolysis, among others. Keep an eye on our site, we will add SU-8 stripping applications notes as we develop and or are notified about them.

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