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SU-8 Photolithography and Its Impact on Microfluidics
Published in Sushanta K. Mitra, Suman Chakraborty, Fabrication, Implementation, and Applications, 2016
Rodrigo Martinez-Duarte, Marc J. Madou
Photoresist stripping, in slightly oversimplified terms, is organic polymer etching. When using the photoresist as sacrificial material, the primary consideration is complete removal of the photoresist without damaging the device under construction. SU-8 photoresist is effectively stripped off with a strong acid such as H2SO4 or an acid-oxidant combination such as H2SO4-Cr2O3 or Piranha (detailed under substrate cleaning), attacking the photoresist but not silicon, silicon oxide, or glass. Other liquid strippers include organic solvent strippers (such as Remover PG from MicroChem and SRGM-Red Stripper from Gersteltec), alkaline strippers (with or without oxidants), and even acetone. These strippers can be used if the postexposure bake is not too long or occurs at a low enough temperature such that solvent molecules are still present in the polymer matrix and act as weak points for the stripper to attack. A hard bake is not recommended if SU-8 is to be removed. High temperatures or long baking times cause the resist to develop a tough “skin,” prevent attack from common strippers, and force a removal step using oxygen plasma.
Polymer Technologies
Published in Ghenadii Korotcenkov, Handbook of Humidity Measurement, 2020
As is seen in Table 19.8, SU-8, polyimide, and Parylene really have great potential for making various MEMS elements. However, the polymer most commonly used for manufacturing microcantilevers and membranes, acceptable for the use of humidity sensors, is SU-8 (Jiguet et al. 2004; Johansson et al. 2005; Lukes and Dickensheets 2013). SU-8, a photopolymerizable epoxy-acrylate polymer (glycidyl-ether-bisphenol-A novolac), is a negative photosensitive polymer used as a structuring material in MEMS and for fabrication purposes within the micro-total-analysis-system (lTAS) area (Carlier et al. 2004). SU-8 is highly transparent in the UV region, allowing fabrication of relatively thick (hundreds of micrometers) structures with nearly vertical side walls. The SU-8 photoresist is most commonly exposed with conventional UV (350–400 nm) radiation, although i-line (365 nm) is the recommended wavelength. SU-8 may also be exposed with e-beam or X-ray radiation. As it can be seen from its formula, the SU-8 polymer has quite low molecular weight (~7000) and thus, when non-cross-linked can easily be dissolved by a number of solvents (e.g., propylene-glycol-methyl ether, gamma-butyrol-acetone, and methyl iso-butyl ketone). Typically, the lithography of SU-8 involves a set of processing steps similar to standard thick photoresists (Kim and Meng 2016): (1) deposition on a substrate (usually via spinning); (2) a softbake to evaporate the solvent; (3) exposure to cross-link the polymer: the exposure of this polymer to UV light generates a strong photoacid, which protonates the epoxy groups of the monomer and starts a cross-linking reaction to create a highly cross-linked polymer (Martinez-Duarte 2014); (4) post-exposure bake to finalize the cross-linking; and (5) development to reveal the cross-linked structure. 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. The last one is good for gas and humidity-sensor design. Parameters of SU-8 are listed in Table 19.11.
Fabricating method of SU-8 photoresist conical nozzle for inkjet printhead
Published in Materials and Manufacturing Processes, 2018
Maocong Yi, Jianbo Feng, Zhifu Yin, Helin Zou
SU-8 photoresist is near-UV negative photoresist with high aspect ratio which is used as structural materials in microelectromechanical system field due to its good chemical and mechanical stability.[17] In this paper, the inkjet printhead nozzle plate was made by SU-8 lithography process. First, the SU-8 2015 photoresist was coated on polydimethylsiloxane (PDMS) substrate using spinner. Then it was patterned by photolithography with appropriate exposure dose and exposure gap. Finally, the nozzle plate was bonded to the open chamber, then the SU-8 conical nozzle can be fabricated which is helpful to inkjet, and this process is simple.