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.