SU -8 is a photo-resist of the company Microchem Corp.. and belongs to the group of negative resists. Like most resists SU-8 consists of three components: a base resin, solvents and photosensitive component. For use SU -8 is mainly used in microsystem technology in ultraviolet LIGA process.
The base resin in this case is EPON Resin, an epoxy resin from Shell Chemical, which has a total of eight epoxy groups in the molecule. EPON consists of a glycidyl ether derivative of bisphenol A. Depending on the design of the paint, this basic resin is dissolved in different solvents. The standard version of γ -butyrolactone (GBL ) is used as a solvent, the contemporary version of SU- 8 2000, however, contains cyclopentanone. The photosensitive component of SU- 8 functions, in contrast to the other (positive ) photo resist, only indirectly related to the solubility of the resist. As a photosensitive component of the paint is a photoinitiator salt (also known as photo Acid generator or PAG ) added. It is a triarylsulfonium hexafluoroantimonate to about 10 % by weight. Triarylsulfonium hexafluoroantimonate is a Lewis acid which is under the influence of UV light, a chain reaction in the photoresist into motion. As a result of this chain reaction, hydrogen ions are separated from the EPON molecule and the vacant binding sites leads to a crosslinking of the paint molecules, the so-called curing.
SU-8 is available in various viscosities, which are controlled by the proportion of solvent in the resist. The viscosity also determines the area of the layer thickness to be achieved with the resist. This is the layer thickness in microns, which is achieved at an engine speed of 3000 min -1, as a designation for each execution of the paint. Common viscosities, for example, SU- 8 2, SU -8 or SU -8 10 100
By means of a baking on a hot plate or in a suitable furnace, the so-called soft or prebake is evaporated at a typical temperature of 95 ° C the largest part of the solvent, whereby the previously liquid resist is solidified by cooling. Heating the coating to above the glass transition temperature of 55 ° C leads to a reflow of the layer. By the exposure of the photo-initiator salt is converted into an acid. During the following, so-called post exposure bake (PEB), the acid induced polymerization, by which it is regenerated. A single photon can thus trigger a series of polymerizations, which have a high photosensitivity of the resist due.