SU-8 photoresist

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.