SU-8 (photoresist)

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structure

SU-8 is a photoresist from Microchem Corp. and belongs to the group of negative photoresist. Like most photoresists, SU-8 consists of three components: base resin , solvent and photosensitive component . SU-8 is mostly used in microsystem technology for ultraviolet LIGA processes.

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 version of the paint, this base resin can be dissolved in different solvents. In the standard version, γ-butyrolactone (GBL) is used as the solvent, while the more modern version SU-8 2000 contains cyclopentanone . In contrast to other (positive) photoresists, the photosensitive component of SU-8 only has an indirect effect on the solubility of the resist. As photosensitive component the resist is a photo initiator (also English Photo Acid Generator called or PAG) added. It is about 10% by weight of a triaryl sulfonium hexafluoroantimonate . Triarylsulfonium hexafluoroantimonate is a Lewis acid that sets a chain reaction in the photoresist in motion when exposed to UV light. As a result of this chain reaction, hydrogen ions are separated from the EPON molecule and the binding sites that are released lead to cross-linking of the paint molecules, known as curing.

SU-8 is available in different viscosities , which are controlled by the proportion of solvent in the resist. The viscosity also determines the range of layer thickness that should be achieved with the resist. The layer thickness in µm, which is achieved at a speed of 3000 min −1 , is used as a designation for the respective version of the paint. Common viscosities are, for example, SU-8 2, SU-8 10 or SU-8 100.

By means of a baking process on a hot plate or in a suitable oven , the so-called soft or prebake, most of the solvent is evaporated at a typical temperature of 95 ° C, whereby the previously liquid resist solidifies after cooling. Heating the lacquer above the glass transition temperature of 55 ° C leads to a reliquefaction of the layer. The exposure converts the photoinitiator salt into an acid . During the subsequent so-called Post Exposure Bake (PEB), the acid induces a polymerization through which it is regenerated. A single photon can thus trigger a series of polymerizations, which makes the lacquer highly photosensitive.

literature

  • Friedemann Völklein, Thomas Zetterer: Practical knowledge of microsystem technology . 2nd Edition. Vieweg + Teubner, 2006, ISBN 978-3-528-13891-2 .