Photoresist
Photoresists (English photoresist ) used in the photolithographic patterning, especially in microelectronics and microsystems technology for the production of structures in the micrometer and sub-micrometer range, as well as in printed circuit board manufacture. The main raw materials for photoresists, polymers ( such as polymethyl methacrylate, novolak Polymethylglutarimid ) and epoxy resins (for example, SU -8), solvent such as cyclopentanone or gamma -butyrolactone, as well as a photosensitive component.
In addition to liquid photoresists there are hard or dry resist ( photo films).
Exposure
When exposure is defined as the selective processing of the photo layer through an exposure mask or photo template (photocopies of the original head image ) with the aim of changing the solubility of this layer locally by a photochemical reaction. The exposure mask consists of a UV-transparent carrier layer, such as quartz glass, and an absorber layer, such as chromium. After the photochemically recoverable solubility change, a distinction photoresists:
Negative resist
The negative photoresist polymerized by exposure and a subsequent annealing step for stabilization, that is, after the development of the exposed areas remain.
These photoresists are mainly used in micro systems technology for the production of the smallest structures in the micrometer and submicrometer range.
Positive resist
In positive resists of the already solidified coating is soluble by irradiation again for respective processing solutions, that is, after development only the areas are left, which are protected by a mask prior to the irradiation and thus are not exposed.
In semiconductor technology, the positive paints used are usually made of resin ( novolac ) together with a photoactive component (eg polymeric diazo compounds ) and a solvent. They are also placed on the substrate by spin coating from the liquid phase. In contrast to negative resists, they are then subjected to a heating step ( pre- bake Sheet ) Here, the solvent and the paint from escaping hardens. Subsequently, the resist is exposed according to the desired structure with UV light. The photoactive component breaks through to the light and the coating becomes soluble in the exposed areas. After exposure, these parts are washed away with a suitable developer solution and the unexposed portions of the photoresist are. After the development is usually followed by an annealing step again (English hard- bake ) to stabilize the photoresist mask.
Development
In the development of patterning the photoresist layer by dissolving the unexposed areas in negative resists and positive resists on the exposed areas is carried out with a suitable solvent ( developer liquid).
Negative resists are insoluble by photopolymerization upon irradiation with UV light. While weak pi bonds in the resist molecules ( intramolecular bonds ) are converted ( intermolecular bonds ) in strong sigma bonds between different molecules of the resist.
Result of the development is the finished adhesive mask.
Distance
After one or more subsequent process steps (eg, doping ) of the photoresist has to be removed. This can be done in two main ways: the wet chemical stripping ( from english to strip, dt, replace ') of the paint, for instance by using acetone, or plasma ashing. In this case, the coating is decomposed or oxidized by using a microwave-excited oxygen plasma, said gaseous reaction products. It is the method most commonly used in the semiconductor industry today.