1. What are photoresists composed of, and how do they work?
Photoresists (also photo coatings) are primarily used in micro electronics and micro system technologies for the production of μm- and sub-μm structures. These resists are generally deposited by spin coating in a range between 250~6000 rpm. Depending on the type of resist, films of 100 nm to 200 μm can be produced with this technique. Thicker films of up to 1 mm are generated with casting procedures.
Alternative coating techniques are:
Positive photoresists as produced by Allresist are composed of a combination of film forming agents such as e.g. cresol novolak resins and light-sensitive components such as e.g. naphthoquinone diazide, which are dissolved in solvents such as e.g. methoxy propyl acetate (equivalent to PGMEA).
The addition of the light-sensitive component to the alkali-soluble novolak leads to a reduced alkalisolubility. After exposure at 308~450 nm (UV range) using an exposure mask, the light-sensitive component is converted into the respective indene carbonic acid derivative which enhances the alkalisolubility of positive resists by a factor of 100. After development, only those areas which were protected by the mask remain, while exposed areas are detached.
For the production of negative photoresists, in addition to novolak resins, bisazides, acid generators, and amine components are dissolved in solvents such as e.g. methoxy propyl acetate. Subsequent exposure leads to crosslinking of the negative coating which then becomes insoluble due to the increased molecular mass. After development, these exposed areas consequently remain on the substrate. In contrast to positive coatings, alkali-soluble novolak resins are not protected by the other components against the alkaline developer, which leads to a removal of unexposed areas.
Image reversal resists are positive resists with an additional amine component. Depending on the manufacturing process, positive or negative images can be generated (→ Question 13).
Further additions are e.g. adhesion promoters, surface smoothers, and dyes.