Photoresist
Photoresists ( English photoresist ) are used in the photolithographic used structuring, in particular in the microelectronics and micro-systems technology for the production of structures in the micrometer and submicrometer range and in circuit board manufacture . The most important raw materials for photoresists are polymers (e.g. polymethyl methacrylate , novolak , polymethylglutarimide ) or epoxy resins (e.g. SU-8 ), solvents such as cyclopentanone or gamma-butyrolactone , and a photosensitive component.
In addition to liquid photoresists, there are also solid or dry resists ( photo films ).
exposure
During exposure, the solubility of the photo layer is locally changed by ultraviolet and an exposure mask or photo template (photocopies of the conductor image original) ( photochemical reaction ). The exposure masks consist of an ultraviolet-permeable carrier layer ( quartz glass , polymer film) and an ultraviolet-impermeable layer (for example chrome or also ink or printing ink). According to the change in solubility, one differentiates:
- Negative resist (Engl. Negative resist ): solubility increases by exposure from
- Positive resist : Solubility increases through exposure.
Negative resist
The negative resist polymerizes through exposure and a subsequent baking step; after development, the exposed areas remain.
These photoresists are mainly used in microsystem technology for the production of the smallest structures in the micrometer and submicrometer range.
Positive varnish
In the case of positive resists, the already solidified resist becomes soluble again for corresponding developer solutions through exposure and after development only those areas remain that were protected from the radiation by a mask.
In semiconductor technology , the positive resists used usually consist of resin ( novolak ) together with a photoactive component (e.g. polymeric diazo compounds ) and a solvent. They are applied to the substrate as a liquid by spin coating. In contrast to negative resists, they are then baked out ( pre-bake ), the solvent escapes and the paint hardens. The varnish is then irradiated with ultraviolet in a structured manner. The ultraviolet breaks the material bond in the lacquer with the photoactive component and the lacquer becomes soluble in the exposed areas. After exposure, these parts are rinsed away with a suitable developer solution and the unexposed parts of the photoresist remain. After development, there is usually another hard-bake to stabilize the photoresist mask. See also photolithography (semiconductor technology) .
development
During development, the photoresist layer is structured by removing the unexposed areas in the case of negative resists or the exposed areas in the case of positive resists using a suitable solvent ( developer liquid ).
Negative resists become insoluble when exposed to ultraviolet radiation as a result of photo polymerization . Weak pi bonds in the resist molecules (intramolecular bonds) are converted into strong sigma bonds between different resist molecules (intermolecular bonds).
The result of the development is the finished adhesive mask or coating for the subsequent etching (deepening or removal of the underlying layer).
distance
After the process steps have taken place (for example doping ), the photoresist mask must be removed again. This is essentially done in two ways:
- by stripping (from English to strip )
- with an organic solvent, e.g. B. acetone (conditionally suitable because of its high vapor pressure, 1-methyl-2-pyrrolidone is more suitable )
- usually with caustic soda , if the substrate allows it
- with special removers (alkaline or neutral liquids)
- by means of plasma incineration : the paint is decomposed or oxidized with the help of a microwave-excited oxygen plasma , whereby only gaseous reaction products are created. It is the most widely used method in the semiconductor industry today.
Web links
Description and recipes of historical photosensitive coatings for etching processes
literature
- E. Roßhaupter, D. Hundt: Photoresists. In: Chemistry in Our Time . 5, No. 5, 1971, pp. 147-153, doi : 10.1002 / ciuz.19710050504 .
Individual evidence
- ↑ Archive link ( Memento of the original from December 17, 2016 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice.
