(DQ) 20-50% weight photosensitive hydrophobic, not water soluble Phenolic Novolak Resin (N) Frequently used for near-UV exposures Water soluble UV exposure destroys the inhibitory effect of DQ Issues: Adhesion, Etch Resistance Negative photoresist A negative photoresist is a type of photoresist in which the portion of the photoresist that is exposed to light becomes insoluble to the photoresist developer. The unexposed portion of the photoresist is dissolved by the photoresist developer. Based on cyclized polyisoprene (rubber) variety of sensitizers (only a few % by weight) free radical initiated photo cross-linking of polymers Issues: potential oxygen inhibition swelling during development long narrow lines can become wavy swelling is an issue for high resolution patterning Example: SU-8 (epoxy-based polymer), good adhesion) Modulation transfer function MTF (modulation transfer function is the ratio of image intensity modulation and object intensity modulation and it is a parameter that indicates the capability of an optical system Differences between positive and negative resist The following table is based on generalizations which are generally accepted in the microelectromechanical systems (MEMS) fabrication industry. Types Based on the chemical structure of photoresists, they can be classified into three types: photopolymeric, photodecomposing, photocrosslinking photoresist. Photopolymeric photoresist is a type of photoresist, usually allyl monomer, which could generate free radical when exposed to light, then initiates the photopolymerization of monomer to produce a polymer. Photopolymeric photoresists are usually used for negative photoresist, e.g. methyl methacrylate and poly(phthalaldehyde)/PAG blends Photocrosslinking photoresist is a type of photoresist, which could crosslink chain by chain when exposed to light, to generate an insoluble network. Photocrosslinking photoresist are usually used for negative photoresist. Photodecomposing photoresist is a type of photoresist that generates hydrophilic products under light. Photodecomposing photoresists are usually used for positive photoresist. A typical example is azide quinone, e.g. diazonaphthaquinone (DQ). For self-assembled monolayer SAM photoresist, first a SAM is formed on the substrate by self-assembly. Then, this surface covered by SAM is irradiated through a mask, similar to other photoresist, which generates a photo-patterned sample in the irradiated areas. And finally developer is used to remove the designed part (could be used as both positive or negative photoresist). Light sources Absorption at UV and shorter wavelengths In lithography, decreasing the wavelength of light source is the most efficient way to achieve higher resolution. Photoresists are most commonly used at wavelengths in the ultraviolet spectrum or shorter (<400 nm). For example, diazonaphthoquinone (DNQ) absorbs strongly