selection for different processes. The primary properties of the photoresist are resolution capability, process dose and focus latitudes required for curing, and resistance to reactive ion etching. Other key properties are sensitivity, compatibility with tetramethylamonium hydroxide (TMAH), adhesion, environmental stability, and shelf life. Resolution Resolution is the ability to differ the neighboring features on the substrate. Critical dimension (CD) is a main measure of resolution. The smaller the CD is, the higher resolution would be. Contrast Contrast is the difference from exposed portion to unexposed portion. The higher the contrast is, the more obvious the difference between exposed and unexposed portions would be. Sensitivity Sensitivity is the minimum energy that is required to generate a well-defined feature in the photoresist on the substrate, measured in mJ/cm2. The sensitivity of a photoresist is important when using deep ultraviolet (DUV) or extreme-ultraviolet (EUV). Viscosity Viscosity is a measure of the internal friction of a fluid, affecting how easily it will flow. When it is needed to produce a thicker layer, a photoresist with higher viscosity will be preferred. Adherence Adherence is the adhesive strength between photoresist and substrate. If the resist comes off the substrate, some features will be missing or damaged. Etching resistance Anti-etching is the ability of a photoresist to resist the high temperature, different pH environment or the ion bombardment in the process of post-modification. Surface tension Surface tension is the tension that induced by a liquid tended to minimize its surface area, which is caused by the attraction of the particles in the surface layer. In order to better wet the surface of substrate, photoresists are required to possess relatively low surface tension. Positive photoresist DNQ-Novolac photoresist One very common positive photoresist used with the I, G and H-lines from a mercury-vapor lamp is based on a mixture of diazonaphthoquinone (DNQ) and novolac resin (a phenol formaldehyde resin). DNQ inhibits the dissolution of the novolac resin, but upon exposure to light, the dissolution rate increases even beyond that of pure novolac. The mechanism by which unexposed DNQ inhibits novolac dissolution is not well understood, but is believed to be related to hydrogen bonding (or more exactly diazocoupling in the unexposed region). DNQ-novolac resists are developed by dissolution in a basic solution (usually 0.26N tetramethylammonium hydroxide (TMAH) in water). Negative photoresist Epoxy-based resists One very common negative photoresist is based on epoxy-based oligomer. The common product name is SU-8