Intro to DNQ-novolacresists

There are three basic ingredients to all commercial DNQ-novolac photoresists: a phenolic novolac resin, a diazonaphthoquinone (DNQ) type dissolution inhibitor, and an organic casting solvent (see Figure 2).² The novolac resin provides the physical properties required in the photoresist such as good film forming characteristics, etch resistance, and thermal stability. The DNQ makes it possible to image the photoresist by providing a photochemical route to modifying the dissolution rate of the resist in aqueous alkaline developers (see Figures 3). The organic solvent has properties that provide the ability to spin coat the resist to form uniform, glassy thin films.

Novolac resins are soluble in a variety of common organic solvents including cyclohexanone, acetone, ethyl lactate, NMP (1-methyl-2-pyrrolidinone), diglyme (diethyleneglycol dimethyl ether), and PGMEA (propyleneglycol methyl ether acetate). In photoresist applications, the casting solvent must be carefully selected to provide good handling, spinning, and film forming properties. The most commonly used solvent today in DNQ-novolac is PGMEA, due to its relatively benign characteristics with respect to human exposure. Resists are generally formulated with polymer loadings of 15 to 30 percent by weight with respect to the solvent content of the resist solution. The viscosity of the solution can be adjusted by varying the polymer to solvent ratio, thus allowing resists to be formulated for coating a variety of film thicknesses.²

Figure 1 - (above) two main components of DNQ-Novolak Resists

Figure 2 - (Below) Development rate of unexposed and fully exposed mixtures of a DNQ type inhibitor and a novolac resin. This type of plot is commonly referred to as a "Meyerhofer plot."⁴