During exposure with matched UV-light, the photo active compound DiazoNaphtoQuinone- (DNQ-) sulfonate (left) releases a N2 molecule and converts into indene carboxylic acid hereby requiring a H2O molecule. Compared to unexposed DNQ-sulfonate, the carboxylic acid yields a resist development rate (alkaline solubility) several orders of magnitude higher.
In order to i) improve the DNQ solubility in the resist, ii) to increase the inhibitor property (dark erosion reduction), and iii) to improve the thermal stability, generally several DNQ-sulfonate molecules are bonded to a so-called backbone-molecule.The photo reaction quantum efficiency defines the number of above-mentioned reactions in relation to the photons absorbed in the resist film. Using a sufficiently transparent resin, a suited photon energy (g, h, i-line with respect to the specific DNQ) and a sufficient H2O-concentration in the resist (rehydration!), the quantum efficiency in DNQ-based positive-tone photo resists achieves values of typically 20-30%.If the resist lacks a minimum concentration of water, the ketone formed during the photoreaction may perform various side reactions (e.g. esterify with the resin or polymerize accompanied by CO2 separation). In both cases, the development rate solely increases by the reduction of the inhibitor (DNQ-sulfonate) concentration. |
Literature: Exposure and Development