The photo sensitive components of our positive photoresists belong to the group of naphthoquinone diazides (NQD). An addition of NQD-derivatives to cresol resins (novolacs) causes a chemical reaction that leads to the formation of naphthoquinone diazide esters with an alkali-solubility reduced by about two orders of magnitude. An exposure to short-wavelength light < 450nm causes chemical decomposition of naphthoquinone diazide esters and a release of nitrogen (Fig.: 1). The reactive carbene-intermediate then reacts with water to the respective indene carboxylic acid derivative (Süss reaction) which dissolve much faster in aqueous-alkaline developers, due to an acidity which is increased by 3 – 4 orders of magnitude. The conversion into indene carboxylic acids can easily be monitored in the absorption spectrum (Fig. 2). An exposure decreases the intensity of the characteristic absorption bands of naphthoquinone diazide esters at 345nm and 395nm considerably, furthermore a pronounced blue shift can be observed.
Fig. 1: Example for a conversion of naphthoquinone diazide esters into indene carboxylic acids (Süss reaction)
Fig. 2: Altered absorption behavior of positive photo resists after exposure to short-wavelength light