Electrochemical evaluation of adhesion between photoresist and invar alloy for fine etching

Shadowmasks used for color cathode ray tubes (C-CRT) have electron pass holes formed by photoetching technique. The accuracy of shapes and dimensions of the holes is controlled by the adhesion between photoresist and the substrate alloy. The adhesion is empirically known to be influenced by the pretreatment of substrate before coating the photoresist. The aim of this study is to evaluate quasiquantitatively the effect of conditions of the pretreatment on the adhesion by electrochemical technique. Thin sheets of Invar alloy (Fe-36 mass% Ni) were used as specimens. As factors that influence the adhesion, the concentration of detergent, the drying time and temperature, and the quantity of spread water were selected. The specimens coated with photoresist after different pretreatments, except for etching patterns, were potentiostatically polarized at 0.3 V (vs. Ag/AgCl, 3.33 kmol·m^-3 KCl) in 0.1 kmol·m^-3Na₂SO₄ + 0.003 kmol·m^-3 NaCl solution. Time-dependent change in current at 0.3 V and the initiation of crevice corrosion at photoresist / substrate interfaces at the same potential were examined. Etching tests were also performed on the specimens coated with photoresist in a FeCl₃ solution of sp.gr. 1.485 at 293 K. In result, it was found that the specimens that showed increasing current during the potentiostatic polarization at 0.3 V suffered from crevice corrosion owing to the bad adhesion of photoresist. These specimens that were susceptible to crevice corrosion also showed small etching factors and large errors in the dimension of etching holes.