Hardness of Rinse Water and Swelling Behavior of Dry Film Photo-Resist

We investigated the swelling behavior of dry film photoresist in rinse process after development by varying the hardness of water. We inspected the appearance of sidewalls and the foot of the resist. We also measured the depth of once swollen resist using a FIB (focused ion beam) and analyzed the chemistry of the resist after rinse using an XPS (X-ray photoelectron spectroscopy). We experimentally proved that divalent cations such as Ca2+ and Mg2+ in hard water could be exchanged with Na+ on the resist surface and quench swelling of the exposed resist in rinse. This study indicates that the use of hard water in rinse process may result in better line definition and resolution in PCB (printed circuit board).