A system for processing semiconductor wafers has process units on a deck of a frame. The process units and the deck have precision locating features, such as tapered pins, for precisely positioning the process units on the deck. Process units can be removed and replacement process units installed on the deck, without the need for recalibrating the load/unload robot. This reduces the time needed to replace process units and restart processing operations. Liquid chemical consumption during processing is reduced by drawing unused liquid out of supply lines and pumping it back to storage.