A system is provided to prepare deionized water having a 100% saturated concentration of a gas, e.g., nitrogen, at a hot temperature, e.g., 50-85° C., and an attendant pressure, e.g., atmospheric pressure, to clean a semiconductor wafer, e.g., of silicon. The gas concentration of a first deionized water portion having a predetermined concentration of the gas at a cold temperature, e.g., 20-30° C., is adjusted in a gassifier chamber having a pressure pump and a pressure sensor, to provide a predetermined under-saturated concentration of the gas at the cold temperature. The temperature of the adjusted gas concentration first water portion is then adjusted by mixing therewith a second deionized water portion having a predetermined concentration of the gas at a predetermined very hot temperature, e.g., 80° C., in a predetermined ratio in a mixer having a temperature sensor. The flows of the first and second water portions are controlled by first and second flow controllers, to form a hot bath at the hot temperature having such saturated gas concentration to clean the wafer, e.g., in a cleaning tank under megasonic vibrations. A controller is connected to the pump, pressure sensor, temperature sensor and first and second flow controllers to control the chamber pressure and the operation of the flow controllers.