A photolithography process may be carried out after cleaning the backside of a wafer by means of an apparatus that includes an illumination module for conducting an optical illumination operation of photolithography to the front side of the wafer, and a cleaning module for conducting a cleaning operation on the wafer backside. The size of the tile divisions are selected to maintain their area equal to, or less, than a predetermined maximum. The second ramp voltage generating circuit generates a second ramp voltage increasing with a gradient gentler than that of the rising edge of a sustain pulse and steeper than that of the first ramp voltage, at the end of each sustain period.