Abstract: The presence of particles on the wafer holder of semiconductor exposure equipment is detected by analyzing any up-and-down movement of the stage during the alignment, for exposure, of a wafer mounted on the wafer holder. The analysis of the up-and-down movement of the stage is conducted based on signals from a motor which moves the stage in the vertical (Z-axis) direction, or based on signals from a sensor indicative of the distance that the stage deviates vertically from a reference position. A feedback mechanism produces information representative of the up-and-down movement of the stage, and a computer processes the signals to produce data indicative of whether particles are present beneath the wafer on the stage.