Deformation measurement of a plate with damping contact

摘要

A direct measurement of the distance between wafer and lens in a wafer scanner is difficult to achieve. An alternative could be to place position sensors at the borders of the chuck and use knowledge of the modes of vibration of the plate to reconstruct the deflection at the point of interest. The fluid film between lens and wafer used in immersion optics acts however as a local damper, changing the dynamics of the plate. This influences the sensor measurements and the reconstructed deflection. Using a finite element analysis of a thin plate it is shown that the mean difference between the measured and the reconstructed deflection can be much higher than when no damper would be present.