Physical spherical phase compensation in reflection digital holographic microscopy

Reflection configured digital holographic microscopy (DHM) can perform accurate optical topography measurements of reflecting objects, such as MEMs, MOEMs, and semiconductor wafer. It can provide non-destructive quantitative measurements of surface roughness and geometric pattern characterization with nanometric axial resolution in real-time. However, the measurement results may be affected by an additional phase curvature introduced by the microscope objective (MO) used in DHM. It needs to be removed either by numerical compensation or by physical compensation.We present a method of physical spherical phase compensation for reflection DHM in the Michelson configuration. In the object arm, collimated light is used for illumination. Due to the use of the MO, the object wavefront may have a spherical phase curvature. In the reference arm, a lens and mirror combination is used to generate a spherical recording reference wave in order to physically compensate the spherical phase curvature of the object wavefront. By controlling the position of the mirror and the sample stage, the compensation process has been demonstrated. The relative positions of the test specimen and the reference mirror must be fixed for the physical spherical phase to be totally compensated. A numerical plane reference wave is preferred for the numerical reconstruction of the phase introduced by the test specimen. Experimental results on wafer pattern recognition are also given.