Phase-modulated differential confocal microscopy with compensating algorithm against specimen-induced aberration

Optical probing with confocal microscopes and related principles is a staple for the fast profilometry of microstructures and free-form objects. Due to the non-interferometric approach, their flexibility and robustness was considered higher for a long time. As the specimen shrink drastically and even single nanometers in axial direction are of concern, formerly overlooked errors become significant. The illuminated specimen itself may impose a wavefront deformation onto the reflected light that causes the instrument to reconstruct a wrong depth from a confocal peak. Here, we demonstrate a method, that can identify suspicious points of the measured profile and furthermore compensate the found deviation significantly. The algorithm is applied and based on the signals from an uniaxial differential confocal microscope with an oscillating pinhole as phase-modulator.