Three-position method for leveling optical surface automatically based on 2D PSD

A new method for leveling optical surface automatically based on 2D Position Sensitive Detector (PSD) was developed and applied in Total Integrated Scatterometer for qualification of superpolished substrates. For light path in the method, incident laser beam is required to be reflected by the sample surface onto PSD, which provides light spot position on photosurface at resolution of 5 μm. Firstly, 2D motorized translation stages under the sample translate it along the two orthogonal translation axes by a distance, and PSD yields correspondingly three positions, from which the sample surface's attitude expressed as analytic equation and normal vector is derived. Secondly, 2D motorized goniometers between the sample and translation stages rotate orderly in each direction of translation axes by angles computed according to rotation transformation to make the sample surface's normal vector vertical. Finally flyback translation is performed to verify the spot on PSD photosurface yields no displacement and the sample surface is level. In the paper, formulas from three positions to the surface's analytic equation and rotation angles are deduced in detail. Performance and error analysis according to the scatterometer's actual dimension shows that the leveling system's adjustable range is ±1.7° and the resolution of two translation axes is separately 21" and 0.5°. Experiment results show that the three-position leveling method's precision is less than 0.1°.