Precise autofocus method based on an annular DOE and a cylindrical lens

With the wide application of lithography technology in the field of large-scale microstructure processing, it is necessary to achieve nano-scale autofocus in the range of hundreds of micrometers, which poses a new challenge to the autofocus method. Therefore, a precise autofocus method based on an annular optical diffractive element (DOE) and a cylindrical lens is proposed in this paper. Different from the traditional centroid method, this method uses an annular DOE and a cylindrical lens to modulate the beam section, so that the shape changes of the spot in the horizontal and vertical directions are inconsistent, that is, an elliptical annular spot is formed. Only one detector is used to detect the long axis and short axis of the spot respectively, and then the coarse focus position and the fine focus position can be calculated respectively. Combining these two results, we can perform autofocus with nanometer precision over a large range. In addition, our autofocus system is simple in structure. The feasibility and practicability of the method are verified by simulation and experiment.