Characteristics of an autofocus system on a grating with period smaller than the focus beam wavelength

摘要

In this paper, characteristics of an autofocus system on a grating with period smaller than the focus beam wavelength are investigated. The through-the-lens autofocus system, which has a visible light source of 670nm wavelength and a light radiation mechanism for causing light to be obliquely incident on a sample surface, has been prepared for experiments. It is shown in experiments that the focusing error for a grating with 0.6-micrometer period is larger than 0.2 micrometer, and polarization of reflected light is changed from circular to elliptic. By performing RCWA simulations, the qualitative correspondence of theoretical expectations with experimental results is obtained. Based on the results of experiments and simulations, methods of reducing the focusing error are proposed. One of the methods is to use the polarization information to correct the focusing error. The method is evaluated experimentally and is shown to achieve autofocus accuracy of +-0.1 micrometer.