Recently, several phase sensitive optical microscopes based on heterodyne techniques have been developed to evaluate small differences in optical path length. Unlike the case of mechanical stylus profiling (e.g. AFM or STM etc.), these techniques are noncontact and nondestructive, and the depth resolution may be obtained to better than 1Å. The differential heterodyne interferometer is suitable for precise measurement of step heights, since its differential configuration can greatly reduce disturbances from the environment. However the DHI has a problem of 2π ambiguity as most phase measuring interferometers. The DHI can measure only the phase from 0 to 2π because of the sinusoidal nature of the optical interference. Thus the measurable step height of the DHI is limited to one quarter of the wavelength of the light source.
In this study, an interferometer based on the differential heterodyne interferometry and the confocal microscopy for measuring large step heights is presented. In order to overcome 2π ambiguity, the proposed interferometer combines the differential heterodyne interferometer (DHI) with the confocal scanning microscope (CSM). CSM can measure large step height. And the differential heterodyne interferometer is less sensitive to environmental disturbance than the other interferometers. Therefore the proposed microscope can measure large step height with high resolution.
In order to get a lateral resolution exceeding the diffraction limit, we need to know the model of response formation. But interpreting the response of the differential heterodyne interferometer requires a complicated algorithm for processing the signal reflected from an object, because its form does not reproduce the profile of a surface under study, and the signal is not always linear with respect to profile parameters. In this study, we developed a method for interpreting the response of the DHI to a groove object.
