The differential heterodyne interferometer (DHI) is suitable for precise measurement of step height, since its differential configuration can greatly reduce disturbances from the environment. However the DHI has a problem of 2p ambiguity as most phase measuring interferometers. The DHI only can obtain the phase from 0 to 2p 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. This study describes a confocal differential heterodyne interferometer (CDHI) for measuring large step heights. The proposed CDHI is based on the differential heterodyne interferometry and confocal scanning microscopy. Using it, the step height of several micrometers can be measured with few nanometers resolution. In addition CDHI has a simple structure and rapid measurement speed.
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