Step-height measurement by means of a dual-frequency interferometric confocal microscope

A novel instrument, the dual-frequency interferometric confocal microscope (DICM), which facilitates the measurement of step features, is investigated. It combines the advantages of the high resolution (subnanometer) of heterodyne interferometry and the relatively large measurement range (approximately 5 microm) of confocal microscopy. The axial response curves of the confocal microscopy system are compared in experiments in which microscopic objects with various numerical apertures and magnifications are used. The results prove that the variation in light intensity is enough to permit discrimination of different orders of interference fringes. The DICM has been successfully utilized to measure the step height of a standard mask, and the experimental results agree well with those measured by scanning probe microscopes. The results also show that the system has good repeatability, with a maximum deviation of 5 nm.