MicroScan: a DMD based optical surface profiler

A Digital Micromirror Device (DMD) consists of an array of e.g. 1024 x 786 small micromirrors. Each of these mirrors has a size of 16 mu m and can be individually switched to an angle of either +100 or - 100. Depending on how these micromirror pixels are switched various reflected patterns or images can be projected digitally when the DMD is properly illuminated. Whereas classical applications for DMDs include digital image- and video-projection, further applications in the field of optical metrology are possible as well. Since the DMD can also act as a pinhole, a new type of optical surface-profiler resembling a confocal microscope but using the DMD for flexible illumination and lateral scanning of the specimen has currently been set up at the Fraunhofer IPT. There are various process-parameters that influence the measurement, among these are e.g. pinhole-size, pinhole-shape, lateral step-size and the number of pinholes used simultaneously. With the presented instrument these process-parameters can not only be easily configured for individual measurements by simply programming the DMD but also adaptive control of these parameters during the measurement may be achieved. The paper explains basics of DMD technology and confocal microscopy, illustrates the principle of the newly designed instrument, discusses difficulties and shows first results achieved with a set-up demonstrator.