Characterization of spatial light modulators for microlithography
The Fraunhofer IMS in Dresden is developing and fabricating spatial light modulators (SLMs) for micro lithography with DUV radiation. The accuracy of analog modulation is very important for the resulting accuracy of the generated features. On the other hand, fabrication tolerances create variations for example in spring constant, zero voltage deflection, and reflectivity. The slightly different response curves of the individual pixels therefore require an individual calibration. The parameters of these are stored in a look-up table so that the proper addressing voltage for the required optical response can be selected. As the deflection angle as well as the size of the SLM pixels are quite small, a direct measurement of the pixel response is not straightforward. An optical system similar to the one in the lithography machine has been set up, where the SLM is operating as a phase grating and the image is generated by a spatial filter. The pixel deflection can be calculated from the aerial image for isolated deflected pixels. The background pixels, that are not calibrated yet, contribute some error to this calculation. However, this error is not very large. Simulations regarding the accuracy of this measurement are discussed, and experimental results are shown.