Abstract

In the 1;1 microlithography projection mask aligners described by Markle⁽¹⁾ and Offner⁽²⁾ (see Figure 1), as little as an eighth of a wave figure error on one of the mirror surfaces can introduce over half a micron lateral image distortion, causing unacceptable misregistration of the wafer patterns. Such a figure error can also shift the image focal plane by over four microns, causing a reduction in the useful depth of focus. Because of the geometry of the projection optics, it is difficult to measure the surface figures with sufficient accuracy to predict the distortions and focal shifts. An instrument was built to measure the distortions of the projection optics with an accuracy of 0.06 microns per site. Routine measurements at a large number of sites along the annular field permitted the identification of typical signatures in the distortion curves. In an attempt to correlate these signatures with mirror fabrication problems, typical mirror figures were modelled with a modified optical designers program. It will be shown that an equivalent surface figure can be calculated from the distortion data by a deconvolution operation. The process is used to predict the focal plane shifts (see Figure 2). The method by which these dummy surfaces are calculated, the effects of the type of polynomial approximations on the predictions, and the choice of the location of the surface will be discussed. The calculated shifts will be compared to the measurements, and the benefits for the manufacturing process will be discussed.

© 1981 Optical Society of America