As overlay requirements for integrated circuits have tightened, the control of mask distortions has become critically important. Rather than make the mask as rigid as possible, a feedback approach has been suggested in which overlay is measured and corrective displacements are thermally generated in a membrane mask. The principle corresponds to that used in adaptive optics, where an active feedback system is used to maintain perfect wave-fronts by distorting the optical surfaces. An important advantage of adaptive masks is that the feedback may be used either to obtain absolute accuracy, or to incorporate desired displacements, for example, to compensate for distortions in previous levels patterned on the wafer. Previous work focused on modeling the temperature distributions required for arbitrary mask displacement distributions. This modeling has now been verified in two very dissimilar experimental arrangements.

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