Adaptive alignment of photomasks for overlay improvement

The International Technology Roadmap for Semiconductors defines a 23 nm overlay error budget at the 65 nm technology node by the year 2007. To meet the stringent requirement, a proof-of-concept adaptive alignment strategy for overlay improvement has been proposed. By positioning a photomask in an adaptive holder that utilizes piezoelectric actuators and passive fixture supports, overlay registration can be significantly improved by deforming the mask with appropriate forces. The desired forces were calculated by solving an inverse problem in which unit loads are applied at actuator sites to determine the mark displacement sensitivities. Tensile loads almost always appeared in the calculated results, indicating a design issue requiring the bonding of corresponding actuators to the mask. Precompressing the mask and superimposing the calculated forces counteracted the tensile loads, but caused degradation of the exact alignment. Design optimization was exploited subsequently to minimize the preload-induced o...