Optical proximity correction (OPC) is widely used in wafer lithography to produce a printed image that best matches the design intent while optimizing CD control. OPC software applies corrections to the mask pattern data, but it generally does not directly compensate for the mask writer and mask process characteristics. The Sigma7500 deep-ultraviolet (DUV) mask writer projects the image of a programmable spatial light modulator (SLM) onto the mask using partially coherent optics similar to wafer steppers, and the residual optical proximity effects of the mask writer are, in principle, correctable with established OPC methods.

This paper discusses LinearityEqualizer, an embedded OPC function developed for the Sigma7500 that enhances mask patterning, employing the Mentor Graphics Calibre OPC engine. It is transparent to the user and does not degrade mask throughput. Along with better resolution of assist features, use of LinearityEqualizer extends the application space of DUV mask writing. Testing shows up to a 4x reduction in the range of systematic CD deviations for a broad array of feature sizes and pitches, and dark assist features are reliably printed down to 120 nm at mask scale.

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