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 in general it
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.
To enhance mask patterning, an embedded OPC function called LinearityEqualizerTM has been developed for the
Sigma7500 that is transparent to the user and which does not degrade mask throughput. It employs the Mentor Graphics
Calibre OPC engine, selected for the computational speed necessary for mask run-time execution. A multi-node cluster
computer applies optimized table-based CD corrections to polygonized pattern data, which is then refractured into a
standard writer format for subsequent data processing. This short-range proximity correction works in conjunction with
ProcessEqualizerTM, a previously developed print-time function that reduces long-range process-related CD errors. OPC
flattens the linearity behavior for all linewidths and pitches, which should improve the total CD uniformity on
production photomasks. Along with better resolution of assist features, this further 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.