Improving the CD linearity and proximity performance of photomasks written on the Sigma7500-II DUV laser writer through embedded OPC
2007
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 compensate for the mask writer and mask process characteristics. The Sigma7500-II deep-UV laser mask writer
projects the image of a programmable spatial light modulator (SLM) using partially coherent optics similar to wafer
steppers, and the optical proximity effects of the mask writer are in principle correctable with established OPC methods.
To enhance mask patterning, an embedded OPC function, LinearityEqualize TM , has been developed for the Sigma7500-
II that is transparent to the user and which does not degrade mask throughput. It employs a Calibre TM rule-based OPC
engine from Mentor Graphics, selected for the computational speed necessary for mask run-time execution. A multinode
cluster computer applies optimized table-based CD corrections to polygonized pattern data that is then fractured
into an internal writer format for subsequent data processing. This embedded proximity correction flattens the linearity
behavior for all linewidths and pitches, which targets to improve the CD uniformity on production photomasks.
Printing results show that the CD linearity is reduced to below 5 nm for linewidths down to 200 nm, both for clear and
dark and for isolated and dense features, and that sub-resolution assist features (SRAF) are reliably printed down to 120
nm. This reduction of proximity effects for main mask features and the extension of the practical resolution for SRAFs
expands the application space of DUV laser mask writing.
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