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As post-litho process effects account for a larger and larger portion of CD error budgets, process simulation terms must be given more weight in the models used for proximity correction. It is well known that for sub-90 nm processes resist and etch effects can no longer be treated as a small perturbation on a purely optical (aerial image) OPC model. The aerial image portion of the model must be combined in a more appropriate way with empirical terms describing resist and etch effects. The OPC engineer must choose a model form which links an optical component with a resist/etch component in a manner that balances efficiency, robustness and fidelity to the aerial image, among other factors. No single way of connecting litho and etch models is ideal in all cases; the best form of linkage depends on the particular litho and etch process to be simulated. In this paper, we provide practical guidelines for linking litho and etch components of a model, using a representative 70 nm process with a large etch bias as an example. This 70 nm case study, which is representative of many sub-90 nm processes that rely on etch to shrink critical features, presents special challenges for OPC modeling. For the process under study, lines were are printed in resist at 120 nm, and the litho model was verified via resist SEM measurements taken at the resist edge. Note that a thresholded aerial image is not well-characterized a distance 25 nm from the resist edge. This is roughly the distance the edge moves back due to the etch step. Although in some cases etch bias can be calculated from aerial image contrast, in general etch bias cannot be predicted from the aerial image because litho and etch are governed by different underlying physics. The model forms available for linking litho and etch range from the efficient “lumped” form, which combines litho and etch simulation in a single model, to a highly accurate two-stage form which separates the two components. In this paper we evaluate the following model forms for applicability to the 70 nm process under study: 1) Aerial image/load kernel combined (“lumped”) model form 2) Aerial image/rule offset “hybrid” model form 3) Separate litho and etch models (2-stage correction)
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