For current and upcoming technology nodes (90, 65, 45 nm and beyond) one of the fundamental enablers of Moore's Law is the use of Resolution Enhancement Techniques (RET) in optical lithography. While RETs allow for continuing reduction in integrated circuits’ critical dimensions (CD), layout distortions are introduced as an undesired consequence due to proximity effects. Complex and costly Optical Proximity Correction (OPC) is then deployed to compensate for CD variations and loss of pattern fidelity, in an effort to improve yield. This, together with other sources for CD variations, causes the actual on-silicon chip performance to be quite different from sign-off expectations.
In current design optimization methodologies, process variation modeling, aimed at providing guardbands for performance analysis, is based on "worst-case scenarios" (corner cases) and yields overly pessimistic simulation results which makes meeting design targets unnecessarily difficult. Assumptions of CD distributions in Monte Carlo simulations, and statistical timing analysis in general, can be made more rigorous by considering realistic systematic and random contributions to the overall process variation.
A novel methodology is presented in this paper for extracting residual OPC errors from a placed and routed full chip layout and for deriving actual (i.e., calibrated to silicon) CD values, to be used in timing analysis and speed path characterization. The implementation of this automated flow is achieved through a combination of tagging critical gates, post-OPC layout back-annotation, and selective extraction from the global circuit netlist. This approach improves upon traditional design flow practices where ideal (i.e., drawn) CD values are employed, which leads to poor performance predictability of the as-fabricated design.
With this more accurate timing analysis, we are able to highlight the necessity of a post-OPC verification embedded design flow by showing substantial differences in the silicon-based timing simulations, both in terms of a significant reordering of speed path criticality and a 36.4% increase in worst-case slack. Extensions of this methodology to multi-layer extraction and timing characterization are also proposed. The paper concludes by showing how the methodology implemented in this flow also provides a general design for manufacturability (DFM) tool template. In particular, by passing design intent to process/OPC engineers, selective OPC can be applied to improve CD variation control based on gates' functions such as critical gates and matching transistors. Furthermore, back-annotated process-based data can be used during early stages of circuit design verification and optimization, driving tradeoffs when significant variability is unavoidable.