Detailed Process Analysis for Sub-Resolution Assist Features Introduction
Sub-Resolution Assists Features (SRAF) is a well known and well described method for process window improvement. The introduction of such a technique is not always an easy task for two reasons. On one hand the SRAF placement rules must be defined very well and on the other hand an empirical resist model must be created, which describes the process.
Model based Optical and Process effects correction (MB-OPC) is using an empirical model so called black box, which must be able to predict properly the printing feature for any kind of complex design configuration. When SRAF are implemented in the design, the degree of freedom for the MB-OPC can be reduced. Beside that effort to predetermine as required as possible the target layer, SRAF placement rules and SRAF printing restrictions will limit the OPC. MB-OPC has to cover both the parameters space corresponding to areas in which SRAF are placed and the parameter space for which no SRAF has been implemented.
Of course, it could also be possible to apply the correction of the proximity effect of a complex design with SRAF by an extensive rule-based OPC. Nevertheless the advantage of MB-OPC exists in the possibility to verify the design after Data Preparation by simulating it with the help of the calibrated model. However one should not trust the simulation alone, always a verification of the design on silicon would be necessary, by comparing simulation to SEM images.
Beside the advantages of MB-OPC also weaknesses exist in the meantime, which could require a combination of rule-based and model-based OPC, so called “hybrid OPC”. Empirical models are very often only able to predict the proximity behavior due to a certain range, which is called the optical range of a model. Distances bigger than this range will be covered by extrapolations. This procedure would be correct, if the proximity behavior was as constant as in the area inside the optical range.
We generated an empirical model with the Calibre Workbench from Mentor Graphics. For the model calibration we chose structures with SRAF placement rules, which we applied to the design as well as SRAF placement rules which were not applied to the design. Afterwards, we performed simulations of critical lines over pitch including SRAF. Beside the MB-OPC, we will also describe in this paper the process steps how to generate the SRAF placement rules. The restrictions resulting from the SRAF rules are presented. Subsequently, the experimental results will show that both for symmetrical and asymmetrical structures an improvement of the process window has been obtained. Also weaknesses become clear, which place either the model or the SRAF rule-set questionable. Finally two solutions will be compared, a pure MB-OPC including the isolated lines outside of the optical range and a combination of MB-OPC with a rule-based OPC table for the isolated lines.
Please disable any pop-up blockers for proper viewing of this Whitepaper.