Model Based OPC Considering Process Window Aspects—A Study

Critical features of a product layout like isolated structures and complicated two-dimensional situations including line ends have often a smaller process window compared to regular highly nested features. It has been observed that the application of optical proximity corrections (OPC) can create yet more aggressive layout situations. Although corrected layouts meet the target contour under optimal exposure conditions, the process window of these structures under non-optimal conditions is thereby potentially reduced. This increases the risk of shorts and opens in the resist images of the designs under non-optimal exposure conditions. The requirement from a lithographer's point of view is to conduct a correction that considers the process window aspect besides the desired target contour. The present study investigates a concept of using the over-dose and under-dose responses of the simulated image of an exposed structure to optimize the correction value. The simulations describing the lithographic imaging process are based on an enhanced variable threshold model (VTRE).

The placement error of the simulated edge of a structure is usually corrected for the nominal dose and focus settings. In the new concept the effective edge placement error is defined as the average of the edge placement errors for the over-dose and the edge placement error for the under-dose conditions. If a specific layout has a very non-symmetric response to over-/under exposure for the evaluated condition, it is prone to a certain failure mechanism (open or short). Hence calculating the average of the edge placement errors will shift the effective correction towards a layout with larger process window.

The paper evaluates this concept for 100nm ground rules and 193nm lithography conditions. Examples of corrected layouts are presented together with experimental data. The limitations of the approach are discussed.