Automatic SRAF size optimization during OPC

Abstract

Sub-resolution assist features (SRAFs) are a common addition to low-k1 masks to improve process window for isolated features. Traditional SRAF placement, which has been widely adopted by the industry, is governed by generation of rules which have been experimentally derived based on exposure and measurement of test patterns. The placement rules must generate SRAFs large enough to improve the process window, but small enough not to print at any point within that process window. This has resulted in tremendous challenges to meet the cost and process window requirements for the advanced technology nodes. Specifically, in the logic products, due to the complex two-dimensional patterns, the placement rules will be quite challenging in order to ensure maximum sizing and no printing. SRAF generation is also plagued by over clean-up due to mask rule checks (MRC) specified. The challenge is to derive the best rules which generate SRAFs without printing through the process window. This paper will explore the possibility of an alternate SRAF placement methodology where the SRAF placement rules can be greatly simplified, and SRAF printability through the required process window conditions will automatically be accounted for during a subsequent Optical Proximity Correction (OPC) step. This methodology has the advantages of simplifying the placement rules while simultaneously ensuring maximum possible SRAF size with no printing within the process window. The SRAF size optimization is performed concurrently with OPC thereby saving valuable time on trying to optimize the rule deck. In this method the side-lobes are automatically suppressed well under the imaging threshold. Experimental verification of the SRAF dependence on its sizing and placement along with printability and main-feature process window will be demonstrated.