Sub-wavelength Lithography and Variability Aware SRAM Characterization

With shrinking of minimum feature size of advanced technology nodes, the impact of litho process variations on the resulting electrical parameters of printed circuits dramatically increases. Litho process variations correspond to random changes in the actual optical conditions (dose and focus) which develop at every mask exposure, hence from die to die. In this way the litho process variations act as a global variability component affecting all devices on a particular die in the same way. In contrast to this, the intrinsic variability of the devices and interconnects originating mostly from local Random Dopant Fluctuations (RDF) and Line Edge Roughness (LER) has a purely spatially uncorrelated component. Yet, it is not clear which of the two limits scaling down variability sensitive circuits such as SRAM beyond 45 nm. This paper presents a tool flow to perform SRAM wide statistical analysis subject to combinations of global litho and local variability components. The tool flow is illustrated in 45 nm industry grade SRAM vehicle. Selected case studies show how this tool flow successfully captures non-trivial statistical interactions between the SRAM cell and the periphery, otherwise less visible when using statistical electrical simulations of the critical path alone.