Fast inverse lithography technology

Many RET technologies, such as rule and model based OPC, the use of sub-resolution assist features, and various PSM methodologies, can be thought of as heuristics employed in an attempt to design improved photomasks. Unfortunately, these traditional approaches are running into severe difficulties at advanced technology nodes (90nm and beyond). We discuss how one can find the optimal photomask by rigorously solving the lithography inverse problem. The design of the optimal mask takes into consideration not only pattern fidelity under nominal conditions, but also the size of the process window and the constraints and costs of mask manufacturing. By formulating the problem in a rigorous mathematical framework, we find highly optimal solutions which do not arise from traditional ad hoc approaches. The resulting masks often provide substantially improved depth-of-focus and exposure latitude, enabling geometries that may be otherwise unattainable.