In advanced nodes, the extension of DUV lithography deep into the sub-wavelength dimensions has led to exploration of many new Resolution Enhancement Techniques (RET). Generally speaking, these RET have enabled higher resolution capabilities using the same exposure wavelength, but at the cost of increasingly complex mask optimization process. One such technique applied to perform Optical Proximity Correction (OPC) is called Inverse Lithography Technique (ILT). It promises the best possible theoretical mask design by solving the inverse problem, where the optical transform from mask to wafer image is solved in reverse using a rigorous mathematical approach . Although the benefits and potentials of ILT in producing a single exposure mask are well documented , its implementation in multiple patterning OPC (MP-OPC) is less explored. In this paper, an ILT mask optimization is applied on a metal layer, consisting of 3 exposures in a litho-etch x 3 (LELELE) process flow. It demonstrates the application of both multi-exposure and etch awareness within the ILT mask correction scheme. This is accomplished by including inter-layer constraints for the resist and the post-etch contours in the objective function of the ILT optimization. The ability to reduce potential interexposure failure modes as well as the associated increase in computational resources will be assessed. Additionally, the results will be compared against a conventional model-based OPC with similar multi-exposure and etch awareness.
The continuous scaling of semiconductor devices is quickly outpacing the resolution improvements of lithographic exposure tools and processes. This one-sided progression has pushed optical lithography to its limits, resulting in the use of well-known techniques such as Sub-Resolution Assist Features (SRAF’s), Source-Mask Optimization (SMO), and double-patterning, to name a few. These techniques, belonging to a larger category of Resolution Enhancement Techniques (RET), have extended the resolution capabilities of optical lithography at the cost of increasing mask complexity, and therefore cost. One such technique, called Inverse Lithography Technique (ILT), has attracted much attention for its ability to produce the best possible theoretical mask design. ILT treats the mask design process as an <i>inverse</i> problem, where the known transformation from mask to wafer is carried out backwards using a rigorous mathematical approach. One practical problem in the application of ILT is the resulting contour-like mask shapes that must be “Manhattanized” (composed of straight edges and 90-deg corners) in order to produce a manufacturable mask. This conversion process inherently degrades the mask quality as it is a departure from the “optimal mask” represented by the continuously curved shapes produced by ILT. However, simpler masks composed of longer straight edges reduce the mask cost as it lowers the shot count and saves mask writing time during mask fabrication, resulting in a conflict between manufacturability and performance for ILT produced masks1,2. In this study, various commonly used metrics will be combined into an <i>objective function </i>to produce a single number to quantitatively measure a particular ILT solution’s ability to balance mask manufacturability and RET performance. Several metrics that relate to mask manufacturing costs (i.e. mask vertex count, ILT computation runtime) are appropriately weighted against metrics that represent RET capability (i.e. process-variation band, edge-placement-error) in order to reflect the desired practical balance. This well-defined scoring system allows direct comparison of several masks with varying degrees of complexities. Using this method, ILT masks produced with increasing mask constraints will be compared, and it will be demonstrated that using the smallest minimum width for mask shapes does not always produce the optimal solution.