Traditional segment-based model-based OPC methods have been the mainstream mask layout optimization techniques in volume production for memory and embedded memory devices for many device generations. These techniques have been continually optimized over time to meet the ever increasing difficulties of memory and memory periphery patterning. There are a range of difficult issues for patterning embedded memories successfully. These difficulties include the need for a very high level of symmetry and consistency (both within memory cells themselves and between cells) due to circuit effects such as noise margin requirements in SRAMs. Memory cells and access structures consume a large percentage of area in embedded devices so there is a very high return from shrinking the cell area as much as possible. This aggressive scaling leads to very difficult resolution, 2D CD control and process window requirements. Additionally, the range of interactions between mask synthesis corrections of neighboring areas can extend well beyond the size of the memory cell, making it difficult to fully take advantage of the inherent designed cell hierarchy in mask pattern optimization. This is especially true for non-traditional (i.e., less dependent on geometric rule) OPC/RET methods such as inverse lithography techniques (ILT) which inherently have more model-based decisions in their optimizations. New inverse methods such as model-based SRAF placement and ILT are, however, well known to have considerable benefits in finding flexible mask pattern solutions to improve process window, improve 2D CD control, and improve resolution in ultra-dense memory patterns. They also are known to reduce recipe complexity and provide native MRC compliant mask pattern solutions. Unfortunately, ILT is also known to be several times slower than traditional OPC methods due to the increased computational lithographic optimizations it performs. In this paper, we describe and present results for a methodology to greatly improve the ability of ILT to optimize advanced embedded memory designs while retaining significant hierarchy and cell design symmetry, therefore, have good turnaround time and CD uniformity. This paper will explain the enhancements which have been developed in order to overcome the traditional difficulties listed above. These enhancements are in the categories of local CD control, global chip processing options, process window benefit, turn-around time and hierarchy retention.
Model-driven target optimization using an ILT hotspot fixer is applied to line collapsing defects of 2-
dimensional randomtest pattern of a very low K1 process. The target is moved by minimizing the process
variation band and the pitches of hotspot points are relaxed.The image quality improvement is thenchecked.
Model driven target optimized NILS and MEEF at the weakest hotspot point are improved to 1.22 and 5.5
from the values 0.79 and 10.6 of a traditional OPCwith advanced solver, respectively. The pattern collapsing
hotspot is then validated to be repaired by optimizing target position. A full hotspot fixer flow including
model-driven target optimization using ILT can also be extended into DFM applications.