Freeform mask optimization using advanced image based M3D inverse lithography and 3D-NAND full chip OPC application

Inverse lithography is increasingly being used as a viable OPC solution to maximize process window (PW), improve CD uniformity (CDU) and minimize the mask error factor (MEF), especially for memory devices. The device yield is typically limited by the process window of a few critical layers, and the Via layer is identified as one of the process window limiters for advanced 3D-NAND devices. To maximize the on-chip yield, ASML has developed advanced image based Mask-3D (M3D) inverse technology that can optimize freeform mask shapes and enhance design printability throughout the mask optimization flow. Mask rule checks (MRC) and sbar printing are optimized simultaneously to deliver the maximum process window. The advanced image based M3D inverse lithography technology (ILT) is used to perform full chip mask correction on the Via layer of a 3D-NAND device. 3D NAND devices contain highly repetitive cell and page buffer patterns. To ensure the full chip device performance, the consistency of the mask correction is important. Our strategy is to use the computationally intensive mask optimization solution from the new advanced image based M3D inverse technology to generate a freeform mask which gives the best lithography performance. We then use Tachyon's Pattern Recognition and Optimization (PRO) engine to propagate the freeform mask solution of the repetitive patterns to the full chip. The periphery of the chip is optimized using conventional OPC methods. The simulation results from the advanced image based M3D inverse technology are compared against the baseline flow, which uses a standard inverse solution. The simulation results from both the flows are further validated on wafer. Significant improvement in overlapping process window (OPW) and CD uniformity is observed using the new advanced inverse technology. The simulation data shows a 32% improvement in depth of focus (DOF), a 5% improvement in the image log slope (ILS) and a 25% reduction in best focus shift (BFS) range. The improvement has also been verified at the wafer-level.