Wire Sizing and Spacing for Lithographic Printability and Timing Optimization

Abstract

As the VLSI feature size has already decreased below lithographic wavelength, the printability problem, due to strong diffraction effects, poses a serious threat to the progress of VLSI technology. A circuit layout with poor printability implies that it is difficult to make the printed features on wafers follow designed shapes without distortions. The development of resolution enhancement techniques (RET) can alleviate the printability problem but cannot reverse the trend of deterioration. Moreover, over-usage of RET may dramatically increase photo-mask cost and increase the cycle time for volume production. Thus, there is a strong demand to consider the subwavelength printability problem in circuit layout designs. However, layout printability optimization should not degrade circuit timing performance. In this paper, we introduce a wire sizing and spacing method to improve wire printability with minimal adverse impact on interconnect timing performance. A new printability model is proposed to handle partially coherent illuminations. The complex printability and timing optimization problem is solved in a two-phase approach. The difficulty of the printability optimization due to its multimodal nature is handled with a sensitivity-based heuristic. A coupling aware timing driven continuous wire sizing algorithm is also provided. Lithographic simulation results show that our approach can improve the printability in term of edge placement error (EPE) by 20%-40% without violating timing, wire width, and spacing constraints.