Optimization of unidirectional standard cell library for routability enhancement

Abstract

Bidirectional cell refers to a standard cell, in which metal-1 (M1) is used for both horizontal and vertical connections. Unidirectional cell, on the other hand, assumes reserved routing, e.g. M1 for only horizontal and M2 for only vertical connections. It has been introduced to take advantage of regular metal patterns, which are easier to print and can overcome the lithography limitations in sub-32nm technology. However, unidirectional cell significantly degrades interconnect routability. In unidirectional cell, M2 is laid out following the cell placement pitch. Since M2 pitch is usually smaller than cell placement pitch, some intra-cell M2 become off-track

which significantly reduces the number of M2 routing channels. Furthermore, complex cells such as mux and flip-flop, requires M3 intra-cell connections because of complex interconnections in a cell

which M3 take part of routing resource and degrade M3 routability. We propose unidirectional standard cell library optimization methods to enhance routability. The number of M1, M2, and M3 routing channels are increased by standard cell layout redesign, post-placement optimization, and intra-cell M2 movement.