Bidirectional cell refers to a standard cell, in which metal-1 is used for both horizontal and vertical connections. Unidirectional cell, on the other hand, assumes reserved routing, e.g. metal-1 for only horizontal and metal-2 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. In unidirectional cell, metal-2 is laid out following the cell placement pitch. Since metal-2 pitch is usually different from placement pitch, some within-cell metal-2 become off track. This significantly degrades metal-2 routability. We propose a unidirectional cell with floating metal-2. After initial cell placement, metal-2 segment within each cell is snapped to nearest metal-2 track and is fixed. In addition, we propose cell redesign and post-placement optimization to enhance metal-1 routability. Metal-1 connections are forced to populate in limited number of tracks, so that remaining tracks are exposed during routing. Combined with post-placement optimization, this allows many longer metal-1 tracks to be available for horizontal connection. Experiments with test circuits show that routing errors are reduced by 7% and 60% with the proposed metal-1 considerations and floating metal-2 together.
Jaewoo Seo and Youngsoo Shin, "Routability enhancement through unidirectional standard cells with floating metal-2," Proc. SPIE 10148, Design-Process-Technology Co-optimization for Manufacturability XI, 101480K (Presented at SPIE Advanced Lithography: March 02, 2017; Published: 28 March 2017); https://doi.org/10.1117/12.2258010.
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