|vlsitechnology.org /IR drop /example 1 /M5,M6 strap widths|
Metal‑5 and Metal‑6 Strap Widths
We want to allocate double width 320λ power straps for metal-5 and metal-6. This is 40 tracks for metal-5 and 20 tracks for metal-6, which is a thick metal layer with double width track spacing. The separation between the next adjacent wires is then 41 tracks or 328λ for metal-5 and 21 tracks or 336λ for metal-6. The same wide metal rules are used.
For metal-5 using a wide metal spacing of 37λ (2.035µm), the max strap width is 106.5λ. We round this down to 106λ (5.83µm) by setting one spacing to 38λ. But a 106λ wide strap only needs a space of 22λ (1.21µm), so the allocation could be reduced to 274λ and adjacent wire spacing 282λ. If we want to keep the wires on grid, then the adjacent wire spacing must be an odd multiple of the track spacing (8λ), so 37 tracks or 296λ is the next multiple.
The power strap allocation is 1 track less, 36 tracks or 288λ (15.84µm). The supply strap widths are 106λ separated by 26λ and 28λ. The power strap allocation is 180% of the metal-2 allocation (160λ), and the power straps' width of 212λ is 132.5% of the metal-2 allocation.
For metal-6 we measure between the 21 tracks of adjacent wires. A separation of 37λ gives a strap width of 108.5λ. We round this down to 108λ by setting one spacing to 38λ
So the metal-6 power strap allocation is 320λ, 200% of the metal-2 allocation, with a width of 216λ which is 135% of the metal-2 allocation.
Note that we chose here to keep the metal-5 supply straps with a 106λ/22λ width/spacing; and the metal-6 straps 108λ/37λ width/spacing. This is a designer choice. We could have increased the metal-5 allocation to bring the supply strap width up to 108λ (or any other value); or decreased the metal-6 allocation to reduce the supply strap width to 106λ.
The supply straps are laid out so that the metal-5 Vdd runs above the metal-3 Vss and vice-versa; and the metal-6 Vdd over the metal-4 Vss, again in order to avoid stacked vias. The Graal screen shot at the bottom right shows the Vss power straps in metal-4, metal-5 and metal-6.