IR Drop

The space allocated for metal-3 power straps can be increased by widening them to run over the standard cells' metal-1 power rails. The metal-3 strap must not run over the cell connectors, the first of which is 20λ or 2½ tracks inside the cell.

The separation between the nearest cell connectors is then

2.5 + 20 + 2.5 = 25 tracks.

The metal-3 allocation is one track less, 24 tracks or 192λ (10.56µm), which is shown in the first Graal screen shot on the right.

We use the wide metal rules to see how wide the supply straps can be.

width   4-6     8-30  32-74  76-106 108-174 176-   
space 4 5 12 22 37 62

We measure over the 25 tracks or 200λ separating the cell connectors.

calculate strap spacing from strap widths and distance to first cell connector If a 12λ wide metal spacing is used, the supply straps will be 80λ wide, which requires a 22λ spacing. If a 22λ spacing is used, then the supply strap is 65λ wide, which only needs a 12λ spacing. In this case, we set the supply strap to 74λ wide, the largest that can use a 12λ spacing, and find the spacing is 16λ.

This arrangement is shown in the Graal screen shot at the bottom right. This shows horizontal metal-3 supply straps 74λ wide spaced 16λ apart and 16λ to the nearest wires over the standard cells.

The actual metal-3 used for power straps is then 148λ (8.14µm), or 92.5% of the allocated 160λ.

metal-3 power strap allocation overlapping standard cell area

horizontal metal-3 power straps and spacing to nearest wires