IR Drop

We find out how much of the power strap allocation of 160λ (8.8µm) can be used as metal-1 supply straps using the wide metal rules table below.

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

Ideally we would like to run the straps as shown in the illustration at the top right. The 160λ allocation is used for two supply straps each 68λ wide separated by 12λ.

The straps run between two standard cell rows. With an allocated space of two rows, the metal-1 rails in the standard cells top and bottom will be the same … both either Vdd or both Vss.

I don't recommend widening the top and bottom supplies above the regular thickness of a standard cell metal-1 power rail. If the metal-1 is made too wide, then there will be wide metal violations to geometries inside the standard cells. This is shown in the drawing on the right, where the metal-1 requires a 12λ spacing (0.66µm) but inside the standard cells it is only 5λ (0.275µm).

Instead the metal-1 structure of the standard cells is maintained, and additional straps are run between them. This arrangement is shown in the Graal screen shot at the bottom right.

calculate strap widths from wide metal spacing and standard cell rail widths The actual metal-1 used for power straps can be found with a simple spreadsheet calculation shown on the left. There is one Vdd and one Vss strap, each 32λ wide, plus the contribution of the standard cell power rails of 24λ giving a total width of 112λ. Thus, of the 160λ allocated to metal-1 power straps, only 112λ or 70% is actual metal.

On the right is a Graal screen shot showing the metal-1 power straps. By using the same metal-1 rails as the standard cells, we can place cells like feedthrus or decoupling caps uder the straps. This can be seen with 2 cells in the middle of the graphic.

wide metal-1 power straps can cause violations

Illustration showing how widening the standard cells' metal-1 power rails can cause design rule violations inside the cells.

metal-1 power strap arrangement in 2 cell tall strap allocation