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Considerations on Routing Power Straps Over Standard Cell Areas
The metal-1 layer is used to make connexions inside the standard cells, and the metal-2 and metal-3 layers are both used to connect the cells' pins with routing over the top of the cells. Any power straps in these layers means that underneath there is no room for standard cells, and thus these straps increase the total die area.
Starting with metal-4, the power straps could run over the top of the standard cell areas. The risk with this approach is that the routing must all be done with the lower metal layers and congestion is more likely. The point at which routing congestion becomes a problem is not clear, so the user must choose how much of the upper metal layer power straps are routed over the top of the standard cells, and how much they are simply stacked on top of the lower layer power straps.
In the context of the methodology presented here, this choice is made thru the values assigned to kan and kwn.
kan and kwn are the power strap coefficients used to multiply p, the reference power strap allocation percentage. p is the amount of metal allocated to the reference metal layer. Multiplying it by kan gives the amount of metal layer n allocated for power supplies; and multiplying it by kwn gives the amount of metal layer n used for power supplies. The allocation includes the spacing and the width and both supplies.
In effect, metal-2 sets the reference power strap percentage. Other metal layers can then be smaller, the same or bigger than the metal-2 power strap. In this case, we set ka2=100%, and the other strap allocations and widths are set by smaller or bigger values of kan and kwn.
For example, in a 6-layer metal system, if we don't want any horizontal power straps, we set ka1=ka3=ka5=0%.
If we set ka2=100% and ka6=200%, then the extra allocation for metal-6 is over the top of the standard cells and not available for routing ordinary wires. Their presence is assumed to not impact the die area and that the metal-1 to metal-5 used to connect the signals underneath also suffer no routing congestion.
Picture courtesy of Philips Semiconductors, now NXP
Vertical metal-4 to horizontal metal-5 supply connections with both power
supplies at double width and overlapping the standard cells.
We assume that the overlap of the standard cells by the upper metal layers does not affect the routing congestion underneath.