A novel method called “M3 post” (illustrated in Fig. 20)
was used for M4 to M2 power hook-up. This method involved
having discontinuous and as opposed to the conventional
method of having a continuous but alternate and
on M3. Direct connections from M2 to M4 due to stacked
vias (V2/V3’s) helped reduce localized (M2–M4) power supply
variations. IR drop was also reduced due to having double the
V2/V3’s as compared to the conventional method. The reduced
current flow in M3 due to this method made it easier to meet Power EM spec. For example, for a typical layout of big buffers,
the worst power EM violation reduced from of 1.57
with a conventional M3 power grid to of 0.93 with
M3 post grid. Similarly, IR drop from M4 to devices improved
from 89.1 mV to 55.8 mV.