Multiple Threshold (Multi Vt) Cell Libraries

With the technologies shrinking to 90nm, 30nm and below one of the
common ways to reduce leakage power is to use multiple Vt libraries.
Subthreshold leakage varies exponentially with the Vt comparated to
the weaker dependance of delay over Vt.

Libraries are offered in different versions each consisting of
standard Vt cells, low Vt cells and high Vt cells independant of each
other. Power and timing is optimized based on these libraries and they
offer good flexibility and opportunity to logic and physical synthesis
tool for optimization process.

Dual Vt synthesis flow has become quite common in 130nm and below
tehnology nodes. In this flow initial synthesis is carried out
targeting primary library which may be a low Vt or high Vt or normal
Vt library, and the second iteraton of synthesis and optimization is
performed based on secondary libraries which are also libraries
consistitng of multiple threshold cells.

Which library has to be used as primary library ?

This depends on the optimization target as per the design requirement.
In general, if optimization target is power performance, first
syntheize the design using the high Vt cell library which achieves
lowest leakage power. In the next iteration of optimization cells in
the critical path has to be replaced by low Vt cells which are faster.
If the optimization target is to meet timing then first use low Vt
cell library to achieve timing and then optimize leakage power using
high Vt cells.