Reducing signoff corners to achieve faster 40 nm SOC design closure

As we go down the technology nodes, a lot of new design variable comes into picture, causing our previous assumptions to fail. This has the result of forcing the use of more timing signoff corners to be take into account their variations at different PVT (Process, Voltage and Temperature) values.

For example, we may have as many as 16 timing sign off corners at C40, thus drastically increasing the turnaround time for the SOC implementation and closure.

In this article we describe what we have done to reduce the timing sign off corners through a better understanding of the smaller geometry variations across multiple corners and how it affects equally good silicon quality. Our analysis indicates that it is possible to reduce the design cycle closure time significantly with only a slight increase in sign off uncertainty.

Accurate Timing Analysis with Corner Definition
An SOC is mainly comprised of millions of sequential elements with combinational cells connected through metal nets which generates a number of timing paths (Figure 1, below). For an SOC to work properly, all the timing paths have to meet certain special timing checks like setup, hold (or any other race condition) etc.

Figure 1: An example of a timing path
Suppose you want an SOC to work at some particular PVT setting determined by specific set of process, voltage and  temperature range values. Temperature range comes from the application requirement and can be as extreme as -40 C to 150 C. Depending on the voltage regulation specs used to derive the SOC constraints and taking into account the other voltage drop effects across the SOC, the voltage range is the first thing that must be determined (for example, 1.08V to 1.32V). The process which determines the quality of silicon doping has a strong dependency on the manufacturing capabilities of the silicon fabrication units.

Depending on the PVT conditions, gate delays and net delays can vary. Silicon designers need to ensure that the SOC’s should work across all the PVTs. This can be done by checking the timing requirements on all the timing paths at different PVT conditions.