ASIC RC Corners or Parasitic Interconnect Corners

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ASIC RC Corners or Parasitic Interconnect Corners

For ASIC timing closure lots of us know we need to check PVT corner. In deep submicron technology node, a foundry may ask to close timing at the following corners.

For setup:

  • SS, low voltage, 125C
  • SS, low voltage, -40C

For hold:

  • SS, low voltage, 125C
  • SS, low voltage, -40C
  • FF, high voltage, -40C
  • FF, high voltage, 125C

But as a matter of fact this is not the whole story. For timing check we also need to specify RC corners. It can be as complicated as:

For setup:

  • SS, low voltage, 125C, Cworst_Ccworst_T and RCworst_Ccworst_T
  • SS, low voltage, -40C, Cworst_Ccworst_T and RCworst_Ccworst_T

For hold:

  • SS, low voltage, 125C, Cworst_Ccworst and RCworst_Ccworst
  • SS, low voltage, -40C, Cworst_Ccworst and RCworst_Ccworst
  • FF, high voltage, -40C, Cworst_Ccworst, Cbest_Ccbest, RCworst_Ccworst, RCbest_Ccbest
  • FF, high voltage, 125C, Cworst_Ccworst, Cbest_Ccbest, RCworst_Ccworst, RCbest_Ccbest

Or it can be simplified as:

  • RCworst_Ccworst_T (worst-case RC, worst-case double-patterning coupling cap, temperature-aware model) for SS, low voltage, -40/125C corner
  • RCbest_Ccbest_T (best-case RC, best-case double-patterning coupling cap, temperature-aware model) for FF, high voltage,-40/125C corner
  • RCtpical for TT 85C corner

PVT, ie SS/low-volt/125C, is for cell models and RC corners, ie, RCworst, is for wire/interconnect models. But what do these RC corners stand for? There are two good blogs about this topic.

The 2nd blog has a good explaining of R, Cg, Cc, and RC delay. The parameters are defined as below

  • W is wire/interconnect width
  • S is wire spacing
  • T is wire thickness
  • H is height, the distance from wire to ground

Cc is wire-to-wire coupling capacitance and Cg is wire-to-ground capacitance. A wire’s delay is roughly determined by RC of that wire. Here is C is Cg+Cc.

The formula are as below:

Note W,S,T,H are not fully uncorrelated. T and H are uncorrelated to each other and to W and S. But W and S are correlated. RC corner tries to model manufacturing wire processing or wire etching variation. So W+S is a constant. If wire is etched out more, W decreases and S increases. If wire is etched out less, W increases and S decreases.

 

The 1st blog has a good explaining of Cbest, Cworst, RCbest, RCworst, and Typical RC corners. Their relationship is as below.

 

 
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