Variation Aware Timing Model of CMOS Inverter for an Efficient ECSM Characterization

In static timing analysis (STA), delay estimation of CMOS standard cells is accomplished by the effective current source model (ECSM) characterization method. ECSM characterization stores the values of threshold crossing points (TCP's) of output voltage in a look-up table for the combination of input transition time (T
_R
) and load capacitance (C
_L
). Due to variability, re-characterization of entire cell library is required to update the look-up tables of TCP's, which is a tedious task. Due to temporal variability mechanisms such as negative bias temperature instability (NBTI) and hot carrier injection (HCI), non-critical paths may become critical. Therefore, a large coverage of TRs in characterization is required. In this work, an analytical timing model for delay estimation of CMOS inverter with large TR coverage is proposed. This model matches well with HSPICE simulation with a maximum error of 3%. With this timing model, the ECSM characterization of CMOS inverter can be accomplished with very few HSPICE simulations compared to traditional approaches. The model coefficients are correlated with device/layout level parameters such as driving strength (W) or number of layout fingers (NF), supply voltage (V
_dd
), threshold voltage (V
_th
), and mobility (u). Furthermore, with this model, re-characterization efforts can be reduced significantly in comparison to conventional approach and variability analysis could be done without exercising Monte-Carlo analysis.