Fine-grained characterization of process variation in FPGAs

As semiconductor manufacturing continues towards reduced feature sizes, yield loss due to process variation becomes increasingly important. To address this issue on FPGA platforms, several variation aware design (VAD) methodologies have been proposed. In this work we present a practical method of process variation characterization (PVC) to facilitate VAD using only intrinsic FPGA resources. The scheme is based on measuring the difference between ring oscillator (RO) delay at different locations within a die, and can be used to perform process variation characterization for LE delays and interconnect delays including direct connection, double wire and hex wires. The difference in loop delays can also be estimated from equations using parameters extracted from primitives and compared with direct measurements. On a Xilinx Spartan-3e device, it was found that the error between the estimated and measured values was on average less than 10%.