A sensitivity analysis of SPICE parameters using an eleven-stage ring oscillator

SPICE is a circuit simulator which predicts node voltages and currents as a function of time from device model parameters. Model parameters are determined by the manufacturing process. Process-induced variations in these parameters occur within a chip or from chip to chip and cause corresponding variations in circuit performance. Values for the model parameters used in simulators are usually obtained from measurements on test structures which are found along the periphery of the circuit or in test chips located at several sites on the product wafer. Because of the spatial separation between test structures and the circuits of interest, differences between measured and simulated performance can occur. This paper presents examples of how well model parameters extracted from a test chip can predict the ac response of a dynamic circuit element (ring oscillator) on the same wafer. Simulation results show which model parameters are critical to performance. A comparison between measurement and simulation results is given and the importance of intra-chip and intra-wafer parameter variations is discussed. For the samples tested, the polysilicon gate linewidth variation was determined to be the primary cause of the ring oscillator frequency variation.