Hypersensitive parameter-identifying ring oscillators for lithography process monitoring

This paper applies process and circuit simulation to examine plausible explanations for measured differences in ring oscillator frequencies and to develop layout and electronic circuit concepts that have increased sensitivity to lithographic parameters. Existing 90nm ring oscillator test chip measurements are leveraged, and the performance of ring oscillator circuit is simulated across the process parameter variation space using HSPICE and the Parametric Yield Simulator in the Collaborative Platform for DfM. These simulation results are then correlated with measured ring oscillator frequencies to directly extract the variation in the underlying parameter. Hypersensitive gate layouts are created by combining the physical principles in which the effects of illumination, focus, and pattern geometry interact. Using these principles and parametric yield simulations, structures that magnify the focus effects have been found. For example, by using 90° phase shift probe, parameter-specific layout monitors are shown to be five times more sensitive to focus than that of an isolated line. On the design side, NMOS or PMOS-specific electrical circuits are designed, implemented, and simulated in HSPICE.