Spatial characterization of process variations via MOS transistor time constants in VLSI and WSI

This paper is the first large-scale experimental characterization of spatia l process variations for a parameter that is directly involved in timing is sues: the MOS transistor time constant. This is achieved by measuring the o scillation period of highspeed (500 MHz) CMOS ring oscillators that are imp lemented at different locations on individual dies and over wafers. Novel p henomena are observed, improving our understanding of how process variation s affect the performance of synchronous systems, particularly in clock dist ribution networks. We observed four components contributing to period varia tions: an environment-dependent component, a process-dependent component of lower spatial frequency, a random component analogous to white noise, and a component depending on the geometry of the power-supply distribution netw ork.