Dynamic models for substrate coupling in mixed-mode systems

In modern monolithic integrated circuits, substrate coupling is a major concern in mixed-mode systems design. Verification of such systems implies the availability of accurate and simulation-efficient substrate coupling models. Traditionally, for frequencies up to a few gigahertz, pure resistive models have been considered sufficient. However, with increasing frequencies of operation, dynamic models become mandatory. The authors motivate the use of dynamic resistive-capacitive (RC) models of substrate coupling as a natural extension to the standard purely resistive models. They propose an extraction methodology that starts with information about the process parameters and circuit´s contact layout, and leads to a contact-to-contact RC element model. The underlying algorithm is based on a finite difference discretisation of the substrate, leading to a large tridimensional mesh which is reduced by means of a fast multigrid algorithm. Unlike standard model order reduction algorithms which can produce models of similar accuracy to state-space descriptions, the proposed method leads to a realisable RC model that can trivially be incorporated into circuit simulation tools. As a first approximation, such a model is shown to correspond to a single time-constant system. Furthermore, it is shown that this time constant can be computed from knowledge of the conductivity and permittivity of a single dominant layer. It is verified that this formulation can accurately model substrate coupling effects for frequencies up to several tens of gigahertz.