EXPERIMENTAL RESULTS AND MODELING TECHNIQUES FOR SUBSTRATE NOISE IN MIXED-SIGNAL INTEGRATED-CIRCUITS

Switching transients in digital MOS circuits can perturb analog circui ts integrated on the same die by means of coupling through the substra te. This paper describes an experimental technique for observing the e ffects of such substrate noise. Various approaches to reducing substra te crosstalk (the use of physical separation of analog and digital cir cuits, guard rings, and a low-inductance substrate bias) are evaluated experimentally for a CMOS technology with a substrate comprised of an epitaxial layer grown on a heavily doped bulk wafer. Observations ind icate that reducing the inductance in the substrate bias is more effec tive than either physical separation or guard rings in minimizing subs trate crosstalk between analog and digital circuits fabricated on epit axial substrates. To enhance understanding of the experimental results , two-dimensional device simulations are used to show how crosstalk pr opagates via the heavily doped bulk. Device simulations are also used to predict the nature of substrate crosstalk in CMOS technologies inte grated in uniform, lightly doped bulk substrates, showing that in such cases the substrate noise is highly dependent on layout geometry. Fin ally, a method of including substrate effects in SPICE simulations for circuits fabricated on epitaxial, heavily doped substrates has been d eveloped using a single-node substrate model.