The Characterizing Substrate Coupling in Deep-submicron Designs Substrate Coupling Ieee Design & Test of Computers

levels of circuit functionality in chips designed for compact consumer electronic products and the widespread growth of wireless communications have triggered the proliferation of mixed analogdigital systems. Single-chip designs combining digital and analog blocks built over a common substrate feature reduced levels of power dissipation, smaller package counts, and smaller package interconnect parasitics. Designing such systems, however, is becoming increasingly difficult owing to coupling problems resulting from the combined requirements for high-speed digital and high-precision analog components. Noise coupling caused by the common chip substrate’s nonideal isolation contributes significantly to the coupling problem in mixed-signal designs.1,2 Fast-switching logic components inject current into the substrate, causing voltage fluctuation. Because substrate bias strongly affects the transistor threshold voltage, voltage fluctuations can affect the operation of sensitive analog circuitry through the body effect. Figure 1a illustrates this coupling mechanism, in which a switching digital node injects current into the substrate (currents J1 and J2 are drawn to ground, but J2 affects the analog transistor bulk potential), causing the local substrate potential Vb to vary at an analog node. Figure 1b illustrates this interaction from the circuit viewpoint. Other known mechanisms for current injection into the substrate include hot-carrier injection and parasitic bipolar transistors.2 The effects of substrate coupling largely depend on the layout specifics. Therefore, accurate analysis of these effects is possible only after extraction of the circuit features and the parasitics. As technology and circuit design advance, substrate noise is beginning to plague even fully digital circuits. In these circuits, the cumulative effect of thousands or millions of logic gates changing state across the chip causes current pulses that are injected and absorbed into the substrate. Those currents are then transmitted to power and ground buses through direct feedthrough and load charge and discharge. Such couplings are highly destructive because pulsing currents, partially injected into the substrate through impact ionization and capacitive coupling, can be broadcast over great distances and picked up by sensitive circuits through capacitive coupling and the body effect. The resulting threshold voltage modulation dynamCharacterizing Substrate Coupling in Deep-Submicron Designs Substrate Coupling