Chapter 13: Impedance Characteristics of Multi-Layer Grids
Overview
The power distribution network spans many layers of interconnect with disparate electrical properties. The impedance characteristics of multilayer power distribution grids and the relevant design implications are the subject of this chapter.
Decoupling capacitors are an effective technique to reduce the effect of the inductance on power distribution networks operating at high frequencies. The efficacy of decoupling capacitors depends on the impedance of the conductors connecting the capacitors to the power load and source. The optimal allocation of the on-chip decoupling capacitance depends on the impedance characteristics of the interconnect. Robust and area efficient design of multi-layer power distribution grids therefore requires a thorough understanding of the impedance properties of the power distributing interconnect structures.
Power distribution networks in high performance digital ICs are commonly structured as a multi-layer grid, as shown in Fig. 13.1. The inductive properties of single layer power grids have been described in Chapter 9. In grid layers with alternating power and ground lines, long distance inductive coupling is greatly diminished due to cancellation, turning inductive coupling in single layer power grids into, effectively, a local phenomenon. The grid inductance, therefore, behaves similarly to the grid resistance: increases linearly with grid length and decreases inversely linearly with grid width ( i.e., the number of lines in the grid). The electrical properties of power distribution grids can therefore be conveniently expressed by a dimension-independent sheet resistance R ? and sheet inductance L ? [276]. The inductance of the power grid layers can be...