Overview

Electro-static discharge (ESD) is one of the most important reliability problems in IC industry. ESD is a transient discharge of the static charge accumulated on a human body, a test device, or a chip through a low-impedance path due to either human handling or a machine contact. ESD damage to a chip include gate oxide breakdown caused by an excessive gate voltage and interconnect burn-out, such as contact spiking, silicide spiking, and metal migration due to excessive heat generated by a transient current in a localized volume of the chip [233, 234]. The aggressive down-scaling of the physical dimension of MOS devices and the rapid increase in the doping of modern CMOS technology have resulted in a significant reduction in both the thickness of the gate oxide and the width of the pn-junctions of MOS devices. The amount of energy needed to damage MOS devices is scaled down with technology. The level of ESD stress generated due to either human handling or device contacting, however, remains unchanged. Modern CMOS circuits are much more vulnerable to ESD stress.

To protect circuits from ESD-induced damage, ESD protection devices must provide a low-impedance path that bypasses the circuits under protection in an ESD event and discharges the static charge to the ground before it damages the protected circuits. These devices must be sufficiently large in size in order to withstand ESD stress and yet fast enough to release ESD stress before it damages the protected circuits. The large size of ESD protection...