characterization, modeling, and design of esd protection circuits

1.2. Characterizing ESD in Integrated Circuits 5
characterization tests [58]. Specific tests are designed to model specific events such as
human handling, machine handling, or field induction.
The most common industrial tests used to measure ESD robustness are the human-body
model (HBM), the machine model (MM), and the charged-device model (CDM) [17,34].
These models will be described in detail in Chapter 2. Briefly, the human-body model,
also known as the finger model, consists of charging a capacitor to a high voltage (say,
2000V) and then discharging the capacitor through a series resistor into an I/O or supply
pin of a packaged IC with another pin grounded and all other pins floating. The capacitor
and resistor values are selected to generate a pulse similar to that generated by an
electrostatically charged human touching the pins of an IC, with a rise time of a few
nanoseconds and a decay time of about 150ns. After an HBM stress is applied between
two pins, the pins are biased at the operating voltage and the consequent leakage current is
measured. If the leakage is greater than some predefined level (say, 1µA) then the package
has failed the (2000V) HBM test. HBM testing is often the sole means of qualifying ESD
reliability because the specifications of the test are standardized industry wide and because
several commercial HBM testers are available.
As in the HBM, in the machine model a capacitor is charged up to a high voltage and then
discharged through the pins of an IC. Unlike the HBM, however, the MM discharges the
capacitor through only a very small, parasitic series resistance, resulting in an oscillatory
input pulse comparable to a pulse generated by a charged metal machine part contacting
an IC pin. Since the series resistance is very small, parasitic inductances and capacitances
of the tester as well as the dynamic impedance of the device under test have a much larger
effect on the shape of the pulse, making a standard, repeatable MM test difficult to
actualize.
While device heating is the primary failure mechanism in the HBM and MM, dielectric
failure is the signature of the charged-device model. Due to the sub-nanosecond rise time
of the CDM pulse, protection devices may not be able to turn on and clamp the input
voltage to a safe level before high-field oxide damage occurs. The CDM test, which
consists of charging a substrate (ground) pin of a package using a voltage source,
removing the voltage source, and then discharging the package by shorting a different pin,
is meant to simulate the electrostatic charging of a package due to improper grounding and
the subsequent discharging when a low-resistance path becomes available. Though much