characterization, modeling, and design of esd protection circuits

2.2. Transmission Line Pulsing 19
industry wide over a number of years. As long as a packaged product is resistant to HBM
tests up to some level of stress, say 4kV, then it is considered to be reliable from an ESD
standpoint. However, as the result of an emphasis on preventing ESD damage from human
handling during production, e.g., by ensuring proper grounding of personnel and
equipment and by using ESD-controlled workstations, the human-body model no longer
represents the dominant failure pattern in the industry [10].
Today the main area of concern is shifting to the charged-device model (CDM), which
introduces a different failure mode from that of the HBM and MM. In this model,
electrostatic charge builds up on a chip due to improper grounding and then discharges
when a low-resistance path becomes available. It is meant to simulate ESD phenomena of
packaged ICs during manufacturing and assembly. For example, a package connected to
the ground pin may be inductively charged up as it is transported along a conveyor belt,
then discharged through any pin touched by a metal handler or test socket [18]. The
characteristic rise time of a CDM pulse is 1ns or less, with a peak current of several amps.
Since the turn-on time of MOS protection circuits is on the order of 1ns, high voltages
have a chance to build up across oxides during a CDM event. Thus, damage to thin oxides
(of the protection device as well as the internal gates being protected) is the signature
failure of CDM events, in contrast with the thermal failure signature of the HBM.
A typical CDM test consists of placing a charge on a substrate (ground) pin using a
voltage source, then disconnecting the voltage source and connecting a different pin
through a low-inductance, low-impedance, 1Ω probe to ground (Fig. 2.4). In another
method referred to as the field-induced model (FIM), a charge is induced on the substrate
by placing the chip on a conducting surface, then discharged through a pin via a lowimpedance
probe. Like the machine model, the CDM has no established standard, and
there is a need for further understanding of the phenomena underlying the model. The
higher ESD sensitivity of shrinking oxides and reduced susceptibility to human handling
will provide the incentive for continued development of the CDM.
2.2 Transmission Line Pulsing
It is obvious from the discussion of the classical characterization models that a single type
of test or figure of merit is not sufficient to guarantee robustness against all EOS/ESD
failures. It is possible for a circuit to pass one type of test, say the human-body model,