characterization, modeling, and design of esd protection circuits

16 Chapter 2. ESD Circuit Characterization and Design Issues
in CMOS technologies and then a discussion of critical parameters in protection circuits
(which can be measured with TLP) and the dependence of these parameters on layout
variations. Finally, the concepts of the chapter are brought together to form an ESD
protection-circuit design methodology.
2.1 Classical ESD Characterization Models and Industrial Testing
The most popular model used in industry to test ESD robustness is the human-body model
(HBM), also known as the finger model. The standardization of this test, first documented
in 1980 and most recently updated in 1989 as military standard MIL-STD 883.C/3015.7
[17], is a result of extensive ESD research since the mid 1970s. In this model a 100pF
capacitor is charged up to a certain voltage and then discharged through a 1500Ω resistor
into an I/O pin of a circuit, with another pin, usually a supply or ground pin, tied to ground
(Fig. 2.2a). According to the MIL-STD specification, the resulting waveform must have a
rise time less than 10ns and a decay time of 150±20ns into a short-circuit load (Fig. 2.2b).
The rise time is dependent upon the parasitic inductance and stray capacitance. For a
HBM voltage of 1500V, the peak current would be approximately 1A. This model is
meant to represent a discharge from a human finger into a pin of a circuit package. Several
commercial testers which meet the military standard are available, e.g., the Hartley
Autozap and IMCS 2400C ESD Sensitivity Test System, making HBM testing relatively
simple.
In a typical reliability test, all the I/O pins on a package are stressed with respect to all
power and ground pins with both polarities of a given HBM voltage using an industrial
tester. In addition, I/O pins may be stressed vs. other I/O pins, and supply pins may be
stressed vs. ground pins. Current-leakage measurements at a specified reverse voltage
(usually the operating voltage) are then performed on the same sets of pins. If a 2kV HBM
test is performed on all pins of a package, and the resulting leakage current of all pins is
below a certain level, say 1µA, then the IC is said to be resistant to 2kV HBM. Obviously,
the HBM failure threshold is dependent upon the chosen failure-current definition. With
the use of this model in designing protection circuits, typical HBM failure thresholds have
improved from 2kV in the early 1980s to about 6kV in the 1990s [2].
The machine model (MM), also called the Japanese model due to its origin, is similar to
the human-body model: a capacitor is charged to a certain voltage and then discharged