characterization, modeling, and design of esd protection circuits

56 Chapter 3. Simulation: Methods and Applications
One of the most powerful features of device simulation is the ability to examine at any
location in the device properties such as temperature, potential, and current density which
are not accessible through real measurements. However, the huge quantity of available
information is also a drawback because simple results must be extracted from the complex
device-simulation models. Although extracting points from a MOSFET snapback I-V
curve is straightforward, extraction of a parameter such as the time to failure for a given
input power is nontrivial because “failure” is not directly defined in simulation. Instead, it
must be determined using some criteria involving the parameters available in the
simulation, such as temperature, J ⋅
E profiles, and sudden drops in device voltage.
Interpretation of simulation results is therefore just as important as accurately defining the
models. In a way this is the converse of ESD testing, in which a simple leakage
measurement determines whether a circuit has failed but the source of the failure cannot
be ascertained without extensive testing and failure analysis.
There are of course limitations to the application of 2D device simulation to studying ESD
circuits. The accuracy of modeling thermal failure is one of the biggest concerns because
there is no way to account for heat dissipation in the third dimension, which becomes
important for long stress times. Section 3.6 discusses the implications of 2D modeling on
predicting thermal failure. Two-dimensional simulation is also unable to examine edges
and corners of devices or to study the susceptibility of semiconductor-metal and metalmetal
contacts and interconnects. Mixed-mode simulations can be used to model the
separate MOSFET devices of a multiple-finger circuit, but there is no way to model the
flow of heat between the closely spaced fingers. For these reasons, the focus of the
simulations is on individual devices of an ESD protection circuit, particularly MOSFETs.
The following sections present physical models and general simulation techniques which
facilitate ESD device simulation and then discuss specific ways in which the models and
techniques can be applied. First is a discussion of the facets of simulation which make
studying ESD possible: implementation of the thermal diffusion equation, temperaturedependent
mobility and impact-ionization models, curve tracing, and mixed-mode
simulation. This is followed by a review of published studies on the application of 2D
device simulation to ESD. Methods used to model the MOSFET I-V curve, thermal
failure, dielectric failure, and latent damage are then discussed.