characterization, modeling, and design of esd protection circuits

166 Chapter 6. Conclusion
6.1 Contributions
An overview of electrostatic discharge issues in the integrated-circuit industry was
constructed to elicit appreciation of the importance of addressing ESD in process
development and circuit design. The phenomenon of ESD was defined and its
implications to ICs were reviewed. ESD failures fall into three main categories: thermal
damage, dielectric damage, and latent failure. Three widely accepted methods used to
characterize ESD sensitivity in ICs are the human-body model, machine model, and
charged-device model tests. Each of these models represents a potential real-world ESD
event, but it was shown that the models offer little insight to the functionality and
weaknesses of an ESD protection circuit and thus that a better characterization scheme is
desirable. Examples of common ESD protection circuits and the theory behind their
design was presented. A review of previous applications of numerical device simulation to
the study of ESD illustrated how simulation can be used to design and analyze protection
circuits and highlighted previously untried simulation methods. A basic protection-circuit
design methodology was outlined and exemplified using results from the transmission-line
pulsing characterization method and two-dimensional simulations. This was followed by
the description of a more complete design methodology based on empirical models
extracted from a fully characterized test-structure design space.
6.1.1 Transmission Line Pulsing
The transmission-line pulsing test method, a relatively new ESD-circuit characterization
scheme, was presented. This test method is superior to the classic characterization models
because it reveals how a protection circuit functions during an ESD stress and quantifies
the failure threshold of a circuit over a wide range of stress times. TLP captures the
transient I-V curve of a stressed device by sampling each current level so briefly that
damage is not incurred. Using TLP, the evolution of leakage current, which is a measure
of the degree of damage, is monitored by measuring the device leakage after each pulse.
This feature aids the determination of critical points at which various types of damage are
created and is especially important in capturing low-level (sub-microamp) leakage which
is a signature of latent failure. The basic setup of a TLP characterization system was
detailed along with an overview of some advanced setup techniques.