characterization, modeling, and design of esd protection circuits

172 Chapter 6. Conclusion
the internal VSS supply while the source is tied to the external VSS supply in order to
reduce substrate noise. The isolation of the source from the substrate results in different
ESD behavior since the discharge current path is altered. In the latter case, a cascoded gate
(also called a stacked gate or split gate) pull-down transistor is used at the I/Os because the
circuit, although designed to operate using a 3.3V supply, must be able to tolerate a 5V
signal on the I/Os in order to meet older circuit-board specifications (a standard pull-down
transistor cannot be used in this case because 5V could develop across the transistor gate,
which is only designed to withstand a 3.3V signal). Stacking two gates in series affects the
ESD response because the snapback voltage and snapback resistance are effectively
doubled.
In addition to applying the design methodology to different types of protection circuits,
determining the feasibility of modeling CDM withstand voltages is also important because
CDM is now the dominant ESD concern in the IC industry. Since CDM stress usually
leads to dielectric damage of gate oxides, a different type of test structure may be required.
For example, by connecting the input of an inverter circuit to the drain of an NMOS pulldown
protection transistor we can determine how effectively the transistor would protect
the input gates of an actual integrated circuit during CDM stress. Test structures might
also be bonded into different types of packages to model the dependence of CDM
robustness on the inductance and resistance of package leads.
An important aspect of the methodology presented in this thesis is that a simple, empirical
approach is taken to model ESD protection circuits. However, in the future we would like
to integrate two-dimensional electrothermal device simulation and circuit simulation into
the process to confirm the trends predicted by the empirical models. In doing so we may
find that a more complex model is needed, i.e., something beyond second-order linear
equations, in which case a more advanced modeling software package would be required.