characterization, modeling, and design of esd protection circuits

38 Chapter 2. ESD Circuit Characterization and Design Issues
impedance outputs are connected to the drains of output-buffer transistors. Design of
output protection is thus more restricted than that of input protection because of low
output-impedance requirements. For example, a well resistor may be placed between an
input pad and the protection MOSFET to reduce the rise time of an ESD pulse, but such a
resistor cannot be placed on an output pad because the increased impedance would exceed
circuit specifications. Also, since the output-protection transistors often double as the
CMOS output buffer, they must meet certain chip-performance specifications. As a result,
output protection relies more on the proper layout of one or two transistors than on the use
of creative circuit designs.
Fig. 2.15a shows a simple diode protection scheme. The diodes are formed by source/
drain diffusions in the p-substrate or n-well. When the circuit is powered up, diode D1 will
become forward biased and conduct current for any input voltage greater than VCC + Vd ,
where Vd is the forward diode drop. Similarly, diode D2 clamps any negative voltage
below VSS - Vd . If the chip is not powered up and an ESD pulse is incident between the
input and, say, VSS , the voltage will be clamped at either the breakdown voltage of the
diode for a positive pulse (note we are neglecting the voltage drop across the dynamic
resistance of the diode) or at -Vd for a negative pulse. The diodes should introduce
minimal leakage current and a negligible parasitic capacitance to the circuit since they are
normally reverse biased. Series resistors can be used in conjunction with diodes (or other
devices) in input protection circuits, as shown in Fig. 2.15b, to create a potential drop from
the pad to the diode and thus reduce the voltage at the input gates. Using a diffused resistor
distributes the resistance and introduces an additional distributed diode, resulting in a
lower gate voltage than that created by a simple polysilicon resistor. Addition of a series
resistor aids circuit protection by slowing down transients (e.g., a machine-model
waveform would be transformed into a HBM-like waveform), but by the same token it can
reduce circuit speed performance by increasing RC time constants.
Although diode circuits are simple to implement and may have provided sufficient ESD
protection in the past, there are a few reasons why they are no longer adequate for protecting
today’s smaller technologies. First, the dynamic resistance of a reverse-biased diode
may be too high to keep voltages clamped at a safe level unless the diode area is very
large. For example, a 250 µm2 area of diode with a typical impedance of 5000 Ω-µm2 has
a resistance of 20Ω and will sustain 20V at a stress current of 1A, a voltage well above the
dielectric threshold of a thin gate oxide. The potential drop can of course be reduced by