Transient Discharge Characteristics of a Diode-string ESD Clamp

Abstract

In this work, we analyze in detail DC and transient characteristics of a diode string as a clamp device for ESD protection based on 2-dimensional (2D) device simulations and mixed-mode simulations. We especially focus on changes in ESD transient discharge characteristics of the device when inserting p-substrate contacts in between each diode in the diode string to provide guidelines in designing diodestring devices assuming usage of a standard CMOS process without triple wells. We show that a usual diode string shows a DC snapback behavior relating parasitic pnpn thyristor trigger to constrain usage of this device only at very low power supply voltage. Next, we show that, by adding p-substrate contacts in between each series diode in the string, the parasitic thyristor action is suppressed to eliminate the DC snapback behavior, but about 2/3 of a PS-mode HBM ESD discharge current flows through parasitic pnpn thyristor paths, which shows that parasitic thyristor action still dominates discharge transient and makes this device quite useful as an ESD clamp device. We explain in detail the mechanisms relating ESD discharge inside the diode-string clamp device.