Turn-on Speed of Grounded Gate Nmos Esd Protection Transistors

The turn-on speed of nMOS transistors (nMOST) is of paramount importance for robust Charged Device Model (CDM) protection circuitry. In this paper the nMOST turn-on time has been measured for the first time in the sub-halve nanosecond range with a commercial e-beam tester. The method may be used to improve CDM-ESD hardness by investigating the CDM pulse responses within circuit. Furthermore it is shown that the CDM results of various protection layouts can be simulated with a SPICE model. Copyright © 1996 Elsevier Science Ltd INTRODUCTION In a highly automated IC production environment, the most relevant ESD hazards are represented by the fast discharge to ground of charge accumulated on a device through the manufacturing equipment. This ESD event is described by the Charged Device Model (CDM) and is characterised by durations and rise-times, in the nanosecond and sub-nanosecond range. The availability of experimental techniques capable of characterising the response of protection circuits subjected to CDM ESD pulses is of crucial importance to improve IC hardness. In CMOS, ESD protection circuits usually employ grounded-gate nMOS transistors (gg-nMOST) as protection elements, because of their low on-resistance and satisfactory holding voltage (due to the parasitic bipolar transistor within their structure). With the introduction of the Charged Device Model [1], the question arises: do most frequently applied ESD protection devices, gg-nMOSTs, trigger fast enough to provide adequate protection against CDM ESD hazards? In recent years some experimental data on the turn-on time has been shown [2-4]. This paper will assess this question thoroughly through measurements and simulations of the turn-on time of gg-nMOSTs with variations of the main layout parameters. Because of the very fast sub-nanosecond transient and the inherent capacitive load of measurement probes, standard equipment is not capable of performing the transient measurements. Therefore, an electron beam testing system, with 150 ps time resolution has been used. In order to obtain a better insight of the experimental results, simulations have been carried out with a SPICE model.