Substrate Pick-Up Impacting on ESD Performances of Cascode NMOS Transistors
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概要
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The cascode NMOS architecture has been tested by the Human Body Model (HBM), Machine Model (MM) and Transmission Line Pulse Generator (TLP) in this paper. For the TLP, detailed silicon data have been analyzed well in many parameters, such as the first triggering-on voltage (Vt1), the first triggering-on current (It1), the holding voltage (Vh), and the TLP I-V curve. Besides the above three kinds of Electrostatic Discharge (ESD) events, the device gate oxide breakdown voltage is also taken into consideration and the correlations between HBM, MM and TLP are also observed. In order to explain the bipolar transistor turning-on mechanisms, two kinds of models have been proposed in this paper. In typical cases, substrate resistance decreases as the technology advances. On the one hand, for processes older than the 0.35µm process, such as 0.5µm and 1µm, ESD designers can use pick-up insertions to trigger integrated circuits (IC) turn on uniformly. The NPN Side Model can dominate ESD performances in such old processes. On the other hand, in 0.18µm and newer processes, such as 0.15µm, 0.13µm, 90nm, etc., ESD designers must use non-pick-up insertion structures. The NPN Central Model can dominate ESD performances in such processes. After combining both models together, the bipolar turning-on mechanisms can be explained as “ESD currents occur from side regions to central regions.” Besides ESD parasitic bipolar transistor turning-on concerns, another reason that ESD designers should use non-pick-up insertions in deep sub-micron processes is the decreasing of the gate oxide breakdown voltage. As IC size scales down, the gate oxide thickness lessens. The thinner gate oxide thickness will encounter a smaller gate oxide breakdown voltage. In order to avoid gate oxide damage under ESD stresses, ESD designers should endeavor to decrease ESD device turn-on resistances. ESD protecting devices with low turn-on resistances can endure larger currents for the same TLP voltage. In this paper, silicon data show that the non-pick-up insertion cascode NMOS transistors turning on resistance is smaller than the pick-up insertion cascode NMOS transistors turning on resistance. Although this paper discovers NPN turning-on mechanisms based on the cascode NMOS structure, ESD designers can adopt the same theories for other kinds of ESD protecting structures, such as one single poly Gate-Grounded NMOS transistor (GGNMOST). ESD designers can use pick-up insertion architecture for NMOS transistors in the low-end processes, but utilize the non-pick-up insertion architecture for GGNMOST in the high-end processes. Then they can obtain the optimized ESD performances.
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