Due to downscaling of modern technologies, integrated circuits (ICs) are more and more vulnerable to the Electrostatic Discharge (ESD) pulses. ESD damage can occur during the manufacturing phase or operational state of the IC. Therefore every input output pin of an IC chip is protected by an ESD protection device. Such devices, operating in breakdown regime, have to sustain current pulses of the orders of Amperes for duration from ns to about 150ns. In some applications, like automotive electronics, where smart power IC technology is used, the ESD protection devices have to sustain also longer pulses of the duration up to 1s but with lower current amplitude. Former studies showed that the current conduction under such pulses occurs in a form of localized current filaments (CFs) which can move along the device active area. In this thesis, physical characteristics of single and multiple CFs and related device failure modes due to thermal breakdown are investigated in smart power technology npn-transistor ESD protection devices. The work is important not only from practical point of view but also gives some new insight into current filament interactions in bistable semiconductor systems with S-shape current voltage characteristics.
The CF behavior was analyzed using transient interferometric mapping (TIM) technique which monitors thermal signals in the devices with ns time and m space resolution. The devices were electrically stressed by different kind of transmission line and solid state pulsers. The spatio-temporal behavior of the CFs in the devices is related to voltage waveforms, making thus a unique relation of internal device behavior to measured electrical parameters. Such knowledge can substantially simplify the device analysis and can be used for optimization of ESD protection devices to enhance their robustness.
The behavior of CFs has been studied in devices of linear, circular and oval geometries as well as in specially designed test structures. Basic properties of single CFs, like speed, size, stopping time, etc. have been analyzed. It was shown that the random position of the filament creation along the device width and the thermal prehistory due to previous current filament passage is crucial to understand the behavior of time to thermal breakdown tTB in the devices. While in the linear devices tTB is random, in the circular ones it is deterministic, making these devices suitable for ESD protection. It was found that complex multiple filament modes can occur at high current, where the thermal breakdown can occur due to redistribution of current between the current filaments, leading to shortening of tTB in comparison to the single CF mode. As well other CF interactions, like meeting and splitting have been studied in details. The thermal prehistory due to multiple filament movement and CF interactions also plays crucial role in failure mechanisms. Furthermore, the CF behavior has been studied as function of ambient temperature and temperature gradient along the device width, mimicking thus the operation of ESD protection devices in powered chips. In linear devices the time to breakdown is shortened step-wise with the increase of ambient temperature which was found to be consistent with the results of thermal simulations.