1.3 Reliability

A reliable MOSFET meets the requirement of correct interaction with other circuit components. Unfortunately, even under nominal operating conditions, this correct interaction cannot be ensured at all times. So-called degradation mechanisms, which are associated with shifts of the device characteristics, endanger the correct interaction and reduce the time-to-failure of integrated application. The physical processes responsible for these mechanisms are in the focus of the device reliability research field, including the characterization, understanding and modeling of, e.g., BTI, hot-carrier degradation (HCD), stress induced leakage current (SILC) and trap assisted tunneling. Only with a deep understanding of how degradation is caused, advanced simulations based on realistic models can ensure a robust circuit design. Moreover, such a fundamental knowledge is a great advantage for future transistor designs to prevent instabilities.

The focus of this thesis lies on the experimental characterization of degradation mechanisms. In this context, numerous methods and sequences have been introduced. Each of these methods and sequences is suitable for the characterization of a certain parameter or physical quantity. For example, charge pumping (CP) allows for the characterization of interface defects, whereas time-dependent defect spectroscopy (TDDS) is suitable for the characterization of single oxide defects. The methods and sequences are discussed in Chapter 3 in detail.

In general, degradation has been shown to consist of a recoverable and a permanent component. While, for example, the recoverable component of BTI is attributed to oxide defects, which capture and emit charge carriers, HCD is suspected to be determined by breakage of passivated Si dangling bonds at the substrate/oxide interface, typically associated with a permanent component of degradation. Both degradation mechanisms are typically studied in an idealized setting and independently from each other [16, 19–24] as discussed in Chapter 2. In particular, for BTI studies no voltage is applied to the drain, leading to homogeneous degradation. With increasing drain bias, the degradation becomes more and more inhomogeneous and the contribution of HCD to the total degradation increases. Even though it is well understood that this mixed degradation corresponds to the situation in real circuits, there is only a limited number of studies available on the impact of the mixed stress conditions [25–27]. Additionally, defect creation, annealing, activation, and deactivation as well as secondarily generated carriers and non-equilibrium effects play an important role.