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Biography
Bianka Ullmann was born in Sofia, Bulgaria, in 1984 and has been in living in Vienna since 1991. She received the degree of Diplomingeneur in physics from the Technische Universität Wien in 2012. She has been working as a project assistant at the Technische Universität Wien since 2013 and joined the Institute for Microelectronics in January 2014.
Characterization of the Permanent Component of MOSFET Degradation Mechanisms
Degradation mechanisms of MOSFETs, such as bias temperature instability and Hot-Carrier Degradation (HCD), are caused by material defects in the amorphous gate oxide. These defects, also called traps, can capture or emit charge carriers from the Si substrate and from the gate oxide, which causes discrete steps of several pA up to μA in the drain to source current. Measurements of this electrical response allow us to characterize trap properties, like characteristic times of capture and emission events. Thus we are able to determine the nature of traps in a more detailed manner.
However, typical capture and emission times of traps vary by many orders of magnitude, from μs to weeks, depending on their properties, the temperature and the bias conditions applied to the MOSFET. For degradation mechanisms, traps with emission times lying outside the experimental window (typically up to 10ks), often referred to as permanent traps, are of special interest for us. They are expected to dominate the distribution of device lifetime. One part of our current research focuses on shifting large capture and emission times into experimentally feasible time slots (measurement time below 1ks), see Fig. 1. For this, we have developed a hardware and software application for temperature control of local polysilicon heater structures within the Time-Dependent Defect Spectroscopy (TDDS) framework. We are able to apply controlled temperature pulses during device recovery within the TDDS sequences of stress recovery cycles. The pulse-like elevation of the temperature during recovery stimulates the charge emission. Thereby, capture and emission events with large characteristic times are moved into the experimental window. Thus we are able to characterize the properties of defects which seem to be otherwise permanent.
From future measurements we expect a significant contribution to the understanding of the nature of traps and of the permanent component of degradation mechanisms, especially in HCD.
Fig. 1: Temperature dependence of the spectral maps after ts=100ms. With increasing temperature, the emission times decrease. As also the capture time constant decreases, the clusters associated with each defect appear after a shorter stress time at higher temperatures. (see Grasser IRPS 2010)
