Mesoscopic phenomena in nanometer scale MOS devices
Creators: name: Wirth, Gilson Inácio
Publisher: name: Universität Dortmund
Year: 1999
Subjects: subject: MOS-device
subject: Mesoscopic phenomena
subject: Quantum transport model
subject: Coulomb blockade
subject: Nanotechnology
subject: FOS: Nanotechnology
subject: FOS: Nanotechnology
subject: Bulk-MOS-system
subject: Temperature effect
subject: RTS
subject: Random telegraph signal
subject: 620

description: The key point in the success of the semiconductor industry is its ability to continuously provide electronic products with decreasing cost per function as well as, at the same time, increasing performance. This is a result of a steady reduction in the feature size combined with a steady rise in density. In this framework, MOSFETs are being aggressively scaled down to dimensions well below 100 nm. However, at a given device dimension, the scaling of the physical processes breaks down and new phenomena that are absent in larger structures can dominate device behavior. The SIA roadmap (1) predicts a minimum feature size of 35 nm and 10 8 transistors per cm 2 for the commercial CMOS technology around 2012. It is now quite obvious that the behavior of the devices that will build up the circuits produced with this technology will significantly deviate from the behavior of their counterparts of greater dimensions. Subsequently, if devices are not properly characterized and modeled, this can turn out to be a barrier for further development in the semiconductor industry. The main scope of this work is to help and speed up the proper characterization and modeling of sub-100 nm MOSFETs. The term mesoscopic phenomena, the main subject of this work, has been introduced to describe the characteristics of systems that are neither microscopic (one or few atoms) nor macroscopic. Meso is borrowed from the Greek, meaning middle. In such systems the wave nature of electron transport or the discrete charge nature of electrons may become relevant. In this thesis negative differential resistance and single electron switching events in the channel of bulk MOSFETs with channel lengths down to 30 nm are demonstrated. First, reproducible unexpected periodic transconductance oscillations in the I D xVG characteristics of nMOSFETs are presented. The oscillations, present from sub-threshold up to strong inversion, are reproducible from sample to sample and with temperature cycling. No dependency of the oscillation period on gate oxide thickness or channel length could be observed and the period of the oscillations does not change in magnetic fields up to 15 T. Various electric transport models for small size MOS systems are analyzed. For several reasons, Coulomb blockade seems to be a rather plausible explanation for the observed effects. This is the first time that such phenomena are reported for a conventional bulk MOS system, suggesting that the electrical behavior of ultra short channel devices may still be an open question. Second, another single electron switching phenomenon is studied. Namely, oxide traps are used as a probe into the local channel surface potential. Studying the bias point dependence of the random telegraph signal (RTS) it is possible to estimate the trap location along the channel. It is shown that the behavior of the RTS does depend upon the properties of the trap and channel electrons, making RTS analysis a valuable tool to study effects as coulomb scattering, electron gas heating and the mechanisms that inuence electrical channel formation in very small area devices. The investigations carried out in this work intend to contribute to the better understanding of the properties of ultra short MOS devices, a necessary issue to produce optimized and reliable systems, leading ultimately to better products. (1) The National Technology Road-Map for Semiconductors, SIA - Semiconductor Industry Association, San Jose, CA, 1997.