"Resistive Switching: Mechanisms and Scaling"
Beta Room, via Golgi 40
July 1st, 2010
Bipolar resistively switching memory elements utilize redox processes on the nanoscale. One type is based on electrochemical oxidation, cation transport in an ionic conductor, and reduction of an electrochemically active electrode metal such as Ag or Cu (electrochemical metallization mechanism, ECM). The other type is based on the drift of oxygen ions in transition metal oxides, a concentration polarization, and a corresponding valence of the cations which leads to an increase or decrease of the electronic partial conductivity (valence change mechanism, VCM). From the conceptional point of view, both types show potentially a scalability to below 10 nm. In addition, both types show faster write/erase times and lower write/erase voltages compared NAND Flash. They are known to be compatible with transistors as select devices and can be integrated into a CMOS process.
For oxides which exhibit switching following the valence change mechanism usually a forming process is needed to transfer the initially highly resistive samples into a state which can be switched by an appropriate electrical stimulus. Morphological modifications after forming are observed on large sized electrodes. After mechanical electrode lift-off conductive AFM methods revealed two types of switching polarities in the vicinity of these features. Using an in-situ SEM technique to observe forming and switching on nano-crossbar elements no obvious morphology changes could be detected.
Nanoimprint lithography is a very useful technology to define patterns in the range of 100nm and less and explore scaling effects. Using single crossbar elements a comprehensive characterization of resistively switching TiO2 memory elements has been performed. Very fast switching in the 10ns range and the capability to write multiple resistance levels using voltage pulses were demonstrated. Currently these investigations are complemented by resistive switching on sputtered NiO films in a very similar device structure.
Microelectronics and Emerging Technologies