(Invited) Current Filamentation in Rram As Measured By High Speed Electrical Thermometry

In this paper, we review our method of measuring the local device temperature in oxide RRAM cells by taking high speed pulsed electrical measurements of the device I-V characteristics. These measurements are taken sufficiently fast (< 5 ns) as to avoid local joule heating of the device, revealing the device’s “true” room temperature I-V characteristics. Repeating these measurements as a function of stage temperature allows the construction of a unique I-V-T map, where T is the device temperature. This calibrated map can then be used under more general I-V interrogation where self-heating is present to extract the actual device temperature from I-V measurements. This method has been applied to a variety of RRAM-type oxide switches, including TiO₂, Ta₂O₅ and HfO₂ devices. Both pristine devices (prior to the initial filament forming breakdown step) as well as devices that have a filament present and can be repeatedly switched between high and low resistance states have been examined. The measurements show that internal device temperatures can range from 50ºC to 400ºC at the onset of the electroforming event, with temperatures above 600ºC when the event is completed. During switching, temperatures within the filaments peak at 300ºC-400ºC during each switching event. Temperature dependent resistivity measurements reveal that the filament has a metallic nature in materials like TiO₂ (resistivity increases with increasing temperature) with a resistivity value of approximately 6000 nΩ-m. The surrounding matrix is semiconducting (resistivity decreases strongly with increasing temperature) and displays highly non-linear I-V curves. By combining the above thermometry technique with modeling of the thermal transport in the filament and its surrounding region, it is possible to get an estimate of the filament radius. Specifically, a unique filament radius is determined that self-consistently explains both the filament I-V behavior and slope of filament temperature vs dissipated power (the filament thermal resistance, R_th). This analysis reveals that the filaments in oxide RRAM memory devices are typically 1 nm in diameter and have a hot zone around them that grows to several nm during the switching event. We are also able to apply this same analysis to the transient (i.e. non-permanent) current filaments that form prior to the permanent filament, during so-called threshold switching events. These measurements confirm for oxides the same picture reported in the literature for chalcogenide phase change materials many years ago - that a transient electrical current filament precedes the permanent structural one in these devices. Moreover, it is most likely that the heating within this transient filament that allows for the microstructural arrangement of the oxide atoms that results in a permanent conducting filament able to participate in the memory switching event.