Conditions for Formation and Rupture of Multiple Conductive Cu and VO Filaments in a Cu/TaOx/Pt Device

In a Cu/TaOx/Pt resistive device multiple conductive Cu bridges can be formed and ruptured successively between the active Cu and inert Pt electrodes. The key parameters to repeatable multibridge switching cycles is the appropriate choice of compliance current window for individual bridge formation and the choice of bipolar and unipolar reset. Controlled formation and rupture of multiple filaments may enable multilevel storage capability in a single CBRAM cell if the repeatibility issue can be successfully addressed. In the past, there were a few reports on multiple filament formation [1,2]. However, its mechanism and how to reliably control the formation and rupture of multiple filaments has been not thoroughly explored. The Cu/TaOx/Pt resistive devices have been fabricated in a crossbar array on a thermally oxidized Si wafer. Metal electrodes and solid electrolyte are deposited by e-beam evaporation and patterned by lift-off technology. The oxygen-deficient TaOx was deposited without O2 injection to the evaporation chamber. The top Cu electrode runs perpendicularly to the bottom Pt electrode. The width of the metal lines varies between 1 μm and 35 μm. Each intersection of Cu and Pt lines is a single cell of a resistive switch. An I-V characteristics of two-bridge formation and rupture is shown in Fig.1. The voltage is swept from 1 to 5V for the set and from -3V to -6V for the reset operation with a sweeping step of 1mV/step. Small 1mV/step is used to resolve the multiple bridge formation and rupture. It can be seen that the first current jump occurs at Vset-1=2.80V and is followed by ohmic current behavior characterized by a Ron-1 =760Ω. Then at Vset2=4.14V another sharp increase in current is observed followed by an ohmic behavior with a different slope of 447Ω. The 2 jump in current is associated with the formation of the 2 Cu bridge. From a parallel resistance formula, the resistance of the 2 bridge is calculated to be Ron-2=1085Ω. Once the second bridge is formed, the current through both parallel resistors drops roughly abruptly by a factor of 2 imposing effectively a lower level of compliance current. For the reset operation the voltage is swept then from -3V to -6V. The 1 bridge is ruptured at -5.32V while the 2 bridge is still intact. From the remaining resistance measured at 780Ω, the ruptured bridge can be clearly identified. The resistance of the ruptured bridge is Ron-1=780x447/(780-447)=1035Ω. Clearly, this is the resistance of the 2 bridge, Ron-2. The remaining unruptured bridge, must be therefore the 1 bridge to be formed in set operation; indeed the measured 780Ω is very close to 760Ω measured in the set operation. Thus the bridge formed as the 2 bridge is the bridge which ruptures first during the reset operation. The remaining bridge is ruptured at -5.61V. In Fig.2 a set operation is shown at a reduced compliance current of Icc=5mA. Here the 1 bridge formation is observed at Vset-1=3.6V and the 2 bridge formation at 4.0V. Due to Icc limitation the second slope observed in Fig.1 and characterized by a resistance of two bridges in parallel can be observed only during the back sweep to 0V. A promising approach for a controlled formation of two bridges consists in formation of 1 bridge with high Ron-1 under a low compliance current of 10-100μA in a first voltage sweep, followed by a formation of the 2 bridge in a second independent voltage sweep. Such a sequential multi-bridge formation is shown in Fig.3. The 1 bridge with Ron-1=1KΩ has been formed at Vset-1=4.3V. During the 2 sweep 2nd bridge is created at Vset-2=3.3V with Ron-2=625Ω. During a back sweep a resistance of the two bridges in parallel can be extracted to be R=375Ω. Both bridges have been identified as Cu conductive nanofilaments by their temperature resistivity coefficient that is close to the respective coefficient for bulk Cu. Once the Vset for the first bridge is stabilized, data indicates that Vset for the 2 bridge will occur within 0.7V – a small enough window to guarantee programming of the 2 bridge. Control of indvidual set and reset operations may pave the way for a deployment of a multi-filament cell for multibit NVM RAM.