Resistive Switching Comparison Between Cu/TaOx/Ru and Cu/TaOx/Pt
Authors: Orlowski Marius K , Fan Ye , Al-Mamun Mohammad , Colon Ben , King Sean W. 
Published: February 2020

Building nonvolatile memory (NVM) directly into a CMOS low-k/Cu interconnect module would reduce latency in connectivity constrained computational devices and reduce chip’s footprint by stacking memory on top of the logic circuits. One strong candidate for NVM is the well-behaved and well-characterized Cu/TaOx/Pt resistive switching device. This device can be operated as a memory cell with copper (Cu) or oxygen vacancy (VO) conductive filaments (CF). Since platinum (Pt) is not an economic choice for industrial production and has been not used in back-end-of-line (BEOL), a BEOL-compatible replacement of Pt is highly desirable. A good candidate for a replacement of Pt is ruthenium (Ru) which has been already deployed in the CMOS BEOL as a possible replacement for Ta/TaN as the liner material. Ru is 45 times less expensive than Pt, and has similar properties as Pt. Pt and Ru are both transition metals with almost identical outer shell structure: Ru has one electron in the 5th orbital and 15 electrons in the 4th orbital, while the larger Pt atom has one electron in the 6th orbital and 17 electrons in the 5th orbital. Cu/TaOx/Pt and Cu/TaOx/Ru devices have been manufactured in the same way. All four layers, Cu, TaOx, Pt, Ru have been deposited by e-beam PVD, with the thicknesses 150 nm, 25 nm, 50 nm, 50 nm, respectively. The only difference in the deposition is the higher melting temperature for Ru of 2250 oC compared with 1768 oC for Pt.

The electric characterization of both devices has shown many similarities and some notable differences. The forming and set voltages for Cu filaments in Ru devices are significantly higher (Vform(Ru)-Vform(Pt)»3V) than in Pt devices. This difference cannot be explained solely in terms of work function difference which is Df (Cu-Pt)=1.3eV and Df (Cu-Ru)=0.1eV. In resistive switching devices the resistance of the conductive filament (CF) is a strong function of the imposed compliance current, Icc. We find that the general behavior of Ron for Cu CF is similar for both devices. For Ru device, we find Ron(Icc)=1.75/Icc
1.04 while for Pt device Ron(Icc)=1.28/Icc
1.03. As shown elsewhere, with the exponent of Icc in the denominator being close to 1, the constants 1.75 V and 1.28 V can be interpreted as the lowest possible voltages under which the device can be set. This usually can be reached as a limiting case for very slow voltage ramp rates. The difference in the minimum set voltage extracted from Ron-Icc characteristics confirms that Vset voltages for Ru devices are significantly higher than for Cu devices. We have inquired as to the nature of the Cu filament in both devices by measuring the temperature coefficient of resistance (TCR); we find TCR(Ru-device)=0.00236K-1 and TCR(Pt-device)=0.00235K-1 for similar values of Ron which, within the accuracy of our measurement, means that the values are identical. We conclude that under a positive voltage stress applied to the Cu electrode, very similar Cu CFs are being formed. Moreover, these values are consistent with TCR values of Cu CFs observed in many other devices. We have also observed that for both devices TCR increases with decreasing Ron reaching a value of 0.0035K-1 at Ron=300 W. (For comparison TCR of bulk Cu is 0.0039K-1.) The major drawback of the Ru device is that while Pt devices can be switched repeatedly back and forth, Ru device are becoming not resettable after a few set-reset operations. The failure of the Ru devices after a few switching cycles is related to the high values of form and set voltages. We attribute this behavior to the reduced stopping power of Ru with respect to Cu diffusion compared with the stopping power of Pt. Because of the reduced stopping power in Ru, the Cu ions are not stopped sharply at the interface but migrate into the Ru electrode. The loss of Cu atoms has to be compensated by additional flux of Cu ions at higher voltages. At the same time, the loss of Cu at the base of the Cu filament, leads to a different geometrical shape of the filament. Whereas with a nearly perfect stopping power, the shape of the filament in the Pt device can be assumed to be conical with sharp tip at the Cu electrode, the shape of the Cu filament in the Ru device is more cylinder-like. It is known that filaments with no pronounced constriction are more difficult to rupture, which explains the limited switching cycles of the Ru device. The results are also compared with similar characteristics of Cu/TaOx/Ta devices.