A CMOS-compatible bipolar analogue switching redox-based ReRAM with TaOx/HfO2 bilayer

Complementary metal-oxide-semiconductor(CMOS)-based von Neumann architecture suffers from intrinsic power and speed inefficiencies, due to the data transfer between processor and the memory unit. This is commonly addressed as von Neumann bottleneck. The processing of the huge amount of information collected with big data requires the transition to novel computing paradigms and architectures, such as bioinspired computing. It relies on memristor technology which allows to co-locate the storage and the processing of data while ensuring low-cost fabrication and high-density of integration. Among the most promising memristive candidates for building future artificial neural networks (ANNs) there are redox-based valance-change mechanism (VCM) resistive random-access memories (ReRAMs). They gained interest for the large amount of distinguishable resistance levels achievable as well as for their scalability. Symmetry, linearity and stochastic nature of the weight update are the major challenges towards their adoption in neuromorphic systems. Stacking a conductive metal oxide (CMO) on top of the insulator layer represents a simple and cost-effective way to solve these issues. However, the switching mechanism in this novel stack configuration, as well as the role played by the CMO, is not fully understood, thus limiting the improvement of device performances. In this master thesis work a CMOS-compatible ReRAM with substoichiometric tantalum oxide (TaOx)/hafnium oxide (HfO2) bilayer sandwiched between titanium nitride (TiN) electrodes is presented. To meet the challenging requirements of CMO materials, the parameters of the TaOx sputtering deposition are carefully controlled. The chamber and target conditions revealed to be fundamental for the reproducibility of the TaOx films. The electrical conductivity of the layer can be finely tuned by means of the pressure, while keeping constant and low the DC power and the oxygen (O2)/argon(Ar) flow ratio. Material properties are investigated through Grazing Incidence X-ray Diffraction (GIXRD), X-ray Reflectivity (XRR) and Circular Transfer Length Method (CTLM). Extensive quasi-static and pulsed electrical characterization have been conducted to assess the performances of the bilayer ReRAM devices, as well as to understand the impact of the CMO conductivity and thickness on the device switching. ReRAM devices embedding a 20 nm thick CMO with sheet resistance Rsh≈90 kΩ/□ showed that a remarkable linear, symmetric and analogue weight update can be reached by sacrificing the dynamic range to roughly a factor 3. Impedance Spectroscopy technique was employed to model the switching mechanism in the bilayer ReRAM devices. By analyzing the equivalent circuit used to describe the device in the different states, the role of the filament and of the defect induced leakage resistance in the TaOx were decoupled.