Probing the resistance switching mechanisms in SiOₓ/Ag RRAM devices

Resistive random access memory (RRAM) devices represent promising candidates for emerging non-volatile data storage applications and neuromorphic computing. In those devices, the resistance of a dielectric -often a binary oxide- is switched between a low resistance state (LRS) and one or more high resistance states (HRS) by the application of an appropriate external electrical bias. This resistance switching could be filamentary, i.e., involves the formation of a conductive filament. This filament can be thought of as chains of conductive oxygen vacancies (intrinsic resistance switching) or metallic atoms from an active device electrode (extrinsic resistance switching). In this thesis, the relationship between device electrode material and its resistance switching mechanism in SiOx (x∼1.9)-based RRAM devices was studied. Although it’s widely reported that RRAM devices with electrochemically active top electrodes, such as Ag, switch extrinsically, I show that both mechanisms and their associated conductive filaments can be triggered during device switching in ambient conditions. Resistance vs temperature measurements and conduction mechanism analysis were used to probe the nature of the formed filaments within device oxide layer. Results show that the two mechanisms can coexist within the device during switching. The type of filament generated by the initial electroforming of the device, however, depends on the polarity of the applied voltage during the electroforming step. This finding could help in optimising those RRAM devices for the different storage applications. Although the two mechanisms were observed under ambient conditions, SiOx/Ag devices showed extrinsic switching behaviour only under vacuum. In such an oxygen-poor environment, the contribution of intrinsic resistance switching mechanism appears to be reduced or probably eliminated. In extrinsically electroformed RRAM devices with Ag top electrodes, a metallic filament is likely to form within the switching layer. Using conductance tomography technique, the metallic filament of those SiOx/Ag devices was partially imaged using a conductive AFM (CAFM) tip

SiOx-based resistive switching memory integrated in nanopillar structure fabricated by nanosphere lithography

textA highly compact, one diode-one resistor (1D-1R) SiOx-based resistive switching memory device with nano-pillar architecture has been achieved for the first time using nano-sphere lithography. The average nano-pillar height and diameter are 1.3 μm and 130 nm, respectively. Low-voltage electroforming using DC bias and AC pulse response in the 50ns regime demonstrate good potential for high-speed, low-energy nonvolatile memory. Nano-sphere deposition, oxygen-plasma isolation, and nano-pillar formation by deep-Si-etching are studied and optimized for the 1D-1R configurations. Excellent electrical performance, data retention and the potential for wafer-scale integration are promising for future non-volatile memory applications.Materials Science and Engineerin

RRAM variability and its mitigation schemes

Emerging technologies such as RRAMs are attracting significant attention due to their tempting characteristics such as high scalability, CMOS compatibility and non-volatility to replace the current conventional memories. However, critical causes of hardware reliability failures, such as process variation due to their nano-scale structure have gained considerable importance for acceptable memory yields. Such vulnerabilities make it essential to investigate new robust design strategies at the circuit system level. In this paper we have analyzed the RRAM variability phenomenon, its impact and variation tolerant techniques at the circuit level. Finally a variation-monitoring circuit is presented that discerns the reliable memory cells affected by process variability.Peer ReviewedPostprint (author's final draft

Impact of laser attacks on the switching behavior of RRAM devices

The ubiquitous use of critical and private data in electronic format requires reliable and secure embedded systems for IoT devices. In this context, RRAMs (Resistive Random Access Memories) arises as a promising alternative to replace current memory technologies. However, their suitability for this kind of application, where the integrity of the data is crucial, is still under study. Among the different typology of attacks to recover information of secret data, laser attack is one of the most common due to its simplicity. Some preliminary works have already addressed the influence of laser tests on RRAM devices. Nevertheless, the results are not conclusive since different responses have been reported depending on the circuit under testing and the features of the test. In this paper, we have conducted laser tests on individual RRAM devices. For the set of experiments conducted, the devices did not show faulty behaviors. These results contribute to the characterization of RRAMs and, together with the rest of related works, are expected to pave the way for the development of suitable countermeasures against external attacks.Postprint (published version