Attilio Belmonte

Three-dimensional observation of the conductive filament in nanoscaled resistive memory devices.

The basic unit of information in filamentary-based resistive switching memories is physically stored in a conductive filament. Therefore, the overall performance of the device is indissolubly related to the properties of such filament. In this Letter, we report for the first time on the three-dimensional (3D) observation of the shape of the conductive filament. The observation of the filament is done in a nanoscale conductive-bridging device, which is programmed under real operative conditions. To obtain the 3D-information we developed a dedicated tomography technique based on conductive atomic force microscopy. The shape and size of the conductive filament are obtained in three-dimensions with nanometric resolution. The observed filament presents a conical shape with the narrow part close to the inert-electrode. On the basis of this shape, we conclude that the dynamic filament-growth is limited by the cation transport. In addition, we demonstrate the role of the programming current, which clearly influences the physical-volume of the induced conductive filaments.

Improvement of data retention in HfO 2 /Hf 1T1R RRAM cell under low operating current

One of the key concerns related to low operating current (<;50μA) of RRAM is the degraded data retention. Most of the retention studies so far focused on high switching current range. In this work, we investigate the retention degradation mechanism at low programming current range (10-40μA) and identify the key parameters that control retention in oxygen vacancy filamentary switching HfO<;sub>2<;/sub>/Hf 1T1R RRAM cells. Based on this understanding we demonstrated significant improvement in retention by adding an additional thermal budget into our process flow. The impact of the Forming process on retention property was also investigated and Forming/SET conditions were optimized to improve the retention without increasing the operation current.

A Thermally Stable and High-Performance 90-nm

In this paper, we optimize the stack of a 90-nm CMOS-friendly WAl₂O₃Cu conductive-bridging random access memory cell integrated in the one-transistor/one-resistor configuration. We show that the excellent Cu buffering properties of a TiW layer inserted at the Al₂O₃Cu interface make it possible, on one hand, to ensure cell integrity after back-end-of-line processing at 400 °C and, on the other, to obtain excellent memory performances. After optimization of the Al₂O₃ layer thickness, the cell exhibits highly controlled set and reset operations, a large memory window, fast pulse programming (0 ns) at low voltage (<;3 V), and low-current (10 μA), and multilevel operation. Finally, 10⁶ cycles of write endurance lifetime with up to a three-decade memory window is demonstrated, and state stability is assessed up to 125 °C.

{\rm Al}_{2}{\rm O}_{3}\backslash{\rm Cu}

We report the improved thermal stability of carbon alloyed Cu0.6Te0.4 for resistive memory applications. Copper-tellurium-based memory cells show enhanced switching behavior, but the complex sequence of phase transformations upon annealing is disadvantageous for integration in a device. We show that addition of about 40 at % carbon to the Cu-telluride layer results in an amorphous material up to 360 °C. This material was then integrated in a TiN/Cu0.6Te0.4-C/Al2O3/Si resistive memory cell, and compared to pure Cu0.6Te0.4. Very attractive endurance (up to 1 × 10(3) cycles) and retention properties (up to 1 × 10(4) s at 85 °C) are observed. The enhanced thermal stability and good switching behavior make this material a promising candidate for integration in memory devices.

-Based 1T1R CBRAM Cell

In this paper, we develop a Quantum-Point-Contact (QPC) model describing the state conduction in a W/Al2O3/TiW/Cu Conductive-Bridging Memory cell (CBRAM). The model allows describing both the voltage- and the temperature-dependence of the conduction. For deep current levels, a resistance component is added in series to the point-contact constriction to account for electron scattering in the residual filament. The fitting of single-particle perturbation also allowed to estimate the number and effective size of the conduction-controlling particles in the QPC constriction. The results clearly point to smaller particles for CBRAM (Cu particles) as compared to oxide-based resistive RAM involving oxygen-vacancy defects, which is discussed as a possible origin of deeper reset level obtained in CBRAM. We also evidence a beneficial impact of this smaller particle size on lower Random-Telegraph-Noise amplitude measured on CBRAM devices.

Influence of Carbon Alloying on the Thermal Stability and Resistive Switching Behavior of Copper-Telluride Based CBRAM Cells

In this work, we present a systematic electrical characterization of TiNHfO2HfTiN RRAM elements from the variability perspective. Variability of both programmed resistance values and switching triggering voltages has been evaluated on small scaled cells in a wide operating current range (2μA till 500μAs), for different oxide stacks, in DC and pulsed conditions. For the first time device-to-device and cycle-to-cycle variability are thoroughly compared as well as the impact of different oxygen vacancy profiles. Increase of variability in low current operation is also elucidated.

Origin of the current discretization in deep reset states of an Al2O3/Cu-based conductive-bridging memory, and impact on state level and variability

Metal-insulator-metal capacitors with SrxTiyOz (STO) dielectric films on TiN, Ru, and RuOx bottom electrodes with TiN top electrodes were studied. Metastable perovskite STO films with compositions in the Sr/(Sr+Ti)∼54–64u2002at.u2009% range were obtained by crystallization at 600u2009°C in N2 of dielectric stacks grown by atomic layer deposition consisting of Sr-rich STO films [Sr/(Sr+Ti)∼64u2002at.u2009%] on thin interfacial TiOx layers. The significant decrease in equivalent oxide thickness (EOT) and STO lattice parameter observed with increasing TiOx thickness indicates full intermixing of the TiOx and STO layers during the crystallization anneal, which results in the formation of an STO layer with higher Ti content and higher dielectric constant. The Sr-rich STO on TiOx stacks crystallize with small grain size, favorable for low leakage (JG). A significant improvement in JG for e-injection from the bottom electrode is obtained when using RuOx, as compared to TiN or Ru. A milder JG improvement with RuOx bottom electrode i...

Intrinsic switching variability in HfO 2 RRAM

In this paper we demonstrate a novel characterization technique for the observation of the conductive filament in conductive bridging memory devices (CBRAM). The conductive filament is observed for a scaled memory element programmed under 10μA operative current. After the electrical programing, the C-AFM tomography enables the 3D analysis of the conductive filament within the switching layer.

Impact of bottom electrode and SrxTiyOz film formation on physical and electrical properties of metal-insulator-metal capacitors

We have demonstrated that by material engineering using different spices to dope HfO2, RRAM cell switching and endurance / retention reliability characteristics can be modulated. The changes in SET/RESET voltages, endurance optimal programming window and retention result mainly from the oxygen scavenging efficiency of Hf cap in presence of different dopants in HfO2. This impacts directly the formation of OEL that controls the RRAM switching characteristics and retention. By utilizing different dopant materials, the operation range of the HfO2 based RRAM can be tailored to be compatible with different selectors and to be adopted for broader applications.

Conductive-AFM tomography for 3D filament observation in resistive switching devices

In this paper we review a dynamic device model for filamentary RRAM in HfO-based dielectrics. We summarize its transient modeling features and its statistical properties. The model explains with satisfactory quantitative resolution all main features of the RRAM switching, not just the voltage, time and temperature dependence, but also statistical fluctuations resulting from atomistic motion and their resulting LRS and HRS-distributions.