The design and the manufacturing of ReRAM test structures allow deeper insight in the performance of the FORMING, RESET, and SET operations at array level, providing details on the process induced variability of the technology, and on the potential sources of failures. Test structures allow also demonstrating the integration capability of the ReRAM technology using a CMOS-compatible process ramping up such non-volatile memory to a maturity level.

In this study, direct experimental materials science evidence of the important theoretical prediction for resistive random access memory (RRAM) technologies that a critical amount of oxygen vacancies is needed to establish stable resistive switching in metal-oxide-metal samples is presented. In detail, a novel in-operando hard X-ray photoelectron spectroscopy technique is applied to non-destructively investigates the influence of the current compliance and direct current voltage sweep cycles on the Ti/HfO2 interface chemistry and physics of resistive switching Ti/HfO2/TiN cells. These studies indeed confirm that current compliance is a critical parameter to control the amount of oxygen vacancies in the conducting filaments in the oxide layer during the RRAM cell operation to achieve stable switching. Furthermore, clear carbon segregation towards the Ti/HfO2 interface under electrical stress is visible. Since carbon impurities impact the oxygen vacancy defect population under resistive switching, this dynamic carbon segregation to the Ti/HfO2 interface is suspected to negatively influence RRAM device endurance. Therefore, these results indicate that the RRAM materials engineering needs to include all impurities in the dielectric layer in order to achieve reliable device performance.

The conducting filament forming voltage of stoichiometric hafnium oxide based resistive switching layers increases linearly with layer thickness. Using strongly reduced oxygen deficient hafnium oxide thin films grown on polycrystalline TiN/Si(001) substrates, the thickness dependence of the forming voltage is strongly suppressed. Instead, an almost constant forming voltage of about 3 V is observed up to 200 nm layer thickness. This effect suggests that filament formation and switching occurs for all samples in an oxidized HfO2 surface layer of a few nanometer thickness while the highly oxygen deficient thin film itself merely serves as a oxygen vacancy reservoir. (C) 2014 AIP Publishing LLC.

The Ti/HfO2 interface plays a major role for resistance switching performances. However, clear interface engineering strategies to achieve reliable and reproducible switching have been poorly investigated. For this purpose, we present a comprehensive study of the Ti/HfO2 interface by a combined experimental-theoretical approach. Based on the use of oxygen-isotope marked Hf*O2, the oxygen scavenging capability of the Ti layer is clearly proven. More importantly, in line with ab initio theory, the combined HAXPES-Tof-SIMS study of the thin films deposited by MBE clearly establishes a strong impact of the HfO2 thin film morphology on the Ti/HfO2 interface reactivity. Low-temperature deposition is thus seen as a RRAM processing compatible way to establish the critical amount of oxygen vacancies to achieve reproducible and reliable resistance switching performances.

We have investigated the resistive switching behavior in stoichiometric HfO2 and oxygen-deficient HfO2−x thin films grown on TiN electrodes using reactive molecular beam epitaxy. Oxygen defect states were controlled by the flow of oxygen radicals during thin film growth. Hard X-ray photoelectron spectroscopy confirmed the presence of sub-stoichiometric hafnium oxide and defect states near the Fermi level. The oxygen deficient HfO2−x thin films show bipolar switching with an electroforming occurring at low voltages and low operating currents, paving the way for almost forming-free devices for low-power applications.

Atomic layer deposition was used for preparation of TiO2 and HfO2 thin films for resistive switching in metal-insulator-metal structures with Pt and TiN top and bottom electrodes, respectively. To obtain stable bipolar resistive switching loops in TiO2-based structures Al2O3 barrier with the thickness of 3 - 5 nm was necessary. HfO2-based structures with the insulator thickness less than 10 nm exhibited stable bipolar resistive switching. Ratio between high resistivity and low resistivity state varied between 20 and 100 depending on structure preparation and composition as well as on parameters of DC current – voltage measurement. Resistive switching effect was demonstrated in metal-insulator-metal structures with HfO2 layers thickness below 3 nm.

The electrical characteristics of different resistance states (virgin, OFF and ON) of a Ti/HfO2/TiN metal-insulator-metal device for resistance random access memory are investigated under different gas ambient. The influence of the atmosphere, the total pressure and the oxygen concentration during electrical measurements is underlined thanks to retention (I-t) and impedance spectroscopy (Z-f) measurements. The total pressure influences the current levels of the three different resistive states: when the total pressure decreases, the current increases, probably due to an increase of the concentration of oxygen vacancies in the HfO2.

Resistive switching in Ti/HfO2/TiN was investigated in-operando by hard x-ray photoelectron spectroscopy. In comparison with the virgin-state, ON- and OFF-states show enhanced Ti/TiOx/HfO2 interface oxidation, resulting from an oxygen-gettering activity of Ti. The formed TiOx layer acts in the resistive switching process as an oxygen reservoir in exchange with the non-stoichiometric HfO2−δ. A Ti1+/Ti3+ valence change redox reaction occurs between OFF- and ON-states. The peak shifts are attributed to space charge potentials created by the varying oxygen vacancy concentration at the interface. A push-pull model of oxygen vacancies as a function of voltage polarity is proposed to describe the mechanism.

This paper deals with the electrical and wideband frequency characterizations of metal-insulator-metal (MIM) capacitors integrating the medium-k material ZrO 2 . First, the in situ material electrical properties are characterized in a frequency range from dc up to 5 GHz by using a microstrip waveguide method. The loss tangent and the permittivity are extracted with frequencies up to 5 GHz. We then investigate the evolution with frequency of the electrical parameters, such as capacitance density, quality factor, temperature coefficient of capacitance, voltage coefficient of capacitance, and cut-off frequency for MIM capacitors which incorporate ZrO 2 dielectric layers with thickness from 10 to 45 nm.

This work reports the bipolar resistive switching behavior of more than 100 back-end-of-line (BEOL) integrated 600×600nm2 TiN/HfO2/Ti/TiN MIM devices in a 4 kbit memory array. Reliable current-voltage switching characteristics were only observed for devices with a thickness ratio of 1 (10 nm HfO2/10nm Ti), indicating the importance of the interface chemistry of the Ti/HfO2 interface. Moreover, the devices show good inter-cell uniformity and thus demonstrate promising prospects for embedded non-volatile memory (eNVM) applications.

The chemical and electronic modifications induced by the electroforming process on the Ti/HfO2/TiN-based resistive switching devices were investigated by non-destructive hard x-ray photoelectron spectroscopy (HAXPES). The results indicate an increase of the titanium top electrode oxidation at the interface with HfO2 after the electroforming process. Additionally, the binding energy values of the HAXPES peaks of the electroformed sample increased as compared to the as-prepared sample. The observed changes between both samples are attributed to the creation of n-type defects, such as oxygen vacancies, in the HfO2 layer near the Ti interface during the electroforming process.

This paper deals with the impact of the top metal electrode on the resistive switching properties of HfO2-based Metal-Insulator-Metal diodes. By screening five different metals as top electrode, Al–Cu–Hf–Pt–Ti, we have demonstrated the feasibility of the resistive switching effect on HfO2. Metals with a low enthalpy of formation of oxides ΔHf0 (Pt and Cu) lead to uni-polar switching whereas easily oxidizable metals with a higher ΔHf0 (Al, Hf and Ti) lead to bipolar switching. Cu-, Hf- and Pt-based devices show a degradation of the top electrode after the forming step by the formation of bubbles whereas such phenomenon was not observed in Al- and Ti-based devices. 200 switching cycles were performed on each device in order to extract the main parameters of the resistive switching effect: ION and IOFF currents in the mA range, ROFF/RON resistance ratio up to 5, Vset and Vreset, voltage levels around 1V, and powers dissipated during read and write operations in the μW and mW range, respectively. For all systems, the reset process dissipates higher power than the set process. From these results, the Ti top adlayer shows the best performance in terms of stability and resistive switching characteristics.

The Ti/HfO 2 /TiN-based system is a very promising candidate for Resistance change Random Access Memory (RRAM). By combining material science studies and integration in a Si CMOS technology, we succeeded to give quantitative insight in the resistive switching mechanism and to process a 4 kbit array with 1T1R RRAM devices. In particular, in-operando hard X-ray photoelectron spectroscopy allows to describe the resistive switching mechanism by a push-pull model of oxygen vacancies as a function of voltage polarity. Moreover, the characterization of integrated 600×600 nm2 TiN/Ti/HfO 2 /TiN 1T1R devices in the pulse-induced mode and the recent realization of a 4 kbit memory array have demonstrated promising performance for embedded non-volatile memory applications.

The titanium tantalum oxide, TiTaO, was fully characterized in situ in an integrated metal–insulator–metal (MIM) configuration on a wide frequency band, from 1 mHz to 30 GHz. First, XPS and XRD analysis show that TiTaO dielectric is amorphous and presents Ti-O, Ta-O, and Ti-Ta bounds. Next, by using dedicated MIM test devices and a specific extraction procedure, both relative permittivity κ and loss tangent tanδ were extracted on the wide frequency band. The results show a dependence on both frequency and thickness. Conduction mechanism of ion impurities such as oxygen vacancies at very low frequencies (below 0.1 Hz), Maxwell-Wagner relaxations due to space charges at electrode interface (between 0.1 and 5 Hz), and dielectric grain boundaries or hopping (between 1 and 100 MHz) are underlined and studied with temperature measurements. The paper shows the usefulness of in situ wideband frequency measurements and that the dielectric permittivity of TiTaO in a 100 nm thick film is decreasing from 86 at 10 kHz to 20 at 10 GHz.

This work addresses a 1T1R RRAM architecture, which allows for the precise and reliable control of the forming/set current by using an access transistor. The 1T1R devices were fabricated in a modified 0.25μm CMOS technology. The memory cells show stable resistive switching in dc as well as pulse-induced mode with an endurance of 103 and 102cycles, respectively. The variation of pulse widths as a function of amplitudes in 1R devices confirmed the set process distribution over a wide range of pulse widths (300ns–100μA), whereas the reset process variation is confined (1–3μs).