Jordi Suñé

Voltage and Power-Controlled Regimes in the Progressive Unipolar RESET Transition of HfO2-Based RRAM

Resistive switching (RS) based on the formation and rupture of conductive filament (CF) is promising in novel memory and logic device applications. Understanding the physics of RS and the nature of CF is of utmost importance to control the performance, variability and reliability of resistive switching memory (RRAM). Here, the RESET switching of HfO2-based RRAM was statistically investigated in terms of the CF conductance evolution. The RESET usually combines an abrupt conductance drop with a progressive phase ending with the complete CF rupture. RESET1 and RESET2 events, corresponding to the initial and final phase of RESET, are found to be controlled by the voltage and power in the CF, respectively. A Monte Carlo simulator based on the thermal dissolution model of unipolar RESET reproduces all of the experimental observations. The results contribute to an improved physics-based understanding on the switching mechanisms and provide additional support to the thermal dissolution model.

Analytic modeling of leakage current through multiple breakdown paths in SiO/sub 2/ films

We have investigated the multiple-event dielectric breakdown of ultra-thin SiO/sub 2/ films used as gate insulators in MOS devices. A theoretical framework, based on the physics of mesoscopic conducting systems, which explains the post-breakdown current-voltage characteristics is presented. In this approach, the breakdown path is treated as a three dimensional quantum point contact in which an effective potential barrier arises as a consequence of the quantization of the transverse momentum of the passing electrons. Hard breakdown corresponds to large spot areas and therefore to completely open conducting channels between the electrodes. On the contrary, soft breakdown is associated with smaller areas and therefore with more restricted conducting paths. An adiabatic formulation of the charge transport process not only yields an analytic expression for the gate leakage current, which improves our understanding of the oxide breakdown physics, but also a compact model well suited for incorporation in circuit simulators. In addition, it is shown that the model accounts for the so-called nonlinear hard breakdown conduction mode in a consistent manner and how its conceptual framework might be adapted to cover the stress-induced leakage current regime.

Analog performance of the nanoscale double-gate metal-oxide-semiconductor field-effect-transistor near the ultimate scaling limits

We explore the prospective behavior of the nanoscale double-gate metal-oxide-semiconductor field-effect-transistor (DG-MOSFET) when used as the basis for building analog circuits. Results for transconductance, output conductance, and related parameters, such as transconductance efficiency, and Early voltage, are presented in the ballistic and diffusive regimes using the non-equilibrium Green’s function method (NEGF) and the scattering theory of the nanoscale MOSFET. Very high transconductance (∼20mS∕μm), approaching the ballistic limit, can be achieved provided that technological improvements further increase the electron mobility in the silicon film. For the ballistic limit a cutoff frequency of about 1–4THz is possible. The transconductance efficiency is not to much affected by length scaling and temperature. The output conductance and Early voltage are severely affected by length scaling as channel length-modulation (CLM) and drain-induced barrier lowering (DIBL) effects become more important, but they...

Soft Breakdown in Ultrathin SiO2 Layers: the Conduction Problem from a New Point of View

In this work, the soft breakdown failure mode in ultrathin (<5 nm) SiO2 layers is experimentally examined by means of current-voltage measurements performed on samples with different gate areas, oxide thicknesses and substrate types. The observed astounding matching which exhibits some of these characteristics, as well as the common features with the final breakdown, leads us to suggest that the current flow in the analysed regime might be largely controlled by the extremely constrictive dimensions of the oxide leakage spots. We present an alternative explanation to the soft breakdown phenomenon which can be naturally extended to the final breakdown conduction stage. In order to illustrate our ideas, a qualitative comparison with the theoretical behaviour of a classical point contact system is also discussed.

Conductance Quantization in Resistive Random Access Memory

The intrinsic scaling-down ability, simple metal-insulator-metal (MIM) sandwich structure, excellent performances, and complementary metal-oxide-semiconductor (CMOS) technology-compatible fabrication processes make resistive random access memory (RRAM) one of the most promising candidates for the next-generation memory. The RRAM device also exhibits rich electrical, thermal, magnetic, and optical effects, in close correlation with the abundant resistive switching (RS) materials, metal-oxide interface, and multiple RS mechanisms including the formation/rupture of nanoscale to atomic-sized conductive filament (CF) incorporated in RS layer. Conductance quantization effect has been observed in the atomic-sized CF in RRAM, which provides a good opportunity to deeply investigate the RS mechanism in mesoscopic dimension. In this review paper, the operating principles of RRAM are introduced first, followed by the summarization of the basic conductance quantization phenomenon in RRAM and the related RS mechanisms, device structures, and material system. Then, we discuss the theory and modeling of quantum transport in RRAM. Finally, we present the opportunities and challenges in quantized RRAM devices and our views on the future prospects.

Analysis of the degradation and breakdown of thin SiO/sub 2/ films under static and dynamic tests using a two-step stress procedure

A two-step stress test is used to analyze the degradation and breakdown of thin SiO/sub 2/ films. The procedure directly relates the degradation of the oxide to the breakdown statistics, without the need of any assumption about the microscopic degradation mechanism. It partially overcomes the problems associated with dynamic tests and allows the direct comparison of different tests (static and dynamic). The evolution of the degradation of 8 nm thick oxides subjected to constant-voltage (CVS), constant-current (CCS), and bipolar square voltage stresses is analyzed using the two step stress method. The results are compared with those of conventional breakdown tests to show the feasibility of the procedure. Our test procedure is also used to study the degradation of 4 nm thick oxides when subjected to CCS. The obtained results suggest that the two-step stress test will also be a powerful tool to analyze the degradation of ultra-thin oxides.

Statistical characteristics of reset switching in Cu/HfO2/Pt resistive switching memory

A major challenge of resistive switching memory (resistive random access memory (RRAM)) for future application is how to reduce the fluctuation of the resistive switching parameters. In this letter, with a statistical methodology, we have systematically analyzed the reset statistics of the conductive bridge random access memory (CBRAM) with a Cu/HfO2/Pt structure which displays bipolar switching property. The experimental observations show that the distributions of the reset voltage (Vreset) and reset current (Ireset) are greatly influenced by the initial on-state resistance (Ron) which is closely related to the size of the conductive filament (CF) before the reset process. The reset voltage increases and the current decreases with the on-state resistance, respectively, according to the scatter plots of the experimental data. Using resistance screening method, the statistical data of the reset voltage and current are decomposed into several ranges and the distributions of them in each range are analyzed by the Weibull model. Both the Weibull slopes of the reset voltage and current are demonstrated to be independent of the on-state resistance which indicates that no CF dissolution occurs before the reset point. The scale factor of the reset voltage increases with on-state resistance while that of the reset current decreases with it. These behaviors are fully in consistency with the thermal dissolution model, which gives an insight on the physical mechanism of the reset switching. Our work has provided an inspiration on effectively reducing the variation of the switching parameters of RRAM devices.

Analysis of the breakdown spot spatial distribution in Pt/HfO2/Pt capacitors using nearest neighbor statistics

The breakdown spot spatial distribution in Pt/HfO2/Pt capacitors is investigated using nearest neighbor statistics in combination with more conventional estimation methods such as the point-event and event-event distance distributions. The spots appear as a random point pattern over the top metal electrode and arise as a consequence of significant localized thermal effects caused by the application of high-voltage ramped stress to the devices. The reported study mainly involves the statistical characterization of the distances between each failure site and the nearest, second nearest, … kth nearest event and the comparison with the corresponding theoretical distributions for a complete spatial randomness (CSR) process. A method for detecting and correcting deviations from CSR based on a precise estimation of the average point intensity and the effective damaged device area is proposed.

Failure Analysis of MIM and MIS Structures Using Point-to-Event Distance and Angular Probability Distributions

Multiple breakdown (BD) spots are generated in large (>10-4 cm2) circular and square area metal-insulator-metal and metal-insulator-semiconductor devices using ramped and constant-voltage electrical stresses. Due to the important local thermal effects that take place at the very moment of the formation of the conductive paths spanning the insulating layer, the failure events become visible on the top metal electrode of the structures as a point pattern. The resulting point-to-event distance and angular histograms are compared with the theoretical distributions corresponding to a complete spatial randomness (CSR) process. The location of the voltage probe tip over the top electrode is considered here as the singular point from which the positions of the BD spots are referred to. In this way, we are able to assess the influence of the probe point on the final BD spot distribution. In most of the cases, this distribution is consistent with CSR, but after prolonged electrical stress, a deviation is detected. This departure from CSR is ascribed to the concentration of the current lines in the top electrode toward the center of the structure. The methods reported here are general and can be used for analyzing the generation of similar point patterns occurring in other structures or material systems.

Modeling of the switching I-V characteristics in ultrathin (5 nm) atomic layer deposited HfO2 films using the logistic hysteron

The current–voltage (I-V) characteristics of Pt/HfO2(5 nm)/TiN resistive switching structures are modeled using an equivalent electric circuit which consists of two antiparallel diodes in combination with a single series resistance, the only difference between the diodes being the threshold functions used to simulate the set and reset events. The switching process is achieved by means of a mathematical entity called the logistic hysteron, which governs the model parameters. The authors show that the model is able to capture the shape of the I-V curves both for positive and negative biases obtained under different current compliance limits for the set process ranging from 0.5 to 10 mA. In order to demonstrate the feasibility of the proposed approach, experimental and model results for the I-V curves are plotted using alternative representations: linear–linear, log–linear, and log–log axis. The role played by the series resistance is discussed in terms of the normalized differential conductance d ln(I)/d ln(V).