Bogdan Govoreanu

10×10nm 2 Hf/HfO x crossbar resistive RAM with excellent performance, reliability and low-energy operation

We report on worlds smallest HfO2-based Resistive RAM (RRAM) cell to date, featuring a novel Hf/HfOx resistive element stack, with an area of less than 10×10 nm2, fast ns-range on/off switching times at low-voltages and with a switching energy per bit of <0.1pJ. With excellent endurance of more than 5.107cycles, large on/off verified-window (>50), no closure of the on/off window after 30hrs/200C and failure-free device operation after 30hrs/250C thermal stress, the major device-level nonvolatile memory requirements are met. Furthermore, we clarify the impact of film crystallinity on cell operation from a scalability viewpoint, the role of the cap layer and bring insight into the switching mechanisms.

VARIOT: a novel multilayer tunnel barrier concept for low-voltage nonvolatile memory devices

Low-voltage low-power nonvolatile floating-gate memory device operation can be achieved by using alternative tunnel barriers consisting of at least two dielectric layers with different dielectric constants k. Low-k/high-k (asymmetric) and low-k/high-k/low-k (symmetric) barriers enable steeper tunneling current-voltage characteristics. Their implementation is possible with high-k dielectric materials that are currently investigated for SiO/sub 2/ replacement in sub-100-nm CMOS technologies. We show that a reduction in programming voltages of up to 50% can be achieved. This would significantly reduce the circuitry required to generate the high voltages on a nonvolatile memory chip, while maintaining sufficient performance and reliability.

Evidences of oxygen-mediated resistive-switching mechanism in TiN\HfO2\Pt cells

In this letter, we study the influence of the Pt top-electrode thickness and of the chamber atmosphere during cell operation on the resistive switching of TiN\HfO2\Pt cells. The oxygen permeability of the Pt electrode directly in contact with the atmosphere significantly affects the resistive switching and the resistance states of the cell. The results provide strong experimental indications that the electroforming operation leads to oxygen-vacancy formation and that the subsequent reset operation relies on the available oxygen species in the filament neighborhood. Significant implications with respect to endurance and retention assessment of resistive-switching memory devices are discussed.

Endurance/Retention Trade-off on

The endurance/retention performance of HfO2/ Metal cap RRAM devices in a 1T1R configuration shows metal cap dependence. For Hf and Ti caps, owning strong thermodynamic ability of extracting oxygen from HfO2, long pulse endurance (>1010 cycles) could be achieved. For Ta cap, owning lower thermodynamic ability of extracting oxygen from HfO2, better retention can be achieved. Therefore, an endurance/retention performance tradeoff is identified on the 40 nm × 40 nm HfO2/Metal cap bipolar RRAM devices. The tradeoff of endurance/retention performance can be explained by a different filament constriction shape depending on metal cap layer as derived from fitting I-V curves in the quantum point contact model. This difference in filament constriction shape is attributed to the thermodynamics difference of metal cap: Hf and Ti have a stronger thermodynamical ability to extract oxygen from HfO2 than Ta. The possibility of tuning the intrinsic reliability performance by changing the cap materials paves a way for optimizing the operation of RRAM devices into the desired specifics.

\hbox{HfO}_{2}/\hbox{Metal}

By tuning the SET/RESET pulse amplitude conditions, the pulse endurance of our 40-nm HfO2/Hf 1T1R resistive-random-access-memory devices demonstrates varying failure behaviors after 106 cycles. For unbalanced SET/RESET pulse amplitude conditions, both low-resistance state (LRS) and high-resistance state (HRS) failures may occur, while varying the pulsewidths influences the LRS/HRS window and the stability of the LRS/HRS states. The failure of the HRS or LRS state during cycling is ascribed to the depletion or excess of oxygen vacancies at the switching interface. Through a dc SET/RESET recovery operation, LRS/HRS states can be recovered after failure, indicating that the distribution of oxygen vacancies can be restored. By optimally balancing the SET/RESET pulse conditions, more than 1010 pulse endurance cycles is achieved.

Cap 1T1R Bipolar RRAM

Trap spectroscopy by charge injection and sensing (TSCIS) is a new, fast and powerful material analysis technique that provides detailed information on the trap density profile and trap energy level in dielectric materials. We show the measurement principle and explain the data analysis. The technique is applied to a number of example materials: SiO₂, Al₂O₃, and Si₃N₄. We show that TSCIS has excellent resolution and is capable of distinguishing between different process-variations.

Balancing SET/RESET Pulse for

An analytic dynamic hour glass model for HfO2 RRAM is demonstrated, describing the reset as a dynamic equilibrium process and the set as a constriction growth limited by ion mobility and current compliance. The dependence on time, voltage and forming conditions is in good constriction growth agreement with experiments. Since the model is fully analytical, it can be implemented in a circuit simulator.

>\hbox{10}^{10}

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.

Endurance in

Transition metal oxide-based resistor random access memory (RRAM) takes advantage of oxygen-related defects in its principle of operation. Since the change in resistivity of the material is controlled by the oxygen deficiency level, it is of major importance to quantify the kinetics of the oxygen diffusion, key factor for oxide stoichiometry. Ab initio accelerated molecular dynamics techniques are employed to investigate the oxygen diffusivity in amorphous hafnia (HfOx, x = 1.97, 1.0, 0.5). The computed kinetics is in agreement with experimental measurements.

\hbox{HfO}_{2}\hbox{/Hf}

An efficient yet accurate model is used for investigating tunneling of minority carriers from the inversion layer of ultrathin MOSFET structures. The model is derived from the concept of the quasibound states lifetimes, which are calculated using a transfer matrix method based on Airy functions. Comparison with experimental data is provided. Performance of high-k materials is discussed and an investigation of their scalability for future CMOS technology nodes is carried out.