Yang Yin Chen

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

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.

Balancing SET/RESET Pulse for

Metal-oxide-based resistive random access memory (RRAM) is a predominant candidate for future non-volatile memories. In this Letter, we report on an innovative technique to observe conductive filaments in these oxide-based RRAM devices. We demonstrate the role of these conductive filaments as responsible for the different ON/OFF resistive states in memory devices by means of Conductive Atomic Force Microscopy (C-AFM). More specifically, C-AFM is used to cycle, de-process, and finally characterizes capacitor-like devices. Different conductive filaments are found for the different memory states. As we show, the ON/OFF state of the devices is associated to changes in morphological and electrical properties of the conductive filaments.

>\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}

A Monte Carlo simulator is developed to investigate the low resistance state (LRS) retention of HfOx based resistive switching memory. The simulator tracks the evolution of oxygen ions and oxygen vacancies by choosing the occurrence of the generation/recombination/migration processes based on their corresponding probability at each simulation step. The current through the resulting conductive filament (CF) configuration is then calculated by a trap-assisted-tunneling solver. The simulation results are corroborated with experimental data. The LRS retention is found to be CF size dependent, and its variability is suggested to be intrinsic due to the stochastic nature of the CF dissolution. The tail bits of the failure time distribution become a limiting factor of the device reliability in a large memory array.

1T1R Bipolar RRAM

Bipolar switching transition metal-oxide (TMO) RRAM devices are intensively studied as possible non-volatile memory for 1x nm node. HfO2 based stacks with excellent operation, good endurance and retention have been proposed [1, 2, 3], with demonstrated scalability down to <;10nm [3]. However, characterization of the reliability failure modes and understanding of the degradation mechanism is urgently needed, especially in the low operation current range relevant for practical application of RRAM devices. Although retention and endurance models in different TMO have been proposed [4, 5, 6], an in-depth understanding of endurance is still lacking for scaled HfO2 RRAM in low current operation.

Dynamic ‘hour glass’ model for SET and RESET in HfO 2 RRAM

A novel procedure to decompose the I- V switching curves of complementary resistive switching (CRS) RRAM cells into the intrinsic switching characteristics of its individual constituting elements is proposed based on the set behavior of HfO2-based bipolar RRAM elements. Analysis of different types of complementary cells indicates that very similar intrinsic switching behaviors occur in strongly different types of bipolar switching RRAM, however with a strong material dependence of the characteristic switching voltage.