Tiancheng Gong

Superior Retention of Low-Resistance State in Conductive Bridge Random Access Memory With Single Filament Formation

Data retention is one crucial reliability aspect of resistive random access memory (RRAM). The retention failure mechanism of the low-resistance state (LRS) for conductive bridge RAM is generally originated from the lateral diffusion of metal ions/atoms from the filament to its surrounding medium. In this letter, we proposed an effective method to improve the LRS retention by controlling the formation of the single filament. For a certain LRS, the effective surface area for metal ions/atoms diffusion in single filament is less than that of multi-filament. Thus, better LRS retention characteristics can be achieved by reducing the metal species diffusion. The validity of this method is verified by the superior retention characteristics of the LRS programmed by current mode, in comparison with voltage programming mode. The former tends to generate a single filament, while the later grows multi-filament. This letter provides a possible way to enhance the retention characteristics of RRAM.

Cu BEOL compatible selector with high selectivity (>107), extremely low off-current (∼pA) and high endurance (>1010)

Selector with high nonlinearity and low leakage current is critical to solve the sneaking current issue in crossbar memory array. In this work, we present a high performance Cu BEOL compatible threshold switching (TS) selector with several outstanding features, such as high nonlinearity (~107), ultra-low off-state leakage current (~pA), robust endurance (> 1010), and sufficient on-state current density (~1 MA/cm2). The observed threshold switching is resulted from the spontaneously rupture of conductive filament in doped HfO2 material. By introducing a tunneling layer in series with the TS layer, the leakage current of the selector is dramatically reduced by more than 5 orders of magnitude. The array level benchmark of this TS selector qualifies its promising potential for 3D storage application.

Demonstration of 3D vertical RRAM with ultra low-leakage, high-selectivity and self-compliance memory cells

Developing high performance self-selective cell (SSC) is one of the most critical issues of the integration of 3D vertical RRAM (V-RRAM). In this work, a four-layer V-RRAM array, with high performance HfO2/mixed ionic and electronic conductor (MIEC) bilayer SSC, was demonstrated for the first time. Several salient features were achieved, including ultra-low half-select leakage (<;0.1 pA), very high nonlinearity (>103), low operation current (nA level), self-compliance, high endurance (>107), and robust read/write disturbance immunity.

Investigation of LRS dependence on the retention of HRS in CBRAM

The insufficient retention prevents the resistive random access memory from intended application, such as code storage, FPGA, encryption, and others. The retention characteristics of high resistance state (HRS) switching from different low resistance state (LRS) were investigated in a 1-kb array with one transistor and one resistor configuration. The HRS degradation was found strongly dependent on the LRS: the lower the resistance of the LRS (RLRS) is, the worse HRS retention will be. According to the quantum point contact model, the HRS corresponds to a tiny tunnel gap or neck bridge with atomic size in the filament. The degradation of HRS is due to the filling or widening of the neck point by the diffusion of copper species from the residual filament. As the residual filament is stronger in case of the lower RLRS, the active area around the neck point for copper species diffusion is larger, resulting in higher diffusion probability and faster degradation of HRS during the temperature-accelerated retention measurement.

Fully CMOS compatible 3D vertical RRAM with self-aligned self-selective cell enabling sub-5nm scaling

In low cost vertical resistive switching memory (VRRAM), the inter-layer leakage becomes a serious problem, primarily resulting from the ultimate scaling in the vertical dimension. In this work, for the first time, we present a novel approach of fabricating 3D VRRAM using self-aligned self-selective RRAM to effectively address such challenge. By successfully suppressing the inter-layer leakage, the scaling limit of VRRAM could be extended beyond 5 nm. Other benefits, such as high nonlinearity (>103), low power consumption (sub-μA), robust endurance and excellent disturbance immunity, were also demonstrated.

Fully BEOL compatible TaOx-based selector with high uniformity and robust performance

We report a novel TaOx-based selector with trapezoidal band structure, formed by rapid thermal annealing in O2 plasma. Salient features were successfully achieved, such as high current density (1MA/cm2), high selectivity (5×104), low off-state current (∼10 pA), robust endurance (>1010), self-compliance and excellent uniformity. The device is composed of fully CMOS-compatible materials and has no thermal budget compatibility concerns. Furthermore, the selector was fabricated in 1kb crossbar array and the integrated 1S1R device shows high nonlinearity in low resistance state (LRS), which is quite effective to solve the sneaking current issue. The demonstrated high performance selector device here shows high potential on manufacturing large scale crossbar array.

Highly uniform and nonlinear selection device based on trapezoidal band structure for high density nanocrossbar memory array

Crossbar array provides a cost-effective approach for achieving high-density integration of two-terminal functional devices. However, the “sneaking current problem”, which can lead to read failure, is a severe challenge in crossbar arrays. To inhibit the sneaking current from unselected cells, the integration of individual selection devices is necessary. In this work, we report a novel TaOx-based selector exhibiting a trapezoidal band structure formed by tuning the concentration of defects in the oxide. Salient features such as a high current density (1 MA·cm–2), high selectivity (5 × 104), low off-state current (~10 pA), robust endurance (>1010), self-compliance, and excellent uniformity were successfully achieved. The integrated one-selector one-resistor (1S1R) device exhibits high nonlinearity in the low resistance state (LRS), which is quite effective in solving the sneaking current issue.

Endurance characterization of the Cu-dope HfO 2 based selection device with one transistor-one selector structure

We investigated the endurance characteristics of a Cu-doped HfO2 selector device in one transistor-one selector (1T1S) structure, which is fully compatible with standard BEOL process. The device exhibits high endurance of 1010 under 10 μΑ compliance current. However, reduced endurance (105) was observed as increasing the compliance up to 100 μΑ. Under the condition of high operation, intrinsic defect in the HfO2 layer was possibly generated and resulted in endurance degradation.

Self-compliance Cu-doped HfO 2 -based selector with low leakage and high reliability

An ideal selector device should have ultra-low leakage current, high reliability and fast recovery speed. In this work, we present a Cu BEOL compatible threshold switching (TS) selector being capable of maintaining ultra-low leakage for hours and recovering low leakage after write operation rapidly. High reliability and uniformity are achieved in both read operation and write operation.