Leqi Zhang

Improvement of data retention in HfO 2 /Hf 1T1R RRAM cell under low operating current

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.

Analysis of Complementary RRAM Switching

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.

Vacancy-modulated conductive oxide resistive RAM (VMCO-RRAM): An area-scalable switching current, self-compliant, highly nonlinear and wide on/off-window resistive switching cell

We report a novel self-compliant and self-rectifying resistive switching memory cell, with area-scalable switching currents, featuring a set current density of ~5nA/nm2 (<;9uA for a 40nm-size cell), high on-state half-bias nonlinearity of 102 and low reset current density of <;0.6nA/nm2 (<;1uA@40nm size). The cell can be operated at below ±4V/10ns, with a large on/off window of >102 and retention extrapolates to 10yr at 101°C. The switching stack is fully based on ALD processes, using common high-k dielectrics and has a thickness of <;10nm, meeting the 3D Vertical RRAM requirements. Moreover, we point out the nonlinearity-low-current operation interdependence and discuss the scaling potential of the areal switching RRAM for reliable sub-μA current operation in the 10nm-cell size realm.

Ultrathin Metal/Amorphous-Silicon/Metal Diode for Bipolar RRAM Selector Applications

We propose a novel metal/silicon/metal (MSM) selector using ultrathin undoped amorphous silicon (a-Si) for resistive-RAM selector application. The new selector behaves as a bidirectional diode, showing a high current drive (~2.2 MA/cm)2, high selectivity (~240 for 1/2 bias), fast switching speed , and excellent endurance ( at target drive current). The doping-free a-Si structure alleviates the dopant-induced variability concerns for ultrascaled devices and eliminates the need for a dopant-activation anneal. Circuit simulations show feasibility of 1-Mb array, with over 25% read margin and 70% write margin, when using the new MSM structure as a selector for a HfO2-based resistive switching memory element.

High-Performance Metal-Insulator-Metal Tunnel Diode Selectors

We report on a novel high-performance metal-insulator-metal tunnel diode, with ultrathin atomic layer deposited Ta2O5, for bidirectional selector applications in resistive switching memory. The diode exhibits high drive current of over 105 A/cm2, high nonlinearity, and fast turn-on and turn-off times in the below-ns range. A very good uniformity for structures down to 40 nm size and excellent ac endurance is demonstrated, well exceeding the stand-alone nonvolatile memory requirements.

Analysis of vertical cross-point resistive memory (VRRAM) for 3D RRAM design

An analysis of 3D VRRAM is presented, taking into account read/write margin, leakage and power consumption. The results give guidelines for array dimensioning (number of layers / in plane array size) and bias conditions. The read margin is more sensitive to the number of layers than the in plane array size in the matrix, while a tradeoff is found between read margin and total leakage/power. Optimized write bias conditions are determined, which improve both write margin and power consumption as compared to the results using the standard bias schemes. A comparison shows that VRRAM is more promising than stacked 3D RRAM, not only from a cost perspective, but also provides better electrical behavior, both for read and write.

One-Selector One-Resistor Cross-Point Array With Threshold Switching Selector

This paper investigates the impact of threshold switching (TS) selector characteristics on the one-selector one-resistor (1S1R) cross-point array performance. TS selector parameter requirements are extracted for 1 Mb array, considering 1S, 1R cell compatibility, read/write margin, and power consumption constraints. The SPICE simulation results show that the threshold voltage (Vth) and the ON-state resistance (Rs) are important selector parameters. Low Vth eliminates 1R disturb issue during the read operation, but this comes at the expense of losing full cell nonlinearity (NL) during the write operation. Increase of Vth and Rs improves the full cell NL and alleviate read disturb issue. However, these reduce 1S1R read window and additional voltages are required for both read and write operations. Compared with selector with nonabrupt current-voltage (I-V) characteristics, the TS selector is more favorable for the low-voltage operation. Finally, different reported TS selectors are evaluated, and the improvement directions are suggested.

High-drive current (>1MA/cm 2 ) and highly nonlinear (>10 3 ) TiN/amorphous-Silicon/TiN scalable bidirectional selector with excellent reliability and its variability impact on the 1S1R array performance

An optimized TiN/amorphous-Silicon/TiN (MSM) two-terminal bidirectional selector is proposed for high density RRAM arrays. The devices show superior performance with high drive current exceeding 1MA/cm2 and half-bias nonlinearity of 1500. Excellent reliability is fully demonstrated on 40nm-size crossbar structures, with statistical ability to withstand bipolar cycling of over 106 cycles at drive current conditions and thermal stability of device operation exceeding 3hours at 125°C. Furthermore, for the first time, we address the impact of selector variability in a 1S1R memory array, by including circuit simulations in a Monte Carlo loop and point out the importance of selector variability for the low resistive state and its implications on the read margin and power consumption.

High-Performance a-IGZO Thin Film Diode as Selector for Cross-Point Memory Application

We present amorphous indium-gallium-zinc oxide Schottky diodes with unprecedented current densities of 104 and 105 A/cm2 at forward biases of 1.5 and 5 V, respectively. The diode presents a high rectification ratio of 1010 at ±2 V, which is essential for suppressing the sneak current of not-selected cells in the memory array. In addition, we show that the diode complies with the demanding performance of memory applications. The device degradation, given by a 30% reduction of its forward current after 104 s of continuous bias stress or 109 pulses cycles, was studied via I-V and C-V measurements and can be attributed to trapping of electrons at deep acceptor levels, which increases the diode built-in potential. Finally, we show that the device is stable upon thermal stress at 300 °C for 1 h, which opens the possibility of further processing and integration with the memory cell.

Tailoring switching and endurance / retention reliability characteristics of HfO 2 / Hf RRAM with Ti, Al, Si dopants

We have demonstrated that by material engineering using different spices to dope HfO2, RRAM cell switching and endurance / retention reliability characteristics can be modulated. The changes in SET/RESET voltages, endurance optimal programming window and retention result mainly from the oxygen scavenging efficiency of Hf cap in presence of different dopants in HfO2. This impacts directly the formation of OEL that controls the RRAM switching characteristics and retention. By utilizing different dopant materials, the operation range of the HfO2 based RRAM can be tailored to be compatible with different selectors and to be adopted for broader applications.