Feng-Ming Lee

Multi-level 40nm WO X resistive memory with excellent reliability

40nm WOX ReRAM has several unique characteristics that are very favorable for MLC application. (1) Although the resistance has strong temperature dependence (as for all ReRAMs) the J-V characteristics can be accurately described, thus all MLC levels are easily modeled. (2) The device is immune to over-erase, thus allow fast MLC programming. (3) The programming is self-converging (as Flash memories) and is independent of history. Thus an algorithm similar to ISPP (Incremental Step Pulse Programming), commonly used by MLC NAND flash, is designed to achieve accurate MLC states. Consequently, fast 50ns switching, 2-bit/cell and 3-bit/cell MLC states with good cycling characteristics and low read disturbance (> 1010) is achieved.

A novel tite buffered Cu-GeSbTe/SiO 2 electrochemical resistive memory (ReRAM)

A novel solid-electrolyte based electrochemical induced conductive bridge (CB) resistive memory (ReRAM) is fabricated and characterized. The new device consists of a Cu-doped GeSbTe ion source, a SiO2 memory layer, and a TiTe ion buffer layer. The ion-buffer layer separates the Cu conducting path from the Cu-ion supply layer thus greatly increases the stability. This tri-layer device greatly improves reliability, yet maintains both low thermal budget BEOL processing and excellent electrical properties.

A Novel Ni/WOX/W Resistive Random Access Memory with Excellent Retention and Low Switching Current

The behavior of WOX resistive random access memory (ReRAM) is a strong function of the top electrode material, which controls the conduction mechanism and the forming process. When using a top electrode with low work function, the current conduction is limited by space charges. On the other hand, the mechanism becomes thermionic emission for devices with a high work function top electrode. These (thermionic) devices are also found to have higher initial resistance, reduced forming current, and larger resistance window. Based on these insights and considering the compatibility to complementary metal–oxide–semiconductor (CMOS) process, we proposed to use Ni as the top electrode for high performance WOX ReRAM devices. The new Ni/WOX/W device can be switched at a low current density less than 8×105 A/cm2, with RESET/SET resistance ratio greater than 100, and extremely good data retention of more than 300 years at 85 °C.

A model for the RESET operation of electrochemical conducting bridge resistive memory (CB-ReRAM)

The RESET operation for electrochemical conducting bridge ReRAM involves complex, dynamic evolution of multiple mechanisms. In this work, the entire RESET process is modeled. Three different types of current are active during the RESET process: (i) ionic current, (ii) tunneling current, and (iii) Ohmic current. While ionic current is the dominant component that electrochemically removes the conducting bridge, we have also confirmed, by simulation and experiments, that tunneling current and Ohmic current consume most of the RESET power. To improve the efficiency and reduce RESET current, we propose a new device structure with a high work function tunneling layer to suppress the tunneling current.

A Novel Varying-Bias Read Scheme for MLC and Wide Temperature Range TMO ReRAM

Resistance of transition metal oxide (TMO) resistive random access memory (ReRAM) depends sharply on temperature, resulting in drastic memory window loss at high temperature. Thus, it is difficult to design the ReRAM that can serve a wide range of operating conditions. It is especially challenging to achieve multi-level-cell (MLC) ReRAM because of the large temperature dependency. This letter investigates both the temperature and read bias dependencies of WOx ReRAM, and found both can be well understood by a modified space-charge limited conduction model. Using this model, we have designed a novel read scheme that varies the read bias according to the device temperature and compensates for the temperature effect on cell resistance. Since TMO ReRAM devices depend on defect states, cell-to-cell and cycle-to-cycle variations are naturally large. An algorithm is designed to address the variability. A 1-Mb WOx ReRAM array is fabricated to both characterize the bias and temperature dependencies and verify the new read scheme. A large and constant memory window is preserved for MLC across a wide temperature range (-40 °C-125 °C), suitable for high-reliability applications.

A novel cross point one-resistor (0T1R) conductive bridge random access memory (CBRAM) with ultra low set/reset operation current

Using the dual Vth characteristics of a multi-layer SiO2/SiO2/Cu-GST conducting bridge (CB) structure we can construct a one-resistor cell without an access device (0T1R). Like 1T Flash memory the Vth is used to store the logic state thus leaving all devices always at high resistance state and a separate isolation device is not needed. The Vth of the cell is determined by the presence of CB in the SiO2 layer only. The CB in the SiO2 is present only temporarily during reading, and is spontaneously dissolved afterward. This spontaneous rupture of the filament in the SiO2 layer greatly reduces the switching current as well as reducing the read disturb. The mechanism for the spontaneous rupture phenomenon is investigated.

Excellent resistance variability control of WOx ReRAM by a smart writing algorithm

TMO ReRAMs, being built on defect states, are intrinsically subject to variability. In this work, cell to cell variability is studied by applying write shots with different current and voltage for Forming, SET and RESET operation, respectively. We found the keys to eliminate tail bits consist of (1) longer write pulse, (2) higher write current and (3) higher write voltage. In order to optimize the performance of write speed, write power and device reliability, we developed a novel resistance control method using a smart writing algorithm. Compared to the conventional ISPP writing scheme, this smart writing algorithm covers much wider switching condition variability and cell-to-cell variation by controlling both current and voltage for ReRAM operation.

A simple new write scheme for low latency operation of phase change memory

The behavior of resistance drift after RESET operation for phase change memory (PCRAM) is investigated. We propose, for the first time, an effective way to accelerate the drift so that the program/read latency may better match that for DRAM for SCM (storage class memory) application. By simply applying an extra annealing pulse after RESET we can quickly anneal out many defects (that are responsible for the drift) and provide a drift-free period that enlarges the read window. A physical model is proposed to understand the defect annealing phenomenon, which predicts the resistance drift behavior well.

A novel retention-enhanced structure and a reset transient model for energy-efficient electrochemical conducting bridge resistive memory

A novel electrochemical conducting bridge structure with an ion buffer layer and a model of the reset transient behavior are proposed. The addition of the ion-buffer layer to the device retards the Cu-atom diffusing toward the Cu-ion supply layer, thus greatly increases the stability and produces excellent electrical properties. An analytical model is proposed to help understand the entire reset process. Three different types of current are investigated during the reset process: (i) ionic current (ii) tunneling current, and (iii) Ohmic current. Results from simulation and experiments show that the tunneling and the Ohmic currents consume most of the RESET power. To improve the operation efficiency and reduce the RESET current, a new device structure with a high work function tunneling layer is proposed to suppress the tunneling current.

Cu-based and WOx-based resistive switching memories (ReRAMs) for embedded and stand-alone applications

Two types of ReRAM are investigated. The first is a CB-ReRAM (conducting bridge ReRAM) that uses metallic ions to form the bridge. This type of device is known to be unstable because the conducting bridge tends to self-destruct. By adding a buffer layer between the electrode and the electrolyte, the retention of the Cu-based CB-ReRAM is greatly improved while the superior electrical performance is preserved. The second is a TMO (transition metal oxide) type ReRAM. The WOX-based TMO ReRAM is very simple and easily integrated, and shows excellent electrical properties for NVM applications. The mechanism of switching and conducting are also investigated in detail.