Wei-Chih Chien

A forming-free WO x resistive memory using a novel self-aligned field enhancement feature with excellent reliability and scalability

A thorough study of the switching mechanism for WO<inf>x</inf> ReRAM gives clues about how to improve its performance and reliability. Consequently, a 60nm WO<inf>x</inf> ReRAM is achieved with excellent characteristics - 50ns fast switching, 10⁶ cycling endurance, large MLC window, low read disturb of > 10⁹, and excellent 150°C/2,000Hrs data retention. Furthermore, the oxidation of the TiN barrier into an insulating TiNO<inf>X</inf> causes the WO<inf>x</inf> to protrude above the remaining TiN and thus creates field enhancement. The boosted electric field eliminates the need for an initial forming step.

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.

Tungsten Oxide Resistive Memory Using Rapid Thermal Oxidation of Tungsten Plugs

A complementary metal oxide semiconductor (CMOS)-compatible WOx based resistive memory has been developed. The WOx memory layer is made from rapid thermal oxidation of W plugs. The device performs excellent electrical properties. The switching speed is extremely fast (?2 ns) and the programming voltage (<1.4 V) is low. For single-level cell (SLC) operation, the device shows a large resistance window, and 108-cycle endurance. For multi-level cell (MLC) operation, it demonstrates 2-bit/cell storage with the endurance up to 10000 times. The rapid thermal oxidation (RTO) WOx resistance random access memory (RRAM) is very promising for both high-density and embedded memory applications.

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.

Multi-Level Switching Characteristics for WOX Resistive RAM (RRAM)

Emerging Central Lab, Macronix International Co., Ltd. Science Park, Hsinchu 300, Taiwan, R.O.C. Phone: +886-3-5786688 E-mail: wcchien@mxic.com.tw Department of Materials Science and Engineering, National Chiao Tung University, Hsinchu 300, Taiwan, R.O.C. Institute of Electronics Engineering, National Tsing Hua University, Hsinchu, 300, Taiwan, R.O.C. Department of Materials Engineering, Tatung University, Taipei, 104, Taiwan, R.O.C.

Current Status and Future Challenges of Resistive Switching Memories

Resistive switching memories (ReRAM), including transition metal oxide memory (TMO-RAM) and conducting bridge memory (CB-RAM), are some of the most promising new technologies that may scale beyond the charge-storage flash memories. Understanding the fundamental operation mechanism is one important challenge to control the key parameters and to choose the ReRAM material. The knowledge on ReRAM reliability is also insufficient to meet the future challenges. This paper will review the recent major approach of WOX ReRAM and Cu-based CB-RAM. Introduction Conventional charge-storage non-volatile memories are approaching their scaling limit [1]. A number of new non-volatile memories have been proposed [2-5] and among them the resistive switching memory is considered one of the most promising candidates [4-5]. To enable this new technology, researchers and engineers are working on four major topics: write power, scaling possibility, ReRAM materials, and cross point array architecture. The write power of ReRAM is lower than several other emerging memories such as PCM and MRAM [6-8]. However, the high forming voltage/current is still a concern, and even the regular write current affects the bandwidth. Although devices smaller than 10nm have been reported [7-8] recently, but array uniformity and reliability are still not proven. And finally, a proliferation of ReRAM materials has yet to converge to a few exhibiting all desired properties. ReRAM Materials Fig. 1 shows the ReRAM feature size evolution compared with charge-storage memory [7-19]. Already ReRAM has passed the scaling limit of charge-storage memories. Fig. 2 shows the relationship between switching current and speed [7-19]. The observation that only a few points are in the low-current/high-speed region indicates that there may exist a tradeoff between operating current and switching speed. The fact that most of the transition metal oxides can be switched not only in bipolar but also in unipolar modes actually triggers a critical issue on how to choose the best material and the best operation mode. Thus fundamental material studies including the switching mechanism, the conducting mechanism, and the material characteristics are the most important topics in ReRAM research. There seems consensus now that the bipolar switching behavior of TMO-RAM and CB-RAM is dominated by the movement of anions (Valency change effect) and cations (Electrochemical effect) (Fig. 3a and 3b) [9,18]. For unipolar opeation, it is believed that the switching mechanism is based on thermochemical effect (Fig. 3c) [20]. However, the detailed physical models still need further improvements and the knowledge for fine tuning the physical parameters to improve the device performances including speed, current, resistance window, data retention, and cycling endurance is still under development. Field Enhancement Structure for WOX ReRAM Since the forming process is an important issue and needs to be solved, a self-aligned field-enhancement device structure is proposed and its 2D simulation for 20 and 100 nm cells are shown in Fig. 4. By oxidizing the TiN liner into an insulating TiNOX the WOX is forced to protrude above the remaining TiN. Fig. 5a shows significantly higher electric field at the center of the WOX when the size of the W plug scales. Fig. 5b shows that the voltage required for the initial forming process falls rapidly when the cell size scales. Therefore, at 60nm or below, the initial forming process is practically eliminated. The 60nm WOX device not only shows forming-free property, but also good electrical performance. Fig. 6a shows the cycling endurance of the 60nm devices is > 10 times, and a 10X resistance window is well maintained by program-verify algorithms. Excellent thermal stability is Fig.1. Evolution for ReRAM and charge-storage memory. ReRAM shows promising scalability beyond 1X nm technology node. Fig.2. Switching current versus switching time for both TMO-RAM and

A low-cost, forming-free WO x ReRAM using novel self-aligned photo-induced oxidation

A novel CMOS compatible photo oxidation (PO) technology is proposed in this paper which, by only using standard DUV photo lithography process, demonstrates a strong oxidation capability to form CMOS compatible WOx. The oxidation occurs through catalytic chemical reaction during the post exposure baking (PEB) process. Based on this unique PO process, a high performance forming free 1T-1R WOx ReRAM is demonstrated. Furthermore, this PO WOx ReRAM can withstand high temperature baking (@ 250°C) for 30 min thus is suitable for embedded systems that require pre-coding, and automotive and other industrial applications.

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.

Unipolar Switching Characteristics for Self-Aligned WO x Resistance RAM (R-RAM)

For the first time a unipolar resistance memory with good performance and reliability is demonstrated. A short (20-50 ns) positive pulse switches the WOx film from low resistance state (LRS) to high resistance state (HRS), while a longer (200-500 ns) positive pulse switches the film from HRS to LRS. Negative pulses, on the other hand, do not produce reversible resistivity changes. Despite the low energy switching, both LRS and HRS are very stable, capable of withstanding 2,500 hours of baking at up to 150 degC- Furthermore, the WOx R-RAM can withstand > 1,000 cycles of LRS/HRS switching, and the device is also highly immune to read disturb. This unipolar device is promising for future 3D high-density NVM storage.