Tai-Yuan Wu

Low power and high speed bipolar switching with a thin reactive Ti buffer layer in robust HfO2 based RRAM

A novel HfO2-based resistive memory with the TiN electrodes is proposed and fully integrated with 0.18 mum CMOS technology. By using a thin Ti layer as the reactive buffer layer into the anodic side of capacitor-like memory cell, excellent memory performances, such as low operation current (down to 25 muA), high on/off resistance ratio (above 1,000), fast switching speed (5 ns), satisfactory switching endurance (>106 cycles), and reliable data retention (10 years extrapolation at 200degC) have been demonstrated in our memory device. Moreover, the benefits of high yield, robust memory performance at high temperature (200degC), excellent scalability, and multi-level operation promise its application in the next generation nonvolatile memory.

Low-Power and Nanosecond Switching in Robust Hafnium Oxide Resistive Memory With a Thin Ti Cap

The memory performance of hafnium oxide (HfOx)-based resistive memory containing a thin reactive Ti buffer layer can be greatly improved. Due to the excellent ability of Ti to absorb oxygen atoms from the HfOx film after post-metal annealing, a large amount of oxygen vacancies are left in the HfOx layer of the TiN/Ti/HfOx/TiN stacked layer. These oxygen vacancies are crucial to make a memory device with a stable bipolar resistive switching behavior. Aside from the benefits of low operation power and large on/off ratio (>100), this memory also exhibits reliable switching endurance (>106 cycles), robust resistance states (200°C), high device yield (~100%), and fast switching speed (<10 ns).

An Ultrathin Forming-Free

Abstract-A forming-free 3-nm-thick HfOx, resistive memory device is demonstrated. The percolation threshold of insulatingto-conductive transition in the ultrathin HfOx, can be lowered by using the Ti capping layer with an adequate post metal annealing. By the reaction between Ti and HfOx,, oxygen ions are depleted from the oxide, and conductive percolative paths, which consist of charged oxygen vacancies, are formed. Without any forming step, the memory can operate with stable bipolar resistance switching by initial positive or negative voltage sweep. Possible scenarios of switching mechanism for the initial state of the device with different sweep directions are proposed. This forming-free device also exhibits an on/off ratio of about ten and excellent memory performances, including high speed (~10 ns), low operation voltages (<; 1.2 V), robust endurance (> 106 cycles), good nonvolatile property (500 min at 85 °C), and 2-b switching per cell.

\hbox{HfO}_{x}

Especially for microcontroller and mobile applications, embedded nonvolatile memory is an important technology offering to reduce power and provide local persistent storage. This article describes a new resistive RAM device with fast write operation to improve the speed of embedded nonvolatile memories.

Resistance Memory With Excellent Electrical Performance

The binary oxide based resistive memories showing superior electrical performances on the resistive switching are reviewed in this paper. The status and challenges of the HfOX based resistive device with excellent memory properties are presented. Several future challenges for the filamentary type switching device are also addressed.

Fast-Write Resistive RAM (RRAM) for Embedded Applications

A bipolar resistive switching memory device with a low power operation (200 μA X 1.3 V) in a W/Ge 0.4 Se 0.6 /Cu/Al structure is investigated. A high quality Ge 0.4 Se 0.6 solid electrolyte is confirmed by X-ray photoelectron spectroscopy. The resistive memory device with a small via size of 0.2 μm has a large threshold voltage of >0.4 V, high resistance ratio (R high /R low ) of > 10 2 , and good uniformity. The switching mechanisms are due to the Cu metallic filament formation and dissolution from the Ge 0.4 Se 0.6 solid electrolyte under positive and negative biases, respectively, which have been confirmed by high resolution transmission electron microscopy image and energy-dispersive X-ray spectroscopy analysis. The strong Cu filament formation can also be investigated by monitoring the erase voltage and erase current. Good endurance of > 10 5 cycles is obtained. Excellent data retention characteristics at 85 °C are observed after 24 h of retention time, owing to the strong Cu metallic filament formation in the Ge 0.4 Se 0.6 solid electrolyte.

Challenges and opportunities for HfO X based resistive random access memory

The dependence of resistive switching of Ta/TaOX/HfOX device governed by general filamentary or novel defects-trapping mechanism on the operation current is demonstrated in this letter. The device with stable resistive switching, high nonlinearity, and robust self-compliance ~ 1 μA is demonstrated, which can be integrated in the vertical RRAM structure. Based on constant current density switching ( ~ 103 A/cm2) governed by defects-trapping transport, where the low and high resistance states attributed to the resistance of Ta/TaOX layer and device initial state, the switching current reduction by scaling down the cell size is proposed in transition metal oxide RRAM.

Bipolar Resistive Switching Memory Using Cu Metallic Filament in Ge0.4Se0.6 Solid Electrolyte

A novel method of fabricating HfOx-based resistive memory device with excellent nonvolatile characteristics is proposed. By using a thin AlCu layer as the reactive buffer layer into the anodic side of a capacitor-like memory cell, excellent memory performances, which include reliable programming/erasing endurance (> 105 cycles), robust data retention at high temperature, and fast operation speed (< 50 ns), have been demonstrated. The resistive memory based on AlCu/HfOx stacked layer in this letter shows promising application in the next generation of nonvolatile memory.

Novel Defects-Trapping

A forming-free resistive memory of TiN/Ti/HfO2/TiN with a thin HfO2 film is demonstrated. The as-fabricated device can be operated without additional forming step to initiate the operation. This device with bipolar operation mode shows high speed (∼ 10 ns), robust endurance (≫ 106 times), good data retention (10-year lifetime), enough resistance ratio, and low power consumption. The simple structure and capability of multi-level operation demonstrate RRAM as a high-density memory in the near future.

{\rm TaO}_{\rm X}/{\rm HfO}_{\rm X}

A 3-D resistive random access memory potentially offers lowest cost per bit and highest bit density memory architecture without the use of transistors in the array. However, without the use of selectors attached to each cell in the array, sneak currents are a key concern, causing signal errors, and excess power dissipation. A nonlinear LRS helps to resolve the issue, but to date, reported LRS nonlinearity values are still insufficient. In this letter, we describe how a 1TNR architecture may be designed and operated to take more advantage of the HRS rather than the LRS nonlinearity, allowing sneak currents to be minimized during write operations, without the use of cell selectors. We show how a recently studied TaOx/HfOx device with highly nonlinear ( ~ 105) HRS can be used in block sizes up to 256 Mb without selectors in a 1T8R architecture with a 25% current margin.