Yuichi Yamazaki

Low Power and High Speed Switching of Ti-doped NiO ReRAM under the Unipolar Voltage Source of less than 3 V

This paper reports on low-power and high-speed resistive switching of a Ti-doped NiO memory, which is based on the switching mechanism of the redox reactions in a filamentary conductive path. A small reset current of less than 100 muA was achieved by controlling the gate voltage of a cell transistor, which acts as an excellent current limiter in the set operation. A fast reset time of less than 5 ns was achieved by doping the Ti into the NiO film. Ti is thought to be effective to not only stabilize the reset process by forming an oxygen reservoir, but also to suppress the abnormal set phenomenon during the reset operation due to the formation of strong Ti-O bonds. Moreover, stable pulse switching with a large resistance change ratio has been successfully demonstrated using a unipolar voltage source of less than 3 V.

Reduction of Reset Current in NiO-ReRAM Brought about by Ideal Current Limiter

In this paper, we fabricated 1T1R NiO-ReRAM test circuits based on 0.18 mum CMOS technology and observed notable suppression of I_reset by imposing current compliance I_comp using a cell transistor. Reducing the stray capacitance between Pt/NiO/Pt and the cell transistor used as a current limiter is crucial in this issue. This enabled the systematic measurement of I_comp dependence of l_reset for I_comp < 1 mA and I_reset ap I_comp was observed for 150 muA les I_comp les950 muA.

Enhanced Thermal Stability in Perpendicular Top-Pinned Magnetic Tunnel Junction With Synthetic Antiferromagnetic Free Layers

Synthetic antiferromagnetic (SAF) free layers consisting of [Co (0.3 nm)/Pd (0.7 nm)]n/Ru/Ta/CoFeB were investigated for use in the free layers of top-pinned magnetic tunnel junctions (MTJs). We found that interlayer exchange coupling properly provides thermal stability when the saturation magnetization of the CoPd layer is less than that of the CoFeB layer. Furthermore, we demonstrated that the thermal stability factor of the top-pinned MTJ with [Co (0.3 nm)/Pd (0.7 nm)]n=1/ Co (0.3 nm)/Ru/Ta/CoFeB- SAF free layers was improved without increasing the intrinsic switching current.

Reliability of MgO Tunneling Barrier for MRAM Device

We investigated the reliability of MgO and AlOx tunneling barrier, for MRAM device. It is found that MgO tunneling barrier has the hydrogen tolerance compared with AlOx tunneling barrier. It also demonstrated that MgO-barrier MTJ has longer lifetime, over 10 years, than AlOx-barrier MTJ by TDDB data

Highly manufacturable multi-level perpendicular MTJ with a single top-pinned layer and multiple barrier/free layers

In this paper, we first report a novel and highly manufacturable multi-level perpendicular magnetic tunnel junction (p-MTJ) with a single top-pinned layer and multiple barrier/free layers. In the proposed p-MTJ structure, all tunnel barriers and free layers lie under a thick pinned layer, which enables good resistance control and large coercivity due to the effect of small film roughness. We also developed a new CoFeB-based free layer and writing scheme to control the stray field and spin-transfer torque. As a result, four stable and well-separated resistance states were obtained in resistance-voltage curves with a 65-nm top-pinned p-MTJ.

A novel MTJ for STT-MRAM with a dummy free layer and dual tunnel junctions

A novel magnetic tunnel junction (MTJ) for embedded memory applications such as spin transfer torque magneto-resistive random access memory (STT-MRAM) is proposed. It consists of a dummy free layer and dual tunnel junctions using perpendicular magnetic anisotropy at the CoFeB/MgO interface. A fabricated MTJ with 53 nm diameter exhibited a high thermal stability factor Δ = 52 and a small switching current Ic0 = 57 μA, resulting in an Δ/Ic0 ratio of 0.91, which is more than twice that of the reference MTJ. This MTJ simultaneously provides an excellent Δ/Ic0 ratio, low-voltage switching (0.34 V at 100 ns), and good manufacturability.

Magnetic tunnel junctions for magnetic field sensor by using CoFeB sensing layer capped with MgO film

We evaluated MgO-based magnetic tunnel junctions (MTJs) for magnetic field sensors with spin-valve-type structures in the CoFeB sensing layer capped by an MgO film in order to obtain both top and bottom interfaces of MgO/CoFeB exhibiting interfacial perpendicular magnetic anisotropy (PMA). Hysteresis of the CoFeB sensing layer in these MTJs annealed at 275 °C was suppressed at a thickness of the sensing layer below 1.2 nm by interfacial PMA. We confirmed that the CoFeB sensing layers capped with MgO suppress the thickness dependences of both the magnetoresistance ratio and the magnetic behaviors of the CoFeB sensing layer more than that of the MTJ with a Ta capping layer. MgO-based MTJs with MgO capping layers can improve the controllability of the characteristics for magnetic field sensors.

Area dependence of thermal stability factor in perpendicular STT-MRAM analyzed by bi-directional data flipping model

We report a statistical analysis of the thermal stability factor (Δ) for the top-pinned perpendicular magnetic tunnel junction (p-MTJ). By using a bi-directional data flipping model, the data retention characteristics of the “0” and “1” states can be fitted separately, including the saturation of failure probability. With the help of a resistance evaluation for the 16-kbit MTJ array, it became clear that the Δ of the “1” state increased as the device area increased, whereas the Δ of the “0” state remains constant regardless of the size. Moreover, we found that the p-MTJ exhibited a much smaller variation of Δ (9.6 ~ 14.3%) compared with the in-plane MTJ. Variations of Δ in both states decreased as the area increased. In combination with an intense magnetic measurement for the discrete monitor devices, the key parameter to increase the Δ and suppress its variation was investigated.

Electrical-field and spin-transfer torque effects in CoFeB/MgO-based perpendicular magnetic tunnel junction

The electric-field (E) dependence of the magnetoresistance (RH) loops for top-pinned perpendicular CoFeB/MgO-based magnetic tunnel junctions (MTJs) in the presence of a spin-transfer torque (STT)-current was measured. The E effects were distinguished from the STT-current effects using a micromagnetic simulation. The coercive field (Hc) decreased and the RH loop shifted as both the positive and negative bias E increased owing to the STT current. Furthermore, E-assisted switching for an MTJ with a diameter of 20 nm, which exhibited a nearly coherent magnetization reversal, was demonstrated using micromagnetic simulation.

Reliability enhancement due to in-situ post-oxidation of sputtered MgO barrier in double MgO barrier magnetic tunnel junction

We have investigated the effects of in-situ post-oxidation (PO) of a sputtered MgO barrier in a double-MgO-barrier magnetic tunnel junction (MTJ) and found that the short error rate was significantly reduced, the magnetoresistance (MR) ratio was increased approximately 18%, and the endurance lifetime was extend. In addition, we found that the distribution of breakdown number (a measure of endurance) exhibits trimodal characteristics, which indicates competition between extrinsic and intrinsic failures. This improvement in reliability might be related to the suppression of Fe and Co diffusion to the MgO barrier, as revealed by electron energy-loss spectroscopy (EELS) analysis.