Katsumi Suemitsu

A 90nm 12ns 32Mb 2T1MTJ MRAM

Since MRAM cells have unlimited write endurance, they can be used as substitutes for DRAMs or SRAMs. MRAMs in electronic appliances enhance their convenience and energy efficiency because data in MRAMs are nonvolatile and retained even in the power-off state. Therefore, 2 to 16Mb standalone MRAMs have been developed [1–4]. However, in terms of their random-access times, they are not enough fast (25ns) [1] as substitutes for all kinds of stand-alone DRAMs or SRAMs. To attain a standalone MRAM with both a fast random-access time and a large capacity, we adopt a cell structure with 2 transistors and 1 magnetic tunneling junction (2T1MTJ), which we previously published for a 1Mb embedded MRAM macro [5]. We need to develop circuit schemes to achieve a larger memory capacity and a higher cell-occupation ratio with small access-time degradation. We describe the circuit schemes of a 32Mb MRAM, which enable 63% cell occupation ratio and 12ns access time.

Improvement of Thermal Stability of Magnetoresistive Random Access Memory Device with SiN Protective Film Deposited by High-Density Plasma Chemical Vapor Deposition

Embedded magnetoresistive random access memory (MRAM) with multi-level interconnects necessitates that magnetic tunnel junction (MTJ) devices have a thermal stability of 350 °C or higher during fabrication. We have improved thermal stability of MRAM devices using SiN protective film deposited by high-density plasma chemical vapor deposition (HDP-CVD) at 200 °C. The MTJ devices with HDP-CVD SiN protective film did not degrade after post-annealing at 350 °C, which suggests that the HDP-CVD process reduced oxide metal on the etched surface of the MTJ devices and that the SiN film blocked H2O diffusion from the interlayer dielectric film during post-annealing at 350 °C. We also fabricated a 1-kbit MRAM array and experimentally demonstrated thermal stability at 350 °C.

Etching Magnetic Tunnel Junction with Metal Etchers

Etch performances of inductory-coupled plasma (ICP) metal etchers with several gas systems are examined under constant ion energy condition to evaluate extendibility to the 300 mm wafer magnetic tunnel junction (MTJ) etch process. The ICP-Ar sputter etch affects little on magnetic properties, and shows about the same magnetoresistive (MR) ratio with conventional Ar ion milling. Major issue is the electrical short by redeposition. The etch uniformity over the wafer and precise etch end-point detection are important. The Cl2 addition to the ICP-Ar etch plasma shows serious pattern deformation and degradation of loop offset (Hoff). Methanol (Me-OH) etch shows slightly lower MR-ratio due to material degradation. However, better Hoff is observed probably due to the ion protection effect by thin carbon layer over the etched surface. Dilution of Me-OH with Ar improves MR ratio. Ar/Me-OH and ICP-Ar etch processes would be the candidate for 300 mm process at present.

Thickness dependence of current-induced domain wall motion in a Co/Ni multi-layer with out-of-plane anisotropy

Thickness dependence of current-induced domain wall (DW) motion in a perpendicularly magnetized [Co/Ni]N multilayered wire containing Ta/Pt capping and Pt/Ta seed layers has been studied. The thickness of the magnetic layer was controlled by the stacking number, N. The threshold current density for driving DW had a local minimum at N = 3 and the velocity of DW motion decreased with N. Estimation of carrier spin polarization from measurements of DW velocity revealed that a thinner Co/Ni stack adjacent to the Pt layers reduced the carrier spin polarization and the strength of adiabatic spin transfer torque.

Effect of Device Temperature on Domain Wall Motion in a Perpendicularly Magnetized Co/Ni Wire

This paper describes experimental results obtained from measuring the dependence of device temperature on the current for domain wall motion in a Co/Ni wire having perpendicular magnetic anisotropy. Devices with different insulating layer thicknesses were prepared in order to control the device temperature. A stable domain wall motion was observed up to the temperature at which perpendicular magnetic anisotropy vanishes. Moreover, the current required for domain wall motion was independent of the device temperature.

Switching-Field Stabilization against Effects of High-Temperature Annealing in Magnetic Tunnel Junctions using Thermally Reliable NixFe100-x/Al-Oxide/Ta Free Layer

We examined the effects of the capping layers of Ta, Al-oxide (AlO), and Ru on the magnetic properties of ultrathin (1–20 nm) NiFe free layers and investigated thermal degeneration of the switching field (Hsw) in magnetic tunnel junctions (MTJs) using these caps in the cell structure of magnetoresistive random access memory (MRAM). The magnetization reversal of the free layer was found to be sensitive to the capping layer that affected various magnetic anisotropies. For postannealing temperatures of up to 350 °C, the Hsw of a NiFe(2 nm)/AlO/Ta free layer was thermally stable, whereas that of conventional NiFe(3 nm)/Ta seriously deteriorated. This is because the AlO capping layer completely prevented the thermal interdiffusion between the NiFe and Ta layers even after annealing at 400 °C. Moreover, a free layer with a small magnetostriction reduced the influence of the stress-relaxation on the free layer caused by postannealing, which also resulted in a thermally stable Hsw in MTJs.

Operating principle of a three-terminal domain wall device with perpendicularly magnetized Ta/CoFeB/MgO free layer and underlying hard magnets

The behavior of a three-terminal domain wall (DW) device with a perpendicularly magnetized CoFeB free layer and underlying hard magnets was investigated. In a Ta/CoFeB/MgO free layer without hard magnets, a current-induced DW motion in the direction of electron flow was observed. In a device having a hard magnet under each end of the free layer, we found that a DW nucleated by injecting current played an important role in the switching of magnetization. We concluded that the switching of magnetization in our device is due to the displacement in the direction of electron flow of the DW created by current. After deriving the principle of operation through experiments, we describe a way to reduce the current required for writing by increasing the thickness of the hard magnets.

Effect of Inserting a Pt Layer into a Perpendicularly Magnetized Co/Ni Multilayered Film in Terms of Current-Induced Domain Wall Motion

The effect of inserting a Pt layer into a Co/Ni multilayered film with a perpendicular magnetization in terms of current-induced magnetic domain wall (DW) motion has been investigated. We found that the current density required to obtain DW motion was approximately 1.9 times greater in Co/Ni/Pt wire than in Co/Ni wire, in spite of the Co/Ni and Co/Ni/Pt multilayered films having almost identical magnetic properties. Estimation of spin polarization from measurements of DW velocity indicated that the decreased spin polarization caused by insertion of the Pt layers accounts for the increased current density required for DW motion in Co/Ni/Pt wire.

Improvement of Thermal Stability of MRAM Device with SiN Protective Film Deposited by HDP CVD

SiN Protective Film Deposited by HDP CVD Katsumi Suemitsu, Yuichi Kawano, Hiroaki Utsumi, Hiroaki Honjo, Ryusuke Nebashi, Shinsaku Saito, Norikazu Ohshima, Tadahiko Sugibayashi, Hiromitsu Hada, Tatsuhiko Nohisa, Tadashi Shimazu, Masahiko Inoue and Naoki Kasai NEC Corporation, Device Platforms Research Laboratories, 1120 Shimokuzawa, Sagamihara, Kanagawa, 229-1198, Japan Phone: +81-42-771-0631 E-mail: k-suemitsu@bp.jp.nec.com Mitsubishi Heavy Industries, LTD., Semiconductor Manufacturing Equipment Office, 1-1-1 Wadasaki-cho, Hyogo-ku, Kobe, Hyogo 652-8585, Japan