Shigeki Takahashi

Lower-current and fast switching of a perpendicular TMR for high speed and high density spin-transfer-torque MRAM

We investigate extremely low programming current and fast switching time of a perpendicular tunnel-magnetoresistance (P-TMR) for spin-transfer torque using a P-TMR cell of 50 nm-diameter. A L10-crystalline ordered alloy is used as a free layer that has excellent thermal stability and a damping constant of about 0.03. The programming current of 49 uA and the switching time of 4 nsec are also demonstrated.

Ion Beam Etching Technology for High-Density Spin Transfer Torque Magnetic Random Access Memory

A spin transfer torque magnetoresistive random access memory (STT-MRAM) is the most promising candidate for a non-volatile random access memory, because of its scalability, high-speed operation, and unlimited read/write endurance. An ion beam etching (IBE) is one of the promising etching methods for a magnetic tunnel junction (MTJ) of the STT-MRAM, because it has no after-corrosion and oxidation problems. In this work, we developed the multiple-step wafer-tilted IBE using computer calculation. Using optimized multiple-step IBE conditions, we fabricated MTJs without barrier-short defects.

Bias voltage and annealing-temperature dependences of magnetoresistance ratio in Ir–Mn exchange-biased double tunnel junctions

Abstract Dual-spin-valve-type double tunnel junctions (DTJs) of sputtered Ir–Mn/Co–Fe/AlO x /Co 90 Fe 10 /AlO x /Co–Fe/Ir–Mn were fabricated using photolithography and ion-beam milling. The DTJs were annealed at various temperatures (150–400°C) to introduce interdiffusion. The magnetoresistance (MR) ratio and DC bias voltage value at which the MR ratio decreases in half value ( V 1/2 ) were measured before and after annealing. A maximum MR ratio and V 1/2 obtained after annealing at ∼320°C was 42.4% and 952xa0mV, respectively, at room temperature. There is a correlation between the loss of the MR ratio and that of V 1/2 above 320°C. The loss of the MR ratio and that of V 1/2 are well explained by considering two phenomena, i.e., interdiffusion of O and Mn at the AlO x /Co–Fe/Ir–Mn interfaces. The mechanism for the loss of MR ratio is not only related to the loss of interface polarization, but is also related to the barrier properties, taking into account the spin-independent two-steps tunneling via defect states in the barrier. These results are consistent with the X-ray photoelectron spectroscopy and cross-sectional transmission electron spectroscopy measurements, which indicate the existence of an Al–Mn–O barrier above 320°C.

Bit yield improvement by precise control of stray fields from SAF pinned layers for high-density MRAMs

A write-operating window with a 100% functional bit yield was successfully obtained by the control of stray fields from synthetic antiferromagnetic (SAF) pinned layers in conventional magnetic random access memories with rectangular magnetic tunneling junction bits. The stray fields were controlled by a newly developed ion-beam etching technique without causing damage and by a precise setting of the SAF pinned layer thickness, and are balanced with Neel coupling fields. As a result, it was found that symmetric switching astroid curves with no offset were obtained and switching distributions were minimized at the zero offset field.

Ion-Beam-Etched Profile Control of MTJ Cells for Improving the Switching Characteristics of High-Density MRAM

The effect of the reduction of the sidewall redeposition layer of magnetic materials is investigated for submicron-patterned magnetic random access memory (MRAM) cells. The sidewall redeposition layer is made at the first etch step of a magnetic tunnel junction (MTJ) with ion beam etching (IBE) in the case that the sidewall angle of a hard mask is steep. By controlling the etched profile at the time of the first IBE step, formation of the redeposition layer is prevented. Functional test results of 1-Kb MRAM arrays show that the sidewall redeposition layer enlarges fluctuation of switching current, and reduces the write operation region. The effect of the sidewall redeposition on the switching characteristics of MRAM arrays is explained qualitatively by micromagnetic simulation solving the Landau-Lifshitz-Gilbert (LLG) equation

Reduction of switching field distributions by edge oxidization of submicron magnetoresistive tunneling junction cells for high-density magnetoresistive random access memories

An edge oxidization effect on magnetization reversals is investigated for submicron-patterned magnetoresistive tunneling junctions (MTJs). By the MTJ edge oxidization which causes the MTJ edge saturation magnetization (Ms) reduction, the switching field distributions (SFDs) for 0.24×0.48μm2 MTJs are reduced to less than 10%. The offset fields and the kinks in resistance-magnetic-field curves are reduced. Micromagnetic simulation results predict that the edge magnetization reversals are suppressed by the MTJ edge Ms reduction and the edge domain size at the remanent states becomes small. Consequently, the edge domain motion suppression by the edge oxidization is effective for decreasing the SFDs.

A Fully Integrated 1 kb Magnetoresistive Random Access Memory with a Double Magnetic Tunnel Junction

A 1 kb magnetoresistive random access memory (MRAM) is demonstrated with a 0.4×1.2 µm2 magnetic tunnel junction (MTJ) and 0.18 µm complementary metal-oxide-semiconductor (CMOS) technology. In this study, a double MTJ, which is designed to have a larger signal margin than a conventional MTJ, is used. The uniformity of resistance for a double MTJ is comparable to that of a single MTJ and is expected to be better than a single MTJ if process conditions are optimized. The magnetic design of the MTJ provided a good astroid curve and resulted in 90% of bits working towards a relatively broad range of bit-line voltage and word-line voltage.

Effect of Interface Buffer Layer on the Reliability of Ultra-Thin MgO Magnetic Tunnel Junctions for Spin Transfer Switching MRAM

Study of the reliability of ultra-thin MgO tunneling barriers for spin transfer switching magnetoresistive random access memory (MRAM) revealed MgO to be an excellent film with little resistance drift. Precise control of CoFeB/MgO/CoFeB interface was found to be important for making highly reliable tunneling barriers.

Spin-RAM for Normally-Off Computer

Spin-RAM technologies for operation speed faster than 30 ns, memory capacity larger than 1 Gbits and practically infinite read/write endurance have been developed by using magnetic tunnel junctions with perpendicular magnetization layers. Combination of Spin-RAM and power-gating technology will enable ultra low power computer called Normally-Off Computer.

Fundamental Studies on Quantum Interconnections between Ballistic Electrons and Cellular Architecture

A novel scheme of quantum interconnections (QI) is proposed with a view to creating next-generation integrated circuits (ICs). The new mode of QI, that is, QI between ballistic electrons and cellular architecture (QIBBC), effectively combines the formerly proposed QI with ballistic and coherent electrons (QIB) and QI with cellular architecture (QIC). The core concept of QIBBC is controlling electrons with electrons, which will lead to the realization of the ultimate device function and density limit in future ICs. Through fundamental analyses, it was found that the velocity control of ballistic electrons is an essential factor for QIBBC.