Masaaki Nakabayashi

Stress stabilization of β‐tantalum and its crystal structure

We studied the relationship between Ta crystal structures and the stress stability of Ta under heating. The stress in Ta which was sputter deposited on SiC was unstable and changed more than 2×109 dyn/cm2 to the compressive side during heating at 200 °C for 30 min in air. In contrast, the stress in Ta which was sputter deposited on SiC whose surface was modified by Ar sputtering was very stable, and the stress change was less than 1.5×108 dyn/cm2 even after 6 h of heating at 200 °C. The x‐ray diffraction patterns of the Ta revealed that stable Ta was strongly (002) oriented β‐Ta, and that unstable Ta was randomly oriented β‐Ta with some α‐Ta. We found that amorphizing the SiC surface or inserting a thin amorphous interlayer enhanced growth of strongly (002) oriented β‐Ta.

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.

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.

Ti-capping technique as a breakthrough for achieving low threshold voltage, high mobility, and high reliability of pMOSFET with metal gate and high-k dielectrics technologies

We have proposed inhibition mechanism of common Al-capping technique for pMOSFET threshold-voltage (Vth) control for the first time, and have established effective Ti-capping technique using metal gate and Hf-based high-k dielectrics. Ti-capping technique can adjust lower Vth than Al-capping one due to the suppression of counter dipole and solid solubility limit in doping. Moreover, Ti-capping technique can improve carrier mobility and negative bias temperature instability (NBTI). We have confirmed that Ti-doped devices achieve higher performance, and the technique is suitable for 32 nm-technology node and beyond.

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.

MgO Based Magnetic Tunnel Junctions With Co

We applied Co20Fe60B20 layers with thicknesses from 1.2 to 1.5 nm to the sensing layers of MgO based magnetic tunnel junctions (MTJs) with spin-valve-type structures annealed at 350 °C for magnetic field sensors. The CoFeB layer on MgO had in-plane magnetic anisotropy and also slight perpendicular magnetic anisotropy that originated at the MgO/CoFeB interface. MR characteristics of high reversibility and linearity were confirmed at a CoFeB thickness of 1.3 nm. We consider that the hysteresis-free properties of the sensing layer were obtained by the balance between in-plane magnetization and other components in magnetization including out-of-plane magnetization.

_{20}

We have designed and demonstrated a top-pinned magnetic tunnel junction (MTJ) with a synthetic antiferromagnetic (SAF) free layer, which eliminated the offset field. The key SAF structure was designed on the basis of a prediction derived from micromagnetic simulations. As a result, fabricated MTJ with an SAF free layer exhibited stable switching without applying an external field.

Fe

Abstract PZT capacitor with direct contact between Si substrate and bottom electrode of the capacitor was obtained with Ir/IrO2/Ir/Ti electrode, by crystallizing sol-gel PZT thin film using RTA (650°CC 30 sec.). Contact resistance for hole diameter of 0.72 μm was 19 Ω. It was observed by cross-sectional TEM that Ti silicide was formed at the interface, but there was not oxygen diffusion from PZT thin film. Fatigue property of the PZT thin film was improved by RTA compared with furnace annealed film (600°CC 60 min.). The absolute value of the remnant polarization was 13 μC/cm2 for both films, but it did not degrade until 108 cycles of switching for the film by RTA, while it degraded before 105 cycles for furnace annealed film.