Masakazu Muraguchi

10 nmf perpendicular-anisotropy CoFeB-MgO magnetic tunnel junction with over 400°C high thermal tolerance by boron diffusion control

We have developed a perpendicular-anisotropy magnetic tunnel junction (p-MTJ) stack with CoFeB free layer and Co/Pt multilayer based synthetic ferrimagnetic (SyF) pinned layer that withstand annealing at a temperature up to 420°C (that compatible with CMOS BEOL process) by controlling boron diffusion. We demonstrated the 10 nmφ p-MTJ with double CoFeB/MgO interface tolerable against 400°C annealing which is a requisite building block for realization of high density spin transfer torque magnetic random access memory (STT-MRAM) in reduced dimensions.

Time-Dependent Ballistic Phenomena of Electron Injected into Half-Ellipse Confined Room

We theoretically studied the time-developing ballistic phenomena of a single-electron confined in a half-ellipse infinite-potential wall by solving the time-dependent Schrodinger equation numerically. We also solved the corresponding Newton equation in order to compare the classical results with the quantum ones, and extracted the quantum features. The ellipse-shaped potential wall completely reflects an electron and causes the focusing ratio of unity in the classical limit. The dispersion of the wave packet of an electron, however, weakens this characteristic nature, and reduces the focusing ratio from unity. Because the dispersion also lets an electron arrive at the collector indistinctly, we define the effective arrival time by finding inflections in the time-dependent profile of the probability density at the collector. Based on the second-derivation technique, we further determine the quantum arrival time (QAT) at which the intrusion of the wave packet occurs dominantly. The comparison of this QAT with the classical arrival time (CAT) determines whether the corresponding ballistic propagation can be discussed on the basis of the quantum consideration or the classical prediction. We further studied how the change in the half-ellipse potential wall shape affects the ballistic phenomena through the change in the ellipticity γ, the system size L and the dispersion degree σ of the wave packet. Using the ellipse-shaped infinite-potential wall, the application of the magnetic field causes irrational cyclotron motion assisted by the ellipse potential, in addition to the rational cyclotron motions. The numerical solution of the time-dependent Schrodinger equation determines the unique cyclotron motion whose peculiarity is caused by the dispersion of the wave packet and is rarely predicted by the classical limit.

Temperature Dependence of Electron Tunneling between Two Dimensional Electron Gas and Si Quantum Dots

Quantum mechanical electron tunneling has potential applications in both science and technology, such as flash memories in modern LSI technologies and electron transport chains in biosystems. Although it is known that one-dimensional quantum electron tunneling lacks temperature dependence, the behavior of electron tunneling between different dimensional systems is still an open question. Here, we investigated the electron tunneling between a two-dimensional electron gas (2DEG) and zero-dimensional Si quantum dots and discovered an unexpected temperature dependence: At high temperature, the gate voltage necessary for electron injection from 2DEG to Si quantum dots becomes markedly small. This unusual tunneling behavior was phenomenologically explained by considering the geometrical matching of wave functions between different dimensional systems. We assumed that electron tunneling would occur within a finite experimental measurement time. Then, the observed electron tunneling is explained only by the contributions of wave packets below the quantum dot with a finite lifetime rather than the ordinary thermal excited states of 2DEG.

First-Principles Study of Time-Dependent Phenomena in Photon-Assisted Tunneling: I. An Electron Injected into Two-Dimensional Lozenge Quantum Dot

We theoretically study the dynamical properties of an electron confined in a two-dimensional (2D) quantum dot (QD) under photon illumination, by solving the time-dependent (TD) Schrodinger equation numerically by the finite difference method in both real space and actual time. To deepen our understanding of the TD features of photon-assisted tunneling (PAT), we employ projection analysis, in which the TD wave function at a QD is decomposed into (static) resonant states by calculating the inner products among them. This analysis further enables the deduction of effective lifetime, by which one can infer the actual period of the electron confined in the QD. The wave number distribution for the transmitted electron is also discussed to examine the propagation of the electron through the system.

Resonating Hartree–Fock Approach for Electrons Confined in Two Dimentional Square Quantum Dots

We studied the applicability of the resonating unrestricted Hartree–Fock (res-UHF) approach for configuration interaction (CI) by considering the electronic structure of two electrons confined in a two-dimensional (2D) square quantum dot (SQD). The UHF solutions for the SQD are nonorthogonal mutually but are appropriate for the basis functions for the res-UHF CI calculation because they represent the conceivable electron-spin configurations rationally. Consequently, the res-UHF CI using these UHF non-orthogonal solutions has an advantage for narrowing down the number of employed Slater determinants, although at the expense of the orthogonality among the Slater bases.

Improvement of Thermal Tolerance of CoFeB–MgO Perpendicular-Anisotropy Magnetic Tunnel Junctions by Controlling Boron Composition

We investigated annealing temperature Ta dependence of tunnel magnetoresistance (TMR) ratio and magnetic properties for perpendicular-anisotropy (CoFe)100-XBX/MgO magnetic tunnel junctions (MTJs) with single (CoFe)100-XBX/MgO interface (s-MTJ) and double CoFeB-MgO interface (d-MTJ) structures with various boron compositions X. High TMR ratio over 100% was observed in the s-MTJ with X= 35 at.% after annealing at 360°C-400°C, whereas the s-MTJ with X = 30 at.% showed the degradation of TMR ratio with the increase of Ta above 360°C, resulting from the decrease of perpendicular anisotropy. The d-MTJ with X = 25 at.% maintained high TMR ratio up to Ta = 400°C owing to its higher perpendicular anisotropy compared with the s-MTJ. The difference of perpendicular anisotropy between the s-MTJ and the d-MTJ can be attributed to higher interfacial anisotropy together with lower saturation magnetization of the d-MTJs. The lower saturation magnetization is attributable to two MgO layers that suppress boron diffusion from CoFeB layers, which was verified by cross-sectional line analysis using electron energy-loss spectroscopy.

1T1MTJ STT-MRAM Cell Array Design with an Adaptive Reference Voltage Generator for Improving Device Variation Tolerance

A device-variation-tolerant spin-transfer-torque magnetic random access memory (STT-MRAM) cell array design with a high-signal-margin reference generator circuit was developed to create high-density 1T1MTJ STT-MRAMs. To realize an appropriate STT-MRAM design, fluctuations in the memory cell characteristics were first measured using a 1-kbit STT-MRAM test chip. Based on these measurements, a reference generator and an STT-MRAM cell array architecture were proposed. This cell array was evaluated in terms of the signal margin for read operation and its tolerance to device variation by means of Monte-Carlo SPICE circuit simulations. The proposed design enables a 50% improvement in the signal margin compared with the conventional cell array circuit.

Low-frequency noise reduction in vertical MOSFETs having tunable threshold voltage fabricated with 60 nm CMOS technology on 300 mm wafer process

In this paper, DC and low-frequency noise (LFN) characteristics have been investigated with actual measurement data in both n- and p-type vertical MOSFETs (V-MOSFETs) for the first time. The V-MOSFETs which was fabricated on 300 mm bulk silicon wafer process have realized excellent DC performance and a significant reduction of flicker (1/f) noise. The measurement results show that the fabricated V-MOSFETs with 60 nm silicon pillar and 100 nm gate length achieve excellent steep sub-threshold swing (69 mV/decade for n-type and 66 mV/decade for p-type), good on-current (281 µA/µm for n-type 149 µA/µm for p-type), low off-leakage current (28.1 pA/µm for n-type and 79.6 pA/µm for p-type), and excellent on–off ratio (1 × 107 for n-type and 2 × 106 for p-type). In addition, it is demonstrated that our fabricated V-MOSFETs can control the threshold voltage (Vth) by changing the channel doping condition, which is the useful and low-cost technique as it has been widely used in the conventional bulk planar MOSFET. This result indicates that V-MOSFETs can control Vth more finely and flexibly by the combined the use of the doping technique with other techniques such as work function engineering of metal-gate. Moreover, it is also shown that V-MOSFETs can suppress 1/f noise ( of 10−13–10−11 µm2/Hz for n-type and 10−12–10−10 µm2/Hz for p-type) to one or two order lower level than previously reported nanowire type MOSFET, FinFET, Tri-Gate, and planar MOSFETs. The results have also proved that both DC and 1/f noise performances are independent from the bias voltage which is applied to substrate or well layer. Therefore, it is verified that V-MOSFETs can eliminate the effects from substrate or well layer, which always adversely affects the circuit performances due to this serial connection.

Demonstration of Yield Improvement for On-Via MTJ Using a 2-Mbit 1T-1MTJ STT-MRAM Test Chip

To realize a high-density spin-transfer-torque magnetic random access memory (STT-MRAM) device comparable with a current dynamic random access memory (DRAM) device, it is a key to develop a new technology for memory cell size reduction. We have already reported a chemical- mechanical-polishing(CMP)-based preparation technology for magnetic tunnel junctions (MTJs) above the via holes that can drastically reduce memory cell area. In this paper, we first introduce the MTJ preparation technology to the mega-bit class STT-MRAM test chip, and demonstrate the improvement of memory-cell operation yield.

Size dependence of electrostatic lens effect in vertical MOSFETs

The size dependence of the electrostatic lens effect in the channel of a nanoscale vertical pillar-type metal–oxide–semiconductor field-effect transistor (V-MOSFET) is studied by quantum dynamics simulation. Our findings indicate that the applicable diameter of the pillar for the efficient current-path control by the electrostatic lens effect on the V-MOSFET is in the range of about 10–30 nm. In the large-diameter pillar (30 nm diameter), the lens effect at the interfaces between the source and the body, and between the body and the source works well owing to the ballistic transport of electrons. On the other hand, in a slim pillar (10 nm diameter or less), the lens effect does not work well, because it is difficult to handle the electron dynamics by the analogy of classical geometrical optics, even though the electrons show ballistic transport. Our results indicate that the proposed technique is applicable for many nanoscale pillar-type devices.