Masaki Kurasawa

A study of dielectric breakdown mechanism in CoFeB/MgO/CoFeB magnetic tunnel junction

We examined the breakdown characteristics of a 1-nm-thick MgO barrier by measuring the time dependent dielectric breakdown (TDDB) and conducting atomic force microscopy (C-AFM) observation. We found that two different local conduction modes, the percolation path and Fowler-Nordheim (F-N) tunneling, contribute to dielectric breakdown. Furthermore, the operating voltage of magnetic tunnel junctions (MTJs) for maintaining reliability over ten years against dielectric breakdown was discussed.

Unipolar resistive switching in CoFeB∕MgO∕CoFeB magnetic tunnel junction

We found that a magnetic tunnel junction (MTJ) shows two types of memory operation. One is spin torque switching, and the other is electric stress induced resistive switching (RS), which is observed in various metal oxides. A MTJ in RS operation exhibits a resistance ratio of greater than 100, an endurance of more than 400cycles, and data retention in excess of 89h at 85°C, which is suitable for programable switch elements. The MTJ can operate as a programable switch, as well as a storage element, in a field programable gate array.

Influence of SRO Top Electrode on Fatigue Characteristics of FRAM

Fatigue characteristics were investigated in PZT capacitors using both Pt/SRO top electrodes and IrO2 top electrodes. Fatigue of PZT capacitors with IrO2 top electrode showed degradation with a 600 oC annealing processes. We assumed that this degradation was caused by the difference in location within the capacitors where defects were located. It is proposed that the capacitors with Pt/SRO top electrode, the defects existed at the interface between Pt and SRO from SIMS analysis. In the capacitor with IrO2 top electrode, on the other hand, the defects existed at the interface between IrO2 and PZT. Fatigue characteristics of PZT capacitors with SRO top electrode showed superiority to those with IrO2 top electrode, because it wasn’t attributed to defects located at the within the top electrode. INTRODUCTION Ferroelectric Random Access Memory (FRAM) are promising nonvolatile RAM (NVRAM) devices for personal electronic equipments, in particular, for mobile applications. Consequently, these devices are required for high density and low power consumption. Thinner ferroelectric films or 3D structures are needed for these purposes, because they are suitable to operate at lower voltage with higher stored charge. In this study, we chose Metal Organic Chemical vapor deposition (MOCVD) method to deposit Pb(Zr,Ti)O3 (PZT) thin film, because this method has numerous advantages like low film porosity, thinner film and good step coverage. Among the requirements for FRAM capacitors, important properties concerned for reliability are high remanent polarization, low leakage current density, high endurance to thermal imprint and fatigue free behavior. In particular, fatigue is explained as the characteristic of ferroelectrics whose remanent polarization decreases with increasing cycling of bipolar applied voltage pulses. It is proposed that this phenomenon is caused by the decrease of switchable domains owing to the pinning phenomenon. This pinning is assumed to occur in the vicinity of grain boundaries, and at the interface between ferroelectrics and electrodes. [1] In the work of Colla et al. [2], fatigue was clarified to be dominated by inhibition of domain nucleation at the electrode surface. In general, the processes of FRAM includes thermal annealing processes to recover its electric characteristics after top electrode sputtering. In ferroelectric film including Pb, for example, PbTiO3, Pb(Zr,Ti)O3 and (Pb,La)(ZrTi)O3, Pb diffuses into electrode and hereby the defects are produced at the interface between PZT and electrodes. The defects at the interface induces the fatigue. In this study, we chose Pt/SRO and IrO2 top electrodes because SRO and IrO2 are good oxide top electrodes in fatigue characteristics. Pt on SRO was used for contact with probe. fatigue characteristics for capacitors with various top electrodes were investigated after annealing at various temperatures in order to better understand the fatigue mechanism. Key Engineering Materials Online: 2004-08-15 ISSN: 1662-9795, Vol. 269, pp 69-74 doi:10.4028/www.scientific.net/KEM.269.69

Preparation and Properties of Epitaxial Ferroelectric Capacitor on Silicon Substrate for FeRAM Application

An all-epitaxial stacked ferroelectric capacitor, from the conductive plug to the top electrode, fabricated on a silicon substrate with a metal-oxide-semiconductor field-effect transistor is described. An SrRuO 3 /Pb(Zr,Ti)O 3 /Ir/TiN/Si(100) heteroepitaxial structure with epitaxially grown thin films on a silicon substrate had good crystallinity. Each thin film was oriented in the (100) direction with a cube-on-cube alignment on the silicon crystal lattice. The switching charges of fabricated epitaxial capacitors were around 36 μ C/cm 2 . The charge was less dependent on size for capacitors larger than 5 × 5 μ m.