Riichiro Takaishi

First demonstration and performance improvement of ferroelectric HfO 2 -based resistive switch with low operation current and intrinsic diode property

We demonstrate, for the first time, a CMOS compatible ferroelectric HfO2-based two-terminal non-volatile resistive switch; HfO2 ferroelectric tunnel junction (FTJ). The device has characteristics of nA-range operation current, self-compliance, and intrinsic diode properties, as well as good device to device uniformity. Simultaneous achievement of these characteristics, which was not reported in the other two-terminal emerging memories, is significant advantage for future non-volatile applications. Accurate understanding of switching mechanism based on first-principles calculations and material characterization enabled us to establish a solid guideline for performance improvement: scaling of both ferroelectric layer and interfacial layer thickness. As a consequence, reduction of operation voltage while maintaining sufficient ON/OFF ratio was successfully demonstrated.

Mechanism of gate dielectric degradation by hydrogen migration from the cathode interface

Abstract Hydrogen migration in a SiO2/Si system is examined in detail by nuclear reaction analysis. Electrical reliability measurements reveal a correlation between hydrogen migration from the cathode interface to the SiO2/Si interface and dynamic degradation of the gate dielectric. In addition, the defect levels generated in the bulk of SiO2 have an energy distribution corresponding to that of oxygen vacancies, as revealed by comparing the measured and simulated stress-induced leakage current. Finally, a model of hydrogen-induced gate dielectric degradation is proposed based on first-principles calculations.

Dynamical observation of H-induced gate dielectric degradation through improved nuclear reaction analysis system

The Nuclear Reaction Analysis (NRA) system was successfully improved in terms of the control of dynamic hydrogen migration and reducing background noise. The proposed new system achieved nondestructive measurements of the hydrogen depth profile with a detection limit of less than 3×1019 atom/cm3. Secondary Ion Mass Spectrometry (SIMS) and NRA with this system were compared in the analysis of the hydrogen depth profile in gate dielectric for the first time and superiority of NRA was demonstrated. In addition, we successfully demonstrated that dynamic hydrogen migration in gate dielectric is strongly correlated with generation of both bulk defects and interface defects of gate dielectrics.

One-to-one correspondence between nm-resolution channel crystallinity and electrical property of poly-Si TFT revealed by NBD two-dimensional imaging

We successfully established the direct correspondence between the whole channel crystallinity at nm-scale and electrical property in one and the same poly-Si thin-film transistors (TFTs). A low-damage focused ion beam (FIB) technique was newly developed for preparing site-specific nanobeam diffraction (NBD) specimen of bare channel of the TFT that was electrically evaluated in advance. We applied NBD two-dimensional imaging (2D1) to the whole channel area for acquiring massive diffraction patterns over 30,000 so that both high spatial resolution (3 nm) and large-area measurement (0.5×0.5 μm2) are achieved. Our NBD-2D1 analysis definitely revealed that drain current in individual TFTs is determined by whether the electrons can travel from the source to the drain without passing through the grain boundaries inside the channel, which cannot be clearly judged by using conventional scanning/ transmission electron microscopy (S/TEM) techniques.

Impact of trap creation at SiO 2 /Poly-Si interface on ultra-thin SiO 2 reliability

The relationship between TDDB characteristics of the devices having ultrathin SiO2 as gate dielectrics and the hydrogen-related trap creation have been re-investigated from the viewpoint of the oxidation process dependence. In order to study the influence of hydrogen on the reliability, deuterium isotope effect has been used. As a result, the Weibull distributions of time-to-breakdown (tBD) depends on the oxidation process condition even under the same oxidation temperature. Trap creation at gate oxide interface strongly correlates to the dielectric breakdown in ultra-thin gate oxides However, this oxidation process dependence could not be explained only by the amount of hydrogen release from SiO2/Si substrate interface From the experimental results of low-voltage SILC, it can be concluded that not only the released hydrogen from SiO2/Si substrate interface but also those from Poly-Si/SiO2 interface correlates to the breakdown mechanisms.

Experimental proof of direct correlation between hydrogen migrated to SiO 2 /Si interface and MOSFET characteristics using high energy 15 N 2+ ion beam

High energy 15 N 2+ ion beam was used not only as NRA (Nuclear Reaction Analysis) for the estimation of hydrogen depth profile but also as the measure to migrate hydrogen to the SiO2/Si interface. In the case of SiO2 formed by wet oxidation (WO), the leakage current and interface trap density (Nit) were dramatically degraded after ion beam irradiation due to the depassivation of hydrogen in the bulk followed by hydrogen migration to the interface. As for SiO2 formed by radical oxidation (RO), the significant hydrogen migration to the interface was not observed and the increased Nit value was almost the same as that of constant current stress case. The direct correlation between hydrogen at the interface and MOSFET degradation was successfully demonstrated.