Takahiko Oasa

Scanning Force/Tunneling Microscopy as a Novel Technique for the Study of Nanometer-Scale Dielectric Breakdown of Silicon Oxide Layer

Scanning force/tunneling microscopy (AFM/STM) was proposed as a novel technique to investigate local dielectric breakdown voltage for the silicon oxide layer. It was manifested that this novel technique could simultaneously measure surface topography and distribution of dielectric breakdown voltage with nanometer-scale resolution. We confirmed that the dielectric breakdown voltage measured with the AFM/STM increased monotonously with the increase in oxide thickness. In addition to the above results, we observed that the oxide layer with visible defect had a lower dielectric breakdown voltage.

Time Dependent Dielectric Breakdown of Thin Silicon Oxide Using Dense Contact Electrification.

We achieved time dependent dielectric breakdown (TDDB) measurement of a thin silicon oxide microscopically using contact electrification. By increasing the external bias voltage, TDDBs of the oxide layer without and with oxide surface roughening were observed sequentially. Charge-to-breakdown in the contact electrification was estimated to be on the order of 10 -5 ∼10 -6 C/cm 2 . This value is higher than that of electrified charge density in the absence of external bias voltage, but is much smaller than the value of∼5×10 -1 C/cm 2 obtained in the conventional TDDB measurement using a metal-oxide-semiconductor (MOS) capacitor. From calculation of the number of injected charges per atom, TDDB measurement using contact electrification is expected to provide a more quantitative evaluation of charge-to-breakdown than that using a MOS capacitor

Heat Treatment and Steaming Effects of Silicon Oxide upon Electron Dissipation on Silicon Oxide Surface.

We investigated heat treatment and steaming effects of silicon oxide upon the surface dissipation of contact-electrified electrons. As a result, we found that the surface diffusion of densely contact-electrified electrons on the silicon oxide surface becomes slower due to the removal of the adsorbed water layer on a silicon oxide layer by means of heat treatment, while it is enhanced by the steamed water layer. From the heat treatment and steaming effects upon the dissipation process, we concluded that the stable state of densely contact-electrified electrons becomes more stable upon removal of the water layer.

Reproducible and Controllable Contact Electrification on a Thin Insulator

The mechanism of contact electrification on an insulator is one of the oldest problems in physics. The major problem is the lack of reliable data on the contact electrification and its dissipation, because of poor experimental reproducibility on even the sign of the contact electrified charge. Here we report a novel microscopic method with a bias voltage to obtain reproducible and controllable contact electrification on a thin insulator, and the first experimental result on the dissipation of the contact electrified charge.

Phase Transition of Contact-Electrified Negative Charges on a Thin Silicon Oxide in Air

We investigated the dense contact-electrified negative charges on a thin silicon oxide surface by the reproducible and controllable contact electrification technique using an atomic force microscope (AFM). Time evolution of the contact-electrified negative charges, which was observed as electrostatic force, showed three dissipation processes. First, the contact-electrified negative charges dissipate slowly, then rapidly and finally, slowly again. It was found by comparison between attractive and repulsive force measurements that the first dissipation process was stable for the applied electric field, whereas the second one was unstable. Analysis of contact voltage dependence and time evolution of the spatial integral of the contact-electrified negative charges revealed the charge sites of silicon oxide for the negative charge. Furthermore, it was found that the time evolution from the first stable dissipation process to the second unstable one was a phase transition from a solid phase to a liquid or gas phase of the contact-electrified negative charges, which was investigated in terms of the nondimensional parameter Γ. By comparison between the spatial distributions of the electrostatic forces measured repulsively and attractively, it was found that the contact-electrified negative charges were very dense and stable in the central region (i.e., solid phase), whereas they were sparse and unstable in the surrounding region (i.e., liquid or gas phase).

Proximity effects of negative charge groups contact‐electrified on thin silicon oxide in air

We investigated proximity effects of negative charge groups contact‐electrified on a thin silicon oxide in air with an initial separation (L) less than a few micrometers using a modified atomic force microscope. As a result, we found the following phenomena. Even for L∼2.0 μm, distributions of two negative charge groups approach each other with time after contact electrification, though this feature is contrary to the expected recession due to the Coulomb repulsive force. For less than L∼1.6 μm, each stable state joins in one negative charge group. These proximity effects seem to be induced by the interplay of the Coulomb repulsive force and the surface diffusion of charges.

Stability of densely contact-electrified charges on thin silicon oxide in air

By changing the polarity of charged trap sites, we investigated the stability of densely contact-electrified charges on thin silicon oxide in air using a modified atomic force microscope. For usual silicon oxides with positively charged trap sites, a stable state is obtained only for negative charge deposition, while for modified silicon oxides with negatively charged trap sites, a stable state is obtained only for positive charge deposition. As a result, we concluded that charged trap sites make densely contact-electrified charges with the same polarity unstable due to the strong Coulomb repulsive force.

Contact Electrification on Thin Silicon Oxide in Vacuum

We investigated the microscopic dissipation of contact electrified charges on a thin SiO2 film in vacuum where a thin layer of water may be adsorbed on the surface using an atomic force microscope (AFM). Charges with narrower spatial distributions were deposited in smaller amounts in vacuum than in air. Moreover, the deposited charge areas in vacuum showed no broadening with time after contact electrification. These demonstrate that the rapid surface diffusion of the charges in air may be caused by a water layer adsorbed on the insulator surfaces.

Parameter Dependence of Stable State of Densely Contact-Electrified Electrons on Thin Silicon Oxide

We investigated the time evolution of a stable state which appeared in the dissipation of contact-electrified electrons. Here, four analytical quantities in the stable state, i.e., initial (electrostatic) force F0, critical force F c, critical time t c at stable-unstable phase transition and time constant τ1 of the stable state, were investigated with respect to parameters of measurement (measurement voltage V s and tip-sample distance Z) and contact electrification (contact voltage V c and contact time t0). As a result, we found that measurement parameters do not affect time evolution of the stable state, whereas contact electrification parameters strongly affect it. Furthermore, we obtained the approximated expression of the electrostatic force as a function of parameters on measurement and contact electrification, and time after contact electrification.

Time Evolution of Contact-Electrified Electron Dissipation on Silicon Oxide Surface Investigated Using Noncontact Atomic Force Microscope

Deposition and observation of contact-electrified electrons on thin silicon oxide surface were performed separately and successfully investigated by the noncontact DC mode measurement of the induced electrostatic force with an atomic force microscope (AFM). It was found that the dissipation of the contact-electrified electrons had three stages with respect to time, which correspond to a stable state, an unstable state and a trapped state at the charge trap site.