Koji Fujisaki

Re-examination of the relationship between packing coefficient and thermal expansion coefficient for aromatic polyimides

Abstract The existence of a possible relationship between molecular packing coefficient and thermal expansion coefficient for various aromatic polyimides was investigated. Rod-like low-thermal-expansion polyimides without side groups were seen to have very high packing coefficients, pointing to free volume as a factor in lowering their thermal expansion coefficients. But the small packing coefficients for low-thermal-expansion polyimides with side groups indicated that this was not so. Also, even if the large packing coefficients tended to increase the Youngs moduli for these polyimides without side groups, the rod-like polyimides with side groups have small packing coefficients and large Youngs moduli. The polyimides with low packing coefficients were found to have very small diffusion coefficients for water vapour.

Fabrication of 3-nm-thick Si3N4 membranes for solid-state nanopores using the poly-Si sacrificial layer process.

To improve the spatial resolution of solid-state nanopores, thinning the membrane is a very important issue. The most commonly used membrane material for solid-state nanopores is silicon nitride (Si3N4). However, until now, stable wafer-scale fabrication of Si3N4 membranes with a thickness of less than 5 nm has not been reported, although a further reduction in thickness is desired to improve spatial resolution. In the present study, to fabricate thinner Si3N4 membranes with a thickness of less than 5 nm in a wafer, a new fabrication process that employs a polycrystalline-Si (poly-Si) sacrificial layer was developed. This process enables the stable fabrication of Si3N4 membranes with thicknesses of 3 nm. Nanopores were fabricated in the membrane using a transmission electron microscope (TEM) beam. Based on the relationship between the ionic current through the nanopores and their diameter, the effective thickness of the nanopores was estimated to range from 0.6 to 2.2 nm. Moreover, DNA translocation through the nanopores was observed.

Electron trap characteristics of silicon rich silicon nitride thin films

Charge localization causes initial retention loss and memory window narrowing after write/erase cycling in a nonvolatile memory device using a metal–oxide–nitride–oxide–semiconductor (MONOS) structure. To overcome these problems, we propose the use of silicon-rich silicon nitride (SRN) thin film as a charge-trapping layer. It was found that most of the electrons injected from the substrate were trapped at the interface between the SRN film and the top oxide and the number of electrons captured by bulk traps of the nitride is negligible. When negative bias is applied to the gate electrode, the electrons trapped at the top interface move back to the bottom interface with SRN. The high effective mobility of the electrons is presumably due to donor-like traps at 0.8 eV below the conduction band bottom of SRN.

Studies on Thermal Cyclization of Polyamic Acids

The imidization reaction of various polyamic acids having different chemical structures has been followed by measuring the weight losses that occurred during dehydro-cyclization. From these studies it was found that when polyamic acids were heated rapidly to a given temperature, the imidization reaction proceeded during the temperature rise but slowed down very markedly after the given temperature was reached. The temperature at which the imidization reaction ended was closely related to the glass transition temperature of the resulting polyimide. Based on these observations, it is concluded that the imidization reaction slows down markedly because the glass transition temperature of the polymer rises as the reaction proceeds, and molecular motion is frozen. In other words, the free rotation of amide bonds in the main chain is frozen. As a result, suitable conformation for imidization cannot take place any more.

Fully CMOS compatible on-LSI capacitive pressure sensor fabricated using standard back-end-of-line processes

A surface micromachined capacitive pressure sensor was fabricated using conventional back-end of line (BEOL) processes in a standard CMOS fabrication line. The combination of standard interlayer dielectric and tungsten was used as sacrificial layers and electrodes, which achieves a large etching selectivity in sacrificial layer removal processes. Measured dependences of capacitance on applied pressure showed a good agreement with simulated results. Although the sensor used metal and amorphous layers in the moving parts (diaphragm), it showed excellent reliability. Sensor characteristics did not change after the deflection test for more than 50M times, temperature cycling test (-55 to 150 deg C, 500 cycles, JEDEC standard) and humidity test (85 deg C, 85% for 100 hr). The process enables us to monolithically integrate MEMS structures with the most advanced CMOS integrated circuits because they use only low temperature processes. Integrating MEMS with high performance digital circuits such as MPU as well as analog circuits enables ultra-tiny one-chip sensor devices.

Thickness-dependent dielectric breakdown and nanopore creation on sub-10-nm-thick SiN membranes in solution

Recently, dielectric breakdown of solid-state membranes in solution has come to be known as a powerful method for fabricating nanopore sensors. This method has enabled a stable fabrication of nanopores down to sub-2 nm in diameter, which can be used to detect the sizes and structures of small molecules. Until now, the behavior of dielectric breakdown for nanopore creation in SiN membranes with thicknesses of less than 10 nm has not been studied, while the thinner nanopore membranes are preferable for nanopore sensors in terms of spatial resolution. In the present study, the thickness dependence of the dielectric breakdown of sub-10-nm-thick SiN membranes in solution was investigated using gradually increased voltage pulses. The increment in leakage current through the membrane at the breakdown was found to become smaller with a decrease in the thickness of the membrane, which resulted in the creation of smaller nanopores. In addition, the electric field for dielectric breakdown drastically decreased when ...