Seiya Kobayashi

Bottom-gate amorphous InGaZnO4 thin-film transistor pH sensors utilizing top-gate effects

We discuss the sensitivity enhancement of bottom-gate type amorphous InGaZnO4 thin-film transistor (a-InGaZnO TFT) pH sensors from the viewpoint of top-gate effects. Comparing the top-gate effects in a-InGaZnO TFTs having TaOx and SiOx ion-sensitive insulators, we draw an analogy between the operations of dual-gate TFTs and TFT pH sensors. Our new concept for enhancing pH sensitivity is characterized by a high capacitance ratio of the ion-sensitive insulator to the bottom-gate insulator and pH sensing utilizing threshold-voltage shifts in bottom-gate transfer characteristics. The close similarity between top-gate effects and pH sensitivity strongly suggests that a common mechanism underlies the phenomena. We discuss the mechanism on the basis of the material properties of a-InGaZnO and the silicon-on-insulator (SOI) model that relates bottom- and top-gate electric fields in fully depleted operations. We believe that the pH-sensitivity enhancement utilizing top-gate effects is one of the potential applications that would make the most of the intrinsic features of a-InGaZnO TFTs.

Development of CNT-Coated Diamond Grains Using Self-Assembly Techniques for Improving Electroplated Diamond Tools

Single crystalline diamond grains covered by nested carbon nanotubes (CNTs) were constructed using self-assembly techniques. The acid-treated CNTs (MWCNTs-COOH) are first adsorbed onto amine-terminated diamond grains, which were chemically functionalized by silane coupling treatments, in N, N-dimethylformamide solution. Then, the drying and readsorption cycle deposited CNT coatings on the diamond grains due to CNT-CNT interactions caused by van der Waals forces. When the diamond grains were bonded to steel substrates by electroplating using nickel sulfamate plating bath, the bonding strength of the CNT-coated diamond grains to the Ni matrix was almost twice as large as that of the normal diamond grains. Hence, the CNT-coated diamond grains are very useful for improving the tool life of electroplated diamond tools.

Fabrication of suspended amorphous indium–gallium–zinc oxide thin-film transistors using bulk micromachining techniques

In this article, we propose a novel application of amorphous indium–gallium–zinc oxide (a-InGaZnO) thin films to micro electromechanical systems (MEMS). For this purpose, we investigated the residual stress in the a-InGaZnO thin films deposited by magnetron sputtering. The films with various residual stresses were characterized using atomic force microscopy, scanning electron microscopy, and X-ray diffraction. We found that the residual stress strongly depended on the sputtering gas pressure. In addition, we discovered a relationship between the residual stress and the microstructure in a-InGaZnO thin films. To gain more insight into the a-InGaZnO thin films with various residual stresses, we fabricated a-InGaZnO thin-film transistors (TFTs), and measured their transfer characteristics. We also fabricated a-InGaZnO TFTs on self-supported membranes, combining the typical TFT fabrication process and bulk micromachining techniques, for the application of a-InGaZnO to MEMS devices.

Depth-Profiling Study on Amorphous Indium–Gallium–Zinc Oxide Thin-Film Transistors by X-ray Photoelectron Spectroscopy

We performed an X-ray photoelectron spectroscopy (XPS) depth-profiling study on the materials used in amorphous indium–gallium–zinc oxide thin-film transistors (a-IGZO TFTs) with Ti and Mo source/drain (S/D) electrodes. The XPS results suggested that there are some differences between the interface regions of Ti/a-IGZO and Mo/a-IGZO for different chemical states of the materials. The chemical states of the back-channel surfaces were also found to be different between the TFTs with Ti and Mo S/D electrodes. In addition, we fabricated indium–gallium–zinc–titanium oxide composite thin films by deposition using multitarget co-sputtering. The electronic structure of these films observed by XPS is similar to that of the Ti/a-IGZO interface region. The fabricated films were found to have a very low resistivity, much lower than that of an a-IGZO film using typical TFT fabrication processes.

5-Axis Capacitive Motion Sensor Fabricated by Silicon Micromachining Technique

We have developed a 5-axis capacitive motion sensor by using silicon bulk-micromachining technique. This sensor has a seismic mass which is vibrated along Z-axis by electrostatic force at the resonant frequency of 1875Hz. The Z-axis acceleration (Az) translates a mass parallel and the X-, Y-axis accelerations (Ax,Ay) tilt a mass. Corioris force induced by 2-axis angular rates (Ωx,Ωy) tilt a mass synchronizing with driving frequency. Therefore the 3-axis accelerations and 2-axis angular rates can be detected selectively because of the difference of frequency. Measured sensitivities of accelerations were approximately 20fF/G in Az, and 6fF/G in Ax and Ay. The sensitivities of angular rates are approximately 3aF/[deg/s] in Ωx and Ωy. The chip size of developed sensor is 8.4mm×8.0mm×1.4mm.