Ken Tsutsui

Two-Dimensional Contact-Type Image Sensor Using Amorphous Silicon Photo-Transistor

We propose a novel two-dimensional contact-type image sensor using amorphous silicon photo-transistors (APTs) in order to form a compact image scanner. Each pixel of the sensor consists of an APT, a storage capacitor, and an amorphous silicon thin-film transistor (a-Si TFT). We have made a prototype 140×240-pixel sensor with a pixel size of 160 µm square. It is confirmed that the prototype APT sensor can detect an image under room light. Exposure for gray scale images is in the range of 10-1 to 7 lxs. This TFT-switched, two-dimensional contact-type image sensor is promising for large-area applications because the process for APTs is fully compatible with that for conventional a-Si TFTs.

Graphoepitaxy of sexithiophene on thermally oxidized silicon surface with artificial periodic grooves

Graphoepitaxial growth of a sexithiophene (6T) thin film was achieved on a thermally oxidized silicon surface with artificial periodic grooves. The surface structure was fabricated by electron beam lithography and the thin film was grown by molecular beam deposition. A well-pronounced, in-plane oriented component ([010]6T‖grooves) was identified by grazing incidence x-ray diffraction, though there also existed some randomly oriented 6T grains. Presence of the graphoepitaxial component was also confirmed by results of the orientational analysis of atomic force microscopy images. It was shown that the in-plane orientation control of organic semiconductors is possible using graphoepitaxy.

Tunable Monolithic DWDM Band-Selection Interleaver Filter Switch on Silicon-on-Insulator Substrate

A tunable band-selection interleaver filter switch formed on a silicon-on-insulator (SOI) substrate for dense wavelength-division-multiplexing (DWDM) system was fabricated, and its fundamental performances were experimentally demonstrated. The device consists of monolithic integration of a Michelson interferometer (MI) structure with two arm-waveguides with different lengths, appropriately designed for ITU-grid separation, a phase-modulation electrode on one arm and tunable wideband Bragg gratings reflectors. An SOI rib waveguide structure with a medium mode size was adopted for low loss and easy fabrication. A bandwidth of the grating, formed by electron beam (EB) lithography and deep reactive-ion etching (Deep-RIE), was rather large of 4 nm at -10 dB transmission level, from which a large coupling coefficient of the Bragg grating of 105 cm-1 was evaluated. A large tuning range of the Bragg grating of 17 nm was obtained. An extinction ratio of the interleaver filter was about 18 dB, and the interleaving switching was also attained with an applied electric power of 50 mW and a switching speed of about 1 ms.

Fabrications of Micro-Channel Device by Hot Emboss and Direct Bonding of PMMA

We have fabricated and evaluated the mechanical, optical and fluidic characteristics a 50µm wide and a 30µm deep micro-channel device produced by hot emboss and direct bonding of PMMA plate with dimensions of 20mm × 20mm × 1mm. The fabricated micro-channel device was evaluated the bond strength, which was confirmed to be high enough for practical use as well as for quite severe cleaning conditions as ultrasonic cleaning in pure water. The optical loss around bonded interface was also evaluated and no increase in the light absorption was observed. The above results confirmed that the hot emboss and direct bonding technologies for micro-channel manufacturing using the PMMA plates realizes high performance micro channel devices.

Thickness Dependent Characteristics of a Copper Phthalocyanine Thin-Film Transistor Investigated by in situ FET Measurement System

The influence of active layer thickness on transport characteristics of a copper phthalocyanine thin-film transistor was investigated by using the in situ FET measurement system with the film deposition continued up to several hundred nm in thickness. The drain current and mobility showed maximum values in the early stage of film growth and then decreased with the increasing film thickness. This result suggests that the over-grown layer affects the transport characteristics of the conductive accumulation layer, for example, owing to change of the electric field in the device.

Current Reduction Mechanism in Organic Thin Film Transistors

Hidden origin of current reduction mechanism in organic thin film transistor (OTFT) structure is investigated based on a device simulation, which makes it possible to derive current–voltage (I–V) characteristics, potential distribution and carrier concentration inside a device by solving Poissons equation and continuity equation. It is made clear that the device performances improve more than twice to three hundred times only by inserting high impurity concentration layer under the source/drain contact region. The cause of the inferior characteristics of conventional OTFT devices is attributed to the deficiency of carriers in the channel region, resulting to a very high potential drop at the source–channel interface, thus leading to an apparent decrease of carrier mobility. These results clearly indicate the direction of OTFT technology development, i.e., development of high carrier concentration, or heavily doped organic semiconductor materials.

Single grain and single grain boundary resistance of pentacene thin film characterized using a nanoscale electrode array

In this paper, we report on a less than 100-nm-wide nanoscale electrode array, with which the field-effect mobilities of a single grain and a single grain boundary in organic thin films are characterized. The method of fabricating the nanoscale electrode array and the evaluation results of the pentacene thin film are described. The nanoscale electrode array was fabricated by EB lithography and a liftoff process. The pentacene thin film was deposited by molecular-beam epitaxy (MBE). The resistances of a single grain and a single grain boundary were estimated to be around 10 and 100 MΩ, respectively, and the field-effect mobility was estimated to be around 1 cm2/(V s), which is almost comparable to the highest value ever reported. These results confirm that a single crystal is essential for high performance organic thin-film transistors.

Fabrication of planar nano-gap electrodes for single molecule evaluation

This paper aims is to fabricate planar nano-gap electrodes for single molecule characterization, which allow to measure the intrinsic nature of single molecule and to derive the number of molecules on them by atomic force microscopy (AFM), whereas conventional non-planar ones do not. Thus, the planar nano-gap electrodes are the essential tool for single molecule electronics research. Currently, the fabrication technology by anodic bonding realized planar electrodes with 300 nm in width and 200 nm in length. Insulation characteristics of the fabricated electrodes were evaluated and were in the order of 1012 Ω, which indicates that these electrodes are suitable for single molecule evaluation.

Investigation of Spatial Resolution in Current-Induced Magnetic Field Detection by Magnetic Force Microscopy

The magnetic field distribution around sub-µm-wide current paths was investigated by magnetic force microscopy (MFM) as a candidate for the current mapping in fine structures. In particular, an undesirable electrostatic force working between an MFM tip and the current path was dynamically eliminated utilizing an extra ac bias to observe the magnetic field correctly. We observed magnetic force signals around current paths consisting of branching or closely aligned metal wires, and the results were compared with results of the numerical simulation of the magnetic field. We found that spatial resolution of magnetic force detection by our method was better than 0.2 µm. The calculation results also indicate that the oscillatory motion of the MFM tip in tapping operation influences spatial resolution.

Quantitative Current Evaluation Through Magnetic Field Detection by Magnetic Force Microscopy

We observed magnetic fields induced by a current passing through a fine path by magnetic force microscopy (MFM) in order to investigate the possibility of evaluating the current with high spatial resolution. In particular, we examined the magnetic field around an artificial current network to provide quantitative data on the method when a torsional displacement was imposed on an MFM cantilever by the magnetic force. Although the torsional displacement signal included some background offset, data calibration could be performed by subtracting the background from the original torsional signal, and the data after calibration was compared with the current value expected from the network structure. The calibrated data showed relatively good consistency with the expected value, which indicates the suitability of MFM observation of the magnetic field in quantitative evaluation of current.