Tomoyuki Takahata

Transparent conductive-polymer strain sensors for touch input sheets of flexible displays

A transparent conductive polymer-based strain-sensor array, designed especially for touch input sheets of flexible displays, was developed. A transparent conductive polymer, namely poly(3, 4-ethylenedioxythiophene):polystyrenesulfonate (PEDOT:PSS), was utilized owing to its strength under repeated mechanical bending. PEDOT:PSS strain sensors with a thickness of 130 nm exhibited light transmittance of 92%, which is the same as the transmittance of ITO electrodes widely used in flat panel displays. We demonstrated that the sensor array on a flexible sheet was able to sustain mechanical bending 300 times at a bending radius of 5 mm. The strain sensor shows a gauge factor of 5.2. The touch point on a flexible sheet could be detected from histograms of the outputs of the strain sensors when the sheet was pushed with an input force of 5 N. The touch input could be detected on the flexible sheet with a curved surface (radius of curvature of 20 mm). These results show that the developed transparent conductive polymer-based strain-sensor array is applicable to touch input sheets of mechanically bendable displays.

Micro-patterning of a conductive polymer and an insulation polymer using the Parylene lift-off method for electrochromic displays

We have developed a process to pattern the conductive polymer of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) film and the insulation polymer of the perfluoro polymer (Cytop) with a Parylene lift-off method for the purpose of fabricating a PEDOT:PSS-based electrochromic display device. Because conventional micro-patterning processes decrease the conductivity of organic electronic polymers due to the destructive solutions of photolithography, PEDOT:PSS and Cytop polymers were patterned by using the dry lift-off film of Parylene. Its patterning resolution of PEDOT:PSS was found to be as low as 20 µm. The insulation layer of Cytop was also patterned on the PEDOT:PSS pattern with the same resolution. This process was able to pattern the 300 µm wide wiring and 1 mm square pixels of PEDOT:PSS and the Cytop to cover the entire area except for the pixels constructed to form an electrochromic display. Finally, the fabricated 4 × 4 pixel device displayed a simple shape, a transverse line.

A wide wavelength range optical switch using a flexible photonic crystal waveguide and silicon rods

We theoretically and experimentally demonstrated that the transmittance of a two-dimensional photonic crystal waveguide (PCW) can be controlled in the 90 nm wavelength range by changing the difference in height between the PCW and silicon column rods that have the same central axis and height as the PCW air holes. Simulations conducted with a three-dimensional finite difference time domain method showed that the transmittance change of a PCW with four air holes in the light propagating direction was −8.5 dB when the height difference was only half the thickness of the slab. We fabricated an optical switch on a single chip by using a single-mask three-step process that consists of electron beam lithography, inductive coupled plasma reactive ion etching and release with hydrofluoric acid vapor. We measured −4.4 dB attenuations at wavelengths ranging from 1470 to 1500 nm.

Liquid-phase packaging of a glucose oxidase solution with parylene direct encapsulation and an ultraviolet curing adhesive cover for glucose sensors.

We have developed a package for disposable glucose sensor chips using Parylene encapsulation of a glucose oxidase solution in the liquid phase and a cover structure made of an ultraviolet (UV) curable adhesive. Parylene was directly deposited onto a small volume (1 μL) of glucose oxidase solution through chemical vapor deposition. The cover and reaction chamber were constructed on Parylene film using a UV-curable adhesive and photolithography. The package was processed at room temperature to avoid denaturation of the glucose oxidase. The glucose oxidase solution was encapsulated and unsealed. Glucose sensing was demonstrated using standard amperometric detection at glucose concentrations between 0.1 and 100 mM, which covers the glucose concentration range of diabetic patients. Our proposed Parylene encapsulation and UV-adhesive cover form a liquid phase glucose-oxidase package that has the advantages of room temperature processing and direct liquid encapsulation of a small volume solution without use of conventional solidifying chemicals.

Three-dimensional silicon fabrication using microloading effects with a rectangular aperture mask

Three-dimensional silicon fabrication is demanded in optical lenses for collecting far infrared radiation. We propose a method for the fabrication of shapes, which have depths varying in the x and y directions, vertical walls and smooth surfaces. A mask design with rectangular apertures was used in a two-stage etching process consisting of anisotropic etching to rough out the three-dimensional shape followed by isotropic etching to smooth the surface. The relationship between the etching depth and area and the aspect ratio (length-to-height ratio) of the rectangular apertures was determined experimentally. Based on this relationship, we describe a procedure for designing rectangular apertures. We fabricated a convex microlens with a diameter of 150 µm and a height of 4.3 µm surrounded by a vertical wall. The arithmetic mean surface roughness of the microlens was 100 nm.

Mechanical properties of few layer graphene cantilever

We report the resonant frequency measurement of few-layer (1-, 2-, and 3-layer) graphene (FLG) cantilevers by optical heterodyne interferometry. The micro-sized FLG cantilevers with and without the diamond-like carbon weights were fabricated using focused ion beam. The Youngs modulus was able to be calculated from the measured resonant frequency. The calculated Youngs modulus was larger than the literature data [1]. This result suggests that the overlapped structure of the FLG cantilever makes the structure rigid.

Measurement of mechanomyogram

We have proposed an approach for measuring mechanomyogram (MMG) by taking advantage of the acoustic impedance matching between liquid and human skin. Using liquid, we could convey the pressure signal of MMG to a piezo-resistive cantilever efficiently. In experiments, the sensor was placed on the skin surface over biceps brachii (a large muscle that is located in the upperarm). The MMG signal, of which the frequency was in range of 10-15 Hz, was able to be detected using silicone oil as the propagating medium. On the other hand, there was no response in the case of using air as the medium. Experimental results also indicated that the proposed sensor was able to detect the vascular oscillations.

Micro mirror arrays for improved sensitivity of thermopile infrared sensors

This papera reports an improved method for higher sensitivity of the thermopile infrared sensors. To increase the sensitivity of thermopile infrared sensors, the thermopile infrared sensors were bonded to the micro mirror arrays (MMA) which focused the infrared radiation in the infrared absorber. As the result, 4-pair thermopiles have demonstrated 220V/W to 350V/W under the pressure of 1Pa atmospheres, thus they are 1.6 times higher sensitivity than without MMA. The fabrication method of MMA is using etching depth difference of DRIE which depends on the aspect ratio with different size mask openings and isotropic RIE.

3D airflow velocity vector sensor

In this paper, we propose a sensor for measuring the three-dimensional (3D) velocity vector of airflows. The sensor, which was a 10 mm spherical figure, had a laminated structure with three channels fabricated inside. The components of the airflow velocity vector were measured respectively by three piezo-resistive cantilevers fabricated in each of the three channels. Experiments with a wind tunnel demonstrated that our sensor can measure not only the velocity amplitude but the 3D velocity direction as well.

A Tactile Sensor Using Piezoresistive Beams for Detection of the Coefficient of Static Friction

This paper reports on a tactile sensor using piezoresistive beams for detection of the coefficient of static friction merely by pressing the sensor against an object. The sensor chip is composed of three pairs of piezoresistive beams arranged in parallel and embedded in an elastomer; this sensor is able to measure the vertical and lateral strains of the elastomer. The coefficient of static friction is estimated from the ratio of the fractional resistance changes corresponding to the sensing elements of vertical and lateral strains when the sensor is in contact with an object surface. We applied a normal force on the sensor surface through objects with coefficients of static friction ranging from 0.2 to 1.1. The fractional resistance changes corresponding to vertical and lateral strains were proportional to the applied force. Furthermore, the relationship between these responses changed according to the coefficients of static friction. The experimental result indicated the proposed sensor could determine the coefficient of static friction before a global slip occurs.