Akihito Nakai

Temperature-controlled transfer and self-wiring for multi-color light-emitting diode arrays

We propose an integration method for arranging light-emitting diode (LED) bare chips on a flexible substrate for multi-color inorganic LED displays. The LED bare chips (240 µm × 240 µm × 75 µm), which were diced on an adhesive sheet by the manufacturer, were transferred to a flexible polyimide substrate by our temperature-controlled transfer (TCT) and self-wiring (SW) processes. In these processes, low-melting point solder (LMPS) and poly-(ethylene glycol) (PEG) worked as adhesive layers for the LED chips during the TCT processes, and the adhesion force of the LMPS and PEG layers was controlled by changing the temperature to melt and solidify the layers. After the TCT processes, electrical connection between the transferred LED chips and the flexible substrate was automatically established via the SW process, by using the surface tension of the melted LMPS. This TCT/SW method enabled us to (i) handle arrays of commercially available bare chips, (ii) arrange multiple types of chips on the circuit substrate by simply repeating the TCT processes and (iii) establish electrical connection between the chips and the substrate automatically. Applying this transfer printing and wiring method, we experimentally demonstrated a 5-by-5 flexible LED array and a two-color (blue and green) LED array.

A stereoscopic display with a vibrating microlens array

In this paper, we demonstrate a prototype of a high-resolution stereoscopic display by vibrating a micro cylindrical lens array. This lens array is fabricated by melting photoresists and form transfer. The height and width of the cylindrical lens measured 12.5 /spl mu/m and 320 /spl mu/m, therefore the focal length is calculated as 1980 /spl mu/m. The lens array vibrates synchronously with the changeover of images which are displayed under the lens array. The brightness is measured at several view angles, and 9 degrees-parallax is observed on condition that the amplitude and cycle of the vibration are 100 /spl mu/m and 120 ms, respectively.

A grasping forceps with a triaxial MEMS tactile sensor for quantification of stresses on organs

This paper reports on a grasping forceps with a triaxial Micro Electro Mechanical Systems (MEMS) tactile sensor on a tip. The laparoscopic surgery is minimally invasive because the incisions are smaller than the open surgery. This results in fast recovery. However, it is a problem in the laparoscopic surgery to damage an organ by localized stress generated by grasping with a thin forceps. To avoid excessive stress applying to the organ, real time evaluation of the stress is important. However, there is no acceptable tool to measure the stress. We propose a grasping forceps with a triaxial MEMS tactile sensor on a tip for a measurement tool. We attached a triaxial MEMS tactile sensor which we have developed on a tip of a grasping forceps. The MEMS sensor can measure not only the pressure but also two directional shear stresses applied to the sensor surface. The sensor size is 7 mm × 7 mm × 2 mm. It is enough small to attach the sensor to the tip of a forceps 12 mm in diameter. In this paper, the characteristics of the forceps with the MEMS sensor during grasping, pushing and pulling actions were evaluated. In these experiments, output of each sensor for pressure and shear stress was proportional to the applied stresses, respectively. Moreover, as an in vivo experiment, we measured the shear stress applied to a pig liver block when it is lifted after being grasped with the forceps. We obtained that the shear stress applied to the liver block increased with the increase of the weight of the liver block.

An all-polymer thin-film scanning optical filter for 3D-image displays

Built a transparent thin-film scanning optical filter, which can be attached to displays to show 3D-images. The scanning motion provides more quality images depending on the viewpoints. Water droplets are put in the filter as microlenses which is driven electrostatically. All the structure of the filter, including conductive elements, is made of transparent polymer films. The electrodes to drive the microlenses are conductive polymer Baytron P/sup /spl reg// patterned with a new technique using selectivity of hydrophobic and hydrophilic regions on the surface. Since the scanning filter consists of multi layers, new functions can be easily added by attaching another layer on top or bottom of the filter. In this paper, an LCD (liquid crystal display) was attached to the bottom as a light source layer.

Temperature-Controlled Transfer and Self-Wiring for Multi-Color Led Display on a Flexible Substrate

We propose an integration method for arranging LED bare chips on a flexible substrate to fabricate a multi-color LED display. LED chips (240 ¿m × 240 ¿m × 75 ¿m) which were arrayed on an adhesive sheet were transferred to a flexible circuit substrate using our temparature-controlled transfer (TCT) and self-wiring (SW) method. Using these methods, we demonstrated a 5-by-5 LED flexible device and a two-color (blue and green) LED device, and observed light emission from the LED chips.

Nanocrystalline porous silicon ultrasonic transmitter with patterned emission area

We propose a nanocrystalline porous silicon (nc-PS) ultrasonic transmitter with a variable directional pattern. The directional pattern is realized by using an interference effect. An ultrasonic wave is emitted from electrodes on the nc-PS layer according to the applied electrical current. The emission areas of the transmitter were patterned in order to use the interference between the emitted ultrasonic waves. To observe the interference effect, our device was designed to have two emission areas. The directional pattern induced by the interference effect depends on the distance between the emission areas, and the frequency and the phase difference of the currents applied to the emission areas. We confirmed that the sharpness and the lobe number of the directional pattern can be changed by the phase difference between two emission areas. We also confirmed that the direction of the main lobe can be changed by the phase difference. With the characteristics of the designable and controllable directional pattern, the proposed device is suitable for distance sensors, especially for proximity sensors.

6-Axis force/torque sensor for spike pins of sports shoes

This paper reports on a method to measure forces and torques acting on spike pins of sports shoes. We fabricated a 2 mm × 2 mm × 0.3 mm 6-axis force/torque sensor chip, which consists of 6 piezoresistive beams. From the resistance changes of the 6 beams, 6 components of forces and torques can be detected. The sensor chip was connected to a flexible printed circuit board (PCB) and covered with thin polydimethylsiloxane (PDMS). Then, the sensor chip and flexible PCB were embedded in spike-pin-shaped epoxy resin. We calibrated the spike-pin-shaped sensor, and confirmed that 6-axis forces and torques were able to be detected by the proposed sensor.

Shear force detector using piezo-resistive beams with sidewall-doping

This paper reports on a shear force detector using piezo-resistive beams with sidewall doping embedded in elastic body. These beams are formed using a 20 μm-thick device Si layer of an SOI (Silicon on Insulator) wafer. Shear force is measured by the resistance change due to the extension/compression of the sidewall. The size of the beams is 180 μm × 15 μm × 20 μm (length × width × thickness). Using these shear force detectors, a triaxial tactile sensor can be fabricated.

Transmissive Scanning Micromirrors for Autostereoscopic Display

In this paper, we report on the transmissive scanning micromirrors for an autostereoscopic display. Double-sided micromirrors actuated by both external magnetic force and Lorentz force were designed and fabricated based on the MEMS technology, and some characteristics of a micromirror were measured. Fabricated micromirrors, a microlens array and an LED matrix were integrated into a prototype of the transmissive autostereoscopic display, and the relationship between the view angle and the light intensity was measured on some conditions. The high feasibility of the autostereoscopic display was proved from the light intensity distribution of this prototype.

6-axis force-torque sensor chip composed of 16 piezoresistive beams

This paper reports the design, a part of the fabrication process, especially ion doping method by oblique ion implantation, and experimental results of a 6-axis force-torque sensor chip composed of 16 piezoresistive beams. The sensor chips area, 2mm square in size, is one-third of that of the minimum 6-axis sensor chip ever reported. It will enhance the mounting density of sensor array and also reduce the cost in case of volume production. These sensor chips were fabricated by MNOIC, 8-inch MEMS foundry in Japan, and their characteristic variations are enough small to make practical use of them.