Kousuke Hikichi

Fabrication and high-speed characterization of SU-8 shrouded two-dimensional microimpellers

In this study, the performance of shrouded two-dimensional microimpellers was measured for application to a micro centrifugal pump used in a portable fuel cell system. Two types of microimpellers were designed, and fabricated by a multi-layer photolithography process using an ultrathick photoresist (SU-8 3000). Microimpellers with a diameter of 10 mm were tested using an air spindle up to maximum rotation speeds determined by the destruction of the microimpellers, which are 350 000 rpm for a purely-radial-outlet blade impeller and 450 000 rpm for a backward blade impeller. The purely-radial-outlet blade impeller showed a higher pressure rise—2.8 kPa at 150 000 rpm, 5.1 kPa at 200 000 rpm and 12.5 kPa at 300 000 rpm. The measured performance satisfies the requirement of the micro centrifugal pump. However, the measured pressure rises are roughly half of the simulated values. The influence of the radial clearance and height difference between the impeller and the diffuser was investigated using an electromagnetic motor and impellers with a diameter of 16 mm. Against expectation, a better pumping performance was obtained with a larger tip clearance, and the height difference did not affect the pumping performance. These unexpected results suggest that the tip clearance and the clearance between the impeller shroud and the stationary housing act as a diffuser.

The wafer-level vacuum sealing and electrical interconnection using electroplated gold bumps planarized by single-point diamond fly cutting

Wafer-level vacuum sealing and electrical interconnection are often critical for microelectromechanical systems (MEMS). This article presents a packaging and integration technology, which is applicable to non-planer (i.e. microstructured) and temperature-sensitive wafers by means of single-point diamond fly cutting of electroplated Au bumps and Au-Au diffusion bonding. The process condition was optimized by L18 Design of Experiments (DOE), and the bonding force and surface pre-treatment were extracted as dominant parameters. The crystal size of the Au bump enlarges after heat treatment for degassing at 300°C, but a smaller crystal size, which is preferred for bonding, is obtained by fly cutting, as observed by electron back scattering diffraction (EBSD). Finally, 100% yield of vacuum-sealing and a high bonding shear strength were achieved.

Wafer-level selective transfer method for FBAR-LSI Integration

This paper reports the wafer-bonding-based integration of heterogeneous devices with different die sizes by laser-assisted selective device transfer technique. 1 mm × 1 mm FBAR (film bulk acoustic resonator) dies supported with a glass wafer were selectively transferred to a wafer of 2 mm × 2 mm BiCMOS sustaining amplifiers by low temperature Au-Au bonding. The FBAR dies left on the support glass wafer were transferred to other BiCMOS wafers, and 4 times of integration were done in total using the same FBAR wafer. The integrated device worked as a chip-size-packaged 2 GHz timing oscillator, and a phase noise of -103 dBc/Hz at 10 kHz offset and -155 dBc/Hz at 1 MHz offset was confirmed.

Bandwidth-tunable SAW filter based on wafer-level transfer-integration of BaSrTiO 3 film for wireless LAN system using TV white space

Cognitive radio technology on TV white spaces has been promoted worldwide to solve the spectrum shortage problem due to the explosive increase in personal communication systems such as smartphones. One of the most difficult challenges to utilize TV white space for personal use is the miniaturization of frequency and bandwidth tunable filters to select vacant TV channels. We have developed a one-chip bandwidth tunable filter by wafer-level transfer-integration of BaSrTiO3 (BST) film varactors and surface acoustic wave (SAW) resonators on a lithium tantalate wafer. The 3 dB bandwidth of the bandwidth tunable filter is tuned between 3.25 MHz and 6.25 MHz by applying 7V to the varactors, while the center frequency was constant at 1.004 GHz, as designed. These filters were installed in our developed prototype cognitive radio systems based on the IEEE802.11af draft and successfully demonstrated wireless LAN communication on TV frequency band.

Wafer–to–wafer transfer process of barium strontium titanate for frequency tuning applications using laser pre-irradiation

This paper describes laser-assisted film transfer technology for barium strontium titanate (BST) deposited on a sapphire substrate. BST is a promising ferroelectric material for varactors, which are required for frequency-tunable RF applications. However, the deposition temperature of BST (600 ~ 700 °C) is too high for surface acoustic wave (SAW) substrates. In this study, BST grown on a sapphire substrate at 650 °C was transferred at low temperature (140 °C) to a borosilicate glass substrate as well as a LiTaO3 substrate. The transferred BST films were characterized as tunable capacitors. A key process in the BST film transfer technology is the laser pre-irradiation of a buffer Pt layer beneath BST from the backside of the sapphire substrate to weaken the BST-to-Pt adhesion. The mechanism of delamination at the BST/Pt interface is discussed using a simple 1D heat transfer model.

Hydroinertia Gas Bearing System to Achieve 470m∕s Tip Speed of 10mm-Diameter Impellers

A microbearing tester driven by an air turbine of 10 mm diameter has been developed, and successfully used to test hydroinertia gas bearings with a shaft of 4 mm diameter. The effects of bearing gas pressure conditions and bearing length to diameter ratio (LID) on the maximum achievable rotation speed were investigated. The maximum rotation speed of 890,000 rpm, which corresponds to the DN number (the product of a shaft diameter in millimeter and a rotation speed in rpm) of 3,560,000, was achieved. At 890,000 rpm, the tip speed of the turbine reaches approximately 470 m/s. Using the bearing system developed, the turbo components of a 100 W class gas turbine and an air pump for 1 kW class fuel cells can be tested.

Laser selective transfer process of barium strontium titanate (BST) on a sapphire substrate

We have been developing the laser selective transfer techniques to transfer Barium Strontium Titanate (BST), which has excellent dielectric properties, prepared on a heat-resistant sapphire substrate to the target substrate. As a sample, Pt/BST/Au structures were made on a sapphire substrate. The laser was irradiated from the backside of the substrate, which weakens Pt/sapphire adhesion. After the laser irradiation, the substrate was separated at Pt/sapphire interface and the BST was transferred to the target substrate. In order to optimize the laser irradiation conditions, the dependences of the transfer process on laser wavelengths (1064nm and 532nm) and pulsewidths (10ns and 200ns) were investigated. After the laser irradiation, we evaluated the shape and number of the BST pixels by adhesion tape test based on ISO2409. Shorter wavelength and longer pulsewidth gave us better results of damageless laser transfer process.We have been developing the laser selective transfer techniques to transfer Barium Strontium Titanate (BST), which has excellent dielectric properties, prepared on a heat-resistant sapphire substrate to the target substrate. As a sample, Pt/BST/Au structures were made on a sapphire substrate. The laser was irradiated from the backside of the substrate, which weakens Pt/sapphire adhesion. After the laser irradiation, the substrate was separated at Pt/sapphire interface and the BST was transferred to the target substrate. In order to optimize the laser irradiation conditions, the dependences of the transfer process on laser wavelengths (1064nm and 532nm) and pulsewidths (10ns and 200ns) were investigated. After the laser irradiation, we evaluated the shape and number of the BST pixels by adhesion tape test based on ISO2409. Shorter wavelength and longer pulsewidth gave us better results of damageless laser transfer process.