Ryoji Okada

Stress Relaxation Mechanism With a Ring-Shaped Beam for a Piezoresistive Three-Axis Accelerometer

A novel stress-relaxation structure with a ring-shaped beam for ensuring stability in a piezoresistive three-axis accelerometer assembled using wafer-level encapsulation and resin-mold packaging is proposed and evaluated in this paper. The reduction of the unstable increase in sensitivity due to buckling against three stress sources, i.e., the thermal stress in device fabrication, wafer bonding, and resin molding, was evaluated by both computer simulation and experiments on test samples. The measured increase in sensitivity due to stress during wafer bonding and resin molding was kept at 1.26 times using the ring-shaped beam, compared with 7.64 times when using a conventional straight beam. In addition, the standard deviation in the sensitivity of resin-molded samples was kept at 0.04 mV/G using the ring-shaped beam, compared with 2.19 mV/G using the straight beam. The sensitivity variation against the temperature change was also kept small and linear; thus, the effect of the ring-shaped beam was confirmed.

Low-insertion-loss compact 3D-MEMS optical matrix switch with 18 input/output ports

The paper describes a new three-dimensional (3D) MEMS optical switch. The switch consists of a collimator array for generating optical beams, and a MEMS mirror array for changing the directions of the beams. Each of the 18 optical ports can be either an input or an output, giving the switch a flexible configuration of 8/spl times/8, 2/spl times/16, and so on. The mirrors and the collimators are assembled passively in a compact package measuring 80/spl times/30/spl times/12 mm/sup 3/. The mean of measured insertion losses of all 64 paths in an 8/spl times/8 switch configuration was 1.1 dB; this confirms the low insertion loss of the developed switch.

Bonding strength characterization of eutectic-based WLP using molecular dynamics and wafer level shear testing

This paper investigated the bonding shear strength of eutectic-based wafer-level-packaging (WLP) for piezoresistive MEMS accelerometers. The bonding conditions were experimentally and analytically determined to realize higher shear strength without a diffusion of the solder material atoms to the adhesion layers: the energy dispersive X-ray (EDX) spectrometry and molecular dynamics (MD) simulations clarified the eutectic reaction of solder materials used in this research. Consequently, the bonding load of 9.8 kN and the temperature of 300 °C was found to be one of rational conditions because of a sufficient shear strength to endure the polishing process after the WLP and few diffusion of solder material atoms to the adhesion layer.

Development of wafer-level-packaging technology for simultaneous sealing of accelerometer and gyroscope under different pressures

This research demonstrates a newly developed anodic bonding-based wafer-level-packaging technique to simultaneously seal an accelerometer in the atmosphere and a gyroscope in a vacuum with a glass cap for micro-electromechanical systems sensors. It is necessary for the accelerometer, with a damping oscillator, to be sealed in the atmosphere to achieve a high-speed response. As the gyroscope can achieve high sensitivity with a large displacement at the resonant frequency without air-damping, the gyroscope must be sealed in a vacuum. The technique consists of three processing steps: the first bonding step in the atmosphere for the accelerometer, the pressure control step and the second bonding step in a vacuum for the gyroscope. The process conditions were experimentally determined to achieve higher shear strength at the interface of the packaging. The packaging performance of the accelerometer and gyroscope after wafer-level packaging was also investigated using a laser Doppler velocimeter at room temperature. The amplitude at the resonant frequency of the accelerometer was reduced by air damping, and the quality factor of the gyroscope showed a value higher than 1000. The reliability of the gyroscope was also confirmed by a thermal cyclic test and an endurance test at high humidity and high temperature.

Fabrication of an optical beam scanning array integrating mirror and lens arrays by passive self-alignment using precisely formed through-holes

An optical beam scanning array (OBSA) integrating two mirror arrays and a collimator-lens array was fabricated. The passive self-alignment inserting a pin into a through-hole was carried out for integration of the mirror arrays and the collimator-lens array. The mirror arrays consist of a mirror substrate with 35 single-axis-rotational mirrors and an electrode substrate for actuating the corresponding mirror by electrostatic force. Each substrate includes two kinds of through-holes: one for passing a collimated-optical beam through, the other for inserting a pin. The conditions for deep-reactive-ion etching (DRIE) of silicon were optimized to form through-holes suitable for passive self-alignment. DRIE from both surfaces of a silicon wafer under the optimized DRIE conditions—a C4F8/SF6 gas-flow ratio of 90% and a bias power in SF6 gas of 110 W—formed precision through-holes with an average undercut of 3.4 µm and an average sidewall angle of 89.9°. The passive self-alignment then yielded an assembly deviation of less than 17.5 µm, which meets the specification of less than 50 µm. This means that passive self-alignment using a through-hole formed under the optimized DRIE conditions can be exploited to make OBSAs without requiring optical alignment of mirrors and lenses.

Study of Production Technique for Metallic Ultra Fine Particles Using Arc Energy

Ultra fine particles (referred to as UFP here) have drawn considerable attention in recent years as possible new function materials. As such, they have been the subject of intensive research in various fields.1 The authors have studied the UFP production technique using arc energy looking closely at the electromagnetic force resulting from arc current. Through this study a method for producing UFP with higher efficiency was developed. The results of that investigation are covered in this report.