Takanori Aono

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.

Wafer-level two-step bonding process for combined sensor with two different pressure chambers

We developed a wafer-level two-step anodic bonding process for a combined sensor with two different pressure chambers: ambient and low vacuum pressure. This two-step bonding process features: a bonding and non-bonding area controlled with a glass bump array, an anodic bonding process with plastic deformation of the glass bumps at high temperature and high loading, and two different pressure chambers formed with a sequence of bonding processes.

Oil-repellent treatment of a flying slider in a hard disk drive

The effectiveness of various oil-repellent treatments was evaluated by measuring the contact angles of mineral oil on the surface of a slider of a hard disk drive. The contact angle on an untreated slider was 20°; it was 51° on a slider dipped in FAS solution (2-(heptadecafluorooctyl)-ethyltrimethoxysilane) and heated to 393 K; 68° on a slider dipped in FASI solution (a octadecafluorooctane solution of FAS treated by hydrolysis and polymerization) and heated to 393 K; 80° on a slider vapor-deposited with FAS solution at 433 K; 80° on a slider dipped in FASI solution after ultraviolet radiation (UV) and heated to 393 K; and 81° on a slider irradiated by UV after being dipped in FASI solution. These results indicate that a compact coating of FAS can be formed, even on the slider surface of a carbon film with few hydroxyl groups, by treating the surface with FAS solution or FASI solution.

Three-axis MEMS inertial sensor for automobile applications

A three-axis microelectromechanical systems (MEMS) inertial sensor measuring two-axis acceleration and angular rate (rotation) has been developed for an electronic stability control (ESC) system for automobiles. We combined the angular rate detection part with the two-axis acceleration detection parts on a single MEMS chip to miniaturize the sensor. The two detection parts were designed to work under different environmental pressures through a two-step wafer level package (WLP) process to achieve the required sensitivity for the gyroscope with excellent vibration immunity for the accelerometers. We report on the design concept and test results of the three-axis MEMS inertial combined sensor and the manufacturing processes of the two-step WLP and through-silicon vias (TSVs).

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.