Teruhisa Akashi

An SOI 3-axis accelerometer with a zigzag-shaped Z-electrode for differential detection

We have developed a fully-differential 3-axis accelerometer with a novel zigzag-shaped Z-electrode, which is used for motion controls of automobiles and robots.

An SOI tactile sensor with a quad seesaw electrode for 3-axis complete differential detection

This paper presents a novel SOI capacitive tactile sensor with a quad-seesaw electrode for 3-axis complete differential detection, which enables integration with a CMOS. For differentially detecting 3-axis forces, the tactile sensor is composed of four rotating plates individually suspended by torsion beams. In this study, to demonstrate the working principle, we fabricated a test device that integrates an SOI substrate with the quad-seesaw electrode and an anodically bondable LTCC substrate with fixed electrodes as an alternative to the CMOS. The experimental results of the test device successfully demonstrated the working principle as well as 3-axis differential detection with a matrix operation.

SOI 3-axis accelerometer with a stress reduction structure

We developed a novel stress reduction structure (SRS) for a capacitive SOI 3-axis accelerometer for the advanced motion control of automobiles and robots. To improve the stability of the sensor output against temperature change, an FEM thermal stress analysis was carried out on a model with a sensor chip, a circuit board, and a package. The FEM results showed that the thermal stress caused by the different coefficients of thermal expansion bent the sensor chip and produced a change in the z-electrode gap. The zero output drift especially in the Z-axis direction by sensor chip bending should be minimized. The SRS was composed of grooves in both the lower and upper Si layers around the detection part of the sensor. Two types of low-stiffness suspensions for stress reduction and for wiring were formed in either the upper or the lower Si layer to connect both sides of the grooves. The experimental results showed that the ratio of zero output drift decreased by a factor of 8.9 by using the proposed SRS.

A mushroom-shaped convex poly-Si structure for preventing z-directional stiction of an SOI-MEMS device

This paper describes a novel convex poly-Si structure capable of preventing a silicon-on-insulator micro-electromechanical- systems (SOI-MEMS) structure that is movable in the z-direction from sticking to a supporting substrate. This structure is referred to as a poly-Si mushroom-shaped structure (pSiMS) because the structure has a convex shape that resembles a pileus of a mushroom. A pSiMS with a height of 0.6 µm and a diameter of 2.8 µm was fabricated on doubly clamped and cantilever beams by the formation of a mold and by refilling the mold with poly-Si. We performed a stiction test and succeeded in preventing 2-mm-long beams from sticking by forming pSiMSs. The test results demonstrated the effectiveness of the pSiMS.

A 2-axis gyroscope with a synchronously-driven dual mass

We report a detuned 2-axis yaw-and-roll gyroscope with a single sensor element. We designed and fabricated a novel structure composed of a synchronously driven dual mass and three types of supporting beams to limit the movable direction of each mass. The designed resonant frequencies agreed well with the measured values, which resulted in 2-axis detuning ratios of -0.7 and -2.2%. The fabricated gyroscope showed a cross-axis sensitivity of +/-2.7%. The test results demonstrated that the gyroscope simultaneously detects yaw and roll rates with low cross-axis sensitivity.

A DRIE compensation mask pattern for fabricating an extremely thick comb electrode

This paper reports a novel DRIE compensation mask pattern for fabricating an extremely thick sense-comb structure. When we attempted to fabricate a 300-μm-thick sense-comb structure to improve the sensitivity of a capacitive sensor, we found that a bridge-shaped silicon residue (bridge) prevented its formation. We first investigated the detailed property and where the bridge appeared by the DRIE. Based on these investigations, we propose a DRIE compensation pattern to remove the bridge. We successfully formed a 300-μm-thick sense-comb structure by adding the compensation pattern to a tip of the comb structure. Experimental results demonstrated the effectiveness of the proposed compensation pattern.

Long-term hermetic vacuum wafer-level packaging with copper-tin thin films

Long-term hermetic vacuum wafer-level packaging (WLP) by bonding with copper-tin (CuSn) thin films is presented. By improving the pretreatment of bonding, the shear strength reached 74 MPa on average with a CuSn 150-μm-wide seal frame. Furthermore, the cavity fabricated by WLP under the optimized conditions maintained its vacuum for 1.8 years. The cavity pressure was directly measured without an extra vibratory element by an ultra-low outgassing residual gas analyzer (RGA) system. The RGA system revealed that the cavity pressure after 1.8 years was 407 Pa. The measured cavity pressure demonstrates that WLP with CuSn films can achieve remarkable vacuum hermeticity.

2D MEMS scanner with a rotation-angle detector for a time-of-flight image sensor

A moving-magnet-type raster-output 2D MEMS scanner with a rotation-angle detector has been developed. To realize high precision and low temperature dependence, we propose the detector composed of four Hall sensors. By operation the outputs of the four Hall sensors, we enhanced the sensitivity and linearity of the detector, and realized the high precision detector. The fabricated detector achieved the detecting linearity of <; ± 0.11° for movable frame, and <; ± 0.07° for movable mirror at room temperature.

Topology optimization method using multistep mapping from 2D photomask to 3D MEMS

We have developed a topology optimization method for designing a two-dimensional (2D) photomask. It employs micromachining-process-like multistep mapping to analytically improve the characteristics of a three-dimensional (3D) MEMS fabricated by the photomask. We used this method to design reinforcing ribs for an electrostatic micromirror to reduce deformation caused by internal residual stress. Employing a dot-array pattern as the initial topology, the optimization calculation converged after 300 iterations. It predicts a 79.1% reduction in the maximum displacement. To verify this result, micromirror structures were fabricated and their deformations were measured. The deformation was reduced by 87.1% in this experiment. The calculation results agree well with the experimental results, demonstrating the effectiveness of the proposed method.