Motohiro Fujiyoshi

Adhesive wafer bonding using a molded thick benzocyclobutene layer for wafer-level integration of MEMS and LSI

This paper describes a wafer bonding process using a 50 µm thick benzocyclobutene (BCB) layer which has vias and metal electrodes. The vias were fabricated by molding BCB using a glass mold. During the molding, worm-like voids grew between BCB and the mold due to the shrinkage of polymerizing BCB. They were completely removed by subsequent reflowing in N2. After patterning Al on the reflowed BCB for the electrodes and via connections, bonding with a glass substrate was performed. Voidless bonding without damage in the vias and electrodes was achieved. Through the process, the control of the polymerization degree of BCB is important, and thus the polymerization degree was evaluated by Fourier transform infrared spectroscopy. The developed process is useful for the wafer-bonding-based integration of different devices, e.g. micro electro mechanical systems and large-scale integrated circuits.

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.

Integration and packaging technology of MEMS-on-CMOS tactile sensor for robot application using molded thick BCB layer and backside-grooved electrical connection

This paper describes a novel integration and packaging process for a chip-size-packaged integrated tactile sensor. A MEMS wafer and a CMOS wafer were bonded with a thick (50 µm thick) BCB (benzocyclobutene) layer, which also works as the dielectric layer of sensing electrodes. The large thickness is advantageous to reduce parasitic capacitance to the CMOS circuit. The thick BCB layer was formed on the CMOS wafer and molded with a glass mold to make a flat surface with via holes. For surface mounting, bond pads are located on the backside of the senor chip by drawing electrical feed lines through the chip edge. To make the feed lines in wafer level, tapered grooves were fabricated along the scribe lines by TMAH wet etching, and half dicing was done along the grooves to access electrodes on the BEOL layer. Finally, the tactile senor was completed and preliminarily evaluated.

Fully-integrated, fully-differential 3-axis tactile sensor on platform LSI with TSV-based surface-mountable structure

The present paper reports a 2.8-mm-square surface-mountable MEMS-on-LSI integrated 3-axis tactile sensor for robot applications, which incorporates sensing and signal processing in a single chip. The MEMS part has a quad-seesaw-electrode structure for fully-differential capacitive 3-axis force sensing. The LSI is an original sensor platform LSI equipped with deep annular-type through silicon vias (TSVs). A multi-project LSI wafer was processed after fabrication in an LSI foundry. The MEMS part and the LSI part were integrated by Au-Au thermocompression bonding. The output is a digital packet and is transferred through the TSV and a bus line. A working test successfully demonstrated the fully-differential capacitive 3-axis force sensing of the fully-integrated tactile sensor.

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 new micro jerk sensor with viscous coupling

We propose a new method for detection of jerk (derivative of acceleration) which is suitable for the MEMS (micro electro mechanical system) sensor. The sensor has two vibrators that arc connected with viscous coupling, and the sensor converts the jerk to the displacement of the second vibrator. In this paper, we explain a new principle of the jerk detection with a two degrees freedom sensor model and analyze the behavior of the sensor model to the viscosity.

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.

A New High-Sensitivity Package-Leak Testing Method for MEMS Sensors

A new high-sensitivity package-leak testing method has been developed for micro electro mechanical system (MEMS) sensors such as a vibrating angular-rate sensor housed in a vacuum package. Procedures of the method to obtain high leak-rate resolution are as follows. (1) A package filled with helium gas is kept in a small and closed chamber to accumulate helium gas leaking out of the package. (2) The chamber is chilled with liquid-N2 after the accumulation to reduce background gases except the helium gas. (3) The helium gas is transported to a mass spectrometer in a short time and recorded as a transient waveform. The leak-rate resolution of 1×10-17 Pa·m3/s was obtained. This resolution was 105 times superior to that of the conventional method.