A Two-Axis Orthogonal Resonator for Variable Sensitivity Mode Localization Sensing.

This paper experimentally demonstrates a mode localization sensing approach using a single two-axis orthogonal resonator. The resonator consists of concentric multi-rings connected by elliptic springs that enable two orthogonal oscillation modes. By electrostatically tuning the anisotropic stiffness between the two axes, the effective coupling stiffness between the modes can be precisely controlled down to near-zero values. This allows the sensitivity of mode localization sensing to be tuned over a wide range. An order of magnitude enhancement in sensitivity is experimentally achieved by reducing the coupling stiffness towards zero. The resonator's simple single-mass structure offers advantages over conventional coupled resonator designs for compact, tunable mode localization sensors. Both positive and negative values of coupling stiffness are demonstrated, enabling maximum sensitivity at the point where coupling crosses through zero. A method for decomposing overlapping resonance peaks is introduced to accurately measure the amplitude ratios of the localized modes even at high sensitivities. The electrostatic tuning approach provides a new option for realizing variable sensitivity mode localization devices using a simplified resonator geometry.

Multi-Ring Disk Resonator with Elliptic Spokes for Frequency-Modulated Gyroscope.

In this paper, we report a multi-ring disk resonator with elliptic spokes for compensating the aniso-elasticity of (100) single crystal silicon. The structural coupling between each ring segments can be controlled by replacing the straight beam spokes with the elliptic spokes. The degeneration of two n = 2 wineglass modes could be realized by optimizing the design parameters of the elliptic spokes. The mode-matched resonator could be obtained when the design parameter, aspect ratio of the elliptic spokes was 25/27. The proposed principle was demonstrated by both numerical simulation and experiment. A frequency mismatch as small as 1330 ± 900 ppm could be experimentally demonstrated, which was much smaller than that of the conventional disk resonator, which achieved as high as 30,000 ppm.

Electrical Design and Evaluation of Asynchronous Serial Bus Communication Network of 48 Sensor Platform LSIs with Single-Ended I/O for Integrated MEMS-LSI Sensors.

For installing many sensors in a limited space with a limited computing resource, the digitization of the sensor output at the site of sensation has advantages such as a small amount of wiring, low signal interference and high scalability. For this purpose, we have developed a dedicated Complementary Metal-Oxide-Semiconductor (CMOS) Large-Scale Integration (LSI) (referred to as "sensor platform LSI") for bus-networked Micro-Electro-Mechanical-Systems (MEMS)-LSI integrated sensors. In this LSI, collision avoidance, adaptation and event-driven functions are simply implemented to relieve data collision and congestion in asynchronous serial bus communication. In this study, we developed a network system with 48 sensor platform LSIs based on Printed Circuit Board (PCB) in a backbone bus topology with the bus length being 2.4 m. We evaluated the serial communication performance when 48 LSIs operated simultaneously with the adaptation function. The number of data packets received from each LSI was almost identical, and the average sampling frequency of 384 capacitance channels (eight for each LSI) was 73.66 Hz.

Design and Fabrication Technology of Low Profile Tactile Sensor with Digital Interface for Whole Body Robot Skin.

Covering a whole surface of a robot with tiny sensors which can measure local pressure and transmit the data through a network is an ideal solution to give an artificial skin to robots to improve a capability of action and safety. The crucial technological barrier is to package force sensor and communication function in a small volume. In this paper, we propose the novel device structure based on a wafer bonding technology to integrate and package capacitive force sensor using silicon diaphragm and an integrated circuit separately manufactured. Unique fabrication processes are developed, such as the feed-through forming using a dicing process, a planarization of the Benzocyclobutene (BCB) polymer filled in the feed-through and a wafer bonding to stack silicon diaphragm onto ASIC (application specific integrated circuit) wafer. The ASIC used in this paper has a capacitance measurement circuit and a digital communication interface mimicking a tactile receptor of a human. We successfully integrated the force sensor and the ASIC into a 2.5 × 2.5 × 0.32.5×2.5×0.3 mm die and confirmed autonomously transmitted packets which contain digital sensing data with the linear force sensitivity of 57,640 Hz/N and 10 mN of data fluctuation. A small stray capacitance of 1.33 pF is achieved by use of 10 µm thick BCB isolation layer and this minimum package structure.

A Tactile Sensor Network System Using a Multiple Sensor Platform with a Dedicated CMOS-LSI for Robot Applications.

Robot tactile sensation can enhance human-robot communication in terms of safety, reliability and accuracy. The final goal of our project is to widely cover a robot body with a large number of tactile sensors, which has significant advantages such as accurate object recognition, high sensitivity and high redundancy. In this study, we developed a multi-sensor system with dedicated Complementary Metal-Oxide-Semiconductor (CMOS) Large-Scale Integration (LSI) circuit chips (referred to as "sensor platform LSI") as a framework of a serial bus-based tactile sensor network system. The sensor platform LSI supports three types of sensors: an on-chip temperature sensor, off-chip capacitive and resistive tactile sensors, and communicates with a relay node via a bus line. The multi-sensor system was first constructed on a printed circuit board to evaluate basic functions of the sensor platform LSI, such as capacitance-to-digital and resistance-to-digital conversion. Then, two kinds of external sensors, nine sensors in total, were connected to two sensor platform LSIs, and temperature, capacitive and resistive sensing data were acquired simultaneously. Moreover, we fabricated flexible printed circuit cables to demonstrate the multi-sensor system with 15 sensor platform LSIs operating simultaneously, which showed a more realistic implementation in robots. In conclusion, the multi-sensor system with up to 15 sensor platform LSIs on a bus line supporting temperature, capacitive and resistive sensing was successfully demonstrated.

3-Axis Fully-Integrated Capacitive Tactile Sensor with Flip-Bonded CMOS on LTCC Interposer.

This paper reports a 3-axis fully integrated differential capacitive tactile sensor surface-mountable on a bus line. The sensor integrates a flip-bonded complementary metal-oxide semiconductor (CMOS) with capacitive sensing circuits on a low temperature cofired ceramic (LTCC) interposer with Au through vias by Au-Au thermo-compression bonding. The CMOS circuit and bonding pads on the sensor backside were electrically connected through Au bumps and the LTCC interposer, and the differential capacitive gap was formed by an Au sealing frame. A diaphragm for sensing 3-axis force was formed in the CMOS substrate. The dimensions of the completed sensor are 2.5 mm in width, 2.5 mm in length, and 0.66 mm in thickness. The fabricated sensor output coded 3-axis capacitive sensing data according to applied 3-axis force by three-dimensional (3D)-printed pins. The measured sensitivity was as high as over 34 Count/mN for normal force and 14 to 15 Count/mN for shear force with small noise, which corresponds to less than 1 mN. The hysteresis and the average cross-sensitivity were also found to be less than 2% full scale and 11%, respectively.

Hetero Acoustic Layer Surface Acoustic Wave Resonator Composed of LiNbO3 and Quartz.

In this work, a shear-horizontal (SH) mode surface acoustic wave (SAW) resonator based on LiNbO3 (LN)/Quartz (Qz) hetero acoustic layer (HAL) structure was studied by simulation and experiment. By this HAL structure, the displacement and electric displacement are well confined in the piezoelectric layer. A lower mechanical loss of Qz than that of lossy amorphous SiO2 further enhances the quality ( Q ) factor. In addition, a negative temperature coefficient of frequency (TCF) of LN is compensated by selecting the crystalline orientation of Qz with a positive TCF. According to simulation results, the Euler angles of (0°, 101°, and 0°) and the normalized thickness of 0.2-0.3 λ (wavelength) for LN are selected to obtain a higher impedance ratio ( Z -ratio) and bandwidth (BW). The Euler angles of (0°, 160°, and 90°) for Qz are selected to obtain the positive maximum TCF. The fabricated resonator exhibits a Z -ratio of 95 dB and a BW of 15.9% in the 700 MHz range. The fit figure of merit (FoM) reaches 410, which is the best level ever reported for an LN-based resonator. The TCF of the resonator is -77 ppm/°C at anti-resonance frequency. A group of resonators composed of LN and LN/Qz with thin and thick electrodes were fabricated to further illustrate the good performance of LN/Qz. The LN/Qz HAL SAW resonator demonstrated in this work exhibits a high Z -ratio, low TCF, and wideband, which has the potential for high-performance wideband filters with steep passband and good temperature characteristics.

Experimental Study of Transverse Mode Suppression on Wideband Hetero Acoustic Layer Surface Acoustic Wave Resonator.

Transverse mode suppression is a great challenge for high-performance surface acoustic wave (SAW) resonators. Conventional methods work well on narrowband resonators, but their performances on wideband resonator have not been demonstrated. In this article, we give an in-depth study on the transverse mode suppression of wideband resonators using 11° YX-LiNbO3 (LN)/70 °Y90°X -quartz (Qz) hetero acoustic layer structure as a platform. Two groups of design, including new dummy electrode and zigzag shape apodization, are proposed. The measured results show that the shape of the dummy electrode is not the dominant factor to affect the transverse mode. The proposed zigzag shape apodization can effectively suppress the transverse, at the same time maintain the quality ( Q ) factor at the same level with the normal type. Additionally, stronger suppression ability can be realized with a tiny tradeoff of Q -factor.

Electroplated Al Press Marking for Wafer-Level Bonding.

Heterogeneous integration of micro-electro mechanical systems (MEMS) and complementary metal oxide semiconductor (CMOS) integrated circuits (ICs) by 3D stacking or wafer bonding is an emerging approach to advance the functionality of microdevices. Aluminum (Al) has been of interest as one of the wafer bonding materials due to its low cost and compatibility with CMOS processes. However, Al wafer bonding typically requires a high temperature of 450 °C or more due to the stable native oxide which presents on the Al surface. In this study, a wafer bonding technique for heterogeneous integration using electroplated Al bonding frame is demonstrated. The bonding mechanism relies on the mechanical deformation of the electroplated Al bonding frame through a localized bonding pressure by the groove structures on the counter wafer, i.e., press marking. The native oxide on the surface was removed and a fresh Al surface at the bonding interface was released through such a large mechanical deformation. The wafer bonding was demonstrated at the bonding temperatures of 250-450 °C. The influence of the bonding temperature to the quality of the bonded substrates was investigated. The bonding shear strength of 8-100 MPa was obtained, which is comparable with the other Al bonding techniques requiring high bonding temperature.

Sputter Deposition and Characterization of Sm-Doped Pb(Mg1/3, Nb2/3)O3-PbTiO3 Epitaxial Thin Film on Si Toward Giant-Piezoelectric Thin Film for MEMS Actuator Application.

To meet the growing demand for better piezoelectric thin films for microelectromechanical systems (MEMSs), we have developed an SM-doped Pb(Mg1/3, Nb2/3)O3-PbTiO3 (Sm-PMN-PT) epitaxial thin film as a next-generation piezoelectric thin film to replace Pb(Zr, Ti)O3 (PZT). The inherent piezoelectricity | e31,f | achieved 20 C/m2, which is greater than those of intrinsic PZT thin films and the best Nb-doped PZT thin film. Besides, the simulation results show that the | e31,f | value of the single Sm-PMN-PT film could be around 26 C/m2. Meanwhile, the breakdown voltage of the as-deposited thin film was higher than 300 kV/cm. These results suggest the high potential of the Sm-PMN-PT epitaxial thin film for piezo-MEMS actuators with large displacement or force.

Surface Acoustic Wave Resonators With Hetero Acoustic Layer (HAL) Structure Using Lithium Tantalate and Quartz.

This article reports a new concept of surface-acoustic-wave (SAW) resonator, which uses shear horizontal (SH) wave confined in a thin LiTaO3 (LT) layer supported by a quartz (Qz) substrate. The LT layer is 35-50°YX LT, and the quartz substrate is 35-60°Y90°X Qz. A negative temperature coefficient of frequency (TCF) of the SH SAW in the LT layer is compensated by the quartz substrate, which shows a wide range of positive TCF depending on the crystalline orientation. Excellent TCFs of 2 and -10 ppm/°C were measured for the series and parallel resonance frequencies, respectively. The strong confinement of the SH SAW in the LT layer results in the best level of resonance characteristics ever reported. The measured impedance ratio reached 84 dB. On the other hand, spurious waves other than the SH SAW are not confined in the LT layer due to the unique properties of quartz, which results in spurious-free characteristics throughout a wide frequency range.

Simple Device to Measure Pressure Using the Stress Impedance Effect of Amorphous Soft Magnetic Thin Film.

A simple micro-machined pressure sensor, based on the stress-impedance (SI) effect, was fabricated herein using typical micro-fabrication technologies. To sense pressure, a 1-µm thin, soft magnetic metallic film of FeSiB was sputtered and used as a diaphragm. Its electrical response (impedance change) was measured under pressure in a frequency band from 5 to 500 MHz. A lumped-element equivalent electric circuit was used to separate the impedance of the soft magnetic metal from other parasitic elements. The impedance change clearly depended on the applied pressure. It was also shown that the impedance change could be explained by a change in relative permeability, according to the theory of the SI effect. The radial stress in the diaphragm and the relative permeability exhibited a linear relationship. At a measurement frequency of 200 MHz, the largest sensor response, with a gauge factor of 385.7, was found. It was in the same order as the conventional sensors. As the proposed device is very simple, it has the potential for application as a cheap pressure sensor.

Development of Sputter Epitaxy Technique of Pure-Perovskite (001)/(100)-Oriented Sm-Doped Pb(Mg1/3, Nb2/3)O3-PbTiO3 on Si.

We have developed a unique sputter deposition technique for a pure-perovskite (001)/(100)-oriented samarium-doped Pb(Mg1/3, Nb2/3)O3-PbTiO3 (Sm-PMN-PT) epitaxial thin film on Si as a future piezoelectric transducer thin film in microelectromechanical systems (MEMSs). This technique bases on the use of a "Pb(Zr,Ti)O3 (PZT)-based seed layer" and "separate sputter deposition." Undesired orientations and phases of such a relaxor-based ferroelectric are usually generated during the sputter deposition. This technique was demonstrated to provide preferential (001)/(100) orientation and pure-perovskite phase to the monocrystalline thin film. The fabricated film had excellent homogeneousness of the content distribution. Considering a practical thickness, a 2- [Formula: see text]-thick monocrystalline thin film was grown on an Si substrate with this technique. Then, the piezoelectricity |e31,f| of the Sm-PMN-PT/PZT stacked film was evaluated through an actuation test of the unimorph cantilever. As a result, it measured 16-17 C/m2, which is almost comparable with intrinsic PZT polycrystalline thin films with high |e31,f| values. Considering that the actuation voltage was divided into Sm-PMN-PT and PZT layers, the inherent piezoelectricity of the Sm-PMN-PT thin film is expected to be higher. Optimization of the phase in the film by tuning the composition ratio also will further improve the piezoelectricity. We believe that this achievement is a great step to discover a giant piezoelectricity relaxor-based thin film beyond PZT for MEMS.

High-Frequency Resonator Using A1 Lamb Wave Mode in LiTaO3 Plate.

Acoustic wave devices utilizing plate waves in thin lithium tantalate (LiTaO3) have the potential of achieving high resonance frequency with a suitable electromechanical coupling factor and a relatively small temperature coefficient of frequency (TCF). The influence of Euler angle and plate thickness on the characteristics of plate waves has been investigated. High-frequency resonators using first antisymmetric Lamb wave mode (A1) in (0°, 42°, 0°) LiTaO3 thin plates have been fabricated and optimized. The resonance frequency is as high as 5 GHz, with a relative bandwidth of 7.3% and an impedance ratio of 72 dB. Finally, the TCF of (0°, 42°, 0°) LiTaO3 has been evaluated. Therefore, it is proved that A1 Lamb wave mode propagating in LiTaO3 at a Euler angle close to (0°, 42°, 0°) is suitable for high-frequency devices.

Fabrication and Characterization of PZT Fibered-Epitaxial Thin Film on Si for Piezoelectric Micromachined Ultrasound Transducer.

This paper presents a fibered-epitaxial lead zirconate titanate (PZT) thin film with intermediate features between the monocrystalline and polycrystalline thin films for piezoelectric micromachined ultrasound transducer (pMUT). The grain boundaries confirmed by scanning electron microscopy, but it still maintained the in-plane epitaxial relationship found by X-ray diffraction analyses. The dielectric constant (εr33 = 500) was relatively high compared to those of the monocrystalline thin films, but was lower than those of conventional polycrystalline thin films near the morphotropic phase boundary composition. The fundamental characterizations were evaluated through the operation tests of the prototyped pMUT with the fibered-epitaxial thin film. As a result, its piezoelectric coefficient without poling treatment was estimated to be e31,f = -10⁻-11 C/m², and thus reasonably high compared to polycrystalline thin films. An appropriate poling treatment increased e31,f and decreased εr33. In addition, this unique film was demonstrated to be mechanically tougher than the monocrystalline thin film. It has the potential ability to become a well-balanced piezoelectric film with both high signal-to-noise ratio and mechanical toughness for pMUT.

Comprehensive Die Shear Test of Silicon Packages Bonded by Thermocompression of Al Layers with Thin Sn Capping or Insertions.

Thermocompression bonding for wafer-level hermetic packaging was demonstrated at the lowest temperature of 370 to 390 °C ever reported using Al films with thin Sn capping or insertions as bonding layer. For shrinking the chip size of MEMS (micro electro mechanical systems), a smaller size of wafer-level packaging and MEMS⁻ASIC (application specific integrated circuit) integration are of great importance. Metal-based bonding under the temperature of CMOS (complementary metal-oxide-semiconductor) backend process is a key technology, and Al is one of the best candidates for bonding metal in terms of CMOS compatibility. In this study, after the thermocompression bonding of two substrates, the shear fracture strength of dies was measured by a bonding tester, and the shear-fractured surfaces were observed by SEM (scanning electron microscope), EDX (energy dispersive X-ray spectrometry), and a surface profiler to clarify where the shear fracture took place. We confirmed two kinds of fracture mode. One mode is Si bulk fracture mode, where the die shear strength is 41.6 to 209 MPa, proportionally depending on the area of Si fracture. The other mode is bonding interface fracture mode, where the die shear strength is 32.8 to 97.4 MPa. Regardless of the fracture modes, the minimum die shear strength is practical for wafer-level MEMS packaging.

Investigation of Piezoelectricity and Curie Temperature of Pb(Mg1/3, Nb2/3)O3-PbTiO3 Epitaxial Thin Film on Si Prepared by Sputter Deposition With Fast Cooling.

This paper reports on an abnormally high Curie temperature ( $T_{c}$ ) of a Pb(Mg1/3, Nb2/3)O3-PbTiO3 (PMN-PT) epitaxial thin film on Si prepared by sputter deposition with fast cooling. This deposition method was previously applied to Pb(Mn, Nb)O3-Pb(Zr, Ti)O3, and a c-axis-oriented epitaxial film with high $T_{c}$ was obtained. Using the same method, a crack-free 2- $\mu \text{m}$ -thick PMN-PT thin film was epitaxially grown on a Si substrate covered with buffer layers. The piezoelectricity, $\vert \text{e}_{31,f}\vert $ , was as large as 18~19 C/m2 under an electric field ranging from 25 to 75 kV/cm. The temperature characteristics of the dielectric constant and crystalline structure were significantly different from those of a bulk single crystal of PMN-PT, and suggested $T_{c}$ higher than 500 °C. The enhanced $T_{c}$ was possibly caused by thermally induced compressive strain received from the Si substrate. This approach can be an effective method for breaking the well-known tradeoff relationship between piezoelectricity and $T_{c}$ of a piezoelectric thin film. 0.

Investigation of Material Constants of CaTiO3 Doped (K,Na)NbO3 Film by MEMS-Based Test Elements.

A CaTiO3-doped (K,Na)NbO3 (KNN-CT) film is a lead-free piezoelectric film that is expected to substitute Pb(Zr,Ti)O3 (PZT) film in piezoelectric micro electro mechanical systems (MEMS). However, the full set of the material constants (elastic constants, piezoelectric constants and dielectric constants) of the KNN-CT film have not been reported yet. In this study, all the material constants of a sputter-deposited blanket KNN-CT film were investigated by the resonance responses of MEMS-based piezoelectric resonators and the phase velocities of leaky Lamb waves on a self-suspended membrane. The phase velocities measured by a line-focus-beam ultrasonic material characterization (LFB-UMC) system at different frequencies were fitted with theoretical ones, which were calculated from the material constants, including fitting parameters. A genetic algorithm was used to find the best-fitting parameters. All the material constants were then calculated. Although some problems arising from the film quality and the nature of deliquescence are observed, all the material constants were obtained exhibiting accuracy within 16 m/s in the phase velocity of leaky Lamb wave.

Bonding-Based Wafer-Level Vacuum Packaging Using Atomic Hydrogen Pre-Treated Cu Bonding Frames.

A novel surface activation technology for Cu-Cu bonding-based wafer-level vacuum packaging using hot-wire-generated atomic hydrogen treatment was developed. Vacuum sealing temperature at 300 °C was achieved by atomic hydrogen pre-treatment for Cu native oxide reduction, while 350 °C was needed by the conventional wet chemical oxide reduction procedure. A remote-type hot-wire tool was employed to minimize substrate overheating by thermal emission from the hot-wire. The maximum substrate temperature during the pre-treatment is lower than the temperature of Cu nano-grain re-crystallization, which enhances Cu atomic diffusion during the bonding process. Even after 24 h wafer storage in atmospheric conditions after atomic hydrogen irradiation, low-temperature vacuum sealing was achieved because surface hydrogen species grown by the atomic hydrogen treatment suppressed re-oxidation. Vacuum sealing yield, pressure in the sealed cavity and bonding shear strength by atomic hydrogen pre-treated Cu-Cu bonding are 90%, 5 kPa and 100 MPa, respectively, which are equivalent to conventional Cu-Cu bonding at higher temperature. Leak rate of the bonded device is less than 10-14 Pa m³ s-1 order, which is applicable for practical use. The developed technology can contribute to low-temperature hermetic packaging.