MEMS development focusing on collaboration using common facilities: a retrospective view and future directions.

I have been developing MEMS (microelectromechanical systems) technology and supporting the industry through collaboration. A facility was built in house on a 20 mm square wafer for use in prototyping MEMS and ICs (integrated circuits). The constructed MEMS devices include commercialized integrated capacitive pressure sensors, electrostatically levitated rotational gyroscopes, and two-axis optical scanners. Heterogeneous integration, which is a MEMS on an LSI (large-scale integration), was developed for sophisticated systems using LSI made in a foundry. This technology was applied for tactile sensor networks for safe robots, multi FBAR filters on LSI, active-matrix multielectron emitter arrays, and so on. The facility used to produce MEMS on 4- and 6-inch wafers was developed based on an old semiconductor factory and has been used as an open hands-on access facility by many companies. Future directions of MEMS research are discussed.

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.

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.

Wave Propagation Direction and c-Axis Tilt Angle Influence on the Performance of ScAlN/Sapphire-Based SAW Devices.

Some previously reported surface acoustic wave (SAW) devices using bulk piezoelectric substrates showed higher acoustic power radiated in either forward or backward wave propagation direction depending on their crystal orientations and are called natural single-phase unidirectional transducers (NSPUDT). While these reports were based on bulk piezoelectric substrates, we report directionality in the c-axis tilted 44% scandium doped aluminum nitride thin piezoelectric film-based SAW devices on sapphire. It is worth noting that our observance of directionality is specifically in Sezawa mode. We produced a c-axis tilt up to 5.5° over the single wafer and examined the directionality by comparing the forward and backward insertion loss utilizing split finger electrodes as a receiver. The wave propagation direction and c-axis tilt angle influence on the performance of SAW devices is evaluated. Furthermore, return loss and insertion loss data are presented for various SAW propagation directions and c-axis tilt angles. Finally, the comparison for both acoustic modes, i.e., Rayleigh and Sezawa, is reported.

Stacked Integration of MEMS on LSI.

Two stacked integration methods have been developed to enable advanced microsystems of microelectromechanical systems (MEMS) on large scale integration (LSI). One is a wafer level transfer of MEMS fabricated on a carrier wafer to a LSI wafer. The other is the use of electrical interconnections using through-Si vias from the structure of a MEMS wafer on a LSI wafer. The wafer level transfer methods are categorized to film transfer, device transfer connectivity last, and immediate connectivity at device transfer. Applications of these transfer methods are film bulk acoustic resonator (FBAR) on LSI, lead zirconate titanate (Pb(Zr,Ti)O3) (PZT) MEMS switch on LSI, and surface acoustic wave (SAW) resonators on LSI using respective methods. A selective transfer process was developed for multiple SAW filters on LSI. Tactile sensors and active matrix electron emitters for massive parallel electron beam lithography were developed using the through-Si vias.

On-Chip Micro-Pseudocapacitors for Ultrahigh Energy and Power Delivery.

Microscale supercapapcitors based on hierarchical nanoporous hybrid electrodes consisting of 3D bicontinuous nanoporous gold and pseudocapacitive manganese oxide deliver an excellent stack capacitance of 99.1 F cm-3 and a high energy density of 12.7 mW h cm-3 with a retained high power density of 46.6 W cm-3.

Advanced LSI-based amperometric sensor array with light-shielding structure for effective removal of photocurrent and mode selectable function for individual operation of 400 electrodes.

We have developed a large-scale integrated (LSI) complementary metal-oxide semiconductor (CMOS)-based amperometric sensor array system called "Bio-LSI" as a platform for electrochemical bio-imaging and multi-point biosensing with 400 measurement points. In this study, we newly developed a Bio-LSI chip with a light-shield structure and a mode-selectable function with the aim of extending the application range of Bio-LSI. The light shield created by the top metal layer of the LSI chip significantly reduces the noise generated by the photocurrent, whose value is less than 1% of the previous Bio-LSI without the light shield. The mode-selectable function enables the individual operation of 400 electrodes in off, electrometer, V1, and V2 mode. The off-mode cuts the electrode from the electric circuit. The electrometer-mode reads out the electrode potential. The V1-mode and the V2-mode set the selected sensor electrode at two different independent voltages and read out the current. We demonstrated the usefulness of the mode-selectable function. First, we displayed a dot picture based on the redox reactions of 2.0 mM ferrocenemethanol at 400 electrodes by applying two different independent voltages using the V1 and V2 modes. Second, we carried out a simultaneous detection of O2 and H2O2 using the V1 and V2 modes. Third, we used the off and V1 modes for the modification of the osmium-polyvinylpyridine gel polymer containing horseradish peroxidase (Os-HRP) at the selected electrodes, which act as sensors for H2O2. These results confirm that the advanced version of Bio-LSI is a promising tool that can be applied to a wide range of analytical fields.

Highly c-axis-oriented monocrystalline Pb(Zr, Ti)O3 thin films on si wafer prepared by fast cooling immediately after sputter deposition.

We successfully developed sputter deposition technology to obtain a highly c-axis-oriented monocrystalline Pb(Zr, Ti)O3 (PZT) thin film on a Si wafer by fast cooling (~-180°C/min) of the substrate after deposition. The c-axis orientation ratio of a fast-cooled film was about 90%, whereas that of a slow-cooled (~-40°C/min) film was only 10%. The c-axis-oriented monocrystalline Pb(Zr0.5, Ti0.5)O3 films showed reasonably large piezoelectric coefficients, e(31,f) = ~-11 C/m(2), with remarkably small dielectric constants, ϵ(r) = ~220. As a result, an excellent figure of merit (FOM) was obtained for piezoelectric microelectromechanical systems (MEMS) such as a piezoelectric gyroscope. This c-axis orientation technology on Si will extend industrial applications of PZT-based thin films and contribute further to the development of piezoelectric MEMS.

Focusing and waveguiding of Lamb waves in micro-fabricated piezoelectric phononic plates.

This paper presents results on the numerical and experimental studies of focusing and waveguiding of the lowest anti-symmetric Lamb wave in micro-fabricated piezoelectric phononic plates. The phononic structure was based on an AT-cut quartz plate and consisted of a gradient-index phononic crystal (GRIN PC) lens and a linear phononic plate waveguide. The band structures of the square-latticed AT-cut quartz phononic crystal plates with different filling ratios were analyzed using the finite element method. The design of a GRIN PC plate lens which is attached with a linear phononic plate waveguide is proposed. In designing the waveguide, propagation modes in square-latticed PC plates with different waveguide widths were studied and the results were served for the experimental design. In the micro-fabrication, deep reactive ion etching (Deep-RIE) process with a laboratory-made etcher was utilized to fabricate both the GRIN PC plate lens and the linear phononic waveguide on an 80 µm thick AT-cut quartz plate. Interdigital transducers were fabricated directly on the quartz plate to generate the lowest anti-symmetric Lamb waves. A vibro-meter was used to detect the wave fields and the measured results on the focusing and waveguiding of the piezoelectric GRIN PC lens and waveguide are in good accordance with the numerical predictions. The results of this study may serve as a basis for developing an active micro plate lens and related devices.

Metallic glass thin films for potential biomedical applications.

We introduce metallic glass thin films (TiCuNi) as biocompatible materials for biomedical applications. TiCuNi metallic glass thin films were deposited on the Si substrate and their structural, surface, and mechanical properties were investigated. The fabricated films showed good biocompatibility upon exposure to muscle cells. Also, they exhibited an average roughness of <0.2 nm, high wear resistance, and high mechanical properties (hardness ∼6.9 GPa and reduced modulus ∼130 GPa). Top surface of the TiCuNi films was shown to be free from Ni and mainly composed of a thin titanium oxide layer, which resulted in the high surface biocompatibility. In particular, there was no cytotoxicity effect of metallic glass films on the C2C12 myoblasts and the cells were able to proliferate well on these substrates. Low cost, viscoelastic behavior, patternability, high electrical conductivity, and the capability to coat various materials (e.g., nonbiocompatible materials) make TiCuNi as an attractive material for biomedical applications.

An optically switchable emitter array with carbon nanotubes grown on a Si tip for multielectron beam lithography.

In this paper we report the design, fabrication and evaluation of a field emitter array of carbon nanotubes (CNTs) on a Si tip with a pn junction. Electron beam emission can be switched on by laser irradiation. The Si tip array is formed on a 5 µm-thick Si membrane. Each emitter consists of CNT emitter tips and a gate electrode. On the apex of the Si tip, CNTs are grown in order to emit electrons at a low extraction voltage. Additionally, the pn junction is formed into emitter tips. Optical switching of an array consisting of nine emitters is demonstrated. The electron beam switching is synchronized with laser irradiation successfully. The emission current and its on/off ratio are approximately 40 nA and 4.

A novel Ti-based nanoglass composite with submicron-nanometer-sized hierarchical structures to modulate osteoblast behaviors.

Owing to recent progress in nanotechnology, the ability to tune the surface properties of metals has opened an avenue for creating a new generation of biomaterials. Here we demonstrate the successful development of a novel Ti-based nanoglass composite with submicron-nanometer-sized hierarchical glassy structures. A first exploratory study was performed on the application of the unique nanostructure to modulate osteoblast behaviors. Our results show that this Ti-based nanoglass composite, relative to conventional metallic glasses, exhibits significantly improved biocompatibility. In fact, a 10 times enhancement in cell proliferation has been achieved. To a great extent, this superior bioactivity (such as enhanced cell proliferation and osteogenic phenotype) is promoted by its unique hierarchical structures combining nanoglobules and submicron button-like clusters from collective packing of these nanoglobules. This nanoglass composite could be widely applicable for surface modifications by means of coating on various materials including BMGs, crystalline metals or ceramics. Therefore, our successful experimental testing of this nanostructured metallic glass may open the way to new applications in novel biomaterial design for the purpose of bone replacement.

Lithium-niobate-based surface acoustic wave oscillator directly integrated with CMOS sustaining amplifier.

In this study, a LiNbO(3)-based SAW resonator was directly integrated with a CMOS sustaining amplifier using new wafer-bonding-based integration technology. The developed integration technology has overcome the large thermal expansion mismatch between LiNbO(3) (14 to 15 ppm/K along the a-axis) and Si (2.6 ppm/K) by temporary wafer supporting and low-temperature Au-Au bonding. Two kinds of bonding, UV polymer bonding for temporary wafer supporting and Au-Au bonding following plasma surface activation, are key process technologies. A 500-MHz one-chip SAW oscillator was prototyped and evaluated. A low phase noise of -122 dBc/ Hz at 10 kHz offset and -160 dBc/Hz at 500 kHz offset was achieved.

LSI-based amperometric sensor for bio-imaging and multi-point biosensing.

We have developed an LSI-based amperometric sensor called "Bio-LSI" with 400 measurement points as a platform for electrochemical bio-imaging and multi-point biosensing. The system is comprised of a 10.4 mm × 10.4 mm CMOS sensor chip with 20 × 20 unit cells, an external circuit box, a control unit for data acquisition, and a DC power box. Each unit cell of the chip contains an operational amplifier with a switched-capacitor type I-V converter for in-pixel signal amplification. We successfully realized a wide dynamic range from ±1 pA to ±100 nA with a well-organized circuit design and operating software. In particular, in-pixel signal amplification and an original program to control the signal read-out contribute to the lower detection limit and wide detection range of Bio-LSI. The spacial resolution is 250 µm and the temporal resolution is 18-125 ms/400 points, which depends on the desired current detection range. The coefficient of variance of the current for 400 points is within 5%. We also demonstrated the real-time imaging of a biological molecule using Bio-LSI. The LSI coated with an Os-HRP film was successfully applied to the monitoring of the changes of hydrogen peroxide concentration in a flow. The Os-HRP-coated LSI was spotted with glucose oxidase and used for bioelectrochemical imaging of the glucose oxidase (GOx)-catalyzed oxidation of glucose. Bio-LSI is a promising platform for a wide range of analytical fields, including diagnostics, environmental measurements and basic biochemistry.

Micromirror with large-tilting angle using Fe-based metallic glass.

For enhancing the micromirror properties like tilting angle and stability during actuation, Fe-based metallic glass (MG) was applied for torsion bar material. A micromirror with mirror-plate diameter of 900 µm and torsion bar dimensions length 250 µm, width 30 µm and thickness 2.5 µm was chosen for the tilting angle tests, which were performed by permanent magnets and electromagnet setup. An extremely large tilting angle of over -270° was obtained from an activation test by permanent magnet that has approximately 0.2 T of magnetic strength. A large mechanical tilting angle of over -70° was obtained by applying approximately 1.1 mT to the mirror when 93 mAwas applied to solenoid setup. The large-tilting angle of the micromirror is due to the torsion bar, which was fabricated with Fe-based MG thin film that has large elastic strain limit, fracture toughness, and excellent magnetic property.

Local electrical modification of a conductivity-switching polyimide film formed by molecular layer deposition.

The electrical modification of a conductivity-switching polyimide film via molecular layer deposition (MLD) is studied for ultrahigh density data storage based on a scanning probe microscope (SPM). A PMDA-ODA (PMDA = 1, 2, 3, 5-benzenetetracarboxylic anhydride, ODA = 4, 4-oxydianiline) film as a recording medium is uniformly formed from a self-assembled monolayer on a Au surface by MLD. It is demonstrated that the conductivity of the film can be changed by applying a voltage between a SPM probe and the film. This conductivity-switching phenomenon is discussed by the molecular orbital approach and considered to be caused by the charge transfer effect or carrier trapping effect of PMDA-ODA.

Imprinted laminate wafer-level packaging for SAW ID-tags and SAW delay line sensors.

We have developed a wafer-level packaging solution for surface acoustic wave devices using imprinted dry film resist (DFR). The packaging process involves the preparation of an imprinted dry film resist that is aligned and laminated to the device wafer and requires one additional lithography step to define the package outline. Two commercial dry film solutions, SU-8 and TMMF, have been evaluated. Compared with traditional ceramic packages, no detectable RF parasitics are introduced by this packaging process. At the same time, the miniature package dimensions allow for wafer-level probing. The packaging process has the great advantage that the cavity formation does not require any sacrificial layer and no liquids, and therefore prevents contamination or stiction of the packaged device. This non-hermetic packaging process is ideal for passive antenna modules using polymer technology for low-cost SAW identification (ID)-tags or lidding in low-temperature cofired ceramic (LTCC) antenna substrates for high-performance wireless sensors. This technique is also applicable to SAW filters and duplexers for module integration in cellular phones using flip-chip mounting and hermetic overcoating.

Etch rate dependence on crystal orientation of lithium niobate.

This paper presents the etch rate of lithium niobate (LiNbO(3)) as a function of crystal orientation. Etching is a fundamental technology needed for the fabrication of new sensors, actuators, and other new devices. In this study, LiNbO(3) spheres 30 mm in diameter were etched in hydrofluoric acid and a mixture of hydrofluoric and nitric acids at different temperatures and different times. The measured data of the etched sphere shape were processed and plotted, giving etch rate diagrams over the entire spheres. Based on the etch rate data obtained, the Wulff-Jaccodine method was used to predict the etched shape of 128° Y-cut and 155° Y-cut LiNbO(3). The predicted etching profiles were compared with those obtained by experiments. A least-square polynomial fit for the data was also developed and was found to be useful in removing some of the variation in the measurements.

Quartz-crystal scanning probe microcantilevers with a silicon tip based on direct bonding of silicon and quartz.

This paper reports on the design, fabrication and characterization of cantilever-shaped quartz-crystal resonators for scanning probe microscopy (SPM) in order to operate under various environments, especially in liquids for biological applications. The cantilevers have functions of self-sensing and self-actuation using piezoelectric effects, and these properties are demonstrated experimentally. Compared to conventional SPM cantilevers, this quartz cantilever is easy to utilize as a SPM based force sensor in liquids because the self-actuating properties can lead to no spurious resonant peaks. In addition, the self-sensing properties will enable its use even in an opaque liquid and can simplify the SPM system. In this research, quartz cantilevers are fabricated on silicon using a silicon-quartz direct bonding technique, and the sharp silicon tip is integrated at the end of the cantilever as well. Additionally, the electrical Q-enhancement (active Q-control) was tested.

Electrostatic actuator probe with curved electrodes for time-of-flight scanning force microscopy.

In this study, we fabricated an electrostatic actuator probe having curved electrodes and evaluated its applicability for use in time-of-flight scanning force microscopy. In this probe, the end position of a cantilever with a tip is switched through electrostatic pull-in effect; the measurement modes can be changed between mass analysis and scanning force microscopy (SFM) modes. We achieved a large displacement of 400 microm for changing working modes. To prevent electrical shortage of the probe and curved electrodes, stoppers were formed along the curved electrodes. Because of the pull-in effect, the spring constant and resonance frequency increased through stiction of the cantilever to the stoppers. Using the fabricated probe, the SFM imaging of a sample featuring a 2-microm-pitch Au grid was demonstrated.