Microstructured silicon membrane with soft suspension beams for a high performance MEMS microspeaker

This study presents a novel approach to MEMS microspeakers design aiming to tackle two main drawbacks of conventional microspeakers: their poor sound quality and their weak efficiency. For this purpose, an acoustic emissive surface based on a very light but very stiff structured silicon membrane was designed and microfabricated. This architecture, for which the membrane undesirable vibration modes were reduced to only three within the microspeaker bandwidth, is promising to let the microspeaker produce high sound quality from 300?Hz to 20?kHz. This silicon membrane is suspended by a whole set of silicon springs designed to enable out-of-plane displacements as large as 300?μm. Different geometries of springs were considered and the material maximum stress was analyzed in each case by finite element modeling. The proposed structure promises an efficiency of 10^?4, that is to say ten times higher than that of conventional microspeakers.     

Annealing temperature effect on structural,optical, morphological and electrical properties of CdS/Si(100) nanostructures

CdS nanostructures have grown on p-type silicon (Si) (100) substrates using sol–gel method. The crystalline quality, surface morphology, optical and electrical properties of the deposited CdS nanostructures have been characterized and analyzed using atomic force microscopy, scanning electron microscopy, X-ray diffraction, thermogravimetric analysis, differential thermal analysis, UV–vis spectroscopy and electrical characterization, respectively. The effect of annealing temperature in the range 200–600 °C on the structural, morphological, optical and electrical properties has been elaborated. The XRD analysis shows that the crystalline quality can be improved by increasing the temperature to 400 °C, but further increase to 600 °C leads to degradation of crystalline quality. The bulk modulus is calculated and showed good agreement with experimental and theoretical results. The optical properties of absorption, reflection, energy band gap and extinction coefficient are obtained by UV–vis spectroscopy. The calculated refractive index and optical dielectric constant have shown good agreement with other results. The electrical and thermal properties are studied for antireflection coating applications.     

Design,fabrication and testing of a piezoelectric energy microgenerator

This article presents the design, fabrication and characterization of a micromachined energy harvester utilizing aluminium nitride (AlN) as a piezoelectric thin film material for energy conversion of random vibrational excitations. The harvester was designed and fabricated using silicon micromachining technology where AlN is sandwiched between two electrodes on top of a silicon cantilever beam which is terminated by a silicon seismic mass. The harvester generates electric power when subjected to mechanical vibrations. The generated electrical response of the device was experimentally evaluated at various acceleration levels. A maximum power of 34.78 μW was obtained for the device with a seismic mass of 5.6 × 5.6 mm² at an acceleration value of 2 g. Various fabricated devices were tested and evaluated in terms of the generated electrical power as well as the resonant frequency.     

Microelectromechanical systems vibration powered electromagnetic generator for wireless sensor applications

This paper presents a silicon microgenerator, fabricated using standard silicon micromachining techniques, which converts external ambient vibrations into electrical energy. Power is generated by an electromagnetic transduction mechanism with static magnets positioned on either side of a moving coil, which is located on a silicon structure designed to resonate laterally in the plane of the chip. The volume of this device is approximately 100 mm³. ANSYS finite element analysis (FEA) has been used to determine the optimum geometry for the microgenerator. Electromagnetic FEA simulations using Ansoft’s Maxwell 3D software have been performed to determine the voltage generated from a single beam generator design. The predicted voltage levels of 0.7–4.15 V can be generated for a two-pole arrangement by tuning the damping factor to achieve maximum displacement for a given input excitation. Experimental results from the microgenerator demonstrate a maximum power output of 104 nW for 0.4g (g=9.81 m s^?1) input acceleration at 1.615 kHz. Other frequencies can be achieved by employing different geometries or materials.     

Design and fabrication of microtunnel and Si-diaphragm for ZnO based MEMS acoustic sensor for high SPL and low frequency application

The purpose of the paper is to design and fabricate a ZnO-based MEMS acoustic sensor for higher sound pressure level (SPL) measurement in the range of 120–200 dB and low frequency infrasonic wave detection. The thickness of silicon diaphragm was optimized for higher SPL using MEMS-CAD-Tool COVENTORWARE. The microtunnel which relates the cavity to the atmosphere was designed and simulated analytically for low cut-off frequency of the sensor in infrasonic band. The resonance frequency of the sensor was obtained using modal analysis. The sensitivity of the sensor was also estimated using COVENTORWARE. The optimized Si-diaphragm thickness for the intended SPL range was determined and found to be 50 μm. The lower cut-off frequency of the sensor for a 10 μm-deep microtunnel was found to be 0.094 Hz. The resonance frequency of the sensor was obtained using modal analysis and found to be 78.9 kHz. Based on simulation results, the MEMS acoustic sensor with 10 μm-deep microtunnel was fabricated. The optimum sensitivity of sensor was calculated using simulated results and found to be 116.4 μVolt/Pa. The lower cut-off frequency of the sensor can be utilized to detect low frequency sounds. The high SPL sensing capability of the device up to 200 dB facilitates detection of high sound pressure level in launch vehicles, rocket motors and weapons’ discharge applications.     

Submicron-scale surface acoustic wave resonators fabricated by high aspect ratio X-ray lithography and aluminum lift-off

A submicron-scale surface acoustic wave (SAW) resonator fabricated by high-aspect-ratio X-ray lithography (XRL) and metal lift-off that operates at microwave frequencies is presented. We demonstrate that XRL is especially well suited for SAW device templating, as long submicron-scale interdigitated transducer structures can be batch patterned with excellent structure quality. 0.4–2.0 μm thick PMMA layers were structured by X-ray lithography shadow projection using silicon nitride-based X-ray masks. Structures with a critical lateral feature size of down to 200–700 nm were processed. The polymer structures served as templates in a subsequent aluminum lift-off process. The metal electrodes were successfully tested as SAW resonators for high frequency applications, e.g. around 1.3 GHz, using calibrated 1-port RF wafer probing measurements. Compared with standard fabrication techniques, the high structure quality of submicron-scale polymer templates made of unusually thick PMMA layers offers additional possibilities to fabricate thicker metal transducers.     

Enhancement of metal coil quality on selective p-silicon based planar electromagnetic coil by thermal annealing process

In this paper we report an optimization of metal quality of planar MEMS electromagnetic coil through thermal annealing process. The study aims to see the effects of annealing process on the quality of metal layer deposited on localized p-type silicon regions. Two annealing process parameters namely isothermal (annealing under time variations in constant temperature) and isochronal (annealing under temperature variations at constant time) were performed on metal contact on highly doped Si substrate and characterized using transfer length method method by measuring the specific contact resistance ρ _C of the metal traces. The measurement results showed that the annealing process have significant influence on physical and electrical characteristics of the metal layer. Analysis showed that the quality of metal layer was significantly improved through the annealing process after treatment at temperature variations between 425–550 °C. An optimum annealing at 525 °C for 15 min was observed and the contact resistance can be reduced significantly up to 400 %. The results also showed that the surface roughness improves while metal contact resistance decreases 40 times when the metal is annealed for more than 10 min. The planar coil structure was designed to reduce the device density of a compact magnetic micro-sensor system.     

Fabrication of an hermetically packaged silicon resonator on LTCC substrate

The design, fabrication and packaging process of silicon resonators capable of the integration of LSI (Large Scale Integration) have been developed on the basis of packaging technology using an LTCC (Low Temperature Co-fired Ceramic) substrate. The structures of silicon resonators are defined by deep reactive ion etching (DRIE) on a silicon on insulator (SOI) wafer and then transferred onto the LTCC substrate and hermetically sealed by anodic bonding technique. The measured resonant frequency of a micromechanical bulk acoustic mode silicon resonator after packaging at 0.02 Pa is 20.24 MHz with a quality factor of 50,600.     

Assessment of the subjective perception of music quality using psychoacoustic approach

It is generally agreed that sound quality is one of the most difficult to measure characteristics of an electroacoustic products such as an earphone or a loudspeaker. A conventional approach used to measure people's subjective perception of these sound reproduction products is to conduct a jury test on a group of experiment participants; however, jury tests require considerable costs, including those of effort and time. As development speed and cost become strategic competitive dimensions, electroacoustic industry needs a more efficient approach to assess their newly developed products for subjective sound quality. This study developed and validated a quantitative model, the tonal harmony level (THL), that can effectively predict people's subjective perceptions of music quality. Participants' subjective perception and preference was measured for four music genres by listening to short music excerpts (8 s) in both ordinal and interval scales. The purpose of using two scales is to examine the consistency between subjective perceptions and to determine the robustness of the subjective measurements. The experimental results were very stable over the two assessment procedures, and the objective THL measure is highly correlated to subjective preference. The analysis suggests that the construction of subjective music quality prediction models should also consider music genre. Among four types of music, musical solos consisted of human vocals accompanied by a few instruments has a distinct pattern from the other three types. Thus, while R² value of the overall regression model is 0.707, the R² values are 0.955 and 0.901 when four music genres are categorized into two groups according to their patterns. When efficiency and accuracy were simultaneously considered, according to the results of this study, the approach of two-group categorization can be adopted.     

Microstructuring of non-conductive silicon carbide by electrical discharge machining

The electrical discharge machining process is an established process for machining materials regardless of their mechanical properties. Thus this process is especially attractive for materials which are hard to machine with conventional machining methods. The only requirement a material has to fulfil is having a certain electrical conductivity. Ceramic materials, (e.g. zirconia, silicon nitride or silicon carbide) exhibit excellent mechanical properties but are mostly electrically non-conductive. This can be compensated by an applied, electrically conductive assisting electrode. With this modification, the electrical discharge machining of non-conductive ceramic material is enabled. In this study the micro electrical discharge machining of non-conductive sintered silicon carbide is investigated. The drilling process shows instabilities due to the excessive generation of carbon products. A stabilisation of the process up to the maximum depth of 420 μm is realized by two approaches: adapting process parameters and adapting the tool electrode geometry. An analysis of the amount of infeed used in a milling process shows that an infeed of 15 μm has the best material removal rate to tool wear rate ratio. A maximum material removal rate of 3.58 × 10^?3 mm³/min is achieved. Detached microstructures with an aspect ratio of 30 are machined. A conducted surface analysis indicates that the present removal mechanism is thermally induced spalling. Furthermore no heat affected zone is present in the machined near-surface area.     

Microelectromechanical systems vibration powered electromagnetic generator for wireless sensor applications

This paper presents a silicon microgenerator, fabricated using standard silicon micromachining techniques, which converts external ambient vibrations into electrical energy. Power is generated by an electromagnetic transduction mechanism with static magnets positioned on either side of a moving coil, which is located on a silicon structure designed to resonate laterally in the plane of the chip. The volume of this device is approximately 100 mm³. ANSYS finite element analysis (FEA) has been used to determine the optimum geometry for the microgenerator. Electromagnetic FEA simulations using Ansoft’s Maxwell 3D software have been performed to determine the voltage generated from a single beam generator design. The predicted voltage levels of 0.7–4.15 V can be generated for a two-pole arrangement by tuning the damping factor to achieve maximum displacement for a given input excitation. Experimental results from the microgenerator demonstrate a maximum power output of 104 nW for 0.4g (g=9.81 m s⁻¹) input acceleration at 1.615 kHz. Other frequencies can be achieved by employing different geometries or materials.

     

On investigation of vibro-acoustics of FDB spindle motors for hard disk drives

The present work investigates vibro-acoustic behaviors of the fluid dynamic bearing (FDB) spindle motors for hard disk drives (HDD) through the sound spectra and the frequency response functions (FRF) of the motor structure. The quantitative evidence on the significance of the acoustic noise originated from the electromagnetic source is deduced from the sound spectra that were measured in two distinct cases of the spinning motor: in the normal operation and at the moment immediately after the power supply was disconnected. It is found that the effect of electromagnetic noise source is more dominant than the combined effect of the mechanical and aerodynamic sources. In addition, it is identified that, within the audible range of frequency, the frequency range of 13.4–20 kHz deems important to the noise problem as it is the main contributor to the acoustic noise for the FDB spindle motors. Moreover, the structural resonances that can be identified via the FRF are found to play an important role in the noise emitted by the motors. The concurrence of resonance and excitation frequencies clearly intensifies the sound spectrum, resulting in high discrete peaks, hence higher decibel level.     

Development and characterization of a microthermoelectric generator with plated copper/constantan thermocouples

This work reports the development and the characterization of a microthermoelectric generator (μTEG) based on planar technology using electrochemically deposited constantan and copper thermocouples on a micro machined silicon substrate with a SiO₂/Si₃N₄/SiO₂ thermally insulating membrane to create a thermal gradient. The μTEG has been designed and optimized by finite element simulation in order to exploit the different thermal conductivity of silicon and membrane in order to obtain the maximum temperature difference on the planar surface between the hot and cold junctions of the thermocouples. The temperature difference was dependent on the nitrogen (N₂) flow velocity applied to the upper part of the device. The fabricated thermoelectric generator presented maximum output voltage and power of 118 mV/cm² and of 1.1 μW/cm², respectively, for a device with 180 thermocouples, 3 kΩ of internal resistance, and under a N₂ flow velocity of 6 m/s. The maximum efficiency (performance) was 2 × 10^?3 μW/cm² K².     

Frequency reconfigurable RF circuits using photoconducting switches

Designs for a frequency switchable dual‐band branch‐line coupler and a reconfigurable S‐band power amplifier input matching network with photoconducting switches are presented. Frequency switching is achieved by increasing the power of the laser applied to the highly resistive silicon wafer and changing the properties of silicon under optical illumination. The advantages of this approach are high‐speed switching, electromagnetic transparency (no interference), and thermal and electrical isolation between the device and the control circuit. A branch‐line coupler frequency shift of 35% and 10% has been achieved from all switches off to all switches on in lower (900 MHz) and upper (1800 MHz) frequency bands, respectively. Frequency switchable class AB power amplifier with silicon switch in the input matching circuit has obtained the frequency tuning range of 2.5–3.5 GHz with no significant loss in efficiency and linearity. © 2009 Wiley Periodicals, Inc. Int J RF and Microwave CAE, 2010.     

An improved bulk acoustic waves chip based on a PDMS bonding layer for high-efficient particle enrichment

In this work, we developed a feasible way to package bulk acoustic waves chip with sandwich structure by inserting a polydimethylsiloxane (PDMS) layer as the adhesive between cover glass and silicon substrate. After spin-coating and curing process, a PDMS layer was formed on one side of the cover glass and then bonded to the silicon substrate with microchannels by oxygen plasma treating. Both simulation and experiment showed that the chip was not leaking and the acoustic waves produced by the piezoelectric transducer could be propagated through the PDMS layer. Finally, a standing wave field was formed in the microchannels. Compared with traditional chip bonded by anodic bonding, simulation results showed that this packaging method did decrease the acoustic pressure in the channel, but the reduction was acceptable. After optimizing the experimental parameters, we successfully aggregated 15-μm silica spheres under a very low input power (21 dBm) at a flow velocity of 1 ml/h, and the enrichment efficiency of silica spheres was greater than 97%.     

Piezoelectric cantilever microphone and microspeaker

A micromachined piezoelectric cantilever transducer, which works both as a microphone and as a microspeaker, has been fabricated and tested. The 2000×2000×4.5 μm³ cantilever has a zinc oxide (ZnO) piezoelectric thin film on a supporting layer of low-pressure chemical-vapor-deposited (LPCVD) low-stress silicon nitride. A highlight of the fabrication process, which may also be relevant for other micromachined structures, is the technique for producing a flat, multilayer cantilever. The measured microphone sensitivity is fairly constant at 3 mV/μbar in the low frequency range and rises to 20 mV/μbar at the lowest resonant frequency of 890 Hz. The 3 mV/μbar sensitivity is the highest reported to date for a microphone with a micromachined diaphragm. When measured into a 2 cm³ coupler with 4 V(zero-peak) drive, the microspeaker output sound pressure level (SPL) is 75 dB at 890 Hz. It increases to approximately 100 dB SPL at 4.8 kHz with 6 V(zero-peak) drive. The measured microphone frequency response agrees well with the results of an ABAQUS simulation     

Characterization of out-of-plane cone metal microneedles and the function of transdermal delivery

The key issue in the research of microneedles is how to fabricate microneedles with low cost and good quality. This paper presents a process for fabrication of cone out-of-plane Ni microneedles and characterizes their properties. The fabrication process consisted of inclined rotational MASK and wafer exposure, fine pattern transfer of polydimethylsiloxane (PDMS) and electroplating. The efficiency of transdermal delivery of baicalin, as well as related mechanical properties, are evaluated using rat skin pretreated by a 10 × 10 microneedle array. The fracture strength of the microneedle is 355 MPa. The cumulative permeability rate improves approximately 100 % due to the effect of the microneedle. The method presented in this paper offers the potential for mass production and wide choice of needle material.     

Design and modeling of a novel microsensor to detect magnetic fields in two orthogonal directions

In this paper, we present the design and modeling of the electrical–mechanical behavior of a novel microsensor to detect magnetic fields in two orthogonal directions (2D). This microsensor uses a simple silicon resonant structure and a Wheatstone bridge with small p-type piezoresistors (10 × 4 × 1 μm) to improve the microsensor resolution. The resonant structure has two double-clamped silicon beams (1000 × 28 × 5 μm) and an aluminum loop (1 μm thickness). The microsensor design allows important advantages such as small size, compact structure, easy operation and signal processing, and high resolution. In addition, the microsensor design is suitable to fabricate using silicon on insulator (SOI) wafers in a standard bulk micromachining process. An analytical model is developed to predict the first bending resonant frequency of the microsensor structure using Macaulay and Rayleigh methods, as well as the Euler–Bernoulli beam theory. Air and intrinsic damping sources of the microsensor structure are considered for its electrical–mechanical response. The mechanical behavior of the microsensor is studied using finite element models (FEMs). For 10 mA of root mean square (RMS) excitation current and 10 Pa air pressure, this microsensor has a linear electrical response, a fundamental bending resonant frequency of 52,163 Hz, and a high theoretical resolution of 160 pT.     

A water-immersible 2-axis scanning mirror microsystem for ultrasound andha photoacoustic microscopic imaging applications

Fast scanning is highly desired for both ultrasound and photoacoustic microscopic imaging, whereas the liquid environment required for acoustic propagation limits the usage of traditional microelectromechanical systems (MEMS) scanning mirrors. Here, a new water-immersible scanning mirror microsystem has been designed, fabricated and tested. To achieve reliable underwater scanning, flexible polymer torsion hinges fabricated by laser micromachining were used to support the reflective silicon mirror plate. Two efficient electromagnetic microactuators consisting of compact RF choke inductors and high-strength neodymium magnet disc were constructed to drive the silicon mirror plate around a fast axis and a slow axis. The performance of this water-immersible scanning mirror microsystem in both air and water were tested using the laser tracing method. For the fast axis, the resonance frequency reached 224 Hz in air and 164 Hz in water, respectively. The scanning angles in both air and water under ±16 V DC driving were ±12°. The scanning angles in air and water under ±10 V AC driving (at the resonance frequencies) were ±13.6° and ±10°. For the slow axis, the resonance frequency reached 55 Hz in air and 38 Hz in water, respectively. The scanning angles in both air and water under ±10 V DC driving were ±6.5°. The scanning angles in air and water under ±10 V AC driving (at the resonance frequencies) were ±8.5° and ±6°. The feasibility of using such a water-immersible scanning mirror microsystem for scanning ultrasound microscopic imaging has been demonstrated with a 25-MHz ultrasound pulse/echo system and a target consisting of three optical fibers.     

A general nonlinear Mason model and its application to piezoelectric resonators

The one‐dimensional electroacoustic behavior of a uniform piezoelectric plate may be modeled by electrical equivalent circuits such as Mason or KLM models. Such models yield mathematically exact solutions to the governing equations, i.e., a set of constitutive equations and the differential equations of electrostatics and mechanics. In this article, we present an extension of the Mason equivalent circuit to the case of arbitrary nonlinear constitutive equations by the use of telegrapher's equations in a mathematically exact way. The resulting model consists of the original Mason circuit, distributed nonlinear voltage sources along its acoustic arm, and a nonlinear voltage source at the electrical terminal. We also show the proof that the circuit meets all the governing equations, and comparison of simulated results and harmonic measurements of FBAR devices. © 2011 Wiley Periodicals, Inc. Int J RF and Microwave CAE, 2011.