Noise in microelectromechanical system resonators

Microelectromechanical system (MEMS) and nanoelectromechanical system (NEMS) based resonators and filters, ranging in frequencies from kHz to GHz, have been proposed. The question of how the stabilities of such resonators scale with dimensions is examined in this paper, with emphasis on the noise characteristics. When the dimensions of a resonator become small, instabilities that are negligible in macro-scale devices become prominent. The effects of fluctuations in temperature, adsorbing/desorbing molecules, outgassing, Brownian motion, Johnson noise, drive power and self-heating, and random vibration are explored. When the device is small, the effects of fluctuations in the numbers of photons, phonons, electrons and adsorbed molecules can all affect the noise characteristics. For all but the random vibration-induced noise, reducing the dimensions increases the noise. At submicron dimensions, especially, the frequency noise due to temperature fluctuations, Johnson noise, and adsorption/desorption are likely to limit the applications of ultra-small resonators.     

Large-area, real-time imaging system for surface acoustic wavedevices

A system for imaging the particle displacement envelope of vibrational (transverse) modes of surface acoustic wave (SAW) devices is described. The modes are being imaged using a schlieren method for visualizing the acoustic power flow with a beam-expanded helium-neon (HeNe) laser. The optical arrangement uses internal reflection from within the quartz substrate to achieve high-efficiency acousto-optic diffraction of the laser light. The use of a CCD camera coupled with a frame grabber and a computer with image calculator software establishes an imaging system for large-area, real-time visualization, recording, accurate measurement, and analysis of vibrational modes of SAW devices. These methods are part of an effort to determine the relationship between acceleration sensitivity and transverse variations in the acoustic-mode shape in SAW resonators. Use of the system in imaging a 98 MHz SAW device is presented as an example     

Dependence of SAW resonator 1/f noise on device size

Experiments were conducted with eight 450-MHz surface acoustic wave (SAW) resonators which demonstrate that a resonator's 1/f noise depends approximately inversely on the active acoustic area of the device. This observation is consistent with a proposed theory that 1/f noise in acoustic resonators is caused by localized velocity or dimensional fluctuations.     

pH-dependent adsorption of Au nanoparticles on chemically modified Si3N4 MEMS devices

Microelectromechanical systems (MEMS) are devices that represent the integration of mechanical and electrical components in the micrometer regime. Self-assembled monolayers (SAMs) can be used to functionalise the surface of MEMS resonators in order to fabricate chemically specific mass sensing devices. The work carried out in this article uses atomic force microscopy (AFM) and X-ray photoemission spectroscopy (XPS) data to investigate the pH-dependent adsorption of citrate-passivated Au nanoparticles to amino-terminated Si₃N₄ surfaces. AFM, XPS and mass adsorption experiments, using ‘flap’ type resonators, show that the maximum adsorption of nanoparticles takes place at pH = 5. The mass adsorption data, obtained using amino functionalised ‘flap’ type MEMS resonators, shows maximum adsorption of the Au nanoparticles at pH = 5 which is in agreement with the AFM and XPS data, which demonstrates the potential of such a device as a pH responsive nanoparticle detector.     

Order formation of colloidal nanoparticles adsorbed on a substrate with friction

We examine the adsorption process and order formation of colloidal nanoparticles on a planar surface with friction. We perform Brownian dynamics simulations with a three-dimensional cell model in which the particle–particle and particle–substrate interactions are modeled on the DLVO theory, and examine the effects of the friction acting between the adsorbed particles and the substrate on the adsorbed structure formed on the substrate. The results obtained are as follows: when the friction is so strong that the adsorbed particles are stuck to the substrate, ordered structures never form, which seems to be quite natural. However, when the magnitude of the frictional force is moderate, an ordered structure can form even with low coverage because the frictional force aids order formation. This is because the friction counterbalances the particles’ Brownian motion, which would otherwise disturb the ordered structure. Furthermore, through a detailed examination of the distribution of the Brownian motion, it is demonstrated that an increase in the friction has a similar effect as a decrease in temperature.     

Guided shear horizontal surface acoustic wave sensors for chemical and biochemical detection in liquids.

The design and performance of guided shear horizontal surface acoustic wave (guided SH-SAW) devices on LiTaO3 substrates are investigated for high-sensitivity chemical and biochemical sensors in liquids. Despite their structural similarity to Rayleigh SAW, SH-SAWs often propagate slightly deeper within the substrate, hence preventing the implementation of high-sensitivity detectors. The device sensitivity to mass and viscoelastic loading is increased using a thin guiding layer on the device surface. Because of their relatively low shear wave velocity, various polymers including poly(methyl methacrylate) (PMMA) and cyanoethyl cellulose (cured or cross-linked) are investigated as the guiding layers to trap the acoustic energy near the sensing surface. The devices have been tested in biosensing and chemical sensing experiments. Suitable design principles for these applications are discussed with regard to wave guidance, electrical passivation of the interdigital transducers from the liquid environments, acoustic loss, and sensor signal distortion. In biosensing experiments, using near-optimal PMMA thickness of approximately 2 microm, mass sensitivity greater than 1500 Hz/(ng/mm2) is demonstrated, resulting in a minimum detection limit less than 20 pg/mm2. For chemical sensor experiments, it is found that optimal waveguide thickness must be modified to account for the chemically sensitive layer which also acts to guide the SH-SAW. A detection limit of 780 (3 x peak-to-peak noise) or 180 ppb (3 x rms noise) is estimated from the present measurements for some organic compounds in water.     

石墨烯谐振式微质量传感器研究进展

微质量传感器可以用于毒害气体,纳米颗粒的检测,被广泛用于环境检测,医疗诊断等领域。近年来,基于石墨烯材料的微质量传感器因为其大的表面积体积比,低电子噪声,及优异的吸附性被广泛研究。本文针对一类基于谐振原理的超高灵敏度石墨烯微质量传感器展开综述,重点论述了石墨烯谐振器的结构,激振拾振方案,质量敏感特性等问题,总结了关于石墨烯谐振式微质量检测的进展及挑战。     

A general method for assembling single colloidal particle lines

We have developed a general method for assembling colloidal particles into one-dimensional lines of single particle thickness. Well-spaced, parallel single particle lines can be readily deposited on a substrate from a dilute Langmuir-Blodgett particle monolayer via a stick-slip motion of the water-substrate contact line. The particle density within the lines is controllable by the particle concentration in the monolayer as well as the pulling speed of the substrate. Lines of a great variety of materials and sizes, ranging from a few nanometers to a few micrometers, have been demonstrated. Multiple depositions create complex patterns such as cross lines, even of different particles. The ability of placing nanoparticles into one-dimensional arrays enables the construction of higher hierarchical device structures. For example, using gold nanoparticle seeds, vertical single nanowire arrays of silicon can be grown replicating the pattern of single particle lines.     

Phase noise in surface-acoustic-wave filters and resonators

Measurements of the phase noise modulation imparted on UHF carriers by surface-acoustic-wave (SAW) filters and resonators have been made using an HP 3047 spectrum analyzer. Three different types of SAW phase noise were observed. One type can be explained by temperature fluctuations. It is characterized by a spectral density of phase fluctuations which decreases as 1/f(2). The predominant noise mechanism in most SAW devices has a 1/f spectral density. The source of this noise is unknown, but it appears to be associated with both acoustic propagation and transduction. In filters fabricated on lithium niobate substrates, a third noise mechanism is evidenced. This mechanism produces nonstationary noise bursts that appear to originate in the transducer region. Experiments have been carried out on substrate materials, transducer metallizations, and over acoustic path lengths. The means by which low-frequency fluctuations are mixed to the carrier frequency have been studied.     

Quantum 1/f effect in resonant biochemical piezoelectric and MEMS sensors

Piezoelectric sensors used for the detection of chemical agents and as electronic nose instruments are based on bulk and surface acoustic wave resonators. Adsorption of gas molecules on the surface of the polymer coating is detected by a reduction of the resonance frequency of the quartz disk, subject also to fundamental quantum 1/f frequency fluctuations. The quantum 1/f limit of detection is given by the quantum 1/f formula for quartz resonators. Therefore, for quantum 1/f optimization and for calculation and improvement of the fundamental sensitivity limits, we must avoid closeness of the crystal size to the phonon coherence length, which corresponds to the maximum error and minimal sensitivity situation, as shown here. Adsorbed masses below the pg range can be detected. Microelectromechanical system (MEMS) resonators have provided a possibility for the nanominiaturization of these sensors. Essential for integrated nanotechnology, these resonant silicon bars (fingers) are excited magnetically or electrically through external applied forces, since they are not piezoelectric or magnetostrictive. The application of the quantum 1/f theory to these systems is published here for the first time. It provides simple formulas that yield much lower quantum 1/f frequency fluctuations for magnetic excitation, in comparison with electrostatically driven MEMS resonators.     

Flexible ultrathin metamaterial absorber for wide frequency band,based on conductive fibers

Flexible and ultrathin wide-band metamaterial absorbers are suggested and demonstrated in the microwave-frequency range. By using resonators of different sizes and conductive fibers on metallic-pattern layer, the total thickness of metamaterial absorber is reduced to be only 1/349 with respect to the operating wavelength at 0.97 GHz. We present the absorption mechanism in terms of the impedance matching with the free space, the distributions of surface current and the three-dimensional distributions for power loss. In simulation, the absorption was over 97% at 0.97–6.12 GHz, and the corresponding experimental absorption band over 97% was 0.87–6.11 GHz. Furthermore, the dielectric substrate of metamaterial absorbers was replaced with flexible substrate in order to have the flexibility and the broadband absorption properties. The absorption band is expanded and the high-absorption performance maintains at the same time. The total thickness of metamaterial absorber comes to be only 1/5194 of the operating wavelength at 0.75 GHz. Our work is expected to contribute to the flexible microwave/electronic devices in the near future.     

碳包覆氧化亚钴纳米颗粒的制备与性能研究

采用直流电弧放电等离子体技术成功制备了碳包覆氧化亚钴纳米颗粒,并对样品的形貌、晶体结构、粒度、比表面积和孔结构采用高分辨透射电子显微镜(HRTEM)、X射线衍射(XRD)、X射线能量色散光谱(XEDS)、拉曼散射光谱(Raman)和N_2吸-脱附等测试手段进行了分析。HRTEM表明该方法制备的碳包覆氧化亚钴纳米颗粒具有典型的核壳结构,颗粒形貌主要为球形或椭球结构,粒度均匀,分散性良好,粒径分布在20~60nm,平均粒径为40nm,外壳碳层的厚度为5nm。XRD证明样品的内核为面心立方结构的氧化亚钴纳米颗粒,外壳为碳层。XEDS图谱表明样品中主要存在Co、O和C元素的特征峰。Raman光谱说明样品中碳外包覆层的石墨化程度较低,发生了红移现象。样品的N_2吸附-脱附等温曲线属Ⅳ型,BET比表面积为33m~2/g,BJH脱附累积总孔孔容和脱附平均孔径分别为0.078cm~3/g和11nm。当量粒径为43nm,与TEM和XRD测得的结果基本一致。     

Monolithic miniaturized quartz microbalance array and its application to chemical sensor systems for liquids

We report on the design, fabrication, and application of novel monolithic miniaturized quartz microbalance (QMB) arrays. Up until now, almost all reported resonator arrays (often designated as "electronic noses" or "electronic tongues", respectively, dependent on their application) are assembled from single QMBs. We fabricate arrays with up to 36 QMBs on a single AT-cut quartz blank. Mass sensitive devices based on AT-cut quartz resonators are suitable as (bio)chemical sensors. A frequency shift caused by mass accumulation on the sensor surface increases theoretically with f/sup 2/, hence the detection limits for the application as chemical sensors should be decreased with increasing frequency. Since the quality factor Q of a quartz crystal decreases with f, the frequency stability is reduced, thus limiting mass sensitivity. The mass sensitivity of resonators with different resonant frequencies was examined by means of electrochemical copper deposition on their surface. Subsequently, the manufactured resonators were coated with different layers (polystyrene, amyl-calix[8]arene, /spl beta/-cyclodextrine). In order to examine the applicability of such coatings as sensitive layers, their sensitivities to toluene in water were investigated. Moreover, arrays with up to four different resonant frequencies on one chip were fabricated for comparing the resonator behavior of the same coating at different frequencies. In another test setup, different layers were sprayed onto an array of microbalances having all the same resonant frequency. This allowed for comparing the different coating behavior under equivalent test conditions. Arrays were tested for viscosity measurement to find an optimum resonant frequency.     

STW gas sensors using plasma-polymerized allylamine

Gas sensors generally consist of two major components: a gas recognition element which provides the specificity and selectivity of the measurement and a physical transducer which translates the gas absorption or desorption event into electronic signal. In this paper, plasma polymerized allylamine (PPAa) film is used as a gas recognition element and a surface transverse wave (STW) device is used as a physical transducer. It is confirmed that STW sensor devices coated with PPAa films provide high sensitivity for moisture. The STW sensor device with a 63 nm PPAa film provided twenty four times higher sensitivity than that of a non-coated STW device. In addition, the chemical structure of PPAa films is characterized by the FT-IR and the contact angle measurement.     

Ionic ligand mediated electrochemical charging of gold nanoparticle assemblies

We demonstrate that ionic surface functionalization is well-suited for controlling the electrochemical charging of nanoparticle assemblies. Gold nanoparticles approximately 2 nm in diameter were functionalized with between 0 and approximately 3.3 cationic thiols per particle and the coupled motion of ions and electrons during redox cycling (charging) was followed in situ using an electrochemical quartz-crystal microbalance. When the electrochemistry is performed using a polycation electrolyte too large to penetrate the nanoparticle film, the degree of reduction possible was found to be dictated by the number of cationic ligands on the particle surface available for charge compensation. This route to reduced particles might be useful for electronic device fabrication, since the negative electronic charge is precisely compensated by immobile cationic ligands.     

Goosebumps‐Inspired Microgel Patterns with Switchable Adhesion and Friction

A substrate mimicking the surface topography and temperature sensitivity of skin goosebumps is fabricated. Close‐packed arrays of thermoresponsive microgel particles undergo topographical changes in response to temperature changes between 25 and 37 °C, resembling the goosebump structure that human skin develops in response to temperature changes or other circumstances. Specifically, positively charged poly[2‐(methacryloyloxy)ethyltrimethylammonium chloride] (PMETAC) brushes serve as an anchoring substrate for negatively charged poly(NIPAm‐co‐AA) microgels. The packing density and particle morphology can be tuned by brush layer thickness and pH of the microgel suspension. For brush layer thickness below 50 nm, particle monolayers are observed, with slightly flattened particle morphology at pH 3 and highly collapsed particles at pH above 7. Polymer brush films with thickness above 50 nm lead to the formation of particle multilayers. The temperature responsiveness of the monolayer assemblies allows reversible changes in the film morphology, which in turn affects underwater adhesion and friction at 25 and 37 °C. These results are promising for the design of new functional materials and may also serve as a model for biological structures and processes.     

An atomic-resolution nanomechanical mass sensor

Mechanical resonators are widely used as inertial balances to detect small quantities of adsorbed mass through shifts in oscillation frequency. Advances in lithography and materials synthesis have enabled the fabrication of nanoscale mechanical resonators, which have been operated as precision force, position and mass sensors. Here we demonstrate a room-temperature, carbon-nanotube-based nanomechanical resonator with atomic mass resolution. This device is essentially a mass spectrometer with a mass sensitivity of 1.3 x 10(-25) kg Hz(-1/2) or, equivalently, 0.40 gold atoms Hz(-1/2). Using this extreme mass sensitivity, we observe atomic mass shot noise, which is analogous to the electronic shot noise measured in many semiconductor experiments. Unlike traditional mass spectrometers, nanomechanical mass spectrometers do not require the potentially destructive ionization of the test sample, are more sensitive to large molecules, and could eventually be incorporated on a chip.     

High Q metal strip SSBW resonators using a SAW design

An experimental study of metal strip surface skimming bulk wave (SSBW) resonators using a surface acoustic wave (SAW) design is presented. Characteristics of SSBW and SAW resonators fabricated with the same photolithographic mask are compared and discussed. High Q low-loss SSBW resonators are achieved using a conventional two-port SAW resonator design and taking special care of the distance L between both interdigital transducers, the metal thickness h/lambda (lambda=acoustic wavelength) and the finger-to-gap ratio. Best overall performance of the SSBW devices in this study is achieved at L=nlambda/2-lambda/4 (compared with L=nlambda/2-lambda/8 for SAW resonators), h /lambda=1.6% (compared with 2% for SAW), and finger-to-gap ratio close to 1. The best device fabricated shows an unloaded Q of 5820 and an insertion loss of 7.8 dB at 766 MHz. The SSBW resonant frequency shows a stronger dependence on the metal thickness than the SAW one. This problem, however, is readily solved by frequency trimming using a CF(4) plasma etching technique. SSBW resonator can be trimmed by 0.2% down in frequency (compared with 0.05% for SAW) without affecting their performance.     

Full-wafer fabrication by nanostencil lithography of micro/nanomechanical mass sensors monolithically integrated with CMOS

Wafer-scale nanostencil lithography (nSL) is used to define several types of silicon mechanical resonators, whose dimensions range from 20?μm down to 200?nm, monolithically integrated with CMOS circuits. We demonstrate the simultaneous patterning by nSL of ～2000 nanodevices per wafer by post-processing standard CMOS substrates using one single metal evaporation, pattern transfer to silicon and subsequent etch of the sacrificial layer. Resonance frequencies in the MHz range were measured in air and vacuum. As proof-of-concept towards an application as high performance sensors, CMOS integrated nano/micromechanical resonators are successfully implemented as ultra-sensitive areal mass sensors. These devices demonstrate the ability to monitor the deposition of gold layers whose average thickness is smaller than a monolayer. Their areal mass sensitivity is in the range of 10(-11)?g?cm(-2)?Hz(-1), and their thickness resolution corresponds to approximately a thousandth of a monolayer.     

碳包覆NiO纳米颗粒的制备与性能

采用直流电弧放电等离子体技术成功制备了碳包覆NiO(NiO@C)纳米颗粒,并对样品的形貌、晶体结构、粒度、比表面积和孔结构采用高分辨透射电子显微镜、X射线衍射、X射线能量色散分析谱仪、拉曼散射光谱和N_2吸-脱附等测试手段进行了分析。实验结果表明:直流电弧等离子体技术制备的NiO@C纳米颗粒具有典型的核壳结构,内核为面心立方结构的NiO纳米颗粒,外壳为碳层。颗粒形貌主要为立方体结构,粒度均匀,分散性良好,粒径分布在30~70nm范围,平均粒径为50nm,外壳碳层的厚度为5nm。NiO@C纳米颗粒BET比表面积为28m~2/g,等效直径为46nm,与TEM和XRD测得的结果基本一致。Raman光谱说明样品中碳包覆层的石墨化程度较低,发生了红移现象。