Quartz Crystal Tuning Fork Research Articles

Size-Effect-Based Dimension Compensations in Wet Etching for Micromachined Quartz Crystal Microstructures.

Microfabrication technology with quartz crystals is gaining importance as the miniaturization of quartz MEMS devices is essential to ensure the development of portable and wearable electronics. However, until now, there have been no reports of dimension compensation for quartz device fabrication. Therefore, this paper studied the wet etching process of Z-cut quartz crystal substrates for making deep trench patterns using Au/Cr metal hard masks and proposed the first quartz fabrication dimension compensation strategy. The size effect of various sizes of hard mask patterns on the undercut developed in wet etching was experimentally investigated. Quartz wafers masked with initial vias ranging from 3 μm to 80 μm in width were etched in a buffered oxide etch solution (BOE, HF:NH4F = 3:2) at 80 °C for prolonged etching (>95 min). It was found that a larger hard mask width resulted in a smaller undercut, and a 30 μm difference in hard mask width would result in a 17.2% increase in undercut. In particular, the undercuts were mainly formed in the first 5 min of etching with a relatively high etching rate of 0.7 μm/min (max). Then, the etching rate decreased rapidly to 27%. Furthermore, based on the etching width compensation and etching position compensation, new solutions were proposed for quartz crystal device fabrication. And these two kinds of compensation solutions were used in the fabrication of an ultra-small quartz crystal tuning fork with a resonant frequency of 32.768 kHz. With these approaches, the actual etched size of critical parts of the device only deviated from the designed size by 0.7%. And the pattern position symmetry of the secondary lithography etching process was improved by 96.3% compared to the uncompensated one. It demonstrated significant potential for improving the fabrication accuracy of quartz crystal devices.

Micro quartz crystal tuning fork based photothermal spectroscopy for trace gas detection

This paper describes a micro-quartz crystal tuning fork (M-QCTF) hermetically sealed in a vacuum environment for the development of low-cost, high-sensitivity photothermal detectors. Compared with conventional QCTF-based photothermal detectors exposed to air, the M-QCTF detector with the approximate stable characteristic resonance frequency (∼32763 Hz) has a small size and high quality factor of 19840. Utilizing the M-QCTF detector and direct absorption spectroscopy (DAS) technology, a gas detection system has been successfully implemented. NO2 was selected as the target gas for evaluating the performance of the M-QCTF based gas sensor. The results indicate a strong linear relationship between the normalized absorbance amplitude of the M-QCTF and the gas concentration, achieving an optimal measurement precision of 117 ppb with an integration time of 230 s. Furthermore, the vacuum packaging structure of the M-QCTF provides advantages such as easy system integration, low cost, and robust anti-interference capability.

Design and implementation of a temperature-insensitive heterogeneous resonant magnetic field sensor

A heterogeneous resonant magnetic field sensor is developed by using a FeGa plate, a quartz crystal double-ended tuning fork (DETF) resonator and a sapphire crystal plate. The double ends of the DETF are bonded to the ends of the magnetostrictive FeGa thin plate through two spacers, forming a resonant magnetic sensitive element. The C-cut sapphire crystal plate as a temperature compensation layer is bonded to the bottom of the magnetic sensitive element. When the temperature fluctuates, the bending deformation caused by the heterogeneous sensitive structure eliminates the thermal stress caused by the thermal expansion effect, thereby reducing the temperature drift of the sensor. The impact of different thicknesses of the sapphire substrate on reducing temperature drift in the sensor is analysed through theoretical calculation and finite element simulation. A sensor prototype was fabricated with a sapphire substrate thickness of 0.45 mm. The experimental results demonstrate that the temperature coefficient of frequency (TCF) of the sensor is reduced from 8.73 Hz/°C to 0.47 Hz/°C in the 0 °C to 60 °C range, and the sensitivity and resolution in the linear region of the sensor are 5.63 Hz/Oe and 42μOe (4.2nT), respectively.

Quartz tuning fork based photoacoustic spectroscopy and sensing

Quartz crystal tuning forks (QTFs) resonators are key components for timing and frequency measurements, due to their high stability, high-quality factors and low power consumptions. Thanks to their piezoelectric properties, QTFs are employed as sensitive element in many fields and systems such as atomic force microscopy, near-field and microwave microscopy, and mass/viscosity sensor. Since 2002, QTFs are also widely used as a sharply resonant acoustic transducer to detect weak photoacoustic excitation for Quartz-Enhanced Photoacoustic Spectroscopy (QEPAS). Among most sensitive optical techniques, QEPAS was demonstrated as the leading-edge technology for addressing these application requirements, providing also modularity, ruggedness, portability and allowing the use of extremely small volumes. QEPAS technique does not require an optical detector, it is wavelength independent, it is immune to environmental noise and can operate in a wide range of temperature and pressure. These factors, together with its proven reliability and ruggedness, represent the main distinct advantages with respect to other laser-based techniques for environmental monitoring and in situ detection. Starting from the basic physical principles governing the QTF physics, I will review the main results achieved by exploiting custom QTFs for QEPAS sensing and as photodetector in LITES setup, with a main focus on real-world applications.

An AlN Piezoelectric Micromachined Ultrasonic Transducer-Based Liquid Density Sensor

As an important physical parameter of liquids, the liquid density plays an important role in many applications. As a result, the development of liquid density sensors is critical. However, current liquid density sensors have drawbacks, such as large size of conventional liquid density sensors, low quality factor of microcantilever beam structures, low sensitivity of quartz crystal tuning forks, and high operating frequency requirement of surface-acoustic-wave Love-mode resonators. In this article, we report an aluminum nitride (AlN) piezoelectric micromachined ultrasonic transducer (PMUT)-based liquid density sensor to mitigate the aforementioned drawbacks of traditional liquid density sensors. The PMUT sensor utilizes CMOS-compatible AlN as the piezoelectric material. This sensor operates by measuring the change in resonant frequency of the PMUT to calculate the liquid density. In an experiment, the resonance of the PMUT sensor is measured by a digital holographic microscope (DHM), which allows direct measurement of the PMUT surface displacement, providing intuitive vibration amplitude and mode. Moreover, the DHM-based optical measurement approach has a higher sensitivity than the conventional electrical impedance-based measurement approach, allowing measurement of weak PMUT vibrations in dense liquids. The quality factor ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${Q}$ </tex-math></inline-formula> ) value of the PMUT sensor is measured to be 49.38 in water. A high sensitivity of 12.71 kHz/( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\text{g}\cdot $ </tex-math></inline-formula> cm <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$^{-{3}}$ </tex-math></inline-formula> ) was determined for this sensor by measuring the resonant frequencies immersed in different glycerol aqueous solutions. We measured 12 PMUTs and the maximum deviation of resonant frequency between different PMUTs is 9.85%. For the first time, the resolution of the PMUT density sensor is determined to be <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$5.9\times 10^{-{4}}\,\,\text{g}\cdot $ </tex-math></inline-formula> cm <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$^{-{3}}$ </tex-math></inline-formula> .

External-Cavity Quantum Cascade Laser-Based Gas Sensor for Sulfur Hexafluoride Detection

The external-cavity quantum cascade laser (ECQCL) is an ideal mid-infrared (MIR) spectral light source for determining large molecular-absorption spectral features with broad transition bands. For this paper, a gas sensor system was developed using a broadband tunable ECQCL and a direct absorption spectroscopy detection scheme with a short path absorption cell of 29.6 cm. For spectral signal detection, a cheap and miniaturized quartz crystal tuning fork- (QCTF) based light detector was used for laser signal detection. The characteristics of the QCTF detector were theoretically simulated and experimentally observed. To demonstrate this sensing technique, sulfur hexafluoride (SF6) was selected as the analyte, which can be used as an effective indicator to identify fault-types of gas-insulated electrical equipment. Preliminary results indicated that a good agreement was obtained between experimentally observed data and reference spectra according to the NIST database and previous publications, and the gas sensor system showed a good linear response to SF6 gas concentration. Finally, Allan–Werle deviation analysis indicated that detection limits of 1.89 ppm for SF6 were obtained with a 1 s integration time, which can be further improved to ~0.38 ppm by averaging up to 131 s.

Modeling and Design of Resonant Magnetic Field Sensors in the Scheme of Differential Magnetostrictive Actuation With Compact Bias Magnetic Circuit

A type of differential resonant magnetic field sensor is developed by employing the composite elements of FeGa plates, piezoelectric quartz crystal double-ended tuning forks (DETFs) and bias permanent magnets. The model and analysis show that the differential design brings the benefits of doubled sensitivity, increased linearity and low temperature drift. Importantly, a small and compact magnetic circuit containing a pair of permanent magnets and dual FeGa plates ensures the generation of differential actuation, which not only reduces the leakage flux but also improve the field-sensitivity and resolution. The magnetic circuit model based on the “lumped parameters” method is established. As an important aid, the distribution of magnetic flux density along the longitudinal direction of the FeGa alloy is simulated by finite element analysis software. In order to avoid the prediction error caused by parameter errors, simulations in different conditions are carried out for comparative analysis. This method ensures the magnetic circuit provides qualified magnetic field. The experimental results show that the differential resonant magnetic field sensor with frequency readout has the characteristics of high sensitivity of 4.4 Hz/Oe, high resolution of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${&lt; }3.04\times 10^{-4}$ </tex-math></inline-formula> Oe, low temperature drift and excellent linear (non-linearity error of 1.3%FS) and low hysteresis (1.5%FS) over its measurement full scale (FS) of ±100Oe.

Quartz crystal tuning fork based 2f/1f wavelength modulation spectroscopy

A compact gas sensing system based on the quartz crystal tuning fork (QCTF) and 2f/1f wavelength modulation spectroscopy (2f/1f-WMS) technique was reported for the first time. An ingenious laser modulation strategy and frequency division multiplexing demodulation algorithm were developed for realizing a single QCTF to detect the first harmonic and second harmonic signals simultaneously. The influence of laser power change and excitation position the QCTF based 2f/1f-WMS technique was first investigated in detail. The results show that, compared with the traditional QCTF-2f method, the reported QCTF based 2f/1f technique has better immunity. To further evaluate this sensing technique, real-time monitoring of ambient water vapor (H2O) was made, the results show that the developed QCTF based 2f/1f-WMS technique can effectively improve the long-term stability and has super anti-interference ability to various environmental disturbance factors, such as light beam jitter, airflow fluctuation, and mechanical vibration, which proves it has great potential in practical field applications, especially for harsh environment.

Temperature-compensated calibration for quartz-enhanced photoacoustic spectroscopy by thermoelastic effects

A light-induced thermoelastic calibration method (TCM) was proposed for the temperature-compensated frequency calibration of quartz enhanced photoacoustic spectroscopy (QEPAS). The temperature effects on the resonance frequency of QEPAS was reported in traditional electrical calibration method (ECM) and TCM. With the temperature varying from 25.6 ℃ to 70.4 ℃, the resonance frequency calibrated by ECM shows a frequency shift of 36.6 ppm. The proposed TCM by using of light-induced thermoelastic wave to drive the resonance of quartz crystal tuning fork (QTF), improved the accuracy of resonance calibration by 2 orders, resulting in a frequency shift of ~ 0.3 ppm. Detailed research demonstrates the TCM is able to work with light intensity of only ~ 50 μW. It is indicated that TCM is a powerful tool for the temperature-compensation of resonance frequency of quartz tuning fork.

Ultrathin two-dimensional Fe-doped cobaltous oxide as a piezoelectric enhancement mechanism in quartz crystal tuning fork (QCTF) photodetectors.

An innovative ultrathin two-dimensional (2D) Fe-doped cobaltous oxide (Fe-CoO) coated quartz crystal tuning fork (QCTF) was introduced for the purpose of developing a low-cost photoelectric detector with a simple configuration. The enhancement mechanism of the piezoelectric signal in the ultrathin 2D Fe-CoO-coated QCTF detector is assumed to be the synergetic photocarrier transfer and photothermal effect of ultrathin 2D Fe-CoO. The ultrathin 2D nanosheet structure of Fe-CoO with a large specific surface area can efficiently absorb and convert light into heat in the QCTF, and the photocarrier transfer from the Fe-CoO nanosheet to the electrode of the QCTF contributes to the enhancement in electricity given the shortened diffusion distance of carriers to the surfaces of the 2D nanosheet. Finite element modeling was adopted to simulate the thermoelastic expansion and mechanical resonance of the QCTF with 2D Fe-CoO coating to support experimental results and analyses. Moreover, the effects of 2D Fe-CoO on the performance of QCTF-based photoelectric detectors were investigated. This Letter demonstrates that ultrathin 2D materials have great potential in applications such as costly and tiny QCTF detectors, light sensing, biomedical imaging, and spectroscopy.

Absorption spectroscopy gas sensor using a low-cost quartz crystal tuning fork with an ultrathin iron doped cobaltous oxide coating

A proof-of-concept photoelectric detector is proposed for use in low-cost, compact, and highly sensitive gas sensors. This detector is based on a quartz crystal tuning fork (QCTF) coated with an ultrathin two-dimensional (2D) iron doped cobaltous oxide (Fe-CoO) film. The Fe-CoO film enhances the photoelectric conversion efficiency of the detector. This detector is combined with wavelength modulation spectroscopy, and its suitability for trace gas sensing is demonstrated using CH4. A minimum detection limit of 0.88 ppmv is achieved with an integration time of 1 s. This corresponds to a normalized noise equivalent absorption coefficient of 2.2 × 10−10 cm-1·W Hz-1/2. Allan variance analysis is used to evaluate the stability of the proposed system, and the results show that an optimum precision of 20 ppbv can be achieved with an integration time of 220 s. Ultrathin 2D materials may be used to develop new photoelectric detectors that have an ultrawide wavelength response range and high photoelectric conversion efficiency. This would provide flexibility and chemical stability at low cost for applications in harsh environments.

Multigas Sensing Technique Based on Quartz Crystal Tuning Fork-Enhanced Laser Spectroscopy.

A compact multigas sensor system based on a single quartz crystal tuning fork (QCTF) and multifrequency synchronous modulation strategy is proposed for trace gas detection. To demonstrate the novel detection technique, three near-infrared continuous-wave (CW) distributed feedback (DFB) diode lasers with center wavelengths of near 1391, 1574, and 1653 nm and a standard 32 kHz QCTF were integrated for simultaneous detection of H2O, CO2, and CH4, respectively. Wavelength modulation spectroscopy with second harmonic detection (WMS-2f) was selected for enhancing sensitivity. Design of the sensor configuration and primary performance between the traditional single-frequency modulation and the proposed tri-frequency modulation were experimentally investigated and compared in detail. The results indicate that the proposed sensing technique has significant advantages of cost effectiveness, portability, and ease-of-use, and detection limits of 1.4, 353, and 3.1 ppm for simultaneously measuring H2O, CO2, and CH4, respectively, are obtained, corresponding to the normalized noise equivalent absorption (NNEA) coefficients of 2.65 × 10-10, 8.09 × 10-10, and 8.28 × 10-10 cm-1 W/√Hz, respectively. Moreover, the use of an erbium-doped fiber amplifier (EDFA) has been demonstrated as an effective method for sensitivity enhancement.

Comparison of digital lock-in amplifier and fast fourier transform algorithms for quartz-tuning-fork enhanced spectroscopy

A near-infrared methane (CH4) gas sensor system was implemented using a quartz crystal tuning fork (QCTF) based photoelectric detector and a fiber-coupled distributed feedback diode laser emitting at 1.653 μm. Wavelength-modulation with second-harmonic (2f) detection method was investigated. For signal processing, two different demodulation algorithms, i.e. Fast Fourier Transformation (FFT) and digital lock-in amplifier (DLIA), were developed and completely compared for harmonic signal extraction. Experimental results show that the SNR of WMS-2f signals using the proposed FFT algorithm can be improved by 2.5 times on average than the common DLIA method, which agrees well with the theoretical value of 2.7 times achieved with the simulated spectra data. In comparison, the proposed FFT algorithm shows better performance on signal processing efficiency and simplification, when a QCTF was used as a photoelectric detector or an acoustic transducer.

Dual-frequency modulation quartz crystal tuning fork-enhanced laser spectroscopy.

An innovative trace gas-sensing technique utilizing a single quartz crystal tuning fork (QCTF) based on a photoelectric detector and dual-frequency modulation technique was demonstrated for the first time for simultaneous multi-species detection. Instead of traditional semiconductor detectors and lock-in amplifier, we utilized the piezoelectric effect and resonant effect of the QCTF to measure the light intensity. A fast signal analysis method based on fast Fourier transform (FFT) algorithm is proposed for overlapping signal extraction. To explore the capabilities of this technique, a gas-sensing system based on two lasers having center emission wavelength of 1.653 µm (a DFB laser diode in the near-IR) and 7.66 µm (an EC QCL in the mid-IR) is successfully demonstrated for simultaneous CH4 spectroscopy measurements. The results indicate a normalized noise equivalent absorption (NNEA) coefficients of 1.33×10-9 cm-1W·Hz-1/2 at 1.653 µm and 2.20×10-10 cm-1W·Hz-1/2 at 7.66 µm, were achieved. This proposed sensor architecture has the advantages of easier optical alignment, lower cost, and a compactness compared to the design of a conventional TDLAS sensor using multiple semiconductor detectors for laser signal collection. The proposed technique can also be expanded to common QEPAS technique with multi-frequency modulation for multiple species detection simultaneously.

Dual-spectroscopy technique based on quartz crystal tuning fork detector

Quartz crystal tuning fork (QCTF) as a sensitive element and valuable component has been widely used for developing mass and stress sensor, pressure and temperature sensor, optoacoustic transducer and photoelectric detector, due to its high quality factor, high stability and precision, and low power consumption. In this paper, QCTF was reported for simultaneous detection of both acoustic signal induced by photoacoustic effect (i.e. photoacoustic spectroscopy) and light signal from molecular absorption process (i.e. direct absorption spectroscopy). A broadband tunable external cavity quantum cascade laser (ECQCL) and two QCTFs with same resonant frequency was demonstrated for dual-spectroscopy technique detection of gas concentration. The results indicated that the proposed dual-spectroscopy technique can be efficiently utilized to enhance sensitivity of QCTF based gas sensors. Comparing to the common QEPAS technique, a sensitivity enhancement of 4–7 times was obtained for the proposed dual-spectroscopy technique.

Atomic Force Microscope‐Based Meniscus‐Confined Three‐Dimensional Electrodeposition

AbstractThe development of a 3D electrochemical deposition system, which combines meniscus‐confined electrodeposition (MCED) with atomic force microscope (AFM) closed‐loop control and has a submicron resolution, is described. Thanks to the high rigidity of the hollow borosilicate glass (or quartz) tip and quartz crystal tuning fork (QTF), combined with the QTF's high force sensitivity, the use of a solution‐filled AFM tip in air is successful. The AFM control enables full automation and in situ growth control. Using this scheme, 3D printing of high‐quality, fully dense, uniform and exceptionally smooth, freestanding straight and overhang pure polycrystalline copper pillars, with diameters ranging from 1.5 µm to 250 nm, and an aspect ratio &gt; 100, is demonstrated. This process may be useful for manufacturing of high‐frequency terahertz antennas, high‐density interconnects, precision sensors, micro‐ and nano‐electromechanical systems, batteries, and fuel cells, as well as for repair or modification of existing micro‐sized or nano‐sized features.

A Controllable DCCS-Based PT Temperature Sensor in High Precision Molecular Spectroscopy Application

In the high precision molecular spectroscopy, the temperature of gas absorption cell is accurately measured. In this paper we discuss the factors affecting the measurement accuracy of temperature sensor employing platinum resistance in a resonant quartz crystal tuning fork (QCTF) detector based wavelength modulation spectroscopy (WMS). A bridge temperature sensor powered by a dual constant current source (DCCS) is proposed. The DCCS was controlled to keep the constant current of 1mA in this work. Furthermore, we used the 3-wire measurements powered by DCCS to eliminate the impact of lead resistances and self heating of the platinum resistance so as to reduce measurement errors. A piecewise linearization model by linear approximation algorithm is employed to evaluate the measurement calibration and increase the measurement accuracy. Detection of trace methane (CH 4 ) was demonstrated using a near infrared distributed feedback diode laser near 1.653 μm and a single pass gas absorption cell with an optical length of 20 cm. An example of the temperature sensor employing Pt100 with 3-wire measurement is developed in the detection of CH 4 with the temperature range from 0°C to 100°C. The controllable DCCS method is demonstrated by the measurement results of the deploying Pt100 sensors. Experimental results show that the accuracy of our proposed temperature sensor is improved in comparison with the conventional platinum resistance thermometer. The measurement accuracy is relatively increased due to employing the piecewise linear approximation model. The results also show good performance for measurement calibration, especially for high temperature region.

Electrostatic forces above graphene nanoribbons and edges interpreted as partly hydrogen-free.

Graphene nanoribbons' electronic transport properties strongly depend on the type of edge, armchair, zigzag or other, and on edge functionalization that can be used for band-gap engineering. For only partly hydrogenated edges interesting magnetic properties are predicted. Electric charge accumulates at edges and corners. Scanning force microscopy has so far shown the centre of graphene nanoribbons with atomic resolution using a quartz crystal tuning fork sensor of high stiffness. Weak long-range electrostatic forces related to the charge accumulation on the edges of graphene nanoribbons could not be imaged so far. Here, we show the electrostatic forces at the corners and edges of graphene nanoribbons are amenable to measurement. We use soft cantilevers and a bimodal imaging technique to combine enhanced sensitivity to weak long-range electrostatic forces with the high resolution of the second-frequency shift. Additionally, in our work the edges of the nanoribbons are mainly hydrogen-free, opening to the route to investigations of partly hydrogenated magnetic nanoribbons.

A Differential Resonant Voltage Sensor Consisting of Piezo Bimorph and Quartz Crystal Double-Ended Tuning Fork Resonators.

A differential resonant voltage sensor with frequency output was developed by bonding two quartz crystal double-ended tuning forks (DETFs) on both sides of a piezo bimorph. The applied voltage induced tensile and compression deformation in the upper and bottom layers of the piezo bimorph, which caused the resonant frequency of the dual DETFs to increase and decrease, respectively. In this case, the differential output of the resonance frequencies of the dual DETFs greatly reduced the effect of temperature drift. In addition, the input resistance of the piezo bimorph reached a few hundred GΩ, which caused almost no influence on the DC voltage under test. The fabricated device showed a linear characteristic over its measurement range of ±700 V with a sensitivity of 0.75 Hz/V, a resolution of 0.007% (0.1 V) and hysteresis of 0.76% of the full range. The quality factor of the DETFs was about 3661 (in air). This novel resonant voltage sensor with its extremely low power consumption is promising for measuring or monitoring DC voltage in various fields.

Fluid Sensing Using Quartz Tuning Forks-Measurement Technology and Applications.

We provide an overview of recent achievements using quartz tuning forks for sensing liquid viscosity and density. The benefits of using quartz crystal tuning forks (QTFs) over other sensors are discussed on the basis of physical arguments and issues arising in real world applications. The path to highly accurate and robust measurement systems is described and a recently devised system considering these findings is presented. The performance of the system is analyzed for applications such as the mixing ratio measurement of fuels, diesel-soot contamination for engine oil condition monitoring, and particle size characterization in suspensions. It is concluded that using properly designed systems enables a variety of applications in industry and research.