A comparsion of force sensors for atomic force microscopy based on quartz tuning forks and length extensional resonators

The force sensor is key to the performance of atomic force microscopy (AFM). Nowadays, most AFMs use micro-machined force sensors made from silicon, but piezoelectric quartz sensors are applied at an increasing rate, mainly in vacuum. These self sensing force sensors allow a relatively easy upgrade of a scanning tunneling microscope to a combined scanning tunneling/atomic force microscope. Two fundamentally different types of quartz sensors have achieved atomic resolution: the 'needle sensor' that is based on a length extensional resonator and the 'qPlus sensor' that is based on a tuning fork. Here, we calculate and measure the noise characteristics of these sensors. We find four noise sources: deflection detector noise, thermal noise, oscillator noise and thermal drift noise. We calculate the effect of these noise sources as a factor of sensor stiffness, bandwidth and oscillation amplitude. We find that for self sensing quartz sensors, the deflection detector noise is independent of sensor stiffness, while the remaining three noise sources increase strongly with sensor stiffness. Deflection detector noise increases with bandwidth to the power of 1.5, while thermal noise and oscillator noise are proportional to the square root of the bandwidth. Thermal drift noise, however, is inversely proportional to bandwidth. The first three noise sources are inversely proportional to amplitude while thermal drift noise is independent of the amplitude. Thus, we show that the earlier finding that quoted optimal signal-to-noise ratio for oscillation amplitudes similar to the range of the forces is still correct when considering all four frequency noise contributions. Finally, we suggest how the signal-to-noise ratio of the sensors can be further improved and briefly discuss the challenges of mounting tips.Comment: 40 pages, 14 figure

A contemporary investigation of force transducers: Past and present scenario

In this paper, retrospective investigation of different types of force transducers, used in different applications (metrological, industrial, scientific etc.) for force measurement, has been done. The paper discusses the complete classification of force transducers based on shape, display and applications. Various types of force transducer have been discussed in the paper including symmetrical, unsymmetrical and alteration types. An attempt has been made to provide a comprehensive investigation related to metrological aspects of force transducer

Photoacoustic-based gas sensing: A review

The use of the photoacoustic effect to gauge the concentration of gases is an attractive alternative in the realm of optical detection methods. Even though the effect has been applied for gas sensing for almost a century, its potential for ultra-sensitive and miniaturized devices is still not fully explored. This review article revisits two fundamentally different setups commonly used to build photoacoustic-based gas sensors and presents some distinguished results in terms of sensitivity, ultra-low detection limits, and miniaturization. The review contrasts the two setups in terms of the respective possibilities to tune the selectivity, sensitivity, and potential for miniaturization.S.P. acknowledges funding from the Community of Madrid under grant number 2016-T1/AMB-1695

Broadband detection of methane and nitrous oxide using a distributed-feedback quantum cascade laser array and quartz-enhanced photoacoustic sensing

Here we report on the broadband detection of nitrous oxide (N2O) and methane (CH4) mixtures in dry nitrogen by using a quartz-enhanced photoacoustic (QEPAS) sensor exploiting an array of 32 distributed-feedback quantum cascade lasers, within a spectral emission range of 1190−1340 cm−1 as the excitation source. Methane detection down to a minimum detection limit of 200 ppb at 10 s lock-in integration time was achieved. The sensor demonstrated a linear response in the range of 200−1000 ppm. Three different mixtures of N2O and CH4 in nitrogen at atmospheric pressure have been analyzed. The capability of the developed QEPAS sensor to selectively determine the N2O and CH4 concentrations was demonstrated, in spite of significant overlap in their respective absorption spectra in the investigated spectral range

Thin-Film AlN-on-Silicon Resonant Gyroscopes: Design, Fabrication, and Eigenmode Operation

Resonant MEMS gyroscopes have been rapidly adopted in various consumer, industrial, and automotive applications thanks to the significant improvements in their performance over the past decade. The current efforts in enhancing the performance of high-precision resonant gyroscopes are mainly focused on two seemingly contradictory metrics, larger bandwidth and lower noise level, to push the technology towards navigation applications. The key enabling factor for the realization of low-noise high-bandwidth resonant gyroscopes is the utilization of a strong electromechanical transducer at high frequencies. Thin-film piezoelectric-on-silicon technology provides a very efficient transduction mechanism suitable for implementation of bulk-mode resonant gyroscopes without the need for submicron capacitive gaps or large DC polarization voltages. More importantly, in-air operation of piezoelectric devices at moderate Q values allows for the cointegration of mode-matched gyroscopes and accelerometers on a common substrate for inertial measurement units. This work presents the design, fabrication, characterization, and method of mode matching of piezoelectric-on-silicon resonant gyroscopes. The degenerate in-plane flexural vibration mode shapes of the resonating structure are demonstrated to have a strong gyroscopic coupling as well as a large piezoelectric transduction coefficient. Eigenmode operation of resonant gyroscopes is introduced as the modal alignment technique for the piezoelectric devices independently of the transduction mechanism. Controlled displacement feedback is also employed as the frequency matching technique to accomplish complete mode matching of the piezoelectric gyroscopes.Ph.D

Multichannel QCM-based system for continuous monitoring of bacterial biofilm growth

© 2020 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes,creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works.Quartz crystal microbalance (QCM) sensors are becoming a good alternative to analytical methods for the measurement of bacterial growth in liquid media culture. For this purpose, two essential resonance parameters allow monitoring of biofilm formation: the series resonance frequency shift and the change of the resistance at this frequency. Nevertheless, several problems arise in determining these parameters, as their relative variation is very small. This means that an accurate procedure must be implemented for the measurement of the QCM resonance parameters, including the automatic calibration of the frequency response effects of the measurement circuits and the automatic compensation of the static electrical capacitance of the QCM. In this paper, a novel multichannel system for on-line monitoring of biofilm formation based on QCM sensors is proposed. QCM resonance parameters are determined from the electrical impedance analysis by means of an auto-balanced impedance bridge. This configuration has allowed the implementation of an affordable multichannel measurement instrument. Obtained results, based on binary mixtures of water-glycerol measurements and real microorganism experiments, are in good agreement with the theoretical behaviour. These results show the great potential of this instrument to be used for monitoring microbial growth and biofilm formation.Peer ReviewedPostprint (author's final draft

Degree-per-hour mode-matched micromachined silicon vibratory gyroscopes

The objective of this research dissertation is to design and implement two novel micromachined silicon vibratory gyroscopes, which attempt to incorporate all the necessary attributes of sub-deg/hr noise performance requirements in a single framework: large resonant mass, high drive-mode oscillation amplitudes, large device capacitance (coupled with optimized electronics), and high-Q resonant mode-matched operation. Mode-matching leverages the high-Q (mechanical gain) of the operating modes of the gyroscope and offers significant improvements in mechanical and electronic noise floor, sensitivity, and bias stability. The first micromachined silicon vibratory gyroscope presented in this work is the resonating star gyroscope (RSG): a novel Class-II shell-type structure which utilizes degenerate flexural modes. After an iterative cycle of design optimization, an RSG prototype was implemented using a multiple-shell approach on (111) SOI substrate. Experimental data indicates sub-5 deg/hr Allan deviation bias instability operating under a mode-matched operating Q of 30,000 at 23ºC (in vacuum). The second micromachined silicon vibratory gyroscope presented in this work is the mode-matched tuning fork gyroscope (M2-TFG): a novel Class-I tuning fork structure which utilizes in-plane non-degenerate resonant flexural modes. Operated under vacuum, the M2-TFG represents the first reported high-Q perfectly mode-matched operation in Class-I vibratory microgyroscope. Experimental results of device implemented on (100) SOI substrate demonstrates sub-deg/hr Allan deviation bias instability operating under a mode-matched operating Q of 50,000 at 23ºC. In an effort to increase capacitive aspect ratio, a new fabrication technology was developed that involved the selective deposition of doped-polysilicon inside the capacitive sensing gaps (SPD Process). By preserving the structural composition integrity of the flexural springs, it is possible to accurately predict the operating-mode frequencies while maintaining high-Q operation. Preliminary characterization of vacuum-packaged prototypes was performed. Initial results demonstrated high-Q mode-matched operation, excellent thermal stability, and sub-deg/hr Allan variance bias instability.Ph.D.Committee Chair: Dr. Farrokh Ayazi; Committee Member: Dr. Mark G. Allen; Committee Member: Dr. Oliver Brand; Committee Member: Dr. Paul A. Kohl; Committee Member: Dr. Thomas E. Michael