Microbalance Principle Research Articles

Periodically Structured Lamé Resonators as High Sensitivity Resonant Mass Sensors

In this work, the electromechanical characterization of periodically structured MEMS Lamé resonators is presented. The resonators are aimed at the development of high-sensitivity resonant mass sensors based on the microbalance principle. Periodic perforations in the resonator decrease its mass while increasing the exposed surface, which is available for mass grafting. As a consequence, the expected sensitivity is significantly increased. Theoretical values for the resonance frequencies, calculated with a simple equivalent material model, are computed and shown to be in very good agreement with the measured values.

A CMOS-compatible bulk technology for the fabrication of magnetically actuated microbalances for chemical sensing

A resonant chemical sensor, fabricated in a standard, CMOS-compatible technology followed by a two-mask post-processing, is presented. A silicon dioxide plate, suspended to two torsional springs, is released from the silicon substrate by means of a TMAH anisotropic wet etch. Two metal loops are integrated on the plate. The first, in conjunction with an external micromagnet, is used to drive the plate into mechanical resonance, while the second loop is used for detection. In accordance with the microbalance principle, a shift of the resonance frequency can be related to the adsorption on the plate of the chemical species under test. The sensor design, and first results on the implementation of the post-processing technology, are presented.

Deposition of zinc oxide thin films for application in bulk acoustic wave resonator

High quality piezoelectric zinc oxide (ZnO) thin films were deposited on (1 0 0)-oriented silicon substrate by reactive rf magnetron sputtering for bulk acoustic wave resonator. In order to improve the ZnO thin films quality, structural and electrical characteristics have been compared before and after annealing in helium (He) by X-ray diffraction and reflection coefficient S 11 measurements. Scanning electron microscopy (SEM) was used to study the crystallographic structure. A previous study has shown that a substrate temperature of 100 °C, a distance between the target and the substrate of 70 mm, and a pressure of 3.35 × 10 −3 Torr in argon and oxygen mixed gas atmosphere, are the optimum conditions to sputter ZnO thin films with good homogeneity and a high degree of crystallinity. These films exhibit an electrical resistivity higher than 10 10 Ω cm and an energy band gap of 3.3 eV at room temperature. X-ray diffraction measurements have shown that ZnO films are highly c-axis-oriented with a full width at half maximum (FWHM) below 0.5°. The decrease of the FWHM after annealing treatment has shown the crystal quality improvement. A growth of c-axis (0 0 2)-oriented ZnO films allows predominant longitudinal wave propagation. Bulk acoustic wave (BAW) resonators have been fabricated by stacking different layers of Al/ZnO/Pt on a silicon substrate which could be used for the fabrication of humidity sensors based on the quartz microbalance principle.

Directly heated quartz crystal microbalance with an integrated dielectric sensor

Cross-sensitivity, especially to humidity, is a common problem in the analysis of chemical gas-sensor signals. To identify the contributions of two gas components at least two suitable signals are needed. This article presents a new chemical sensor unit which is a combination of a quartz crystal microbalance and a dielectric sensor. Thus it yields information about both the mass and the dielectricity of the same chemically sensitive film. The differences in absorption isotherms and drift behaviour are discussed. To make this a universal element for gas sensing, a heater and a temperature gauge are also integrated directly on the quartz by thin-film technology. This allows the application of temperature modulation techniques to the quartz microbalance principle.

Versatile microcontrolled gas sensor array system using the quartz microbalance principle and pattern recognition methods

A new gas sensor array system for the detection of special gases or solvent vapours using the microbalance principle is presented. Different coatings (e.g. polymers, cage structures, self-assembled monolayers) as sensitive materials on the quartz surface allow the sensor array configuration. This gas analyzer system and the knowledge of effective chemical sensitive coatings open a great variety of special application cases, e.g. in the environmental and chemical industry or biotechnology. After a short discussion of the theoretical background, experimental results with calixarene coated quartzes and the first results with a gas chromatographic column are shown. Finally a brief description of investigations with pattern recognition methods using the principal component analysis and neutral network is given.

Using the quartz microbalance principle for sensing mass changes and damping properties

A brief summary of the most important development steps of a special crystal sensing technique is presented. Then, a lumped-element equivalent circuit model is introduced that describes the near-resonance electrical characteristics of a quartz-crystal microbalance (QCM) simultaneously loaded by a surface mass layer and contacting analyte. This model predicts that surface mass accumulation causes a shift of the resonance peak while increasing the density—viscosity product of the sensitive layer. These two effects cause both a translation and a decrease of the resonance peak. Experimental results are given which demonstrate interactions between sensitive materials (adsorbates) coated on the quartz crystal and various solvent vapours (analytes). Different polymers, modified ethanolamines and compounds with long alkyl-chains are used as coatings. Two of these coated sensors show an anomalous effect: the exposure to solvent vapours results in an increase of frequency because of the effective mass decrease. The apparent discrepancy implies that the physical properties of the film must change, requiring a more detailed characterization of the coated quartz.