Lateral-Mode Vibration of Microcantilever-Based Sensors in Viscous Fluids Using Timoshenko Beam Theory

To more accurately model microcantilever resonant behavior in liquids and to improve lateral-mode sensor performance, a new model is developed to incorporate viscous fluid effects and Timoshenko beam effects (shear deformation, rotatory inertia). The model is motivated by studies showing that the most promising geometries for lateral-mode sensing are those for which Timoshenko effects are most pronounced. Analytical solutions for beam response due to harmonic tip force and electrothermal loadings are expressed in terms of total and bending displacements, which correspond to laser and piezoresistive readouts, respectively. The influence of shear deformation, rotatory inertia, fluid properties, and actuation/detection schemes on resonant frequencies ( ) and quality factors ( ) are examined, showing that Timoshenko beam effects may reduce and by up to 40% and 23%, respectively, but are negligible for width-to-length ratios of 1/10 and lower. Comparisons with measurements (in water) indicate that the model predicts the qualitative data trends, but underestimates the softening that occurs in stiffer specimens, indicating that support deformation becomes a factor. For thinner specimens, the model estimates quite well, but exceeds the observed values for thicker specimens, showing that the Stokes resistance model employed should be extended to include pressure effects for these geometries.[2014-0157

Unconventional Uses of Microcantilevers as Chemical Sensors in Gas and Liquid Media

The use of microcantilevers as (bio)chemical sensors usually involves the application of a chemically sensitive layer. The coated device operates either in a static bending regime or in a dynamic flexural mode. While some of these coated devices may be operated successfully in both the static and the dynamic modes, others may suffer from certain shortcomings depending on the type of coating, the medium of operation and the sensing application. Such shortcomings include lack of selectivity and reversibility of the sensitive coating and a reduced quality factor due to the surrounding medium. In particular, the performance of microcantilevers excited in their standard out-of-plane dynamic mode drastically decreases in viscous liquid media. Moreover, the responses of coated cantilevers operating in the static bending mode are often difficult to interpret. To resolve these performance issues, the following emerging unconventional uses of microcantilevers are reviewed in this paper: (1) dynamic-mode operation without using a sensitive coating, (2) the use of in-plane vibration modes (both flexural and longitudinal) in liquid media, and (3) incorporation of viscoelastic effects in the coatings in the static mode of operation. The advantages and drawbacks of these atypical uses of microcantilevers for chemical sensing in gas and liquid environments are discussed

Resonant Magnetic Field Sensors Based On MEMS Technology

Microelectromechanical systems (MEMS) technology allows the integration of magnetic field sensors with electronic components, which presents important advantages such as small size, light weight, minimum power consumption, low cost, better sensitivity and high resolution. We present a discussion and review of resonant magnetic field sensors based on MEMS technology. In practice, these sensors exploit the Lorentz force in order to detect external magnetic fields through the displacement of resonant structures, which are measured with optical, capacitive, and piezoresistive sensing techniques. From these, the optical sensing presents immunity to electromagnetic interference (EMI) and reduces the read-out electronic complexity. Moreover, piezoresistive sensing requires an easy fabrication process as well as a standard packaging. A description of the operation mechanisms, advantages and drawbacks of each sensor is considered. MEMS magnetic field sensors are a potential alternative for numerous applications, including the automotive industry, military, medical, telecommunications, oceanographic, spatial, and environment science. In addition, future markets will need the development of several sensors on a single chip for measuring different parameters such as the magnetic field, pressure, temperature and acceleration

Alternative piezoresistor designs for maximizing cantilever sensitivity.

Over the last 15 years, researchers have explored the use of piezoresistive microcantilevers/resonators as gas sensors because of their relative ease in fabrication, low production cost, and their ability to detect changes in mass or surface stress with fairly good sensitivity. However, existing microcantilever designs rely on irreversible chemical reactions for detection and researchers have been unable to optimize symmetric geometries for increased sensitivity. Previous work by our group showed the capability of T-shaped piezoresistive cantilevers to detect gas composition using a nonreaction-based method – viscous damping. However, this geometry yielded only small changes in resistance. Recently, computational studies performed by our group indicated that optimizing the geometry of the base piezoresistor increases device sensitivity up to 700 times. Thus, the focus of this work is to improve the sensitivity of nonreaction-based piezoresistive microcantilevers by incorporating asymmetric piezoresistive sensing elements in a new array design. A three-mask fabrication process was performed using a 4 silicon-on-insulator wafer. Gold bond pads and leads were patterned using two optical lithography masks, gold sputtering, and acetone lift-off techniques. The cantilevers were patterned with electron-beam lithography and a dry etch masking layer was then deposited via electronbeam evaporation of iron. Subsequently, the silicon device layer was deep reactive ion etched (DRIE) to create the vertical sidewalls and the sacrificial silicon dioxide layer was removed with a buffered oxide etch, completely releasing the cantilever structures. Finally, the device was cleaned and dried with critical point drying to prevent stiction of the devices to the substrate. For the resonance experiments, the cantilevers were driven electrostatically by applying an AC bias, 10 Vpp, to the gate electrode. A DC bias of 10 V was placed across the piezoresistor in series with a 14 kÙ resistor. The drive frequency (0 – 80 kHz) was swept until the cantilever resonated at its natural frequency, which occurred when the output of the lock-in amplifier reached its maximum. These devices have been actuated to resonance under vacuum and their resonant frequencies and Qfactors measured. The first mode of resonance for the asymmetric cantilevers was found to range between 40 kHz and 63 kHz, depending on the piezoresistor geometry and length of the cantilever beam. The redesigned piezoresistive microcantilevers tested yielded static and dynamic sensitivities ranging from 1-6 Ù/Ìm and 2-17 Ù/Ìm displacement, respectively, which are 40 –730 times more sensitive than the best symmetric design previously reported by our group. Furthermore, the Q-factors ranged between 1700 and 4200, typical values for MEMS microcantilevers

Sensors and actuators, Twente

This paper describes the organization and the research programme of the Sensor and Actuator (S&A) Research Unit of the University of Twente, Enschede, the Netherlands. It includes short descriptions of all present projects concerning: micromachined mechanical sensors and actuators, optical sensors, recording media and sensors based on magnetic materials, FET-based sensors and systems and the integration of electronic functions and systems in sensor chips

Effect of Support Compliance on the Resonant Behavior of Microcantilever-Based Sensors in Viscous Fluids

Resonant microcantilevers are often considered for use in chemical sensing and biosensing applications. However, when excited in the conventional transverse flexural mode, their performance in liquids is severely compromised. Theoretical and experimental studies have shown that the detrimental effects of the liquid may be mitigated by operating the microcantilever in lateral flexure, especially for microbeams having smaller length-to-width (L/b) ratios. However, for these most promising geometries the predictions of existing models tend to diverge from experimental data for resonant frequency (fres) and quality factor (Q). A likely reason for these discrepancies is support compliance, which has been neglected in existing models. Thus, the derivation of an analytical model for the lateral-mode dynamic response of a microcantilever in a viscous fluid, including the effects of support compliance, is warranted and is the focus of this dissertation. Analytical solutions for natural frequency and Q are first obtained for the free-vibration case, followed by solutions for the forced-vibration response when the cantilever is excited by an imposed harmonic relative rotation near the support (simulating electrothermal actuation). Values of fres and Q are extracted from the response spectra for the tip deflection and the bending strain near the support. The support compliance (required as model input) is analytically related to device dimensions by employing dimensional analysis and 3-D FEA. The analytical results for the resonant characteristics are also related to sensor performance metrics (sensitivity and limit of detection), thus permitting one to exploit the potential of lateral-mode microcantilever-based liquid-phase sensors. The impact of support compliance, fluid resistance, and beam dimensions on the free- and forced-vibration response are explored, as are the differences associated with the two output signals. Comparisons of results with experimental data show a marked improvement over the previous rigid-support models for smaller L/b values. For the practical ranges of parameters considered the model indicates that, at smaller L/b values, support compliance may reduce Q by up to ~14% and fres and mass sensitivity (Sm) by up to ~21%. Conversely, for L/b\u3e15 the support compliance effects are no more than 2% on Q and 4% on fres and Sm

Development of a three-axis MEMS accelerometer

While originally developed to deploy air bags for the automotive industry, Microelectromechanical Systems (MEMS) based accelerometers have found their way into everything from video game controllers to cells phones. As prices drop and capabilities improve, it is expected that the use of accelerometers will further expand in the coming years. Accelerometers currently have the second highest MEMS sales volume, trailing only pressure sensors [1]. In this work several single and three-axis accelerometers are designed, fabricated, and tested under a variety of conditions. The designed accelerometers are all based off of the piezoresistive effect, where the value of a resistor changes with applied mechanical stress [2]. When accelerated, the inertia of a suspended proof mass causes stress on piezoresistors placed on support arms. The corresponding changes in these resistor values are then converted to an output voltage using a Wheatstone bridge. To sense acceleration independently in all three axes, structures with three distinct modes of vibration and three sets of Wheatstone bridges are used. Devices were fabricated at the Semiconductor and Microsystems Fabrication Laboratory (SMFL), located at RIT. A modified version of the RIT bulk MEMS process was used, consisting of 65 steps, 7 photolithography masks, bulk silicon diaphragm etch, and top hole release etch [3]. Unfortunately the finished chips show poor aluminum step coverage into contact vias and over polysilicon lines. This results in open circuits throughout the chip, prohibiting proper operation. Process corrections have been identified, and with proper fabrication the designs are still expected to yield working devices. Since the finished accelerometers were not functional, several commercial accelerometers have been tested to characterize sensitivity, linearity, cross-axis sensitivity, frequency response, and device lifetime