On the nonlinear dynamics of a piezoresistive based mass switch based on catastrophic bifurcation

This research investigates the feasibility of mass sensing in piezoresistive MEMS devices based on catastrophic bifurcation and sensitivity enhancement due to the orientation adjustment of the device with respect to the crystallographic orientation of the silicon wafer. The model studied is a cantilever microbeam at the end of which an electrostatically actuated tip mass is attached. The piezoresistive layers are bonded to the vicinity of the clamped end of the cantilever and the device is set to operate in the resonance regime by means of harmonic electrostatic excitation. The nonlinearities due to curvature, shortening and electrostatic excitation have been considered in the modelling process. It is shown that once the mass is deposited on the tip mass, the system undergoes a cyclic fold bifurcation in the frequency domain, which yields a sudden jump in the output voltage of the piezoresistive layers; this bifurcation is attributed to the nonlinearities governing the dynamics of the response. The partial differential equations of the motion are derived and discretized to give a finite degree of freedom model based on the Galerkin method, and the limit cycles are captured in the frequency domain by using the shooting method. The effect of the orientation of the device with respect to the crystallographic coordinates of the silicon and the effect of the orientation of the piezoresistive layers with respect to the microbeam length on the sensitivity of the device is also investigated. Thanks to the nonlinearity and the orientation adjustment of the device and piezoresistive layers, a twofold sensitivity enhancement due to the added mass was achieved. This achievement is due to the combined amplification of the sensitivity in the vicinity of the bifurcation point, which is attributed to the nonlinearity and maximizing the sensitivity by orientation adjustment of the anisotropic piezoresistive coefficients

Effect of Hydrodynamic Force on Microcantilever Vibrations: Applications to Liquid-Phase Chemical Sensing

At the microscale, cantilever vibrations depend not only on the microstructure’s properties and geometry but also on the properties of the surrounding medium. In fact, when a microcantilever vibrates in a fluid, the fluid offers resistance to the motion of the beam. The study of the influence of the hydrodynamic force on the microcantilever’s vibrational spectrum can be used to either (1) optimize the use of microcantilevers for chemical detection in liquid media or (2) extract the mechanical properties of the fluid. The classical method for application (1) in gas is to operate the microcantilever in the dynamic transverse bending mode for chemical detection. However, the performance of microcantilevers excited in this standard out-of-plane dynamic mode drastically decreases in viscous liquid media. When immersed in liquids, in order to limit the decrease of both the resonant frequency and the quality factor, and improve sensitivity in sensing applications, alternative vibration modes that primarily shear the fluid (rather than involving motion normal to the fluid/beam interface) have been studied and tested: these include in-plane vibration modes (lateral bending mode and elongation mode). For application (2), the classical method to measure the rheological properties of fluids is to use a rheometer. However, such systems require sampling (no in-situ measurements) and a relatively large sample volume (a few milliliters). Moreover, the frequency range is limited to low frequencies (less than 200Hz). To overcome the limitations of this classical method, an alternative method based on the use of silicon microcantilevers is presented. The method, which is based on the use of analytical equations for the hydrodynamic force, permits the measurement of the complex shear modulus of viscoelastic fluids over a wide frequency range

Multiphysics modelling and experimental validation of microelectromechanical resonator dynamics

The modelling of microelectromechanical systems provides a very challenging task in microsystems engineering. This field of research is inherently multiphysics of nature, since different physical phenomena are tightly intertwined at microscale. Typically, up to four different physical domains are usually considered in the analysis of microsystems: mechanical, electrical, thermal and fluidic. For each of these separate domains, well-established modelling and analysis techniques are available. However, one of the main challenges in the field of microsystems engineering is to connect models for the behavior of the device in each of these domains to equivalent lumped or reduced-order models without making unacceptably inaccurate assumptions and simplifications and to couple these domains correctly and efficiently. Such a so-called multiphysics modelling framework is very important for simulation of microdevices, since fast and accurate computational prototyping may greatly shorten the design cycle and thus the time-to-market of new products. This research will focus on a specific class of microsystems: microelectromechanical resonators. MEMS resonators provide a promising alternative for quartz crystals in time reference oscillators, due to their small size and on-chip integrability. However, because of their small size, they have to be driven into nonlinear regimes in order to store enough energy for obtaining an acceptable signal-to-noise ratio in the oscillator. Since these resonators are to be used as a frequency reference in the oscillator circuits, their steady-state (nonlinear) dynamic vibration behaviour is of special interest. A heuristic modelling approach is investigated for two different MEMS resonators, a clamped-clamped beam resonator and a dog-bone resonator. For the clamped-clamped beam resonator, the simulations with the proposed model shows a good agreement with experimental results, but the model is limited in its predictive capabilities. For the dogbone resonator, the proposed heuristic modelling approach does not lead to a match between simulations and experiments. Shortcomings of the heuristic modelling approach serve as a motivation for a first-principles based approach. The main objective of this research is to derive a multiphysics modelling framework for MEMS resonators that is based on first-principles formulations. The framework is intended for fast and accurate simulation of the steady-state nonlinear dynamic behaviour of MEMS resonators. Moreover, the proposed approach is validated by means of experiments. Although the multiphysics modelling framework is proposed for MEMS resonators, it is not restricted to this application field within microsystems engineering. Other fields, such as (resonant) sensors, switches and variable capacitors, allow for a similar modelling approach. In the proposed framework, themechanical, electrical and thermal domains are included. Since the resonators considered are operated in vacuum, the fluidic domain (squeeze film damping) is not included. Starting from a first-principles description, founded on partial differential equations (PDEs), characteristic nonlinear effects from each of the included domains are incorporated. Both flexural and bulk resonators can be considered. Next, Galerkin discretization of the coupled PDEs takes place, to construct reduced-order models while retaining the nonlinear effects. The multiphysics model consists of the combined reduced-order models from the different domains. Designated numerical tools are used to solve for the steady-state nonlinear dynamic behaviour of the combined model. The proposed semi-analytical (i.e. analytical-numerical) multiphysics modeling framework is illustrated for a full case study of an electrostatically actuated single-crystal silicon clamped-clamped beam MEMS resonator. By means of the modelling framework, multiphysics models of varying complexity have been derived for this resonator, including effects like electrostatic actuation, fringing fields, shear deformation, rotary inertia, thermoelastic damping and nonlinear material behaviour. The first-principles based approach allows for addressing the relevance of individual effects in a straightforward way, such that the models can be used as a (pre-)design tool for dynamic response analysis. The method can be considered complementary to conventional finite element simulations. The multiphysics model for the clamped-clamped beam resonator is validated by means of experiments. A good match between the simulations and experiments is obtained, thereby giving confidence in the proposed modelling framework. Finally, next to themodelling approach for MEMS resonators, a technique for using these nonlinear resonators in an oscillator circuit setting is presented. This approach, called phase feedback, allows for operation of the resonator in its nonlinear regime. The closedloop technique enables control of both the frequency of oscillation and the output power of the signal. Additionally, optimal operation points for oscillator circuits incorporating a nonlinear resonator can be defined

Design of Novel Sensors and Instruments for Minimally Invasive Lung Tumour Localization via Palpation

Minimally Invasive Thoracoscopic Surgery (MITS) has become the treatment of choice for lung cancer. However, MITS prevents the surgeons from using manual palpation, thereby often making it challenging to reliably locate the tumours for resection. This thesis presents the design, analysis and validation of novel tactile sensors, a novel miniature force sensor, a robotic instrument, and a wireless hand-held instrument to address this limitation. The low-cost, disposable tactile sensors have been shown to easily detect a 5 mm tumour located 10 mm deep in soft tissue. The force sensor can measure six degrees of freedom forces and torques with temperature compensation using a single optical fiber. The robotic instrument is compatible with the da Vinci surgical robot and allows the use of tactile sensing, force sensing and ultrasound to localize the tumours. The wireless hand-held instrument allows the use of tactile sensing in procedures where a robot is not available

Piezoelectric Transducers Based on Aluminum Nitride and Polyimide for Tactile Applications

The development of micro systems with smart sensing capabilities is paving the way to progresses in the technology for humanoid robotics. The importance of sensory feedback has been recognized the enabler of a high degree of autonomy for robotic systems. In tactile applications, it can be exploited not only to avoid objects slipping during their manipulation but also to allow safe interaction with humans and unknown objects and environments. In order to ensure the minimal deformation of an object during subtle manipulation tasks, information not only on contact forces between the object and fingers but also on contact geometry and contact friction characteristics has to be provided. Touch, unlike other senses, is a critical component that plays a fundamental role in dexterous manipulation capabilities and in the evaluation of objects properties such as type of material, shape, texture, stiffness, which is not easily possible by vision alone. Understanding of unstructured environments is made possible by touch through the determination of stress distribution in the surrounding area of physical contact. To this aim, tactile sensing and pressure detection systems should be integrated as an artificial tactile system. As illustrated in the Chapter I, the role of external stimuli detection in humans is provided by a great number of sensorial receptors: they are specialized endings whose structure and location in the skin determine their specific signal transmission characteristics. Especially, mechanoreceptors are specialized in the conversion of the mechanical deformations caused by force, vibration or slip on skin into electrical nerve impulses which are processed and encoded by the central nervous system. Highly miniaturized systems based on MEMS technology seem to imitate properly the large number of fast responsive mechanoreceptors present in human skin. Moreover, an artificial electronic skin should be lightweight, flexible, soft and wearable and it should be fabricated with compliant materials. In this respect a big challenge of bio-inspired technologies is the efficient application of flexible active materials to convert the mechanical pressure or stress into a usable electric signal (voltage or current). In the emerging field of soft active materials, able of large deformation, piezoelectrics have been recognized as a really promising and attractive material in both sensing and actuation applications. As outlined in Chapter II, there is a wide choice of materials and material forms (ceramics: PZT; polycrystalline films: ZnO, AlN; polymers and copolymers: PVDF, PVDF-TrFe) which are actively piezoelectric and exhibit features more or less attractive. Among them, aluminum nitride is a promising piezoelectric material for flexible technology. It has moderate piezoelectric coefficient, when available in c-axis oriented polycrystalline columnar structure, but, at same time, it exhibits low dielectric constant, high temperature stability, large band gap, large electrical resistivity, high breakdown voltage and low dielectric loss which make it suitable for transducers and high thermal conductivity which implies low thermal drifts. The high chemical stability allows AlN to be used in humid environments. Moreover, all the above properties and its deposition method make AlN compatible with CMOS technology. Exploiting the features of the AlN, three-dimensional AlN dome-shaped cells, embedded between two metal electrodes, are proposed in this thesis. They are fabricated on general purpose Kapton™ substrate, exploiting the flexibility of the polymer and the electrical stability of the semiconductor at the same time. As matter of fact, the crystalline layers release a compressive stress over the polymer, generating three-dimensional structures with reduced stiffness, compared to the semiconductor materials. In Chapter III, a mathematical model to calculate the residual stresses which arise because of mismatch in coefficient of thermal expansion between layers and because of mismatch in lattice constants between the substrate and the epitaxially grown films is adopted. The theoretical equation is then used to evaluate the dependence of geometrical features of the fabricated three-dimensional structures on compressive residual stress. Moreover, FEM simulations and theoretical models analysis are developed in order to qualitative explore the operation principle of curved membranes, which are labelled dome-shaped diaphragm transducers (DSDT), both as sensors and as piezo-actuators and for the related design optimization. For the reliability of the proposed device as a force/pressure sensor and piezo-actuator, an exhaustive electromechanical characterization of the devices is carried out. A complete description of the microfabrication processes is also provided. As shown in Chapter IV, standard microfabrication techniques are employed to fabricate the array of DSDTs. The overall microfabrication process involves deposition of metal and piezoelectric films, photolithography and plasma-based dry and wet etching to pattern thin films with the desired features. The DSDT devices are designed and developed according to FEM and theoretical analysis and following the typical requirements of force/pressure systems for tactile applications. Experimental analyses are also accomplished to extract the relationship between the compressive residual stress due to the aluminum nitride and the geometries of the devices. They reveal different deformations, proving the dependence of the geometrical features of the three-dimensional structures on residual stress. Moreover, electrical characterization is performed to determine capacitance and impedance of the DSDTs and to experimentally calculate the relative dielectric constant of sputtered AlN piezoelectric film. In order to investigate the mechanical behaviour of the curved circular transducers, a characterization of the flexural deflection modes of the DSDT membranes is carried out. The natural frequency of vibrations and the corresponding displacements are measured by a Laser Doppler Vibrometer when a suitable oscillating voltage, with known amplitude, is applied to drive the piezo-DSDTs. Finally, being developed for tactile sensing purpose, the proposed technology is tested in order to explore the electromechanical response of the device when impulsive dynamic and/or long static forces are applied. The study on the impulsive dynamic and long static stimuli detection is then performed by using an ad hoc setup measuring both the applied loading forces and the corresponding generated voltage and capacitance variation. These measurements allow a thorough test of the sensing abilities of the AlN-based DSDT cells. Finally, as stated in Chapter V, the proposed technology exhibits an improved electromechanical coupling with higher mechanical deformation per unit energy compared with the conventional plate structures, when the devices are used as piezo-actuator. On the other hand, it is well suited to realize large area tactile sensors for robotics applications, opening up new perspectives to the development of latest generation biomimetic sensors and allowing the design and the fabrication of miniaturized devices

Optical MEMS sensors for wall-shear stress measurements

Ph. D. Thesis.This research reports on the development and experimental characterisation of optical sensors based on Micro-Electro-Mechanical-Systems (MEMS) technologies for walls hear stress quantification in turbulent boundary-layer flows. The MEMS sensors are developed to measure the instantaneous wall-shear stress directly via a miniature flush-mounted floating element, which is on the order of hundreds of microns square. The floating element is suspended flush to the wall by up to four specially designed micro-springs. As the flow passes over the wall, the sensor’s floating element moves, allowing direct measurement of the local forces exerted by the flow on the wall. A new optical transduction scheme based on the Moiré fringe pattern is developed alongside with an optical pathway to measure the instantaneous wall-shear stress using a single photodetector. Using this new optical technique consists of a lens array and fibre optics that provides the ability to detect the wall-shear stress using different sensing element sizes, leads to miniaturisation of sensors. Utilising the lens array, the focused light spot size is controlled, providing the opportunity of scanning the Moiré fringe pattern area on the sensors with different sensing element sizes. The microfabrication process of the devices are carried out by using a four mask bulk Silicon-on-Insulator (SOI) process and a BF33 wafer, where each device is placed at the center of a 5 mm × 5mm chip. Two generations of sensor packaging are developed to accommodate the sensors’ dies as well as the sensors’ optoelectronics, whilst the floating element is flush-mounted to the surface. The MEMS sensors calibration is carried out in a laminar flow rig over a wall-shear stress range of 0 to 5.32 Pa, where the results indicate a sensitivity range of 38 to 740 nm/Pa, an accuracy range of 1.4 to 2.36% and a repeatability range of 0.68 to 1.96%. The value of the of minimum detectable wall-shear stress for the developed MEMS wall-shear stress sensors varies in a range of 17 to 593 µPa, resulting in a minimum and maximum dynamic range value of 79 dB and 109 dB, respectively. The results from the dynamic characterisation indicate a resonant frequency range of 1 to 8.3 kHz. In a series of wind tunnel experiments over a range of Reτ = 560 to 1320, the instantaneous wall-shear stress within the turbulent boundary-layer flow is measured simultaneously by the MEMS sensors and an by either hot-wire anemometry or laser Doppler velocimetry using the near-wall velocity gradient technique. Excellent agreement is observed in the time series and statistics across these three independant measurement techniques.Faculty of Science, Agriculture and Engineering (SAgE), Newcastle Universit

Reliability testing and stress measurement of QFN packages encapsulated by an open-ended microwave curing system

In this paper, the influence of microwave curing on the reliability of a representative electronic package is examined by reliability testing and measurement of residual stresses. A LM358 voltage regulator die was mounted to an open Quad Flat No-leads package (QFN) for reliability testing. For the stress measurement, a specifically designed stress measurement die was mounted to the QFN package. The chips were encapsulated with Hysol EO1080 thermosetting polymer material. Curing was performed using an open-ended microwave oven system equipped with in situ temperature control. Three different temperature profiles for microwave curing were selected according to the requested degree of cure and chemical composition of the cured material. A convection cure profile was selected for the control group samples. Thermal cycling and HAST tests were performed on a total number of 80 chips. 95 QFN packages with stress measurement chips were also manufactured. Increased lifetime expectancy of the microwave cured packaged chips was experimentally demonstrated and measured between 62% to 149% increased lifetime expectancy after Temperature Cycling Test (TCT), and between 63% and 331% after highly Accelerated Ageing Test (HAST) and TCT compared to conventionally cured packages. Analysis of specifically designed stress test chips showed significantly lower residual stresses ranging from 26 MPa to 58.3 MPa within the microwave cured packages compared to conventionally cured packaged chips which displayed residual stresses ranging from 54 MPa to 80.5 MPa. This article therefore provides additional confidence in the industrial relevance of the microwave curing system and its advantages compared to traditional convection oven systems

Micro implantable pressure sensors for lifetime monitoring of intracranial pressure : this dissertation is submitted for the degree of Doctor of Philosophy, School of Engineering and Advanced Technology, Massey University

Permission for the re-use of Figures was obtained from Oxford University Press and BMJ Publishing Group Ltd.The elevation of intracranial pressure (ICP) associated with traumatic brain injury (TBI), hydrocephalus and other neurological conditions is a serious concern. If left untreated, increased pressure in the brain will reduce cerebral blood flow (CBF) and can lead to brain damage or early death. Currently, ICP is monitored through invasive catheters inserted into the brain along with a shunt. However, insertion of catheters or shunts is an invasive procedure that introduces vulnerability to infection. In principle, the risk of infection would be overcome by a fully implantable pressure monitoring system. This would be particularly valuable for hydrocephalus patients if lifetime monitoring was available. An implantable pressure monitoring system relies on a thin flexible membrane as part of the pressure sensor. The thin film membrane displaces under load and correspondingly induces a change in a relevant electrical quantity (resistance, or capacitance). Micro-electro-mechanical system (MEMS) is the technology that helps in creating micro/nano-mechanical structures integrated with signal conditioning electronics. These micro structures can be inserted into the brain, where the thin film is exposed to a corrosive fluid (saline/blood) at a temperature of approximately 37 ◦C. The miniaturization in MEMS permits examination, sensing and monitoring from inside the patient for longer durations. However, the accuracy, particularly in terms of sensor drift over long durations, is a key concern. In general, the issue of drift is attributed to the aging and mechanical fatigue of thin film structures, particularly the thin flexible membrane. Therefore, it is essential to analyze the thin film deflection and fatigue behaviour of MEMS pressure sensors for achieving long-term reliability and accuracy. Thus, the high-level goal of this research is to identify a viable approach to producing a flexible membrane suitable for deployment as a lifetime implantable pressure measuring system. In this context, finite-element modelling (FEM) and finite-element analysis (FEA) of thin film deflection and fatigue behaviour have been conducted. The FEM was implemented in COMSOL Multiphysics with geometries resembling a capacitive type pressure sensor with titanium (Ti) thin film membrane deposited onto the silicon substrate. The mechanical behavior of thin film structures including stresses, strains, elastic strain energy density, and thin film displacements of several thicknesses (50 μm, 25 μm, 4 μm, 1 μm, 500 nm, 200 nm) have been studied. In addition, fatigue physics module has been added to the FEM to analyze the fatigue life of thin film structures. The FEA results in the form of fatigue usage factors have been plotted. Finally, to analyze the effect of fluid pressure transmission of the thin film membrane inside the closed skull, fluid-structure interaction has been modelled. The model represents a 2D fluid medium with the thin film membrane. The velocity magnitude, displacement, shear rate (1/s) and kinetic energy density (J/m3) of 4 μm and 25 μm thick Ti films has been plotted. From this analysis, 4 μm thin film membrane showed good tradeoff for thickness, pressure transmission, and mechanical behaviour. To validate the FEM, a custom designed acoustic-based thin film deflection and fatigue life experiments have been set up. The experimental design comprised of: (1) A voice coil-based multimedia speaker and subwoofer system to assist in displacing the thin film membranes, (2) A laser displacement sensor to capture the displacements, (3) A spectrum analyzer palette for generating random vibrations, (4) Dataloggers to record the input vibrations and thin film displacements, and (5) Scanning electron microscopy (SEM) to visualize the surface topography of thin film structures. Thin film titanium (Ti) foils of 4 μm and 25 μm thick were obtained from William Gregor Ltd, Ti-shop, London. The thin-film specimens were clamped to 3mm acrylic substrates and bonded to the subwoofer system. The Gaussian random vibrations generated from the spectrum analyzer loaded the voice coil of the multi-media speaker system, which assists in displacing the thin films. The SEM surface observations are divided into two regions: (1) Pre-cycle observation, where the thin film surfaces are observed before the application of any load, and (2) Post-cycle observation, where the thin films surfaces are observed after application of cyclic loadings. Based on the understanding of the FEM and experimental studies, a conceptual framework of MEMS pressure sensor has been developed. In this part of the work, initially, underlying concepts of complementary-metal-oxide silicon (CMOS) circuit simulation, MEMS modelling, and CMOS layout design have been discussed. Next the MEMS fabrication process involving deposition (sputtering), etching, and final packaging have been discussed. Finally, an optimized design process of the membrane-based sealed cavity MEMS pressure sensors has been outlined

MEMS sensors for wall shear stress and flow vector measurement

The accurate measurement of airflows is an important area of experimental aerodynamics. MEMS technology has been applied to the measurement of wall shear stress and freestream velocity vectors. Existing methods of measuring wall shear stress vary greatly and have different strengths and weaknesses, making them each applicable to specific situations. Probes designed for measuring 3D velocity components are relatively large in diameter, introducing significant disturbances into the airflow. The tip diameters of such probes are typically of the order of several millimetres and the minimum diameter is around 1 mm. A sensor for measuring wall shear stress, consisting of a surface fence structure 5 mm long, 750 μm high and 20 μm thick was developed. The fence, and main body on which it was mounted, were fabricated from the photo-definable polymer SU8 with an integrated gold resistive strain gauge to measure the pressure-induced deflection. Wind tunnel testing gave a voltage output of 0.18 mV for a shear stress of approximately 0.35 Pa. This concept was then adapted and an in-plane cantilever sensor was developed. The cantilever sensor was manufactured from SU-8 with an integrated resistive strain gauge of NiCr. The pressure-induced deflection of the cantilever, calibrated by the integrated strain gauge, could be related to the wall shear stress on the surface. The sensor gave a response of 9.6x10(^-4) (mV/V) μm under mechanical deflection. For a 2 mm long, 400 μm wide cantilever when tested on a flat plate in a wind tunnel, a response of 1 mV for a shear stress of 0.35 Pa was seen. Four cantilever sensors were arranged orthogonally to create a new type of probe for measuring flow direction and velocity, which could also measure total pressure. The probe was shown to be able to measure these variables and with further development had the potential to allow the fabrication of a smaller probe tip than that possible by conventional methods