Collapse Voltage Research Articles

Fingerprint Imaging System Based on Capacitive Micromachined Ultrasonic Transducer by Using Impediography Method Including Direct Touch and Waveguide Methods.

Fingerprint imaging is a powerful tool in biometric identification systems. This paper proposes a fingerprint imaging method that involves applying an impedance signal to a fingerprint via ultrasonic impediography using a capacitive micromachined ultrasonic transducer (CMUT). We fabricated CMUT sparse array which has a center frequency of 6.25 MHz and collapse voltage of 50 V by local oxidation of silicon process. We develop a prototype by using waveguide and ultrasonic impediography method based on the CMUT. By using a waveguide made from a hard material, the prototype can ensure device protection and image lateral resolution improvement. The proposed technique successfully images the acoustic input impedance differences between the ridges and valleys of a fingerprint. The images created using direct touch and waveguide models are compared and analyzed. In addition, we present a phenomenon in the waveguide system in terms of energy loss by using numerical simulation. Therefore, this study facilitates ultrasonic fingerprint impediography measurement based on the basic direct touch and quartz glass waveguide system.

Optimization of a Collapsed Mode CMUT Receiver for Maximum Off-Resonance Sensitivity

We propose an airborne collapse capacitive micromachined ultrasonic transducer (CMUT) as a practical viable ultrasound transducer capable of providing a stable performance at the off-resonance frequencies. Traditional practice is to bias the CMUT plate close to collapse voltage to achieve high coupling coefficient and sense the incoming ultrasound as an open-circuit receive voltage signal of the transducer or short-circuit receive current (SCRC). Maintaining CMUT plate in the vicinity of collapse threshold is rather difficult. In this paper, an analytic approach to design an airborne collapsed-mode CMUT for maximum off-resonance sensitivity is presented. We use small-signal circuit model to evaluate the performance of a collapsed CMUT for varying operating conditions. CMUT operational parameters that yield the highest off-resonance SCRC are directly obtained from performance design curves. Collapsed CMUT plate is then biased in a critical biasing region that produces a stable and maximum off-resonance sensitivity. We experimentally verify and measure a stable sensitivity of a fabricated collapsed CMUT cell of −60 dB V/Pa at 100 kHz when biased between 50 to 65 V. We characterize our linear circuit model performance against the measured performance of collapsed CMUT in air within 4-dB tolerance. [2018-0058]

Analysis of spring softening effect on the collapse voltage of capacitive MEMS ultrasonic transducers

This paper explained the dependency of collapse voltage on semiconductor device structural features (membrane diameter, membrane thickness and the vertical distance between the electrodes) and physical characteristics (mechanical residual stress of the silicon nitride membrane) considering the electro-mechanical model of MEMS based Capacitive micromachined ultrasonic transducer (CMUT). To have sensitivity comparable to that of piezoelectric ultrasonic transducers (UTs), CMUTs need to be biased close to the collapse voltage. Maximum efficiency is achieved in the conventional mode of operation by biasing the device close to the collapse voltage. The total acoustic output pressure is determined by the efficiency of the device. Hence a careful investigation of the same is decidedly required. Finite element method (FEM) model by PZFlex and analytical model of single element CMUT with 0.75 µm thick silicon nitride membranes suspended on 0.5 µm thick cavity were developed showing resonance frequency at 5 MHz. Through these analyses, it is observed that membrane and vacuum gap thickness are both directly proportional to collapse voltage, while radius of the membrane and also its area are inversely proportional to collapse voltage. Initially the spring softening effect of the membrane has been neglected. Later the effect has been included in the proposed model and analyzed. It has been shown that the spring softening effect cannot be neglected for accurate CMUT modeling. A capacitive micromachined ultrasound transducer can be realized with the model described in this paper.

Quantitative imaging performance of frequency-tunable capacitive micromachined ultrasonic transducer array designed for intracardiac application: Phantom study

Commercially available intracardiac echo (ICE) catheters face a trade-off between viewing depth and resolution. Frequency-tunable ICE probes would offer versatility of choice between penetration or resolution imaging within a single device. In this phantom study, the imaging performance of a novel, frequency-tunable, 32-element, 1-D CMUT array integrated with front-end electronics is evaluated. Phased-array ultrasound imaging with a forward-looking CMUT probe prototype operated beyond collapse mode at voltages up to three times higher than the collapse voltage (-65 V) is demonstrated. Imaging performance as a function of bias voltage (-70 V to -160 V), transmit pulse frequency (5–25 MHz), and number of transmit pulse cycles (1–3) is quantified, based on which penetration, resolution, and generic imaging modes are identified. It is shown that by utilizing the concept of frequency tuning, images with different characteristics can be generated trading-off the resolution and penetration depth. The penetration mode provides imaging up to 71 mm in the tissue-mimicking phantom, axial resolution of 0.44 mm, and lateral resolution of 0.12 rad. In the resolution mode, axial resolution of 0.055 mm, lateral resolution of 0.035 rad, and penetration depth of 16 mm are measured. These results show what this CMUT array has the potential versatile characteristics needed for intracardiac imaging, despite its relatively small transducer aperture size of 2 mm × 2 mm imposed by the clinical application.

Low temperature capacitive micromachined ultrasonic transducers (cMUTs) on glass substrate

The possibility of fabricating capacitive micromachined ultrasonic transducers (cMUTs) on glass substrates may open the way for new fields of application in which the transparency of the substrate is advantageous. In this study, we demonstrate that a low-temperature process can be carried out to achieve cMUTs on glass substrates. Limited to temperatures lower than 400 °C, the process is based on the use of nickel as a sacrificial layer. The cMUT electromechanical behavior is studied and the performance compared to those obtained with silicon substrates. The cMUTs fabricated on glass substrate showed performance comparable with the Si ones. A slight shift in the resonance frequency and collapse voltage was observed. It is shown that these differences arise from the residual mechanical stresses in the substrate.

An improved analytical and finite element method model of nanoelectromechanical system based micromachined ultrasonic transducers

This paper accurately models the silicon nitride membrane displacement profile of nanoelectromechanical system (NEMS) capacitive micromachined ultrasonic transducer (CMUT). The general analytical model is proposed for the linear and non-linear region of operation with the choice of the appropriate spring constant value. Calculated results are validated with 2-D finite element method (FEM) solver PZFLEX. Full 3-D simulations are carried out to show that for a constant area the circular geometry gives maximum displacement followed by hexagonal and square membranes, respectively. Variation of the membrane displacement with its thicknesses, radii, the electrode separation and voltages are plotted. These demonstrate how a CMUT membrane's displacement can be optimized with a change in structural geometries and biases which is a necessary condition for obtaining the required sensitivity. The model evaluates the collapse voltage very close to experimental and FEM results. The dependency of the resonances on the thickness of the membrane is evaluated. Though this phenomenon is not noticed in the available literature, the FEM results support it. The FEM strain profile also reveals that at the operating biases the structure is working well under its strain limit as predicted by the analytical model.

Lumped Electromechanical Modeling of Capacitive Micromachined Ultrasonic Transducers

Conventional plane piston lead zirconium titanate (PZT) based transducers perform poorly for audio speakers and as capacitance receivers due to the lack of proper matching layer materials. Conventional designs have relatively large gaps, 50-100 μm, and use the air in the gap as the restoring force of the vibrating electrode. With the help of silicon micromachining, Capacitive Micromachined Ultrasonic Transducers (CMUTs) are defined. The gaps are made to be as small as 500Å, and the restoring force of the vibrating electrode is the stiffness of the electrode itself. In this paper, a single CMUT element is represented by the lumped electromechanical model. It is realized with comparison of the experimental and simulated outcomes that a single spring constant value is not sufficient to faithfully describe the entire region of operation of CMUT. The search results in two optimized values of spring constants, one fits well to describe the low bias voltage region and the other the collapse voltage regime, where the membrane touches the substrate and is unable to produce vibrations. The membrane displacement behavior under static bias is analyzed. It is found that the displacement at the centre of the membrane is a function of not only the bias but depends highly on the membrane structural geometries and physical characteristics. CMUT membrane’s displacement can be optimized with a change in the above which is a necessary condition for obtaining the required sensitivity. Numerical calculations are done with the help of MATLAB and Finite Element Method (FEM) simulations are aimed with help of PZFlex.

Capacitive micromachined ultrasonic transducer for ultra-low pressure measurement: Theoretical study

Ultra-low pressure measurement is necessary in many areas, such as high-vacuum environment monitoring, process control and biomedical applications. This paper presents a novel approach for ultra-low pressure measurement where capacitive micromachined ultrasonic transducers (CMUTs) are used as the sensing elements. The working principle is based on the resonant frequency shift of the membrane under the applied pressure. The membranes of the biased CMUTs can produce a larger resonant frequency shift than the diaphragms with no DC bias in the state-of-the-art resonant pressure sensors, which contributes to pressure sensitivity improvement. The theoretical analysis and finite element method (FEM) simulation were employed to study the relationship between the resonant frequency and the pressure. The results demonstrated excellent capability of the CMUTs for ultra-low pressure measurement. It is shown that the resonant frequency of the CMUT varies linearly with the applied pressure. A sensitivity of more than 6.33 ppm/Pa (68 kHz/kPa) was obtained within a pressure range of 0 to 100 Pa when the CMUTs were biased at a DC voltage of 90% of the collapse voltage. It was also demonstrated that the pressure sensitivity can be adjusted by the DC bias voltage. In addition, the effects of air damping and ambient temperature on the resonant frequency were also studied. The effect of air damping is negligible for the pressures below 1000 Pa. To eliminate the temperature effect on the resonant frequency, a temperature compensating method was proposed.

Characterization of capacitive micromachined ultrasonic transducers

This paper describes numerical and experimental characterization of capacitive micromachined ultrasonic transducers (CMUTs) for ultrasound transmission. Simulations based on a finite elements method to model the electromechanical behaviour of CMUTs and to determine the dimensions of elementary cells are presented. In particular, we analyse how the collapse voltage and the capacitance are affected by different parameters of a circular cell and by different bias voltages. The fill factor is defined as the ratio of the top electrode radius to the membrane radius and we study the influence of the fill factor in the performances of CMUTs. The fabrication process of a CMUT uses anodic bonding of a SOI wafer on a borosilicate glass substrate and we compare experimental results with numerical results in terms of eigenfrequencies, bandwidth, quality factor and capacitance for non-metallized and metallized membranes.

Optimized Bias Voltage Modulation Sequence for cMUT and Nonlinear Contrast Imaging

Rationale and aim: In the field of ultrasonic contrast agent imaging, Capacitive Micromachined Ultrasonic Transducer (cMUT) are of great interest because of their wide frequency bandwidth. However, due to their intrinsic nonlinear behaviour, their use with classical nonlinear imaging techniques (pulse inversion, amplitude modulation, harmonic imaging..) is still limited. Different approaches have been proposed to suppress the nonlinear part of the emitting signal from a cMUT. Recently, a new imaging sequence called Bias Voltage Modulation (BVM) has shown very good potential but is limited to the conventional regime of the probe (well below the collapse voltage). Thus the probe's sensitivity is not optimized. Materials and methodsAn improved version of the BVM sequences is proposed to allow the use of bias voltages close to the collapse voltage. The principle is to change the coefficients associated with the successive pulses of the sequence. Experiments have been performed using a 128-element cMUT probe connected to an open scanner. For microbubbles measurements, Sonovue contrast agents have been imaged through a flow phantom. A contrast to tissue ratio (CTR) and a signal to noise ratio (SNR) were calculated to assess the efficacy of the new approach. ResultsThe application of BVM sequence with new coefficients show an increase of the CTR of 19dB at high bias voltage (90% of the collapse) compared to the classical BVM sequence. Due to the high sensitivity of the probe in this regime, the SNR is also increased by 9dB. These results reveal that the BVM sequence can be optimized further and applied at bias voltage close to the collapse and thus fully exploit the potential of the cMUT technology for contrast agent imaging.

CMUTs with high-K atomic layer deposition dielectric material insulation layer.

Use of high-κ dielectric, atomic layer deposition (ALD) materials as an insulation layer material for capacitive micromachined ultrasonic transducers (CMUTs) is investigated. The effect of insulation layer material and thickness on CMUT performance is evaluated using a simple parallel plate model. The model shows that both high dielectric constant and the electrical breakdown strength are important for the dielectric material, and significant performance improvement can be achieved, especially as the vacuum gap thickness is reduced. In particular, ALD hafnium oxide (HfO2) is evaluated and used as an improvement over plasma-enhanced chemical vapor deposition (PECVD) silicon nitride (Six)Ny)) for CMUTs fabricated by a low-temperature, complementary metal oxide semiconductor transistor-compatible, sacrificial release method. Relevant properties of ALD HfO2) such as dielectric constant and breakdown strength are characterized to further guide CMUT design. Experiments are performed on parallel fabricated test CMUTs with 50-nm gap and 16.5-MHz center frequency to measure and compare pressure output and receive sensitivity for 200-nm PECVD Six)Ny) and 100-nm HfO2) insulation layers. Results for this particular design show a 6-dB improvement in receiver output with the collapse voltage reduced by one-half; while in transmit mode, half the input voltage is needed to achieve the same maximum output pressure.

Progresses in cMUT device fabrication using low temperature processes

In this paper, we present an original fabrication process of capacitive micromachined ultrasonic transducers (cMUTs) using a low temperature method for high frequency medical imaging applications. The process, which is limited to 400 °C, is based on surface micromachining. The material choices are adapted in order to respect the thermal specifications allowing monolithic integration. Thus, we have found alternative methods to replace the usual high temperature steps in cMUT elaboration. In this way, a nickel silicide layer, presenting good physical and electrical characteristics, is used as a bottom electrode. The membrane, silicon nitride, is deposited using a 200 °C PECVD process. Then, a metallic layer is chosen as a sacrificial layer, in order to achieve the cavity. For that, nickel has been chosen due to its low roughness and its high etching selectivity during the excavation. After their fabrication, the transducers have been tested to verify their functionality and, thus, to validate this low temperature process. Device physical properties have been determined by electrical and optical measurement in air. We evaluated resonance frequency, collapse voltage and electromechanical coupling coefficient in accordance with the simulation. Eventually, low charging effects and low initial deflections can predict good long-term use and ageing of the cMUTs.

OPTIMIZED MECHANICAL DESIGN OF CAPACITIVE MICROMACHINED SWITCH: A CAD-BASED NEURAL MODEL

In this paper, the authors propose the neural modeling of optimized design for pull-in instability reduction of micromachined capacitive shunt radio frequency (RF) micro electro mechanical system (MEMS) switch. Prediction of effective dielectric constant and hence the critical collapse voltage that represents the bridge position instability for two typical bridge geometrics have been derived using artificial neural network (ANN). The effects of residual stress, length of center conductor and the gap between the bridge and center conductor of switch in lowering the driving voltage have been studied. Based on the neural model results, we have observed the reduction of 3 V in critical collapse voltage for an increase of 10 μm in length of center conductor. We have also noted the strong variation in voltage (reduction of 0.8 V for 1 MPa residual stress reduction) with respect to residual stress change. We achieved the reduction of 1.5 V in collapse voltage by reducing the gap between the bridge and the center conductor by 0.1 μm. Among the two structures considered, the structure with lower width of the center conductor proved as an optimum in achieving low critical collapse voltage. Further, the performance of trained neural network with the training datasets derived from MATLAB simulation has been evaluated in terms of convergence speed and mean square error.

Sensitive Area Clustering Based on Voltage Stability and Contingency Analyses with Impact of Line Stability Indices

With a growing concern for maintaining stable voltage operation within the network, many critical issues on voltage stability related studies have been raised in the area of power system research. The problem of voltage stability is one of the main concerns in the operation of power systems. Due to the excessive load demand and non-linearity in loads leads to voltage instability and ultimately leading to voltage collapse. Voltage collapse has to be given the utmost priority because it led to cascading blackouts in the whole network. A voltage stability index is that which examines the voltage stability condition formulated based on line in a power system and identifies sensitive bus of the system to maintain the stable system, which will be great importance. With the help of Voltage stability index, the point of voltage collapse, critical line and critical voltage with respect to load bus are identified. Also this paper deals the development of the sensitive area clustering technique based on the results obtained from voltage stability analysis and contingency analysis. The performance of this index is presented and the effectiveness of the analyzed methods is demonstrated through simulation studies in IEEE 30 bus test systems.

Membrane Displacement Behavior of Capacitive Micromachined Ultrasonic Transducers under Static Bias

The present paper focuses on the theory of vibration of a circular micro-machined ultrasonic transducer to model a single cell of Capacitive Micromachined Ultrasonic Transducers (CMUTs) and it describes the displacement behavior of membrane with respect to applied static bias following Masons analysis of a suspended membrane under tension. Analysis of the critical parameters like collapse voltage and membrane deflection is made and compared with experimental results to verify the validity of the model.

An improved lumped element nonlinear circuit model for a circular CMUT cell

This paper describes a correction and an extension in the previously published large signal equivalent circuit model for a circular capacitive micromachined ultrasonic transducer (CMUT) cell. The force model is rederived so that the energy and power is preserved in the equivalent circuit model. The model is able to predict the entire behavior of CMUT until the membrane touches the substrate. Many intrinsic properties of the CMUT cell, such as the collapse condition, collapse voltage, the voltage-displacement interrelation and the force equilibrium before and after collapse voltage in the presence of external static force, are obtained as a direct consequence of the model. The small signal equivalent circuit for any bias condition is obtained from the large signal model. The model can be implemented in circuit simulation tools and model predictions are in excellent agreement with finite element method simulations.

Accurate Determination of Pull-in Voltage for MEMS Capacitive Devices with Clamped Square Diaphragm

Accurate determination of the pull-in, or the collapse voltage is critical in the design process. In this paper an analytical method is presented that provides a more accurate determination of the pull-in voltage for MEMS capacitive devices with clamped square diaphragm. The method incorporates both the linearized modle of the electrostatic force and the nonlinear deflection model of a clamped square diaphragm. The capacitor structure has been designed using a low stress doped poly silicon diaphragm with a proposed thickness of 0.8 μm and an area of 2.4 mm2, an air gap of 3.0 μm, and a 1.0 μm thick back plate. The value of pull-in voltage calculated using equation is about 6.85V and the finite element analysis (FEA) results show that the pull-in occurs at 6.75V. The resulting pull-in voltage and deflection profile of the diaphragm are in close agreement with finite element analysis results.

Detailed investigation of capacitive micromachined ultrasonic transducer design space

In this study, we present general design guidelines for MEMS ultrasonic transducers, capacitive micromachined ultrasonic transducers (CMUTs), obtained from finite element models with a statistical design of experiment approach. The design space is illustrated using 2-D response surfaces as a function of various geometrical parameters and material selections. Important transducer figure of merits such as operation frequency, fractional bandwidth, collapse voltage and transformer ratio are investigated. Our study illustrates that while keeping geometrical parameters constant, changing the material selection to silicon carbide from silicon dioxide results in a 2.4 times increase in the transformer ratio, and more than 2.5 times increase in the operating frequency while keeping the bandwidth the same. Furthermore, our studies indicate that for a CMUT membrane with an aspect ratio of 4, using a spring constant value obtained analytically results in an error of 160% for the collapse voltage and transformer ratio values.

Deep-collapse operation of capacitive micromachined ultrasonic transducers

Capacitive micromachined ultrasonic transducers (CMUTs) have been introduced as a promising technology for ultrasound imaging and therapeutic ultrasound applications which require high transmitted pressures for increased penetration, high signal-to-noise ratio, and fast heating. However, output power limitation of CMUTs compared with piezoelectrics has been a major drawback. In this work, we show that the output pressure of CMUTs can be significantly increased by deep-collapse operation, which utilizes an electrical pulse excitation much higher than the collapse voltage. We extend the analyses made for CMUTs working in the conventional (uncollapsed) region to the collapsed region and experimentally verify the findings. The static deflection profile of a collapsed membrane is calculated by an analytical approach within 0.6% error when compared with static, electromechanical finite element method (FEM) simulations. The electrical and mechanical restoring forces acting on a collapsed membrane are calculated. It is demonstrated that the stored mechanical energy and the electrical energy increase nonlinearly with increasing pulse amplitude if the membrane has a full-coverage top electrode. Utilizing higher restoring and electrical forces in the deep-collapsed region, we measure 3.5 MPa peak-to-peak pressure centered at 6.8 MHz with a 106% fractional bandwidth at the surface of the transducer with a collapse voltage of 35 V, when the pulse amplitude is 160 V. The experimental results are verified using transient FEM simulations.

Capacitive Micromachined Ultrasonic Transducer Based Gas Sensor Modeling and Simulation

Micro Electro Mechanical Systems (MEMS) based Ultrasonic Transducers presents several advantages for some applications such as ease of array fabrication, unique thermal manipulation capabilities. Micromachining allows us to miniaturize device dimensions and produce capacitive transducers whose performance is comparable with their piezoelectric counterpart. Computer simulation has been widely used for MEMS based system. The main advantage of computer simulation is to provide design optimization by varying geometry, layer dimension and materials of the device without actual fabrication. This systematic approach can save time and cost of device fabrication and experimentation. In this work, a model of Capacitive Micromachined Ultrasonic Transducer (CMUT) is simulated in COVENTORWARE software. The solver is used to analyze the electrical current flow, voltage and heat and stress distribution in the device. The result is used to predict the temperature and stress as a function of the applied voltage across heating layer. The displacement results are used in the mechanical analysis to predict the mechanical behaviour of the CMUT. Here two different types of dielectric materials Si3N4 and Zr02 are used. The optimization is performed mainly based on resonant frequency and collapse voltage. Then we use this model as a gas detector (Sensor) to detect gas like Methane.