Collapse Voltage Research Articles

Rational design of CMUTs with annular electrodes for high ultrasonic emission via ESSE enabled stiffness adjustment

Capacitive micromechanical ultrasonic transducers (CMUTs) with high transmitting acoustic pressure are in urgent demand in the rapidly growing field of air-coupled and therapeutic ultras ound. However, most current CMUTs can rarely balance the performance improvement and batch fabrication capacity, which severely impedes their practical applications. This paper proposes novel CMUTs with annular electrodes that can implement significant improvement in multiple performances while featuring a simple structure and batch fabrication feasibility. The annular electrode configurated between the membrane edge and center areas can effectively soften the corresponding-area membrane stiffness through electrostatic spring softening effects, and leave the stiffness of the membrane central area unchanged, finally enabling the membrane to produce a piston-like deformation, thus improving average displacement and output acoustic pressure. A finite element method was employed to analyze the effect of annular electrodes on the CMUT main performance. The results demonstrated that the novel structure could achieve prominent enhancement in multiple performances, such as an average to maximum displacement rate of 0.46 (about 0.32 for conventional CMUTs), maximum improvements of 300%, 255%, and 11% in average displacement, acoustic pressure, and electromechanical coupling coefficients compared to those of conventional ones. Deep analyses of the variation of the main performances including acoustic pressure, receiving sensitivity, and collapse voltage suggested that an optimal electrode coverage range of 36% ∼ 55% can be used to achieve relatively high comprehensive performances. Meanwhile, the proposed CMUTs feature a simpler structure and fabrication process in comparison with previous ones, showing great promise in air-coupled and therapeutic ultrasound applications.

Real-time 3D plane-wave imaging using annular capacitive micromachined ultrasonic transducer array

Imaging methods and 2D sparse arrays have been extensively researched to realize real-time 3D volumetric imaging with higher frame rates. In this study, we developed a 3D plane-wave imaging method using an annular capacitive micromachined ultrasonic transducer (CMUT) array as a breakthrough technique for realizing 3D imaging with a limited number of elements. We adopted the double oxidation and wafer bonding processes for creating the cavity of a CMUT with a 2 µm silicon membrane. To ensure surface uniformity before the bonding process, we implemented an additional reactive ion etching process after the second oxidation. The characterization results showed that the resonant frequency of the proposed device was 6.2 MHz with 65 V collapse voltage. The demonstration of real-time 3D volumetric imaging was conducted in oil using a Vantage 64 ultrasound research system. A plane-wave imaging scheme was used to detect wire phantoms immersed in the oil, with 289 plane waves transmitted over a range of 20° from the center of the annular array. The reconstructed data were used to create three orthogonal cross sections of the target region. The − 6 dB lateral and axial resolutions were 0.56 mm and 0.35 mm for the wires positioned at 18 mm from the proposed device, which were narrow enough to detect a wire phantom of 380 µm diameter. The normalized frame rate was calculated and compared with that in a previous study. We verified that plane-wave imaging has advantages in real-time imaging when compared with the classic phased array and synthetic phased array. Thus, the annular CMUT array is a promising mechanism for real-time 3D imaging with higher frame rate than that possible with previous methods.

Effective Suppression of Amorphous Ga2O and Related Deep Levels on the GaN Surface by High-Temperature Remote Plasma Pretreatments in GaN-Based Metal-Insulator-Semiconductor Electronic Devices.

Gallium nitride (GaN) has been considered one of the most promising materials for the next-generation power and radio-frequency electronic devices, as they can operate at higher voltage, higher frequency, and higher temperature, compared with their silicon (Si) counterparts. However, the fresh GaN surface is susceptible to the natural oxidation composed of Ga2O3, Ga2O, and other intermediate oxidation states. Moreover, the oxidized GaN surface no longer features the distinct atomic step-terrace morphology, resulting in a degraded interface when gate or passivation dielectrics are deposited without appropriate pretreatment. It is responsible for the degraded performance of GaN-based devices such as current collapse and threshold voltage instability. In this study, the proposed high-temperature (500 °C) remote plasma pretreatments (RPPs) can play a significant role in addressing the issue of the deteriorated GaN surface exposed to air. Atomic step-terrace morphology was recovered after 500 °C-RPP due to the removal of oxides and suboxides. First-principles calculations verified that Ga2O at the GaN surface leads to interface states at ∼2.9 eV (EC-E ∼ 0.4 eV) in the bandgap, which is consistent with the increase of interface states at the EC-E range of 0.4-0.9 eV measured through constant-capacitance deep-level transient spectroscopy. Meanwhile, deep interface states and surface-related current collapse are well suppressed in GaN metal-insulator-semiconductor devices. These improved properties by 500 °C-RPP are generalizable to a broader range, including pre-gate and pre-passivation treatment, of which a decent surface/interface is desirable for high-performance GaN-based devices.

High Efficiency Over 70% at 3.6-GHz InAlN/GaN HEMT Fabricated by Gate Recess and Oxidation Process for Low-Voltage RF Applications

In this work, high-efficiency InAlN/GaN high electron mobility transistor (HEMT) is fabricated by recess and oxidation process under the gate (RAO) for low-voltage RF applications. Compared with conventional HEMT fabricated on the same heterojunction, RAO-HEMT exhibits a significant decrease in gate leakage, attributed to the thin oxide layer formed around and under the gate as well as adjustment of the location of the peak electric field and confinement of the strong electric field in the oxide. Due to the alleviated electric field as well as adoption of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\text{N}_{{2}}\text{O}$ </tex-math></inline-formula> for plasma treatment, suppressed RF dispersion including the extremely low current collapse and negligible knee voltage walkout is obtained for RAO-HEMT. Although RAO-HEMT delivers lower saturation current as the result of the partial oxidation of the barrier under the gate, it shows a similar ON-resistance to conventional HEMT, which contributes to the reduction in knee voltage. Besides, higher <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${f}_{T}/{f}_{\text {MAX}}$ </tex-math></inline-formula> is achieved for RAO-HEMT, as the result of the higher dc transconductance and suppressed RF transconductance collapse as well as larger output resistance. Eventually, benefiting from the improvement in gate leakage, RF dispersion, knee voltage, and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${f}_{\text {MAX}}$ </tex-math></inline-formula> by RAO, RAO-HEMT delivers both higher power-added-efficiency (PAE) and output power ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${P}_{\text {OUT}}$ </tex-math></inline-formula> ) than conventional HEMT. At 3.6 GHz, RAO-HEMT provides a high PAE of 71% (74%) at <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${V}_{{\text {DS}}}$ </tex-math></inline-formula> of 6 V (9 V), which performs better than most of the existing Si, GaAs, or GaN RF devices in PAE, revealing the great potential of InAlN/GaN RAO-HEMT in low-voltage low-power consumption RF applications.

Assessing the Microfabrication-Related Variability of the Performance of CMUT Arrays

This paper addresses the assessment of the variability of CMUT arrays&#x2019; electro-mechanical and acoustic performance, as related to the tolerance of the CMUT vertical dimensions due to the microfabrication process. A 3-factors 3-levels factorial sensitivity analysis is carried out to compute the main effects and the interaction effects of the moving plate thickness, the passivation layers thickness, and the sacrificial layer thickness, on the CMUT resonance frequency, collapse voltage, and static capacitance, as well as on the transmission and reception sensitivity amplitude and bandwidth and time delay in water-coupled condition. The analysis is performed by means of FEM simulations of the CMUT static behavior and dynamic response, and the findings are compared to experimental data.

Fabrication of 32 × 32 2D Capacitive Micromachined Ultrasonic Transducer (CMUT) Arrays on a Borosilicate Glass Substrate With Silicon-Through-Wafer Interconnects Using Sacrificial Release Process

Close integration of transducer arrays with supporting electronic circuits is essential in achieving efficient and compact ultrasound systems. An integral part of hybrid integration of 2D CMUT array to CMOS electronics is the introduction of through-glass-via (TGV) interconnects in glass substrates as an integral part of the 2D CMUT array fabrication. Micro-cracks around via locations, via discontinuity, and poor coplanarity between the vias and glass substrate are some of the challenges with laser-drilled, paste-filled copper-through-glass-via (Cu-TGV) interconnects. This study provides a detailed fabrication process for making <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$32\times 32$ </tex-math></inline-formula> -element 2D CMUT arrays on a composite glass substrate incorporating silicon-through-glass vias (Si-TGV) as interconnects using sacrificial release approach. On one column of a fabricated 2D CMUT array, we measured a mean resonant frequency of 5.6 MHz in air and an average device capacitance of 1.5 pF. With the introduction of a buried top electrode in the device structure, we achieved a collapse voltage of 93 V, which is considerably lower than the collapse voltage measured in our previously demonstrated 2D CMUT arrays with top electrode on top of the nitride plate. The fabricated array is flip-chip bonded on a custom-designed driving integrated circuit to demonstrate the complete system operation. We measured a peak-to-peak pressure of 1.82 MPa at 3.4 MHz, and 5 mm from the array surface in a 0.33 mm focal spot size. [2021-0101]

A Review on Analytical Modeling for Collapse Mode Capacitive Micromachined Ultrasonic Transducer of the Collapse Voltage and the Static Membrane Deflections

Analytical modeling of capacitive micromachined ultrasonic transducer (CMUT) is one of the commonly used modeling methods and has the advantages of intuitive understanding of the physics of CMUTs and convergent when modeling of collapse mode CMUT. This review article summarizes analytical modeling of the collapse voltage and shows that the collapse voltage of a CMUT correlates with the effective gap height and the electrode area. There are analytical expressions for the collapse voltage. Modeling of the membrane deflections are characterized by governing equations from Timoshenko, von Kármán equations and the 2D plate equation, and solved by various methods such as Galerkin’s method and perturbation method. Analytical expressions from Timoshenko’s equation can be used for small deflections, while analytical expression from von Kármán equations can be used for both small and large deflections.

A study of the foil explosion in vacuum using spectral streak camera diagnostics

The foil explosion in vacuum was studied experimentally using a spectral streak camera. The experiment was performed with Cu, Al, Ni, and Ti foils of thickness 6 μm at a current density rise rate of ∼6 × 105 A/(cm2 ns). For all exploded foils, a plasma corona was formed in the gas desorbed from the foil surface when the voltage across the discharge gap reached the collapse voltage. In the Cu, Al, and Ni foil explosions, the specific deposited energy εdep was sufficient for complete melting but insufficient for vaporization of the foil material. However, the streaked spectra obtained for these foil explosions contained the foil material lines that appeared 80–200 ns after tcoll. The authors hypothesize that these lines appeared as a result of the decaying of the metastable liquid state of the exploded foil metal, which took place at a certain time after the core was shunted. In the Ti foil explosion, the deposited energy was insufficient to melt the metal, the transition to the metastable liquid state did not occur, and the discharge plasma spectrum did not contain lines of the foil material.

Broad bandwidth air-coupled micromachined ultrasonic transducers for gas sensing

The present work aims to develop ultra-wide bandwidth air-coupled capacitive micromachined ultrasonic transducers (CMUTs) for binary gas mixture analysis. The detection principle is based on time-of-flight (ToF) measurements, in order to monitor gas ultrasound velocity variations. To perform such measurements, CMUTs were especially designed to work out of resonance mode, like a microphone. The chosen membrane size is 32 × 32 µm2 and gap height is 250 nm. The resonance frequency and collapse voltage were found at 8 MHz and 58 V respectively. As mentioned, the CMUTs were exploited in quasi-static operating mode, in a very low frequency band, from 1 MHz to 1.5 MHz frequencies. The transducer impulse response was characterised, and a −6 dB relative fractional frequency bandwidth (FBW) higher than 100% was measured, enabling to use CMUT for the targeted application. Additionally, a measuring cell has been designed to hold the fabricated CMUT emitter and receiver prototypes facing each other. The volume inside the cell was kept lower than 3 mL and the surface of emitter/receiver was 1.6 × 8 mm2. To validate the general principle of the proposed technique, two binary gas mixtures of CO2/N2 and H2/N2, with varying concentrations, have been tested. The results are very promising with a measured limit of detection (LOD) of 0.3% for CO2 in N2 and 0.15% for H2 in N2.

Closed-Form Expressions on CMUTs With Layered Anisotropic Microplates Under Residual Stress and Pressure.

Capacitive micromachined ultrasonic transducers (CMUTs) are promising in the emerging fields of personalized ultrasonic diagnostics, therapy, and noninvasive 3-D biometric. However, previous theories describing their mechanical behavior rarely consider multilayer and anisotropic material properties, resulting in limited application and significant analysis errors. This article proposes closed-form expressions for the static deflection, collapse voltage, and resonant frequency of circular-microplate-based CMUTs, which consider both the aforementioned properties as well as the effects of residual stress and hydrostatic pressure. These expressions are established by combining the classical laminated thin plate (CLTP) theory, Galerkin method, a partial expansion approach for electrostatic force, and an energy equivalent method. A parametric study based on finite-element method simulations shows that considering the material anisotropy can significantly improve analysis accuracy (~25 times higher than the theories neglecting the material anisotropy). These expressions maintain accuracy across almost the whole working voltage range (up to 96% of collapse voltages) and a wide dimension range (diameter-to-thickness ratios of 20-80 with gap-to-thickness ratios of ≤2). Furthermore, their utility in practical applications is well verified using numerical results based on more realistic boundary conditions and experimental results of CMUT chips. Finally, we demonstrate that the high accuracy of these expressions at thickness-comparable deflection results from the extended applicable deflection range of the CLTP theory when it is used for electrostatically actuated microplates.

Experimental studies of the shunting discharge developing during a foil explosion in vacuum

An experimental study of Al and Ti foil explosions was performed at a current density rise rate of ∼6 × 105 A/(cm2·ns) using a framing camera (3-ns exposure time) and a streak spectral camera. Framing photography has shown that during an Al or a Ti foil explosion, a low-density plasma corona is formed 10–20 ns before a time at which the voltage across the discharge gap reaches the collapse voltage, t = tcoll. Spectral measurements have shown that atomic hydrogen and carbon lines occurred at t = tcoll for both Al and Ti foil explosions. The spectral radiation of the foil material was detected only for Al foil explosions 50–100 ns after t = tcoll. The experimental data and the estimates made in the study indicate that the formation of a plasma corona during a foil explosion occurs in the gas desorbed from the foil surface.

An Efficient Model of Nanoelectromechanical Systems Based Ultrasonic Sensor With Fringing Field Effects

In this work, modeling of parasitic capacitance due to fringing field and its influence on the performance of nano-electro-mechanical-system (NEMS) capacitive micromachined ultrasonic sensor (CMUT) is reported. The fringing effect was modeled with the help of empirical approach which is based on equivalent parallel plate capacitor employing circular membrane approximation. While accommodating the fringing field, the finiteness of the electrode diameter was taken care of. Two important performance parameters, electromechanical coupling coefficient and collapse voltage, were modeled. Influence of the fringing field from a finite dimensional electrode on various structural parameters was also investigated. Capacitance of a single cell, including the static deformation produced by polarizing voltage was evaluated numerically. As an important figure of merit, it was observed that the coupling factor is strongly dependent on the bias voltage, radius, membrane thickness, and also on the electrode separation. The factor approaches unity for higher bias (close to collapse) and large diameter thin membranes. Comparative study of percentage errors for direct and fringing model shows a significant improvement in the case of fringing approach, as expected. The error percentage for the evaluation of the collapse voltage shows a reduction of more than 10%.

Effect of High-k Passivation Layer on High Voltage Properties of GaN Metal-Insulator-Semiconductor Devices

In this paper, the GaN-based MIS-HEMTs with Si <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> single-layer passivation, Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> /SiN <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> bilayer passivation, and ZrO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /SiN <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> bilayer passivation are demonstrated. High-k dielectrics are adopted as the passivation layer on MIS-HEMTs to suppress the shallow traps on the GaN surface. Besides, high permittivity dielectrics passivated MIS-HEMTs also show an improved breakdown voltage characteristic, and that is explained by 2-D simulation analysis. The fabricated devices with high-k dielectrics/SiN <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> bilayer passivation exhibit higher power properties than the devices with plasma enhanced chemical vapor deposition-SiNx single layer passivation, including smaller current collapse and higher breakdown voltage. The Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> /SiN <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> passivated MIS-HEMTs exhibit a breakdown voltage of 1092 V, and the dynamic R <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">on</sub> is only 1.14 times the static R <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">on</sub> after off-state VDS stress of 150 V. On the other hand, the ZrO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /SiN <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> passivated MIS-HEMTs exhibit a higher breakdown voltage of 1207 V, and the dynamic Ron is 1.25 times the static R <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">on</sub> after off-state VDS stress of 150 V.

Condenser MEMS Microphone

The market for MEMS microphones is growing every year. The aggregate average annual growth rate is 11.7%. This is due to the increase in devices using voice control, the increase in the number of microphones in smartphones, and the increasing role of MEMS microphones in the Internet of Things. A MEMS microphone consists of a MEMS element for converting acoustic pressure, a preamplification chip, a board with an acoustic inlet, and a cover. The MEMS conversion element is a variable capacitor with polysilicon plates. The main parameters of MEMS microphones are the signal-to-noise (S/N) ratio, non-linear distortion coefficient, and non-uniformity of the amplitude-frequency characteristics. When developing a MEMS microphone, computer simulation is used in the ANSYS software package. Using the ANSYS Mechanical program, the design is calculated and optimized to achieve the necessary sensitivity parameters of the MEMS conversion element. Using electrostatic analysis, the CV characteristics are calculated to determine the active capacitance and collapse voltage of MEMS-CE membranes. The influence of the housing dimensions, position, size and number of acoustic holes, the sizes above the membrane and under membrane volumes of the microphone frequency response is calculated in order to determine the effect of the housing parameters on the MEMS microphone’s characteristics. An experimental prototype of the first domestic MEMS microphone is developed and manufactured at the NPK Technological Center. New designs are being actively designed to improve the technical characteristics of MEMS microphones.

Collapse voltage analysis of central annular ring metallized membrane based MEMS micromachined ultrasonic transducer

In this paper we have explored central annular ring metallized capacitive micromachined ultrasonic transducer (CMUT) structure. The metallization is done partially and it is patterned in such a way that it forms a flat ring on the surface of the membrane. This will decrease the price of the device as the need of metallization is less. Moreover, it is seen that the maximum displacement that the membrane can attain just before collapsing does not change even for partial metallization. It is perceived that with decreasing the area of the electrode the collapse voltage increases and vice versa. Changing the gap height has a huge impact on the collapse voltage. We have also examined the effect of different membrane materials on collapse voltage. Changing the membrane thickness hardly affects the value of collapse voltage. The electrode thickness is infinitesimally small as compared to the membrane thickness and is neglected in the analytical modeling approach. The analytical results are compared with three-dimensional (3-D) finite element method (FEM) model results. Excellent agreements between them are observed.

Comparative Study Between Partially and Fully Recessed‐Gate Enhancement‐Mode AlGaN/GaN MIS HEMT on the Breakdown Mechanism

In this paper, two enhancement‐mode (E‐mode) AlGaN/GaN recessed‐gate MIS devices with the in situ N2 plasma treatment, known as partially recessed‐gate metal insulator semiconductor (MIS) device and fully recessed‐gate MIS device, are designed. Among them, the partially recessed‐gate MIS device shows excellent characteristics that the threshold voltage of +3.5 V, saturation current density of 722 mA mm−1, peak transconductance of 212 mS mm−1, and the power figure of merit of 4.39 × 108 W cm−2 which is the highest value in E‐mode AlGaN/GaN MIS device with the threshold voltage greater than 3 V. Moreover, the current collapse and breakdown voltage of partially recessed‐gate MIS device and fully recessed‐gate MIS device are investigated, and Silvaco simulation is carried out to analyze the difference of breakdown mechanism between two structure devices. Through the analysis of five main breakdown leakage paths, we can get the conclusion that the factor causing the breakdown voltage difference between partially recessed‐gate MIS device and fully recessed‐gate MIS device is the difference in the width of depletion region of the two devices.

Design and Analysis of Capacitive Micromachined Ultrasonic Transducer

Background:Objective:To simulate a Micromechanical systems (MEMS) based CMUT working as a transmitter with the existing design and provide comparison within the possible architectural geometries.Methods:FEM simulation software COMSOL is used to simulate the 3D model of the transducer radiating in the air. The classical thin-plate theory is employed to solve for CMUT with a circular shape which is sufficient when the ratio of the diameter to thickness of the plate is very large, an aspect common in CMUTs. The Galerkin-weighted residual technique is used to get a solution for thin plate equation with the presumption that the deflections are small in comparison to the thickness of the plate.Results:The resonant frequency of CMUT with different geometries have been calculated. The deflection of membrane with applied DC bias is shown along with collapse voltage calculation. The generated ultrasound is shown with the AC bias superimposed on the DC bias. The capacitance change with the increasing DC voltage is discussed. The deflection of membrane is maximum as the resonance frequency is proved.Conclusion:The review of Capacitive Micromachined Ultrasonic Transducer architectures with different shapes is highlighted. The working behavior of CMUT with suitable dimension is simulated in 3D providing researcher data to wisely choose the CMUT prior to the fabrication. The CMUT is prioritized on various characteristics like wafer area utilization, deflection percentage within the cavity and durability of the transducer.

Micro-electro-mechanical-system based capacitive ultrasonic transducer as an efficient immersion sensor

In this paper we have analyzed the influence of the membrane inherent residual stress along with its Young’s modulus for evaluating the characteristics of a capacitive micromachined ultrasonic transducer (CMUT) for medical imaging and sensor applications. The membrane displacement profile, the frequency response characteristic and the coupling coefficient are evaluated and a close conformity with the PZFlex Finite Element method (FEM) simulations has been confirmed. The variation of the coupling coefficient with the different structural parameters indicates the use of thinner membranes with lower transduction gap as efficient transducers. Moreover, coupling factor increases as bias increases and an investigation of the collapse voltage is essential. Various coupling mediums are analyzed to find the transducer equivalent resistance and quality factor to predict its utility as a sensor for gaseous and liquid coupling mediums.

An Analysis Method for Capacitive Micromachined Ultrasound Transducer (CMUT) Energy Conversion during Large Signal Operation.

With Capacitive Micromachined Ultrasound Transducers (CMUTs) increasingly being used for high intensity, large signal ultrasound applications and several drive methods being proposed, the efficiency of these devices in this operation regime have not been quantitatively evaluated. Since well-known frequency and capacitance-based coupling coefficients definitions are not valid for large signal, nonlinear operation, an energy-based definition should be used. In this paper, an expression for mechanical energy in a CMUT is obtained based on the assumption that CMUT is a linear time varying capacitor in all regimes of operation. This expression is evaluated by the help of an experimentally verified nonlinear CMUT model to define an energy conversion ratio (ECR) which can be considered as a coupling coefficient valid for all regimes of operation. This parameter is validated in the small signal regime and then used to evaluate CMUT performance with various large drive signals. The quantitative modeling results show that CMUTs do not need DC bias to achieve high efficiency large signal transduction: AC only signals at half the operation frequency with amplitudes beyond the collapse voltage can provide efficiencies (ECR) above 0.9 with harmonic content below −25 dB. Based on these results, ECR variation with membrane geometry and parasitic capacitance are given as examples for device optimization. The overall modeling approach is also qualitatively validated by experiments.

Capacitive Micromachined Ultrasonic Transducer (CMUT): Analytical Evaluation of Membranes Performance Under Fabrication Related Stress

Kapasitif Mikro İşlenmiş Ultrasonik Çevirgeç (kMUÇ)’in alıcı ve verici performansı gap (kavite) yüksekliği, zar kalınlığı ve zar çapı gibi birçok parametreye bağlıdır. YHigh power transmission from CMUT (capacitive micromachined ultrasonic transducer) surface depends on output pressure and membrane displacement. Moreover, fabrication process related stress on membrane should also be considered because it affects CMUT performance in terms of collapse voltage, resonance frequency and gap distance. Therefore, stress on membrane becomes important criteria for CMUT modelling and fabrication. Surface micromachining and wafer bonding technologies are widely used for CMUT fabrications. These fabrication processes include several depositions and etching steps that those induce stress on CMUT membrane. Fabrication process related stress are classified as compressive or tensile. In this study, three common CMUT membranes, Si3Ni4, Poly-Si and SiC, were selected for analytic calculations and displacement and output pressure of these CMUT membranes were evaluated under built in stress. It was shown that stress on membrane has significant effect on membrane deflection and pressure from device surface for three membranes. As a result, stress on vibrating membrane should be minimized and optimized for reliable and high performance device fabrication when considering wide range of CMUT applications.