Audren Boulmé

Fast time-domain modeling of fluid-coupled cMUT cells: from the single cell to the 1-D linear array element

We report a fast time-domain model of fluid-coupled cMUTs developed to predict the transient response-i.e., the impulse pressure response-of an element of a linear 1-D array. Mechanical equations of the cMUT diaphragm are solved with 2-D finite-difference schemes. The time-domain solving method is a fourth-order Runge-Kutta algorithm. The model takes into account the electrostatic nonlinearity and the contact with the bottom electrode when the membrane is collapsed. Mutual acoustic coupling between cells is introduced through the numerical implementation of analytical solutions of the impulse diffraction theory established in the case of acoustic sources with rectangular geometry. Processing times are very short: they vary from a few minutes for a single cell to a maximum of 30 min for one element of an array. After a description of the model, the impact of the nonlinearity and the pull-in/pull-out phenomena on the dynamic behavior of the cMUT diaphragm is discussed. Experimental results of mechanical displacements obtained by interferometric measurements and the acoustic pressure field are compared with simulations. Different excitation signals-high-frequency bandwidth pulses and toneburst excitations of varying central frequency-were chosen to compare theory with experimental results.

A capacitive micromachined ultrasonic transducer probe for assessment of cortical bone

A wide range of ultrasound methods has been proposed to assess the mechanical strength of bone. The axial transmission technique, which consists of measuring guided elastic modes through the cortical shell of long bones such as the radius or tibia, has recently emerged as one of the most promising approaches of all bone exploration methods. Determination of dispersion curves of guided waves is therefore of prime interest because they provide a large set of input data required to perform inverse process, and hence to evaluate bone properties (elastic and geometric). The cortical thickness of long bones ranges from approximately 1 to 7 mm, resulting in wide inter-individual variability in the guided wave response. This variability can be overcome by using a single probe able to operate with a tunable central frequency, typically within the 100 kHz to 2 MHz frequency range. However, there are certain limitations in the design of low-frequency arrays using traditional PZT technology; these limitations have triggered active research to find alternative solutions. Capacitive micromachined ultrasonic transducers (cMUTs) have the potential to overcome these limitations and to improve axial transmission measurement significantly. The objective of this study was to design and construct a new cMUT-based axial transmission probe and to validate the approach. We report all the steps followed to construct such a prototype, from the description of the fabrication of the cMUT (based on a surface micromachining process) through probe packaging. The fabricated device was carefully characterized using both electrical and optical measurements to check the homogeneity of the device, first from cMUT to cMUT and then from element to element. Finally, axial transmission measurements carried out with the prototype cMUT probe are shown and compared with results obtained with a PZT-based array.

A strategy to predict and reduce baffle effects in linear array of CMUTs

This paper presents a cMUT array model based on matrix formulation like for the theory of linear systems. The model is compared with experimental data from which the baffle effect phenomenon in cMUT arrays is clearly identified. We propose to use the eigen-mode decomposition of linear matrices to understand and interpret origins of these phenomena. The impact of normal modes on the radiation directivity of cMUT array is clearly analyzed. The role of some geometric parameters on the baffle effect is studied too.

Design of broadband linear micromachined ultrasonic transducer arrays by means of boundary element method coupled with normal mode theory

In view of the maturity of fabrication processes for capacitive micromachined ultrasonic transducers (cMUTs), engineers and researchers now need efficient and accurate modeling tools to design linear arrays according to a set of technological specifications, such as sensitivity, bandwidth, and directivity pattern. A simplified modeling tool was developed to meet this requirement. It consists of modeling one element as a set of cMUT columns, each being a 1-D periodic array of cMUTs. Model description and assessment of simulation results are given in the first part of the paper. The approach is based on the theory of linear systems so the output data are linked to input data through a large matrix, known as an admittance matrix. In the second part of the paper, we propose reorganization of matrix equations by applying the normal mode theory. From the modal decomposition, two categories of eigenmodes are highlighted, one for which all cMUTs vibrate in phase (the fundamental mode) and the others, which correspond to localized subwavelength resonances, known as baffle modes. The last part of the paper focuses mainly on the fundamental mode and gives several design strategies to optimize the frequency response of an element.

Dual mode cMUTs fabricated with LPCVD sacrificial release process

This paper aims to develop an integrated dual mode ultrasonic transducers (based on cMUTs technology) for applications of targeted drug delivery dedicated to small animal experiments. Two functions are designed on the same transducer: one for high frequency imaging and the other for thermal activation of liposomes. Tests and performances evaluation are reported.

cMUT technology applied to galvanic isolation: Theory and experiments

This study relies on double-sided chips where capacitive Micromachined Ultrasonic Transducers (cMUT) are manufactured on both sides of a silicon substrate in order to perform galvanic isolation between two electrical circuits. The operating principle is based on the propagation of acoustic waves from one transducer to the other, providing a high level of electrical isolation. The work presented here focuses mainly on the assessment of the power efficiency, from an experimental and from a theoretical point of view. Encouraging power efficiency values were reached, showing a significant efficiency gain when taking advantage of the coupling with the substrate.

Electroacoustic response of cMUT-based linear arrays: Role of inactive elements

This paper presents a strategy to model a full cMUT-based linear array. All elements of the array, active and passive, are taken into account. Development focuses mainly on the introduction in our model of the longitudinal dispersive wave, which propagates only in the fluid and at the surface of arrays. Simulations are compared with experiments to assess model validity.

Control and monitoring of dynamic collapse event in cMUTs arrays for therapeutic applications

In this paper, the displacement behavior of the cMUT is analyzed thanks to a time domain model. The nonlinear components in the response spectrum with different excitation parameters are compared. A method based on the addition of a series impedance to the cMUT, which permits to decrease the harmonic components and enhance the maximum displacement swept by the diaphragm, is applied in a therapeutic context. The benefits of such a method are pointed out and discussed. After that, experimental measurements are performed to confirm these advantages, and discussed.

Assessment of imaging capability of 16×16 2D cMUT transducer arrays

Among the different transducers technologies suitable for medical imaging, the cMUTs are known to offer an attractive alternative to conventional piezoelectric ones, with features including wider bandwidth, wider directivity, and readiness for integration with electronics. This last point is of particular interest for 2D matrix arrays. Here, we present the acoustic and imaging performances of these devices initially dedicated for external cardiac imaging (frequency range [1.5–4 MHz]).

Modeling a CMUT covered with passivation layer by means of boundary element method

This paper presents the development of a model to predict the response of CMUTs arrays covered by a viscoelastic material layer, i.e. the passivation layer. Theoretical approach developed here combines two models: on one side a CMUT model based on boundary element method and, on the other side a model of propagation in elastic media for computing the Green function associated to the semi-infinite loading medium. A first confrontation between theory and experiments is presented through a set of electrical impedance measurements obtained with one prototype of CMUT array element. Several loading media with variable viscoelastic properties were tested. It results that for media which rheological behavior obeys to Newtonian model, even in the MHz frequency range, the model validation was achieved. For the others which rheological behavior is poorly understood, it was necessary to implement more complex models like Kelvin-Voigt or Maxwell, in order to improve concordance between theory and experiments.