Arne Rønnekleiv

CMUT array modeling through free acoustic CMUT modes and analysis of the fluid CMUT interface through Fourier transform methods

A method for analyzing capacitive micromachined ultrasonic transducer (CMUT) arrays and arrays of elements composed of several CMUTs is proposed. It is based on a combination of a free acoustic mode description of an isolated CMUT, and the coupling of these modes to the fluid in which waves should be excited or detected through an impedance matrix that will depend on frequency. The parameters of the model describing the isolated CMUT is independent of frequency and excitation of neighbor CMUTs, whereas the acoustic impedance matrix describing the coupling to the fluid will depend on both the excitation of neighbor CMUTs and frequency. Hence, this splitting of the calculations has a potential for saving computer time. The analysis gives transfer functions from excitations that vary harmonically with time and space along the array surface to CMUT parameters as current, mode excitations, or output acoustic pressure. Based on this, the response of essentially arbitrary excitations of the CMUTs may be obtained. The method is used to analyze an infinitely large array of circular CMUTs on a rectangular grid. The CMUTs are assumed to be operating in collapsed mode. Sharp resonances are shown to occur that could be significantly damped by adding series resistors to the CMUTs or increasing the water viscosity.

Fabrication and Characterization of CMUTs realized by Wafer Bonding

This paper presents the fabrication process and electrical characterization of Capacitive Micromachined Ultrasonic Transducers (CMUTs) realized by fusion bonding. The transducer array with electrical connections was realized by four photolithography steps. The electrical characterization of two different geometries is presented. The smallest structure have 120 nm deep circular cavities with radius of 5.7 mum separated by a pitch of 12.5 mum and are vacuum-sealed by a 100 nm silicon nitride membrane. The dimensions in the second geometry are twice as large except of the cavity gap which is the same. The electrical measurements of the input impedance were carried out by a vector network analyzer on one row of 52 elements, each with four CMUTs. In non-collapsed mode in air the small CMUTs have a resonance frequency of 30 MHz with a Q value of 70 when DC biased by 30 V. When immersed in oil, the resonance frequency was reduced to 8 MHz and the peak was broadened as expected. The large CMUT structures show somewhat distorted resonances at about 13 MHz in air, with Q-values up to 50 to 60 for the better devices. Simulations indicate resonance frequencies that are about 20% lower. Hence some process is taking place in these devices that are not included in the theory. No hysteresis was observed in any of the measurements

Enhancement of SAW laser probe measurements by signal processing

Many different types of the SAW laser probe have been used for several years to characterize SAW devices as well as inherent properties of particular materials. Our laser probe is of a modified knife-edge type. Operating with a linear response, it has a high dynamic range. The probe provides full phasor information of the detected signal, and directional properties of the detection process makes it possible to determine the surface tilt components in two spatially orthogonal directions. We first sum up the basic characteristics of the probe. We show several examples where we take advantage of these features combined with signal processing techniques such as the fast Fourier transform. This enables us to concentrate on distinct properties of the devices under test. We thus exploit the spatial frequency domain and, in the basic detection process, the directions of the wavecrests, to enhance measurements of particular properties. Examples include results obtained from measurements on a selection of components containing various structures.

5F-5 Reducing Fluid Coupled Crosstalk Between Membranes in CMUT Arrays by Introducing a Lossy Top Layer

Capacitive micromachined ultrasound transducers (CMUTs) promise high transducer performance for several ultrasound applications. Likely the most promising opportunities will be found in applications which require arrays with a large numbers of individual elements for precise beam steering and focusing such as in ultrasound imaging. In off-axis beam steering neighbor elements operate at different phase. This leads to deformation gradients along the surface of the array which cause local tangential forces acting upon the adjacent medium. In the case of an adjacent lossless fluid this results in local high-Q resonances which have significant detrimental impact on the transducer array performance in off-axis operation. It is therefore of paramount importance to control and reduce the excitation of such resonances to an acceptable level through the design. The present paper presents one approach to this. Simulations show that by introducing a soft intermediate surface layer which is only a few per cent of its longitudinal wavelength in thickness, and which has high shear deformation losses - of the order of 0.4 in loss tangent, quite acceptable results may be obtained without adding more than 0.5-1 dB in transmit losses. Although not readily commercially available, it may be possible to develop adequate polymer materials for this purpose. It is shown also that such a layer may be mechanically protected by an additional stiff layer without significant degradation of the ultrasound transducer performance

Co-optimization of CMUT and receive amplifiers to suppress effects of neighbor coupling between CMUT elements

Capacitive micromachined ultrasound transducers (CMUTs) promise high transducer performance for several ultrasound applications. When using the CMUT array for medical applications where a focused ultrasound image with a 90 degree image sector is needed, we need a large number of individual elements. In off-axis beam steering, neighbor elements operate at different phase. This leads to unwanted acoustic effects caused by the interaction with the fluid medium outside the array. We see high-Q resonances close to the center frequency of the array at off-axis angles, which we want to reduce. We propose to use Transimpedance Amplifiers (TIAs) and Charge Sampling Amplifiers (CSAs) where we can easily adjust the input impedance, which opens up the possibility to design amplifiers that are optimized for an ultrasound system with CMUTs. Simulations show that a low impedance path results in suppression of the effects of resonances for both CSAs and TIAs and that co-optimization is important since the frequency of the CMUT array affects the Q-factor of the unwanted resonances. Even though we introduce an impedance mismatch the noise figure is still at an acceptable level. We present simulations in water, blood plasma with estimated data, and olive oil and show that the viscosity of the medium greatly influences the presence of resonances. This indicates that effects that might be present in human tissue may be much reduced in olive oil or other vegetable oils.

Backing requirements for CMUT arrays on silicon

Capacitive Micromachined Ultrasonic Transducer (CMUT) have been subject to research by several research groups during the last two decades. Despite many potential advantages over traditional piezoelectric ultrasound transducers, the CMUT technology has not yet made a proper commercial breakthrough. One issue which we believe need more investigation is the control of the acoustic crosstalk, both through the silicon substrate and through the adjacent fluid medium.The work presented in this thesis concerns the modeling of immersed CMUT arrays and the investigation of two different acoustic crosstalk effects which may harm the transducer response. The CMUT array is modeled with an analytical model describing the motion of the single acoustically isolated CMUT cell as a combination of free acoustic vibration modes. Several CMUT cells may be connected to form larger elements, and the vibration modes of adjacent CMUTs are coupled through the fluid outside, by an acoustic impedance matrix. In addition, the model may also include the motion of the silicon substrate and the in uence from the source impedance of the electronics.The first crosstalk effect which we focus on in this work, is the generation of surface acoustic waves (SAWs) along the surface of the silicon substrate supporting the CMUT array. If the SAWs are not damped in any way, they may couple to waves in the fluid at certain steering angles, and cause a drop in transmission eciency at these angles. In the presented simulation we show that the silicon substrate must be limited in thickness in order for the backing material to be able to damp the SAWs. If the CMUT array is mounted on top of a stack of silicon substrates containing transmit and receive circuitry, the thickness and composition of both the bonding materials and the silicon substrates must be taken into account. We have compared three different bonding techniques, surface liquid interdi usion (SLID) bonding, anisotropic conductive adhesive (ACA) and direct fusion bonding, and we show that fusion bonding is the technique which is best suited for high frequency CMUT arrays with several IC chips underneath. The penetration depth of the SAWs is frequency dependent, and we show that a high frequency CMUT array with center frequency around 30 MHz, stacked on top of three circuit layers, should have a total silicon thickness below 100 m. If the acoustic backing material instead is placed between the CMUT array and the rst circuit chip, other bonding techniques than fusion bonding may also be used, and the thicknesses of the IC chips are no longer of importance. However, the transmission of the electric signal through or around the backing layer might be a challenge.The other crosstalk effect which has been investigated in this thesis, is the inter-element coupling at the CMUT- fluid interface. We refer to this effect as dispersive guided modes, and show that the excitation of local resonances in the interface region may affect the overall transmission from the array at frequencies well within the 100 % bandwidth of the transducer. These waves are not damped by the acoustic backing material. Many CMUT designers choose to have larger distances between CMUT cells in neighbor elements than between CMUTs within an element. We denote this as double periodicity. We have compared the e ect of the dispersive guided modes in arrays with the same distance between all the CMUT cells, regardless if they are in the same or in adjacent elements (denoted as single periodicity), and arrays with double periodicity. Simulations show that the response from arrays with double periodicity is affected by the crosstalk even at broadside radiation, whereas the effect becomes apparent at o -axis beam steering for arrays with single periodicity.Through simulations, we have shown that it is possible to mechanically damp the dispersive guided modes substantially by introducing a lossy coating material of a few micrometer thickness. The PDMS material RTV516 from GE Silicones seems to have material properties which are well suited for such damping. We have also shown that introducing a low electric source impedance in the transmit electronics may reduce the e ect of the local CMUT- uid resonances on the detected signal.

Minimizing the bottom reflection in Ultrasonic CMUT Transducer backing using low profile structuring

Capacitive Micro-machined Ultrasonic Transducer (CMUT) transducers need an acoustic backing to ensure that any acoustic signal which propagates from the transducer into the substrate is absorbed in the backing. The backing should be made such that it does not give a false echo in the signal received by or transmitted from the transducer. Ideally, this backing material should provide high attenuation and it should match the acoustic impedance of the CMUT supporting structure (usually silicon). To avoid the echoes described above, a thick backing layer is required. But in many cases, there is little space available under the transducer so that it is difficult to accommodate a sufficiently thick layer of material with realistic propagation losses, to ensure that no signal is reflected back to the transducer. In this paper, we discuss irregular structures at the bottom surface that are used to scatter the waves. The proposed structure scatters the waves into waves with significantly changed propagation directions, giving long propagation paths back to the transducer. The structure is analyzed using FEM simulations for a simple 2D case. Different ways of implementing the structure is also discussed.

Analysis of charge effects in high frequency CMUTs

This paper presents a theoretical analysis of charge effects in silicon nitride membranes in Capacitive Micromachined Ultrasonic Transducers (CMUTs) working at 30 MHz in air. The analysis was motivated by observations of drift in the resonance frequency to the CMUTs during electrical and heterodyne interferometric measurements while keeping the DC voltage level constant over time. After applying a DC voltage of -40 V for 3 hours, a drift of 0.6 MHz is observed. Analysis showed that this corresponded to a transfer of 15 per cent of the electrode charge into the silicon nitride membrane. While adjusting the voltage to keep the coupling factor constant equal to 0.5 and comparing a charge ratio of 0 and 1, the electrical Q-factor is increased by 8.5 % while the mechanical Q-factor is decreased by 7.8 % for a cavity height of 100 nm. The mechanical resonance frequency is reduced by 6.9 %. However, these changes are moderate and do not impact the response of the CMUT significantly in wide band transducer operation.

Design of micromachined resonators for fish identification

The ID tag presented here was designed to give a tag of small size that could be produced at a low prize, and that could be read remotely in live fish, even in seawater. The last condition precludes use of electromagnetic waves for interrogation of the tags, and acoustic interrogation is then a clear alternative. The solution presented is a passive tag with a set of acoustic resonances that may be detected acoustically. The tag operates in the 200 to 400 kHz range. The identity of the tag is given by a unique combination of resonances in this frequency range. For the tags presented here there are five resonances per tag. If five or more resonances are chosen from a predetermined set of say 17 resonance frequencies, a total number of at least 3000 to 4000 different tags are available. This is adequate for classification of fish at the batch level in fish farms, or of local wild fish tribes. The resonators on a tag consists of a thin, nominally 500 nm thick silicon nitride membrane suspended over separate evacuated cavities, made by bulk silicon micromachining. The resonators were designed to have Q-factors in the range 27 to 35 with viscous losses in the water neglected. The resonators have been measured in water and in dead or live anesthetized fish from distances up to 30 cm. Sharp resonances in fair accordance with the tag design were achieved. Some alterations of the tag response with change of the angular orientation of the tag relative to the ultrasound beam are seen. This is also theoretically expected.

Surface acoustic wave resonator from thick MOVPE-grown layers of GaN(0001) on sapphire

We report on fabrication and measurement of a surface acoustic wave resonator prepared on ∼10m thick GaN(0001) films. The films were grown by metal-organic vapor phase epitaxy on a c-plane sapphire substrate. The surface morphology of the films were examined with scanning electron and atomic force microscopy. A metallic bilayer of Al/Ti was subsequently evaporated on the nitride film surface. Definition of the resonator interdigital transducers, designed for a wavelength of λ=7.76m, was accomplished with standard UV lithography and lift-off. S-parameter measurements showed a resonator center frequency f 0 =495MHz at room temperature, corresponding to a surface acoustic wave velocity of 3844m/s. The insertion loss at center frequency was measured at 8.2dB, and the loaded Q-factor was estimated at 2200. Finally, measurements of the resonator center frequency for temperatures in the range 25–155°C showed a temperature coefficient of -18ppm/°C. The intrinsic GaN SAW velocity and electromechanical coupling coefficient were estimated at ν SAW =383 1m/s and K 2 =1.8±0.4·10 −3 .