Tissue harmonic imaging requires good control of the nonlinearity in the ultrasound probe, as transmitted second harmonics from the probe may interfere with tissue harmonics and degrade image quality. We have studied the nonlinearity in four different medical ultrasound probes by measuring the capacitive part of their electrical impedances under varying electric fields, at frequencies well below and above the resonances. The probes were made with two different piezoelectric materials, piezoceramic PZT and single-crystal PMN-PT, with either soft backing operating at half-wavelength resonance or hard backing operating at quarter-wavelength resonance. When the applied electric field amplitude E was increased from 0.05V/mu m to 0.5V/mu m, we observed an increase in both the capacitance at high frequency, interpreted as clamped conditions, and at low frequency, interpreted as free conditions. This is a nonlinear phenomenon as these capacitances will not change in the linear regime. The increase in free capacitance was from 4 to 10 times larger than the increase in clamped capacitance for all the investigated probes. This indicates a stronger nonlinearity for the free capacitance. At the low-frequency excitation, we observed distortion in the current passing through the acoustic stack corresponding to a relative second harmonic level of -20 dB. We conclude that the nonlinear impedance of the acoustic stack in the investigated probes was primarily caused by nonlinearities in the mechanical coefficients, while contributions from dielectric nonlinearity were negligible.

Tissue harmonic imaging is often the preferred ultrasound imaging modality due to its ability to suppress reverberations. The method requires good control of the transmit stage of the ultrasound scanner, as harmonics in the transmitted ultrasound pulses will interfere with the harmonics generated in the tissue during nonlinear propagation, degrading image quality. In this study, a medical ultrasound probe used in tissue harmonic imaging was experimentally characterized for transmitted second-harmonic distortion to identify and compare the sources of nonlinear distortion in the probe and transmit electronics. The system was tested up to amplitudes above what is found during conventional operation, pushing the system to the limits in order to investigate the phenomenon. Under these conditions, second-harmonic levels up to -20 dB relative to the fundamental frequency were found in the ultrasound pulses transmitted from the probe. The transmit stage consists of high-voltage transmit electronics, cable, tuning inductors, and the acoustic stack. The contribution from the different stages in the ultrasound transmit chain was quantified by separating and measuring at different positions. Nonlinearities in the acoustic transducer stack were identified as the dominating source for second harmonics in the transmitted ultrasound pulses. Contribution from other components, e.g., transmit electronics and cable and tuning circuitry, were found to be negligible compared with that from the acoustic stack. Investigation of the stack's electrical impedance at different driving voltages revealed that the impedance changes significantly as a function of excitation voltage. The second-harmonic peak in the transmitted pulses can be explained by this nonlinear electrical impedance distorting the driving voltage and current.

Understanding the nonlinearity in ultrasound probes is important for tissue harmonic imaging, as 2nd harmonic components transmitted from the probe may interfere with harmonics from the tissue and degrade the image quality. The aim of this paper was to identify the main sources of nonlinearity in a medical ultrasound probe. This was done by investigating the capacitive part of its electrical impedance under high excitation voltages, at frequencies well below and well above resonance. We found that when the excitation voltage amplitude was increased from 10 V to 110 V, the free capacitance increased by 50 %, while the clamped capacitance remained unchanged. We also observed an increase with voltage in the loss tangent under free conditions, but not under clamped conditions. We conclude that the nonlinear electrical impedance of the acoustic stack was associated with the mechanical motion and piezoelectric coupling, while contributions from dielectric nonlinearity was negligible.

Dual-frequency ultrasonic transducers, capable of operating over distinct frequency bands, may be conveniently used in different applications such as harmonic imaging. This paper presents the design and fabrication of a dual-frequency hybrid transducer prototype, consisting of a Capacitive Micromachined Ultrasound Transducer (CMUT) placed on top of a piezoelectric transducer stack. The low-frequency (LF) transducer, i.e. a PZT disk supported by a backing and matched using a quarter-wavelength layer, was designed for transmit (TX) and receive (RX) operation around 7 MHz in immersion, while the high-frequency (HF) transducer, i.e. a single-element Reverse-Fabricated CMUT, was designed for RX operation around 14 MHz. Finite Element Modeling (FEM) was used to investigate the behavior of the proposed hybrid transducer stack. A hybrid transducer prototype was then fabricated. Two additional transducer prototypes, i.e. a single-element piezoelectric and a single-element CMUT, were fabricated using the same materials. The three transducer prototypes were electrically and acoustically characterized. Characterization results were compared with simulations showing that the proposed hybrid transducer approach can be used to increase the RX frequency bandwidth, thus demonstrating its potential in nonlinear imaging applications.

Tissue harmonic imaging is often the preferred ultrasound imaging modality due to its ability to suppress reverberations. The method requires good control on the transmit stage of the ultrasound scanner, as harmonics in the transmitted ultrasound pulses will interfere with the harmonics generated in the tissue during nonlinear propagation, degrading the image quality. In this study, a medical ultrasound probe used in tissue harmonic imaging was experimentally characterized for transmitted 2nd harmonic distortion. To investigate the phenomenon and push the system to the limit, transmit powers well above conventional operation level were tested. We observed transmit levels at the 2nd harmonic frequency up to -20 dB relative to the fundamental frequency. The transmit stage consists of high-voltage output electronics, cable, tuning network, and the acoustic transducer stack. By separating and measuring at different stages in the ultrasound transmit chain, the main source of 2nd harmonics in transmitted ultrasound pulses was identified as nonlinearities in the acoustic stack.

Purpose This paper aims to demonstrate low-temperature bonding for piezoelectric materials at temperatures well below the relevant Curie temperatures so as to avoid depolarization of the piezoelectric material during bonding. Design/methodology/approach Au-coated test samples of lead zirconate titanate (PZT) are bonded to a WC-based resonant backing layer with In–Bi eutectic material in which the In–Bi metal system is a preform or thin, evaporated layers. The bonded samples are characterized using electrical impedance spectroscopy and cross-section microscopy. The first technique verifies the integrity of polarization and reveals the quality of the bondline in a non-destructive manner, particularly looking for voids and delaminations. The latter technique is destructive but gives more precise information and an overview of the structure. Findings Successful low-temperature (115°C) bonding with intact PZT polarization was demonstrated. The bondlines show a layered structure of Au/Au–In intermetallic compounds (with Bi inclusions)/Au, capable of withstanding temperatures as high as 271°C before remelting occurs. For bonded samples using In–Bi preform, repeatable bonds of high quality (very little voiding) were obtained, but the bonding time is long (1 h or more). For bonded samples using evaporated thin films of In–Bi, bonding can be performed in 30 min, but the process needs further optimization to be repeatable. Originality/value Low-temperature solid-liquid interdiffusion (SLID) bonding is a novel technique, merging the fields of low-temperature solder bonding with the SLID/transient liquid phase (TLP) approach, which is normally used for much higher temperatures.

Au-Sn Solid-Liquid Interdiffusion (SLID) bonding of a standard piezoelectric material (PZT) to tungsten carbide (WC) has been investigated. Both materials have a bare bonding surface with an absolute roughness up to 1.5 μm. Bonded samples were characterized by means of acoustic coupling between the PZT and the WC, through electrical impedance measurements. Furthermore, the electrical impedance measurements were employed as a novel method for non-destructive characterization of voids in bond-lines. The results from the non-destructive impedance measurements were compared with the traditional cross-sectional microscopy to inspect voids in SLID bonds, showing good correspondence. Successful Au-Sn SLID bonding of a PZT to a WC is achieved at a temperature higher than 300 °C and a heating rate as high as 120 °C/minute. The bond-line consists of a layered structure Au / Au-Sn (ζ phase) / Au, in accordance with previous studies. Prior to the cross-sectional microscopy, the difference in the bond-lines is clearly observed by the non-destructive electrical impedance measurements. The present study has demonstrated the feasibility of adopting Au-Sn SLID bonding to samples with a high surface roughness.

This paper presents a bonding technique based on gold-tin (Au-Sn) intermetallics to bond an active layer to a backing material in a piezoelectric ultrasonic transducer stack. The bonding process was performed at 310°C. The temperature ramping rate was found to play an important role to the bond quality. Three different bonding temperature profiles P1, P2 and P3 corresponding to a ramping rate 120°C/min., 45°C/min. and 20°C/min., respectively, were investigated. The bonded samples were characterized by electrical impedance measurements in air and cross-sectional microscopy inspection. The most reproducible bondlines were found to be with the heating rate 120°C/min. The resulted bondlines show a layered structure of Au/Au-Sn intermetallic/Au with a corresponding average thickness of 5 µm/17 µm/5 µm, respectively. Based upon energy dispersive spectroscopy (EDS) analysis, the obtained Au-Sn intermetallic compounds are found in good agreement with those from literature. A 64-element one dimensional array formed by Au-Sn SLID method was fabricated and electrical impedance across elements was measured. The results show a good uniformity across elements in the array, and good accordance to Finite Element simulations from COMSOL. This shows the feasibility of using SLID bonding technology to assembly stacks of piezoelectric ultrasonic transducers.

This paper studies a design concept of a dual frequency ultrasonic transducer, in which we use different technologies for each band. A hybrid structure consisting of a piezoelectric stack used for the lower frequency (LF) band and a Capacitive Micromachined Ultrasound Transducer (CMUT) array placed on the top surface used for the higher frequency (HF) band is demonstrated by Finite Element Method (FEM) simulations. Simulated results show that LF structure introduces ripples into the HF. Simulations also show that using an RTV lens on top of the device is sufficient to damp the vibrations from the CMUTs interfering to the LF band. An array to test the LF band of the design was fabricated and measured electrical impedances show a good correspondence to simulations. The work shows a potential of combining piezoelectric and MUTs technologies in ultrasound transducer design for use in medical applications.

Fully digitized 2D ultrasound transducer arrays require one ADC per channel with a beamforming architecture consuming low power. We give design considerations for per-channel digitization and beamforming, and present the design and measurements of a continuous time delta-sigma modulator (CTDSM) for cardiac ultrasound applications. By integrating a mixer into the modulator frontend, the phase and frequency of the input signal can be shifted, thereby enabling both improved conversion efficiency and narrowband beamforming. To minimize the power consumption, we propose an optimization methodology using a simulated annealing framework combined with a C++ simulator solving linear electrical networks. The 3rd order single-bit feedback type modulator, implemented in a 65 nm CMOS process, achieves an SNR/SNDR of 67.8/67.4 dB across 1 MHz bandwidth consuming 131 mu W of power. The achieved figure of merit of 34.2 fJ/step is comparable with state-of-the-art feedforward type multi-bit designs. We further demonstrate the influence to the dynamic range when performing dynamic receive beamforming on recorded delta-sigma modulated bit-stream sequences.

Interconnection technology based on anisotropic conductive film (ACF) has been selected to assemble a transducer array on a flexible substrate in ultrasound imaging applications. The process of bonding and subsequently dicing a transducer on a flexible substrate was evaluated. The results show good integrity of ACF interconnects in test samples undergoing the assembly process. The interconnects, composed of a transducer element and a flex pad, retain high mechanical shear strength and low electrical resistance even after the tough dicing process. Furthermore, a high process yield promising for implementation in industry has been obtained. This work has demonstrated the feasibility of using ACF for assembling ultrasound transducers on system substrates.

Flip-chip interconnection technology based on anisotropic conductive film (ACF) has been selected to assemble multiple chips (i.e. ASICs - Application-Specific Integrated Circuits) on a flexible substrate in an electro-acoustic module. The chips are relatively large with a high number of I/O pins distributed over its area (on the order of 100 mm2). In this work, the processes of bonding one single chip, two chips simultaneously and four chips simultaneously to a substrate using ACF were characterized and compared. The results show similar effect of bond force on electrical resistance of ACF interconnects for bonded samples with one single chip, two chips and four chips. High bond yield values (more than 99 %) are obtained for all groups of samples bonded with a normalized bond force varying from 50 N/chip to 500 N/chip. In general, higher bond force, hence higher bond pressure, provides better ACF interconnects in terms of lower electrical resistance and higher bond yield. This work has demonstrated the feasibility of bonding multiple chips, each of which has a relatively large area with a high number of I/O pins, to a flexible substrate using ACF.

Bonding and integration technologies in ultrasound probes are challenging due to space limitations. Flexible substrates, with the benefits of being bendable, small and allows 3D integration, is an excellent choice for signal transferring in such systems. We have chosen to use anisotropic conductive film (ACF) as interconnection technology and the quality and the reliability of a flex-ACF-flex bonding have been studied. The relationship between bonding force and contact resistance have been characterized including the effect of thermal cycling tests and humidity tests. The study shows that flex-ACF-flex is a robust bonding technique which has very good performance also after environmental tests and is well suited for interconnection in ultrasonic transducer probes.

A low-temperature bonding process for ultrasound transducers is presented: compatible with poling requirements, manufacturability and reliability.In this work, we demonstrate that a thermosonic bonding process can provide a reliable, metallurgical bond at moderate temperatures, even down to room temperature, with bonding times in the order of seconds. Bonding parameters (temperature, compression force, ultrasonic energy) were optimized by evaluating shear strength on Au stud bump bonded Si chips. Model systems have been bonded, mimicking a complete Electro-Acoustic Module (EAM), including a stack of IC emulator / flex interconnection / interface part of the ultrasound transducer.