Capacitive Micromachined Ultrasonic Transducer Array Research Articles

Monolithic Multiband CMUTs for Photoacoustic Computed Tomography With In Vivo Biological Tissue Imaging.

Among the biomedical imaging modalities, photoacoustic computed tomography (PACT) was one of the emerging hybrid techniques in recent years. In designing the PACT imaging system, a finite-bandwidth transducer is one of the limited factors for the overall performance. As the target size is inversely proportional to the dominant frequency components of the generated photoacoustic (PA) signal, a broad bandwidth transducer is desired for different scales' imaging. In this paper, a monolithic multiband capacitive micromachined ultrasonic transducer (CMUT) array was designed and fabricated for the reception of the wideband PA signals so as to provide high-resolution images with high-frequency CMUT arrays and present the high signal-to-noise-ratio major structure with low-frequency CMUT arrays. To demonstrate its performance, a phantom experiment was conducted to show and evaluate the various qualities of multiresolution images. In addition, an in vivo mouse model experiment was also carried out for revealing the multiscale PA imaging capability with the multiband CMUTs on biological tissues. From the obtained results, the images from different CMUT arrays could show the structures of the mouse brain in different scales. In addition, the images from the high-frequency CMUT arrays were able to reveal the major blood vasculatures, whereas the images from low-frequency CMUT arrays showed the gross macroscopic anatomy of the brain with higher contrast.

Design and Fabrication of a High-Power Air-Coupled Capacitive Micromachined Ultrasonic Transducer Array With Concentric Annular Cells

One shortcoming of capacitive micromachined ultrasonic transducers (CMUTs) for commercial use is the weak output power relative to traditional ultrasonic transducers. Recently, we reported an annular CMUT cell with an improved transmit efficiency over a conventional circular cell in air. Extending this paper, we designed and fabricated a nine-element concentric CMUT array to enhance the transmit power and offer depth focusing. The proposed 200-kHz array has an aperture diameter of 2.13 cm and a fill factor of 81%. A pillar-free etching process was developed to create the deep large-area cavities of the proposed wafer-bonded CMUT. The fabricated device was characterized for the static and dynamic performance using a profilometer and a laser Doppler vibrometer, respectively. The average maximum plate dynamic displacements of the CMUT driven by 20-Vpp ac excitation voltage at dc biases of 100 and 150 V were measured to be 1.28 and $1.97~\mu \text{m}$ , respectively. Accordingly, the surface output power densities were calculated to be 0.40 and 0.96 KW/m2. We investigated the crosstalk between neighboring cells and the plate-cracking phenomenon, and provided relative suggestions for improvement. This paper demonstrates the feasibility of the concentric annular-cell CMUT array design for air-coupled applications.

Quantitative comparison of PZT and CMUT probes for photoacoustic imaging: Experimental validation

Photoacoustic (PA) signals are short ultrasound (US) pulses typically characterized by a single-cycle shape, often referred to as N-shape. The spectral content of such wideband signals ranges from a few hundred kilohertz to several tens of megahertz. Typical reception frequency responses of classical piezoelectric US imaging transducers, based on PZT technology, are not sufficiently broadband to fully preserve the entire information contained in PA signals, which are then filtered, thus limiting PA imaging performance. Capacitive micromachined ultrasonic transducers (CMUT) are rapidly emerging as a valid alternative to conventional PZT transducers in several medical ultrasound imaging applications. As compared to PZT transducers, CMUTs exhibit both higher sensitivity and significantly broader frequency response in reception, making their use attractive in PA imaging applications. This paper explores the advantages of the CMUT larger bandwidth in PA imaging by carrying out an experimental comparative study using various CMUT and PZT probes from different research laboratories and manufacturers. PA acquisitions are performed on a suture wire and on several home-made bimodal phantoms with both PZT and CMUT probes. Three criteria, based on the evaluation of pure receive impulse response, signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR) respectively, have been used for a quantitative comparison of imaging results. The measured fractional bandwidths of the CMUT arrays are larger compared to PZT probes. Moreover, both SNR and CNR are enhanced by at least 6dB with CMUT technology. This work highlights the potential of CMUT technology for PA imaging through qualitative and quantitative parameters.

(Invited) How to Integrate MEMS on Foundry-Fabricated CMOS Backplanes

A large variety of new applications for Internet of Things (IoT), Industrie 4.0 and other consumer products are pushing volume and manufacturing of microelectromechanical systems (MEMS) into a new phase. Due to massive use of these sensors and actuators in mobile and industrial applications smaller systems combined with low power consumption and higher versatility are required. MEMS are transducers that sense or control physical, chemical or optical quantities creating devices for applications in the area of 3D-motion tracking, pressure sensing, light shaping or detection of irradiation. The combination of MEMS with an application specific integrated circuit (ASIC) based on CMOS technology enables the entire system to interact with outside world. These ASICs are providing required features such as analog-to-digital conversion, amplification, filtering, information processing and storage as well as communication to the outside world. There are different solutions available to combine MEMS with customized CMOS circuits. One choice known as system-in-package (SiP) is the manufacturing of MEMS and CMOS on separate wafers and its subsequent integration in a multi-chip module using interposer/ rewiring technologies. Key advatanges of this technique are high flexibility, high modularity and a complete decoupled manufacturing of CMOS and MEMS. This enables a rapid development and leads to low development costs. And, there is no impact of different sizes between CMOS and MEMS chip. In order to enhance the integration density, to drive system feature size down, to suppress potential parasitic capacitances and to reduce power consumption an integrated manufacturing on the same substrate is a must and known as system-on-chip (SoC) solution. On one hand this approach can be still realized by a multi-wafer processing and a final integration using bonding technologies. This technology is often named as heterogeneous integration. Key advantage of multi-wafer technology is the possibility to use high-performance MEMS materials such as monocrystalline silicon as actuation/sensing material and its integration on CMOS wafers. Disadvantages are the required alignment accuracies when using a metallic bond or limitation for integration densities when using via-last approach. A more consistent way is a complete monolithic integration of MEMS on CMOS substrates. Especially when large transducer arrays are required (e.g. bolometers, micro-mirror arrays, CMUT arrays) a monolithic integration of MEMS on CMOS is the best solution. All advantages are now on hand – high integration densities, low parasitic capacitances and an effective usage of CMOS area. Fraunhofer IPMS applies surface and bulk micromachining technologies for the integration of MEMS on customized CMOS backplanes. To reduce development costs and time-to-market IPMS combines standard CMOS processes offered from CMOS foundries with a subsequent integration of MEMS part in our MEMS fab. Due to the application specific design of the CMOS backplane this concept allows a perfect match between ASIC and MEMS functionalities. In a typical project flow CMOS and MEMS will be manufactured and tested in parallel to shorten development time. After ASIC design is finished it will be tested using Multi- Project- Wafer run (MPW) in a CMOS foundry. Using these MPW runs a complete characterization of CMOS functionality can be realized in an early project state at low costs. Parallel to CMOS testing an entire process development of MEMS part can be done on passive devices in our MEMS fab. If CMOS and MEMS characterization is completed the MEMS part will be monolithically integrated on these customized CMOS backplanes. But there are still some challenges for the integration of MEMS on foundry fabricated CMOS backplanes, e.g.: Modifications of last CMOS/ interconnect/ ILD layers to realize an appropriate interface to MEMS Clarification of PCM (process control monitoring) test environment, depth of test structures and possible arising contamination issues In-chip surface topology after interconnect processing to define necessary actions to achieve required planarity for further processing Tuning mix-and-match lithography to achieve high overlay accuracies (Picture 1: Reached overlay accuracy between last 0,18µm CMOS and first MEMS layer of about 30- 40nm after correction step using i-line stepper) Required chip design features when using sacrificial layer technology IPMS offers a wide range of surface and bulk micromachining technologies which are particularly suitable for the fabrication of sensors and actuators on pre-fabricated CMOS wafers using monolithic integration. Especially surface micromachining using inorganic sacrificial layer technology allows the realization of rather complex MEMS structures on CMOS backplanes. Based on examples such as spatial light modulators (SLM) and capacitive micromachined ultrasonic transducers (CMUT) we will present solutions for the integration of surface micromachined MEMS on customized CMOS substrates. Figure 1

Directivity Theory and Analysis of 2D Capacitive Micro-Machined Ultrasonic Transducer Array

Directivity Theory and Analysis of 2D Capacitive Micro-Machined Ultrasonic Transducer Array

Large-Scale Nonlinear Lumped and Integrated Field Simulations of Top-Orthogonal-to-Bottom-Electrode CMUT Architectures.

Capacitive micromachined ultrasonic transducers (CMUTs) promise many advantages over traditional piezoelectric transducers such as the potential to construct large, cost-effective 2-D arrays. To avoid wiring congestion issues associated with fully wired arrays, top-orthogonal-to-bottom electrode (TOBE) CMUT array architectures have proven to be a more practical alternative, using only 2N wires for an N ×N array. Optimally designing a TOBE CMUT array is a significant challenge due to the range of parameters from the device level up to the operating conditions of the entire array. Since testing many design variations can be prohibitively expensive, a simulation approach accounting for both the small and large-scale array characteristics of TOBE arrays is essential. In this paper, we demonstrate large-scale TOBE CMUT array simulations using a nonlinear CMUT lumped-circuit model. We investigate the performance of the array with different CMUT design parameters and array operating conditions. These simulated results are then compared with measurements of TOBE arrays fabricated using a sacrificial release process.

Supply-Doubled Pulse-Shaping High Voltage Pulser for CMUT Arrays.

A supply-doubled pulse-shaping high voltage (HV) pulser is presented for medical ultrasound imaging applications, particularly those that use capacitive micromachined ultrasonic transducers (CMUT). The pulser employs a bootstrap circuit combined with dynamically-biased stacked transistors, which allow HV operation above process limit without lowering device reliability. The new pulser overcomes supply voltage limitation of conventional unipolar pulsers by generating output signals that are almost twice the supply level. It also can generate three-level pulses to further optimize the transmit pressure signals. A proof-of-concept prototype has been implemented in 0.18-μm HV CMOS/DMOS technology with 60 V devices. Measurement results show that the HV pulser can safely generate controllable three-level pulses with up to 85 Vpp from 45 V supply. Acoustic measurements are conducted connecting the pulser to a CMUT with 2 pF capacitance and 8.3 MHz center frequency. The pulse shape has been adjusted for the CMUT under test to generate maximum pressure output and the results are in good agreement with a large signal CMUT model.

Fabrication of Vacuum-Sealed Capacitive Micromachined Ultrasonic Transducers With Through-Glass-Via Interconnects Using Anodic Bonding

This paper presents a novel fabrication method for vacuum-sealed capacitive micromachined ultrasonic transducer (CMUT) arrays that are amenable to 3D integration. This paper demonstrates that MEMS structures can be directly built on a glass substrate with preformed through-glass-via (TGV) interconnects. The key feature of this new approach is the combination of copper through-glass interconnects with a vibrating silicon-plate structure suspended over a vacuum-sealed cavity by using anodic bonding. This method simplifies the overall fabrication process for CMUTs with through-wafer interconnects by eliminating the need for an insulating lining for vias or isolation trenches that are often employed for implementing through-wafer interconnects in silicon. Anodic bonding is a low-temperature bonding technique that tolerates high surface roughness. Fabrication of CMUTs on a glass substrate and use of copper-filled vias as interconnects reduce the parasitic interconnect capacitance and resistance, and improve device performance and reliability. A 16×16-element 2D CMUT array has been successfully fabricated. The fabricated device performs as the finite-element and equivalent circuit models predict. A TGV interconnect shows a 2-Ω parasitic resistance and a 20-fF shunt parasitic capacitance for 250-μm via pitch. A critical achievement presented in this paper is the sealing of the CMUT cavities in vacuum using a PECVD silicon nitride layer. By mechanically isolating the via structure from the active cells, vacuum sealing can be ensured even when hermetic sealing of the via is compromised. Vacuum sealing is confirmed by measuring the deflection of the edge-clamped thin plate of a CMUT cell under atmospheric pressure. The resonance frequency of an 8-cell 2D array element with 78-μm diameter circular cells and a 1.5-μm plate thickness is measured as 3.32 MHz at 15-V dc voltage (80% V pull-in ). [2016-0200].

Fabrication and characterization of a high frequency and high coupling coefficient CMUT array

Fabrication and initial measurement results of a bisbenzocyclobutene (BCB) based capacitive micromachined ultrasonic transducer (CMUT) linear phased array has been presented. The 32 channel 40 MHz CMUT array has been fabricated using a BCB based adhesive wafer bonding technique where BCB has been used as the diaphragm structural material, the bottom insulation layer, and also as the interelectrode insulator. The array has an aperture size of 0.62 mm. Each element consists of 264 square shaped cells. Each CMUT cell was designed to have a diaphragm sidelength of 10 μm with a cavity thickness of 0.75 μm. A low resistivity (0.01 Ω-cm) silicon wafer has been used as the bottom electrode. Series resonance of the array elements has been measured as 48 MHz which is within 20% agreement with the 3D FEA values simulated using IntelliSuite™, a MEMS design tool. Electromechanical coupling coefficient has been extracted to be 0.66 from the electrical input impedance measurement using an Agilent vector network analyzer with 20 V bias voltage. The measured static capacitance of individual cell is in excellent agreement with the 3-D FEA model with a deviation of 1.27%. The fabricated array can be used for anterior segment ophthalmic imaging, cardiovascular imaging, dermatology, small animal imaging, and biometric authentication. The technique can be used to fabricate high performance CMUT arrays for other applications.

Ex Vivo HIFU Experiments Using a $32 \times 32$ -Element CMUT Array.

High-intensity focused ultrasound (HIFU) has been used as noninvasive treatment for various diseases. For these therapeutic applications, capacitive micromachined ultrasonic transducers (CMUTs) have advantages that make them potentially preferred transducers over traditional piezoelectric transducers. In this paper, we present the design and the fabrication process of an 8 ×8 -mm 2 32 ×32 -element 2-D CMUT array for HIFU applications. To reduce the system complexity for addressing the 1024 transducer elements, we propose to group the CMUT array elements into eight HIFU channels based on the phase delay from the CMUT element to the targeted focal point. Designed to focus at an 8-mm depth with a 5-MHz exciting frequency, this grouping scheme was realized using a custom application-specific integrated circuit. With a 40-V dc bias and a 60-V peak-to-peak ac excitation, the surface pressure was measured 1.2 MPa peak-to-peak and stayed stable for a long enough time to create a lesion. With this dc and ac voltage combination, the measured peak-to-peak output pressure at the focus was 8.5 MPa, which is expected to generate a lesion in a minute according to the temperature simulation. The following ex vivo tissue experiments successfully demonstrated its capability to make lesions in both bovine muscle and liver tissue.

Bilateral CMUT Cells and Arrays: Equivalent Circuits, Diffraction Constants, and Substrate Impedance.

We introduce the large-signal and small-signal equivalent circuit models for a capacitive micromachined ultrasonic transducer (CMUT) cell, which has radiating plates on both sides. We present the diffraction coefficient of baffled and unbaffled CMUT cells. We show that the substrate can be modeled as a very thick radiating plate on one side, which can be readily incorporated in the introduced model. In the limiting case, the reactance of this backing impedance is entirely compliant for substrate materials with a Poisson's ratio less than 1/3. We assess the dependence of the radiation performance of the front plate on the thickness of the back plate by simulating an array of bilateral CMUT cells. We find that the small-signal linear model is sufficiently accurate for large-signal excitation, for the purpose of the determining the fundamental component. To determine harmonic distortion, the large-signal model must be used with harmonic balance analysis. Rayleigh-Bloch waves are excited at the front and back surfaces similar to conventional CMUT arrays.

Frequency Tuning of Collapse-Mode Capacitive Micromachined Ultrasonic Transducer

The information in an ultrasound image depends on the frequency that is used. In a clinical examination it may therefore be beneficial to generate ultrasound images acquired at multiple frequencies, which is difficult to achieve with conventional transducers. Capacitive micromachined ultrasonic transducers (CMUTs) offer a frequency response that is tunable by the bias voltage. In this study we investigate this frequency tunability for ultrasonic imaging. We characterized a CMUT array operated at bias voltages up to three times higher than the collapse-voltage. All elements of the array were connected to a single transmit and receive channel through a bias circuit. We quantified the transmit-receive and transmit sensitivity as a function of frequency for a range of bias voltages. Impulse response measurements show that the center frequency is modifiable between 8.7MHz and 15.3MHz with an applied bias voltage of −50V to −170V. The maximum transmit sensitivity is 52kPa/V at a center frequency of 9.0MHz with an applied bias voltage of −105V. The −3dB transmit range in center frequency accessible with the variable bias voltage is 6.7–15.5MHz. This study shows that a collapse-mode CMUT can operate efficiently at multiple center frequencies when the driving pulse and the bias voltage are optimized. We demonstrate the usefulness of frequency tuning by comparing images at different optimal combinations of driving frequency and bias voltage, acquired by linearly moving the transducer across a tissue mimicking phantom.

Capacitive micromachined ultrasonic transducer arrays incorporating anodically bondable low temperature co‐fired ceramic for small diameter ultrasonic endoscope

A process for the fabrication of a capacitive micromachined ultrasonic transducer (CMUT), with a resonance frequency of 2.88 MHz, using an anodically bondable custom-designed low temperature co-fired ceramic (LTCC) substrate with a 60 μm via diameter made of Au is presented. Fabrication of CMUTs with a high fill factor on the LTCC via with a pitch distance of 150 μm is good candidate for medical applications of a miniaturised ring shape CMUT endoscope. Electrical connection between the front side where the device is fabricated and the backside of the LTCC can be achieved with horizontal and vertical interconnects. Investigation of limitations with respect to the device dimensions and experimental fabrication results are discussed. CMUT arrays on LTCC provide a simplified fabrication process with advantages of LTCC architecture compared with typical glass counterparts used in the microelectromechanical systems field.

Towards a Reduced-Wire Interface for CMUT-Based Intravascular Ultrasound Imaging Systems.

Having intravascular ultrasound (IVUS) imaging capability on guide wires used in cardiovascular interventions may eliminate the need for separate IVUS catheters and expand the use of IVUS in a larger portion of the vasculature. High frequency capacitive micro machined ultrasonic transducer (CMUT) arrays should be integrated with interface electronics and placed on the guide wire for this purpose. Besides small size, this system-on-a-chip (SoC) front-end should connect to the back-end imaging system with a minimum number of wires to preserve the critical mechanical properties of the guide wire. We present a 40 MHz CMUT array interface SoC, which will eventually use only two wires for power delivery and transmits image data using a combination of analog-to-time conversion (ATC) and an impulse radio ultra-wideband (IR-UWB) wireless link. The proof-of-concept prototype ASIC consumes only 52.8 mW and occupies 4.07 [Formula: see text] in a 0.35- [Formula: see text] standard CMOS process. A rectifier and regulator power the rest of the SoC at 3.3 V from a 10 MHz power carrier that is supplied through a 2.4 m micro-coax cable with an overall efficiency of 49.1%. Echo signals from an 8-element CMUT array are amplified by a transimpedance amplifier (TIA) array and down-converted to baseband by quadrature sampling using a 40 MHz clock, derived from the power carrier. The ATC generates pulse-width-modulated (PWM) samples at 2 × 10 MS/s with 6 bit resolution, while the entire system achieved 5.1 ENOB. Preliminary images from the prototype system are presented, and alternative data transmission and possible future directions towards practical implementation are discussed.

Fabrication of a Curved Row–Column Addressed Capacitive Micromachined Ultrasonic Transducer Array

Capacitive micromachined ultrasonic transducers (CMUTs) exhibit wide-bandwidth and can be made into tiny 2-D arrays that are integrable with electronics. A simple implementation of such 2-D array is the row–column addressed CMUT array. However, one critical limitation of a row–column array is its narrow field of view, which can be improved by curving the array. This paper reports a novel fabrication method that realizes a flexible 32 + 32 elements row–column addressed CMUT array using a two stage fabrication process. First, a functional row–column CMUT array is fabricated with a double silicon-on-insulator wafer bonding process. The second stage involves encapsulating the device with a polymer coating, yet leaving the bottom silicon substrate exposed for wet etching. The bottom buried oxide layer will also act as an etch stop layer. Pitch-catch experiments are presented to verify the functionality. Our final device, excluding the encapsulation layer, is less than $5~\mu \text{m}$ thick with a 12.8-mm $\times \,\, 12.8$ -mm aperture. The final flexible device exhibits a 4.5-MHz center frequency with a −3-dB fractional bandwidth of 82%. [2015-0317]

Self and Mutual Radiation Impedances for Modeling of Multi-Frequency CMUT Arrays.

Multi-frequency capacitive micromachined ultrasonic transducers (CMUTs) consist of interlaced large and small membranes for multiband operation. In modeling these devices, accurate and computationally efficient methods are required for computing self- and mutual-acoustic-radiation impedances. However, most previous works considered mutual-acoustic impedance between radiators of identical size. A need was thus found to revisit the fundamental framework for mutual-acoustic impedance for its applicability to radiators, especially flexural disks, of differing size. The Bouwkamp integral method is used to achieve infinite series expressions for self- and mutual-acoustic radiation impedances. Polynomial-fitting-based approximate relations of the mutual-acoustic impedance are developed for arbitrary array geometries and are in good agreement with exact expressions. The derived mutual-acoustic impedance is incorporated into equivalent circuit models of multi-frequency CMUTs showing excellent agreement with finite element modeling. The results demonstrate that mutual-acoustic interactions significantly impact device performance. The framework presented here may prove valuable for future design of multi-frequency arrays for novel multiscale imaging, superharmonic contrast imaging, and image therapy applications.

Fabrication of Vacuum-Sealed Capacitive Micromachined Ultrasonic Transducer Arrays Using Glass Reflow Process.

This paper presents a process for the fabrication of vacuum-sealed capacitive micromachined ultrasonic transducer (CMUT) arrays using glass reflow and anodic bonding techniques. Silicon through-wafer interconnects have been investigated by the glass reflow process. Then, the patterned silicon-glass reflow wafer is anodically bonded to an SOI (silicon-on-insulator) wafer for the fabrication of CMUT devices. The CMUT 5 × 5 array has been successfully fabricated. The resonant frequency of the CMUT array with a one-cell radius of 100 µm and sensing gap of 3.2 µm (distance between top and bottom electrodes) is observed at 2.84 MHz. The Q factor is approximately 1300 at pressure of 0.01 Pa.

Design of capacitive micromachined ultrasonic transducer (CMUT) linear array for underwater imaging

Purpose – The purpose of this paper was to develop a novel capacitive micromachined ultrasonic transducer (CMUT) reception and transmission linear array for underwater imaging at 400 kHz. Compared with traditional CMUTs, the developed transducer array offers higher electromechanical coupling coefficient and higher directivity performance. Design/methodology/approach – The configuration of the newly developed CMUT reception and transmission array was determined by the authors’ previous research into new element structures with patterned top electrodes and into directivity simulation analysis. Using the Si-Silicon on insulator (Si-SOI) bonding technique and the principle of acoustic impedance matching, the CMUT array was fabricated and packaged. In addition, underwater imaging system design and testing based on the packaged CMUT 1 × 16 array were completed. Findings – The simulation results showed that the optimized CMUT array configuration was selected. Furthermore, the designed configuration of the CMUT 1 × 16 linear array was good enough to guarantee high angular resolution. The underwater experiments were conducted to demonstrate that this CMUT array can be of great benefit in imaging applications. Practical implications – Based on our research, the CMUT linear array has good directivity and good impedance matching with water and can be used for obstacle avoidance, distance measurement and imaging underwater. Originality/value – This research provides a basis for CMUT directivity theory and array design. CMUT array presented in this paper has good directivity and has been applied in the underwater imaging, resulting in a huge market potential in underwater detection systems.

Performance Assessment of CMUT Arrays Based on Electrical Impedance Test Results

A quantitative quality assessment method for capacitive micromachined ultrasonic transducer (CMUT) arrays based on electrical impedance measurements made at wafer level is proposed. The resonant response in air of CMUTs is described and then used to obtain a quantitative quality value. Several issues that could occur in CMUT arrays as well as their effect on the output of the assessment are analyzed. This method has been applied on a wafer containing several CMUT designs and the obtained results are summarized. [2014-0354]

Design and performance analysis of capacitive micromachined ultrasonic transducer (CMUT) array for underwater imaging

In this paper, a Receipt & Transmission CMUT array has been developed for underwater imaging purposes. To guide the array design more intuitively, directivity fuctions of CMUT array are deduced in accordance with its structure feature. According to the simulation results, the optimized CMUT array configuration is selected. Through employing a Si-SOI bonding technique, the CMUT array has been fabricated. Finally, the underwater imaging system design and performance testing are completed. The ź6 dB center frequency is centered at 460 kHz and has a relative bandwidth of 130 %. The ź3 dB main beam width is about 4°. These values imply that the designed CMUT array has wide bandwidth and fine directivity. Using sector scanning and echo processing, obvious position and image of the obstacles has been reconstructed. This further demonstrates that CMUT array presented in this paper can be of great benefit in underwater detection systems.