Fabrication Of Capacitive Micromachined Ultrasonic Transducer Research Articles

Multilayer Masking Technology for Fabricating Airborne CMUTs With Multi-Depth Fluidic Trenches

This paper presents a novel multilayer masking technology for fabricating airborne capacitive micromachined ultrasonic transducers (CMUTs) with multi-depth fluidic trenches, in which a plurality of masking layers for different etch depths are selectively patterned prior to the real etching to keep a planar wafer surface. The multi-depth fluidic trenches are utilized to effectively control the squeeze film within the CMUT gap and tune the fractional bandwidth (FBW) of CMUT. As such, the FBW can be changed over a wide range by only adjusting the trench height. With further lowering the gap height, the FBW can be extremely widened up to 168%. The receive sensitivity has also been significantly improved through the process of local oxidation of silicon to reduce the parasitic capacitance. The proposed multilayer masking technology enables the fabrication of airborne CMUTs with different FBWs and sensitivities on the same wafer and further combine these devices into arrays for high-resolution imaging and photoacoustic or thermoacoustic applications. These devices also provide a low minimum detectable pressure (MDP), as low as 1.37 mPa in the FBW of 13.5% for the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$8.5~\mu \text{m}$ </tex-math></inline-formula> deep fluidic trenches and 1.52 mPa in the 17.2% FBW for the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$16.2~\mu \text{m}$ </tex-math></inline-formula> trenches. Furthermore, the multilayer masking technology demonstrates the capability of building microelectromechanical systems (MEMS) with multi-depth micro/nanostructures. [2021-0090]

Use of Supporting Post Width to Increase the CMUT’s Resonant Frequency

A new method is proposed to increase the resonant frequency of a Capacitive Micromachined Ultrasonic Transducer (CMUT) device while keeping the radii unchanged for devices fabricated by sacrificial release (SR) method. The resonant frequency of a CMUT cell is determined by the properties of the membrane such as the Young&#x2019;s modulus, membrane dimensions and effective membrane mass. Effective Young&#x2019;s modulus is used to calculate the resonant frequency of CMUT fabricated by SR method. The supporting post structure will affect the boundary conditions at the edge of the CMUT membrane and in turn affect the deflection together with the effective Young&#x2019;s modulus of the membrane. The perturbation method is used to derive the solution for the governing von K&#x00E1;rm&#x00E1;n equations. The results show that the thicker is the supporting post width, the higher is the resonant frequency. This method can also be used to simplify the mask design and CMUT fabrication process.

Wide Bandwidth and Low Driving Voltage Vented CMUTs for Airborne Applications.

This paper presents a novel method to increase the bandwidth (BW) of airborne capacitive micromachined ultrasonic transducers (CMUTs). This method introduces a gaseous squeeze film as a damping mechanism, which induces a stiffening effect that lowers the pull-in voltage and improves the sensitivity. The optimal behavior of this stiffening effect versus the damping mechanism can be controlled by creating optimized fluidic trenches of various heights within the gap. The fractional BW can be controlled from 0.89% to 8.1% by adjusting the trench height while lowering the pull-in voltage to less than 54 V at the gap height of 1.0 [Formula: see text]. To achieve the largest sensitivity and lowest pull-in voltage at a given BW, we have developed a multi-parameter optimization method to adjust all combinations of design parameters. A novel multiple hard-mask process flow has been developed to enable fabrication of CMUTs with different cavity and trench heights on the same wafer. These devices provided an equivalent noise pressure level of 4.77 μ Pa/ √ Hz with 6.24-kHz BW for 7.6- [Formula: see text] deep fluidic trenches and 4.88 μ Pa/ √ Hz with 7.48-kHz BW for 14.3- [Formula: see text] deep fluidic trenches. This demonstration of the wide-BW CMUTs with high sensitivity and low pull-in voltage makes them applicable to medical and thermoacoustic imaging, nondestructive testing, and ultrasonic flow metering.

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.

Fabrication and Packaging of CMUT Using Low Temperature Co-Fired Ceramic.

This paper presents fabrication and packaging of a capacitive micromachined ultrasonic transducer (CMUT) using anodically bondable low temperature co-fired ceramic (LTCC). Anodic bonding of LTCC with Au vias-silicon on insulator (SOI) has been used to fabricate CMUTs with different membrane radii, 24 µm, 25 µm, 36 µm, 40 µm and 60 µm. Bottom electrodes were directly patterned on remained vias after wet etching of LTCC vias. CMUT cavities and Au bumps were micromachined on the Si part of the SOI wafer. This high conductive Si was also used as top electrode. Electrical connections between the top and bottom of the CMUT were achieved by Au-Au bonding of wet etched LTCC vias and bumps during anodic bonding. Three key parameters, infrared images, complex admittance plots, and static membrane displacement, were used to evaluate bonding success. CMUTs with a membrane thickness of 2.6 µm were fabricated for experimental analyses. A novel CMUT-IC packaging process has been described following the fabrication process. This process enables indirect packaging of the CMUT and integrated circuit (IC) using a lateral side via of LTCC. Lateral side vias were obtained by micromachining of fabricated CMUTs and used to drive CMUTs elements. Connection electrodes are patterned on LTCC side via and a catheter was assembled at the backside of the CMUT. The IC was mounted on the bonding pad on the catheter by a flip-chip bonding process. Bonding performance was evaluated by measurement of bond resistance between pads on the IC and catheter. This study demonstrates that the LTCC and LTCC side vias scheme can be a potential approach for high density CMUT array fabrication and indirect integration of CMUT-IC for miniature size packaging, which eliminates problems related with direct integration.

Optimization on benzocyclobutene-based CMUT fabrication with an inverse structure

It has been demonstrated that the adhesive wafer bonding technique benefits the fabrication of capacitive micromachined ultrasonic transducers (CMUTs) in the aspects of structural flexibility, CMOS process compatibility, and fabrication yield. Recently, the feasibility of fabricating CMUTs using benzocyclobutene (BCB) based polymer as both the adhesive and structural material was reported. However, the issues of voids at the wafer bonding interface, material deformation and electrode distance control need to be addressed. In this paper, we present an improved fabrication process that allows a void-free device with reduced polymer deformation. The chemical reactions during the BCB curing process were revealed using Fourier transform infrared spectroscopy. Microscopy and cross-sectional SEM images were employed to illustrate the structure improvement in terms of voids and polymer deformation. In addition, an inverse structure that allows an accurate control of the effective electrode distance was presented. The cross-sectional SEM images and the impedance measurement showing the dynamic response of the CMUT demonstrated the feasibility of this inverse structure. The differences between the inverse structure and the conventional structure were also discussed and compared.

Low temperature CMUT fabrication process with dielectric lift-off membrane support for improved reliability

This paper reports an improved CMOS compatible low temperature sacrificial layer fabrication process for capacitive micromachined ultrasonic transducers (CMUTs). The process adds the fabrication step of silicon oxide evaporation which is followed by a lift-off step to define the membrane support area without a need for an extra mask. This simple addition improves reliability by reducing the electric field between the top and bottom electrodes everywhere except the moving membrane without affecting the vacuum gap thickness. Furthermore, the parasitic capacitance which degrades the CMUT receive performance is reduced. A 1D CMUT array suitable for intracardiac echocardiography (ICE) imaging with 9 MHz center frequency is fabricated using this method. Detailed electrical and acoustic testing indicates adequate performance of the devices for ICE in agreement with simulations. Long term output pressure testing with more than 2 × 1011 pulsing cycles and environmental testing demonstrate the efficacy of the approach for improved reliability as compared to devices without the additional membrane support layer.

Practical CMUT Fabrication With a Nitride-to-Oxide-Based Wafer Bonding Process

The introduction of wafer-bonding technique to capacitive micromachined ultrasonic transducer (CMUT) fabrication offered advantages over the surface micromachining technique, such as simplified fabrication, better membrane uniformity, and increased active area. The conventional wafer-bonding CMUT process employs silicon-on-insulator (SOI) wafers to produce the vibrating membrane layer. However, the use of SOI wafers can lead to poor membrane thickness uniformity despite the high cost. Furthermore, an additional local-oxidation (LOCOS) process should be used for SOI-based wafer bonding process to improve breakdown voltage and reduce parasitic capacitance. This paper reports a nitride-to-oxide bonding process for CMUT fabrication, designed to achieve similar results as the LOCOS-added process reported by Park et al. , but without the need for SOIs and fabrication complexity. A fully functional CMUT designed for collapse-mode operation was fabricated and an immersion center frequency was measured to be 4.2 MHz with a 90% bandwidth. [2016-0210]

Fabrication of High-Efficiency CMUTs With Reduced Parasitics Using Embedded Metallic Layers

The transmit and receive sensitivity of the capacitive micromachined ultrasonic transducer (CMUT) is proportional to the active device capacitance formed by the vacuum gap of the device, and an insulation layer between the gap and the device electrode. In the sacrificial release process of CMUT fabrication, this insulation layer cannot be made arbitrarily thin due to conformality issues. In this paper, we propose and prove the applicability of a micromachining technique by which metallic sacrificial islands are embedded inside grooves etched on the substrate, yielding topology free surfaces. This obviates the conformality requirement, and enables the growth of a thinner insulation layer which reduces the effective gap height, and, hence, improves sensitivity. Embedded metalic layers, which provide a flat surface for subsequent process steps, have also been used as the back electrode of the CMUT, which facilitated the manufacturing of devices with reduced stray capacitance on thermally oxidized wafers. CMUT devices were fabricated using the proposed technique, and their parameters were measured to justify the performance improvement. While the dc bias requirement is reduced by 19%, the output sensitivity of the device is 10% higher than that of the conventional CMUT, and spurious capacitance is decreased by 70%.

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].

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.

A three-mask process for fabricating vacuum-sealed capacitive micromachined ultrasonic transducers using anodic bonding.

This paper introduces a simplified fabrication method for vacuum-sealed capacitive micromachined ultrasonic transducer (CMUT) arrays using anodic bonding. Anodic bonding provides the established advantages of wafer-bondingbased CMUT fabrication processes, including process simplicity, control over plate thickness and properties, high fill factor, and ability to implement large vibrating cells. In addition to these, compared with fusion bonding, anodic bonding can be performed at lower processing temperatures, i.e., 350°C as opposed to 1100°C; surface roughness requirement for anodic bonding is more than 10 times more relaxed, i.e., 5-nm rootmean- square (RMS) roughness as opposed to 0.5 nm for fusion bonding; anodic bonding can be performed on smaller contact area and hence improves the fill factor for CMUTs. Although anodic bonding has been previously used for CMUT fabrication, a CMUT with a vacuum cavity could not have been achieved, mainly because gas is trapped inside the cavities during anodic bonding. In the approach we present in this paper, the vacuum cavity is achieved by opening a channel in the plate structure to evacuate the trapped gas and subsequently sealing this channel by conformal silicon nitride deposition in the vacuum environment. The plate structure of the fabricated CMUT consists of the single-crystal silicon device layer of a silicon-on-insulator wafer and a thin silicon nitride insulation layer. The presented fabrication approach employs only three photolithographic steps and combines the advantages of anodic bonding with the advantages of a patterned metal bottom electrode on an insulating substrate, specifically low parasitic series resistance and low parasitic shunt capacitance. In this paper, the developed fabrication scheme is described in detail, including process recipes. The fabricated transducers are characterized using electrical input impedance measurements in air and hydrophone measurements in immersion. A representative design is used to demonstrate immersion operation in conventional, collapse-snapback, and collapse modes. In collapsemode operation, an output pressure of 1.67 MPa pp is shown at 7 MHz on the surface of the transducer for 60-Vpp, 3-cycle sinusoidal excitation at 30-V dc bias.

Monolithic CMUT-on-CMOS Integration for Intravascular Ultrasound Applications

One of the most important promises of capacitive micromachined ultrasonic transducer (CMUT) technology is integration with electronics. This approach is required to minimize the parasitic capacitances in the receive mode, especially in catheter-based volumetric imaging arrays, for which the elements must be small. Furthermore, optimization of the available silicon area and minimized number of connections occurs when the CMUTs are fabricated directly above the associated electronics. Here, we describe successful fabrication and performance evaluation of CMUT arrays for intravascular imaging on custom-designed CMOS receiver electronics from a commercial IC foundry. The CMUT-on-CMOS process starts with surface isolation and mechanical planarization of the CMOS electronics to reduce topography. The rest of the CMUT fabrication is achieved by modifying a low-temperature micromachining process through the addition of a single mask and developing a dry etching step to produce sloped sidewalls for simple and reliable CMUT-to-CMOS interconnection. This CMUT-to-CMOS interconnect method reduced the parasitic capacitance by a factor of 200 when compared with a standard wire-bonding method. Characterization experiments indicate that the CMUT-on-CMOS elements are uniform in frequency response and are similar to CMUTs simultaneously fabricated on standard silicon wafers without electronics integration. Ex- periments on a 1.6-mm-diameter dual-ring CMUT array with a center frequency of 15 MHz show that both the CMUTs and the integrated CMOS electronics are fully functional. The SNR measurements indicate that the performance is adequate for imaging chronic total occlusions located 1 cm from the CMUT array.

Fabrication of Capacitive Micromachined Ultrasonic Transducers via Local Oxidation and Direct Wafer Bonding

We present the successful fabrication of capacitive micromachined ultrasonic transducers (CMUTs) with an improved insulation layer structure. The goal is to improve device reliability (electrical breakdown) and device performance (reduced parasitic capacitance). The fabrication is based on consecutive thermal oxidation steps, on local oxidation of silicon (LOCOS), and on direct wafer bonding. No chemical-mechanical polishing step is required during the device fabrication. Aside from the advantages associated with direct wafer bonding for CMUT fabrication (simple fabrication, cell shape flexibility, wide gap height range, good uniformity, well-known material properties of single-crystal materials, and low intrinsic stress), the main vertical dimension (electrode separation) is determined by thermal oxidation only, which provides excellent vertical tolerance control ( <;10 nm) and unprecedented uniformity across the wafer. Thus, we successfully fabricated CMUTs with gap heights as small as 40 nm with a uniformity of ±2 nm over the entire wafer. This paper demonstrates that reliable parallel-plate electrostatic actuators and sensors with gap heights in the tens of nanometer range can be realized via consecutive thermal oxidation steps, LOCOS, and direct wafer bonding without chemical-mechanical polishing steps.

CMUT characterization by interferometric and electric measurements

Capacitive micromachined ultrasonic transducers (CMUTs) with 5.7 mum radius, realized by wafer bonding, have been characterized by both optical and electrical measurements. These measurements are performed by our heterodyne interferometer and a network analyzer, respectively. The results from this article will be used to improve the fabrication of next-generation CMUTs. We have investigated the spread in resonance frequency of different CMUT membranes along the array. Q-factors have been obtained using both measurement methods, and the results from the 2 methods have been numerically compared. The relation between applied dc voltage and resonance frequency has been studied. Temperature measurements show that increasing temperature leads to a small decrease in the resonance frequency of the CMUTs; however, the decrease is small enough to ensure stable operation with small variations in room temperature. The heterodyne interferometer is used to inspect the vibration pattern of the CMUTs' higher harmonic modes. These modes are located at approximately 60 MHz in air. To the authors' knowledge, vibration modes at frequencies >40 MHz have not been previously studied.

Surface micromachined capacitive ultrasonic transducer for underwater imaging

This study presents the primary design, fabrication process and device measurement of a Capacitive Micromachined Ultrasonic Transducer (CMUT) for underwater acoustic imaging. Theoretical analysis and computer simulations of the CMUT are performed. The CMUT fabrication uses the full surface micromachining techniques of the Micro Electro Mechanical System (MEMS). These techniques are Low Pressure Chemical Vapor Deposition (LPCVD), photolithography, Reactive Ion Etching System (RIE) dry etching, sacrificial layer wet etching, metal thermal evaporation coating and Plasma‐Enhanced Chemical Vapor Deposition (PECVD). Several important issues regarding fabrication are discussed. The measured input impedance of the CMUT is in agreement with the theoretical prediction. The received signal has a 35 dB signal‐to‐noise ratio indicating that practical applications of the immersion CMUT are feasible and that the radiation pattern measurement of the CMUT array has good beamforming characteristics for underwater imaging.

Fabricating capacitive micromachined ultrasonic transducers with wafer-bonding technology

Introduces a new method for fabricating capacitive micromachined ultrasonic transducers (CMUTs) that uses a wafer bonding technique. The transducer membrane and cavity are defined on an SOI (silicon-on-insulator) wafer and on a prime wafer, respectively. Then, using silicon direct bonding in a vacuum environment, the two wafers are bonded together to form a transducer. This new technique, capable of fabricating large CMUTs, offers advantages over the traditionally micromachined CMUTs. First, forming a vacuum-sealed cavity is relatively easy since the wafer bonding is performed in a vacuum chamber. Second, this process enables better control over the gap height, making it possible to fabricate very small gaps (less than 0.1 /spl mu/m). Third, since the membrane is made of single crystal silicon, it is possible to predict and control the mechanical properties of the membrane to within 5%. Finally, the number of process steps involved in making a CMUT has been reduced from 22 to 15, shortening the device turn-around time. All of these advantages provide repeatable fabrication of CMUTs featuring predictable center frequency, bandwidth, and collapse voltage.