Capacitive Micromachined Ultrasonic Transducer Devices Research Articles

Li-Ion Cell in Operando Monitoring and Prognostication Using CMUT Devices

Capacitive micromachined ultrasonic transducer (CMUT) devices are ideal solutions for in operando battery monitoring applications especially for consumer electronics. Their minute size (2mm x 1mm x 0.5mm) allows direct placement on the battery without affecting the form factor of the device, the volume production techniques used for their manufacture allow for a very low-cost device, and they can be easily integrated with front-end integrated circuits either in a single device or in a discrete multichip form. Together with industry partners we have developed CMUT devices that we use to continuously monitor the behavior of a battery while it is cycling.For this work we used CMUTs (close-up shown in Fig. 1(a)) developed to operate with a low bias voltage (less than 30 V) which reduces the complexity of the driving hardware. The impulse response and its frequency content are shown in Fig. 1(b). In collapse mode, the peak frequency is 4.0 MHz and the one-way –3 dB BW is 76%, which allows flexibility in setting the drive conditions to interrogate the battery. Our testing also shows that the CMUTs show good device-to-device uniformity and they have a very stable performance in variable temperature environments (e.g. 0℃ - 60℃ ).The cell is interrogated in pulse-echo mode. The CMUT is excited by a wide-band unipolar pulse and the returned analog signal is filtered and amplified. The signal is then digitized and processed to extract features that allow us to quantify the state of the battery. Figure 1(c) depicts such signal features change under baseline cell cycling conditions (three cycles at a charge rate of C/4). These changes are the result of mechanical material property changes in the cell e.g., electrode stiffness and density [1] which directly affect the acoustic wave propagation inside the cell. Figure 1(d) depicts signal feature change due cell overcharge (red line). Ultrasound is especially sensitive to gas generation which occurs as the cell voltage extends beyond normal operating range [2]. Signal features provide early warning notice that cell is outside of normal operating conditions.Figure 1. a) Close-up image of CMUT device. b) CMUT impulse response and corresponding frequency content. c) Signal features (RMS and Time of Flight (ToF)) and thickness change during C/4 charge and discharge. d) Signal features during cell constant current overcharge. Cell overcharged after 4.4V (red line). Chang, W. et al. Measuring effective stiffness of Li-ion batteries via acoustic signal processing. Mater. Chem. A 8, 16624–16635 (2020).Appleberry, M. C. et al. Avoiding thermal runaway in lithium-ion batteries using ultrasound detection of early failure mechanisms. Journal of Power Sources 535, 231423 (2022). Figure 1

An Improved Static Membrane Deflection Analysis for Collapse Mode CMUT Fabricated by Sacrificial Release Method

To improve the accuracy of the static membrane deflection calculation for the collapse mode capacitive micromachined ultrasonic transducer (CMUT) fabricated by sacrificial release (SR) method. The beam theory was used and the substrate supporting force can be modeled by a coefficient to the load on the membrane. The surrounding supporting post bending can be modeled by applying the elastic support boundary conditions (ESBCs) for the peripheral part of the membrane. CMUT devices have been fabricated using the SR process to validate this analytical analysis. Young’s modulus of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\text {Si}_{{3}} \text {N}_{{4}}$ </tex-math></inline-formula> was derived as 200 GPa by a method using center deflections of several different radii single-layer conventional devices under 1 atm. The analytical deflection results and contact radius results for the collapse mode CMUT membranes were testified by experimental results under 1 atm and were found matching the experimental results. This showed that the analysis can provide improved results for SR fabricated CMUT devices by including the substrate supporting effect and the post bending effect.

Sensitivity Optimization of Wafer Bonded Gravimetric CMUT Sensors

Optimization of the mass sensitivity of wafer bonded resonant gravimetric capacitive micromachined ultrasonic transducers (CMUTs) is presented. Gas phase sensors based on resonant gravimetric CMUTs have previously been demonstrated. An important figure of merit of these sensors is the sensitivity which, for typical CMUT geometries, is increased by decreasing the radius of the CMUT cell. This paper investigates how to minimize the radius of CMUT cells fabricated using the wafer bonding process. The design and process parameters affecting the radius of the CMUT and hereby the sensitivity are studied through numerical simulations and atomic force microscopy measurements. An excellent fit was obtained between the simulations and measured profiles with a low relative error of ≤ 5%, thus validating the simulation model. Two types of CMUTs are designed and fabricated using the design and process rules determined herein, with experimentally determined mass sensitivities of 0.46 Hz/ag and 0.44 Hz/ag, respectively. The two CMUT devices have cavities made using the local oxidation of silicon (LOCOS) and reactive ion etching (RIE) process. For the LOCOS process, it was found that the smallest radius can be obtained by choosing a Si <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> oxidation mask and lowering the pad SiO2 thickness, vacuum gap height, and Si bump height. For the RIE process, the vertical dimensions do not influence the horizontal dimensions and consequently, equivalent rules do not exist.

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 and Characterization of Capacitive Micromachined Ultrasonic Transducers with Low-Temperature Wafer Direct Bonding.

This paper presents a fabrication method of capacitive micromachined ultrasonic transducers (CMUTs) by wafer direct bonding, which utilizes both the wet chemical and O2 plasma activation processes to decrease the bonding temperature to 400 °C. Two key surface properties, the contact angle and surface roughness, are studied in relation to the activation processes, respectively. By optimizing the surface activation parameters, a surface roughness of 0.274 nm and a contact angle of 0° are achieved. The infrared images and static deflection of devices are assessed to prove the good bonding effect. CMUTs having silicon membranes with a radius of 60 μm and a thickness of 2 μm are fabricated. Device properties have been characterized by electrical and acoustic measurements to verify their functionality and thus to validate this low-temperature process. A resonant frequency of 2.06 MHz is obtained by the frequency response measurements. The electrical insertion loss and acoustic signal have been evaluated. This study demonstrates that the CMUT devices can be fabricated by low-temperature wafer direct bonding, which makes it possible to integrate them directly on top of integrated circuit (IC) substrates.

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.

Fabrication of Capacitive Micromachined Ultrasonic Transducer Arrays With Isolation Trenches Using Anodic Wafer Bonding

This paper presents a novel process for the fabrication of capacitive micromachined ultrasonic transducer (CMUT) arrays with isolation trenches using anodic bonding technique. The developed fabrication process is easy, stiction-free, repeatable, reliable, requires low-temperature (<;400°C), and gives high yield. It is a four-mask process. The process is best suited for air-coupled CMUT devices, and can also be used for vacuum-sealed devices. The CMUT device is designed and fabricated successfully using the developed process, which requires silicon-on-insulator (SOI) wafer for membrane and cavity formation, and Pyrex glass wafer for cavity and bottom electrode formation. The process is used for single-cell as well as for development of 1 × 5, 5 × 5, and 10 × 10 CMUT arrays. CMUT cells were designed using MEMSCAD tool CoventorWare. The fabricated devices have been characterized using nanovibration analyzer, and the resonance frequency is found to be ~1.5 MHz.

PECVD low stress silicon nitride analysis and optimization for the fabrication of CMUT devices

Two technological options to achieve a high deposition rate, low stress plasma-enhanced chemical vapor deposition (PECVD) silicon nitride to be used in capacitive micromachined ultrasonic transducers (CMUT) fabrication are investigated and presented. Both options are developed and implemented on standard production line PECVD equipment in the framework of a CMUT technology transfer from R & D to production. A tradeoff between deposition rate, residual stress and electrical properties is showed.The first option consists in a double layer of silicon nitride with a relatively high deposition rate of ~100 nm min−1 and low compressive residual stress, which is suitable for the fabrication of the thick nitride layer used as a mechanical support of the CMUTs. The second option involves the use of a mixed frequency low-stress silicon nitride with outstanding electrical insulation capability, providing improved mechanical and electrical integrity of the CMUT active layers. The behavior of the nitride is analyzed as a function of deposition parameters and subsequent annealing. The nitride layer characterization is reported in terms of interfaces density influence on residual stress, refractive index, deposition rate, and thickness variation both as deposited and after thermal treatment. A sweet spot for stress stability is identified at an interfaces density of 0.1 nm−1, yielding 87 MPa residual stress after annealing. A complete CMUT device fabrication is reported using the optimized nitrides. The CMUT performance is tested, demonstrating full functionality in ultrasound imaging applications and an overall performance improvement with respect to previous devices fabricated with non-optimized silicon nitride.

Diamond-based capacitive micromachined ultrasonic transducers in immersion

Diamond is a superior membrane material for capacitive micromachined ultrasonic transducers (CMUTs). By using ultrananocrystalline diamond (UNCD) membrane and plasma-activated wafer bonding technology, a single diamond-based circular CMUT is demonstrated and operated in immersion for the first time. The diamond-based CMUT, biased at 100 V, is excited with a 10-cycle burst of 36 V(p-p) sine signal at 3.5 MHz. Pressure generated on a 2-D plane coincident with the normal of the CMUT is measured using a broadband hydrophone. Peak-to-peak hydrophone voltage measurements along the scan area clearly indicate the main lobe and the side lobes, as theoretically predicted by our directivity function calculations. The peak-to-peak hydrophone voltage on the axial direction of the CMUT is found to be in agreement with our theoretical calculations in the Fraunhofer region (-45 mm <y <-15 mm). The spectrum of the diamond-based CMUT is measured for a dc bias of 100 V, and ac excitation with 30-cycle bursts of 9, 36, and 54 V(p-p) sine signal. A peak response at 5.6 MHz is measured for all ac amplitudes. Overall, diamond is shown to be an applicable membrane for CMUT devices and applications.

A photoacoustic imager with light illumination through an infrared-transparent silicon CMUT array

A novel hardware design and preliminary experimental results for photoacoustic imaging are reported in this paper. This imaging system makes use of an infrared-transparent capacitive micromachined ultrasonic transducer (CMUT) chip for ultrasound reception and illuminates the image target through the CMUT array. The cascaded arrangement between the light source and transducer array allows for a more compact imager head and results in more uniform illumination. Taking advantage of the low optical absorption coefficient of silicon in the near infrared spectrum as well as the broad acoustic bandwidth that CMUTs provide, an infrared-transparent CMUT array has been developed for ultrasound reception. The center frequency of the polysilicon-membrane CMUT devices used in this photoacoustic system is 3.5 MHz, with a fractional bandwidth of 118% in reception mode. The silicon substrate of the CMUT array has been thinned to 100 μm and an antireflection dielectric layer is coated on the back side to improve the infrared-transmission rate. Initial results show that the transmission rate of a 1.06-μm Nd:Yag laser through this CMUT chip is 12%. This transmission rate can be improved if the thickness of silicon substrate and the thin-film dielectrics in the CMUT structure are properly tailored. Imaging of a metal wire phantom using this cascaded photoacoustic imager is demonstrated.

Label-Free Biodetection Using Capacitive Micromachined Ultrasonic Transducers (CMUTs) and Its Application for Cardiovascular Disease Diagnostics

In this study, Capacitive Micromachined Ultrasonic Transducers (CMUTs) were used to detect protein using two different schemes involving antibody detection and biomarker or antigen detection. For antibody detection, anti-human IgG Fab-specific was used as an antibody that specifically adsorbs on human IgG functionalized CMUT devices and anti-goat IgG was chosen as a non-specific negative control. We achieved a significant higher signal for the specific antibodies (~10-500 ng/ml) on CMUT devices in comparison to the non-specific negative control at low concentration. For antigen detection, we were interested in a cardiovascular disease biomarker and chose the tumor necrosis factor alpha (TNF-α) as the analyte. We found a linear calibration curve for TNF-α at very low concentrations and demonstrated that this label-free biodetection has a general applicability for assays including diagnostics of cardiovascular disease.

Square Diaphragm CMUT Capacitance Calculation Using a New Deflection Shape Function

A new highly accurate closed-form capacitance calculation model has been developed to calculate the capacitance of capacitive micromachined ultrasonic transducers (CMUTs) built with square diaphragms. The model has been developed by using a two-dimensional polynomial function that more accurately predicts the deflection curve of a square diaphragm deformed under the influence of a uniform external pressure and also takes account of the fringing field capacitances. The model has been verified by comparing the model-predicted deflection profiles and capacitance values with experimental results published elsewhere and finite element analysis (FEA) carried out by the authors for different material properties, geometric specifications, and loading conditions. New model-calculated capacitance values are found to be in excellent agreement with published experimental results with a maximum deviation of 1.7%, and a maximum deviation of 1.5% has been observed when compared with FEA results. The model can help in improving the accuracy of the design methodology of CMUT devices and other MEMS-based capacitive type sensors built with square diaphragms.

A Miniature Capacitive Micromachined Ultrasonic Transducer Array for Minimally Invasive Photoacoustic Imaging

This paper describes the development of a capacitive micromachined ultrasonic transducer (CMUT) array for minimally invasive photoacoustic imaging (PAI). Integrated on a miniature silicon bar that is approximately 100 μm thick and 2.8-5 mm × 8-18 mm in area, this CMUT array can be implanted into a tissue or placed inside an organ without causing major tissue disruption. Close proximity to the target tissue allows this CMUT array to pick up a relatively weak ultrasound signal generated in a photoacoustic process and to provide diagnostic information inaccessible from a noninvasive transducer. For invasive PAI, silicon-based CMUTs offer an additional significant advantage: The silicon and the dielectric membrane of CMUTs are relatively transparent to near infrared, and the shadowing problem associated with the piezoelectric ultrasonic transducers can be minimized or avoided. A two-layer polysilicon surface micromachining process was used to fabricate this device, followed by a double-sided deep-silicon-etching process for shaping the silicon substrate into a thin probe. Experimental characterization found that the center frequency of the CMUT devices with a 46-μm-diameter 1.0-μm-thick polysilicon membrane was 5.0 MHz, with a fractional bandwidth of 116% in reception mode. The PAI of the nerve cord of a lobster using this miniature CMUT array was demonstrated.

A Capacitive Micromachined Ultrasonic Transducer Array for Minimally Invasive Medical Diagnosis

A capacitive micromachined ultrasonic transducer (CMUT) array for minimally invasive medical diagnosis has been developed. Unlike traditional ultrasonic transducers, which generally use a bulky piece of substrate, this transducer array was integrated on a 40--thick micromachined silicon substrate into a probe shape with a typical shank width of 50-80 and a shank length of 4-8 mm. For 1-D arrays, 24-96 CMUT devices were integrated on one such silicon probe and formed an accurately configured phase array. In addition to miniaturization, reduction of the substrate thickness also decreases the intertransducer crosstalk due to substrate Lamb waves. Due to its miniature size, this array can be placed or implanted close to the target tissue/organ and can perform high-resolution high-precision diagnosis and stimulation using high-frequency ultrasounds. The issue of conflict between resolution and penetration depth of ultrasonic diagnosis can therefore be resolved. A two-layer polysilicon surface micromachining process was used to fabricate this device. Suspended polysilicon membranes of diameters ranging from 20 to 90 and thicknesses from 1.0 to 2.5 were used to generate and detect ultrasounds of frequencies ranging from 1 to 10 MHz. B-mode imaging using this transducer array has been demonstrated.

A Capacitive Micromachined Ultrasonic Transducer Array for Minimally Invasive Medical Diagnosis

A capacitive micromachined ultrasonic transducer (CMUT) array for minimally invasive medical diagnosis has been developed. Unlike traditional ultrasonic transducers, which generally use a bulky piece of substrate, this transducer array was integrated on a 40--thick micromachined silicon substrate into a probe shape with a typical shank width of 50-80 and a shank length of 4-8 mm. For 1-D arrays, 24-96 CMUT devices were integrated on one such silicon probe and formed an accurately configured phase array. In addition to miniaturization, reduction of the substrate thickness also decreases the intertransducer crosstalk due to substrate Lamb waves. Due to its miniature size, this array can be placed or implanted close to the target tissue/organ and can perform high-resolution high-precision diagnosis and stimulation using high-frequency ultrasounds. The issue of conflict between resolution and penetration depth of ultrasonic diagnosis can therefore be resolved. A two-layer polysilicon surface micromachining process was used to fabricate this device. Suspended polysilicon membranes of diameters ranging from 20 to 90 and thicknesses from 1.0 to 2.5 were used to generate and detect ultrasounds of frequencies ranging from 1 to 10 MHz. B-mode imaging using this transducer array has been demonstrated.

Building CMUTs for imaging applications from top to bottom

This paper focuses on an innovative fabrication process of a capacitive micromachined ultrasonic transducer (CMUT) array for medical imaging. Our approach consists in inverting the function of each layer to build the CMUT capacitive cell starting from the membrane, made of silicon nitride, up to the bottom electrode, which is supported by an appropriate acoustic backing to absorb any ultrasound energy transmitted towards the backside of the array and to prevent the ringing of the pulse-echo signal. The fabricated devices exhibited a large bandwidth (>100%) and an increased sensitivity, in respect to the classical CMUT devices. Using the fabricated devices as linear probes, echographic images are obtained.