Capacitive Micromachined Ultrasonic Transducer Research Articles

Simulation Analysis and Performance Testing Investigation of Capacitive Micromachined Ultrasonic Transducer

In various applications of ultrasonic waves, the ultrasonic transducer is the key device of ultrasonic testing and ultrasonic imaging. Compared with the traditional piezoelectric transducer, the capacitive micromachined ultrasonic transducer (CMUT) has many striking advantages, such as low impedance, high bandwidth, easy integration and low cost, and it is expected to become a next generation of mainstream products. In this paper, a CMUT structure for underwater-imaging applications is designed, and the finite element model is established by using COMSOL software, then the modal analysis, harmonic response analysis, electromechanical coupling analysis and transient analysis are carried out. As a consequence, the key parameters of CMUT are obtained, namely resonance frequency, voltage collapse and electromechanical coupling coefficient. For the processed CMUT line array consisting of 16 elements, a test system is built and the emission performance, receiving performance, directivity, bandwidth and preliminary imaging of the designed transducer are tested and analyzed. The results show that the designed CMUT array can meet the requirements of underwater-imaging applications.

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.

A new compact analytical model of nanoelectromechanical systems-based capacitive micromachined ultrasonic transducers for pulse echo imaging

A compact analytical model of a capacitive micromachined ultrasonic transducer (CMUT) exhibiting a damped oscillatory output for pulse echo imaging is proposed. The analysis is improved herein by taking into account the acoustic damping coefficient, proportional to the velocity of vibration of the CMUT membrane equation of motion. Solutions for both radial and temporal variables of the modeled equation are obtained. The result is found to be a comprehensively damped oscillation that would enable an enhanced time interval for a pulse echo imaging sensor system. Finite element method modeling of the CMUT was also performed using PZFlex, a finite element analysis-based simulation tool. The results obtained from the analytical model are in good agreement with those from PZFlex.

A robust control approach for MEMS capacitive micromachined ultrasonic transducer

An optimal composite nonlinear feedback control method with integral sliding mode is presented in this paper. The controller extends the travel range of the micro-electromechanical system capacitive micromachined ultrasonic transducer (CMUT). Moreover, enhanced transient response and precise tracking performance is achieved. It is known that CMUT is inherently unstable which results in pull-in phenomenon and it is very sensitive to small perturbations, so one of the major problems is to stabilize the CMUT beyond the pull-in limit with the external disturbances. In addition, the input saturation problem is significant to CMUT. Based on that, a robust control scheme is derived using composite nonlinear feedback control law combined with integral sliding mode control law. Then all the tuning parameters for the proposed control method are converted into a minimization problem and solved by particle swarm optimization algorithm automatically. We verified the effectiveness through extending the travel range of the CMUT gap by three control methods which are proportional integral derivative, composite nonlinear feedback and the method we proposed. The stability and small range tracking performance with three control methods is compared on the pull-in position of CMUT. The simulations show that the proposed control method has desired tracking performance and robustness to external disturbance with input saturation.

A Column-Row-Parallel Ultrasound Imaging Architecture for 3D Plane-wave Imaging and Tx 2nd-Order Harmonic Distortion (HD2) Reduction.

We propose a Column-Row-Parallel imaging frontend architecture for integrated and low-power 3D medical ultrasound imaging. The Column-Row-Parallel architecture offers linear-scaling interconnection, acquisition and programming time with row-by-row or column-by-column operations, while supporting volumetric imaging functionality and fault-tolerance against possible transducer element defects with per-element controls. The combination of column-parallel selection logic, row-parallel selection logic, and per-element selection logic reaches a balance between flexible imaging aperture definition and manageable imaging data / control interface to a 2D array. A 16×16 CMUT-ASIC Column-Row-Parallel prototype is fabricated and assembled with a flip-chip bonding process. It facilitates the 3D plane-wave coherent compounding algorithm for volumetric imaging with a fast frame rate of 62.5 Hz and 46% improved lateral resolution with 10-angle compounding and a field of view volume of 2.3mm in both azimuth and elevation, 8.5mm in depth. At a hypothetically scaled up 64x64 array size, the frame rate can still be kept at 31.2 Hz for a volume of 40mm in both azimuth and elevation, 150mm in depth. An interleaved checker board pattern with in-phase (I) and quadrature (Q) excitations is also demonstrated for reducing CMUT second harmonic distortion (HD2) emission by up to 25 dB at the loss of 3 dB fundamental energy reduction. The method reduces nonlinear effects from both transducers and circuits and is a wide band technique that is applicable to arbitrary pulse shapes.

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.

Probe development of CMUT and PZT row–column-addressed 2-D arrays

This paper presents the characterization of two prototyped fully integrated 62 + 62 row–column-addressed (RCA) 2-D transducer array probes, which are based on capacitive micromachined ultrasonic transducer (CMUT) and on piezoelectric transducer (PZT) technology, respectively. Both transducers have integrated apodization to reduce ghost echoes and were designed with similar acoustical features i.e. 3 MHz center frequency, λ/2-pitch and 24.8 mm × 24.8 mm active footprint. The transducer arrays were assembled in a 3-D printed probe handle with electromagnetic shield and integrated electronics for driving the 128-channel coaxial cable to the scanner. The electronics were designed to allow all elements, both rows and columns, to be used interchangeably as either transmitters or receivers. The transducer characterization i.e. bandwidth, phase delay, surface pressure, sensitivity, insertion loss, and acoustical crosstalk, were based on several single element measurements, including pressure and pulse-echo, and were evaluated quantitatively and comparatively. The weighted center frequency was 3.0 MHz for both probes and the measured −6 dB fractional bandwidth was 109 ± 4% and 80 ± 3% for the CMUT and the PZT probe, respectively. The surface pressures of the CMUT and PZT were 0.55 ± 0.06 MPa and 1.68 ± 0.09 MPa, respectively, and the receive sensitivities of the rows (receiving elements) were 12.9 ± 0.7 μV/Pa and 13.7 ± 2.1 μV/Pa.

Fabrication and characterization of SU-8-based capacitive micromachined ultrasonic transducer for airborne applications

We present a successful fabrication and characterization of a capacitive micromachined ultrasonic transducer (CMUT) with SU-8 as the membrane material. The goal of this research is to develop a post-CMOS compatible CMUT that can be monolithically integrated with the CMOS circuitry. The fabrication is based on a simple, three mask process, with all wet etching steps involved so that the device can be realized with minimal laboratory conditions. The maximum temperature involved in the whole process flow was 140°C, and hence, it is post-CMOS compatible. The fabricated device exhibited a resonant frequency of 835 kHz with bandwidth 62 kHz, when characterized in air. The pull-in and snapback characteristics of the device were analyzed. The influence of membrane radius on the center frequency and bandwidth was also experimentally evaluated by fabricating CMUTs with membrane radius varying from 30 to 54 μm with an interval of 4 μm. These devices were vibrating at frequencies from 5.2 to 1.8 MHz with an average Q-factor of 23.41. Acoustic characterization of the fabricated devices was performed in air, demonstrating the applicability of SU-8 CMUTs in airborne applications.

A Fast-Switching (1.35-μs) Low-Control-Voltage (2.5-V) MEMS T/R Switch Monolithically Integrated With a Capacitive Micromachined Ultrasonic Transducer (CMUT).

This paper describes the design and fabrication of an electrostatic MEMS switch that can be co-fabricated on the same substrate with a capacitive micromachined ultrasonic transducer (CMUT) as a transmit/receive (T/R) switch. The structure of the switch is modified from a single CMUT cell. An interrupted transmission line is defined across the center of the cell with control electrodes on both sides to pull a movable plate down. The plate has an insulation layer underneath and a metal bump is formed on the insulation layer and aligned to the transmission line gap, so that the switch could be turned on by pulling down the plate with electrostatic force and making the metal bump close the gap in the transmission line. The switch was designed using a finite-element model (FEM) and fabricated on a glass substrate using anodic bonding. A static characterization was first performed on a switch test structure, which shows the dc switching voltage was 68 V and the on-resistance was 50 Ω. The RF in and RF out isolation was measured as approximately 66 dB and insertion loss was approximately 4.5 dB for frequency range commonly used for medical ultrasound imaging. Then we performed the dynamic characterization in immersion. By setting the dc bias at 67 V, we found that the switch could be operated with a control-voltage as low as 2.5 V. The switching and release times are related to the rise time and the fall time of the control signal, respectively. The minimum switching time was measured as 1.34 μs with a control signal rise time of 300 ns, and the minimum release time was measured as 80 ns with a control signal fall time of 20 ns. We further demonstrated that a 1-kHz control signal with the optimized rise and fall times can be used to conduct and block a sinusoidal signal with 1-MHz frequency and 300-mV pp amplitude, as well as unipolar pulses with 5-V pp amplitude, 500-ns pulse width, and 2-kHz repetition rate. The presented MEMS switch could potentially eliminate the high-voltage process requirement for the on-chip front-end electronics of a CMUT-based ultrasound imaging system and thus improve the overall system efficiency.

Backward-Mode Photoacoustic Imaging Using Illumination Through a CMUT With Improved Transparency.

In this paper, we describe a capacitive micromachined ultrasonic transducer (CMUT) with improved transparency for photoacoustic imaging (PAI) with backside illumination. The CMUT was fabricated on a glass substrate with indium-tin oxide bottom electrodes. The plate was a 1.5- silicon layer formed over the glass cavities by anodic bonding, with a 1- silicon nitride passivation layer on top. The fabricated device shows approximately 30%-40% transmission in the wavelength range from 700 to 800 nm and approximately 40%-60% transmission in the wavelength range from 800 to 900 nm, which correspond to the wavelength range commonly used for in vivo PAI. The center frequency of the CMUT was 3.62 MHz in air and 1.4 MHz in immersion. Two preliminary PAI experiments were performed to demonstrate the imaging capability of the fabricated device. The first imaging target was a 0.7-mm diameter pencil lead in vegetable oil as a line target with a subwavelength cross section. A 2-mm-diameter single CMUT element with an optical fiber bundle attached to its backside was linearly scanned to reconstruct a 2-D cross-sectional PA image of the pencil lead. We investigated the spurious signals caused by the light absorption in the 1.5- silicon plate. For pencil lead as a strong absorber and also a strong reflector, the received echo signal due to the acoustic excitation generated by the absorption in silicon is approximately 30 dB lower than the received PA signal generated by the absorption in pencil lead at the wavelength of 830 nm. The second imaging target was a "loop-shape" polyethylene tube filled with indocyanine green solution ( ) suspended using fishing lines in a tissue-mimicking material. We formed a 3-D volumetric image of the phantom by scanning the transducer in the - and -directions. The two experimental imaging results demonstrated that CMUTs with the proposed structure are promising for PAI with backside illumination.

Evaluation of an Ultrasound-Guided Focused Ultrasound CMUT Probe for Targeted Therapy Applications

Capacitive Micromachined Ultrasonic Transducer (CMUT) technology, which has been widely studied in the field of medical imaging, possesses strong design flexibility due to its manufacturing process. Many applications could benefit from this unique feature, especially those that require different operating ultrasonic frequencies. This article reports on the characterization of the therapeutic low-frequency field provided by an ultrasound-guided focused ultrasound CMUT probe that is connected to a custom ultrasonic scanner for hyperthermia applications. The study begins by mapping the focused ultrasonic beam in the vicinity of the focal spot and a parametric analysis providing the maximum peak-to-peak (PTP) pressure delivered by the probe under different acoustic conditions. The measured maximum PTP pressure at the targeted operating frequency of 1 MHz is 3 MPa, with a maximum of 3.6 MPa at 1.25 MHz. Based on an in vitro setup found in the literature, the temperature elevation at the focal point was measured, showing results in agreement with the targeted applications (max. ΔT = 7.5°C). The article concludes with a reliability study considering the delivered pressure and the self-heating of the CMUT probe: the results show the good stability of the pressure amplitude over 1.8 × 109 cycles at a duty cycle of 40%, with a moderate internal heating of 3°C.

Nonlinear dynamics and stability of microsystems engineering elements

Subject of Research. Static and dynamic problems of coupled electroelasticity are considered for electrostatic (capacitive) transducers used in sensors and actuators based on nano- and microsystem technology of various applications. Method. Above-mentioned problems are analyzed by mathematical apparatus of nonlinear mechanics and bifurcation theory as well as modern numerical methods, including numerical continuation techniques for nonlinear boundary-value problems of mathematical physics. Main Results. Comparative analysis of analytical and numerical methods was performed for nonlinear static and dynamic boundary-value problems of electroelasticity for microsystems and nanosystems engineering. Basic discrete (nonlinear nano/micro-electromechanical oscillators) and distributed (membranes, plates) electromechanical models were considered. Equilibrium forms, their stability and bifurcations were studied for afore-named elastic systems under the influence of electric fields of various configurations. Bifurcation diagrams were derived depending on key physical parameters. Nonlinear dynamic problems for elastic systems at time-varying electric fields were also considered. Practical Relevance. The present research is of considerable practical significance because it reveals and analyzes qualitatively the elastic elements properties and characteristics that are important for nano/micro-system design, such as equilibria structure and stability, amplitude and force response of the system, etc. Used methods and mathematical formulations can be applied in the design process of micromechanical accelerometers and gyroscopes, pressure sensors, micro-pumps, capacitive micro-machined ultrasound transducers, radio-frequency and optical switches, electromagnetic energy harvesting systems and biomedical devices.

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.

A fluid-coupled transmitting CMUT operated in collapse mode: Semi-analytic modeling and experiments

An electro-mechanical, semi-analytic, reduced-order (RO) model of a fluid-loaded transmitting capacitive-micromachined ultrasound transducer (CMUT) operated in collapse mode is developed. Simulation of static deflections, approximated by a linear combination of six mode shapes, are benchmarked toward state-of-the-art models and validated with digital holography microscope measurements of a fabricated CMUT device. The dynamic response of a detached single CMUT cell and an array of CMUT cells is predicted and analyzed for the effect of mutual radiation. The step-wise validation shows that our model predicts the static response including hysteresis behavior of a collapse-mode CMUT with a high accuracy. The dynamic response and frequency-tunability are modeled with a satisfactory accuracy. The developed RO model is computationally efficient and in general faster than finite element methods. It is concluded that the presented RO model allows fast parameter analysis and is a powerful tool for CMUT pre-design.

Acoustical Streaming in Microfluidic CMUT Integrated Chip Controls the Biochemical Interaction Rate

Acoustical fluid mixing and streaming in microfluidic chips enhance the detection and fluid routeing capabilities in the lab-on-chip devices. Capacitive micromachined ultrasound transducers (CMUT) are easy to integrate into a closely packed environment, and they can be simultaneously used as integrated sensors and micropumps/mixers. In this paper, particular focus is given to examininge the impact of the acoustical fluid mixing to the kinetics of biochemical interaction. CMUT interdigital transducers for 10-MHz operation in water were designed and fabricated using the surface micromachining technique. Devices use Scholte type waves for biochemical detection and acoustical streaming. They also have the ability to control the directionality of acoustical streaming by +/−90° phase shift. The impact of acoustical streaming to the liquid diffusion kinetics in the microchannel and to the kinetics of adsorption of the bovine serum albumin (BSA) to the gold surface was investigated experimentally. For microfluidic experiments, CMUTs were assembled with 100 $\mu \text{m}$ deep microchannels. It was determined that acoustical streaming can improve the diffusion rate through the microchannel. Also, it was shown that BSA adsorption rate can be controlled by changing the phase shift during excitation of the Sholte type waves. [2016–0224]

An Experimental Protocol for Assessing the Performance of New Ultrasound Probes Based on CMUT Technology in Application to Brain Imaging.

The possibility to perform an early and repeatable assessment of imaging performance is fundamental in the design and development process of new ultrasound (US) probes. Particularly, a more realistic analysis with application-specific imaging targets can be extremely valuable to assess the expected performance of US probes in their potential clinical field of application. The experimental protocol presented in this work was purposely designed to provide an application-specific assessment procedure for newly-developed US probe prototypes based on Capacitive Micromachined Ultrasonic Transducer (CMUT) technology in relation to brain imaging. The protocol combines the use of a bovine brain fixed in formalin as the imaging target, which ensures both realism and repeatability of the described procedures, and of neuronavigation techniques borrowed from neurosurgery. The US probe is in fact connected to a motion tracking system which acquires position data and enables the superposition of US images to reference Magnetic Resonance (MR) images of the brain. This provides a means for human experts to perform a visual qualitative assessment of the US probe imaging performance and to compare acquisitions made with different probes. Moreover, the protocol relies on the use of a complete and open research and development system for US image acquisition, i.e. the Ultrasound Advanced Open Platform (ULA-OP) scanner. The manuscript describes in detail the instruments and procedures involved in the protocol, in particular forthe calibration, image acquisition and registration of US and MR images. The obtained results prove the effectiveness of the overall protocol presented, which is entirely open (within the limits of the instrumentation involved), repeatable, and covers the entire set of acquisition and processing activities for US images.

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

An Optimization and Comparative Study of Air-Coupled CMUT Cells With Circular and Annular Geometries.

Air-coupled capacitive micromachined ultrasonic transducers (CMUTs) with annular cell geometry have recently been reported to have a promising transmit sensitivity. This paper reports three optimization schemes, which further improve the transmit sensitivity and also help achieve a reasonable comparison between the novel annular and conventional circular cells. Lumped element models of both cell types with laminate plate structures are presented. Based on these models, a design optimization flowchart was constructed to facilitate analytical optimization on the three schemes. Circular and annular CMUTs with a common 97-kHz natural resonance frequency were fabricated and characterized to verify the efficacy of the optimization principle. Using the optimization flowchart, annular and circular cells with frequencies ranging from 100 to 300 kHz were analytically optimized and then compared. The comparison results demonstrate that, given the same dc bias and ac excitation voltage, the output power density at the plate surface of the optimized annular cell is double that of the optimized circular cell. Additionally, when generating the same surface power density, an optimized annular cell requires either half the dc bias or half the ac excitation voltage of an optimized circular cell. This paper provides a practical optimization framework for CMUT cell design and demonstrates the superiority of annular cells for air-coupled applications.

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]