The microfabrication of capacitive ultrasonic transducers (original) (raw)
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Surface micromachined capacitive ultrasonic transducers
IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control, 2000
The current state of a novel technology, surface microfabricated ultrasonic transducers, is reported. Experiments demonstrating both air and water transmission are presented. Air-coupled longitudinal wave transmission through aluminum is demonstrated, implying a 110 dB dynamic range for transducers at 2.3 MHz in air. Water transmission experiments from 1 to 20 MHz are performed, with a measured 60 dB SNR at 3 MHz. A theoretical model is proposed that agrees well with observed transducer behavior. Most significantly, the model is used to demonstrate that microfabricated ultrasonic transducers constitute an attractive alternative to piezoelectric transducers in many applications.
Micromachined capacitive ultrasonic transducers fabricated using silicon on insulator wafers
Microelectronic Engineering, 2002
In this paper we present the fabrication process of capacitive micromachined ultrasonic transducers by means of surface micromachining techniques using silicon on insulator (SOI) substrates as starting material. The use of SOI substrates provides a structural layer, in which the free-standing membranes are fabricated, of crystal silicon with excellent mechanical properties, and a sacrificial layer of buried oxide that can be etched selectively with respect to silicon. The process we developed has allowed to obtain transducers with resonance frequency at 6 MHz in air.
Precision Engineering, 2002
The fabrication of capacitive ultrasonic transducers by means of surface micromachining techniques and a low-temperature process is presented. Investigation of main process steps is reported. The use of polyimide as sacrificial layer, possible as the process is at low temperature, guarantees precise control of active transducer cells, thanks to its etching selectivity against the structural materials employed, and to the lithographic definition of the sacrificial layer into islands before the deposition of the membrane layer (pre-patterning). Control of the mechanical properties of free-standing membranes has been gained with the optimization of silicon nitride deposition and following thermal annealing steps.
New fabrication process for capacitive micromachined ultrasonic transducers
2003
In this paper, we introduce a new method to fabricate Capacitive Micromachined Ultrasonic Transducers (CMUT) that uses a wafer-bonding technique. The transducer membrane and cavity are defined separately on a Silicon-On-Insulator (Sol) wafer and on a prime quality silicon wafer, respectively. Using silicon direct bonding in a vacuum environment, the two wafers are bonded forming the transducer. This new process offers many advantages over surface micromachining on the fabrication of the transducers with different cavity and membrane configurations. ChlUTs with different dimensions have been successfully fabricated and characterized. For the first time, sub-MHz operation is achieved with CMUTs. The test results show that the new process is a promising method to fabricate CMUTs for operation in air and water at different frequency ranges.
Highly integrated 2-D capacitive micromachined ultrasonic transducers
Two dimensional (2-D) silicon based capacitive micromachined ultrasonic transducer (cMUT) arrays are fabricated efficiently using standard integrated circuit (IC) processing techniques. Furthermore, high density interconnects are implemented using through-chip vias which bring the signal from the front surface of the transducer chip to the back side. The transducer chip then can be flip-chip bonded to a signal processing chip. This results in a very compact two chip hybrid package that can be easily sealed for immersion applications. The electrical interconnects are realized by high aspect ratio vias (50 μm in diameter, 550 μm in depth) that are etched into the silicon substrate by a deep reactive ion etcher (DRIE). The vias are then coated by polysilicon which is subsequently doped to achieve good conductivity. The transducer arrays are then built on the front side using the conventional process developed for cMUTs. Measurements indicate that the through wafer vias have a typical r...
Silicon Micromachined Ultrasonic Transducers
Japanese Journal of Applied Physics, 2000
This paper reviews capacitor micromachined ultrasonic transducers (cMUTs). Transducers for air-borne and immersion applications are made from parallel-plate capacitors whose dimensions are controlled through traditional integrated circuit manufacturing methods. Transducers for airborne ultrasound applications have been operated in the frequency range of 0.1-11 MHz, while immersion transducers have been operated in the frequency range of 1-20 MHz. The Mason model is used to represent the cMUT and highlight the important parameters in the design of both airborne and immersion transducers. Theory is used to compare the dynamic range and the bandwidth of the cMUTs to piezoelectric transducers. It is seen that cMUTs perform at least as well if not better than piezoelectric transducers. Examples of single-element transducers, linear-array transducers, and two-dimensional arrays of transducers will be presented.
The characterization of capacitive micromachined ultrasonic transducers in air
Ultrasonics, 2002
Surface micromachined, capacitive ultrasonic transducers have been fabricated using a low thermal budget, CMOS-compatible process. They exhibit interesting properties for transduction in air at frequencies in excess of 1 MHz, when driven from a standard ultrasonic voltage source. Experiments are described using 1 mm square devices in air, operating in both pitch-catch and pulse-echo modes. The dependence on d.c. bias voltage is examined, together with calibration measurements using 1/8 in. microphones. The radiated beam profile, and the farfield directivity pattern, have been measured for both broad bandwidth and one-burst excitation, using a scanned miniature receiver. A 16 element square array is also presented, which has been used to measure the beam cross-sections from a focussed source.
2002 IEEE Ultrasonics Symposium, 2002. Proceedings.
Capacitive micromachined ultrasonic transducers (CMUTs) have long been studied. Past research has shown that CMUTs indeed have remarkable features such as wide bandwidth and high efficiency. This paper introduces an inclusion to the CMUT technology that uses the wafer-bonding technique to fabricate membranes on silicon. This new technology enables the fabrication of large membranes with large gaps, and expands the frequency span of CMUTs to 10 kHz in the low end. CMUT devices with different frequency spans are fabricated using both technologies, and tested. Electromechanical coupling efficiency, k T 2 , value as high as 0.85 and fractional immersion bandwidth as wide as 175 % are measured.
Advances in Capacitive Micromachined Ultrasonic Transducers
Micromachines, 2019
Capacitive micromachined ultrasonic transducer (CMUT) technology has enjoyed rapid development in the last decade. Advancements both in fabrication and integration, coupled with improved modelling, has enabled CMUTs to make their way into mainstream ultrasound imaging systems and find commercial success. In this review paper, we touch upon recent advancements in CMUT technology at all levels of abstraction; modeling, fabrication, integration, and applications. Regarding applications, we discuss future trends for CMUTs and their impact within the broad field of biomedical imaging.