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Therapeutic ultrasound guided by magnetic resonance imaging (MRI) is a noninvasive treatment that potentially reduces mortality, lowers medical costs, and widens accessibility of treatments for patients. Recent developments in the design and fabrication of capacitive micromachined ultrasonic transducers (CMUTs) have made them competitive with piezoelectric transducers for use in therapeutic ultrasound applications. In this paper, we present the first designs and prototypes of an 8-element, concentric-ring, CMUT array to treat upper abdominal cancers. This array was simulated and designed to focus 30–50 mm into tissue and ablate a 2–3 cm diameter tumor within 1 hour. Assuming a surface acoustic output pressure of 1 MPa peak to peak (8.5 W/cm2) at 2.5 MHz, we simulated an array that produced a focal intensity of 680 W/cm2 when focusing to 35 mm. CMUT cells were then designed to meet these frequency and surface acoustic intensity specifications. These cell designs were fabricated as 2.5 mm by 2.5 mm test transducers and used to verify our models. The test transducers were shown to operate at 2.5 MHz with an output pressure of 1.4 MPa peak to peak (16.3 W/cm2). With this CMUT cell design, we fabricated a full 8-element array. Due to yield issues, we only developed electronics to focus the four center elements of the array. The beam profile of the measured array deviated from the simulated because of crosstalk effects; the beamwidth matched within 10% and sidelobes increased by 2 times, which caused the measured gain to be 16.6 compared to 27.4.

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.[ 2010-0197]Index Terms-Capacitive micromachined ultrasonic transducer (CMUT), direct wafer bonding, electrical breakdown, local oxidation of silicon (LOCOS), parasitic capacitance, patterning of silicon via oxidation.

Distributed sensing of gas-phase chemicals using highly sensitive and inexpensive sensors is of great interest for many defense and consumer applications. In this paper we present ppb-level detection of dimethyl methylphosphonate (DMMP), a common simulant for sarin gas, with a ppt-level resolution using an improved capacitive micromachined ultrasonic transducer (CMUT) as a resonant chemical sensor. The improved CMUT operates at a higher resonant frequency of 47.7 MHz and offers an improved mass sensitivity of 48.8 zg/Hz/μm(2) by a factor of 2.7 compared to the previous CMUT sensors developed. A low-noise oscillator using the CMUT resonant sensor as the frequency-selective device was developed for real-time sensing, which exhibits an Allan deviation of 1.65 Hz (3σ) in the presence of a gas flow; this translates into a mass resolution of 80.5 zg/μm(2). The CMUT resonant sensor is functionalized with a 50-nm thick DKAP polymer developed at Sandia National Laboratory for dimethyl methylphosphonate (DMMP) detection. To demonstrate ppb-level detection of the improved chemical sensor system, the sensor performance was tested at a certified lab (MIT Lincoln Laboratory), which is equipped with an experimental chemical setup that reliably and accurately delivers a wide range of low concentrations down to 10 ppb. We report a high volume sensitivity of 34.5 ± 0.79 pptv/Hz to DMMP and a good selectivity of the polymer to DMMP with respect to dodecane and 1-octanol.