S.T. Hansen

Silicon Micromachined Ultrasonic Transducers

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.

High-frequency CMUT arrays for high-resolution medical imaging

The paper describes high-frequency 1D CMUT arrays designed and fabricated for use in electronically scanned high-resolution ultrasonic imaging systems. Two different designs of 64-element linear CMUT arrays are presented. A single element in each array is connected to a single-channel custom front-end integrated circuit for pulse-echo operation. The first design has a resonant frequency of 43 MHz in air, and operates at 30 MHz in immersion. The second design exhibits a resonant frequency of 60 MHz in air, and operates at 45 MHz in immersion. Experimental results are compared to simulation results obtained from the equivalent circuit model and nonlinear dynamic finite element analysis; a good agreement is observed between these results. The paper also briefly discusses the effects of the area fill factor on the frequency characteristics of CMUTs, which reveals that the transducer active area should be maximized to obtain a wideband response at high frequencies.

Air-coupled nondestructive evaluation using micromachined ultrasonic transducers

Nondestructive evaluation techniques which use conventional piezoelectric transducers typically require liquid coupling fluids to improve the impedance mismatch between piezoelectric materials and air. Air-coupled ultrasonic systems can eliminate this requirement if the dynamic range of the system is large enough such that the losses at the air-solid interfaces are tolerable. Capacitive micromachined ultrasonic transducers (cMUTs) have been shown to have more than 100 dB dynamic range when used in bistatic transmission mode. This dynamic range, along with the ability to transmit ultrasound efficiently into air, makes cMUTs ideally suited for air-coupled nondestructive evaluation applications. These transducers can be used either in through transmission experiments at normal incidence to the sample or to excite and detect guided waves in aluminum and composite plates. In this paper, we present results of a pitch-catch transmission system using cMUTs that achieves a dynamic range in excess of 100 dB. The pair of transducers is modeled with an equivalent electrical circuit which predicts the transmission systems insertion loss and dynamic range. We also demonstrate the feasibility of Lamb wave defect detection for one-sided nondestructive evaluation applications. A pair of cMUTs excites and detects the so mode in a 1.2 mm-thick aluminum plate with a received signal-to-noise ratio of 28 dB without signal averaging.

Wideband micromachined capacitive microphones with radio frequency detection

Silicon microphones based on capacitive micromachined ultrasonic transducer membranes and radio frequency detection overcome many of the limitations in bandwidth, uniformity of response, and durability associated with micromachined condenser microphones. These membranes are vacuum-sealed to withstand submersion in water and have a flat mechanical response from dc up to ultrasonic frequencies. However, a sensitive radio frequency detection scheme is necessary to detect the small changes in membrane displacement that result from utilizing small membranes. In this paper we develop a mathematical model for calculating the expected output signal and noise level and verifies the model with measurements on a fabricated microphone. Measurements on a sensor with 1.3 mm2 area demonstrate less than 0.5 dB variation in the output response between 0.1 Hz to 100 kHz under electrostatic actuation and an A-weighted equivalent noise level of 63.6 dB(A) SPL in the audio band. Because the vacuum-sealed membrane structure ha...

Capacitive micromachined ultrasonic Lamb wave transducers using rectangular membranes

This paper details the theory, fabrication, and characterization of a new Lamb wave device. Built using capacitive micromachined ultrasonic transducers (CMUTs), the structure described uses rectangular membranes to excite and receive Lamb waves on a silicon substrate. An equivalent circuit model for the transducer is proposed that produces results, which match well with those observed by experiment. During the derivation of this model, emphasis is placed on the resistance presented to the transducer membranes by the Lamb wave modes. Finite element analysis performed in this effort shows that the dominant propagating mode in the device is the lowest order antisymmetric flexural wave (A/sub 0/). Furthermore, most of the power that couples into the Lamb wave is due to energy in the vibrating membrane that is transferred to the substrate through the supporting posts of the device. The manufacturing process of the structure, which relies solely on fundamental IC-fabrication techniques, is also discussed. The resulting device has an 18 /spl mu/m-thick substrate that is almost entirely made up of crystalline silicon and operates at a frequency of 2.1 MHz. The characterization of this device includes S-parameter and laser vibrometer measurements as well as delay-line transmission data. The insertion loss, as determined by both S-parameter and delay-line transmission measurements, is 20 dB at 2.1 MHz. When configured as a delay-line oscillator, the device functions well as a sensor with sensitivity to changes in the mass loading of its substrate.

CMUT ring arrays for forward-looking intravascular imaging

The paper describes an annular CMUT ring array designed and fabricated for the tip of a catheter used for forward-looking intravascular imaging. A 64-element, 2-mm average diameter array was fabricated as an experimental prototype. A single element in the array is connected to a single-channel custom front-end integrated circuit for pulse-echo operation. In conventional operation, the transducer operates at around 10 MHz. In the collapsed regime, the operating frequency shifts to 25 MHz and the received echo amplitude is tripled. The SNR is measured as 23 dB in a 50-MHz measurement bandwidth for an echo signal from a plane reflector at 1.5 mm. We also performed a nonlinear dynamic transient finite element analysis for the transducer, and found these results to be in good agreement with experimental measurements, both for conventional and collapsed operation.

Characterization of capacitive micromachined ultrasonic transducers in air using optical measurements

Capacitive micromachined ultrasonic transducers (CMUTs) are efficient transmitters and receivers for air-coupled nondestructive evaluation applications. In this paper, we present optical measurements on CMUTs with circular and rectangular membranes. Use of laser interferometer permits accurate measurement of individual membrane displacements as well as characterization of mode shapes on the membrane. When optical displacement measurements are combined with electrical impedance measurements, all elements of the equivalent circuit model can be evaluated. In particular, an estimate of the loss in the transducer is possible. Loss mechanisms include structural losses and squeeze-film effects of air behind the membrane, the latter of which has both frequency-shifting and dissipative effects. Recently fabricated circular and rectangular membrane designs typically exhibit a real loss on the order of 500 rayls/spl plusmn/200 rayls. The degree of resonant frequency shift for unsealed membranes in air depends strongly on the particular geometry, but can be as much as 30% of the transducers resonant frequency in vacuum. Measurements of CMUT loss due to air are compared to squeeze-film theory and are included in an equivalent circuit model. This model is compared against the measurement results.

Defect imaging by micromachined ultrasonic air transducers

Capacitive micromachined ultrasonic transducers (cMUTs) are shown to have over 100 dB dynamic range in air. This enables fast imaging of internal defects of solid structures with high signal-to-noise ratio. The high dynamic range is the result of a resonant structure with a fractional bandwidth limited to about 10%. Better temporal resolution is required to differentiate the defects in the depth dimension, which demands higher bandwidth devices. In this paper we present an optimized pulse-echo electronics system for cMUTs in air. Simulations suggest that dynamic ranges in excess of 100 dB are attainable in pulse-echo operation using commercially available discrete components. Transmission experiments through aluminum and composite plates verify more than 100 dB dynamic range and demonstrate the ability of cMUTs to image defects in air at 2.3 MHz. We also present a variation on cMUT design which improves the useful bandwidth of the device, permitting greater depth resolution in pulse-echo imaging.

Improved modeling and design of microphones using radio frequency detection with capacitive micromachined ultrasonic transducers

Broadband acoustic sensing, over several decades of frequency, has traditionally been difficult to achieve. An alternative approach to conventional condenser microphones is to use capacitive micromachined ultrasonic transducer (CMUT) membranes with a sensitive radio frequency (RF) detection method. Since the resonant frequency of a typical CMUT membrane is several megahertz, the membrane response to acoustic frequencies below resonance, from DC to several hundred kilohertz, is constant. This paper presents the theory, modeling, and sensitivity predictions of the RF detection method. Electrical thermal noise is now incorporated in the model and ultimately limits the sensitivity. In addition, we present experimental results showing the flat frequency response, from 0.1 Hz to 100 kHz, of a microphone using RF detection. Present measurements demonstrate a sensitivity of 53 dB/Pa/Hz, though improvements to the design are expected to achieve sensitivities approaching 100 dB/Pa/Hz.

Wideband micromachined acoustic sensors with radio frequency detection

Silicon micromachining techniques permit batch fabrication of microphones that are small, reproducible, and inexpensive. However, many such sensors have limited bandwidth or are too fragile to be used in a humid, wet, or dusty outdoor environment. Microphones using capacitive micromachined ultrasonic transducer (CMUT) membranes and radio frequency (RF) detection overcome some of the problems associated with conventional micromachined microphones. CMUT membranes can be vacuum-sealed and still withstand atmospheric pressure and submersion in water. In addition, the membrane mechanical response is very flat from dc up to hundreds of kilohertz. A very sensitive RF detection scheme is necessary to detect the small changes in membrane displacement that result from utilizing smaller membranes. In this paper, we present the theory and recent experimental results of RF detection with CMUT membranes. Measurements of a sensor with 1-mm2 area demonstrate a flat output response of the acoustic sensor from a fraction of 1 Hz to over 100 kHz, with a sensitivity at 1 kHz of 65 dB/Pa in a 1-Hz noise bandwidth.