Air-Coupled Nondestructive Evaluation Using Micromachined Ultrasonic Transducers

Nondestructive evaluation techniques Amlrurn , which use conventional piezoelectric transducers tvDicallv reauire 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 a t the air-solid interfaces are tolerable. Capacitive miFig, Schematic cross-section of a single cMUT mem. cromachined ultrasonic transducers (cMUTs) have brane been shown t o 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. transmission system using cMUTs that achieves a In this paper, we present results of a pitch-catch dynamic range in excess of 100 dB. The pair of transducers is modeled with an equivalent electrical circuit which predicts the transmission system's 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. INTRODUCTION Many capacitive micromachined ultrasonic transducers (cMUTs) have been developed for efficient excitation and detection of ultrasound in air [l], [2]. Although such a system may efficiently transfer energy to the air, signal losses at the air-solid interfaces remain. Therefore, a system with a large dynamic range is still necessary for defect detection in the sample. Figure 1 shows the structure of a single membrane of a cMUT. A single element consists of a 50 pmradius, 1 pm-thick metalized silicon-nitride membrane suspended above a silicon substrate. Approximately Fig. 2. Magnified view of cMUT transducer with membrane radii of 50 @m 12,000 such membranes are electrically connected in parallel to form a 1 cm2 transducer, a section of which is shown in Fig. 2. Application of an alternating signal superimposed on a DC bias voltage moves the memhrane and generates ultrasound in air. Reception of ultrasonic waves is analogous to generation, as movement of the membrane varies the charge between the capacitor plates formed by the membrane and substrate. The resulting transducer resonates between 2.3 and 2.5 MHz depending on other process parameters, and is capable of a dynamic range of 100 dB. EQUIVALENT CIRCUIT MODELING An equivalent circuit model of the cMUT is useful both for transducer design as well as for examining the characteristics and limitations of a defect detection system. The model in Fig. 3 shows a small-signal equivalent of a cMUT operating in air (31, largely based on analysis by Mason (41. Quantities on the left side of Fig. 3 represent electrical quantities, while elements on the right side are mechanical quantities. A 0-7803-5722-1/99/$10.00