David E. Dausch

Theory and operation of 2-D array piezoelectric micromachined ultrasound transducers

Piezoelectric micromachined ultrasound transducers (pMUTs) are a new approach for the construction of 2-D arrays for forward-looking 3-D intravascular (IVUS) and intracardiac (ICE) imaging. Two-dimensional pMUT test arrays containing 25 elements (5 times 5 arrays) were bulk micromachined in silicon substrates. The devices consisted of lead zirconate titanate (PZT) thin film membranes formed by deep reactive ion etching of the silicon substrate. Element widths ranged from 50 to 200 mum with pitch from 100 to 300 mum. Acoustic transmit properties were measured in de-ionized water with a calibrated hydrophone placed at a range of 20 mm. Measured transmit frequencies for the pMUT elements ranged from 4 to 13 MHz, and mode of vibration differed for the various element sizes. Element capacitance varied from 30 to over 400 pF depending on element size and PZT thickness. Smaller element sizes generally produced higher acoustic transmit output as well as higher frequency than larger elements. Thicker PZT layers also produced higher transmit output per unit electric field applied. Due to flexure mode operation above the PZT coercive voltage, transmit output increased nonlinearly with increased drive voltage. The pMUT arrays were attached directly to the Duke University T5 phased array scanner to produce real-time pulse-echo B-mode images with the 2-D pMUT arrays.

In Vivo Real-Time 3-D Intracardiac Echo Using PMUT Arrays

Piezoelectric micromachined ultrasound transducer (PMUT) matrix arrays were fabricated containing novel through-silicon interconnects and integrated into intracardiac catheters for in vivo real-time 3-D imaging. PMUT arrays with rectangular apertures containing 256 and 512 active elements were fabricated and operated at 5 MHz. The arrays were bulk micromachined in silicon-on-insulator substrates, and contained flexural unimorph membranes comprising the device silicon, lead zirconate titanate (PZT), and electrode layers. Through-silicon interconnects were fabricated by depositing a thin-film conformal copper layer in the bulk micromachined via under each PMUT membrane and photolithographically patterning this copper layer on the back of the substrate to facilitate contact with the individually addressable matrix array elements. Cable assemblies containing insulated 45-AWG copper wires and a termination silicon substrate were thermocompression bonded to the PMUT substrate for signal wire interconnection to the PMUT array. Side-viewing 14-Fr catheters were fabricated and introduced through the femoral vein in an adult porcine model. Real-time 3-D images were acquired from the right atrium using a prototype ultrasound scanner. Full 60° × 60° volume sectors were obtained with penetration depth of 8 to 10 cm at frame rates of 26 to 31 volumes per second.

5I-4 Piezoelectric Micromachined Ultrasound Transducer (pMUT) Arrays for 3D Imaging Probes

Piezoelectric micromachined ultrasound transducers (pMUTs) have been fabricated and characterized in this work as a new approach for manufacture of 2D arrays for forward-looking 3D intravascular (IVUS) and intracardiac (ICE) imaging. Two-dimensional pMUT arrays containing 25 elements (5 times 5 arrays) were bulk micromachined in silicon substrates. The devices consisted of suspended PZT thin film membranes formed by deep reactive ion etching. Membrane widths ranged from 50 to 200 mum, with element pitch from 100 to 300 mum. Acoustic transmit properties were measured in de-ionized water with a calibrated hydrophone placed at a range of 20 mm. Measured transmit frequencies for the pMUT elements ranged from 4-11 MHz. Element capacitance for pMUTs ranged from 30 to over 400 pF depending on element size. Smaller element sizes generally produced higher acoustic transmit output than larger elements, and thicker PZT layers also produced higher output per unit electric field applied. Due to flexure mode operation above the PZT coercive voltage, transmit output also increased nonlinearly with increased drive voltage. The pMUT arrays were attached directly to the system amplifiers of the Duke University T5 Phased Array Scanner by a modified cable assembly in order to produce real-time pulse-echo B-mode images with the 2D pMUT arrays

Improved pulse-echo imaging performance for flexure-mode pMUT arrays

Piezoelectric micromachined ultrasound transducers (pMUTs) are potential candidates for catheter-based ultrasound phased arrays. pMUTs consist of lead zirconate titanate (PZT) thin film membranes formed on silicon substrates and are operated in flexure mode by driving the PZT film above its coercive field to induce flextensional motion. The fundamental operation of pMUT devices has been demonstrated; however, pulse-echo imaging has been limited to date. The objective of this work was to optimize transducer design for improved pulse-echo imaging performance. Flexure mode operation was optimized by (1) increasing transmit voltage above the PZT coercive field to induce ferroelectric domain switching, and (2) using partial cycle transmit pulses to increase the polarization in the PZT thin film and increase receive signal. As a result, pulse-echo images of tissue were obtained. 1-D arrays operating at 5 MHz were capable of resolving targets in a commercial tissue phantom as well as human anatomy. Real-time 3-D imaging was also demonstrated using 2-D arrays at 5 and 12.5 MHz. These results suggest that pMUTs have sufficient performance for application in ultrasound imaging with frequency range suitable for catheter-based phased-array transducers.

Reduced voltage artificial eyelid for protection of optical sensors

The fabrication, testing and performance of a new device for the protection of optical sensors will be described. The device consists of a transparent substrate, a transparent conducting electrode, insulating polymers, and a reflective top electrode layer. Using standard fabrication techniques, arrays of apertures can be created with sizes ranging from micrometers to millimeters. A stress gradient resulting from different coefficients of thermal expansion between the top polymer layer and the reflective metal electrode, rolls back the composite thin film structure from the aperture area following the chemical removal of a release layer, thus forming the open condition. The application of a voltage between the transparent conducting and reflective metal electrodes creates an electrostatic force that unrolls the curled film, closing the artificial eyelid. Fabricated devices have been completed on glass substrates with indium tin oxide electrodes. The curled films have diameters of less than 100micrometers with the arrays having fill factor transparencies of over 70%. Greater transparencies are possible with optimized designs. The electrical and optical results from the testing of the artificial eyelid will be discussed.

Electrostatic operation and curvature modeling for a MEMS flexible film actuator

In this paper we develop a mathematical model to simulate the actuation of a multilayer metallic strip. In the first step of the model development, we employ previous theory to quantify the radius of curvature in the unimorph due to differing thermal coefficients in the constituent materials. The resulting radius of curvature is subsequently used to compute the voltage required to uncurl the actuator. Numerical experiments were performed with the model and the trends were found to be in agreement with experimental data.

11F-3 Performance of Flexure-Mode pMUT 2D Arrays

We report on the performance characteristics of 2D pMUT arrays. 2D arrays suitable for diagnostic ultrasound imaging containing 81 elements were bulk micromachined in silicon substrates. The devices consisted of suspended PZT thin film membranes formed by deep reactive ion etching. Membrane widths ranged from 50 to 200 mum, with element pitch from 100 to 300 mum. Center frequency, fractional bandwidth, acoustic pressure output and pulse-echo insertion loss were measured in a de-ionized water tank. Pulse-echo images were made using the Duke University T5 Phased Array Scanner. Measured resonance frequencies for the flexure-mode pMUT elements in transmit mode were in the range of 4-13 MHz, and different vibrational modes were observed. Larger membrane widths tended to operate with standard plate-mode vibration, whereas smaller membranes were relatively thickness independent and operated in a transverse resonant mode proportional to membrane length. Acoustic pressure output was 322 Pa/V for a 75 mum pMUT single element measured at 8.6 MHz by a calibrated hydrophone at 22 mm range. This compared to 382 Pa/V for a commercial 2D bulk ceramic array with 350 mum elements operating at 3.5 MHz at the same distance. Pulse-echo insertion loss measured by reflection from an aluminum block at 22 mm range in the water tank was -106 to -124 dB per element for 75 mum elements. It was observed that the pMUTs were more efficient in transmit than receive mode, as transmit insertion loss was -38 dB per element. Receive sensitivity was increased by applying a dc bias to the pMUT elements prior to receiving acoustic reflection. Because flexure-mode pMUTs operate above the coercive voltage in transmit mode, receive bias increased the polarization of the piezoelectric film prior to transducer receive. Receive sensitivity increased by as much as 18 dB with up to 20 V dc bias. Pulse- echo B-mode images were produced with 9x9 element (1.1 mm x 1.1 mm) pMUT arrays operating at 7 MHz, producing axial resolution of 1 mm at 10 mm depth in a tissue phantom.

High speed, large motion electrostatic artificial eyelid

The fabrication, testing and performance of a new device for the protection of optical sensors are described. The device consists of a transparent substrate, a transparent conducting electrode, insulating polymers, and a reflective top electrode layer. Using standard fabrication techniques, arrays of apertures can be created with sizes ranging from micrometers to millimeters. A stress gradient resulting from different thermal coefficients of expansion between the top polymer layer and the reflective metal electrode, rolls back the composite thin film structure from the aperture area following the chemical removal of a release layer. The application of a voltage between the transparent conducting and reflective metal electrodes creates an electrostatic force that unrolls the curled film, closing the artificial eyelid. Fabricated devices have been completed on glass substrates with indium tin oxide electrodes. The curled films have diameters of less than 100 /spl mu/m with the arrays having fill factor transparencies of over 80%.

Integration of photonic bandgap composites with piezoelectric actuators for rejection wavelength tuning

Physically robust photonic bandgap (PBG) composites based on electrostatically stabilized polymeric colloidal particles are presented. The glass transition (Tg)of the composites can be varied over a large temperature range through the selection of the monomer(s) used to fabricate the composite. Composites with a subambient Tg exhibited a mechanochromic response and were integrated with a peizoelectric actuator to produce a prototype device which exhibited a fully reversible tunable rejection wavelength, capable of a ca. +/- 86 nm (172 nm full range)stop band shift.

Electrostatic flexible film actuators as IR choppers for pyroelectric detectors or microbolometers

Flexible film electrostatic MEMS actuators can be used as micromachined IR choppers for pyroelectric and microbolometer sensors. The flexible actuators act as tightly curled shutters, providing transmission of IR radiation to the sensor elements when open and reflection of the IR when closed. These actuators consist of a polymer/metal film stack which is microfabricated and released from a substrate. Thermal and mechanical stress in the film stack causes the actuator to curl when released, and the film can be uncurled by applying an electric field between the curled film and the substrate. Tightly curled actuators in the range of 50 μm to 1 mm square have been fabricated, and arrays have been produced and operated. Operating voltage is in the range of 50 - 300 V with frequencies > 5 kHz. The performance of these actuators is presented, and their applicability to IR choppers is discussed.