Jonathan J. Bernstein

A micromachined comb-drive tuning fork rate gyroscope

The development of single-crystal and polysilicon tuning fork gyroscopes of very small size and low cost using microfabrication technology is reported. These tuning fork gyroscopes are extremely rugged, inherently balanced, and easy to fabricate. For a 1-mm gyroscope, projected performance is 10 to 100 degrees /hr for bias stability and for resolution in a 60-Hz bandwidth. To date, 5000 degrees /hr in a 60 Hz bandwidth has been demonstrated. The principle of operation, projected and measured performance and fabrication of silicon, polysilicon, and nickel units and associated electronics and control issues are discussed.<<ETX>>

Micromachined high frequency ferroelectric sonar transducers

Millimeter-sized ferroelectric monomorph sonar transducers have been built using sol-gel PZT on micromachined silicon wafers. First generation transducer arrays with diaphragms varying in size from 0.2 to 2 mm were tested. Second generation 8/spl times/8 arrays have also been built and tested in water in the frequency range of 0.3 to 2 MHz. Improvements to the sol-gel process have yielded high-quality, crack-free PZT films up to 12 /spl mu/m in thickness, which leads directly to higher sensitivity and figure of merit for acoustic transducers. The longitudinal piezoelectric coefficient d/sub 33/ is 140 to 240 pC/N, measured through a double beam laser interferometer. Remanent polarization of 28 /spl mu/C/cm/sup 2/, a coercive field of 30 kV/cm, and dielectric constant of 1400 were measured on 4-/spl mu/m thick films. Test results are presented, including frequency response, beam patterns, and sensitivity. High-resolution acoustic images have been generated using these transducers and a four-element underwater acoustic lens. Potential applications for these transducers include high-frequency imaging sonars, medical ultrasound, ultrasonic communication links, and flaw detection (NDT).

Low-noise MEMS vibration sensor for geophysical applications

The need exists for high-sensitivity, low-noise vibration sensors for various applications, such as geophysical data collection, tracking vehicles, intrusion detectors, and underwater pressure gradient detection. In general, these sensors differ from classical accelerometers in that they require no direct current response, but must have a very low noise floor over a required bandwidth. Theory indicates a capacitive micromachined silicon vibration sensor can have a noise floor on the order of 100 ng//spl radic/Hz over 1 kHz bandwidth, while reducing size and weight tenfold compared to existing magnetic geophones. With early prototypes, we have demonstrated Brownian-limited noise floor at 1.0 /spl mu/g/Hz, orders of magnitude more sensitive than surface micromachined devices such as the industry standard ADXL05.

Two-axis magnetically-driven MEMS scanning catheter for endoscopic high-speed optical coherence tomography.

A two-axis scanning catheter was developed for 3D endoscopic imaging with spectral domain optical coherence tomography (SD-OCT). The catheter incorporates a micro-mirror scanner implemented with microelectromechanical systems (MEMS) technology: the micro-mirror is mounted on a two-axis gimbal comprised of folded flexure hinges and is actuated by magnetic field. The scanner can run either statically in both axes or at the resonant frequency (>= 350Hz) for the fast axis. The assembled catheter has an outer diameter of 2.8 mm and a rigid part of 12 mm in length. Its scanning range is +/- 20 in optical angle in both axes with low voltages (1 approximately 3V), resulting in a scannable length of approximately 1 mm at the surface in both axes, even with the small catheter size. The catheter was incorporated with a multi-functional SD-OCT system for 3D endoscopic imaging. Both intensity and polarization-sensitive images could be acquired simultaneously at 18.5K axial scans/s. In vivo 3D images of human fingertips and oral cavity tissue are presented as a demonstration.

DIELECTRIC, FERROELECTRIC, AND PIEZOELECTRIC PROPERTIES OF LEAD ZIRCONATE TITANATE THICK FILMS ON SILICON SUBSTRATES

This article reports the fabrication of thick films of lead zirconate titanate (PZT) on platinum‐buffered silicon substrates by screen printing. Crack‐free films, up to 12 μm on a single pass, show a dielectric permittivity of 200, tangent losses of 0.05, remanent polarization of 2.5 μC/cm2, and coercive field of 40 kV/cm. The field‐induced longitudinal piezoelectric coefficient d33 at 40 kV/cm dc bias and 4 kV/cm alternating field corresponded to 50 pC/N. The magnitude of the piezoelectric voltage coefficient g33, computed from the strain coefficient and dielectric permittivity, under the same conditions, was found to be 36×10−3 V m/N, higher than that of a poled PZT bulk ceramic in comparison. These results are promising for a broad variety of sensor applications.

Ferroelectric and antiferroelectric films for microelectromechanical systems applications

Abstract In this paper we have introduced several types of ferroelectric and antiferroelectric thin (thickness 1 μm) films developed in our group for sensor, actuator and transducer applications in microelectromechanical systems (MEMS). Ferroelectric lead zirconate titanate (PZT) films of up to 12 μm in thickness have been prepared on Pt-buffered silicon substrates, which allows for the conventional, through-thickness polarization, and of up to 5 μm in thickness on insulating layer (ZrO 2 ) passivated silicon substrates, which allows for the novel, in-plane polarization. The in-plane poled films make it possible to develop d 33 -mode rather than d 31 -mode bending devices, which immediately leads to two-times improvement in device performance because d 33 ≈2 d 31 . It also can greatly increase the voltage sensitivity of bending devices because the film thickness and electrode spacing are separated as independent variables, and thus, smaller film capacitance can be obtained by using wider electrode spacing even for fixed film thickness. In addition to PZT ferroelectric films, we have also developed antiferroelectric films as an alternative for high-strain microactuators. The strain level of the antiferroelectric films can reach more than 0.4%, and both digital and analog actuation can be realized by modifying the compositions of the films. As an example for MEMS applications, micromachined, unimorph-type two-dimensional transducer arrays have been fabricated based on both the through-thickness and in-plane polarized PZT films, which can be used for miniaturized, high-resolution acoustic imaging such as hand-held divers sonar system, medical ultrasound imaging, and non-destructive testing.

Dielectric hysteresis from transverse electric fields in lead zirconate titanate thin films

Excellent symmetric dielectric hysteresis is observed from lead zirconate titanate (PZT) thin films using transverse electric fields driven by interdigitated surface electrodes. The 1-μm-thick PZT films with a Zr/Ti ratio of 52/48 are prepared on ZrO2 buffered, 4-in.-diam silicon wafers with a thermally grown SiO2 layer. Both the ZrO2 buffer layer and PZT film are deposited by using a similar sol–gel processing. Remanent polarization of about 20 μC/cm2 with coercive field less than 40 kV/cm is obtained as measured using a triangle wave at 50 Hz. Thicker films are being developed and retention for the transversely polarized state is currently under study. One of the objectives of this study is to develop a large array of d33-driven unimorph sensing elements for a high-resolution acoustic imaging system.

Electromagnetically actuated mirror arrays for use in 3-D optical switching applications

This paper presents an electromagnetic MEMS mirror technology for use in 3-D optical switching applications. These mirrors may be actuated through large angles at low voltage and low current. Multiple coils on the backs of the mirrors interact with permanent magnetic fields to provide two-axis orthogonal actuation. A custom package brings the MEMS mirror array and magnets into close proximity. Actuation is linear versus drive current on both axes, and displays negligible charging and drift. These mirrors have achieved greater than 10/spl deg/ mechanical rotation per mA in each axis. The mirror rotation angle is hysteresis free to less than the 0.01/spl deg/ measurement accuracy.

Sensing characteristics of in-plane polarized lead zirconate titanate thin films

The sensing characteristics of in-plane polarized lead zirconate titanate (PZT) thin films were studied and compared with the through-thickness polarized PZT films. The in-plane polarized PZT films were deposited on ZrO2-passivated silicon substrates and had interdigitated electrode systems on the top surface; hence, they can be polarized in the film plane. This in-plane polarization configuration separates the electrode spacing and film thickness as independent variables; thus, the voltage sensitivity can be increased by using wider electrode spacing even for fixed film thickness. The results show that for films with a thickness of 1 μm the voltage sensitivity of in-plane polarized PZT films can be more than 20 times higher than that of the conventional, through-thickness polarized PZT films which were deposited on Pt-buffered silicon substrates.

In vivo 3D human vocal fold imaging with polarization sensitive optical coherence tomography and a MEMS scanning catheter

We present in-vivo 3D human vocal fold images with polarization sensitive optical coherence tomography (PS-OCT). Characterizing the extent and location of vocal fold lesions provides useful information in guiding surgeons during phonomicrosurgery. Previous studies showed that PS-OCT imaging can distinguish vocal fold lesions from normal tissue, but these studies were limited to 2D cross-sectional imaging and were susceptible to sampling error. In-vivo 3D endoscopic imaging was performed by using a recently developed 2-axis MEMS scanning catheter and a spectral domain OCT (SD-OCT), running at 18.5 frames/s. Imaging was performed in the operating room with patients under general anesthesia and 3D images were acquired either by 2D scanning of the scanner on the sites of interest or by combining 1D scanning and manual sliding to capture whole length of the vocal fold. Vocal fold scar, polyps, nodules, papilloma and malignant lesions were imaged and characteristics of individual lesions were analyzed in terms of spatial distribution and variation of tissue structure and birefringence. The 3D large sectional PS-OCT imaging showed that the spatial extent of vocal fold lesions can be found non-invasively with good contrast from normal tissue.