Skip to main content
eScholarship
Open Access Publications from the University of California

Scholarly Works (12 results)

  • Thesis
  • Peer Reviewed
UC Berkeley Electronic Theses and Dissertations (2018)

A variety of physical mechanisms have been exploited to capture electronic images of a human fingerprint, including optical, capacitive, pressure, and acoustic mechanisms. Compared to other technologies, ultrasonic fingerprint sensors have two major advantages (1) they are insensitive to contamination and moisture on the finger (2) they have the ability to measure images at multiple depths hundreds of microns from the sensor surface. With the maturity of the thin film piezoelectric materials technology and MEMS-CMOS eutectic wafer-bonding process, piezoelectric micromachined ultrasonic transducers (PMUTs) arrays with electrical addressing to individual pixel have been proposed and developed for ultrasonic fingerprint sensors.

This research focuses on the modeling and characterization of the PMUT-based ultrasonic fingerprint sensors. First of all, a 51.7% fill-factor, 110 x 56 rectangular PMUTs array is demonstrated with improved pressure output, receive sensitivity, and image resolution. With a customized CMOS design, time-gated images collected at two imaging depths was demonstrated to construct overlaid patterns separated axially by 127 um. Using another 65 x 42 circular PMUTs array ultrasonic fingerprint sensor design, transmit beamforming is implemented. The measured TX pressure output with beamforming is 25 kPa and the 3 dB beam-width is 50 um, a 1.6 x increase in pressure and 6.4 x decrease in beam-width relative to non-beamformed measurements. Beamforming increases the receive voltage by a factor of 1.4 and the SNR by 7 dB. On the other hand, the mode shapes of the dense rectangular PMUTs array with shared mechanical anchors are more complicated than a single-MUT dynamic model. Finite element model was used to model the mode shapes exhibited by the entire array, while experimental measurements of the mode-shape via laser Doppler vibrometry (LDV) was used to compute the volume velocity, which correlates to the measured far field pressure. In order to have an accurate prediction of the 2D near field pressure output from dense transducer arrays, time domain acoustic simulation using k-wave MatLab toolbox and vibration displacement measurements using LDV were utilized. Effects of mechanical crosstalk, acoustic surface waves, and anchor motions were also characterized. To further improve the fingerprint sensors performance, equivalent circuit model of circular PMUTs operating at the same resonant frequency in air is formulated. PMUT displacement is proved to be inversely proportional to the device thickness. Sub-micro thick piezoelectric layer devices were designed and fabricated to demonstrate the thickness scaling benefits.

    Cover page: Piezoelectric Micromachined Ultrasonic Transducers for Ultrasonic Fingerprint Sensors
    • Article
    • Peer Reviewed
    UC Davis Previously Published Works (2007)
    The static and dynamic characteristics of a bimorph deformable mirror (DM) for use in an adaptive optics system are described. The DM is a 35-actuator device composed of two disks of lead magnesium niobate (PMN), an electrostrictive ceramic that produces a mechanical strain in response to an imposed electric field. A custom stroboscopic phase-shifting interferometer was developed to measure the deformation of the mirror in response to applied voltage. The ability of the mirror to replicate optical aberrations described by the Zernike polynomials was tested as a measure of the mirror's static performance. The natural frequencies of the DM were measured up to 20 kHz using both stroboscopic interferometry as well as a commercial laser Doppler vibrometer (LDV). Interferometric measurements of the DM surface profile were analyzed by fitting the surface with mode-shapes predicted using classical plate theory for an elastically supported disk. The measured natural frequencies were found to be in good agreement with the predictions of the theoretical model.
      Cover page: Characterization of a bimorph deformable mirror using stroboscopic phase-shifting interferometry
      • Article
      • Peer Reviewed
      UC Davis Previously Published Works (2005)
      The wave front corrector is one of the three key elements in adaptive optics, along with the wave front sensor and the control computer. Low cost, compact deformable mirrors are increasingly available. We have tested the AOptix bimorph deformable mirror, originally developed for ultra-high bandwidth laser communication systems, to determine its suitability for vision science applications, where cornea and lens introduce optical aberrations. Measurements of the dynamic response of the mirror to a step input were obtained using a commercial Laser Doppler Vibrometer (LDV). A computer-controlled Twyman-Green interferometer was constructed to allow the surface height of the deformable mirror to be measured using Phase-Shifting Interferometry as a function of various control voltages. A simple open-loop control method was used to compute the control voltages required to generate aberration mode shapes described by the Zernike polynomials. Using this method, the ability of the deformable mirror to generate each mode shape was characterized by measuring the maximum amplitude and RMS error of each Zernike mode shape up to the fifth radial order. The maximum deformation amplitude was found to diminish with the square of the radial order of the Zernike mode, with a measured deformation of 8 microns and 1.5 microns achieved at the second-order and fifth-order Zernike modes, respectively. This deformation amplitude appears to be sufficient to allow the mirror to correct for aberrations up to the fifth order in the human eye.
        Cover page: Characterization for vision science applications of a bimorph deformable mirror using phase-shifting interferometry
        • Article
        • Peer Reviewed
        UC Davis Previously Published Works (2017)
        Wake-up receivers have been proposed to provide continuous sensing in a wide variety of Internet of Things applications. To achieve impedance transformation and passive amplification between the antenna and rectifier node, current wake-up receivers employ large magnetic transformers that exhibit both low Q and inductance per unit area in planar CMOS technology. The adoption of piezoelectric MEMS resonators to replace magnetic transformers offers an area-efficient, high-Q, monolithic solution that allows large voltage amplification for ultra-low-power applications. AlN Contour Mode Resonators (CMRs) are an emerging class of piezoelectric MEMS that offers frequency reconfiguration via lithography.
          Creative Commons 'BY-SA' version 4.0 license
          • Article
          • Peer Reviewed
          UC Davis Previously Published Works (2005)
          This paper reports the results of interferometric characterization of a bimorph deformable mirror (DM) designed for use in an adaptive optics (AO) system. The natural frequencies of this DM were measured up to 20 kHz using both a custom stroboscopic phase-shifting interferometer as well as a commercial Laser Doppler Vibrometer (LDV). Interferometric measurements of the DM surface profile were analyzed by fitting the surface with mode-shapes predicted using classical plate theory for an elastically-supported disk. The measured natural frequencies were found to be in good agreement with the predictions of the theoretical model. Copyright © 2005 by ASME.
            Cover page: Optical Characterization of a Bimorph Deformable Mirror
            • Article
            • Peer Reviewed
            UC Davis Previously Published Works (2018)
            Advances in the magnetic sensing technology have been driven by the increasing demand for the capability of measuring ultrasensitive magnetic fields. Among other emerging applications, the detection of magnetic fields in the picotesla range is crucial for biomedical applications. In this work Picosense reports a millimeter-scale, low-power hybrid magnetoresistive-piezoelectric magnetometer with subnanotesla sensitivity at low frequency. Through an innovative noise-cancelation mechanism, the 1/f noise in the MR sensors is surpassed by the mechanical modulation of the external magnetic fields in the high frequency regime. A modulation efficiency of 13% was obtained enabling a final device's sensitivity of ~950 pT/Hz1/2 at 1 Hz. This hybrid device proved to be capable of measuring biomagnetic signals generated in the heart in an unshielded environment. This result paves the way for the development of a portable, contactless, low-cost and low-power magnetocardiography device.
              Cover page: Hybrid GMR Sensor Detecting 950 pT/sqrt(Hz) at 1 Hz and Room Temperature
              • Article
              • Peer Reviewed
              UC Davis Previously Published Works (2017)
              A new wake-up receiver (WuRx) designed in 0.13 μm CMOS process is hybrid-integrated with a micro-electromechanical system (MEMS) RF resonator. While only consuming 166 nW of power (18.8 nW for the comparator and 147 nW for periphery), the WuRx is capable of detecting RF signals as weak as -60 dBm. In order to achieve this sensitivity, a passive voltage-boosting network comprised of a high quality factor (high-Q) piezoelectric MEMS resonator is utilized, in place of an inductor, to amplify the received voltages applied to a CMOS rectifier. The output of the rectifier is then applied to a low-power comparator used to set a logical bit that indicates when an RF signal has been detected. The trade-offs in designing the voltage boosting network are explained and an equation for the optimum number of rectifier stages is derived.
                Creative Commons 'BY-SA' version 4.0 license
                • Article
                • Peer Reviewed
                UC Davis Previously Published Works (2005)
                The purpose of this study was to evaluate the performance of a bimorph deformable mirror from AOptix, inserted into an adaptive optics system designed for in-vivo retinal imaging at high resolution. We wanted to determine its suitability as a wavefront corrector for vision science and ophthalmological instrumentation. We presented results obtained in a closed-loop system, and compared them with previous open-loop performance measurements. Our goal was to obtain precise wavefront reconstruction with rapid convergence of the control algorithm. The quality of the reconstruction was expressed in terms of root-mean-squared wavefront residual error (RMS), and number of frames required to perform compensation. Our instrument used a Hartmann-Shack sensor for the wavefront measurements. We also determined the precision and ability of the deformable mirror to compensate the most common types of aberrations present in the human eye (defocus, cylinder, astigmatism and coma), and the quality of its correction, in terms of maximum amplitude of the corrected wavefront. In addition to wavefront correction, we had also used the closed-loop system to generate an arbitrary aberration pattern by entering the desired Hartmann-Shack centroid locations as input to the AO controller. These centroid locations were computed in Matlab for a user-defined aberration pattern, allowing us to test the ability of the DM to generate and compensate for various aberrations. We conclude that this device, in combination with another DM based on Micro-Electro Mechanical Systems (MEMS) technology, may provide better compensation of the higher-order ocular wavefront aberrations of the human eye.
                  Cover page: Bimorph deformable mirror: an appropriate wavefront corrector for retinal imaging?
                  • Article
                  • Peer Reviewed
                  UC Berkeley Previously Published Works (2009)
                  We present a free-space optical interconnect system capable of dynamic closed-loop optical alignment using a microlens scanner with a proportional integral and derivative controller. Electrostatic microlens scanners based on combdrive actuators are designed and characterized with vertical cavity surface emitting lasers (VCSELs) for adaptive optical beam tracking in the midst of mechanical vibration noise. The microlens scanners are fabricated on silicon-on-insulator wafers with a bulk micromachining process using deep reactive ion etching. We demonstrate dynamic optical beam positioning with a 700 Hz bandwidth and a maximum noise reduction of approximately 40 dB. Eye diagrams with a 1 Gb/s modulation rate are presented to demonstrate the improved optical link in the presence of mechanical noise.
                    Cover page: Robust free space board-to-board optical interconnect with closed loop MEMS tracking
                    • Article
                    • Peer Reviewed
                    UC Davis Previously Published Works (2017)
                    A monolithic piezoelectric MEMS-CMOS resonant transformer that can be used in ultra-low-power high-efficiency RF sensing applications is presented for the first time. The MEMS-CMOS resonant transformer is based on a 59 MHz 2-port Aluminum Nitride (AlN) Contour Mode Resonator (CMR) bonded to a 0.18 μm NMOS-based rectifier for voltage boosting and RF-to-DC conversion. The integrated device is fabricated in a foundry-based process by conductive eutectic wafer bonding. To amplify the voltage, the AlN CMR is designed to attain a large quality factor (Q=1150) and a relatively low dielectric capacitance (C0=1.51 pF) in relation to the number of rectifier stages (n=20). As a result, a ten-fold voltage gain MEMS-CMOS resonant transformer is demonstrated in this work.
                      Creative Commons 'BY-SA' version 4.0 license
                      Top