Hyunkyu Park

CMOS-compatible AlN piezoelectric micromachined ultrasonic transducers

Piezoelectric micromachined ultrasonic transducers for air-coupled ultrasound applications were fabricated using aluminum nitride (AlN) as the active piezoelectric layer. The AlN is deposited via a low-temperature sputtering process that is compatible with deposition on metalized CMOS wafers. An analytical model describing the electromechanical response is presented and compared with experimental measurements. The membrane deflection was measured to be 210 nm when excited at the 220 kHz resonant frequency using a 1Vpp input voltage.

Characterization for vision science applications of a bimorph deformable mirror using phase-shifting interferometry

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.

Low friction liquid bearing mems micromotor

This paper examines the performance of rotating microdevices incorporating a liquid bearing to couple a rotating element to a fixed substrate. Liquid bearing technology promises to significantly improve the durability and lifetime of micromechanical motors. Here, the fluid is confined between the rotor and stator using surface patterning of a hydrophobic layer. Magnetic actuation of 10 mm diameter silicon rotor is used to characterize the liquid bearing motor at rotation rates up to 1800 rpm. Bearings with fluid thickness from 20–200 microns are characterized. A minimum torque of 0.15 µN-m is required to overcome static friction and initiate rotation. At rotation rates above 720 rpm, the rotor wobble is less than ±1 mrad and the bearing exhibits viscous friction with a drag coefficient of 1.2 × 10−3 µN-m/rpm.

Single-Crystal PMN-PT MEMS Deformable Mirrors

This paper describes microelectromechanical systems deformable mirrors (DMs) fabricated from Pb(Mg1/3Nb2/3)O3-PbTiO3 single crystal (PMN-PT) for use in ocular adaptive optics. The DM is a piezoelectric unimorph with 35 actuators on a 13-mm circular membrane. Each actuator inside the 8-mm pupil achieves a static stroke of over 5 μm at 10 V. Dynamic measurements prove that the DM can be operated up to a 2.27-kHz bandwidth. The large stroke with low driving voltage and high operating bandwidth confirm that the DM is a promising candidate for use as a wavefront corrector in vision science applications. The measured piezoelectric properties of the PMN-PT are in close agreement with factory specifications, demonstrating that the piezoelectric properties of single-crystal PMN-PT are not degraded by the bonding and lapping process used here. The large 13-mm-diameter 30-μm-thick membrane is produced by constructing silicon rings to protect the membrane from the considerable compressive stress present in the SiO2 layer of a silicon-on-insulator wafer. In the prototype mirror, residual stress in the electrode metal results in an initial peak-to-valley surface flatness of 3.3 μm which is reduced to 0.7 μm by iterative computation of the control voltages applied to the electrodes.

An electrohydrodynamically driven microfabricated actuator for the study of miniature ion propulsion engine and electric wind devices

For the study of miniature ion propulsion engine and electric wind devices, we have developed an electrohydrodynamically (EHD) driven microfabricated actuator. It consumes a maximum power of 100 mW and has a maximum resultant driving force of 0.45 /spl mu/N in the first observed driving mode. The actuator consists of a mass/spring configuration fabricated with dual ion drives for propulsion. DC partial electrical discharge produces and accelerates the ions. Electric wind is generated by the momentum transfer from the ions to the air. Momentum is also transferred by virtue of the formation of intermittent space charge near the ionization zone. A selection between single and dual ion drives allows observation of various oscillation modes beginning at 896 Hz. The maximum out-of-plane oscillation amplitude measured was approximately 2 /spl mu/m.

Bimorph deformable mirror: an appropriate wavefront corrector for retinal imaging?

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

OPTICAL CHARACTERIZATION OF A BIMORPH DEFORMABLE MIRROR

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

Wideband Mechanical Excitation by a Microcorona-Driven Vibrating Element

We have designed, fabricated, and tested a microcorona driven (MCD) vibrating element. The vibrating element consists of a mass plate at the end of a cantilever. The proof mass is selectively driven by either one or two microcorona ionizers. During the dc negative corona discharge, the build-up of negative space charge electrostatically repelled the cathodes on the mass plate against the mechanical elastic force of the spring, damping force, and electrostatic force. This resulted in the wideband mechanical self-excitation of the MCD vibrating element. Using laser Doppler vibrometry (LDV), two resonance frequencies of out-of-plane modes were measured experimentally at peak values of 896 and 1312 Hz, and it was consistent with the ANSYS finite element modal analysis results at ~823 and 1323 Hz, respectively. The transition from Trichel pulse mode to diffuse glow mode resulted in a discontinuity in the experimental plot of proof mass velocity versus applied voltage. The MCD vibrating element consumed a maximum power of ~100 mW and had a maximum resultant driving force of ~0.45 μN in the first observed driving mode. The maximum out-of-plane oscillation amplitude measured was to be ~2 μm.

MEMS deformable mirrors for adaptive optics using single crystal PMN-PT

A MEMS-based deformable mirror constructed using single-crystal PMN-PT for use in ophthalmologic adaptive optics is presented. The fabrication process and the results of characterization of the DM are described. A large stroke and high operating bandwidth assure that the DM can be a promising wavefront corrector.

Fabrication and Characterization of MEMS Deformable Mirrors for Adaptive Optics

A bimorph deformable mirror (DM) for use in ophthalmologic adaptive optics is presented. The fabrication process and the results of characterization of the DM are described. Interferometric measurements of the DM surface shape and voltage-to-displacement characteristics are shown. The response of the DM to a step voltage input is measured using a commercial laser Doppler vibrometer (LDV). Experimental measurements of the DM are compared with both finite-element and analytical models. Analysis of the experimental measurements compared to the theoretical model will be used to design and fabricate an optimized DM for vision science.Copyright