Mark N. Horenstein

Microelectromechanical deformable mirrors

A new class of silicon-based deformable mirrors is described. These devices are capable of correcting time-varying aberrations in imaging or beam forming applications. Each mirror is composed of a flexible silicon membrane supported by an underlying array of electrostatic parallel plate actuators. All structural and electronic elements were fabricated through conventional surface micromachining using polycrystalline silicon thin films. A layout and fabrication design strategy for reducing nonplanar topography in multilayer micromachining was developed and used to achieve nearly flat membrane surfaces. Several deformable mirrors were characterized for their electromechanical performance. Real-time correction of optical aberrations was demonstrated using a single mirror segment connected to a closed-loop feedback control system. Undesirable mirror contours caused by residual stress gradients in the membrane were observed.

Continuous-membrane surface-micromachined silicon deformable mirror

The authors describe the development of a new type of micromachined device designed for use in correcting optical aberrations. A nine-element continuous deformable mirror was fabricated using surface micromachining. The electromechanical behavior of the deformable mirror was measured. A finite-difference model for predicting the mirror deflections was developed. In addition, novel fabrication techniques were developed to permit the production of nearly planar mirror surfaces.

Computation of Corona Space Charge, Electric Field, and V-I Characteristic Using Equipotential Charge Shells

A charge simulation technique incorporating discretized equipotential charge shells in the volume is used to approximate the electric field and space charge around a single conductor in corona and to compute the voltage-current relationship for the discharge. No iteration is required in the solution method. Results are compared to corona in coaxial geometry, for which analytical treatment is also possible, and to experimental V-I measurements in line-to- plane geometry.

Adaptive optic correction using microelectromechanical deformable mirrors

A micromachined deformable mirror (?-DM) for optical wavefront correction is described. Design and manufacturing approaches for ?-DMs are detailed. The ?-DM employs a flexible silicon membrane supported by mechanical attachments to an array of electrostatic parallel plate actuators. Devices are fabricated through surface micromachining using polycrystalline silicon thin films. ?-DM membranes measuring 2 mmx2 mmx2 ?m, supported by 100 actuators are described. Figures of merit include stroke of 2 ?m, resolution of 10 nm, and frequency bandwidth dc to 7 kHz in air. The devices are compact, inexpensive to fabricate, exhibit no hysteresis, and use only a small fraction of the power required for conventional DMs. Performance of an adaptive optics system using a ?-DM is characterized in a closed-loop control experiment. Significant reduction in quasistatic wavefront phase error is achieved. Advantages and limitations of ?-DMs are described in relation to conventional adaptive optics systems and to emerging applications of adaptive optics such as high-resolution correction, small-aperture systems, and optical communication.

Differential capacitive position sensor for planar MEMS structures with vertical motion

A capacitive sensor has been developed for measuring the vertical deflection of bridge-type micro-electromechanical (MEMS) silicon actuators. The sensor requires no electrodes above the actuator surface and does not require the actuator diaphragm to be used as a signal electrode. Sets of interdigitated electrodes, one for ac signal injection and the other for signal sensing, are placed beneath the actuator membrane. As the actuator deflects, the capacitance between the interdigitated finger electrodes is altered, leading to a change in the time-varying charge induced on the sense fingers. This change in induced charge is monitored by a current-to-voltage converter, thereby providing a measure of actuator displacement in the direction perpendicular to the silicon substrate. Signal voltages on the order of 10 mV per 1 μm of deflection are observed for deflections in the 1-μm range.

Characterization of Electrodynamic Screen Performance for Dust Removal from Solar Panels and Solar Hydrogen Generators

The direct solar energy conversion in gigawatt scales by photovoltaic, photothermal, and photoelectrochemical processes is of national and global importance in meeting energy needs. Dust depositions on solar panels and solar concentrators cause efficiency loss from 10% to 30% depending upon the surface mass concentration of dust requiring manual cleaning with water. Such a cleaning process is expensive for large-scale installations where water is scarce. Transparent electrodynamic screens, consisting of rows of transparent parallel electrodes embedded within a transparent dielectric film, can be used for dust removal for their application as self-cleaning solar collectors. When the electrodes are activated by phased voltage, the dust particles on the surface of the film become electrostatically charged and are removed by the traveling wave generated by applied electric field. Over 90% of deposited dust is removed within 2 min, using a very small fraction of the energy produced by the panels. No water or mechanical movement is involved. An analysis of the electrodynamic removal mechanisms based on electrostatic and dielectrophoretic forces opposed by the adhesion forces due to van der Waals and image forces is presented.

Development of microelectromechanical deformable mirrors for phase modulation of light

A silicon-based, surface micromachined, deformable mirror device for optical applications requiring phase modulation, including adaptive optics and pattern recognition systems is described. The mirror will be supported on a massively parallel system of electrostatically con- trolled, interconnected microactuators that can be coordinated to achieve precise actuation and control at a macroscopic level. Several genera- tions of individual actuators as well as parallel arrays of actuators with segmented/continuous mirrors have been designed, fabricated, and tested. Deflection characteristics and pull-in behavior of the actuators have been closely studied. Devices have been characterized with regard to yield, repeatability, and frequency response. An electromechanical model of the system has been simulated numerically using the shooting method, and good correlation with experimental results has been ob- tained. A twenty-channel parallel control scheme has been developed

Ion-beam machining of millimeter scale optics

An ion-beam microcontouring process is developed and implemented for figuring millimeter scale optics. Ion figuring is a noncontact machining technique in which a beam of high-energy ions is directed toward a target substrate to remove material in a predetermined and controlled fashion. Owing to this noncontact mode of material removal, problems associated with tool wear and edge effects, which are common in conventional machining processes, are avoided. Ion-beam figuring is presented as an alternative for the final figuring of small (<1-mm) optical components. The depth of the material removed by an ion beam is a convolution between the ion-beam shape and an ion-beam dwell function, defined over a two-dimensional area of interest. Therefore determination of the beam dwell function from a desired material removal map and a known steady beam shape is a deconvolution process. A wavelet-based algorithm has been developed to model the deconvolution process in which the desired removal contours and ion-beam shapes are synthesized numerically as wavelet expansions. We then mathematically combined these expansions to compute the dwell function or the tool path for controlling the figuring process. Various models have been developed to test the stability of the algorithm and to understand the critical parameters of the figuring process. The figuring system primarily consists of a duo-plasmatron ion source that ionizes argon to generate a focused (approximately 200-microm FWHM) ion beam. This beam is rastered over the removal surface with a perpendicular set of electrostatic plates controlled by a computer guidance system. Experimental confirmation of ion figuring is demonstrated by machining a one-dimensional sinusoidal depth profile in a prepolished silicon substrate. This profile was figured to within a rms error of 25 nm in one iteration.

Electrostatic effects in micromachined actuators for adaptive optics

Abstract The electrostatic properties of double-cantilevered micro-electromechanical silicon actuators are examined for possible use in large-scale, deformable mirror arrays. Devices display typical nonlinear deflection versus voltage characteristics and electromechanical instability at elevated voltages. When the polysilicon membrane is suspended over a substrate with an insulating layer, the effects of electrostatic charge migration and contact charging alter actuator performance.

Characterization of electrodynamic screen performance for dust removal from solar panels and solar hydrogen generators

The direct solar energy conversion in gigawatt scales by photovoltaic, photothermal, and photoelectrochemical processes is of national and global importance in meeting energy needs. Dust depositions on solar panels and solar concentrators cause efficiency loss from 10% to 30% depending upon the surface mass concentration of dust requiring manual cleaning with water. Such a cleaning process is expensive for large-scale installations where water is scarce. Transparent electrodynamic screens, consisting of rows of transparent parallel electrodes embedded within a transparent dielectric film, can be used for dust removal for their application as self-cleaning solar collectors. When the electrodes are activated by phased voltage, the dust particles on the surface of the film become electrostatically charged and are removed by the traveling wave generated by applied electric field. Over 90% of deposited dust is removed within 2 min, using a very small fraction of the energy produced by the panels. No water or mechanical movement is involved. An analysis of the electrodynamic removal mechanisms based on electrostatic and dielectrophoretic forces opposed by the adhesion forces due to van der Waals and image forces is presented.