Matthew Hart

Stroboscopic interferometer system for dynamic MEMS characterization

We describe a computer-controlled stroboscopic phase-shifting interferometer system for measuring out-of-plane motions and deformations of MEMS structures with nanometer accuracy. To aid rapid device characterization, our system incorporates (1) an imaging interferometer that records motion at many points simultaneously without point-by-point scanning, (2) an integrated computer-control and data-acquisition unit to automate measurement, and (3) an analysis package that generates sequences of time-resolved surface-height maps from the captured data. The system can generate a detailed picture of microstructure dynamics in minutes. A pulsed laser diode serves as the stroboscopic light source permitting measurement of large-amplitude motion (tens of micrometers out-of-plane) at kilohertz frequencies. The high out-of-plane sensitivity of the method makes it particularly suitable for characterizing actuated micro-optical elements for which even nanometer-scale deformations can produce substantial performance degradation. We illustrate the capabilities of the system with a study of the dynamic behavior of a polysilicon surface-micromachined scanning mirror that was fabricated in the MCNC MUMPS foundry process.

A raster-scanning full-motion video display using polysilicon micromachined mirrors

Abstract As portable computers become more widespread, there is increasing need for lightweight, low-power, inexpensive video displays with high information content. Many established display technologies are useful for large-format displays, but do not satisfy the weight and power requirements of demanding portable display applications. Micromachined raster-scanning displays, however, look attractive for portable computing applications. We describe the operation of a raster-scanning full-motion video display constructed using surface micromachined mirrors. The 41×52 pixel display is interfaced directly to a computer video card. Display resolution is limited by dynamic deformation of the mirror surface. Mirror-scan irregularity is shown to be negligible compared to diffraction from the mirror aperture and dynamic deformation of the mirror surface. The line-scan micromirror has been operated for more than 45 billion cycles with less than 1% change in the mirror resonant frequency.

Stretched-film micromirrors for improved optical flatness

We have developed a new tensile optical-surface (TOS) process to produce optically flat micromirrors capable of scanning at high frequencies. A polysilicon membrane is stretched across a stiff, single-crystal silicon-rib structure. This structure increases the stiffness of the mirror without significantly increasing its mass. The low mass makes possible high operating frequencies without deformation that could significantly compromise the optical performance of the mirror. Electrostatic comb drives, made of thick single-crystal silicon, provide large forces that enable mirror operation at tens of kHz.

Fast surface profiling by spectral analysis of white-light interferograms with Fourier transform spectroscopy

We present a fast white-light interference method for measuring surface depth profiles at nanometer scales. Previously reported white-light profilers have relied either on path difference scanning or on spectral analysis of the reflection from a fixed interferometer. We show that by performing this spectral analysis with an imaging Fourier transform spectrometer, the high speed of spectral techniques may be combined with the simple data interpretation characteristic of the scanning method. Giving experimental results from a profiler based on this principle, we show that real-time visualization of surface profiles is possible and we report measurements with a repeatability of approximately 5 nm rms. We also demonstrate good agreement with stylus profiler measurements.

Performance of a high-stroke segmented MEMS deformable-mirror technology

This paper presents a MEMS DM that is a hybrid of surface micromachining and bulk micromachining. The combination of fabrication techniques resulted in a DM that has demonstrated 7.6 μm stroke at 125 V, 98.6% fill factor, and excellent optical quality of better than 16 nm rms after packaging. Preliminary cyclic testing over 110 hours and 107 cycles showed no noticeable changes to the actuator positions after cycling.

Stroboscopic interferometer with variable magnification to measure dynamics in an adaptive-optics micromirror

Interferometry has proven a powerful tool to measure out-of-plane movements in MEMS with high accuracy, In this paper, we demonstrate a setup for stroboscopic interferometry that combines the precise data registration obtained by combining phase-shifting techniques with the high spatial resolution and aperture that are characteristic for an optical microscope.

Segmented MEMS deformable-mirror technology for space applications

This paper presents MEMS deformable-mirror technology under development at Iris AO. The hybrid approach uses surface-micromachining techniques to fabricate actuator arrays. High-fill-factor mirror arrays are flip-chip bonded on top of these actuator arrays. The single-crystal-silicon mirror segments provide robust substrates for optical coating with excellent surface quality (6-20 nm rms surface-figure errors). The hexagonally close-packed segments are 350 μm on a side, and can thus provide high-spatial frequency corrections in a small form factor. High-stroke actuation of greater than >7.5 μm has been experimentally verified while keeping actuation voltages within reasonable bounds (<130 V). Three electrodes under each actuator allow for piston/tip/tilt motion. An open-loop controller has been demonstrated to position a 37-segment array resulting in a flattened array with only 19 nm rms of surface figure error.

Stroboscopic phase-shifting interferometry for dynamic characterization of optical MEMS

Macro-scale optical components with surface flatness better than 25 nm over large areas (more than 1 X 1 mm) are widely available. However, the flatness of optical MEMS devices (for example micro-mirrors and -diffraction gratings) is often considerably worse. In addition to static deformation caused by film stresses and stress gradients, dynamic mechanical effects, such as air drag and excitation of higher-order resonant modes, cause surface deformations that are difficult to predict using theoretical or finite- element models. These deformations can cause significant degradation to optical performance. Dynamic measurements of nanometer-scale displacements across the entire surface of a micro-mirror are difficult or impossible to perform with conventional MEMS metrology techniques such as SEM, AFM, and optical microscopy. Stroboscopic interferometry, however, can be used to measure time-slice images that show 3D motion of fast-moving MEMS devices, with vertical resolution better than 1 nm. In this paper, we report the application of this technique to dynamic characterization of fold-up surface- micromachined structures and show how the method can be used to provide new insights into the optical and mechanical behavior of scanning micro-mirror devices.

Segmented MEMS deformable-mirror for wavefront correction

Advances in microelectromechanical systems (MEMS) technology are enabling the design and fabrication of new class of deformable mirrors (DM) for adaptive optics (AO). This paper presents a MEMS DM design that is suitable for a large range of applications. More than 7.5 μm of stroke has been demonstrated for this DM with 125 V drive. Other minor design variations have shown 1.63 μm step responses of 120 μs and 140 μs rise and fall times with only a 36 V drive. The DMs have excellent optical quality of 6-20 nm rms that varies with temperature only 0.56 nm/°C peak-to-valley.

Advanced wavefront correction technology for the next generation of adaptive optics equipped ophthalmic instrumentation

Adaptive optics (AO) is becoming increasingly important in improving system resolution in flood illuminated fundus cameras, confocal laser scanning ophthalmoscopes (cSLO) and optical coherence tomography (OCT). For the latter two cases, AO also provides an increase in the throughput light levels. The flood and cSLO modalities have allowed for the routine, in-vivo visualization of individual cone photoreceptor cells and real time blood flow measurements of single leukocyte cells. Most recently, evidence of the rod mosaic has also been observed. A key component in all of these systems is the deformable mirror (DM) that provides the correction of the high order aberrations. The majority of these systems to-date have utilized large, expensive DMs originally designed for astronomy. This paper details ongoing work at Iris AO, Inc in which advanced fabrication techniques based on microelectromechanical systems (MEMS) are being leveraged. This approach yields extremely compact DMs that offer higher performance and lower cost, coupled with the ability for batch fabrication. The Iris AO design uses an array of individually addressable hexagonal segments than can each be moved in three orthogonal directions. Such a design allows for superior ocular wavefront fitting performance and very high stroke (>10 microns). Additionally, our DMs can be fabricated with diameters that are an order of magnitude smaller than conventional non-MEMS techniques.