Devices, Structures, and Processes for Optical MEMS


High-precision microlenses have been fabricated utilizing hydrophobic effects and polymer-jet printing technology. The lenses are formed precisely at desired locations on a wafer using a polymer-jet system in which hydrophobic effects define the lens diameter and surface tension creates a high-quality optical surface. They have 200–1000-μm diameters and 343–7862-μm focal lengths. At 635 nm, wavefront aberrations (measured using a Shack-Hartmann sensor of λ/100 accuracy) are λ/5-λ/80, and the variation in focal length is less than 4.22%. Using WYKO NT3300, the maximum deviation of the surface profile from an ideal circle was measured to be approximately 0.15 μm for most cases (range ~ 0.05–0.23 μm). The average p-v optical-path-length-difference values were 0.14, 0.25, 0.33, and 0.46 μm for 200 μm-, 400 μm-, 600 μm-, and 1mm-diameter microlenses, respectively.

Using micromachining techniques, polarization beam splitters (PBS), important optical components to separate the orthogonal TE and TM components of light, have been batch-processed and characterized. The devices were fabricated from thin-film, low-stress silicon nitride membranes and showed excellent performance. Measurements using NANO Deep-UV System showed that the final thickness of the membranes varied from 418.8 to 419.5 nm. Using a WYKO NT3300, the radius-of-curvature of a typical nitride membrane was measured to be 51 m: the membrane is virtually flat. Very good performance has been demonstrated by the new MEMS-PBS structures at 635 nm: extinction ratios (for reflected and transmitted light) of (21dB, 10dB), (21dB, 14dB), and (21dB, 16dB) for single-, double-, and triple-layer systems, respectively with corresponding insertion losses of 3, 10, and 13%.

A new, straightforward, CMOS-compatible, three-mask process is used to fabricate high-performance torsional microscanners driven by self-aligned, vertically offset comb drives. Both the moving and fixed combs are defined using the same photolithography mask and fabricated in the same device layer, a process allowing the minimum gap between comb fingers to be as small as twice the alignment accuracy of the photolithography process. The fabricated microscanners have torsional resonant frequencies between 58 Hz and 24 kHz and maximum optical-scanning angles between 8 and 48° with actuation voltages ranging from 14.1 to 67.2 Vac-rms. The yields on two separate fabrication runs have been better than 70%. To demonstrate an application for these scanners, laser-ablation patterns suitable for ocular cornea surgery have been generated. First, a two-dimensional scanning system has been assembled by orienting, two identical microscanners at right angles to one another. Next, when driven by two 90o out-of-phase 6.01-kHz sine waves, the cross-coupled scanners produce circular patterns having radii fixed by the amplitude of the driving voltage. Then, a small pattern from the surface topography found on a US Roosevelt dime has been chosen and emulated to generate a cumulative ablation pattern that compares favorably with similar emulations reported by earlier researchers who used larger, more-complicated ablation systems.

Various wavefront sensing techniques to evaluate their suitability for precisely characterizing high-order wavefront aberrations of large magnitude have been examined. An appropriate wavefront sensor must be able to characterize corneal scarring, tear film effects, LASIK flap wrinkles, and keratoconics and post-LASIK corneas. The following sensors are discussed: phase-shifting interferometry (including sub-Nyquist interferometry and two-wavelength interferometry), curvature sensing, phase-diversity method, lateral shearing interferometry, star test, Ronchi test, and knife-edge test. Their fundamental operating principles and theoretical limitations as well as their past and current wavefront-sensing abilities are presented and discussed.

An addressable array (5-by-5) of high-quality microlenses suitable for application in a Shack-Hartmann (SH) sensor in a micro-optical system has been demonstrated. Specific lenses in the array can be addressed using a new selection scheme (that we have designed, built, and tested) in which the mechanical resonant frequencies of individual lens-support carriages are varied. Thus, by changing the frequency of the drive voltage, only two electrical connections per row are required in the lens system to identify the selected lens by its resonating focal image. Using this lens-identification method will allow us to improve the dynamic range of SH sensors by a factor of 12-46 above values reported for conventional SH designs.

A MEMS-based, phase-shifting interferometer (MBPSI) that is much faster than conventional phase-shifting interferometers (PSI) has been demonstrated. The conventional interferometers use piezoelectric actuators to obtain phase-shifted signals. In contrast, the MEMS-based system takes measurements using a comb-driven vertically resonating micromachined mirror that is illuminated by synchronized laser pulses (1 μsec duration at λ = 660 nm). The MBPSI employs a four-frame phase-shifting technique (four CMOS-imager frames for each profile measurement) at a rate of 23 profile measurements-per-second (43.5 msec per measurement). At this rate, the MBPSI can capture more than 700 PSI measurements of a time-varying phenomenon in a 30.5-second interval which compares to 1 measurement in 1 second in conventional systems. The MBPSI in Twyman-Green configuration has accurately tracked the fast-changing, transient motion of a PZT actuator, with a precision of ± λ/220 (± 3 nm). Measurements to check the repeatability of the system, performed in a 20-minute period, show that it is accurate to within ± 10 nm.

Jeffrey Bokor
James R. Graham
Publication date: 
May 31, 2007
Publication type: 
Ph.D. Dissertation
Choo, H. (2007). Devices, Structures, and Processes for Optical MEMS. United States: University of California, Berkeley.

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