Single-axis rotational micromirrors actuated by comb finger structures have been designed in view of their application in reflective scanning picoprojectors for laser beam displacement along two perpendicular directions to obtain a raster scan scheme. A resonant mirror operating at a frequency around 25 kHz, suitable for horizontal scans, as well as a linear mirror, suitable for vertical scan at the typical video refresh rate (60 Hz), have been fabricated by Silicon-on-Insulator technology and are illustrated in this paper. We have in particular exploited the potentialities of semiconductor laser self-mixing interferometry, a powerful technique for characterizing the dynamic response of MEMS, for detecting the electromechanical response of both kinds of micromirrors. We report the results of the spot optical measurements performed on resonant and linear mirrors aimed at detecting the frequency of the fundamental rotational mode as well as of the in-plane and out-of-plane modes, close in frequency to the fundamental mode. We have experimentally demonstrated that the fabricated devices are suitable for high-resolution miniaturized projectors, in terms of frequency response and scanning angle.

Optical Detection of the Electromechanical Response of MEMS Micromirrors Designed for Scanning Picoprojectors

Three-dimensional imaging solutions provide depth perception that cannot be achieved with traditional two-dimensional systems. A 3D optical inspection system can be implemented using structured light as 3D sensing technology. In this paper, we show that structured light can be projected onto the measured object by means of an innovative scanning microsystem that employs a single-axis torsional MEMS mirror for fast steering a near infra-red laser beam on a diffractive silicon microstructure. As a proof of principle of the functionality of the designed microsystem, the generated line patterns are shone on 3D objects and deformation of the projected lines is detected with a CMOS camera. From line deformation, the object depth is calculated and found in accordance to the geometrical size. The developed miniaturized solution overcomes typical drawbacks of other scanning technologies such as large size and heavy weight.

Infrared structured light generation by optical MEMS and application to depth perception

Semiconductor manufacturers continue to increase the component densities on computer chips by reducing the device dimensions to less than 10 nm. This trend requires faster, more precise, and more robust optical metrology tools that contain complex and high-precision optics with challenging imaging requirements. Here, we present dark-field digital holographic microscopy as a promising optical metrology technique that uses optics with acceptable complexity. A theoretical analysis and an experimental demonstration of this technique are presented, showing the impact of the coherence length of the light source on the field of view. Finally, we also present the first holographically obtained images of metrology targets.

Impact of coherence length on the field of view in dark-field holographic microscopy for semiconductor metrology: theoretical and experimental comparisons

In this work, we have applied optical low-coherence reflectometry (OLCR), implemented with infra-red light propagating in fiberoptic paths, to perform static and dynamic analyses on piezo-actuated glass micro-membranes. The actuator was fabricated by means of thin-film piezoelectric MEMS technology and was employed for modifying the micro-membrane curvature, in view of its application in micro-optic devices, such as variable focus micro-lenses. We are here showing that OLCR incorporating a near-infrared superluminescent light emitting diode as the read-out source is suitable for measuring various parameters such as the micro-membrane optical path-length, the membrane displacement as a function of the applied voltage (yielding the piezo-actuator hysteresis) as well as the resonance curve of the fundamental vibration mode. The use of an optical source with short coherence-time allows performing interferometric measurements without spurious resonance effects due to multiple parallel interfaces of highly planar slabs, furthermore selecting the plane/layer to be monitored. We demonstrate that the same compact and flexible setup can be successfully employed to perform spot optical measurements for static and dynamic characterization of piezo-MEMS in real time.

Testing of Piezo-Actuated Glass Micro-Membranes by Optical Low-Coherence Reflectometry

In this work, we present the results of the opto–electro–mechanical characterization of tunable micro-lenses, Tlens®, performed with a single-spot optical measuring system. Tested devices are composed of a transparent soft polymer layer that is deposited on a supporting glass substrate and is covered by a glass membrane with a thin-film piezoelectric actuator on top. Near-infrared optical low-coherence reflectometry is exploited for both static and low-frequency dynamic analyses in the time domain. Optical thickness of the layers and of the overall structure, actuation efficiency, and hysteretic behavior of the piezo-actuator as a function of driving voltage are obtained by processing the back-reflected signal in different ways. The use of optical sources with relatively short coherence lengths allows performing interferometric measurements without spurious resonance effects due to multiple parallel interfaces, furthermore, selecting the plane/layer to be monitored. We finally report results of direct measurements of Tlens® optical power as a function of driving voltage, performed by redirecting a He-Ne laser beam on the lens and monitoring the focused spot at various distances with a digital camera.

Characterization of Tunable Micro-Lenses with a Versatile Optical Measuring System

In this paper, we illustrate the design and testing of new silicon microstructures, fabricated by means of a conventional planar process. These "Venetian-blind" structures consist of arrays of narrow, rectangular suspended masses (width =31 /spl mu/m, length =400 /spl mu/m, thickness =15 /spl mu/m), which can be tilted using electrostatic actuation. Characterization of their static and dynamic behavior was performed with optical methods. The diffraction patterns in monochromatic light were analyzed and vibration measurements were performed by means of semiconductor laser feedback interferometry: experimental data on the tilt angle as a function of the applied voltage and on the resonance frequencies are reported. A maximum tilt angle of approximately 1.9/spl deg/ was obtained with a driving voltage in the range of 70-95 V. All the tested devices showed resonance frequencies higher than 80 kHz, which is fast enough (i.e., switching time in the millisecond range) for future use in optical interconnections. Numerical analyses were performed to evaluate the coupled electromechanical behavior of the microstructures, confirming the observed experimental behavior.

F. Carli

Papers

Testing of "Venetian-Blind" silicon microstructures with optical methods