In future space missions for Universe and Earth Observation, scientific return could be optimized using MOEMS devices. Large micromirror arrays (MMA) are used for designing new generation of instruments. In Universe Observation, multi-object spectrographs (MOS) are powerful tools for space and ground-based telescopes for the study of the formation and evolution of galaxies. This technique requires a programmable slit mask for astronomical object selection; 2D micromirror arrays are perfectly suited for this task. In Earth Observation, removing dynamically the straylight at the entrance of spectrographs could be obtained by using a Smart Slit, composed of a 1D micro-mirror array as a gating device. We are currently engaged in a European development of micro-mirror arrays, called MIRA, exhibiting remarkable performances in terms of surface quality as well as ability to work at cryogenic temperatures. MMA with 100 × 200 μm2 single-crystal silicon micromirrors were successfully designed, fabricated and tested down to 162 K. In order to fill large focal planes (mosaicing of several chips), we are currently developing large micromirror arrays to be integrated with their electronics. 1D and 2D arrays are built on wafer with Through Wafer Vias in order to allow routing of the device on wafer backside, foreseeing integration with dedicated ASICs. The yield of these devices as well as contrast enhancement have been successfully implemented.
Previously, the realization and closed loop control of a MEMS scanner integrating position sensors made with piezoresistive sensors was presented. It consisted of a silicon compliant membrane with integrated position sensors, on which a mirror and a magnet were assembled. This device was mounted on a PCB containing coils for electromagnetic actuation. In this work, the reliability of such system was evaluated through thermal and mechanical analysis. The objective of thermal analysis was to evaluate the lifetime of the MEMS scanner and is consisting of temperature cycling (-40°C to 100°C) and accelerated electrical endurance (100°C with power supplied to all electrical components). The objective of mechanical analysis was to assess the resistance of the system to mechanical stress and is consisting of mechanical shock and vibration. A high speed camera has been used to observe the behavior of the MEMS scanner. The use of shock stopper to improve the mechanical resistance has been evaluated and had demonstrated a resistance increase from 250g to 900g. The minimum shock resistance required for the system is 500g for transportation and 1000g for portative devices.
A MEMS scanner with a high level of motion freedom has been developed. It includes a 2D mechanical tilting capability of +/- 15°, a piston motion of 50μm and a focus/defocus control system of a 2mm diameter mirror. The tilt and piston motion is achieved with an electromagnetic actuation (moving magnet) and the focus control with a deformation of the reflective surface with pneumatic actuation. This required the fabrication of at least one channel on the compliant membrane and a closed cavity below the mirror surface and connected to an external pressure regulator (vacuum to several bars). The fabrication relies on 3 SOI wafers, 2 for forming the compliant membranes and the integrated channel, and 1 to form the cavity mirror. All wafers were then assembled by fusion bonding. Pneumatic actuation for focus control can be achieved from front or back side; function of packaging concept. A reflective coating can be added at the mirror surface depending of the application. The tilt and piston actuation is achieved by electromagnetic actuation for which a magnet is fixed on the moving part of the MEMS device. Finally the MEMS device is mounted on a ceramic PCB, containing the actuation micro-coils. Concept, fabrication, and testing of the devices will be presented. A case study for application in an endoscope with an integrated high power laser and a MEMS steering mechanism will be presented.
An integrated position sensor for a dual-axis electromagnetic tilting mirror is presented. This tilting mirror is composed of a silicon based mirror directly assembled on a silicon membrane supported by flexible beams. The position sensors are constituted by 4 Wheatstone bridges of piezoresistors which are fabricated by doping locally the flexible beams. A permanent magnet is attached to the membrane and the scanner is mounted above planar coils deposited on a ceramic substrate to achieve electromagnetic actuation. The performances of the piezoresistive sensors are evaluated by measuring the output signal of the piezoresistors as a function of the tilt of the mirror and the temperature. White light interferometry was performed for all measurement to measure the exact tilt angle. The minimum detectable angle with such sensors was 30µrad (around 13bits) in the range of the minimum resolution of the interferometer. The tilt reproducibility was 0.0186%, obtained by measuring the tilt after repeated actuations with a coil current of 50mA during 30 min and the stability over time was 0.05% in 1h without actuation. The maximum measured tilt angle was 6° (mechanical) limited by nonlinearity of the MEMS system.
Multi-object spectroscopy (MOS) is a powerful tool for space and ground-based telescopes for the study of the formation
and evolution of galaxies. This technique requires a programmable slit mask for astronomical object selection. We are
engaged in a European development of micromirror arrays (MMA) for generating reflective slit masks in future MOS,
MMA with 100 × 200 μm2 single-crystal silicon micromirrors were successfully designed, fabricated and tested. Arrays
are composed of 2048 micromirrors (32 x 64) with a peak-to-valley deformation less than 10 nm, a tilt angle of 24° for
an actuation voltage of 130 V. The micromirrors were actuated successfully before, during and after cryogenic cooling,
down to 162K. The micromirror surface deformation was measured at cryo and is below 30 nm peak-to-valley.
These performances demonstrate the ability of such MOEMS device to work as objects selector in future generation of
MOS instruments both in ground-based and space telescopes. In order to fill large focal planes (mosaicing of several
chips), we are currently developing large micromirror arrays integrated with their electronics.
This paper reports the fabrication of a 20×20 micro mirror array (MMA) designed for high optical power application (5- 8kW/m2). Each pixel can attain a 2D mechanical tilt angle of +/- 4° in any arbitrary axis with an applied voltage of 150V. A novel packaging architecture is proposed to increase the ratio of mirror surface to packaging surface based on fully vertically integration process of the actuation (vertical electrodes), electrical interconnections (TSV) and signal processing (electronic). All components have a pitch smaller than the mirror surface. A detailed assessment of the fabrication process - including 3D wafer level assembly, through silicon via (TSV), electronic integration, and characterization methodology is presented with experimental results.
We report on the advances towards the design and fabrication of a system consisting of two 10mm mirrors, one actuated magnetically and the other electrostatically. The system will be used for beam steering. The maximum resonant frequencies and deflection angle of each of the actuators will be reviewed and compared.
We show the first results of a linear 100-micromirror array capable of modulating the phase and amplitude of the spectral
components of femtosecond lasers. Using MEMS-based reflective systems has the advantage of utilizing coatings tailored
to the laser wavelength range. The innovative features of our device include a novel rotational, vertical comb-drive actuator
and an X-shaped, laterally reinforced spring that prevents lateral snap-in while providing flexibility in the two degrees of
freedom of each mirror, namely piston and tilt. The packaging utilizes high-density fine-pitch wire-bonding for on-chip
and chip-to-PCB connectivity. For the first deployment, UV-shaped pulses will be produced to coherently control the
dynamics of biomolecules.