Programmable Micro Diffraction Gratings (PMDG) are one dimensional arrays of individual mirrors, the height of which can be adjusted to produce a desired spectrum at a given angle. They can function both as spatial light modulators and as reconfigurable generators of high-resolution spectra. Fig. 1a shows the way the PMDG is actuated, while Fig. 1b presents an example of applications for PMDGs, the generation of synthetic spectra [12]. MEMS PMDGs have been used in microspectrometers, compact projection displays, optical communication systems and miniaturized external cavity lasers [1-4] because of their optical properties and compactness.
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, called MIRA. The 100 x 200 μm2 micromirrors are electrostatically tilted providing a precise angle. The main requirements are cryogenic environment capabilities, precise and uniform tilt angle over the whole device, uniformity of the mirror voltage-tilt hysteresis and a low mirror deformation.
A first MMA with single-crystal silicon micromirrors was successfully designed, fabricated and tested. A new generation of micromirror arrays composed of 2048 micromirrors (32 x 64) and modelled for individual addressing were fabricated using fusion and eutectic wafer-level bonding. These micromirrors without coating show a peak-to-valley deformation less than 10 nm, a tilt angle of 24° for an actuation voltage of 130 V. Individual addressing capability of each mirror has been demonstrated using a line-column algorithm based on an optimized voltage-tilt hysteresis. Devices are currently packaged, wire-bonded and integrated to a dedicated electronics to demonstrate the individual actuation of all micromirrors on an array. An operational test of this large array with gold coated mirrors has been done at cryogenic temperature (162 K): the micromirrors were actuated successfully before, during and after the cryogenic experiment. The micromirror surface deformation was measured at cryo and is below 30 nm peak-to-valley.
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,
called MIRA.
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.
Programmable MEMS diffraction gratings are used for spectroscopic applications because of their potential in tailoring visible and infrared spectra. A fully programmable MEMS diffraction grating (FPMDG), where every micro-mirror can move independently in a range 0 - λ/2, where λ is the wavelength of light, leads to a better control of the intensity for each wavelength in the synthetized spectrum – the intensity can take any value from 0 (micro-mirror λ/4-condition) to the maximum (no micro-mirror displacement). The FPMDG chip contains 64 micro-mirrors which are actuated electrostatically. Rigid Si micro-mirrors are connected to side electrodes via linkage arms, permitting the micro-mirror to follow a pure vertical displacement, reducing the micro-mirror bending throughout actuation. Microfabrication is based on a 4 mask photolithography process, using SOI and Pyrex wafers. Each micro-mirror of the FPMDG chip can move by 1.25μm at voltages below 100 V. Two families of micro-mirrors, 50μm or 80μm wide, show negligible cross-talk during actuation. The micro-mirror bowing is as small as 0.14 μm over 700 μm and remains unchanged throughout actuation. Extinction ratios of up to 100 have been achieved by actuating only 3 adjacent micro-mirrors. The measurements have shown high stability and good reproducibility over time. Finally, FPMDGs are used to demonstrate shaping of the input spectrum: the intensity in a particular wavelength region is controlled through independent actuation of a set of adjacent micro-mirrors. The result is attenuation or cancellation of the corresponding wavelengths.
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, called MIRA. The 100 x 200 μm2 micromirrors are electrostatically tilted providing a precise angle. The main
requirements are cryogenic environment capabilities, precise and uniform tilt angle over the whole device, uniformity of
the mirror voltage-tilt hysteresis and a low mirror deformation.
A first MMA with single-crystal silicon micromirrors was successfully designed, fabricated and tested. A new generation
of micromirror arrays composed of 2048 micromirrors (32 x 64) and modelled for individual addressing were fabricated
using fusion and eutectic wafer-level bonding. These micromirrors without coating show a peak-to-valley deformation
less than 10 nm, a tilt angle of 24° for an actuation voltage of 130 V. Individual addressing capability of each mirror has
been demonstrated using a line-column algorithm based on an optimized voltage-tilt hysteresis. Devices are currently
packaged, wire-bonded and integrated to a dedicated electronics to demonstrate the individual actuation of all
micromirrors on an array. An operational test of this large array with gold coated mirrors has been done at cryogenic
temperature (162 K): the micromirrors were actuated successfully before, during and after the cryogenic experiment. The
micromirror surface deformation was measured at cryo and is below 30 nm peak-to-valley.
Multi-object spectroscopy (MOS) allows measuring infrared spectra of faint astronomical objects that provides
information on the evolution of the Universe. MOS requires a slit mask for object selection at the focal plane of the
telescope. We are developing MEMS-based programmable reflective slit masks composed of 2048 individually
addressable micromirrors. Each micromirror measures 100 × 200 μm2 and is electrostatically tilted by a precise angle of
at least 20°. The main requirements for these arrays are precise and uniform tilt angle over the whole device, uniformity
of the mirror electromechanical behavior, a flat mirror deformation and individual addressing capability of each mirror.
This capability of our array is achieved using a line-column algorithm based on an optimized tilt angle/voltage hysteresis
of the electrostatic actuator.
Micromirror arrays composed of 2048 micromirrors (32 × 64) and modeled for individual addressing were fabricated
using fusion and eutectic wafer-level bonding. These micromirrors without coating demonstrated a peak-to-valley
deformation less than 8 nm and a tilt angle of 24° for an actuation voltage of 130 V. A first experiment of the linecolumn
algorithm was demonstrated by actuating individually 2 × 2 micromirrors.
In order, to avoid spoiling of the optical source by the thermal emission of the instrument, the micromirror array has to
work in a cryogenic environment. Therefore, these devices were characterized in a cryogenic environment at -111°C and
several lines of micromirrors were tilted successfully under these conditions.
Multi-object spectroscopy is a powerful tool for space and ground-based telescopes for the study of the formation of
galaxies. This technique requires a programmable slit mask for astronomical object selection. We are developing
MEMS-based programmable reflective slit masks for multi-object spectroscopy that consist of micromirror arrays on
which each micromirror of size 100 x 200 μm2 is electrostatically tilted providing a precise angle. The main requirements
for these arrays are cryogenic environment capabilities, precise and uniform tilt angle over the whole device, uniformity
of the mirror voltage-tilt hysteresis and a low mirror deformation. A first generation of MEMS-based programmable
reflective slit masks composed of 5 x 5 micromirrors was tested in cryogenic conditions at 92 K. Then, first prototypes of
large arrays were microfabricated and characterized, but the reliability of these arrays had to be improved. To increase
the reliability of these devices, a third generation of micromirror arrays composed of 64 x 32 micromirrors is under
development. This generation was especially designed for individual actuation of each mirror, applying a line-column
algorithm based on the voltage-tilt hysteresis of the actuator. The fabrication process was optimized and is now based on
multiple wafer level bonding steps. Microfabricated devices have micromirror with a peak-to-valley deformation less
than 3 nm. The mirrors can be tilted at 20° by an actuation voltage lower than 100 V. First experiments showed that our
micromirrors are well suited for the line-column addressing of each micromirror.
We are developing MEMS-based programmable reflective slit masks for future generation infrared multi-object
spectroscopy (MOS) for space and ground-based telescopes. These devices are composed of monocrystalline silicon
micromirrors of size 200 × 100 um2 which can be tilted by electrostatic actuation yielding a tilt-angle of 20°. An
electromechanical clamping mechanism has been demonstrated providing uniform tilt-angle within one arc minute
precision over the whole array (5 × 5 micromirrors). Slit masks of different sizes have been produced; the largest one
measures 25 × 22 mm2 and is composed of 20'000 micromirrors. Thanks to the architecture and the fabrication process
of these slit masks; the micromirror peak-to-valley deformation (PTV) is uniform over the device and was measured
being below 10 nm for uncoated micromirror. A slit mask of size 5 × 5 micromirrors was successfully tested in cryogenic
conditions at 92 K; the micromirrors were actuated before, during and after the cryogenic experiment. To achieve for the
large arrays a better fabrication yield and a higher reliability, the architecture, the process flow, the assembly and the
electronics are being optimized. Optical characterizations as well as experiments of the large devices are underway.
Multi-object spectroscopy (MOS) is a powerful tool for space and ground-based telescopes for studying the formation of
galaxies. This technique requires a programmable slit mask for astronomical object selection. A first sample of MEMS-based
programmable reflective slit masks with elements of size 200×100 μm2 has been successfully tested in cryogenic
conditions at 92 K. Devices of larger size were microfabricated, the largest chip measures 25×22 mm2 and is composed
of 200×100 electrostatic actuated micromirrors. These devices are composed of two chips: the electrode chip and the
mirror chip, which are processed separately and assembled consecutively. The mirror chip is bonded on top of the
electrode chip and microfabricated pillars on the electrode chip provide the necessary spacing between the two parts. A
process flow utilizing refilling techniques based on borophosphosilicate glass (BPSG) deposition and reflow was
developed. Programmable reflective slit masks based on this fabrication process were microfabricated and characterized.
These devices exhibit a micromirror deformation of 11 nm peak-to-valley and an actuation voltage of 145 V for a tilt
angle of 9°. Preparation of samples for MOS experiments are underway.
Next-generation infra-red astronomical instrumentation for space and ground-based telescopes requires MOEMS-based
programmable slit masks for multi-object spectroscopy (MOS) which has to work in cryogenic environment. A first
prototype of micromirror arrays (MMA) of 5×5 single-crystal silicon micromirrors was successfully designed,
fabricated and tested. 100×200 μm2 micromirrors can be tilted by electrostatic actuation yielding 20° mechanical tiltangle.
The MMA were successfully actuated before, during and after cryogenic cooling, below 100 K. A MMA is
composed of two different chips fabricated on silicon on insulator (SOI) wafers: the mirror chip and the electrode chip.
The array was obtained by assembling these two chips. For the assembly step of large array (100×200 micromirrors) we
needed high precision alignment as well as the suppression of manual handling. Therefore we developed a technique of
assembly for such devices and we designed and fabricated a dedicated XYZ tip/tilt stage. This stage allows aligning the
electrodes towards the micromirrors with a micrometer precision. Large MMA of 100×200 micromirrors, measuring
22 mm×25 mm, for large field of view were microfabricated and assembled using the above setup. No additional
deformations were observed due to the assembly step. The peak to valley (PTV) deformation of the micromirrors was
found to be 14 nm PTV. The first actuation tests were carried out.
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