1.

INTRODUCTION

Micro-Opto-Electro-Mechanical Systems (MOEMS) are based on the micro-electronics fabrication process. They are designed for a wide range of applications like sensors, switches, micro-shutters, beam deflectors, and micro-deformable mirrors. The main advantages of micro-optical components are their compactness, scalability, and specific task customization using elementary building blocks. As these systems are easily replicable, the price of the components is decreasing dramatically when their number is increasing. They will be widely integrated in next-generation astronomical instruments, especially for space missions, as they allow remote control.

MOEMS have not yet been used in space astronomy, but this technology will provide the key to small, low-cost, light, and scientifically efficient instruments, and allow impressive breakthroughs in tomorrow’s observational astronomy.

The NASA’s Origin Program and equivalent programs in Europe bring into fashion what astronomy always wanted to do, explaining where we are coming from by studying the formation of the galaxies and their evolution, as well as the formation and evolution of the planets around nearby stars.

The following gives three applications of MOEMS in observational astronomy:

2.

PROGRAMMABLE SLIT MASKS

Multi-Object Spectrographs (MOS) are the major astronomical instruments for studying primary galaxies and remote and faint objects, whose light spectra are strongly shifted to infrared wavelengths. Current object selection systems are limited and/or difficult to implement in next generation MOS for space and ground-based telescopes. Programmable multi-slit masks are a very suitable solution for this task. A promising solution is the use of MOEMS devices such as micromirror arrays (MMA) or micro-shutter arrays (MSA), which both allow the remote control of the multi-slit configuration in real time. Next generation ground-based MOS will also benefit from these developments. The typical size of these micro-elements is around 100µm, and MMAs [1,2] are designed for generating reflecting slits, while MSAs [3] generate transmissive slits. MSA has been selected to be the multi-slit device for Near Infrared Multi-Object Spectrograph (NIRSpec) for the James Webb Space Telescope (JWST), and is under development at the NASA’s Goddard Space Flight Center. They use a combination of magnetic effect for shutter opening, and electrostatic effect for shutter latching in the open position [3].

In Laboratoire d’Astrophysique de Marseille, we have developed over several years different tools for the modelling and the characterization of these MOEMS-based slit masks, during the design studies on JWST-NIRSpec. [4]. Within the framework of the JRA Smart Focal Planes (part of the European FP6 Opticon program), the LAM and the Institut de Micro-Technologies (IMT) of University of Neuchatel (Switzerland) have engaged a collaboration in order to demonstrate a European MOEMS-based slit mask. We develop and microfabricate a novel MMA suited for MOS. Based on our simulations and measurements, we have fixed several parameters: one micromirror per astronomical object, which corresponds to the baseline for NIRSpec, 20° tilted micromirror used for the “ON” position, minimizing the parasitic light, flat mirror surface, uniform tilt angle over the whole array for optimising the optical design of the instrument, a fill factor of more than 90% and a low driving voltage below 100 V. The micro mirror array has also to work at cryogenic temperatures.

We have designed a new micro-mirror array architecture. The micro-mirrors are realized on a mirror chip and actuated electrostatically by a separate electrode chip, fabricated independently and assembled subsequently. The suspension of the mirrors is made of flexion cantilevers underneath the mirror. A clamping mechanism has also been developed in order to set a very precise ON position from micro-mirror to micromirror (Fig. 1). Due to the electrostatic force the mirror rotates upwards (b) until the first landing pad, which is attached to the mirror, hits the electrode (c). Then the mirror starts rotating in the inverse direction until it hits the second landing pad, which is attached to the mirror frame (d), and remains electrostatically locked in this position. Thus the final tilting angle is mostly independent on the actuation voltage and relies merely on a homogenous spacing between the mirror and electrode chips.

Fig. 1

Schematic view of the actuation of the mirror and electrostatic locking mechanism

A first small test array of 5×5 micro-mirrors was successfully micro fabricated using a combination of bulk and surface micromachining. The 100 × 200 µm² mirrors are defined by deep reactive ion etching in the 10µm thick device layer of a silicon-on-insulator (SOI) wafer, whereas the suspension of the mirrors is defined by a patterned poly-silicon layer hidden on the backside of the mirrors (Fig. 2) [5].

Fig. 2

SEM image of a small 5×5 micro-mirror array

At LAM, a dedicated characterization bench has been developed for the complete analysis of MOEMS devices, actuators or micro-mirrors as well as full arrays. This modular Twyman-Green interferometer allows high in-plane resolution (4µm) or large field of view (40mm). Out-of-plane measurements are performed with phase-shifting interferometry with a low-coherence source, showing very high resolution (standard deviation<1nm). Features such as optical quality or electro-mechanical behavior are extracted from these high precision three-dimensional component maps. Range is increased without loosing accuracy by using two-wavelength phase-shifting interferometry authorizing large steps measurements. Dynamic analysis like vibration mode and cut-off frequency is also measured with time-averaged interferometry [6].

Using our characterization benches, preliminary characterization is done on this first test array of 100×200 µm² micro-mirrors. The surface quality of the micro-mirror is measured by phase-shifting interferometry, and a total aberration of 10 nm peak-to-valley is measured, with 1nm roughness. These mirrors can be electrostatically tilted by 20° at an actuation voltage of 90V. In many MOS observations, astronomers need to have the spectrum of the background nearby the studied object. For this purpose, the programmable slit mask must be able to be in a “long slit” mode where several adjacent mirrors parallel to the long side of the mirror must tilt by the same angle. Our locking mechanism is designed in order to ensure this goal. The locking performance has been measured on single micro-mirrors and an angle difference of only 1 arc-minute has been obtained on our first prototype.

The cryogenic compatibility is crucial for the application in an infrared (IR) MOS. The operating temperature must be below 100 K for near and mid IR and below 40 K for far IR. Our MMA is designed such that all structural elements have a matched coefficient of thermal expansion (CTE) in order to avoid deformation or even flaking within the device when cooling it down to the operating temperature. The mirrors are covered with a gold layer for IR operation, whereas gold has a different CTE than silicon. As the silicon mirror is 10 µm thick and the coating is 60 nm thin, we estimate that the induced deformation will be small.

For characterising the surface quality and the performance of our MMA’s at low temperature, we have developed a cryo chamber optically coupled to a high-resolution Twyman-Green interferometer [6]. The interferometer provides a sub-nanometer accuracy by using phase-shifting technique. The cryo-chamber allows pressure down to 10^-6 mbar and temperatures down to 60 K. A dedicated PCB interface and mechanical support were developed, which allows working in vacuum and which avoids additional stress on the MMA device, which itself is packaged in PGA chip carrier. The PGA is inserted in a ZIF-holder integrated on the PCB board (Fig. 3).

Fig. 3

Cryogenic set-up mounted on the interferometric bench.

The micromirrors could be successfully actuated before, during and after cryogenic cooling at 92K (Fig. 4) [7]. We could measure the surface quality of the gold coated micromirrors at room temperature, below 100K and being actuated: there is a slight increase of the deformation from 35 nm to 50nm PtV, due to CTE mismatch between silicon and gold layer (Fig. 4). This small deformation is still well below the requirement for MOS application at IR. This value could be decreased if needed by using double-side coated mirrors, easily feasible in our process flow.

Fig. 4

Testing of a micro-mirror array at 92K (0V and 90 V applied); 100×200 µm² mirrors surface deformation at room temperature and at 92K

Large arrays of 20’000 micromirrors per array are under development.

3.

DEFORMABLE MIRRORS

Next generation space telescopes as well as future giant telescopes rely on the availability of highly performing wavefront correction systems. These systems require deformable mirrors with very challenging parameters, including number of actuators up to 250 000 and interactuator spacing around 500µm. MOEMS-based devices permit the development of a complete generation of new deformable mirrors. We are currently developing a micro-deformable mirror (MDM) based on an array of electrostatic actuators with attachment posts to a continuous mirror on top, in collaboration with a microtechnology laboratory, LAAS (Toulouse, France).

The originality of our approach lies in the elaboration of layers made of polymer materials, in order to reach high strokes for low driving voltages. We have chosen to develop a process based on SU8 photoresist, used up to now only as micromolds for electroplating or masters for hot-embossing. This polymer exhibits a low Young modulus (E= 6GPa) compared to typical polysilicon structures (E= 158GPa). The same material is also well suited for the continuous mirror realization of a MDM, leading to a complete MOEMS device made with SU8 material.

Mirrors with very efficient planarization and active actuators have been demonstrated; the first polymer piston-motion actuator has been realized with a 10µm thick structural layer made of a 580µm square plate, four 580µm-long and 100µm-wide spring arms, and an air-gap of 10µm (Fig. 5). We measure a pure piston motion, with a maximal displacement larger than 2µm for 30 Volts. We have developed a model by Finite-Element Modeling, considering the Mindlin plate equations, and the results are in close agreement with the measurements. We use our dedicated surface characterization bench in the time-averaged interferometry configuration, and a 6.5kHz resonance frequency was measured (Fig. 6). This frequency is well suited for AO systems which require operating frequency up to 1.5-2 kHz.

Fig. 5

View of a polymer actuator with a 580µm plate by scanning electron microscope

Fig. 6

Frequency response of the polymer-based actuator. A theoretical curve of a second-order system with f_r=6.5kHz and Q=0.7 is fitted

Polymer materials are usually not suited for space applications. However, SU8 material has no record in terms of space qualification; then an extensive measurement plan is scheduled in the near future. An alternative will be the encapsulation of the device for space applications.

We have also designed a first MDM prototype and realized it at the Memscap foundry in the USA. The architecture is based on an array of nine piston actuators attached to a continuous membrane via attachment posts, with all layers in polysilicon (Fig. 7a). The piston actuator is a 200*200µm² square plate with four 100µm long-spring arm. The design has been optimized for decreasing the print-through. Spring arms are interlacing and additional platforms are put to fill the edges. A maximum deflection of 350 nm is obtained for 35 V, for the central actuator. Influence functions have been measured on this MDM prototype. The deflection obtained when applying 25 V on the different actuators successively is shown in Fig. 7b. [8].

Fig. 7

a) Micro-deformable mirror prototype with 3×3 actuators (200×200µm² for each actuator) and a continuous membrane on top. Structural material is polysilicon; b) Influence functions of LAM prototype (one actuator, middle right, is stuck on the substrate)

AO systems are based on linear matrices operations. In the case of deformable mirrors driven by electrostatic forces, the actuation is highly non linear, due to the dependence of the stroke in V², and a dependence with the inverse of the instant gap to the square during actuation. Then, in order to “linearize” the actuation of the deformable mirror, we have developed a dedicated 14-bit electronics. After a calibration procedure and by fitting the response by a polynomial, we are able to drive the actuator in terms of stroke and not any more in terms of voltage. The actual location of the actuator versus expected location of the actuator gives a nearly perfect response with a standard deviation of 3.5nm [8]. Effect of the adjacent actuators location on this linearized response (crossed non-linearities) is under study.

Based on our FEM models, a complete deformable mirror made with polymer materials is under development [8]. In a further step, driving electronics could be integrated in the Si substrate.

4.

PROGRAMMABLE MICRO-DIFFRACTION GRATINGS (PMDG)

Programmable Micro-Diffraction Gratings (PMDG) are a new type of micro-opto-electro-mechanical systems (MOEMS), opening new observational capabilities in future astronomical instrumentation. LAM is involved with Thales Alenia Space in an ESA study. Within this study, key parameters have been defined and characterization benches at LAM have been modified specifically for the PMDG device characterization. A PMDG device realized by Silicon Light Machines (SLM) in USA has been purchased (Fig. 8) and measurements have been realized.

Fig. 8

PMDG device realized by SLM.

Programmable gratings are based on a serial of parallel ribbons with alternating fixed and movable ribbons. Typical dimensions are 5µm-wide and 200µm-long ribbons. By using electrostatic force, ribbons are actuated. This structure is common in the RF-MEMS used for switching very quickly signals along lines. However, in PMDG, the number of parallel ribbons is very high (up to several thousands), they must be identical from ribbon to ribbon and their surface quality has to be at the level of optical quality.

When all ribbons are in the same plane, this device is a reflector and the light is sent back (Fig. 9). When movable ribbons are pulled down, the shape of the reflecting surface is like a square grating, diffracting the light in the different diffraction orders. If the stroke is equal to λ/4, the light is completely switched in the diffraction directions; for values ranging from 0 to λ/4, only part of the light is diffracted. A few ribbons are efficient enough to diffract the light.

Fig. 9

PMDG principle.

By using a mask in the Fourier plane, it is possible to block out the reflected light in the dark state while in the bright state, light is sent out. Two modes are then foreseen:

Most applications are based on 0^th and 1^st orders operation. Zero^th order operation means that 0^th order is transmitted and all other orders blocked, and first order operation means that 1^st order (+1 and/or −1) is transmitted and all others are blocked. Either configuration could be used according to the requested performances. The diffraction efficiency is usually better in the 0^th order while the 1^st order operation gives a better contrast easily (0^th order with optimised optical architecture could deliver also a good contrast). The number of ribbons used by PMDG “pixel” determines the separation between the 0^th order and the 1^st orders. For a complete separation and then a better contrast, usually three ribbon-pairs (periods) are needed, i.e. 6 ribbons.

A PMDG component is a good candidate for making a tuneable spectrometer. The principle is to place the component in the focal plane of a first diffracting stage (using a grating for instance) and to use the PMDG as a wavelength selector by reflecting the selected wavelengths and by switching-off the others (by diffracting them). It becomes then possible to realise a programmable and adjustable filter in λ and Δλ (Fig 10).

Fig. 10

Principle of wavelengths selection by using a PMDG. Selected wavelengths are reflected while others are diffracted and blocked by a mask in Fourier plane.

The Darwin mission will search, detect and characterize exo-planets, using high-contrast nulling interferometry, coupled with spectroscopic observation. The search of possible life forms on targeted exo-planets leads to implement a spectrometer able to detect absorption peak of H₂O, CO₂ and O₃ molecules (Fig. 11). The Darwin mission, presently under study at Thales Alenia Space, operates in the far IR range (6 µm to 20 µm) with a very small field of view. These characteristics are very favourable with the use of a PMDG device

Fig. 11

Typical spectra in DARWIN mission, exoplanet without and with life signatures H₂O, CO₂ and O₃.

By implementing a PMDG device in a spectrograph, the detector array is replaced by a single detector [9]. Our observational strategy becomes:

This new strategy shows the following advantages:

One of LAM benches dedicated to the characterization of MOEMS has been redesigned and re-arranged to a Darwin-like demonstrator breadboard. We have simulated from Fig. 11 typical exo-planet spectra and implemented them in the PMDG device, using the analog mode. Whole demonstrator throughput response including wavelength dependence is included in our model. Optical response in the spectral channel is shown in Fig. 12; it reproduces very finely the expected spectrum from Fig. 11. Exo-planet spectra without and with life signatures are shown. [9]

Fig. 12

Exo-planet spectra without and with life signatures.

The major application of PMDG technology for space missions is the realization of spectrometers, wideband sharp-edge filters or dichroïc plates with fully adjustable transmission, by means of the PMDG analog control. A new generation of instruments is foreseen that offers new functionalities which cannot be achieved by means of conventional optics:

5.

CONCLUSION

MOEMS devices are still under development, but they show great advantages for future astronomical instrumentation. Main applications are programmable slit masks for multi-object spectroscopy, micro-deformable mirrors for wavefront correction, and programmable diffraction gratings for programmable spectrographs.

We are involved since several years in the studies of different aspects of MEMS-based slit mask, as MMA and MSA modeling, original spectrograph concepts and micro-optical components characterization. We are now developing in Europe, with IMT, a new class of components based on micro-mirror arrays. The first results show that the 100 × 200 µm² micro-mirrors can achieve most of the requirements including a very good surface quality, which is necessary for high contrast spectroscopy. The device architecture and materials have been selected for future space-compatible applications. Our demonstration of operation below 100K is a key result. Large arrays of 20’000 micromirrors per array are under development.

Micro-deformable mirrors (MDM) are a new generation of deformable mirrors for highly performing wavefront correction systems. We are currently developing in collaboration with LAAS, a MDM made of polymer materials, and based on an array of electrostatic actuators attached to a continuous mirror. Efficient planarization and active actuators have been demonstrated; the first polymer piston-motion actuator exhibits a stroke of 2µm for 30V and a resonance frequency of 6.5kHz. Realization of a complete polymer-based MDM is under way. Configurations for use in space applications are possible.

Programmable diffraction gratings show new functionalities for future space instruments, including spectrometers, wideband sharp-edge filters or dichroïc plates with fully adjustable transmission. Successful implementation in Darwin-like mission has been demonstrated on a breadboard. Exo-planet spectra without and with life signatures could be clearly measured.

These MOEMS devices are not limited for integration in astronomical instrumentation in space. Applications in physics (spectroscopy, beam shaping) or biology (ophthalmology) are also foreseen.