3.1 -

The MEMS technologies

Most of the MEMS technologies rely on a few basic processes:

The first two technologies are classified as “volume micromachining technologies” because they enables to fabricate rather thick (20 to 1000 μm) microstructures, and the last technologies are mainly used for “surface micromachining” since they can be used to machine thin layers of material (say less than 20 μm). For surface micromachined MEMS, all the fabrication technologies of microelectronics were available, however sacrificial layer technologies have been added to fabricate moving objects, and also new specific materials which are not used is microelectronics are needed to perform mechanical actions (piezoelectric or magnetic materials for instance). The basic materials and structures are adapted to the fabrication of specific actuators or sensors.

Actuation of microstructures require a driving force which is easily controlled by an electrical input. Since they only require a metal layer deposition, the easiest to manufacture use:

Electrically controlled forces can be generated through other physical effects such as piezoelectric effect, magnetic effect. However, these means are more difficult to implement because they require thin film deposition of specific materials (piezoelectric layers or magnetic layers) which are more difficult to obtain than simple metal layers.

Most of microactuators are based on the following actuation methods:

Many other methods (Shape Memory Alloys, phase changes of a liquid, friction control of a moving object …) have also been experimented.

3.2 -

Specific additional technologies for MOEMS

Manufacturing of smooth optical surfaces: although the top surface of a silicon or glass wafer is of good optical quality, after micromachining, the surface smoothness of cavities or sidewalls is often perturbed, and the resulting roughness is too large for optical applications. This is a real manufacturing challenge: it happens that almost all optical quality surfaces that have been demonstrated up to now rely on the initial optical polishing of the wafer surface.

Optical thin film deposition: in order to integrate optical quality mirrors, it is often necessary to deposit optical multilayer coatings that act as mirrors or anti-reflection layers. Depending on the wavelength, various optical materials are known for these multilayers. However, it is necessary to choose optical materials that are fully compatible with the micromachining process, which is not always possible. Moreover, the optical multilayer coatings often produce stress in the microstructure, which can deform considerably the optical surface and lead to a beam distortion.

Integrated optics: planar waveguides and optical fiber technologies are expected to provide a powerful mean to design high performance optical systems. It is thus very useful to implement a waveguide technology which can be monolithically integrated with the micromechanical parts of the system.

Optical sources and detectors: the goal of MOEMS is to integrate on the same wafer a complete optical system, it is thus interesting to be able to bond or to manufacture directly some active devices, such as photodiodes or laser diodes.

4.1 -

Beam scanning and switching

One of the most basic action on light beams consists in modifying the direction (or more generally the amplitude distribution) of the beam. The MOEMS that are probably among the first which were demonstrated are scanning micromirrors (1). For instance, thin metal or silicon layers can be patterned to produce small mirrors suspended by torsion beams, and actuated by a electrostatic forces. The applications of such devices are very wide: the simplest is the usage of these micromirrors as scanning elements for bar-code readers, or laser-based vision systems. Using the same technologies, it is easy to manufacture arrays of micromirrors which have been used to demonstrate video displays (2,3). More recently, an optical switching matrix for optical fiber communications has been demonstrated with a matrix of micromirrors (4).

Guided optics micromechanical switches have also been demonstrated. Some of these switches are based on the movement of optical fibers actuated by microelements (5,6): the input fiber is moved in order to couple light into one or another receiving fiber. Another principle is based on the movement of an integrated optical waveguide located on a moving cantilever beam, which can launch light into one or another receiving waveguide (7). A third principle consists in inserting a mirror oriented at 45 degrees, at the crosspoint of two waveguides or optical fibers, in order to switch the light at 90 degrees from its initial direction (8,9).

4.2 -

Amplitude modulation

In this case, we consider the amplitude modulation of a beam without intentional modification of the beam shape. Several light amplitude modulators have been demonstrated. The simplest way is based on the principle of the mechanical chopper: in that case suspended mechanical microstructures which are actuated perpendicularly to the beam are used as variable shutters (10). Another principle, which can be used for coherent beams, is based on Fabry-Perot interferometers (11): in this case the interferometer is composed of a fixed mirror, and a thin membrane which is parallel and very close to the fixed mirror. The membrane moves toward the fixed mirror under electrostatic actuation, which changes by λ/4 the optical path inside the micro-interferometer, in order to switch the device from a reflective to a transparent state. A light modulator based on this principle has been designed for optical fiber communication links. This modulator is placed at the receiving end of the fiber, and acts as a variable reflector: when driven by an electrical signal, it modulates the reflection of the incoming beam, and thus produces a return signal without light emitting device at the receiving port (12).

4.3 -

Beam shaping

An important application of MOEMS is that they open the possibility to manufacture cheap adaptive optics systems. These systems are based on 2D arrays of micromirrors which can move independently in a direction perpendicular to the substrate, and thus correct locally phase imperfections on the beam wavefront (13). Such active mirrors are already used in astronomy, however their applications could be much wider if they can be manufactured at low cost.

4.4 -

Spectral filtering

This is again a very important application of MOEMS. Several tunable microfilters based on the principle of Fabry-Perot micro-interferometers have been demonstrated (14), and also a few grating based microspectrometers have been manufactured (15). A very promising device is the photodiode equipped with an integrated tunable Fabry-Perot filter. Such devices have been demonstrated with III-V compounds micromachining (16,17), and act as a very small microspectrometer for chemical sensing.

4.5 -

Laser wavelength control

Micromechanical devices can often be used to control the wavelength of lasers. An external optical system including a mirror, a Fabry-Perot cavity or a diffraction grating with electromechanical actuation can be associated to a laser diode for such a purpose. This kind of device could lead to high performance lasers sources with a wide tuning range (18,19,20), or a better control of the pulse rate (21).

4.6 -

Optical alignment and packaging

One of the most fascinating capability of MOEMS for field applications is the numerous possibilities that they offer to simplify the alignment and packaging of optical systems. To illustrate this concept, let us consider the old idea of manufacturing silicon V-grooves for the positioning of optical fibers: these V-grooves can be manufactured within a sub-micron tolerance and they can be employed to align precisely a single mode fiber with another optical system, without additional adjustments. More generally, modern microfabrication methods have the unique capability to enable micron order alignments at the fabrication level, without additional adjustments. This feature is precious because it enables the fabrication of complete optical systems in a very compact way, on a single substrate, and one can get rid of the numerous adjustments which are generally a cause of drift and instability in most of the optical systems. Several microdevices were designed to take advantage of microfabrication capabilities in order to improve optical alignments (22,23). In addition, several authors have shown the concept of micro-optical benches, which include a complete optical system on a single substrate, and for which a very limited number of adjustments are needed (24). Moreover, when the machining technology is not precise enough to obtain a perfect alignment, it is possible to integrate small actuators with the optical element in order to correct the slight residual misalignment of the sensitive device (25)

5.1 -

A small weight

One of the most obvious advantage of Microsystems for space applications is their small weight. When the work can be done with the same quality and robustness by a light system, its advantage is immediate in terms of launching cost for space applications. For instance, a small scanning micromirror can be as efficient as a large one (and even better) for a total weight which can be more than one order of magnitude smaller.

5.2 -

A volume fabrication for a small additional cost

Another cost advantage of Microsystems comes from the fact that it is possible to fabricate numerous systems for almost the same cost. Therefore, it becomes possible to design systems which are based on arrays of microdevices. For instance, adaptive optics are sophisticated systems that one could not afford except if they are small and inexpensive enough. Moreover, this feature opens the possibility of a rather systematic redundancy which is very useful for space applications. By using MOEMS technology, the additional cost of such a redundancy can be quite easy to support.

5.3 -

A small energy consumption

Many optical systems using microactuators will consume less energy if they are small enough, because the energy required to move an object usually increases with its dimensions. Of course, this property will also depend on the energy losses in the actuation system: for instance, electrostatic actuators, which are voltage controlled with almost no current flow, generally exhibit less losses than electromagnetic actuators, because the latter are based on large currents and suffer from Joule effect losses.

5.4 -

Robustness

As mentioned before, it is possible to design rather sophisticated microsystems containing only a very limited number of optical adjustments. Since adjustments are generally sources of misalignments, this feature generally leads to more simple and more robust optical devices.

In addition, one should notice that the small size of these micro-optical devices generally leads to other interesting features:

5.5 -

A small response time

The small mass of microdevices is clearly an advantage when quick movements of the device are required. When a step driving force is applied, inertial forces limit the response time for the displacement of the object. In the case of small objects, the position can be modulated at very high speeds. For instance some micro Fabry-Perot modulators can easily work with a frequency bandwidth of 1 MHz (11).

5.6 -

Problems associated with radiations

It is worth reporting that some electrostatically driven microdevices can suffer from reliability problems in space because of the presence of radiations. Electrical charges are generated by radiations on the moving parts of electrostatic actuators, which leads to strong attractive forces between the fixed and the moving part. Therefore, the moving part is brought in close contact with the fixed one, and sometimes sticks to it permanently. Some further work should be done to get rid of this problem.