Patent Publication Number: US-2002009256-A1

Title: Variable optical attenuator and beam splitter

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
     [0001] This application is a continuation-in-part of U.S. pat. application Ser. No. 09/619,013, filed Jul. 19, 2000, the contents of which are incorporated herein by reference. The present application further claims the benefit of priority from: U.S. Provisional Application No. 60/144,628 filed Jul. 20, 1999, the contents of which are incorporated herein by reference; U.S. Provisional Application No. 60/170,492 filed Dec. 13, 1999, the contents of which are incorporated herein by reference; U.S. Provisional Application No. 60/170,494 filed Dec. 13, 1999, the contents of which are incorporated herein by reference; and U.S. Provisional Application No. 60/250,516 filed Nov. 30, 2000, the contents of which are incorporated herein by reference. 
    
    
     
       FIELD AND BACKGROUND OF THE INVENTION  
       [0002] The present invention relates to the field of microelectromechanical systems (MEMS) and, more particularly, to microelectromechanical or micromechanical devices that actuate a moving element between operative positions to provide, for example, an attenuating or beam splitting operation.  
       [0003] A MEMS device is a micro-device that is generally manufactured using integrated circuit fabrication or other similar techniques and therefore has the potential for cost-effective, large-scale production. A MEMS device is a high precision system used to sense, control, or actuate on very small scales by combining mechanical, electrical, magnetic, thermal and/or other physical phenomena. It typically includes a tiny mechanical device element such as a sensor, mirror, valve, or gear that is embedded in or deposited on a semiconductor chip or substrate. These systems may function individually, or they may be combined in array configurations to generate effects on a larger scale. Advantageously, a MEMS device may be monolithically integrated with driving, control, and/or signal processing microelectronics to improve performance and further reduce the cost of manufacturing, packaging, and testing the device. As used herein, the term “MEMS device” is intended to embrace devices that are physically small and have at least one component produced using micromachining or other microfabrication techniques, and the term MEMS device includes microactuators, micromechanical devices, and micromachine devices.  
       [0004] Due to their considerable technological potential, the use of MEMS is currently being pursued in many different fields. In particular, high precision MEMS are receiving an increasing amount of interest in the fiber-optics field because of their capability to overcome several limitations associated with prior art technologies: see generally Motamedi et al., “Micro-opto-electro-mechanical devices and on-chip optical processing”,  Optical Engineering,  vol. 36, no. 5, p. 1282 (May 1997), the contents of which are incorporated herein by virtue of this reference.  
       [0005] In fiber-optic communication systems, information is transmitted as a light or laser beam along a glass or plastic wire, known as a fiber. A significant amount of electronic communication and information transfer is effected through fiber-optic lines due to their much broader bandwidth and lower susceptibility to electromagnetic interference compared to conventional copper or metal wires. For example, much of the Internet and many long distance telephone communication networks are connected with fiber-optic lines. Optical communication requires a variety of devices for controlling the light beam; a variable optical attenuator and a beam splitter are common devices used for such control. Variable optical attenuators are used in such networks to control the power of an optical signal, and have in the past been implemented in various designs. The present invention relates to beam attenuation in optical communications networks and, more particularly, to a micromechanical valve that can be actuated (moved) in a controlled series of small steps, thus providing a function of controlled attenuation of a light beam. If the valve is provided with a reflecting surface (mirror) and a beam hits it at an angle, the valve can act as a beam splitter.  
       [0006] Past designs of variable optical attenuators include (but are not limited to) inserting a rotatable disk between a pair of optical fibers (U.S. Pat. No. 4,591,231 to Kaiser et al.), changing the orientation between two fibers, inserting a thin film with a changing optical density in the beam path (U.S. Pat. No. 4,904,044 to Tamulevitch), or using a flexural mirror (U.S. Pat. No. 5,915,063 to Colbourne et al.). Recently, variable attenuators have been miniaturized and implemented using MEMS techniques, for example by Bergman and Bishop in U.S. Pat. No. 6,163,643, by Aksyuk et al. in U.S. Pat. No. 6,148,124, and by Robinson in U.S. Pat. No. 6,137,941. Since the present invention also uses MEMS attenuators, it can only be compared to these recent devices. Both Aksyuk&#39;s and Bergman and Bishop&#39;s patents use focusing optics for a beam which is basically parallel to a substrate, and a vertical attenuator moving along the substrate and cutting the beam path. Thus both are limited to a linear configuration, and lack the flexibility allowed by two- or three-dimensional stacking of attenuator arrays, as potentially enabled by the present invention. Robinson&#39;s device is a pivotable MEMS mirror mounted on a substrate, which controls the attenuation of a focused perpendicular light beam by deflecting varying amounts of the beam, and is again useful only in a two-dimensional system, since an array of mirrors is mounted on one planar substrate. All three patents suffer from a major disadvantage in that they provide a low movement resolution, i.e. a limited number of intermediate positions or “steps” in the movement of the attenuator device, unlike the preferably 300 closely spaced (about 1 micrometer apart) steps of the present invention. Thus, the mirror in Bergman and Bishop&#39;s patent is located at the focal plane, and has very limited movement range. In Aksyuk&#39;s patent, the attenuator can assume a very small number of intermediate positions, and the attenuation is in fact quite constant, and not very variable. The attenuation in Robinson&#39;s system is driven by pulse modulation, a method that can lead to loss of information, a major disadvantage in a communications system. None of the previous MEMS-type attenuators allows the controlled shaping of the cross section of a beam, e.g. the peripheral blocking of any desired portion of a Gaussian shaped beam, as does one embodiment of the attenuator of the present invention. Present products thus have several limitations regarding geometry, flexibility of function, design, performance, etc.  
       [0007] In addition, in prior art MEMS devices that actuate a moving element, the design of the actuator and the mechanical coupling of the actuator to the moving element typically generate a significant amount of dynamic friction during actuation: see for example Akiyama et al., “A Scratch Drive Actuator with Mechanical Links for Self-Assembly of Three-Dimensional MEMS”,  Journal of Microelectromechanical Systems,  vol. 6, no. 1, p. 10 (March 1997). As such devices are operated over time, the dynamic friction tends to wear the device components and reduce the reliability and positioning accuracy of the device. Similarly, the moving element of these MEMS devices are generally attached to the substrate or a support component of the device by means of weights, springs, clamps, or other like mechanisms. Again, because these parts are in physical contact with one another, there is dynamical friction during actuation and the parts may wear, leading to reduced device accuracy. Positioning accuracy and repeatability are particularly important for variable attenuators and splitters (e.g. in returning repeatably to a position dictated by a look-up table or LUT), and these attributes are, as emphasized above, generally lacking in prior art.  
       [0008] There is therefore a need for an improved MEMS device capable of rapidly and efficiently actuating to a series of controlled, closely spaced positions, a generally flat moving element such as a mirror or absorber to provide, for example, a variable attenuator or beam splitter operation. It would further be desirable if such a MEMS device were not susceptible to wear from dynamic friction effects and exhibited minimal insertion loss.  
       SUMMARY OF THE INVENTION  
       [0009] The present invention provides a MEMS device having a generally planar moving element disposed in parallel to the surface of a substrate; and an actuator operatively engageable with the moving element for selectively step-moving the element in a series of successive, closely spaced operative positions in a plane parallel with the surface of the substrate. The substrate includes a zone that is penetrable by the optical beam, such zone preferably being an aperture formed within the substrate or comprising an optically transparent material. The moving element preferably travels in a linear path. Specifically, the present invention employs novel control and structural features to provide a light beam variable attenuation function presently unavailable from any known competitive MEMS techniques. The invention takes advantage of a novel feature in the valve technology described in the parent application, in which the actuation of the valve from an operative position yielding an open aperture to an operative position yielding a close aperture can be done in a plurality of very small steps, for example any number between 1 and 300 steps. In the particular example discussed hereinbelow, 300 steps are the maximum needed for a full movement of the valve. It is understood that this number is for exemplary purposes only, and that the number of maximum steps can vary in a wide range, from ca. 10 to ca. 500.  
       [0010] In a preferred embodiment, the device is particularly suitable for use as an optical attenuator where the moving element alters the intensity and/or the shape of an optical beam. In this case, the moving element preferably comprises an absorber, although it can also comprise a mirror or non-absorber material. In another preferred embodiment, the device is particularly suitable for use as a beam splitter, and in this case, the moving element preferably comprises a mirror.  
       [0011] In a preferred embodiment, the actuator comprises a plurality of elongated actuating beams spaced perpendicularly to and along a travel path of the moving element. Each beam extends substantially parallel to the surface of the substrate and has a tip, and a base that is rigidly fixed with respect to the substrate. The actuator further includes a beam actuator that controllably moves the actuating beams so that the beams that are positioned along the portion of the travel path in which the moving element is located intermittently engage the moving element and thereby move the moving element in a step-wise fashion in a desired direction along the travel path. The beams are preferably conductive and the beam actuator preferably comprises, for each actuating beam: at least one first electrode mounted to the substrate and positioned vertically from that actuating beam with respect to the surface of the substrate; at least one second electrode mounted to the substrate and positioned horizontally from the actuating beam with respect to the surface of the substrate; and circuitry for controllably generating a first electric field between the at least one first electrode and the actuating beam to move that actuating beam in a vertical direction with respect to the surface of the substrate, and a second electric field between the at least one second electrode and the actuating beam to move that actuating beam in a horizontal direction with respect to the surface of the substrate.  
       [0012] Where the travel path is linear and has first and second edges, the plurality of actuating beams preferably comprises a first set of actuating beams spaced along the first edge of the travel path; and a second set of actuating beams spaced along the second edge of the travel path, the beam actuator controllably moving the tips of the beams in the first set synchronously with the tips of the beams in the second set. In each of the first and second sets, adjacent ones of the actuating beams that are located along the edge of the portion of the travel path in which the moving element is located may rotate out of phase so that the intermittent engagement of the moving element by adjacent tips in each set is successive. Alternatively, where the moving element rests on static support members fixed to the substrate, in each of the first and second sets, the actuating beams that are located along the edge of the portion of the travel path in which the moving element is located may rotate in phase so that the intermittent engagement of the moving element by said beams in each set is simultaneous.  
       [0013] Other actuators may also be used. In all embodiments, the moving element preferably includes a conductive component, and the device further comprises at least one substrate electrode and circuitry for generating an electric field between the conductive component and the substrate electrode or electrodes to hold the moving element by means of static friction (also referred to as virtual or artificial gravity).  
       [0014] The device is preferably fabricated using micromachining techniques, and with the moving element fabricated in a position parallel to the surface of the substrate. More preferably, surface micromachining techniques are employed in which a plurality of material layers are sequentially deposited and etched. Arrays of the devices may also be provided on a common substrate, each device having its own moving element and actuator. Other objects, features, and advantages of the present invention will become apparent upon reading the following detailed description of the embodiments with the accompanying drawings and appended claims.  
       [0015] According to the present invention there is provided a MEMS device comprising: (a) a substrate having a surface; (b) at least one generally planar moving element disposed in parallel to the surface of the substrate; and (c) an actuator operatively engageable with the at least one moving element for selectively moving the at least one element between a series of successive closely spaced operative positions in a plane horizontal to the surface of the substrate.  
       [0016] According to the present invention there is provided an array of MEMS devices on a substrate having a surface, each of the MEMS devices comprising: (a) at least one generally planar moving element disposed in parallel to the surface of the substrate; and (b) an actuator operatively engageable with the at least one moving element for selectively moving the at least one element between successive, closely spaced operating positions in a plane horizontal to the surface of the substrate. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0017] The objects and advantages of the present invention will be better understood and more readily apparent when considered in conjunction with the following detailed description and accompanying drawings which illustrate, by way of example, preferred embodiments of the invention and in which:  
     [0018]FIG. 1 is an isometric view of the general configuration of a MEMS device in accordance with the present invention;  
     [0019]FIG. 1A is a cross-sectional view of the device taken along the line  1 A- 1 A in FIG. 1;  
     [0020]FIG. 2 shows the shape and motion of a moving element of the device in a preferred embodiment of the present invention;  
     [0021]FIG. 3 shows an alternative shape and motion of the moving element;  
     [0022]FIG. 4 shows the substrate of an optical attenuator or beam splitter MEMS device;  
     [0023]FIG. 4A shows a transparent zone etched through the substrate and surrounded by a non-transparent zone.  
     [0024] FIGS.  5 A- 5 C illustrate the operation of the MEMS device of the present invention as a variable attenuator;  
     [0025] FIGS.  6 A- 6 B illustrate the operation of the MEMS device of the present invention as a variable beam splitter/attenuator;  
     [0026]FIG. 7 shows a preferred configuration for holding the moving element to the actuator of the device;  
     [0027]FIG. 8 is a top plan view of the MEMS device of the present invention comprising preferred actuator that uses actuating beams;  
     [0028] FIGS.  9 A- 9 B show cross-sectional side views of the device and actuator of FIG. 8;  
     [0029] FIGS.  10 A- 10 B illustrate the relative positioning of an actuating beam and corresponding electrodes for electrostatically actuating the beam;  
     [0030] FIGS.  11 A- 11 F show and illustrate the operation of the actuator of FIG. 8;  
     [0031] FIGS.  12 A- 12 B illustrate the operation of an actuator based on a variation of the actuator of FIG. 8;  
     [0032]FIG. 13 illustrates a top view of a possible adaptation to the actuator of FIGS.  12 A- 12 B to ensure that the moving element&#39;s motion is linear;  
     [0033] FIGS.  14 A- 14 D illustrate the operation of another possible actuator for use in the MEMS device of the present invention;  
     [0034] FIGS.  15 A-C illustrate a preferred two-valve configuration of the variable attenuator MEMS device of the present invention;  
     [0035] FIGS.  16 A- 16 I illustrate possible steps in fabricating the MEMS device of the present invention.  
     [0036]FIG. 17 shows an isomeric view of a two-dimensional attenuating/splitting device comprising a 3×3 array of variable attenuator/splitter MEMS devices. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
     [0037]FIG. 1 shows an isometric view of the general configuration of a MEMS valve device  100  as disclosed in the parent application. The device  100  includes a substrate  102  having a surface  104 . A moving or switching element  106  has a generally flat main portion is disposed in parallel to the substrate  102 , above the surface  104 . As described in detail below, moving element  106  may also have support wings, legs or other appendage-like members that are connected to the main portion of element  106  (not shown in FIG. 1). A cross-sectional view of the device  100  taken along the line  1 A- 1 A in FIG. 1 is shown in FIG. 1A. Referring to FIGS. 1 and 1A, the main portion of moving element  106  has a first major surface  108  facing away from substrate  102  and a second major surface  110  that faces substrate  102 , and more specifically surface  104  of substrate  102 .  
     [0038] As shown, moving element  106 , or more specifically the main portion thereof, is preferably separated from substrate  102  by a short distance h. As described in detail below, when device  100  performs an attenuation or beam splitting operation, element  106  is selectively moved to a different operative position in the horizontal plane located a distance h above substrate  102 . While moving between operative positions in the horizontal plane, i.e. during actuation, moving element  106  may temporarily leave the horizontal plane. Furthermore, moving element  106  may be located on the surface  104  of substrate  102  above an aperture therein (i.e. h may be positive or may equal zero), moving element  106  may be recessed within an aperture of substrate  102  (i.e. h may be slightly negative), or moving element  106  may be located on the other side of substrate  102  (i.e. h may have a relatively large negative value). In all embodiments, however, moving element  106  is disposed horizontally or in parallel to substrate  102 .  
     [0039] MEMS device  100  is particularly suitable for use as an attenuator or a beam splitter in a fiber optic communication network. When used as an attenuator and/or a beam splitter, moving element  106  is used to selectively absorb, attenuate, reflect or otherwise alter or modulate the properties and/or path of a light beam. Consequently, moving element  106  may include means for altering the characteristics of a wave incident upon it, i.e. reduce the wave intensity, change the wave shape or reflect (split) part or all of the wave into a direction different from the original direction. Such means may include (but not be limited to) element  106  being an absorber, optical mirror, attenuator, filter, or beam splitter, or element  106  having a shape such as a V-shaped leading edge, for example, or any other suitable shape.  
     [0040] As shown in the embodiment of FIG. 2, moving element  106  may be rectangular and may move in a linear direction within a travel path, defining a range of travel, in the horizontal plane. For example, element  106  may have a travel path along the line defined by arrows  112  or the line defined by arrows  114 . More generally, moving element  106  may move in any linear direction within the horizontal plane. In an alternate embodiment shown in FIG. 3, the moving element may be sector-shaped, as shown at  116 , or circular shaped (not shown), and may move in a radial or pendulum-like motion about a point  120 , as shown by arrows  118 . As a further alternative, the motion of element  106  may be a combination of rotational and translational motion. As indicated, the main portion of moving element  106  is generally flat but otherwise may be of a shape other than those shown in FIGS. 2 and 3, such as circular or elliptical. Substrate  102  is a semiconductor wafer substrate that may be fabricated using well-known integrated circuit processing techniques. The substrate is preferably silicon based, but other materials such as glass, polymers, or metals may also be used. An actuator, which may comprise microelectronic components, is preferably built in or on substrate  102  and serves to actuate the desired movement of moving element  106 , as described in detail below. Substrate  102  is preferably produced with atom smooth surfaces and a high degree of parallelism and linearity. As shown in FIG. 4, in the case of an optical attenuator or beam splitter, substrate  102  may include a first zone  130  through which an input light beam  150  from an exemplary optical fiber  155  does not penetrate, and a second zone  140  that is transparent to light beam  150 . A baseline  135  separates the zones  130  and  140 . The actuation of element  106  preferably occurs at least partially above the second zone  140 , and in a direction parallel to or perpendicular to baseline  135 . The second zone  140  may, for example, comprise a transparent glass. Alternatively, the substrate may simply be absent in zone  140 , as long as sufficient structural support for device  100  is otherwise provided. For instance, zone  140  may be a hole or aperture etched through substrate  102 , and which is surrounded by zone  130  (e.g. see FIG. 4A). Generally, zones  130  and  140  may be located on substrate  102  in any number of ways, and it is also possible for substrate  102  to have more than one zone  130  and/or zone  140  which are not contiguous. For example, two non-penetrable substrate zones  130  may be separated by a single penetrable zone  140 . As a further alternative all of substrate  102  may comprise an optically transparent material such as glass.  
     [0041] FIGS.  5 A-C illustrate the operation of device  100  as a variable attenuator, in which moving element  106  is either an absorber or a mirror. In FIG. 5A, moving element  106  is in a first (“open”) position and light beam  150  that preferably impinges perpendicularly (normally incident) on substrate travels unimpeded through transparent zone  140 . When moving element  106  is translated, parallel to substrate  102 , to one of n (more “closed”) positions defined by the number of basic steps in the stepped movement of the actuator (n being preferably 300 steps), e.g. step number 150 of 300 as shown in FIG. 5B, part of light beam  150  is attenuated, either absorbed in moving element  106 , or reflected away. An attenuated beam  152  that passes through zone  140  thus includes less intensity, or is truncated and changed in shape, to a degree dependent on the position of moving element  106  relative to zone  140 . For example, in FIG. 5C, element  106  is shown positioned at the end of its travel range (step 300 out of 300 in the preferred embodiment discussed), totally blocking (attenuating) light beam  150 . The attenuation is almost continuously variable due to the high resolution of the movement of element  106  (a distance of about 1 μm between discrete operative positions, as explained below), providing a feature unavailable in any competitive MEMS device.  
     [0042]FIGS. 6A and 6B illustrate the operation of device  100  as a beam splitter/attenuator  210  in which moving element  106  is preferably a mirror. As with the attenuator above, in FIG. 6A, moving element  106  is in a first (“open”) position and light beam  150  from an input source  170  travels unimpeded through transparent zone  140  in substrate  102  as a continuing, first output beam  150 ′. When moving element  106  is translated, parallel to substrate  102 , to one of n (more “closed”) positions, e.g. step number 150 as shown in FIG. 6B, a portion of light  150  reflects off (is split from) of the surface of mirror  106  and is directed as a secondary, second output beam  150 ″. In this mode of operation, light beam  150  is not normally incident upon the surface of mirror  106  but rather has an angle of incidence (i.e. the angle between the normal to the mirror surface and the light) that is greater than zero. In a preferred embodiment, the angle of incidence of light beam  150  is about 45°. As will be appreciated by those skilled in the art, when beam splitter  210  is actuated from the first position to a series of successively more closed positions, the amount of light in continuing beam  150 ′ decreases almost continuously, while the amount of light split into secondary beam  150 ″ increases almost continuously. Thus, one achieves both a variable attenuation of beam  150  in the direction of first output beam  150 ′, and increased transmission of secondary beam  150 ″. In other words, the moving element alters the characteristics of an optical beam in a stepped varying fashion when moving between successive operative positions.  
     [0043] Furthermore it will be appreciated by those skilled in the art, that beam splitter  210  can be actuated in the opposite direction, from the first position in which the aperture  140  is totally covered to a series of successively more opened positions. In this case the main beam is the reflected beam  150 ″ and the secondary beam is the continuing beam  150 ′. The amount of light in continuing beam  150 ′ increases almost continuously, while the amount of light into the main beam  150 ″ decreases almost continuously. Thus, one achieves both a variable attenuation of beam  150  in the direction of main output beam  150 ″, and increased transmission of secondary beam  150 ′. In other words, the moving element alters the characteristics of an optical beam in a stepped varying fashion when moving between successive operative positions.  
     [0044] It should be noted that the light beams may travel through any of the attenuators described above in the reverse direction to that illustrated, that is with the input and output beams reversed.  
     [0045] The present invention may use a number of different types of actuation approaches for selectively changing the position of each moving element  106  in device  100 . Generally, the actuator transforms electrical or thermal energy into controllable motion (as indicated above, at least part of the actuator is preferably located in or on substrate  102 ). The preferred actuation approach may depend on the type of moving element used. In particular, the actuator may be based on the following types of actuation principles: thermomechanical; shape memory alloys (SMA) with thermal actuation; electromagnetic; electrostatic; or piezoelectric (other actuation principles such as those based on magnetic, diamagnetic, mechanical, or phase change principles may also be used).  
     [0046] These microactuation principles are well known in the art: see generally R. G. Gilbertson et al, “A survey of Micro-Actuator Technologies for Future Spacecraft Missions”,  Practical Robotic Interstellar Flight: Are PVe Ready? Conference,  New York, (August-September 1994), the contents of which are incorporated herein by virtue of this reference. Briefly, thermomechanical actuation is based on the physical expansion or contraction that occurs in materials when they undergo temperature variations. Shape memory alloy (SMA) effect actuation is based on changes in material properties that arise in some metal alloys (such as nitinol) when they are cycled above or below a certain transition temperature. SMA effect shape changes are generally much greater and occur over a much smaller temperature range compared to thermal expansion/contraction. Both these types of thermally driven actuators require cooling, either passive or active, to reverse their actuation action.  
     [0047] Electromagnetic actuation is based on electric current moving through a conducting material. Advantages of electromagnetic actuation include the very rapid generation of forces and operation that is relatively independent of temperature. However the efficiency of electromagnetic actuation decreases significantly on the micro-scale, and it may be difficult to fabricate and appropriately position small electromagnetic coils in a MEMS device. Electrostatic actuation is based on the attraction of oppositely charged objects and repulsion between similarly charged objects. Electrostatic forces also arise very rapidly and are relatively temperature-independent. Electrostatic actuation is also highly efficient over small distances. Piezoelectric actuation is based on the mechanical force and motion that arise from the dimensional changes generated in certain crystalline materials when subjected to voltage or charge. Typical piezoelectric materials include quartz, lead zirconate titanate, and lithium niobate. Piezoelectric materials respond very quickly and with high forces to changes in voltage potentials.  
     [0048] Generally, the actuator should provide for stable and accurate positioning of the moving element  106  at each of its operative (or n stable state) positions. In addition to using one of the above mentioned actuation principles to move the element  106  from one operative position to another, the same or a different principle may be used to maintain the moving element in one of its n stable states. Preferably, electrostatic means are used to hold the moving element in its desired position as described in connection with FIG. 7 below.  
     [0049] The preferable attachment technique illustrated in FIG. 7, provides an electrostatic attraction between moving element  106  and the substrate, actuator, and or support structure (i.e. the fixed components) of device  100 . In FIG. 7, moving element  106  comprises a conductive component  330  and a functional component  340  (e.g. a mirror or an optical absorber). If necessary, components  330  and  340  of moving element  106  may be separated by an additional insulating layer. Optionally, element  106  can comprise a single component of a material capable of providing both the conducting and the desired optical function.  
     [0050] As shown in FIG. 7, moving element  106  rests on two posts  350  located on top of substrate surface  104 . Posts  350  may be formed by etching within the substrate  102  or may be deposited on top of substrate  102  during fabrication, for example. Two electrodes  360  are also located on top of substrate surface  104  (alternatively electrodes  360  may be located underneath or within surface  104 ). Although electrodes  360  are shown to be positioned between posts  350 , they may generally be positioned anywhere along surface  104  as long as they are at least approximately underneath moving element  106  (for instance, electrodes  360  may be positioned outwardly of posts  350  in FIG. 7). By applying, for example, suitable voltage difference  370 , the conducting component  330  can be made more positively charged and substrate electrodes  360  more negatively charged (or vice versa), resulting in an electrostatic field that maintains moving element  106  against posts  350 . For example, conducting component  330  can be charged to a voltage above a certain reference level (i.e. ground), and substrate electrodes  360  can be charged to a voltage below that reference level. Signal  370  can again be provided by suitable microelectronic circuitry located in or on substrate wafer  102 . In an alternative embodiment, fixed electrodes  360  are oppositely charged by connecting a potential difference between them. Localized charges are thereby induced on conducting component  330  so that element  106  is electrostatically sustained and attached to posts  350 .  
     [0051] Again, it should be noted that the upper direction in FIG. 7 is not necessarily against the direction of gravity, and device  100  can be positioned in any orientation, with the electrostatic force between electrodes  360  and conducting component  330  providing a “virtual gravity” effect on moving element  106 . A further advantage of the attachment configuration of FIG. 7 is the absence of a direct electrical contact between moving element  106  and the substrate electrodes  360 . Additionally, moving element  106  is not restricted to particular connecting points, and the attachment force provided by the potential difference  370  can be adjusted as desired. As a result, this preferred attachment mechanism for element  106  permits device  100  to function in any orientation, without relying on gravity and without requiring the use of springs (or other connection components) that may produce dynamic friction during actuation, resulting in wear, or the use of bearing-like parts that are difficult to fabricate in micro dimensions.  
     [0052] In one preferred implementation, the actuator may comprise a number of independently controllable (or actuable) members for selectively engaging moving element  106 . Each member preferably has a base end connected to substrate  102  an another free end or tip that is selectively or operatively engageable with moving element  106 . The members, or their free ends, may be controllably moved by way of any one of the actuation principles mentioned above (e.g. electrostatic, piezoelectric, thermomechanical, etc.) to carry moving element  106  in a desired direction. In doing so, the actuator members may engage moving element  106  in succession or simultaneously depending on the specific details of actuator operation.  
     [0053] For example, FIG. 8 shows a top plan view of a preferred configuration of MEMS device  100  having an actuator  400  having two sets  410  and  420  of actuating beams  430 . Beams  430 , which act as fingers or cantilevers, are generally elongated, and preferably of a rectangular or square cross-section, at least near the tips thereof. Each set  410 ,  420  comprises a number of beams  430 , although, for clarity of illustration in FIG. 8, only two beams  430  are shown in each of sets  410  and  420 . However, the presence of additional beams is intended to be indicated by the ellipses, as shown, so that, in general, beams  430  extend along substantially the entire travel path of element  106 , preferably near the edge or side of that path. The line of travel of element  106  is represented by the double-headed arrow  404 , and the associated travel path of moving element  104  has edges at  406 , as shown in FIG. 8. For example, in one preferred embodiment moving element  106  is of 300 μm in length (L), 300 μm in width (W), and about 2 μm in thickness and travels a horizontal distance of about 1 μm between operative positions (e.g. the “stable positions” of the attenuator or beam splitter). For the exemplary dimensions, each set  410 ,  420  of actuating beams may have between 15-20 equally spaced-apart beams  430 , each having a length of 150 μm and a 2 μm by 2 μm cross-section. However, any number of beams of different shapes and sizes may be used, depending on the size and application of device  100  and element  106 , and the above example is in no way intended to be restrictive.  
     [0054] As shown in the top plan view of FIG. 8, and more clearly in the cross-sectional side views FIGS.  9 A- 9 B, moving element  106  preferably includes wings  126  extending perpendicular to the line of travel of element  106  from opposite ends thereof. Each wing  126  is supported by a subset of the beams  430  in set  410  or set  420 . At different positions within its range of travel (i.e. along its travel path), element  106  is supported by different subsets of beams  430 . By actuating the beams  430 , or more specifically the distal ends or tips of beams  430 , in a systematic and controlled manner, element  106  is moved in a desired direction. Electrodes  360  located in or on substrate  102  serve to hold or attach element  106  in place. For this purpose, element  106  may include a conductive component as described in connection with FIG. 7 (but not shown in FIG. 8). Where MEMS device  100  is an optical variable attenuator or beam splitter, the portion of substrate  102  between electrodes  360  may be penetrable, i.e. transparent, to light, as described above in connection with FIG. 4.  
     [0055] In each set  410 ,  420 , the base end of each beam  430  is preferably connected to a single anchor or base portion  460  on substrate  102 . Alternatively, however, the base of each beam  430  may be connected to an individual anchor portion that is separately connected to substrate  102 . Other configurations may also be used to rigidly fix the base of each actuating beam  430  with respect to substrate  102 . As shown in FIGS.  9 A- 9 B, moving element  106  may include fin-like legs  128  extending toward substrate  102 , and similarly, each beam  430  may include a fin like leg  432  at the tip of the beam (i.e. the end of the beam away from base portion  460 ) also extending toward substrate  102 . These legs ensure that there is no physical contact between beams  430  or moving element  106  and the electrodes on the surface  104  of substrate  102  (or substrate  102  itself). Legs  128  and  432  thereby serve to avoid any sticktion or short circuits, but may be omitted if this is not a concern.  
     [0056] Beams  430  may be actuated by any suitable actuation principle, however, electrostatic actuation is preferably used, and therefore actuating beam  430  are preferably conductive. As illustrated in FIG. 8, to provide electrostatic actuation, each beam  430  has a bottom electrode  440  and a side electrode  450  associated therewith. The corresponding bottom electrode  440  preferably lies along substrate  102 , underneath each beam  430 , as is shown in FIGS.  10 A- 10 B. FIG. 10B further illustrates the positioning of a corresponding side electrode  450  for each actuating beam  430 . Side electrode  450  preferably includes a support  455  so that side electrode  450  is generally at the same height as beam  430  with respect to the surface  104  of substrate  102 . The tip of the actuating beam can be actuated away from substrate  102  by making both the beam  430  and bottom electrode  440  more positively (or negatively) charged than a reference. The tip may also be actuated away from the substrate by its inherent elasticity, or by being pulled electrostatically by another electrode. Conversely, the tip of the actuating beam can be actuated towards substrate  102  by making one of beam  430  and bottom electrode  440  more positively charged than a reference and the other more negatively charged than the reference. Similarly, the tip of the actuating beam can be actuated in the direction towards side electrode  450  by making one of beam  430  and side electrode  450  more positively charged than a reference and the other more negatively charged than the reference; while by making both beam  430  and side electrode  440  more positively (or negatively) charged than a reference, the tip of the actuating beam can be actuated in the direction away from side electrode  450 . As above, the tip may also be actuated away from side electrode  450  by its inherent elasticity, or by being pulled electrostatically by another electrode. As will be appreciated by those skilled in the art, integrated circuitry for generating the above-described electrostatic forces, for example using voltage pulse signals, may be readily and conveniently provided in substrate  102 . Furthermore, to provide the desired actuation of beams  430 , electrodes could be positioned at both sides of an actuating beam, and it is also possible to provide an electrode above each actuating beam  430  (in addition to or instead of bottom electrode  440 ).  
     [0057] As illustrated in FIGS. 8 and 10A- 10 B, corresponding bottom and side electrodes  440  and  450  preferably extend in parallel along a considerable portion of each beam  430 . The stress in beams  430  is low during actuation since only relatively small displacements are required. Also, the tips of beams  430  preferably remain generally parallel to the surface  104  of substrate  102 , as illustrated by FIG. 9A, which shows a cross-sectional side view of the MEMS device of FIG. 8 with opposing beams  430  in an unactuated position, and FIG. 9B, which shows the same cross-sectional side view with the opposing beams  430  in actuated towards substrate  102 . Furthermore, it should be pointed out that a side electrode  450  is generally positioned in close proximity to its corresponding beam  430 , while being far enough way from the next closest beam  430  so that any electrostatic force between the side electrode  450  and the next closest beam is negligible. In this manner, a particular side electrode only serves to actuate the beam corresponding thereto.  
     [0058] FIGS.  11 A- 11 F illustrate the operation of beam actuator  400  illustrated in FIG. 8. In general, beams  430  in set  410  are actuated synchronously or in tandem with corresponding beams in set  420 , so that moving element  106  is transported in a straight path, as shown in FIG. 8. FIGS.  11 A- 11 F show the actuation of the tips of four beams  430 - 1 ,  430 - 2 ,  430 - 3 , and  430 - 4  in one of sets  410  or  420 . As indicated, each set  410  and  420  may include any number of beams  430 , but generally only a subset of those beams holds mirror element wing  126  at any one time.  
     [0059] In FIG. 11A, the tips of beams  430 - 1 ,  430 - 2 ,  430 - 3 , and  430 - 4  are in a first level position in which all four beam tips are at the same height above substrate  102  and all four beam tips are supporting wing  126  of moving element  106 . Preferably, when moving element  106  is in a desired operative position, beams  430 - 1 ,  430 - 2 ,  430 - 3 , and  430 - 4  are in such a level position. Referring to FIG. 11B, upon actuation, the tips of beams  430 - 2  and  430 - 4  begin to move away from substrate  102  so that only members  430 - 2  and  430 - 4  support wing  126 . Subsequently, the tips of beams  430 - 2  and  430 - 4  begin to be actuated to the left in FIG. 11B. As the tips of beams  430 - 2  and  430 - 4  move to the left in FIG. 11B, wing  126  is transported in the same direction. Since a corresponding actuation takes place simultaneously with respect to the other wing  126  of moving element  106 , element  106  is thereby transported by actuator  400  in the same desired direction.  
     [0060] Next, referring to FIG. 11C, the upward actuation of the tips of beams  430 - 2  and  430 - 4  ceases and those beam tips move back down and toward substrate  102  until a second level position is reached in FIG. 11D. As with the first level position of FIG. 11A, all four beam tips support wing  126  of moving element  106  in the second level position of FIG. 11D, and moving element  106  lies in the same horizontal plane as in FIG. 11A. The tips of beams  430 - 1  and  430 - 3  are then actuated up and away from substrate  102  so that they begin to support wing  126  on their own. The tips of beams  430 - 1  and  430 - 3  are subsequently actuated to the left as shown in FIG. 11E, with wing  126  and element  106  moving in tandem. At the same time, the sideways or leftward actuation of beams  430 - 2  and  430 - 4  ends and these beam tips retreat, without affecting the movement of element  106  (see FIG. 11E), to their unactuated level position in FIG.  11 A. The upward actuation of the tips of beams  430 - 1  and  430 - 4  ends and these beam tips move down and toward substrate  102  until, in FIG. 11F, the tips of all four beams  430 - 1 ,  430 - 2 ,  430 - 3 , and  430 - 4  are all again in a level position. This process repeats itself until moving element  106  has moved to the desired operative position. As wing  126  moves outside the range of a particular beam tip, e.g. that of beam  430 - 4  in FIG. 11F, the actuation of that beam tip may end. Correspondingly, when wing  126  has moved on top or within range of another beam tip, e.g. that of a beam immediately to the left of beam  430 - 1  in FIG. 11F, that beam tip begins to be actuated as described above.  
     [0061] As illustrated in FIGS.  11 A- 11 F, the tips of alternate beams effectively undergo a rotation-like motion to successively and repeatedly actuate moving element  106 . In the illustrated embodiment of FIGS.  11 A- 11 F, the rotation of the tips of the first pair of beams  430 - 1  and  430 - 3  and the rotation of tips of the second pair of beams  430 - 2  and  430 - 4  (both counter-clockwise in FIGS.  11 A- 11 F) are out of phase so that each pair successively acts to transport moving element in the desired direction. The amount of motion in each step depends on the horizontal amplitude of the beams. For example, a 2×2 μm beam that is 150 μm long preferably has a horizontal and vertical amplitude of about 1 μm (or less). Also, although, as illustrated in FIGS.  11 A- 11 F, the rotation-like motion of the beams is preferably rectilinear, it may also be circular or elliptical, for instance. Furthermore, to move element  106  in the reverse direction, the rotation of the beam tips can simply be reversed.  
     [0062] If more than four beams  430  are underneath wing  126  of element  106  at a given position, the actuated motion of the beam tips may be more complex. For example, with six beam tips underneath wing  126  at a given position, the beam tips may be actuated as three separate groups or pairs whose rotation-like motions are generally 120° out of phase with one another.  
     [0063] As described, electrostatic beam actuation is preferably used because of the efficiency and ease of implementation of electrostatic forces in a microelectromechanical system. In particular, by controlling or modulating the timing and duration of voltage pulse signals applied to beams  430 , bottom electrodes  440 , and side electrodes  450 , the tips of the actuating beams may be controllably rotated in a clockwise or anti-clockwise direction, translating moving element  106  as described above. Associated control circuitry used for this purpose is preferably microelectronically implemented within MEMS device  100 , using convention integrated circuit fabrication techniques well known in the art. The frequency and phase relationship between applied voltage pulse signals, controls the direction and travelling speed of the movement of element  106 . However, as indicated above, in the beam actuator embodiments described above and variations thereof, the rotation-like-actuation of the beams  430  can be achieved by any of the different actuation methods described in the parent application and above. For example, beams  430  may comprise a piezoelectric crystalline material. In this case, by applying appropriate voltage pulse signals to the piezoelectric beams, they may be manipulated to mechanically bend in the horizontal and vertical directions, and thereby transport moving element  106  in a desired direction in the manner just described. As will be appreciated, the orientation and structure of beams  430  may vary, in particular depending on the type of actuation method used.  
     [0064] Thus, generally, in actuator  400  a plurality of elongated actuating beams  430  are spaced perpendicularly to the travel path of the moving element  106 . Each beam  430  extends above and preferably substantially parallel to surface  104  of substrate  102 , and each beam has a base rigidly fixed with respect to substrate  102  (i.e. via anchor portion  460 ) and a tip that is preferably proximate or near an edge  406  of the moving element&#39;s travel path. Actuator  400  controllably causes the tips of the actuating beams  430  to rotate, so that the tips of the actuating beams that are located along the edge of the portion of the travel path in which the moving element is located intermittently engage the moving element. By intermittently engaging moving element  106  during their rotation, the tips serve to actuate the moving element in a desired direction along the travel path. Furthermore, it will be appreciated that actuator  400  can be adapted to actuate element  106  along other types of travel paths. For example, if moving element  106  is sector-shaped and moves in a radial or pendulum-like motion about a point  120  (see FIG. 3), beams  430  may be positioned to extend perpendicularly to and along substantially the entire radial travel path of element  106  (with element  106  rotatably fixed with respect to substrate  102  at point  120 ). In this case, only a single set of actuating beams  430  is required since the travel path only has a single, arc-shaped, outer edge.  
     [0065] To summarize, and using the clockwise movement as exemplary, the preferred movement of a valve over one cycle includes four principal steps: 1) separating the valve from its rest element by raising it above the substrate; 2) moving the valve to the right; 3) lowering the valve toward the substrate; and 4) placing the valve on the rest element, separating the beams from the valve, and moving the beams back to the left. A similar movement can occur counterclockwise. Step 4 does not return the valve to its original position, but leaves it displaced laterally by one “step” of typically 1 μm from the starting point of the cycle. This movement is similar to that of the tracks of a half-track vehicle. The actuation allowing such “half-track” movement is typically driven by three-level (preferably +60V, 0V and −60V) voltage pulses from four independent sources. The alternating positive and negative voltage cycles are symmetrical and fulfill an important function of discharging (or preventing the build-up of) any parasitical charges that may be created on the actuators by the use of a single type polarity.  
     [0066] FIGS.  12 A- 12 B illustrate a possible modification to the operation of the actuator  400 . In the actuator embodiment of FIGS.  12 A- 12 B, the tips of the beams  430 - 1 ,  430 - 2 ,  430 - 3  in beam set  410  (and beam set  420 ) rotate in unison, i.e. all in phase with one another. When moving element  106  is in a static operative position, wings  126  and/or element  106  are supported by fixed posts  470 . Posts  470  preferably extend upwardly from surface  104  of substrate  102 , but optionally posts  470  may be replaced with static beams that are not actuated. As shown in FIG. 12A, during actuation, the tips of beams  430 - 1 ,  430 - 2  and  430 - 3  begin a rotation-like motion in which they are first actuated upwards (away from substrate  102 ) so that the tips rise above the level of posts  470 , lifting wing  126  off posts  470 . Subsequently, the beam tips are actuated in parallel to substrate  102 , transporting moving element  106  in a desired direction (to the left in FIG. 12A). As the rotation-like motion of the tips of beams  430 - 1 ,  430 - 2 ,  430 - 3  continues, the upward actuation of the tips ends, so that the beam tips retreat or fall below the level of static posts  470  (see FIG. 12B). When this occurs, wing  126  is again held and supported by posts  470 , although now at a different position in the horizontal plane above substrate  102 . The sideways actuation of the tips of beams  430 - 1 ,  430 - 2 ,  430 - 3  also ceases at this stage. This rotation-like cycle is repeated until moving element  106  has been re-positioned to a desired operative location.  
     [0067]FIG. 13 illustrates a possible adaptation of an actuator  400  operating as described in connection with FIGS.  12 A- 12 B which serves to ensure that the motion of element  106  is linear and that element  106  is not undesirably tilted. As described above, element  106  is actuated at opposite ends by two synchronously operating sets  410  and  420  of beams  430  extending from base portions  460 . As shown in FIG. 13, the tips of beams  430  in each set  410  and  420  are connected to a connecting support beam  480  that supports and holds a wing  126  of element  106 . Connecting support beams  480  increase the cumulative actuation force generated by the individual tips of beams  430  and also act to further synchronize the operation and movement of the beam tips. As a result, moving element  106  is evenly held and supported from both sides. In the embodiment of FIG. 13, the tips of all beams in each group are actuated in phase during actuation of moving element  106 , regardless of the position of element  106  within its range of travel. One or more additional synchronization beams  490 , linking the connecting support beams  480 , may also be used to further synchronize the actuation operation of each set  410 ,  420  of beams  430 . Preferably, at least two synchronization beams  490  are used, one near each end of beams  480  (only one beam  490  is shown in FIG. 13).  
     [0068] Furthermore, as will be appreciated, it is also possible to rotate moving element  106  in the horizontal plane by, for example, operating the two sets  410 ,  420  of beams  430  out of phase. Other types of more complicated movements of element  106  may also be achieved by combining additional sets of beams in different possible configurations and synchronizing those beam sets accordingly.  
     [0069] FIGS.  14 A- 14 D illustrate the operation of another possible actuator  500  for use in MEMS device  100  of the present invention. In this embodiment, when element  106  is in an operative position, e.g. any position out of 300 stable positions defined between a fully “closed” and a fully “open” position for a variable attenuator or beam splitter MEMS device  100 , element  106  is held on static posts  510  extending from surface  104  of substrate  102 , as shown in FIG. 14A. Alternatively, moving element  106  may have legs  510  that rest on surface  104  of substrate  102 . Actuator  500  further includes beams  520  whose tips are located above and apart from moving element  106  (or a wing or other appendage thereof) when the latter is in an un-actuated or operative state. As with actuator  400  described above, beams  520  are preferably attached to substrate  102  by way of an anchor or base portion (not shown). Upon actuation, moving element  106  is raised from the posts  510  and attaches to beams  520 . Preferably, beams  520  are conductive allowing an attractive electrostatic force to be generated between beams  520  and a conducting component of element  106  (also not shown). However, magnetic attraction may also be used for this purpose. Beams  520  are preferably relatively rigid in vertical direction, so that the tips of beams  520  do not bend substantially when attracting element  106 . Once element  106  is attached to the tips of beams  520 , the tips of beams  520  are actuated in a desired horizontal or sideways direction (FIG. 14B). The combination of the attraction of element  106  and actuation of the tips of beams  520  moves element  106  in a desired direction.  
     [0070] Once element  106  is moved to a desired horizontal location, the attraction between beams  520  and element  106  is ended so that moving element  106  is released and again held by posts  510 , as shown in FIG. 14C. The actuation of the tips of beams  520  also ceases so that they return to their unactuated position. If element  106  is to be moved further in the same direction, the above actuation steps are repeated. Once again, it should be clear that in this embodiment, as with all of the embodiments of the present invention, the terms “up”, “down”, “lower”, “upper”, “top”, and “bottom” are used merely for illustrative purposes, and that MEMS device  100  can operate independently of its overall orientation.  
     [0071] Another preferred embodiment of the present invention uses two or more moving elements or valves in conjunction with one aperture. The two or more moving elements or valves are positioned on different planes above, and preferably on opposite sides of the aperture, each valve covering only part of the aperture. An example of such a two-valve unit  600 , in which the shape of the valve (in addition to the degree of overlap with the aperture) is also involved in controlling the attenuation function is shown in FIG. 15.  
     [0072] The two-valve unit  600  uses two moving elements similar to element  106  located in two different layers. FIG. 15A shows in top view a first valve  106 ′. Valve  106 ′ is located for example to the left and above the plane of a transparent zone  140 . FIG. 15B shows a top view of a second valve  106 ″, located for example to the right and above zone  140 , in a plane higher than and parallel with valve  106 ′. Valves  106 ′ and  106 ″ have respective leading edges  602 ′ and  602 ″ which may be straight, as in the preferred embodiment of an element  106 . Preferably, edges  602 ′ and  602 ″ are shaped with shapes which provide a uniform attenuation to a light beam. For example, as shown in FIGS.  15 A-B, edges  602 ′ and  602 ″ are each shaped as a “V”. If a perpendicular Gaussian shaped beam is directed toward, and centered in zone  140 , its peripheral sections can be progressively and uniformly blocked by controllably moving both valves one toward the other, using the innovative stepped movement of the present invention. In a preferred stable position, only the central part of the beam is allowed to pass through zone  140 . Such an intermediate, partially blocking or attenuating position is shown in FIG. 15C. Alternatively, if the starting position is one in which the two valves completely block the beam, by controllably moving them apart from each other, the peripheral sections of the beam are increasingly allowed through zone  140 . Those skilled in the art will appreciate that more than two valves, moving in different but parallel planes and having different leading edge profiles and different movement directions, can be used to control the attenuation function.  
     [0073] The attenuation function as a function of the movement is not linear, but there are a number of ways to overcome this non-linearity. For example, a LUT can be used to store the values of the necessary movement of each valve for a given attenuation factor.  
     [0074] The fabrication of MEMS device  100  and its various components may be achieved using conventional macromachining, mesomachining, or micromachining techniques. Preferably, micromachining technology including the well-known photolithography, deposition, and etching fabrication methods used in the microelectronics and micromachining industries is used to manufacture all of the components of device  100 . See generally, Chertkow et al., “Opportunities and Limitations of Existing MicroFabrication Methods for Microelectromechanical Devices”, Proc. 25 th  Israel Conf. on Mechanical Engineering, Technion City, Haifa, Israel, p. 431 (May 1994) and Petersen, “Silicon as a Mechanical Material”,  Proceedings of the IEEE,  vol. 70, no. 5 (May 1982), the contents of which are hereby incorporated herein by virtue of this reference. Batch manufacturing of MEMS devices in integrated circuit fabs or foundries permits the production of large volumes of devices at extremely low cost.  
     [0075] Micromachining fabrication technology includes both bulk and surface micromachining processes. With bulk micromachining techniques, microstructures are formed by etching away the bulk of a silicon wafer to produce the desired structure. On the other hand, surface micromachining techniques build up the structure in layers of thin films on the surface of a suitable wafer substrate. Typically, films of a structural material and a sacrificial material are deposited and etched in sequence. Generally. the more mechanical layers used during surface micromachining, the more complex the structure and the more difficult fabrication becomes. Once the desired structure has been formed, the sacrificial material is etched away to release the structure. Due to its mechanical properties and compatibility with modem integrated circuit fabrication processes, polysilicon, i.e. polycrystalline silicon, is preferably used as the MEMS structural material. Polysilicon is strong, flexible, fatigue-resistant, and highly compatible with integrated circuit fabrication techniques.  
     [0076] MEMS device  100  is preferably built using this type of sacrificial polysilicon surface Micromachining technology, which as described above, enables the mass production of complex microelectromechanical systems, by themselves or integrated with microelectronics. FIGS.  16 A- 16 I illustrate a preferred method of fabricating the mechanical structure of the MEMS device  100 , including actuator  400 , of FIG. 8 using surface micromachining techniques. More specifically, FIGS.  16 - 16 I show a cross-sectional side view of device  100  during the various steps in the fabrication process.  
     [0077] Before micromachining begins, substrate  102  is selected and prepared. Generally, substrates of different materials, dimensions, thickness, and surface preparation may be used, although the physical dimensions of substrate  102  may be dictated by the purpose and operation of device  100 . Furthermore, as described above, in the case of an optical switch device part of substrate  102  may be removed (bulk etched) to provide a transparent or penetrable zone  140  in substrate  102  (see FIG. 4). Furthermore, where MEMS device  100  is an optical switch and moving element  106  is a mirror, the surface preparation of substrate  102  (including surface  104 ) is preferably of high quality so that the reflective surface  108  (see FIG. 1) of the mirror can also be provided with a high degree of surface quality, especially in terms of flatness and parallelism.  
     [0078] Once a suitable substrate  102  has been prepared, a first polysilicon layer  610  is deposited on the surface  104  thereof. Polysilicon layer  610  is photolithographically patterned before undergoing chemical etching. As is well known in integrated circuit fabrication processes, a two-dimensional mask may be used to define the patterns to be etched. As illustrated in FIG. 16A, the deposition and patterning of polysilicon layer  610  forms bottom electrodes  440  and substrate electrodes  360  used for electrostatic attachment. In FIG. 16B, an oxide (e.g. silicon dioxide) layer  620  is deposited on top of substrate  102  and the remaining polysilicon layer  610 . Oxide layer  620  is then patterned and etched to provide slots  660  for the subsequent deposition of anchor portions  460 , dimples  670  for fin legs  432  of beams  430 , and dimples  680  for fin legs  128  of moving element  106 . This is shown in FIG. 16C.  
     [0079] In FIG. 16D, a second polysilicon layer  630  is deposited on top of oxide layer  620  and into slots  660 ,  670 , and  680  to form anchor portions  460 , fin legs  432 , and fin legs  128  respectively. Further patterning and etching of polysilicon layer  630  produces beams  430  and moving element  106 , as shown in FIG. 16E. Where moving element  106  is a mirror, its top surface  108  may be coated with gold or aluminium, for example, using standard deposition and patterning methods to render surface  108  reflective. As indicated, to minimize losses, any mirror or other optical element used in MEMS device  100  should be designed to be very smooth. Furthermore, as described in detail above, in the case of an optical switch, the mirror is provided above substrate  102  in an area in which input light beams will be directed, below which substrate  102  is either absent or transparent. However, for other types of MEMS devices moving element  106  may be fabricated in other positions above substrate  102 .  
     [0080] In FIG. 16F, a further oxide layer  640  is deposited, as shown. Patterning and etching of layer  640  is carried out to provide slots  690  for wings  126  of moving element  106 . Polysilicon layer  650  is subsequently deposited, as shown in FIG. 16G; and patterning and etching of layer  650  results in wings  126 , as shown in FIG. 16H. At this stage, the deposition and patterning of the mechanical layers is complete. As a result, in FIG. 16I, the remainder of oxide layers  620  and  640  is chemically removed, leaving behind the desired polysilicon mechanical structures. Alternatively, release of the mechanical structures may be accomplished by etching steps.  
     [0081] In general, fabrication of the associated microelectronics (not shown) for MEMS device  100  may be performed simultaneously with, before, or after, the above-described surface machining steps. It will be appreciated that alternative and further fabrication steps will be required for different types of actuators and/or different types of actuation and/or attachment principles. In addition, different configurations and applications of MEMS device  100  may alter or vary the fabrication details and materials used. Furthermore, other fabrication processes may also be used, although it is highly preferable that the fabrication of moving element  106  take place above the highly smooth and planar surface  104  of substrate  102 , as explained above.  
     [0082] It will be appreciated that the present invention is capable of providing attenuating/splitting devices with a number of inputs M and outputs N for a variety of applications, by employing a plurality of variable attenuators/splitters. Preferably, the moving elements in the attenuators/splitters are actuated and move in directions that are parallel to one another. For example, the attenuators/splitters may share a common substrate so that the moving elements of each attenuator/splitter are generally coplanar. FIG. 17 shows an isomeric view of such a two-dimensional attenuating/splitting device comprising a 3×3 array  800  of devices  100 . Array  800  is shown having only 9 devices  100  configured in a 3×3 array by way of example only, and for convenience of presentation. It is understood that an array of the present invention can have a plurality of other than nine variable beam splitter/attenuators  100 , and that devices  100  do not necessarily have to be configured in an array comprising equal numbers of elements in each row or column. Devices  100  provide a 3×3 array of inputs and outputs arranged in rows and columns. Each optical attenuator/splitter is shown in FIG. 17 in an OFF position in which input beam  150  travels through penetrable zone  140  of substrate  102  comprising a hole or aperture formed within the substrate.  
     [0083] It will be appreciated that the MEMS device of present invention, which includes a generally flat moving element such as a mirror disposed horizontally above a smooth wafer substrate, provides several advantages. The device  100  allows for a fast actuation response, low losses, compact structure, and relatively large actuation displacements, unlike prior art devices that form the moving element by etching into the substrate wafer. The actuation of the moving element in the present invention effectively occurs in parallel to the substrate as a translation, thus minimizing any air resistance and providing more favourable actuation performance from the standpoint of inertia and energy considerations. Importantly, because of the high degree of planarity of substrate  102  and moving element  106  during fabrication, the design and positioning of the moving element in the present invention avoids small deviations that can significantly affect device operation accuracy, as may occur in prior art devices in which a moving element or mirror is disposed vertically with respect to the substrate or in prior art devices in which the moving element tilts with respect to the substrate. As indicated, MEMS device  100  may have a relatively long travel path, so that there is no overlap between operative positions of moving element  106  in terms of the location of these positions in the plane above substrate  102 .  
     [0084] While preferred actuation embodiments uses actuating beams to translate the moving element from one operative or “stable” position in a horizontal plane above the substrate to another operative position in that horizontal plane, actuators based on other actuation techniques can also be used. In addition the physical phenomenon used to generate the required actuation forces may be based on various physical principles including: thermomechanical; shape memory alloys (SMA) and thermal actuation; electromagnetic; electrostatic; elasticity; or piezoelectric, magnetic, diamagnetic, mechanical, or material phase change. Also, while the moving element is preferably held by static friction induced by an electrostatic or magnetic force, as described above, other support and attachment configurations for the moving element may also be used.  
     [0085] MEMS device  100  may be advantageously implemented for other applications relating to fiber optic communication, such as optical switches, valves, collimators, and the like. In particular, as indicated in the parent application, MEMS device  100  of the present invention can be used as an optical switching element. Use of the preferred actuators and preferred mechanisms for attaching moving element  106  permits the device to be actuated and used as variable attenuators and beam splitters with milisecond speed of movement between operative positions, and with minimal dynamical friction thereby reducing wear and increasing reliability.  
     [0086] Thus generally, while the invention has been described in conjunction with specific embodiments, it is evident that numerous alternatives, modifications, and variations will be apparent to those skilled in the art in light of the foregoing description.