Abstract:
The invention provides a micro-electro-mechanical-system (MEMS) mirror device, comprising: a mirror having a 2-dimensional rotational articulated hinge at a first end, and having a 1-dimensional rotational articulated hinge at a second end opposite the first end; a movable cantilever connected to the mirror through the 1-dimensional rotational articulated hinge; a support structure connected to the mirror through the 2-dimensional rotational articulated hinge and connected to the movable cantilever; whereby movement of said movable cantilever causes rotation of the mirror in a first axis of rotation, and the mirror is also rotatable about a second torsional axis of rotation perpendicular to said first axis of rotation.

Description:
RELATED APPLICATION 
   This application claims the benefit of prior U.S. provisional application No. 60/464,972 filed Apr. 24, 2003. 

   FIELD OF THE INVENTION 
   The invention relates to a MEMS (micro-electro-mechanical-system) two dimensional mirror with articulated suspension structures for high fill factor arrays. 
   BACKGROUND OF THE INVENTION 
   A MEMS (Micro-Electro-Mechanical-System) device is a micro-sized mechanical structure having electrical circuitry fabricated together with the device by various microfabrication processes mostly derived from integrated circuit fabrication methods. The developments in the field of microelectromechanical systems (MEMS) allow for the bulk production of microelectromechanical mirrors and mirror arrays that can be used in all-optical cross connect switches, 1×N, N×N optical switches, attenuators etc. A number of microelectromechanical mirror arrays have already been built using MEMS production processes and techniques. These arrays have designs that fall into approximately three design categories. 
   A first category consists of conventional 2D gimbal mirrors with each mirror surrounded by a frame. The conventional 2D gimbal mirror is one of the most common types of MEMS 2D micromirrors. An example is shown in  FIG. 6 . It consists of a central mirror  10  that is connected to an outer frame  12  with torsional hinges  14 . The outer frame  12  is in turn connected to the support structure  16  with another set of torsional hinges  18 . There are four electrodes under the central mirror  10  that can be actuated resulting in a 2D tilt of the mirror-frame assembly. One such device is disclosed under U.S. Patent Application Publication No.: US2002/0071169 A1, publication date Jun. 13, 2002. One of the shortcomings of this design is the inability to achieve high fill factors (that is the spacing between two consecutive mirrors or the ratio of the active area to the total area in an array) in a mirror array. An example of a high fill factor would be &gt;90% active mirror portion along one dimension. 
   A second category consists of 2D/3D mirrors with hidden hinge structures. With significant advances made in Spatial Light Modulators, a number of 2D micromirror devices have been designed with various types of hidden hinge structure. Examples of these are disclosed in U.S. Pat. No. 5,535,047, U.S. Pat. No. 5,661,591, U.S. Pat. No. 6,480,320 B2. 
   A schematic of an example of such a device is shown in  FIG. 7 . Although this device structure can yield high fill factor arrays, the fabrication processes are very complex. For more discussion on the Spatial Light Modulators and Digital Mirror devices with hidden hinge structure, references are made to U.S. Pat. No. 5,061,049, U.S. Pat. No. 5,079,545, U.S. Pat. No. 5,105,369, U.S. Pat. No. 5,278,652, U.S. Pat. No. 4,662,746, U.S. Pat. No. 4,710,732, U.S. Pat. No. 4,956,619, U.S. Pat. No. 5,172,262, and U.S. Pat. No. 5,083,857. 
   A third category consists of 2D mirrors each mounted on a single moving flexible post. An example of a MEMS tilt platform supported by a flexible post  30  as shown in  FIG. 8 . The post  30  extends within a moat  32  or trench formed in the substrate or supporting material  34 . The post  30  can be made sufficiently long and flexible to act as an omnidirectional hinge, bending to allow the mirror  36  to be positioned with two degrees of freedom. 
   Some of the shortcomings of this design are process complexity, post flexibility, wiring, and tilt eccentricity. A few of such devices have been disclosed in U.S. Pat. No. 5,469,302, US Patent Application Publication No. US 2002/0075554 A1. Furthermore, the control for these devices becomes complex and is a substantial part of the device cost. 
   SUMMARY OF THE INVENTION 
   Some of the advantages realized in some but not necessarily all embodiments include: 
   high fill factor linear arrays. Fill factors as high as 99% may be achieved in some embodiments along one dimension; 
   almost negligible coupling between two tilt axes; 
   inexpensive and simple control. Even an open loop/look up table control is a possibility; 
   simple fabrication process can be used to fabricate the device; and 
   the cantilever part of the device can also be used for capacitive, magnetic or optical sensing of mirror position. 
   According to one broad aspect, the invention provides a micro-electro-mechanical-system (MEMS) mirror device, comprising: a mirror having a 2-dimensional rotational articulated hinge at a first end, and having a 1-dimensional rotational articulated hinge at a second end opposite the first end; a movable cantilever connected to the mirror through the 1-dimensional rotational articulated hinge; a support structure connected to the mirror through the 2-dimensional rotational articulated hinge and connected to the movable cantilever; whereby movement of said movable cantilever causes rotation of the mirror in a first axis of rotation, and the mirror is also rotatable about a second torsional axis of rotation perpendicular to said first axis of rotation. 
   In some embodiments, the 2-dimensional rotational articulated hinge comprises: a first 1-dimensional rotational articulated hinge having a first mounting point at a first end and having a second end; a second 1-dimensional rotational articulated hinge having a second mounting point at a first end and having a second end, the second end of the first 1-dimensional rotational articulated hinge being connected to the second end of the second 1-dimensional rotational articulated hinge; a third 1-dimensional rotational articulated hinge connected to the second ends of the first and second articulated 1-dimensional rotational hinges; whereby the first 1-dimensional rotational articulated hinge and the second 1-dimensional rotational articulated hinge define the first axis of rotation between the first and second mounting points, and the third 1-dimensional rotational articulated hinge and the 1-dimensional rotational articulated hinge at the second end of the mirror define the second torsional axis of rotation perpendicular to the first axis of rotation. 
   In some embodiments, each 1-dimensional rotational articulated hinge comprises a respective articulated beam having a large thickness to width aspect ratio. 
   In some embodiments, each 1-dimensional rotational articulated hinge comprises a respective articulated beam having a large thickness to width aspect ratio, the beam being formed of a material or materials selected from a group consisting of silicon, polysilicon, Silicon Nitride, Silicon dioxide, and metallic depositable materials. 
   In some embodiments, the beams are formed of a unitary construction. 
   In some embodiments, the beams, the mirror, and the movable cantilever are formed of a unitary construction. 
   In some embodiments, a device is provided in which the mirror has an angular range of motion at least 0.3 degrees in each axes. 
   In some embodiments, the device further comprises electrodes for applying electrostatic force to the mirror so as to move the mirror in the first and second axes of rotation. 
   In some embodiments, the electrodes comprise two electrodes each for applying a respective electrostatic force to the mirror so as to move the mirror in a respective direction in the second axis of rotation, and at least one electrode for applying electrostatic force to the movable cantilever so as to move the mirror in the first rotational axis. 
   In some embodiments, said at least one electrode comprises two electrodes mounted on the support structure each for applying a respective electrostatic force to the moving cantilever so as to move the mirror in a respective direction in the first rotational axis. 
   In some embodiments, said support structure comprises a first region on a first side of the movable cantilever to which is mounted a first of said two electrodes for applying electrostatic force to the movable cantilever, and a second region opposite the moving cantilever to the first region to which is mounted a second of said two electrodes for applying electrostatic force to the movable cantilever. 
   In some embodiments, the device further comprises: a rigid extension of the movable cantilever extending beyond where the support structure is connected to the movable cantilever in a direction opposite to the mirror; whereby movement of the extension of the movable cantilever causes a corresponding opposite movement of the movable cantilever. 
   In some embodiments, the device comprises a first electrode for applying electrostatic force to the mirror so as to move the mirror in a first direction in the first axis of rotation, and a second electrode for applying electrostatic force to the mirror so as to move the mirror in a second direction in the first axis of rotation. 
   In some embodiments, the first electrode for applying electrostatic force to the mirror so as to move the mirror in a first direction in the first axis of rotation is on the support structure proximal the moving cantilever, and the second electrode for applying electrostatic force to the mirror so as to move the mirror in a second direction in the first axis of rotation is on the support structure proximal the extension of the moving cantilever. 
   In some embodiments, the moving cantilever and the rigid extension of the moving cantilever are together pivotably mounted to the support structure. 
   In some embodiments, the moving cantilever and the rigid extension of the moving cantilever are together rigidly mounted to a portion of the support structure which is sufficiently flexible to allow the moving cantilever and the rigid extension of the moving cantilever to rotate in the first axis of rotation. 
   In some embodiments, moments of inertia of the rigid extension of the moving cantilever substantially balance moments of inertia of the moving cantilever and mirror. 
   In some embodiments, the device in which the mirror is made of silicon plated with a metal. 
   In some embodiments, the metal comprises Au, Al or Cu layers. 
   In some embodiments, the plurality N of devices is arranged side by side to form a 1×N MEMs array, where N≧2. 
   In some embodiments, the plurality N×M of devices is arranged in N rows of M devices thereby forming an N×M MEMs array, where N≧2 and M≧2. 
   In another embodiment, the mirror is used for optical switching and the movable cantilever is used for capacitive, magnetic or optical sensing of mirror position. 
   According to another broad aspect, the invention provides an optical switch comprising: a plurality of optical ports; a plurality of devices each adapts to switch light between a respective pair of said optical ports. 
   According to another broad aspect, the invention provides a 2-dimensional rotational articulated hinge for connection to a support structure and a device to be rotated, the hinge comprising: a first 1-dimensional rotational articulated hinge having a first mounting point at a first end and having a second end; a second 1-dimensional rotational articulated hinge having a second mounting point at a first end and having a second end, the second end of the first 1-dimensional rotational articulated hinge being connected to the second end of the second 1-dimensional rotational articulated hinge; a third 1-dimensional rotational articulated hinge having a first end connected to the second ends of the first and second articulated 1-dimensional rotational hinges and having a second end; whereby the first 1-dimensional rotational articulated hinge and the second 1-dimensional rotational articulated hinge define a first axis of rotation between the first and second mounting points, and the third 1-dimensional rotational articulated hinge defines a second torsional axis of rotation perpendicular to the first axis of rotation between the first end and second end of the third 1-dimensional rotational articulated hinge. 
   In some embodiments, each 1-dimensional rotational articulated hinge comprises a respective articulated beam having a high thickness to width aspect ratio. 
   In some embodiments, the beams are formed of a unitary construction. 
   In some embodiments, the beams are formed of a material or materials selected from a group consisting of silicon, polysilicon, Silicon Nitride, Silicon dioxide, and Metallic depositable materials. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Preferred embodiments of the invention will now be described with reference to the attached drawings in which: 
       FIG. 1A  and  FIG. 1B  provide two views of a conventional 1 dimensional MEMS mirror with an articulated suspension structure; 
       FIG. 2  shows the device of  FIG. 1  in two rotational states; 
       FIG. 3A  is a plan view of a two dimensional articulated rotational hinge provided by an embodiment of the invention; 
       FIG. 3B  illustrates a MEMS mirror featuring the two dimensional rotational articulated hinge of  FIG. 3A ; 
       FIG. 4A  is a view of a mirror with a two dimensional rotational articulated hinge and moving cantilever mounting system provided by an embodiment of the invention; 
       FIGS. 4B and 4C  provide a cutaway and side sectional view of a mirror with a two dimensional rotational articulated hinge and moving cantilever mounting system provided by another embodiment of the invention; 
       FIG. 4D  is a view of a mirror with a two dimensional rotational articulated hinge and moving cantilever mounting system provided by another embodiment of the invention; 
       FIG. 5  is a one dimensional MEMS array of devices like the device of  FIG. 4A ; 
       FIG. 6  is a view of a conventional two dimensional gimbal mirror with a supporting frame; 
       FIG. 7  is a representative sketch of a MEMS mirror with a hidden hinge structure; and 
       FIG. 8  is a representative sketch of a 2D mirror mounted on a single moving flexible post. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   A known 1D MEMS torsional mirror supported by articulated suspension springs/hinges is shown in  FIGS. 1A and 1B . This arrangement consists of a support structure  30  within which is mounted a mirror  34  connected to the support structure  30  through two articulated hinges  36 . Typically, the entire mirror plus articulated hinges arrangement is made of a single piece of silicon. The articulated hinges  36  consist of a silicon beam with a high aspect ratio of length to width thereby allowing torsional rotation. Using articulation allows a long silicon beam to be provided in a very small space. Also shown are a pair of address electrodes  38  and  40 . These would be connected to control systems capable of applying voltages to the electrode. Typically the mirror arrangement would be attached to ground. The mirror  34  can be rotated around its rotational axis (θx)  32  by applying electrostatic force on either side of the mirror using the electrodes  38 , 40 . This is shown in  FIG. 2 . Generally indicated at  50  is the mirror in a first configuration where the mirror has been rotated counter clockwise about the rotational axis  32  and generally indicated at  52  shows the same arrangement in which the mirror has been rotated clockwise about the rotational axis  32 . 
   To facilitate 2D rotation of a mirror, that is rotation in both (θx) and (θz), θz being orthogonal to the main torsional tilt (θx), an embodiment of the invention provides a 2D rotatable articulated hinge. A top view of a new articulated hinge is shown in  FIG. 3A . The 2D rotatable articulated hinge includes a first articulated hinge portion  60  and a pair of second articulated hinges  62 , 63 . Each of the second articulated hinges  62 , 63  is connectable to a support structure indicated generally at  64  and is also connected to the first articulated hinge  60 . Each of the three articulated hinges  60 , 62 , 63  is similar to the conventional articulated hinge  36  of  FIG. 1A . Namely each articulated hinge consists of a silicon beam with high aspect ratio thickness to width. The entire arrangement consisting of the three articulated hinges  60 , 62 , 63  is preferably made from a single unitary piece of silicon. In other embodiments, the arrangement is made of a deposited material such as polysilicon, Silicon Nitride, Silicon dioxide, and Metallic depositable materials. Other materials may be employed. Preferably the construction is unitary in the sense that no assembly is required. However, the beams may be made of multiple materials, for example in a layered structure. The first articulated hinge  60  allows rotation along a first torsional axis (θx) while each of the second articulated hinges  62  and  63  allow rotation about a second axis (θz). 
   Referring now to  FIG. 3B , shown is a first example use of the articulated hinge of  FIG. 3A . Here the articulated hinge is generally indicated by  70  and is connected to a mirror  72  at the opposite end of which there is another 1D articulated hinge  74 . Preferably the entire arrangement of  FIG. 3B  is made from a single piece of silicon. The arrangement as shown in  FIG. 3B  allows the mirror  72  to rotate about the main rotational axis (θx) and the additional rotational axis (θz) which is orthogonal to the main rotational axis. 
   In a preferred embodiment of the invention, the arrangement of  FIG. 3B  is employed in an apparatus illustrated by way of example in  FIG. 4A . Here, again the 2D rotation articulated hinge  70  is shown connected to the mirror  72  and 1D rotational articulated hinge  74 . A support structure is generally indicated by  76 . The 2D rotational articulated hinge  70  is connected in two places  78 , 79  to the support structure. The 1D rotational articulated hinge  74  is connected to the support structure  76  through a cantilever  80 . The cantilever is preferably simply another piece of silicon which is connected to the support structure  76  at  82  in a manner which allows substantially no rotation of this cantilever about the main rotational axis (θx). However, the cantilever  80  does have some flexibility, and in particular, the end  87  of the cantilever  80  most remote from the connection  82  to the support structure is capable of some up and down motion. To allow additional flexibility of the cantilever  80 , parts may be removed. In the illustrated example, the cantilever  80  includes a gap  89  near the mounting point  82  to support structure  76 . This reduces the amount of force necessary to cause the up and down motion of point  87 . 
   To control rotation in the torsional axis (θx), electrodes are provided  84 , 85  which operate similar to the electrodes through  38 , 40  of  FIG. 1A . This allows the control of the rotation of the mirror  72  about the main torsional axis. Also shown is an electrode  86  beneath the cantilever structure  80  which controls the up and down motion of the end  87  of the cantilever  80  most remote from the connection  82  to the support structure  76 . The up and down motion of this point  87  causes rotation of the mirror  72  about the additional rotational axis (θz), thus making the mirror tilt in both axes either simultaneously or independently. 
   Any suitable dimensions for the articulated hinges may be employed. Different numbers of articulations can be employed. The more articulations included in a given articulated hinge, the less will be the required force to cause rotation about the respective axis. In an example implementation, the dimensions of the various hinges are as follows: 
   Hinge  62  and  63 : {75 um (L), 1.5 um (W), 15 um (T), 5 um (Gap) and 3 (articulations)}; 
   Hinge  60  and  74 : {75 um (L), 1.5 um (W), 15 um (T), 5 um (Gap) and 11 (articulations)} 
   In preferred embodiments, both for the embodiment of  FIG. 4A  and subsequently described embodiments, some or all of the entire structure used to make the mirror, cantilevers and articulated hinges is connected to ground, and behaves like an electrode. For example if these components are made of doped silicon they become conductive. In this way, by applying a voltage to an electrode (for example electrode  84  of  FIG. 4A ) the mirror behaves as the second electrode without the need to deposit a second designated electrode. 
   In some embodiments, in order to provide the most flexible control over the rotation over the additional rotational axis (θz), an additional support structure is provided on top of the cantilever  80  with an additional electrode so that a force could be applied to cause the end of  87  of the cantilever  80  to move upwards. However, in some applications, this additional degree of freedom may not be required. An example of this is shown in  FIG. 4B  (and the side view in  FIG. 4C ) which is very similar to  FIG. 4A , with the exception of the additional support structure  91  and additional electrode  93  which allow an electrostatic force to be applied to the cantilever structure to move it both up and down. Note the view of  FIG. 4B  only shows half of the structure. 
   The embodiment of  FIG. 4A  has employed the use of electrodes through which electrostatic forces can be applied to control rotation in the two rotational axes. More generally, any other type of force could also be employed in either or both of these rotational axes. For example thermal, magnetic, thermal bimorph or piezo-electric forces can be employed to achieve the required rotation and control. 
   This combination of the 2D rotational articulated hinge, an articulated torsional mirror, and a moving cantilever results in a fully functional 2-D MEMS mirror. The cantilever can be deflected in either up or down directions depending on the arrangement of electrodes or force application, thus making the torsional mirror rotate about the second axis θz in either direction. For most electrostatic applications, the cantilever can be deflected downwards only to reduce the number of I/O&#39;s and control complexity. 
   A number of mirrors can be placed side by side to make a linear mirror array with minimal spacing between two mirrors. An example of this is shown in  FIG. 5  where a linear array of four 2D torsional mirrors  90 , 92 , 94 , 96  with 2D rotational articulated hinges and cantilevers is shown. An arbitrary number could be included in such an array. Another embodiment provides a two dimensional array of N×M such mirror devices. 
   One of the main advantages of the structure of  FIG. 4A  is the minimal coupling between the two tilt axes. This device structure can be used in any number of applications. It can be used as a single mirror for any appropriate application of a single or multi-array configuration. The arrangement achieves a high fill factor for mirror arrays (that is the spacing between two consecutive mirrors in an array is minimized) and is very simple to fabricate. The spacing between two mirrors can be as low as few microns or as limited by microfabrication processes. 
   Another embodiment of the invention will now be described with reference to  FIG. 4D . This embodiment is very similar to that of  FIG. 4A . This embodiment includes an additional cantilever  97  mounted over further support structure  98  to which an additional electrode  99  is affixed. Cantilever structures  80  and  97  together pivot about mounting points to the support structure  76 . In operation, with this arrangement an electrostatic force can be applied between the electrode  87  and cantilever  80  to move point  87  in a downward direction. Similarly, an electrostatic force can be applied between electrode  99  and the underside of cantilever  97  to cause the end  87  of cantilever  80  to move upwards. Thus, the arrangement of  FIG. 4D  provides the same flexibility as the arrangement of  FIG. 4B  provided earlier in that both upwards and downwards mobility in the second axis of rotation (θz) is possible. The attachment of the cantilever structure composed of combined elements  80  and  97  to the support structure can either be pivotable, or rigid. In the event of a rigid connection, the support structure  76  would need to have some flexibility to allow the upwards and downwards motion of the two cantilever portions on either side of support structure  76 . 
   In another embodiment, the arrangement of  FIG. 4D  is implemented with a balanced cantilever structure. With this embodiment, the moments of inertia on either side of the support structure  76  are substantially equalized. In one embodiment, this is achieved by making the second cantilever portion  97  substantially longer than the cantilever portion  80  such that the moments of inertia of the second cantilever portion  97  about the support structure  76  offsets the moment of inertia of the components on the other side of the support structure. 
   The device can be fabricated with existing MEMS fabrication processes. A few of the suitable processes that are commercially available are “Optical IMEMS” R  from Analog Devices Inc (see Thor Juneau, et al, 2003, ‘Single-Chip 1×84 MEMS Mirror Array For Optical Telecommunication Applications’, Proceeding of SPIE, MOEMS and Miniaturized Systems III, 27–29 January 2003, Vol. 4983, pp. 53–64.), SOI MUMPS (http://www.memsrus.com/figs/soimumps.pdf) from Cronos (MEMScAP subsidiary). A custom process can also be put together to fabricate the device. 
   It is to be understood that in a system application, a control system would be provided to control the rotation of the mirror in the two degrees of freedom. This would be controlled through the proper application of the forces through the various electrodes. The control system will preferably be an open loop system with a voltage look-up table for various tilt position or a closed loop system with capacitance or optical sensing. 
   The mirrors in the above employed embodiments need to have a reflective coating, for example of Au, Al, or Cu in one of more layers. The mirrors are used to perform the main switching of beams of light. However, it is to be understood that the cantilever portion could also have a reflective coating. The cantilever and/or mirror components could be used for capacitive or optical sensing. For example, the mirror components might be used for switching, while the cantilever components are used to perform sensing with signals generated to perform feedback control over the orientation of the mirrors in the additional rotational axis (θz). 
   Numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein.