Abstract:
The present invention provides a compliant mechanism that can be used to make a variety of devices, such as tunable optical devices that are more reliable, more cost effective and/or exhibit better performance than prior art devices.

Description:
RELATED APPLICATIONS  
       [0001]    This application claims priority to U.S. Provisional Application No. 60/284,943, filed Apr. 20, 2001. This application is also a continuation-in-part of U.S. patent application Ser. No. 10/085,143 (Attorney Docket No. SMT-0039), filed Mar. 1, 2002 entitled “Compliant Mechanism and Method of Forming Same,” which is a continuation-in-part of U.S. patent application Ser. No. 09/811,612 filed Mar. 20, 2001, entitled “Electrostatically-Actuated Tunable Optical Components Using Entropic Materials”, which is a continuation-in-part of U.S. patent application Ser. No. 09/766,687 filed Jan. 19, 2001, entitled “Tunable Fabry-Perot Interferometer Using Entropic Materials.” U.S. patent application Ser. No. 09/811,612 also claims priority to U.S. Provisional Application Nos. 60/190,111, filed Mar. 20, 2000 and 60/211,529, filed Jun. 15, 2000. U.S. patent application Ser. No. 10/085,143 also claims priority to U.S. Provisional Application Nos. 60/284,943, filed Apr. 20, 2001 and 60/303,772, filed Jul. 10, 2001. All of the above applications are hereby incorporated by reference in there entirety. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    This invention relates to a mechanism that can be used to make a variety of devices where precise positioning of a device element is desirable. Examples include tunable optical elements such as mirrors, lenses, filters, prisms and diffraction gratings for use in tunable optical devices.  
           [0004]    2. Background of the Related Art  
           [0005]    There is a continuing need for precise positioning of optical elements in devices for various applications, such as optical systems including imaging systems and telecommunications networks. Such precise positioning offers benefits such as tunable devices and simplified packaging.  
           [0006]    Existing technologies for precise positioning of optical elements are either to costly, unreliable, or do not exhibit the performance needed for present and/or future systems requirements.  
         SUMMARY OF THE INVENTION  
         [0007]    An object of the invention is to solve at least the above problems and/or disadvantages and to provide at least the advantages described hereinafter.  
           [0008]    The present invention provides a compliant mechanism that can be used to make a variety of devices that are more reliable, more cost effective and/or exhibit better performance than prior art devices. The present invention further provides an actuated compliant mechanism for precisely positioning optical elements in optical devices.  
           [0009]    Additional advantages, objects, and features of the invention will be set forth in part in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from practice of the invention. The objects and advantages of the invention may be realized and attained as particularly pointed out in the appended claims. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]    Preferred embodiments of the invention will be described in detail with reference to the following drawings in which like reference numerals refer to like elements wherein:  
         [0011]    [0011]FIG. 1 is a cross-sectional view of a compliant mechanism, in accordance with an embodiment of the present invention;  
         [0012]    [0012]FIG. 2 is a cross-sectional view of an actuated device utilizing the compliant mechanism of FIG. 1, in accordance with one embodiment of the present invention;  
         [0013]    [0013]FIGS. 3A and 3B are plan views of a preferred embodiment of first and second sets of electrodes for implementing the first and second actuators, respectively, shown in FIG. 2;  
         [0014]    [0014]FIGS. 4A through 4E are cross-sectional views of steps in one preferred method of fabricating the compliant mechanism  100  of FIG. 1; and  
         [0015]    [0015]FIG. 5 is a plan view according to one embodiment. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0016]    FIGS.  1  shows one embodiment of the compliant mechanism  100  of the present invention. The compliant mechanism  100  includes an island  105 , which is suspended from a frame  115  using a compliant member  110 , which is attached to the frame  115  and the island  105 . The junction or interface where the island  105  meets the compliant member  110  is the first interface  120 . The junction or interface where the compliant member  110  meets the frame  115  is the second interface  125 . The exterior surface of the frame  115  includes an outer frame edge  130 .  
         [0017]    The island  105  is preferably formed from a material that is more rigid than the compliant member  110 , and preferably has a higher Young&#39;s modulus than the compliant member  110 . Examples of such materials include polymers and other organic materials. The compliant member  110  is preferably an elastic material, preferably a polymer with a Young&#39;s modulus smaller than the Young&#39;s modulus of the island  105  and frame  115 , and preferably has a relatively high elastic limit. In one preferred embodiment, the Young&#39;s modulus of the compliant member  110  is preferably less than 1 G Pascal. In embodiments where it is desirable to maintain the rigidity of the island  105  while the compliant member  110  is deformed, it is preferred that the Young&#39;s modulus of the island  105  is at least two orders of magnitude larger than the Young&#39;s modulus of the compliant member  110 . Materials such as elastomers can offer Young&#39;s modulus as much as five orders of magnitude less than the Young&#39;s modulus of a typical silicon substrate. The frame  115  is preferably formed from a rigid material, which may be the same material used for the island  105 .  
         [0018]    To have the capability to achieve large motion or displacements, the compliant member  110  should preferably display linear-elastic behavior over a wide range of frequencies and over a substantial portion of the deformation range at low actuation forces. Entropic materials, such as elastomers, aerogels and other long-chained polymers, are one type of material which provides such behavior.  
         [0019]    The term “island” is used for convenience in describing the invention and the preferred embodiments herein. The term does not imply isolation or separation from all elements of a device. Instead, it describes an element that is sufficiently separated from a support structure, such as the frame  115 , so that the element can move relative to such support structure. The element may be movably or rigidly attached to one or more other elements in a device in certain embodiments.  
         [0020]    An advantageous feature of certain embodiments of the invention is that the island is capable of having individual actuators and/or sensor elements placed on the island, so that each island may be individually actuated or sensed. Providing one or more rigid, discretely controlled islands with multiple actuators and/or sensors on each island allows for precision movement and/or sensing of each island. Such embodiments are advantageous for use with optical elements, but also can be used in other applications where precession movement and/or sensing is needed.  
         [0021]    In general, the island  105  is formed of a first material, the compliant member  110  is formed of a second material and the frame  115  is formed of a third material. The first, second, and third materials may be the same, similar, or different materials or any combination thereof. It should be noted, however, that one preferred embodiment includes forming the island  105  and the frame  115  from a single wafer of etchable material causing the island  105  and the frame  115  to consist of the same material.  
         [0022]    The island  105  has what is generally referred to as a “neutral position” which is the position the island  105  tends towards when not subjected to external forces. Thus, the island  105  tends to remain in a neutral position until an external force is applied to the island  105  which displaces the island  105  from the neutral position.  
         [0023]    An optical component  420  may be supported by the island  105  of the compliant mechanism  100 . The optical component  420  can include any variety of optical components or elements such as a fully reflective mirror, a partially reflective mirror, a hologram, a diffraction grating, a lens, prism, filter, various waveplates, etc. Note that other components requiring precise positioning can also be placed on the island  105  in substitution for, or in addition to the optical component  420 .  
         [0024]    The components of the compliant mechanism can be formed from a variety of materials. As described above, the island  105  is formed of a first material, which can be opaque, translucent, or transparent to electromagnetic radiation. The first material may also be an electrical conductor or an electrical insulator. Furthermore, the first material may be rigid or flexible. The optical component  420  may be intrinsic with the island  105  or affixed to the island  105  by any of various means well known in the art, such as bonding by various adhesives, or metallic bonding such as soldering, etc. The optical component  420  may also be formed using standard silicon or glass fabrication/processing techniques.  
         [0025]    An aperture  525  (shown by dashed lines) may optionally extend through the optical component  420 , the island  105  and the compliant member  110 . The aperture  525  may have any symmetric or asymmetric shape. Alternatively, in lieu of forming an aperture  525  through the island  105  and the optical component  420 , the island  105  may be formed from a material that is transparent to the wavelength of the light that will be impinging on the island  105  to allow light to pass through the island as desired.  
         [0026]    [0026]FIG. 2 is a cross-sectional view of an actuated device  200 , in accordance with one embodiment of the present invention. The actuated device  200  includes a compliant mechanism  100 , which is disposed adjacent to an actuator support  210 . The compliant mechanism  100  includes an island  105 , which is attached to and supported by a compliant member  110 . The compliant member  110  is also attached to a frame  115 . Attached to the compliant member  110 , underneath the island  105 , is a first actuator  220 . The first actuator  220  can include any number and configuration of magnetic, electrostatic, or mechanical force transducers, but are preferably electrodes configured for electrostatic actuation.  
         [0027]    In the embodiment shown, the compliant member  110  has a prescribed thickness. The compliant member  110 , however, could have any thickness. For example, it could have a thickness approximately equal to a width of the trench. The thickness of the compliant member  110  is preferably used to maintain the longitudinal stiffness. For example, as the thickness of the compliant member  110  is increased, there is increased longitudinal stiffness due to the properties of shear deformation. Thus, the compliant member  110  behaves more in shear mode as it becomes thicker.  
         [0028]    In the embodiment shown, an optical component  420  is supported by the island  105 . The optical component  420  can be a mirror, grating, or any other type of optical component. For example, the optical component  420  can be a dielectric stack mirror deposited onto the top surface of the island  105 . The optical component  420  can also be formed intrinsically with the island  105 .  
         [0029]    The actuator support  210  includes an actuator frame  230  onto which is attached a second actuator  240 . The second actuator  240  can include any configuration of force transducers which cooperatively function with the first actuator  220 , but are preferably electrodes for electrostatic actuation. The compliant mechanism  100  is attached to the actuator support  210  by spacers  250 . The spacers  250  serve to maintain a predetermined spacing between the second actuator  240  and the first actuator  220  when the actuators are not actuated.  
         [0030]    In operation, the first and second actuators  220  and  240  can be controlled to apply a force to the island  105 , thereby moving the island  105 . The compliant member  110  exerts a restoring force to the island  105 , which tends to urge the island  105  back into alignment with the frame  115  when the actuating force is removed.  
         [0031]    In a preferred embodiment, the first and second actuators  220  and  240  comprise electrodes that are configured to generate an electrostatic force when a command signal is applied to the first and second actuators  220  and  240 . The command signal applied to the first and second actuators  220  and  240  can be configured to create a repulsive or an attractive electrostatic force between the first and second actuators  220  and  240 .  
         [0032]    A feature of certain embodiments the present invention is that the actuation mechanism, comprised of the first and second actuators  220  and  240  in the embodiment of FIG. 2, is on a side of the compliant mechanism  100  opposite the optical component  420 . This effectively separates the “drive cavity”, which is the area between the compliant mechanism  100  and the actuator support  210 , from any optical cavity that may be formed with the optical component  420 . For example, the optical component  420  may be a mirror, and a second mirror may be positioned in a parallel relationship with optical component  420  to form a resonant optical cavity. The design of the actuated device  200  allows for independent optimization of the actuation mechanism and/or the optical cavity.  
         [0033]    The island  105  represents a suspended mass, and the compliant member  110  represents a spring supporting the mass represented by the island  105 . Thus, the island  105  and compliant member  110  combination is a mechanically resonant structure.  
         [0034]    The mass of the island  105  and/or the spring constant of the compliant member  110  can be adjusted to obtain a predetermined resonant frequency. This can be useful if, for example, one wants to avoid movement of the island when the entire actuated device is physically moved at relatively low frequencies.  
         [0035]    One way to adjust the resonant frequency of the island  105  and compliant member  110  combination is to adjust the mass of the island  105 . However, there may be a limit as to how small the island  105  can be made because of the physical size of the optical component  420  that is supported by the island  105 . As shown in FIG. 2, one way of removing mass from the island  105  is to create voids  260  (represented by dashed lines) in the island  105  by etching trenches or wells in the island  105 . The voids  260  may be created by any means known in the art.  
         [0036]    As discussed above, one of the preferred actuation methods is electrostatics. This is accomplished by making the first actuator  220  on the compliant mechanism  100  and the second actuator  240  on the actuator frame  230  electrodes that are configured to receive command signals that, in turn, generate attractive electrostatic forces between the actuators  220  and  240 .  
         [0037]    [0037]FIGS. 3A and 3B are plan views of a preferred embodiment of first and second sets of electrodes  300  and  400  for implementing first and second actuators  220  and  240 , respectively. In this embodiment, three electrodes  300 A- 300 C make up the first set of electrodes  300 , and a single common electrode  400 A is used for the second set of electrodes  400 . It should be appreciated that this arrangement could be reversed, so that the three electrodes  300 A- 300 C could be placed on the actuator frame  230 , while the common electrode  400 A is placed on the compliant member  110 , underneath the island  105 .  
         [0038]    A particularly advantageous feature of certain preferred embodiments is to have three separately controlled actuator elements in the apparatus, each of which can be used to apply an independent force to a portion of the island  105 . As shown in FIG. 3A, an optimal configuration is to employ three electrodes underneath the island  105 . The voltage applied to each independent electrode generates an independent force, generally perpendicular to the surface of the island  105 , centered at the geometric mid-point of the electrode segment. Each electrode segment has a distinct center of force. The electrode segments can be arranged such that these three centers of force are distributed advantageously across the surface of the island  105 , enabling the actuators to actuate the island  105  to move into any desired position. The use of three centers of force provides for accurate, deterministic positioning for systems with three degrees of freedom.  
         [0039]    As discussed above, first and second sets of electrodes  300  and  400  are configured to generate an electrostatic force when a command signal (voltage) is applied thereto. The command signal can be configured to create a repulsive or an attractive electrostatic force between the sets of electrodes  300  and  400  However an attractive electrostatic force is the preferred mode of operation.  
         [0040]    During displacement, up and down motion of the island  105 , and therefore the spacing of the gap between the first and second actuators  220  and  240 , can be controlled by applying a voltage between the three electrodes  300 A- 300 C and the counter-electrode  400 A. The three-electrode structure shown in FIG. 3A for the first set of electrodes  300  allows for control of the tilt of the island  105 , and therefore the optical component  420  mounted thereto, with respect to the frame  115 . This is accomplished by selectively applying a stronger voltage to one or more of the three electrodes  300 A- 300 C. Although, in this embodiment, three electrodes are used for the first set of electrodes  300 , a different electrode pattern and a different number of electrodes can be used while still falling within the scope of the present invention.  
         [0041]    In order to control tilt and gap spacing of the island  105 , it is preferable to have a sensing mechanism that will indicate how much tilt and gap spacing is present. In one embodiment, the tilt and gap spacing is determined using optical feedback.  
         [0042]    [0042]FIGS. 4A through 4E are cross-sectional views of steps in one preferred method of fabricating the compliant mechanism  100  of FIG. 1. It should be appreciated that, although FIGS.  4 A- 4 E illustrate the fabrication of a single compliant mechanism  100 , the fabrication process is designed so that a plurality of compliant mechanisms can be fabricated simultaneously on a single wafer. The method is preferably implemented with standard photolithographic processing techniques.  
         [0043]    [0043]FIG. 4A through 4E provide an example of a particularly advantageous feature of preferred embodiments of the invention. Embodiments of the present invention are particularly suitable for manufacturing in quantity by manufacturing multiple compliant mechanisms in parallel from a single wafer of material, such as silicon. Further, embodiments of the present invention provide for manufacturing each layer separate from the other layer and subsequently assembling the layers into multi-layer mechanisms. Such separate manufacture of each layer allows for materials and processing steps to differ substantially in each layer.  
         [0044]    As shown in FIG. 4A, the fabrication method begins by providing a double-side polished silicon wafer  500 , which is preferably approximately half a millimeter thick. As discussed above, although silicon is used in one preferred embodiment of the present invention, any of the materials known in the art that are compatible with micro-electromechanical manufacturing techniques may be used. The silicon wafer  500  has a first side  510  and a second side  520  Both of these sides are preferably polished.  
         [0045]    Next, as shown in FIG. 4B, the second side  520  of the silicon wafer is coated with a compliant material layer  530 , preferably by spin coating to a desired thickness. Then, as shown in FIG. 4C, the first side  510  of the silicon wafer is coated with a photoresist layer  540 , which is patterned to form an etch mask with openings  550  over the locations of the trench that will eventually be formed.  
         [0046]    Then, as shown in FIG. 4D, a continuous trench  560  is etched down to the layer of compliant material  110 , preferably using deep reactive ion etching (DRIE). The layer of compliant material  110  acts as an etch stop. The etching of the trench  560  forms the island  105  and the frame  115  of the compliant mechanism  100 . Once the trench  560  is etched, the photoresist layer  540  is removed, as shown in FIG. 4E.  
         [0047]    Although not shown, it should be appreciated that an optical component, or any other type of component, may be fabricated on the portion of the silicon wafer  500  that will eventually become the island  105 , using any fabrication techniques known in the art, preferably prior to coating the silicon wafer  500  with the photoresist layer  540 . Further, if the compliant mechanism  100  is to be used in an actuated device, such as the actuated device  200  shown in FIG. 2, electrodes may be fabricated on the compliant member  110 , using any fabrication techniques known in the art.  
         [0048]    It will be understood that persons of skill in the art might consider addtional steps such as, for example, adding another layer between the compliant material and the silicon wafer  500 .  
         [0049]    In certain preferred embodiment the thickness of the compliant layer can be selected so that the thickness is approximately as large or larger than the wideth of the trench. This will enhance the longitudinal stiffness of the device by causing motion perpendicular to the island increasingly to generate sheer deformation in the compliant layer. This has advantages described in the co-pending applications.  
         [0050]    In a preferred embodiment of a tunable filter, it is desirable not to have compliant material such as elastomer in the optical path of the device. This can be accomplished a number of ways, such as, for example, by exerzing the material after it is placed on the substrate in the regions where it is not desrired. Alternatively, one can take advantage of the properties of materials as elastomer by curing the compliant materials only in the areas where it is desirable to have compliant material. Accordingly, after elastomer in spun on or otherwise applied, a mask or other technique can be used to avoid curing the region of the compliant material near the optical path. Effectively, this leaves an aperture through the elastomer.  
         [0051]    As shown in FIG. 5, an optical apature is preferably formed on the elastomer. Under the elastomer, an outer perimeter of the island and an inner perimeter of the frame are shown. Between the inner and outer perimeters is the trench.  
         [0052]    The foregoing embodiments and advantages are merely exemplary and are not to be construed as limiting the present invention. The present teaching can be readily applied to other types of apparatuses. The description of the present invention is intended to be illustrative, and not to limit the scope of the claims. Many alternatives, modifications, and variations will be apparent to those skilled in the art. In the claims, means-plus-function clauses are intended to cover the structures described herein as performing the recited function and not only structural equivalents but also equivalent structures.