Abstract:
Tabs or stops are integrated into a membrane structure to prevent its snapdown. Features comprising two surfaces separated by a distance equal to the maximum desired range of movement are produced. When the two surfaces contact, the motion of the structure is arrested or greatly diminished by increasing its rigidity. For an electrostatically actuated MEMS structure, these features can be used to limit the range of motion such that pull-in or snapdown is avoided, greatly enhancing the reliability of the device. One key design feature is that the two contacting surfaces are maintained at the same electrical potential avoiding problems associated with electrostatic cavity discharge.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    Microelectromechanical system (MEMS) membranes are used in a spectrum of optical applications. For example, they can be coated to be reflective and then paired with a stationary mirror to form a tunable Fabry-Perot (FP) cavity/filter. They can also be used as stand-alone reflective components to define the end of a laser cavity, for example. Optical surfaces, such as curved, binary, or diffractive surfaces, can be fabricated on the membranes to create movable mirrors and lenses.  
           [0002]    The MEMS membranes are typically produced by depositing a membrane structure over a sacrificial layer, which has been deposited on a support structure. This sacrificial layer is subsequently etched away, or otherwise removed, to produce a suspended membrane structure in a release process. Often the membrane layer is a metal or silicon compound and the sacrificial layer can be polyimide, for example.  
           [0003]    Typically, membrane deflection is achieved by applying a voltage between the membrane and a fixed electrode on the support or an air bridge structure, for example. Electrostatic attraction moves the membrane in the direction of the fixed electrode as a function of the applied voltage. This results in changes in the reflector separation of the FP filter, cavity length in the case of a laser, or lens/mirror position.  
         SUMMARY OF THE INVENTION  
         [0004]    One chronic problem associated with MEMS membranes in general is stiction. Specifically, if deflected sufficiently to contact an adjoining surface, the membranes can “snap-down” or adhere to that surface because of atomic-level forces, for example. One example is a suspended membrane structure that is designed to provide out of plane motion using electrostatic actuation. If the applied voltage exceeds that required to deflect the membrane to its instability point (roughly one third of the initial gap), then the membrane can snapdown. If the atomic-level bonding forces exceed the restoring force of the membrane structure, the membrane will remain “stuck” to the fixed electrode. Another scenario that produces a similar result is triggered by an acceleration load, when the load is sufficient to deflect the membrane to its full extent, as in a shock test.  
           [0005]    One path to solving stiction problems includes the addition of surface features and/or coatings to the membrane, or the stationary surface adjacent the membrane, to allow the membrane to recover from a snapdown event. The contact area between the two surfaces can be reduced so that the bonding forces are reduced. Roughening the surfaces is an example of this approach as is producing discrete protrusions on either surface. A number of risks, however, are inherent with this solution. Surface roughening is not appropriate for all applications. Stiction bumps can become damaged in the event of snapdown since the electrical potential across the electrostatic cavity will be discharged through the small contact area of the bump. This can lead to bump damage or bump welding.  
           [0006]    Another path focuses on reducing the surface energy of contacting surfaces by using a chemical treatment. Antistiction coatings, however, do not appear to be a robust solution, merely incrementally improving the survivability of membranes to snapdown—the coatings can also be relatively slow acting. They may also be incompatible with required optical coatings, such as dielectric antireflective (AR) coatings or highly reflective (HR) coatings for example, or damage active semiconductor devices because of organic content.  
           [0007]    The present invention concerns the integration of tabs or stops that prevent snapdown of a deflectable membrane structure. Features comprising two surfaces separated by a distance equal to the maximum desired range of movement are produced. When the two surfaces contact, the motion of the structure is arrested or greatly diminished. For an electrostatically actuated MEMS structure, these features can be used to limit the range of motion such that pull-in or snapdown is avoided, greatly enhancing the reliability of the device. One key design feature is that the two contacting surfaces can be maintained at the same, or near the same, electrical potential avoiding problems associated with electrostatic discharge.  
           [0008]    The above and other features of the invention including various novel details of construction and combinations of parts, and other advantages, will now be more particularly described with reference to the accompanying drawings and pointed out in the claims. It will be understood that the particular method and device embodying the invention are shown by way of illustration and not as a limitation of the invention. The principles and features of this invention may be employed in various and numerous embodiments without departing from the scope of the invention. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]    In the accompanying drawings, reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale; emphasis has instead been placed upon illustrating the principles of the invention. Of the drawings:  
         [0010]    [0010]FIG. 1 is a perspective, exploded view of a tunable filter comprising an optical membrane device, according to the present invention;  
         [0011]    [0011]FIG. 2 is an elevation view of the distal side of the inventive optical membrane device showing the optical port;  
         [0012]    [0012]FIG. 3 is a perspective of view of another implementation of the membrane structure with tabs according to the present invention;  
         [0013]    [0013]FIG. 4 is a close-up perspective view showing the inventive tab structure; and  
         [0014]    [0014]FIGS. 5A through 5E illustrate a process for fabricating the membrane structure with the tabs. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0015]    [0015]FIG. 1 shows a Fabry-Perot tunable filter  100  comprising an optical membrane device  110 , which has been constructed according to the principles of the present invention.  
         [0016]    Generally, in the FP filter device  100 , a spacer device  114  that separates the mirror device  112  from the membrane structure  214  to thereby define a Fabry-Perot (FP) cavity.  
         [0017]    The optical membrane device  110  comprises handle material  210 . Preferably, the handle material is wafer material such as from a silicon wafer, which has been singulated into the illustrated device.  
         [0018]    An optical membrane layer  212  is added to the handle wafer material  210 . The membrane structure  214  is formed in this optical membrane layer  212 . This optical membrane layer is currently between 5 and 10 micrometers in thickness. Preferably, it is between 6 and 8 micrometers in thickness.  
         [0019]    An insulating layer  216  separates the optical membrane layer  212  from the handle wafer material  210 . During manufacture, this insulating layer functions as a sacrificial/release layer, which is partially removed to release the membrane structure  214  from the handle wafer material  210 . In the preferred embodiment, this insulating layer is between 3 and 6 micrometers in thickness. In the current embodiment, it is greater than 3 micrometers, preferably greater than 3.5 μm, in thickness, but less than 5 micrometers.  
         [0020]    In a current embodiment, the membrane layer  212  is silicon. Currently, the membrane layer is manufactured from a silicon wafer that has been bonded to the insulating layer  216  under elevated heat and pressure. Other alternatives are polycrystalline silicon or silicon nitride, which have been deposited on the insulating layer.  
         [0021]    In the current embodiment, the membrane structure  214  comprises a body portion  218 . The optical axis  10  of the device  100  passes concentrically through this body portion  218  and orthogonal to a plane defined by the membrane layer  212 . A diameter of this body portion  218  is preferably 300 to 600 micrometers, currently it is about 500 micrometers.  
         [0022]    Tethers  220  of the membrane structure  214  extend radially from the body portion  218  to the membrane structure&#39;s outer portion  222 , which comprises the ring where the tethers  220  terminate. In the current embodiment, a spiral tether pattern is used.  
         [0023]    An optical coating dot  230  is typically deposited on the body portion  218  of the membrane structure  214 . In the implementation as a Fabry-Perot filter or other reflecting membrane, the optical dot  230  is preferably a highly reflecting (HR) dielectric mirror stack. This yields a highly reflecting, but low absorption, structure that is desirable in, for example, the manufacture of high finesse Fabry-Perot filters. In applications relying on transmission, both sides of the membrane structure  214  are a typically coated with dielectric AR coatings.  
         [0024]    In the illustrated embodiment, artifacts of the manufacture of the membrane structure  214  are etchant or release holes  232 . These holes allow an etchant to pass through the body portion  218  of the membrane structure  214  to assist in the removal of the insulating layer  216  during the release process.  
         [0025]    In the illustrated embodiment, metal pads  234  are deposited on the proximal side of the membrane device  110 . These are used to solder bond, for example, the spacing structure  114  onto the membrane device  110 . Of course, it could be avoided if the spacing structure  214  is formed to be integral with the membrane device  110 , instead of integral with the mirror device  112 , as shown.  
         [0026]    Bond pads  235  are useful when installing the filter  100  on a micro-optical bench, for example. Also provided are a membrane layer wire bond pad  236  and a handle wafer wire bond pad  238 . The membrane layer bond pad  236  is a wire bonding location for electrical control of the membrane layer  212 . The handle wafer bond pad  238  is a wire bond pad for electrical access to the handle wafer material  210 .  
         [0027]    According to the present invention, tabs or stops  250  are provided that prevent snapdown of a deflectable membrane structure  214  against the handle material  210 . The tabs  250  comprise a feature that is connected to the membrane structure  214 , or adjacent the structure, that comes into contact with another portion of the membrane layer  212  when the membrane structure  214  has deflected a predetermined, maximum desirable, distance.  
         [0028]    In the embodiment of FIG. 1, the tabs  250  are attached to the tethers  220  of the membrane structure  210  and extend to overhang the outer portion  222  of the membrane structure  214 . When fully deflected, the tabs  250  of the membrane structure contact the outer portion to thereby increase the rigidity of the structure, thus preventing further deflection and snapdown against the handle wafer  210 .  
         [0029]    [0029]FIG. 2 shows an optical port  240 . It is provided, in some embodiments, extending from a distal side of the handle wafer material  210  to the membrane structure  214 . Whether or not this optical port  240  is required depends upon the transmissivity of the handle wafer material  210  at the optical wavelengths over which the membrane structure  110  must operate. Typically, with no port, the handle wafer material along the optical axis must be AR coated.  
         [0030]    Specifically, the optical port  240  has generally inward sloping sidewalls  244  that end in the port opening  246 . As a result, looking through the distal side of the handle wafer material, the body portion  218  of the membrane structure  214  is observed, with a concentric optical coating  230 .  
         [0031]    [0031]FIG. 3 shows exemplary tab  250  from the side. Specifically, the tabs  250  each comprise a base or vertical offset portion  252  and an overhanging or cantilevered portion  254 . The cantilevered portion  254  extends in the direction of the adjacent tether or the outer portion  222 . The offset portion  252  sets the vertical distance between the overhanging portion  254  and the adjacent tether or outer portion and thus the amount the membrane will deflect before experiencing increased rigidity because of the tabs  250 .  
         [0032]    [0032]FIG. 4 shows another embodiment of the membrane structure  214  with tabs  250  extending between tethers  220 . For example, tab  250 - 1  is fabricated to be connected to tether  220 - 1  and engages tether  220 - 2  upon full out-of-plane deflection of membrane structure  214 .  
         [0033]    [0033]FIGS. 5A through 5E illustrate a process for fabricating a membrane structure  214  with tabs or stops  250 , according to the principles of the present invention.  
         [0034]    With reference to FIG. 5A, the exemplary process steps are tailored for a device formed using an silicon-on-insulator (SOI) wafer as the starting material, with a covering of oxide  510 . The buried oxide layer  216 , between the handle material  210  and the membrane layer  212 , is used as the sacrificial release layer.  
         [0035]    The membrane layer  212  is patterned to define the membrane body  218  and the tethers  220 . Reactive ion etching (RIE) or deep RIE can be used for this purpose. The etch stops on the buried oxide layer.  
         [0036]    Referring to FIG. 5B, a conformal layer  514  is used to fill the vias  512 , which define the tethers  220  and which the tabs  254  will bridge in some embodiments. TEOS is a good candidate for this purpose since it is very conformal and can be etched away during the release step. The width of the vias  512  is limited by the thickness of TEOS that can be deposited in some implementations. The vias  512  can be less than twice the deposited thickness so that they are completely filled. In one embodiment, the thickness of the deposited film sets the gap between the tabs and the stops or the thickness of the base or offset  252 . If film thicknesses exceeding the desired gap height are required for refill purposes, then it is possible to use a blanket etch or polishing process (or combination of the two) to thin the film deposited on the top surface of the wafer.  
         [0037]    Referring to FIG. 5C, the deposited oxide layer  514  is patterned to reveal anchor points  518  to which the tabs will be anchored.  
         [0038]    Referring to FIG. 5D, a tab layer  520  is deposited conformally over the wafer. LPCVD polysilicon deposition is capable of providing conformal coverage for the feature sizes indicated. A two micrometer polysilicon layer is used.  
         [0039]    Referring to FIG. 5E, the deposited polysilicon  520  is patterned to define the tab structures  250 . RIE etching is currently used for this purpose. The geometry of the tabs  250  is typically tailored to minimize the contact area between the tab  254  and the stop region that they contact. An optical port  101  is also formed, along with an electrical via  522  to the handle material  210 .  
         [0040]    Finally, the membrane structure with the attached tabs  250  is released using a hydrofluoric acid to etch away the sacrificial oxide layer  216  and oxide layer  514 .  
         [0041]    The current method allows stops to be attached to the top of the membrane structure. This has two primary advantages: 1) the processing remains on the top surface of the MEMS device allowing more flexibility in setting the maximum range of movement as well as the choice of materials and ordering of process steps; and 2) the tabs and the stops can be produced such that they are at the same electrical potential, avoiding large current flows when contact is made.  
         [0042]    While this invention has been particularly shown and described with references to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims. For example, the description has been relative embodiments where the out-of-plane movement that is to be controlled is in the direction of the support structure. Movement away from the support structure is controlled by tabs that are connected to the outer portion of the membrane structure and that are cantilevered over the released portion of the structure, such as the body or tethers.