Abstract:
A MOEMS Fabry-Perot tunable filter includes an optical membrane structure. Two electrostatic cavities are provided, one on either side of the membrane structure. As a result, electrostatic attractive forces can be exerted on the optical membrane to enable deflection in either direction, typically, along the optical axis. This is useful in calibrating the tunable filter during operation to a λ∘ set point. It is also useful in controlling the membrane to avoid unstable operation.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    Micro-optical electromechanical system (MOEMS) 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. They can be used as tip/tip-tilt mirrors in switches  
           [0002]    The MOEMS 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 to produce a suspended membrane structure in a release process. Often the membrane layer is a 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 structure. 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 or cavity length in the case of a laser.  
         SUMMARY OF THE INVENTION  
         [0004]    In typical operation in an FP filter, a mode within an order of operation of the tunable filter is scanned across some spectral band of interest. For example, in dense wavelength division multiplexing (DWDM) systems such as defined by the current ITU grid, channel slots are defined between approximately 1490 nanometers (nm) and 1620 nm, for the L, C, and S bands. If the filter were designed to scan, for example, the C band, it would be desirable to place a mode of the filter at 1570 nm at a zero Volts condition, for example, and then scan that mode by deflecting the membrane to approximately 1530 nm by ramping the electrostatic drive voltage. Similarly, if a full L, C and S band scan is to be performed, it would be desirable to spectrally place the mode of the filter at approximately 1490 nm for zero Volts and then scan it to approximately 1611 nm by ramping the voltage.  
           [0005]    For reasons associated with MOEMS filter fabrication, however, the operation of the tunable filter is slightly more complex. The spacing between orders of operation for a filter is termed the filter&#39;s free spectral range (FSR). This FSR is typically determined by a spacer layer(s) between the optical membrane and stationary reflector. The location of the filter&#39;s modes typically depends upon the curvature of the reflectors of the FP cavity. In order to determine the location of the filter passband at rest, when there is no electrostatic field in the cavity, both the membrane/reflector spacing and reflector curvatures need to be specified to high levels of accuracy, for example, within a few nanometers.  
           [0006]    Another related issue concerns the physical distance over which the membrane can be deflected. There are typically limitations in the voltages that are available to electrostatically deflect the membrane. For example, many times the systems are designed to run only on a few Volts. Moreover, there can be limitations associated with the stability of the membrane. For example, general electrostatic deflection cavity design parameters specify that a membrane should typically only be deflected over approximately one third of the cavity width to avoid unstable operation.  
           [0007]    The present invention concerns a MOEMS Fabry-Perot tunable filter. As is common, it includes an optical membrane structure. Two electrostatic cavities are provided, however, one on either side of the membrane structure. As a result, electrostatic attractive forces can be exerted on the optical membrane to enable deflection in either direction, typically, along the optical axis. This is useful in calibrating the tunable filter during operation to a desired λ∘ set point. It is also useful in controlling the membrane to avoid unstable operation.  
           [0008]    In general, according to one aspect, the invention concerns a triple electrode MOEMS Fabry-Perot tunable filter. It comprises an optical membrane, including a membrane electrode. A first stationary electrode supports an electrical field to deflect the optical membrane in a first direction and a second stationary electrode supports an electrical field to deflect the membrane in a second direction.  
           [0009]    In one embodiment, the membrane electrode comprises a conductive layer on the optical membrane. In the typical embodiment, the optical membrane structure itself is conductive to thereby function as the membrane electrode, however. This can be achieved in the context of semiconductor-based devices by controlling the doping of the layer that is used to form the optical membrane structure.  
           [0010]    In one embodiment, the optical membrane structure is vaulted to form a hemispherical Fabry-Perot cavity. In another embodiment, it is substantially planar to form a flat reflector of a hemispherical Fabry-Perot cavity.  
           [0011]    According to other aspects of the present embodiments, the first stationary electrode comprises a conductor layer on a support wafer. Alternatively, in another embodiment, a support wafer structure is rendered conductive to function as a first stationary electrode.  
           [0012]    According to another aspect of one of the embodiments, a reflector of a Fabry-Perot filter comprises a dielectric mirror structure that is deposited on a support wafer structure. Alternatively, an optical port can be formed in a support wafer structure and a mirror attached to the support wafer structure, typically with an intervening spacer layer, to define a Fabry-Perot cavity between the mirror and the membrane structure.  
           [0013]    In general, according to another aspect, the invention also features a tunable filter. This filter comprises a support wafer structure and an optical membrane structure, including a membrane electrode that is attached to the support wafer structure. A first electrostatic cavity is provided between the support wafer and a proximal side of the optical membrane structure. A second stationary electrode defines a second electrostatic cavity on a distal side of the membrane structure.  
           [0014]    In general, according to still another aspect, the invention concerns a triple electrode optical membrane. This membrane comprises an optical membrane structure, including a membrane electrode. First and second stationary electrodes are provided to deflect the membrane in either direction along an axis that is orthogonal to a surface of the membrane.  
           [0015]    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  
       [0016]    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:  
         [0017]    [0017]FIG. 1 is a schematic diagram illustrating a three-electrode optical membrane structure according to the present invention;  
         [0018]    [0018]FIG. 2 is a schematic cross-sectional view showing a Fabry-Perot tunable filter having two electrostatic cavities on either side of the optical membrane; and  
         [0019]    [0019]FIG. 3 is a schematic cross-sectional view showing a second embodiment of a two electrostatic cavity Fabry-Perot filter according to the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0020]    [0020]FIG. 1 shows an optical membrane structure having two electrostatic cavities, which has been constructed according to the principles of the present invention. Specifically, an optical membrane structure  150  with electrode is provided on an optical axis  10 . Usually this optical axis extends orthogonally to a plane of the membrane. Typically, this optical membrane structure will be coated to be optically reflective. Common coating techniques include metal layers and/or multilayer dielectric mirrors. Dielectric mirrors have the advantage of providing high reflectivity with low absorption, as typically required in high finesse devices.  
         [0021]    A first electrostatic cavity  136  and a second electrostatic cavity  138  are provided on a proximal and a distal side, respectively, of the optical membrane structure  150 . In the illustrated embodiment, the optical membrane structure is electrically grounded. The voltage between the optical membrane structure  150  and a first stationary electrode  102  is controlled by a first stationary electrode voltage source  106 . The second electrostatic cavity  138  extends between the optical membrane structure  150  and a second stationary electrode  104  on the distal side of the membrane structure  150 . The voltage across the second electrostatic cavity  138  is controlled by a second stationary electrode voltage source  108 .  
         [0022]    During operation, in one configuration, the optical membrane structure  150  with the two electrostatic cavities  136 ,  138  is used such that the first electrostatic cavity  136  and the second electrostatic cavity  138  function in a pull-pull mode. For example, the first stationary electric voltage sources  106  is used to bias the membrane in one direction, while the second source  108  is then used modulate the membrane position by pulling it the membrane through its zero voltage position and further deflect it beyond that position. Alternatively, the first voltage source  108  as the drive source and the second source is used as the bias source.  
         [0023]    [0023]FIG. 2 shows a MOEMS Fabry-Perot tunable filter  100 , which implements the principles of the present invention.  
         [0024]    Generally, the Fabry-Perot tunable filter  100  comprises a support structure  124 . In the present implementation, this support structure is fabricated from semiconductor wafer material, such as doped silicon wafer material.  
         [0025]    The optical membrane structure  150  is connected to the support structure  124 . In the illustrated embodiment, a first electrostatic cavity spacer layer  122  separates the membrane structure  150  from the support structure  124  and thus defines the first electrostatic cavity  136 . In the current implementation, this first spacer layer  122  is manufactured from a sacrificial release layer that is partially removed during the fabrication process to release the membrane structure  150  from the support structure  124 .  
         [0026]    The first spacer layer  122  is preferably manufactured from a nonconducting material. In the present implementation, it is silicon oxide. Alternatively, other materials are used in other embodiments. For example, silicon nitride is an alternative, along with polyamide, for example.  
         [0027]    In the present embodiment, the first spacer layer  122  is between 2 and 5 μm in thickness. Presently, the thickness is between 3.5 and 4.5 μm thick. With a nominal thickness of about 4 μm.  
         [0028]    A second spacer layer  120  defines the second electrostatic cavity  138 . In one implementation, the second electrostatic cavity spacer layer  120  is another silicon oxide layer.  
         [0029]    In present embodiment, it is less than 2 micrometers (μm) in thickness. In the present embodiment, the second spacer layer  120  is about 1 μm thick.  
         [0030]    A mirror spacer structure  116  separates the second spacer layer  122  from a concave mirror structure  118 . Preferably, the internal surface  134  of this mirror structure  118  has been coated to be highly reflective. Presently, a seven to twenty-one layer dielectric mirror is formed on the inner surface  134  to provide a high finesse, low absorption optical reflector.  
         [0031]    The total thickness of the second electrostatic cavity spacer layer  120  and the mirror spacer layer  116  define the length of the Fabry-Perot cavity between the inner surface  134  of the mirror structure  118  and a membrane mirror structure  132  that is deposited on the membrane structure  150 . This membrane mirror structure  132  is also preferably a high reflectivity, low absorption multilayer dielectric mirror structure, such as a seven to twenty-one layer dielectric stack. In the preferred embodiment, the FP cavity length is 10 to 40 μm, preferably about 10 to 20 μm, with a nominal mirror separation of about 16 μm in the current embodiment. Generally, however, these distances are dependent on the desired FSR.  
         [0032]    In the illustrated embodiment, the optical beam that is launched into the Fabry-Perot cavity  130  enters the cavity  130  from the left. In the illustrated embodiment, an optical port  126  is manufactured in the support structure  124 . This may or may not be necessary depending upon the transmissivity of the support structure  124  to the wavelengths of interest.  
         [0033]    In the preferred embodiment, an antireflection (AR) coating  128  is deposited on the proximal side of the membrane structure  150  in order to reduce reflections at the first membrane interface to maximize radiation coupled into the Fabry-Perot cavity  130 .  
         [0034]    Further, according to the illustrated embodiment, the Fabry-Perot cavity  130  is what is sometimes referred to as a hemispherical cavity, with the first reflector, defined by the membrane  150 , being substantially planar and a curved reflector being defined by the surface  134  of the mirror structure  118 .  
         [0035]    According to further aspects of the illustrated embodiment, a drive voltage or tuning voltage is established between a preferably grounded membrane structure  150  and the support structure  124 . The drive voltage can be either directly applied to the support structure  124  in the case where the support structure is conductive, being manufactured from doped wafer material. Alternatively, electrodes can be deposited on the distal side of the support structure  124 , i.e., the part of the support structure that faces the first electrostatic cavity  136 .  
         [0036]    According to ftrther aspects of the illustrated embodiment, a bias voltage generator  110  is connected to the mirror spacer structure  116 . Again, this structure has either bulk conductivity, or alternatively conductive electrodes are placed, such as deposited on, a proximal side of the mirror spacing structure  116  on the side of the structure that faces the second electrostatic cavity  138 .  
         [0037]    This preferred embodiment has advantages surrounding the fact that the FP cavity does not overlap the electrostatic cavity that is used as the drive voltage cavity. Therefore, the parameters of the electrostatic cavity are optimized without impacting the optimization of the FP cavity.  
         [0038]    In other embodiments, the drive voltage  112  is applied to the mirror spacing structure  116  and the bias voltage applied to the support structure  124 . In the present implementation, because of the relative thicknesses of the cavity spacer layer  122  and the second cavity spacer layer  120 , drive voltage is preferably applied to the support structure  124 .  
         [0039]    In still another embodiment, drive voltages can be applied to both the support structure  124  and the mirror structure spacer layer  116  so that the membrane is deflected in a pull-pull arrangement.  
         [0040]    One artifact of fabrication to note is the fact that the optical port  126  in the support structure  124  can be used as a shadow mask in the deposition of the membrane AR coating  128 . Similarly, before the installation of the mirror structure  118 , the optical port  140  in the mirror spacer structure  116  can be used as a shadow mask in the deposition of the membrane HR coating  132 .  
         [0041]    Depending on the rigidity of the layer that is used to form the membrane structure  150 , tethers can be provided between a center portion of the membrane structure that is coincident with the optical axis  10  and the part of the membrane structure that is between spacer layers  122  and  120 . In one embodiment, radial tethers are used. In another embodiment, spiral tethers are used extending between the outside portion of the membrane structure inward to the center of optical membrane structure.  
         [0042]    According to one fabrication method for the first embodiment, the wafer assembly starts with a base handle wafer (Wafer A), which will become the supporting structure  124  for the entire device. Currently it is produced from a standard n-type doped silicon wafer. Typically, this silicon wafer is 75 mm-150 mm in diameter and 400-500 microns thick. The wafer is oxidized to a depth of typically 2-4.5 microns to form the first spacer layer  122 . The maximum tuning range is ˜33% of this oxide thickness and the required maximum tuning voltage is inversely proportional to the square of the thickness (as is typical for electrostatic drives).  
         [0043]    A second n-type doped silicon wafer (Wafer B) is bonded to the handle wafer using elevated temperature and mechanical pressure. This second wafer will become the electrostatically deflectable membrane  150 . It is typically mechanically ground to a thickness of 4-7 μm. After grinding, the surface of the second wafer is oxidized to a thickness of typically 0.5-4.0 μm. This second oxide layer will be patterned to be the second cavity spacer layer  120 . A preferred thickness of 3.0 to 4.0 μm will yield the ability to bias the membrane position by about 1.0 μm as is required for typical WDM applications in which the maximum wavelength is about 1620 nanometers. A membrane and tether pattern is preferably etched into the oxide grown on the second wafer.  
         [0044]    A third n-type doped silicon wafer is bonded to the oxide on second, again using elevated temperature and mechanical pressure. This wafer will become the second spacer layer  116  that defines the free spectral range of the FP filter cavity in combination with the thickness of the second oxide layer  120 . The third wafer “buries” the membrane-patterned oxide on Wafer B and is subsequently ground to a thickness of typically 6-20 μm, which is appropriate to the mirror-to-mirror spacing of the curved mirror-flat mirror Fabry-Perot optical cavity.  
         [0045]    Typically, the optical port  126  is patterned and etched into the support wafer  124 , using a combination of isotropic and anisotropic etching. The oxide, first spacer layer  122  is used as an etch stop. Alternatively, the optical port etch step can be omitted, as silicon is partially transparent at infrared wavelengths. In such case, an anti-reflective (AR) coating is applied the outer surface of Wafer A to minimize reflection from the air-silicon interface.  
         [0046]    The resulting structure is subjected to an isotropic oxide etchant to “release” the membrane and tether structure from the first spacer oxide layer  122  and the etch-stop oxide is removed from the openings forming the spacer and contact. A typical implementation would use concentrated HF followed by methanol, followed by a drying step using supercritical carbon dioxide. A high reflectivity (HR) multi-layer dielectric mirror  132  is deposited through the spacer opening  140  onto the membrane interior surface using the port  140  as a mask. An anti-reflection (AR)  128  coating is similarly deposited through the optical port  126  onto the exterior surface of the membrane. Both of these coatings must be designed for the wavelength bands of interest.  
         [0047]    Next, electrical contacts are deposited on the backside of support wafer  124  and in the contact opening to the membrane wafer layer  150 , typically using aluminum or a refractory metal.  
         [0048]    A concave, highly polished micro-mirror  118 , preferably made using a mass-transport process, is installed on top of the spacer layer. The mirror has an HR coating on its interior surface (and AR coating on its exterior surface) so it forms a precision, high finesse optical cavity in conjunction with the membrane structure  150 . High parallelism and accurate spacing is maintained because of the uniformity of the spacer grinding thickness. The mirror attachment can be performed using AuSn attachment layers.  
         [0049]    For the second embodiment, the wafer assembly again begins with the attachment of Wafers A and B and the subsequent oxidation of Wafer B to form the second cavity spacer layer.  
         [0050]    Setting aside this structure, a separate, 400-500 micron thick, n-type doped silicon wafer (Wafer C) of the same size as the bonded wafers is oxidized (or receives deposited oxide which is subsequently densified) to a thickness, typically 0.5-1.0 micron. Wafer C is patterned and the oxide thickness is etched to form a deflectable mirror membrane and tether pattern. Another n-type doped silicon wafer (Wafer D) is bonded to Wafer C. (Wafer D “buries” the membrane-patterned oxide on Wafer C). Wafer D is ground to a thickness suitable for an electrostatically deflectable silicon membrane thickness, typically 4-8 microns. The ground surface of Wafer D is bonded to the oxidized surface of Wafer B using elevated temperature and mechanical pressure. Wafer C is subsequently ground to a thickness of typically 6-20 micron, appropriate to the mirror-to-mirror spacing of the curved mirrorflat mirror Fabry-Perot optical cavity.  
         [0051]    Optical port  126  is patterned and etched into Wafer A. A spacer and electrical contact pattern is etched into Wafer C, using the oxide as the etch stop layer, followed by an anisotropic silicon etch to transfer the diaphragm and tether pattern from oxide into the underlying silicon. A typical implementation would use directional reactive ion etching for this step. The etch stop oxides are removed from the openings, releasing the diaphragm and tethers without wet etchant, thus avoiding the stiction caused by liquid surface tension.  
         [0052]    The remaining steps are similar to those in the previous embodiment. A high reflectivity (HR) mirror layer is deposited through an appropriate shadow mask and the spacer opening onto the membrane surface, and an AR coating is deposited on the optical port side of the membrane. Electrical contacts are deposited on the surfaces of Wafer A and Wafer D, typically of aluminum or a refractory metal.  
         [0053]    A curved mirror with an HR coating and patterned metallization (typically Ti-Au, 0.5 micron thickness) is attached to the spacer layer.  
         [0054]    [0054]FIG. 3 shows another embodiment of the triple electrode Fabry-Perot filter according to the present invention. In this embodiment, no optical port is typically provided along the optical axis  10  in the support structure  124 . Further, the stationary mirror structure  134  is deposited directly on the support structure  124 .  
         [0055]    To yield the hemispherical Fabry-Perot cavity configuration, the membrane structure  150  is vaulted in the direction away from the stationary mirror structure  134 . This pimpling can be accomplished by controlling coating stress in the structure  150 . The first spacer layer  122  that separates the vaulted membrane structure  150  from the port structure  124  defines both the electrostatic cavity  136  and the FP optical cavity between the stationary mirror structure  134  and a mirror structure that is formed as part of the membrane structure  150 .  
         [0056]    A second spacer layer  120  separates the membrane structure  150  from the second, cantilever electrode structure  142 . In the illustrated embodiment, an optical port is provided in this electrode structure  142 . The second electrostatic cavity  138  extends between the distal side of the membrane structure  150  and the cantilevered portions of the second electrode structure  142 .  
         [0057]    Again, depending on the rigidity of the layer that is used to form the membrane structures  150 , tethers can be provided between a center portion of the membrane structure that is coincident with the optical axis  10  and the part of the membrane structure that is between spacer layers  122  and  120 . In one embodiment, radial tethers are used. In another embodiment, spiral tethers extend between the outside portion of the membrane structure and the center optical membrane.  
         [0058]    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.