Abstract:
A multi-cavity micro-optical Fabry-Perot filter uses electrostatically deflected MEMS membranes. The filter comprises a first electrostatically deflectable membrane device. A curved mirror structure is formed on its optical membrane. Similarly, a second electro-statically deflectable membrane device is provided, which has a second curved mirror structure on the membrane. The spacer is used to separate the first membrane device from the second membrane device. The spacer supports a mirror between the first and second curved mirror structures. Wafer-level and device level assembly techniques are also described.

Description:
BACKGROUND OF THE INVENTION 
     Single cavity Fabry-Perot (FP) tunable filters are commonly used in spectral monitoring applications. High finesse devices have impulse-like spectral filter functions that can be scanned across a band of interest in order to determine the spectral optical energy distribution. 
     One of the most common applications for FP filters is in wavelength division multiplexing (WDM) systems. In commercially available WDM systems, the channel assignments/spacings can be tight, 100 GigaHertz (GHz) to 50 GHz, based on the International Telecommunication Union (ITU) grid. Further, the number of potential channels, channel slots, in a link can be large. Observation of the ITU Grid suggests 100&#39;s of channels per link in the L α , C α , and S α , bands that stretch from about 1491 nanometers (nm) or 200 Terahertz (THz) to about 1612 nm or 186 THz. Additional channels in this range are provided by the 50 GHz offset of the L β , C β , and S β  bands. Still other systems are being proposed that have assignments/spacings in the 10 to 20 GHz range. In each of these systems, the channels are confined to their channel slot or frequency assignment to an absolute accuracy of less than 10 GHz, in some cases. In order to verify the proper operation of these WDM systems, FP-based optical channel monitors are required with pass bands of 10 GHz and less. 
     When even sharper filter functions, i.e., smaller pass bands, are required, multiple Fabry-Perot filters can be deployed in a cascade configuration. Two cascaded filters effectively double the sharpness of the net filter function. Moreover, careful co-design of the two filter cavities can yield substantial improvements in the side mode suppression. 
     Expanding the applications for FP tunable filters, beyond the standard monitoring applications or to applications requiring narrowed passbands, requires effort in the design of a class of FP filters called multi-cavity FP tunable filters. These filters have multiple discrete coupled optical cavities. Selection of the mirror reflectivities for the end mirrors and the mirror separating the cavities, along with control over the cavity lengths, leads to the ability to provide filters that have controllable passband profiles during the design stage and dynamically during operation. Most commonly, the filters are designed to have a top-hat pass band profile, which can be used to selectively route single channels or blocks of contiguous channels in a fully or partially populated WDM signal. 
     SUMMARY OF THE INVENTION 
     Critical to the deployment of multi-cavity FP filters is the fabrication of microelectromechanical system (MEMS)/micro-optical electromechanical system (MOEMS) filter designs. In the past, macro-scale multi-cavity FP filters have been manufactured. 
     Generally, however, these devices do not have the form factor required for communications applications. Moreover, they typically lack mechanical robustness and have poorer performance. Smaller optical fiber-based multi-cavity FP filters have been proposed for communications applications. The drawbacks here are associated with the difficulties in depositing highly reflecting HR dielectric mirror coatings on the fiber ends and control of other cavity parameters such as end-mirror curvatures. Moreover, fiber-based cavity FP filters typically use piezoelectric-based actuators, which typically suffer from electromechanical instability. 
     The present invention is directed to a multi-cavity micro-optical Fabry-Perot filter. It uses electrostatically deflected MEMS membranes. Such devices can have excellent mechanical/optical characteristics. 
     In general, according to one aspect, the invention features a multi-cavity Fabry-Perot filter. The filter comprises a first electrostatically deflectable membrane device. A curved mirror structure is formed on its optical membrane. Similarly, a second electro-statically deflectable membrane device is provided, which has a second curved mirror structure on the membrane. The spacer is used to separate the first membrane device from the second membrane device. The spacer supports a mirror between the first and second curved mirror structures. 
     According to one embodiment, the first and second membrane devices each comprise a support, a device layer in which a deflectable membrane is formed, and a sacrificial layer, which separates the support from the device layer. The sacrificial layer is selectively removed to release the membrane. 
     An optically curved surface is formed on the deflectable membrane. Typically, the optical surface is a concave mirror with a continuous surface profile. In alternative embodiments, however, diffractive or Fresnel-type mirror profiles can be used. In order to provide the mirror structures, an optical coating is deposited on the optically curved surfaces of the membrane devices. The optical coating is typically a multi-layer dielectric mirror. 
     According to other aspects of the present embodiment, the spacer comprises two spacer layers between which a dielectric mirror layer is located. Regions of these first and second spacer layers are preferably removed surrounding an optical axis to thereby expose the dielectric mirror layer of the flat mirror to the device&#39;s optical cavities. This produces a suspended dielectric mirror in a region surrounding the optical axis of the filter. 
     In some embodiments, a support frame is provided around the spacer. This support frame is preferably integral with the spacer and defines one or two blind holes into which one or both of the membrane devices are installed. Thus, each membrane device is aligned using preferably lithographically-formed features of the support frame/spacer to thereby align the membrane devices relative to each other. In one embodiment, registration features are provided in at least one or both of the blind holes. The membrane devices are abutted against these features to yield a robust alignment system. Spring or biasing elements can also be fashioned in the support frame/spacer to ensure good abutment of the devices against these registration features. 
     In general, according to another aspect, the invention features a process for assembling multi-cavity Fabry-Perot filters. Generally, this process relies on the production of a precursor structure that will be subsequently singulated, by die sawing for example, into singulated filters. Typically, a first die of electrostatically membrane devices is attached to a first side of a spacer, which includes the flat mirror, followed by the attachment of a second die of electrostatically deflectable membrane devices to a second side of the spacer. 
     Preferably, to facilitate the die saw singulation, a perimeter around the membranes of the membrane devices is preferably sealed during the attachment or subsequent to the attachment of the dies to the spacer. Further, optical ports on the backsides of the dies are also preferably filled prior to the die saw operation. A good candidate for the fill material is a photoresist, which is later removed, after the sawing is complete, and any particulate matter washed away. 
     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 
     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: 
     FIG. 1 is an exploded perspective view of the dual cavity Fabry-Perot tunable filter according o the present invention; 
     FIG. 2 is a partial-perspective view of a membrane device spacer according to the present invention; 
     FIG. 3 is a perspective view of the constructed Fabry-Perot filter of the present invention with hidden lines shown in phantom; 
     FIG. 4 is a partial side cross-sectional view through the optical axis of the dual cavity Fabry-Perot filter illustrating the operation of the spacer; 
     FIG. 5 is an exploded perspective view of another embodiment in which a frame is provided on the spacer to facilitate alignment of the membrane devices and handling of the spacer during fabrication; 
     FIG. 6 is a plan view of a die of membrane devices prior to die level assembly; 
     FIG. 7 is a perspective partial cut-away view illustrating the assembly of the membrane device die with the intervening spacer; and 
     FIG. 8 s a plan view of the assembled membrane devices and spacer to thereby form a precursor structure that is ready for die saw singulation. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 shows a dual cavity Fabry-Perot (FP) tunable filter  100 , which has been constructed according to the principles of the present invention. 
     Generally, the FP filter  100  comprises two membrane devices  110 - 1 ,  110 - 2 . Each membrane device comprises a curved mirror structure  250 , which is supported on a deflectable optical membrane  214 . According to the preferred embodiment, the membrane  214  is electrostatically deflected to yield out-of-plane movement. 
     A spacer  410  is installed between the two membrane devices  110 - 1 ,  110 - 2 . In the illustrated embodiment, this spacer  410  comprises two spacer layers  412 ,  414  that define the length of the two optical cavities. Sandwiched between the two spacer layers  412 ,  414  is a dielectric mirror layer  416 . 
     In the illustrated embodiment, regions of the spacer layers  412  and  414  are removed surrounding the optical axis  10 . This yields a suspended, dielectric, flat or relatively flat, mirror  420 , which functions as the center mirror structure in the dual cavity Fabry-Perot filter  100 . 
     One configuration, and associated fabrication technique, for the membrane devices  110  is described in U.S. Pat. appl. Ser. No. 09/804,618, filed on Mar. 12, 2001, entitled “MEMS Membrane with Integral Mirror/Lens”, by Flanders, et al. This application is incorporated herein by this reference in its entirety. 
     Generally, as described in that application, each membrane device  110  comprises a support  210 , which is typically manufactured from silicon handle wafer material. A device or membrane layer  212  is installed or deposited on the support  210  with an intervening sacrificial or release layer  216 . 
     The release layer  216  also defines the electrostatic cavity between the membrane  214  and the support  210 , in the illustrated implementation. Electrical access to the support  210  is provided by wire bond pad  336 . Electrical access to the device layer  212  and thus the membrane  214  is provided via the device layer bond pad  334 . Alternatively, electrical access to the device layer is obtained by a port through the backside of the support. 
     In the illustrated implementation of the membrane device  110 , the MEMS release structure comprises a spiral pattern of tethers  220  that extend between an outer portion  222  of the membrane  214 , which portion is supported by remnants of the sacrificial layer, and the membrane body  218 . At the center of the membrane body  218  is the curved mirror structure, which generally comprises a reflective layer  230  that has been deposited over a curved surface  250 , which has been fabricated in the membrane. The reflective layer  230  is preferably a dielectric mirror comprising multiple alternating high and low index thin film layers. The curved reflective optical surface or mirror structure  250  is provided in the center of the mirror layer  230 . This curved optical surface is preferably a concave surface to thereby yield a mirror structure with a concave optical surface. 
     Surrounding the membrane  214  on the device layer  212  are metalizations  234 . These are used in the bonding of the membrane device  210  to the spacer  410 . 
     In the current implementation, the membrane devices  110 - 1 ,  110 - 2  are substantially identical to each other, as illustrated by the similar reference numerals. Thus, membrane device  110 - 1  illustrates the backside features of the membrane devices  110 . Specifically, an optical port  240  is preferably etched through the backside of the membrane devices  110  thereby provide optical access to the backside of the membranes  214 . 
     FIG. 2 illustrates the details of the current embodiment of the spacer  410 . Typically, the spacer  410  comprises the two spacer layers  412 ,  414  that generally define the lengths of the two optical cavities of the dual cavity Fabry-Perot filter  100 . Generally, these spacer layers  412 ,  414  are between 10 and 50 microns in thickness to thereby yield similarly sized optical cavities. The center dielectric mirror layer  416  is relatively thinner, typically being a few micrometers to less than one micrometer in thickness. 
     FIG. 3 shows the assembled Fabry-Perot filter  100 . Specifically, it shows the two membrane devices  110 - 1 ,  110 - 2 , which are separated by the intervening spacer  410 . 
     FIG. 4 is a partial side cross-sectional view illustrating the relationship between the optical and electrostatic cavities in the constructed FP filter  100 . 
     Specifically, the spacer layers  412 ,  414  define the two optical cavities  12 ,  14 . The optical cavities  12 ,  14  extend between the center mirror  416  and the two concave mirror structures  250  that face each other along the optical axis  10 . 
     As described previously, these concave mirrors  250  are formed in their respective membranes  214  using mirror coatings  230  over a curved surface that has been formed in the membrane layer. Deflection of these membranes is achieved via electric fields that are established in their respective electrostatic cavities  216 ′ that are created by the removal of the sacrificial layers  216 . Optical access to the respective backsides of each of the membranes  214  is provided by the optical ports  240  that extend through the supports  210  of each membrane devices  110 - 1 ,  110 - 2 . The voltage difference between the supports  210  and the respective membranes  214  yields electrostatic deflection of the membranes  214  in the direction of the support and thus into the respective electrostatic cavities  216 ′. In this way, the optical lengths of each of the optical cavities  12 ,  14  are independently modulated to thereby provide the tuning function for the Fabry-Perot filter  100 . In the current embodiments, the electrostatic cavities are between 3 and 6 microns in length. These distances enable optical tuning across the free spectral range for 1,000 to 2,000 nanometer light, while limiting the actuation voltages required for the membrane. 
     Fabrication Strategies 
     FIG. 5 illustrates one strategy for the fabrication and assembly of the tunable filter  100 . Specifically, a frame  510  is provided that surrounds the spacer  410 . In one implementation, this frame  510  is integral with the spacer  410 . Specifically, the spacer  410  is provided as a blind-hole structure that is etched or otherwise fabricated into the frame  510 . In one implementation, this is accomplished by deep reactive ion etching (DRIE). 
     In conjunction with the formation of the blind-hole, registration features  512  are provided, in one implementation, to facilitate the alignment of the membrane device  110 - 1  into the blind-hole in the frame  510 . Spring elements  514  are further provided in some instances to improve the registration or abutment of the membrane device  110 - 1  against the registration features  512 . 
     The principle objective behind providing the frame  510  is to facilitate the handling of the relatively thin spacer  410 . As a result, the blind-holes can be provided on both sides of the frame  510  or one of the membrane devices  110  can be flush mounted whereas the other one can be inserted into a relatively deeper blind-hole. 
     In the illustrated implementation, the membrane devices  110  have front-side bond pads  336 ,  334 . Metal traces  516  are provided between connection pads  522 . Upon assembly, the connected pads  522  electrically contact the respective bond pads  336 ,  334  of the membrane devices  110  and thereby enable electrical access via wire bond pads  520  on the sides of the frame  510 . The traces  516  function as jumpers between the wire bond locations  336 ,  334  on the membrane devices, which become concealed during assembly, and the bond pads  520  on the spacer  510 . 
     Another application/assembly strategy is illustrated in FIGS. 6-8. Generally, these figures illustrate a scheme for wafer or die level assembly. 
     Specifically, as illustrated in FIG. 6, multiple membrane devices  110  are provided on a single die  610 . 
     Next, as illustrated in FIG. 7, the dies  610  are assembled with the intervening spacer  412 . 
     One difficulty associated with this die level fabrication is the difficulty in handling the relatively thin spacer  412 . In one embodiment, a precursor spacer is bonded to one of the dies  610 ; the spacer layer adjoining the die  610  is of the desired thickness. The other spacer layer is then polished back to the desired thickness. This allows assembly using a more mechanically robust spacer, while still achieving the desirable optical cavity size. 
     Under current embodiments, the dies are metal bonded to each other. Specifically, ball bump metalizations  612 , as illustrated in FIG. 6, or thick metal depositions can be used. 
     In an alternative embodiment, larger metalizations are used as illustrated by reference numeral  614  in FIG.  7 . Preferably, these metalizations are continuous around an entire perimeter of the membrane  214 . This pattern has the effect of sealing the optical cavity. As a result, as illustrated in FIG. 8, when the optical filters  100  are separated by, for example, die sawing along lines  620 , slurry from the sawing process does not enter and contaminate the optical cavities. Moreover, to completely protect the MEMS devices during the die saw process, the optical ports  240  are also preferably filled prior to die saw. In one example, the fill material is photoresist. When the die saw process is completed and the tunable filters  100  are fully singulated, the photoresist fill is removed in a circulated acetone bath, for example. 
     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.