Abstract:
An article and method including an optical fiber coupled to a microstructure which is easily manufactured. The microstructure is comprised of a plurality of layers including a receptacle layer and a movable layer. The optical fiber has a high reflectivity terminal end forming the non-movable layer. The receptacle layer has an opening aligned with the movable layer for receiving the terminal end of the optical fiber. The optical fiber is inserted into the opening and the receptacle layer and fixed in relation to the microstructure, so as to form cavity between the terminal end of the fiber and the movable layer of the microstructure. The movable layer is physically adapted for moving relative to the non-movable layer under the influence of an actuating force. Additionally a method of manufacture of forming a microstructure comprising a receptacle layer having an opening therein and a movable layer, inserting a terminal end of an optical fiber into the opening, and affixing the optic fiber to the microstructure.

Description:
FIELD OF THE INVENTION 
     The present invention relates to micro-structural optical devices having a movable membrane for generating optical interference effects and the like. 
     BACKGROUND OF THE INVENTION 
     Microstructural devices having a movable membrane for creating optical interference effects may be used in a variety of applications. For example, such devices are useful as high speed, inexpensive optical modulators for optical communications systems. 
     The theory underlying the performance and mechanical response of fiber optical microstructures configured for measuring pressure and temperature is generally described in Greywall et. al, U.S. Pat. No. 5,831,262 (hereinafter Greywall et. al &#39;262). Additional description concerning the design of micromechanical optical modulators may be found in U.S. Pat. No. 5,500,761 and U.S. Pat. Nos. 5,654,819 and 5,589,974. 
     The microstructure may be suitably structured for example as a Fabrey-Perot device having equal reflectivity mirrors as generally described in Aratani&#39; et al., “Process and Design Considerations for Surface Micromachined Beams for a Tunable Interferometer Array in Silicon”, Proc. IEEE Micro. Electromech. Workshop, Ft. Lauderdale, Fla., Feb. 7-10, 1993, pp. 230-235. 
     Using such devices in optical systems requires optical coupling to waveguides such as optical fibers. Optical coupling, however, may be problematic. Greywall et al. described an Article Comprising An Optical Fiber Attached To A Micromechanical Device in Greywall et. al &#39;262 in which an optical fiber is integrally attached to a microstructure. In Greywall et al. &#39;262 a layer of cement and a layer of glass are index matched to the index optic fiber, i.e. the layers and the fiber have the same index of refraction. The glass layer provides support for an adjacent layer of the microstructure. Integration of the fiber with the microstructure was taught to eliminate or reduce the interference effects that would otherwise occur if the fiber end and the microstructure were spaced. 
     The fabrication of the microstructure as described in Greywall et al. &#39;262 includes silicon nitride or polysilicon which is deposited on the first side of a silicon wafer. A “pill” of readily etchable sacrificial material is then deposited on the first layer. A second layer side composed of silicon nitride or polysilicon, is deposited on top of the pill and then a layer of glass is deposited over the second layer. The wafer is etched from the second side to the first layer. Also, holes are etched into the first layer through a conductive layer if present. The etchant is delivered through the holes to the pill of the sacrificial material sandwiched between the first and the second layer. The sacrificial material is etched away, releasing the first layer. 
     Since the manufacturing procedure of Greywall et al. &#39;262 requires use of a pill of readily sacrificial material and removal of the sacrificial layer through the holes etched through the first layer, the “pill” must be precisely deposited over the first layer if the gap formed after the sacrificial layer is removed is to have the needed dimensions. In addition, the procedure includes etching holes in order to remove the sacrificial layer. The holes in turn may interfere with the structural properties of the movable membrane or may have to be plugged. 
     Greywall et al. &#39;262 also describes that the terminal end of the fiber be connected to the flat surface of the glass via a layer of cement between the terminal end of the fiber and the glass. As such, the cement composition must be index matched and have suitable bonding properties to attach the fiber to the glass. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to an article having an optical fiber coupled to a microstructure which is easily manufactured. The microstructure is comprised of a plurality of layers including a receptacle layer and a movable layer. 
     The optical fiber has a high reflectivity terminal end forming the non-movable layer. The receptacle layer has an opening aligned with said movable layer for receiving the terminal end of the optical fiber. The optical fiber is inserted into the opening in the receptacle layer and is fixed in relation to the microstructure, so as to form a cavity between the terminal end of the fiber forming the non-movable layer and the movable layer of the microstructure. The movable layer is physically adapted for moving relative to the non-movable layer under the influence of an actuating force. 
     The microstructure may also include a spacer layer which defines the distance between the movable layer and the non-movable layer. 
     The present invention can be used as an optical microphone with remote sensing capability, an article to measure pressure and an article to measure temperature. 
     Additionally, the present invention relates to a method of manufacture of the article by forming a microstructure comprising a movable layer and a receptacle layer having an opening therein, inserting a terminal end of an optical fiber into said opening, and affixing the optic fiber to the microstructure. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Further features of the invention will become apparent from the following detailed description of the specific embodiments when read in conjunction with the accompanying drawings, in which: 
     FIG. 1 shows a first embodiment of an article according to the present invention wherein the microstructure is configured to modulate an optical signal in response to sonic energy. 
     FIG. 2 shows the calculation of optical cavity for a ¾λ thick silicon nitride movable layer and a ¼λ thick high reflectivity coating on the terminal end of the optical fiber acting as the non-movable layer. 
     FIGS. 3A to  3 F show the representative process flow for the method of manufacture of the present invention. 
     FIG. 4 shows an embodiment of an article according to the present invention wherein the microstructure is configured to modulate an optical signal in response to temperature changes. 
     FIG. 5 shows the article of the present invention where a ferrule is used about the optic fiber. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The present invention is directed to an article having an optical fiber  18  coupled to a microstructure  8 . The article can be used to modulate an optical signal in response to physical events such as sonic energy, temperature or pressure. 
     Turning to the drawings illustrating preferred embodiments of the present invention, FIG. 1 shows a cross sectional view of an optical microphone with remote sensing capabilities. The article of manufacture includes an optical fiber  18  having a highly reflective coating forming a non-movable layer  28  on the face of the terminal end. The terminal end of the optical fiber  18  is fixed within the microphone of FIG. 1 which includes a movable layer  22  responsive to sonic energy. 
     The microstructure  8  preferably comprises a silicon wafer  10  having a portion removed to form a chamber  32 . Defining the top of the chamber  32  is a movable layer  22 . A receptacle  16  is formed through the use of a receptacle layer  14  in the microphone  8  above the movable layer  22  to accept and align the non-movable layer  28  of optic fiber  18  with the movable layer  22  of the microphone  8 . 
     A cavity  24  exists between the non-movable layer  28  of the optical fiber  18  and the movable layer  22  of the microphone  8 . The distance between the nonmovable layer  28  and the movable layer  22  is defined by a spacer layer  12  deposited on the surface of the silicon wafer  10  of the microphone  8 , as further described below. 
     The non-movable layer  28  on the face of the terminal end of the optical fiber  18  is aligned with the movable layer  22  of the microstructure  8  through the use of the receptacle  16 . The receptacle  16  is created by a receptacle layer  14  deposited on the spacer layer  12  of the microstructure  8 . The receptacle layer  14  is preferably made of phosphosilicate glass film or a similar composition and preferably has a thickness sufficient to properly align a fiber optic cable  18 . The receptacle layer  14  has an opening  15  therein which is aligned with the cavity  24 , the non-movable layer  22  and the chamber  32  of the microstructure  8 . 
     In its most preferred embodiment, the movable layer  22  of the microstructure  8  is a portion of a film layer  20  formed over the wafer layer  10 , spacer layer  12  and receptacle layer  14  including the openings in the spacer layer  12  and receptacle layer  14 . As shown, the opening in the spacer layer  12  is of a smaller diameter, and therefore extends further into the opening  15 , than the opening in the receptacle layer  14 . The film layer  20  covers the receptacle layer  14  and the spacer layer  12 , arranged in the described stepwise fashion, including the exposed tops and sides of these layers  12  and  14 . The film layer  20  in the area where the chamber  32  is formed acts as the movable layer  22  in the preferred embodiment. 
     Determination of various dimensions of the layers, coatings and gaps are dependent on the wavelength (λ) of the light passed through the optical fiber. Generally, applications up to 300 meters from the device use a light of 0.850 micron wavelength, however, if the distance is over 300 meters a light having a wavelength of about 1.30 micron is standard. 
     The spacer layer  12  is preferably thermally grown to a specific thickness in relation to the wavelength of light used in the optical microstructure  8  as described above. The distance between the movable layer  22  and the non-movable layer  28  at equilibrium is preferably determined by the formula            m                 λ     2     ±                             
     wherein m is a whole number and ε is between 0 and λ/10 with λ/20 being most preferred. In this most preferred embodiment, the equilibrium is on the steepest slope of the graph of FIG. 2 to provide the greatest response in reflectivity when an actuating force is applied to the movable layer  22 . 
     The opening  15  in the receptacle layer  14  is created so that, after the film layer  20  is deposited on the microstructure  8 , the optical fiber  18  is easily aligned in its x and y coordinates when inserted. Since the spacer layer  12  extends into the opening beyond the receptacle layer  14 , it forms a stop  13  against which the terminal end of the optical fiber  18  rests to align the terminal end of the optic fiber in the z coordinate. This arrangement allows for passive packaging techniques to be used wherein the x and y positions of the optical fiber  18  are defined by the opening  15  layer  14  and the z position is defined stop  13  of the spacer layer  12 , preferably both covered by the film layer  20 . The terminal end of the optical fiber  18  is preferably fixed in the receptacle  16  by cement or solder  30  about at least a portion of the perimeter of the fiber  18 . 
     In the preferred embodiment the film layer  20  is deposited over the receptacle layer  14  and spacer layer in stepwise fashion. The film layer  20  is preferably silicon nitride or some other reflective material whereas it comprises the movable layer  22 . It is preferred that the film layer  20  can have a refractive index of about 1.9 to 2.2, and most preferably 2.0. The film layer  20  has a thickness being an odd multiple of ¼λ, and preferably is about ¼λ or ¾λ thick. 
     In determining the thickness of the film layer  20 , and therefore the movable layer  22 , there is a trade off between the manufacture process and the responsiveness of the movable layer  22  where a thicker layer is easier to produce but a thinner layer achieves greater responsiveness to the actuation energy. 
     Moreover, the movable layer  22  of the microphone  8  may be etched to provide damping holes  26 . Damping holes  26  are provided to control ringing in the movable layer  22 . The size, shape, spacing and numbering of damping holes  26  may be modified based on the microphone characteristics. 
     The optical fiber  18  may be single mode, multimode, plastic coated, silica fiber and the like. Alternatively, as shown in FIG. 5, a ferrule  40  can be used about the fiber  18  to fix the fiber  18  to the microstructure  8 , as is well known in the art, when the movable layer  22  is larger than the diameter of the optical fiber  18 . 
     A coating is placed on the face of the terminal end of the optical fiber  18  which forms the non-movable layer  28  in a Fabrey-Perot cavity structure. The non-movable layer  28  can be a zinc sulfide film or other highly reflective material preferably having a refractive index of about 2.0 to 3.0 and most preferably about 2.3-2.4. The thickness of the coating is about ¼λ in the preferred embodiment but can be adjusted according to the physical properties of the coating used, including the refractivity thereof, and size of the cavity  24 . 
     The optical fiber  18  is preferably metalized on its perimeter to provide a wettable surface for solder  30  for fixing the optical fiber  18  to the microstructure  8 . The optical fiber  18  is placed into the opening  15  in the receptacle layer  14  and is brought into contact with the film layer  20  on the spacer layer  12  for positioning in the x, y and z coordinates. The lateral position of the optical fiber  18  is set by lithographically defined opening in the receptacle layer  14  while the distance between the movable layer  22  and the non-movable layer  28 , or z position, is generally set by the thickness of the spacer layer  12 . The fiber  18  is then soldered to the microstructure  8  about the perimeter of the fiber  18  without the need for adhesive between the movable layer  22  and the non-movable layer  28 . 
     Alternatively, when the movable layer  22  is larger than the diameter of the optical fiber  18  and a ferrule  40  is used, the ferrule  40  has a metalized perimeter to be soldered to the metalized film layer  20  of the microstructure  8 . In such an embodiment, the ferrule  40  is brought into contact with the film layer  20  on the spacer layer  12  to set the z position (the space between the movable  22  and non-movable  28  layers). See FIG.  5 . The fiber  18  is fixed to the microstructure  8  by solder  30  contacting the metalized film layer  20  about the perimeter of the ferrule  40 , again without the need for adhesive between the movable layer  22  and the nonmovable layer  28 . 
     In another embodiment of the present invention shown in FIG. 4, the article is configured to measure temperature. For the temperature measuring device the manufacturing method is modified to form a closed chamber  32 . This may be accomplished by capping said portion of the silicon wafer beneath the optical cavity  24  by bonding a silicon wafer cap  36  with material of suitable optical property over the chamber  32  and filling it with a gas. 
     More specifically, a closed chamber  32  is formed within the wafer  10  beneath the movable layer  22 . The chamber  32  is evacuated and capped. The chamber  32  thereby isolates the cavity  24  from variations in ambient pressure. The movable layer  22  only moves in response to changes in temperature that alter the pressure in the gas filled cavity  24 . As a result, the reflectivity of the microstructure changes. Such a change in reflectivity can be correlated to the ambient temperature. 
     In another embodiment of the present invention, the article is configured to measure pressure. In the case of this embodiment there are no damping holes  26  and the cavity  24  is sealed. In operation the article as configured to measure pressure is exposed to fluid pressure and moves toward the non-movable layer  28 . The change in position of the movable layer  22  causes a change in the cavity  24  resulting in a change in the reflectivity of the microstructure. The change in reflectivity can be correlated to the pressure exerted on the movable layer  22 . 
     In yet another embodiment, not shown, the article can function as an optical modulator through the use of electrodes on the movable layer  22  and the nonmovable layer  28  with a voltage therebetween to provide a modulated optical signal as would be known in the art. 
     The preferred method for fabricating the present article of manufacture is shown in a representative process flow for an optical microphone in FIGS. 3A-3F. The fabrication process will be modified as necessary to accommodate structural requirements of other embodiments of the invention as described herein. 
     As shown in FIG. 3A, the microstructure includes a silicon wafer  10  which acts as a foundation or base layer. A spacer layer  12  of silicon dioxide is thermally grown over the silicon wafer  10  to a thickness equal to the desired size of the cavity  24 . 
     A receptacle layer  14  is deposited on the spacer layer  12  as shown in FIG.  3 B and the receptacle layer  14  is then patterned through lithography or other suitable technique to permit alignment of the optical fiber  18  during attachment to the microstructure  8 . The spacer layer  12  is pattern etched to open the optical cavity  24 . See FIG.  3 C. 
     As shown in FIG. 3D, a film layer  20  of silicon nitride or some other suitable composition is deposited over the structure and forms the movable layer  22  of the optical cavity  24 . The movable layer  22  may be etched patterned to provide damping holes  26  as desired for the particular application of the microstructure  8 . 
     A metal ring (not shown) is preferably added to the top surface of the microstructure  8  surrounding the opening  16  to provide a wettable surface to solder the optical fiber  18 , or ferrule  40  when used, to the microstructure  8 . 
     The silicon wafer  10  is then bulk micromachine etched to release the mechanically active area of the movable layer  22  thereby forming a chamber  32  as shown in FIG.  3 E. Etchants typically found suitable for this application include potassium hydroxide (KOH) and ethylene diamine pyrocatechol (EDP). 
     The terminal end of the optical fiber  18  is coated with a highly reflective material such as zinc sulfide to form the non-movable layer  28  of the Fabrey Perot cavity structure. The coating  28  is preferably about ¼λ thick. 
     The optical fiber  18  is then metalized to form a wettable surface for soldering to the microstructure  8 . The optical fiber  18  is placed in the opening  16  of the receptacle layer  14  and into contact with the stop  13  of the spacer layer  12  with the film layer  20  thereon. The fiber  18  is aligned in the x, y, and z, directions and is then fixed to the microstructure with a metal ring of solder  30 . The solder  30  wets the top surface of the structure, and the perimeter of the optical fiber  18  about the opening  16 . See FIG.  3 F. 
     Once again, when the movable layer  22  is larger than the diameter of the fiber  18  a ferrule  40  is used. The ferrule  40  is metalized for soldering to the microstructure  8  and contacts the stop  13  of the spacer layer  12  for alignment in the z direction. 
     In operation, sonic energy from the lower surface of the microstructure  8  causes the movable layer  22  to move from an equilibrium position in relation to the non-movable layer  28  thereby changing the gap in the cavity  24 . FIG. 2 shows the calculation of the reflectivity vs. the gap of the optical cavity for a ¾λ thick silicon nitride membrane and a ¼λ thick high reflectivity coating on the face of the terminal end of the optical fiber  18 . 
     The present invention allows for the use of passive attachment techniques since the lateral position of the fiber  18  is set by the lithographically defined opening  15  in the receptacle layer  14 , while the z position is set by the thickness of the spacer layer  12 . In this regard, active alignment requires that light be passed through the fiber and measured to align the fiber with a microstructure  8 . However, passive alignment requires only mechanical techniques without optical measurements requiring light through the optical fiber  18 . 
     Variations to the above described product and method will make themselves apparent to one skilled in the art reading this disclosure. All such modifications are intended to fall within the spirit and scope of the present invention, limited only by the appended claims. All patents and publications cited herein are incorporated by reference.