Abstract:
A device capable of receiving an optical fiber through an orifice in the housing of the device. The housing having a gold surface over a substrate material. An indium layer is located on the gold surface. A solder joint is formed with the indium layer covering the gold surface to have indium silver solder surrounding the fiber and maintaining the fiber in a desired position with the housing. The indium layer can be formed of pure indium and the solder joint is formed of indium silver solder having about 97% indium and about 3% silver. The fiber or fibers may be metallized with a pure indium coating. The indium silver solder joint can provide an improved hermetic seal at fiber entrances and exits of the device.

Description:
BACKGROUND 
     Microelectromechanical systems or MEMS have electromechanical structures typically sized on a millimeter scale or smaller. These structures are used in a wide variety of applications including for example, sensing, electrical and optical switching, and micron scale (or smaller) machinery, such as robotics and motors. MEMS devices are very sensitive to environmental exposure. As such, MEMS structures are encased in hermetically sealed packages. 
     In the case of optomechanical switches and other opto-coupling devices, optical fibers must enter the package without allowing the interior of the package to be exposed to the environment. This includes possible exposure through any space that may exist between the optical fiber and the sleeving that surrounds the fiber. To prevent this, the sleeving typically is removed and the fiber sealed with a sealing material at an orifice to the package. 
     The hermetic seal is of particular concern in optomechanical devices. This is because any surface contaminants on the devices can affect mechanic properties, including increasing stiction; can affect optical properties, including reducing the reflectance of optical structures; and can affect electromagnetic interactions. These concerns are in addition to the typical concerns over exposure of electrical components to the environment. Thus, in optomechanical devices, a robust hermetic sealing is of particular importance. 
     The packages typically are formed of a nickel iron compound plated with a gold exterior layer. A nickel layer can be plated on the nickel iron compound to facilitate adhesion of the gold to the package. 
     Typically, a solder process is used to hermetically seal the package. One example of such a process is disclosed in U.S. Pat. No. 5,692,086, by Beranek et al., entitled OPTICAL FIBER LOCKING SUBMOUNT AND HERMETIC FEEDTHROUGH ASSEMBLY, issued Nov. 25, 1997, herein incorporated by reference in its entirety. Difficulties with conventional solder processes include fiber degradation caused by higher temperatures of such processes. 
     Therefore, what is needed is a low temperature solder process which provides a robust hermetic seal and favorable device performance and yields. 
     SUMMARY 
     In a possible implementation in accordance with the present invention, a device is provided capable of receiving an optical fiber through an orifice in the housing of the device. The housing has a gold surface over a substrate material. An indium layer is located on the gold surface. A solder joint is formed with the indium layer covering the gold surface to have indium silver solder surrounding the fiber and maintaining the fiber in a desired position with the housing. 
     In one embodiment, the indium layer is deposited by electroplating and is formed of pure indium. The solder joint may be formed of indium silver solder having about 97% indium and about 3% silver. 
     The indium silver solder can have a melting point below that of the indium layer. This allows the indium layer to act as protective barrier over the gold surface, inhibiting the indium silver solder from dissolving the gold surface. The indium layer adheres well to the gold and the indium silver solder, in turn, adheres well to the indium layer. 
     Further, the indium silver solder adheres well to the gold surface if some or all of the indium is dissolved during the solder process. If part or all of the indium layer is dissolved, the indium silver solder is less reactive when it does contact the gold surface. As a result, indium silver solder reduces the reaction rate with the gold surface if the indium layer is sacrificed or removed by the indium silver solder. The silver in the indium silver solder functions to reduce the reaction of the indium in the solder with the gold surface. Thus, the indium silver solder further inhibits removal of the gold surface even if the solder comes in contact with the gold surface. This is particularly beneficial in situations where the temperature, or the temperature uniformity or gradient, at the solder site is difficult to control precisely. 
     As a result, compared with a pure indium solder process not employing a gold protection layer such as an indium layer, the combination of the indium layer with the indium silver solder can increase the process window, from about 1-2 minutes to about 10 minutes. In addition, and as a result of a larger process window, the indium silver solder used in conjunction with an indium layer over the gold surface allows for greatly improved production yields with less process constraints. 
     In a preferred embodiment, the solder joint formed on the indium layer covering the gold surface provides an improved hermetic seal at both the fiber entrances, and exits, of the device. 
     It is possible in some embodiments to remove an intermediate portion of the fiber sleeve and place the sleeveless portion within the entrance or exit tunnel through the side wall of a device housing where the solder joint will be formed. Removing an intermediate portion of the sleeve conveniently allows the multiple fibers of a ribbon-type cable to be generally maintained in relative alignment during the solder process. It also conveniently allows alignment during an indium fiber/fibers metalization process if applicable. 
     The fiber or fibers may be metallized with a pure indium coating. The indium metalization provides good wetting with the indium silver solder and adheres well to a glass fiber. If metallized with pure indium, no additional flux is necessary to provide a good solder joint. Similarly, no flux is necessary on a pure indium layer to provide a good solder joint with the interior tunnel surface. 
     If the metalization layer is formed by stripping and metallizing a bare fiber, preferably the metalization layer does not extend under the sleeving. If such is the case, a sheath, such as epoxy, may be formed to extend over the sleeving, the bare fiber, and onto an adjacent portion of the metallized fiber for strain relief if desired. 
     As such, in some embodiments, the sheath seals the interface between the sleeving and the metalization layer, and covers the bare portion of the fiber. It also provides rigidity at the interface between the sleeving and the metalization layer to help protect against breakage of the fiber at this interface during cable installation through the housing wall, and during positioning and soldering of the cable. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a partial cut away side view of fiber within a walled orifice of a possible embodiment in accordance with the present invention. 
     FIG. 2 shows a partial cut away side view of a possible embodiment of a fiber extending through an orifice of a sidewall of a device package in accordance with the present invention. 
     FIG. 3 shows a partial cut away side view of a possible embodiment of a metallized fiber in accordance with some embodiments of the present invention. 
     FIG. 4 shows a possible indium plating process in accordance with the present invention. 
    
    
     DESCRIPTION 
     FIG. 1 shows a fiber  110  surrounded by a sleeving  120 . The fiber extends into an orifice  130  in a wall  140 . The orifice  130  has an associated interior orifice surface  135  within the orifice  130 . The fiber  110  has a metalization coating  115 . A solder joint (not shown) can be formed in the space surrounding the metallized fiber to secure the fiber  110  to the interior orifice surface  135 . The orifice  130  and the interior orifice surface  135  may be a ferrule, a tunnel, a channel, a tube, or other such structure, in, or coupled to, a housing wall  140  of a device package (not shown). 
     The interior orifice surface  135  has a gold surface  142  over a substrate  144 . In one embodiment, the substrate  144  is a nickel iron compound. In such an embodiment, a nickel plating layer  146  may be formed over the substrate  144  to facilitate adhesion of the gold surface  142  to the wall  140  as shown in FIG.  1 . 
     Indium solder may be used to form the solder joint (not shown). It has been discovered by the present inventors, however, that unless practiced in a narrow process window, such as one minute or less, the indium solder can dissolve the gold layer  142  during the solder process. This exposes the nickel plating layer  146  to the indium solder, causing oxidation of the nickel, which results in a poor bond with the wall  140 . This has been observed to cause yields of about 10% or lower. 
     Instead, the gold surface  142  of the interior orifice surface  135  is coated with an indium layer  148 , and an indium silver solder is used to form the solder joint. Indium silver solder can have a lower melting point than indium, about  148  degrees Celsius in the case of 97% indium 3% silver solder. In contrast, indium has a higher melting point of about 156 degrees Celsius. 
     The indium silver solder (not shown) should be selected to have a melting point below that of the indium layer  148 . As such, the indium silver solder should have a silver content of about 1% to about 5%. This allows the indium layer  148  to act as protective barrier over the gold surface  142 . This inhibits the indium silver solder from dissolving the gold surface  142 . If the silver content in the indium silver solder is, or becomes too great, the melting point of the indium silver solder can rise above that of the indium layer  148  and reduce the protective benefits provided by the indium layer  148 . 
     The indium layer  148  adheres well to the gold surface  142  and can be plated to about 50-70 microns using an electrolyte solution. The indium silver solder, in turn, adheres well to the indium layer  148 , or to the gold surface  142  if some or all of the indium is dissolved during the solder process. 
     In addition to being inhibited from coming in contact with the gold surface  142 , indium silver solder is less reactive if it does contact the gold surface  142 . As a result, indium silver solder reduces the reaction rate with the gold surface  142  even if the indium layer  148  is sacrificed or removed by the indium silver solder. The silver in the indium silver solder functions to reduce the reaction of the indium in the solder with the gold surface  142 . Thus, the indium silver solder further inhibits removal of the gold surface  142  even if the solder comes in contact with the gold surface  142 . This is particularly beneficial in situations where the temperature, or the temperature uniformity or gradient, at the solder site is difficult to precisely control. 
     As compared to a pure indium solder process without a gold protection layer, the combination of the indium layer  148  with indium silver solder having about 3% silver can increase the process window from about 1-2 minutes to about 10 minutes. In addition, and as a result of a larger process window, the indium silver solder used in conjunction with an indium layer  148  over the gold surface  142  allows for greatly improved production yields with less process constraints. 
     In one embodiment, as shown in FIG. 2, a NiFe substrate  243 , such as 46% NiFe, has a nickel plating layer  246  with an overlying gold surface  242 . The gold surface  242  covers the entire housing  205  to provide a relatively non-reactive coating within an interior chamber (not shown) of a device package. A non-reactive interior housing surface is desirable around MEMS devices, and of particular importance in optomechanical MEMS devices, to help ensure proper performance of the device. 
     In this embodiment, an intermediate portion of the sleeve  220  is removed, and the sleeveless portion placed within a tunnel or a ferrule  237  through a side wall  241  of a device housing  205 . Removing an intermediate portion of the sleeve conveniently allows the multiple fibers of a ribbon-type cable to be generally maintained in relative alignment during the solder process. It also conveniently allows alignment during a fiber/fibers metalization process, discussed further below, if the fiber is not pre-metallized. 
     The fiber or fibers  210  may be metallized with an indium coating  215 . The indium metalization  215  provides good wetting with the indium silver solder and adheres well to a glass fiber  210 . If metallized with pure indium, no additional flux is necessary to provide a good solder joint. Similarly, no flux is necessary on a pure indium layer  248  to provide a good solder joint with the interior ferrule surface  235 . 
     The indium layer  248  improves the reliability of solder flow application and allows the indium silver solder  247  to be applied by heating a portion of the wall  241  adjacent the ferrule  237  and contacting an indium silver solder wire to a portion of the wall  242  near the ferrule  237 . In some embodiments, the size of the ferrule and/or its relationship to the size of the fiber and the sleeving is selected so that surface tension will cause the molten solder to remain substantially within the interior of the orifice. 
     It is contemplated that fibers could be metallized with other materials such as for example gold, silver, copper, nickel, nickel/gold or the like, or a combination thereof. Indium metalization, however, allows a low temperature fiber metalization process. Indium silver solder has a low Young&#39;s Modulus so reduces thermal stressing of the fiber during the metalization process. It also allows the fiber to maintain good optical properties and provides a good solder joint as discussed above. 
     Further, indium does not require evaporation or sputtering type of processes and machinery for the metalization process. As such, indium metalization can allow higher throughput and require fewer process steps than other metalization processes. If silver is used, the silver content of the indium silver solder should be selected so that any silver added to the solder from the metalization layer, or any other source, does not cause the solder melting point to rise above that of the indium layer. 
     Furthermore, it is contemplated that pure indium solder and a sacrificial silver layer on the gold surface could be used to inhibit the gold surface  242  from being dissolved without using the indium layer  215 . Nevertheless, the lower temperature and measured silver content in the solder provided by indium silver solder wire is preferred. 
     It is further contemplated that a pre-metallized fiber, such as a gold metallized fiber, could be provided with an overlying indium coating to provide a similar protection of the gold metalization layer of a gold metallized fiber, if desired. 
     Turning to FIG. 3, metalization of the fiber  310  may be performed by applying molten indium onto a glass fiber  310  to form a metalization layer  315 . Indium wets to glass, so prior to indium metalization of the fiber  310 , the fiber should be cleaned, but no flux should be applied. This is because the flux would remove the surface oxide on the glass that facilitates the adhesion of the indium to the fiber  310 . 
     As discussed above, it is possible to metallize the fiber at an intermediate portion of the cable after that portion of the sleeving has been removed as illustrated in FIG.  3 . In such an embodiment, the metalization layer  315  may not always extend under the sleeving  320 . A sheath  312 , such as epoxy, may be formed to extend over the sleeving  320  and an adjacent portion of the metalization layer  315 . 
     In this embodiment, the sheath  312  seals the interface between the sleeving  320  and the metalization layer  315  and covers a bare surface portion  311  of the fiber  310 . Furthermore, it provides rigidity at the interface between the sleeving  320  and the metalization layer  315 . As such, in this embodiment, the sheath  312  can help protect against breakage of the fiber at this interface, during installation of a cable into the housing, and during the solder process itself. It also is possible to have epoxy (not shown) secure the sleeving to the housing  205  shown in FIG.  2 . This may be done after installation and soldering. 
     Although FIGS. 1 &amp; 2 illustrate an interior orifice surface surrounding the fiber, some embodiments may have a solder joint used to maintain fibers in a desired position with the housing without soldering to a wall that goes around the fiber. For example, a fiber, or fibers, may have a solder joint with a planar or other non-completely surrounding surface. As such, the solder joint could be formed at other than an entry or an exit to a package. 
     The indium layer  248  within the ferrule  237  shown in FIG. 2, may be applied with an electrolytic process as illustrated in FIG. 4. A gold plated housing may be selectively coated with XP-2000, available from Pyramid Plastics, Inc., located in Hope, Arkansas, or with other commercially available masking products, to form a mask  407 . The masked housing  400  may then be lowered into an indium plating solution, such as a solution of indium sulfamate available from Indium Corp. of America, located in Utica, N.Y. This allows indium plating to occur within the ferrule  237 , shown in FIG. 2, without plating the entire surface of the housing  205 . Pure indium may be used as the anode with the housing  400  being the cathode as illustrated in FIG.  4 . 
     Typically, the package  400  is not completely submersed in the indium plating solution, but is lowered only enough to submerse the ferrule, multiple ferrules, or other desired solder point. Although shown in FIG. 2 with the indium layer  248  coating only the interior surface  235  of the ferrule  237 , the masking and electrolytic application can result in the indium layer  248  coating adjacent portions of the housing outside of the interior surface of the ferrule  237 . A 50-70 micron thick indium plating has been found to reduce the gold dissolution rate and provide good wetting property to the molten indium silver solder. 
     Some embodiments in accordance with the present invention can be used with a variety of optomechanical type switches. A few examples of optomechanical switches are shown in: U.S. patent application Ser. No. 09/063,664, filed on Apr. 20, 1998, by Li Fan, entitled MICROMACHINED OPTOMECHANICAL SWITCHES, now abandoned; and U.S. patent application Ser. No. 09/483,268, filed on Jan. 13, 2000, by Fan, et al., entitled MICROMACHINED OPTOMECHANICAL SWITCHING DEVICES, both herein incorporated by reference in their entireties. The embodiments of the present invention are not limited to electromechanical optical switches but can be applied to any device coupled to a fiber optic line. 
     While the preferred methods and embodiments of the present invention have been described in detail above, many changes to these embodiments may be made without departing from the true scope and teachings of the present invention. The present invention, therefore, is limited as claimed below and the equivalents thereof.