Metal fusion bonding – Process – Metal to nonmetal with separate metallic filler
Reexamination Certificate
1998-12-31
2001-04-17
Ryan, Patrick (Department: 1725)
Metal fusion bonding
Process
Metal to nonmetal with separate metallic filler
C228S056300, C228S246000
Reexamination Certificate
active
06216939
ABSTRACT:
The invention pertains to a method for making a hermetically sealed package, which package comprises a housing, a lid, and a feedthrough for at least one stripped optical fiber.
A method for making a hermetically sealed package is known from U.S. Pat. No. 4,779,788. The method disclosed in this publication involves feeding an optical fiber through a hole in one of the metal walls of the package. Subsequently, excess solder in a molten state is provided outside the hole to form a solder body surrounding and adhering to the optical fiber. The solder body is also connected with the outside of the metal housing and with at least part of the length of the wall of the hole. During cooling the molten solder shrinks and the free surface of the solder body also shrinks under the influence of surface tension. The solder is drawn towards the hole and onto and around the glass fiber, thus hermetically sealing in the fiber.
It is common practice to use an array of optical fibers instead of one single fiber. An array of, e.g., seven optical fibers requires the making of at least seven holes and seven hermetical feedthroughs, for instance by the method disclosed in U.S. Pat. No. 4,779,788. Accordingly, the process for manufacturing flat packages is complicated and expensive. Not surprisingly, the hermetical feedthroughs are responsible for the greater part of the total costs involved in said manufacturing process.
U.S. Pat. No. 5,061,035 concerns a method for making a hermetically sealed fiber array comprising a bundle of optical fibers provided with a solderable metal coating (preferably nickel and gold) to ensure adhesion to a solder. The optical fiber bundle is placed in a supporting structure having a front face and a coated inner surface so that the end face of the optical fiber bundle is flush with the front face of the supporting structure. The assembly is then heated and fluxless solder is applied to the end face and allowed to wick between the individual fibers and the inner surface of the support. Preferably, the solder is sucked in by means of a vacuum on the back face of the supporting structure.
U.S. Pat. No. 4,174,491 relates to a method in which a metallized optical fiber is placed in a groove in a mount substrate. A keeper substate which is also provided with a groove is placed on top of the mount substrate so that the grooves snugly hold the optical fiber. The grooved substrates are welded together by means of a solder coating provided on their grooved surfaces. Subsequently, the interspace between the optical fiber and the substrates is filled up with a low fusing solder.
DE 28 29 548 discloses the deposition of a thin layer of indium and lead on both an upper and a lower substrate's surface. These layers are used to weld the said substrates together. After this welding step, a lid is hermetically welded onto the upper substrate. The assembly further comprises an optical fiber which is fed through grooves in the upper and lower substrates. After sealing of the lid, the space between the optical fiber on the one hand and the upper and lower substrates on the other hand is filled up with a low fusing solder.
U.S. Pat. No. 5,412,748 discloses a similar method. A fiber feedthrough between a cap and a submount is hermetically sealed in a separate process step after the cap has been soldered to the submount. It is described how an auxiliary solder is used for filling the space between the optical fiber and the cap. This auxiliary sealing solder can be formed by melting a solder having a relatively low melting point and injecting the solder into the gap using a capillary phenomenon.
It is an object of the present invention to provide a method for hermetically sealing packages which involves comparatively few and simple process steps. This object is achieved by the following process steps in the method described in the first paragraph:
placing the optical fiber or fibers and at least one solder preform between the sealing surface of the lid and the sealing surface of the housing
sealing the assembly, at least around the optical fiber or fibers, by applying pressure and heat so as to press the fiber or fibers into the solder.
With this method, all the optical fibers in an array can be sealed simultaneously in a very efficient and effective manner. The seam obtained with the method according to the invention is (practically) impervious to moisture and other gases and easily meets the current standards.
Further, the hermetic fiber feedthrough(s) can be formed using conventional package housings or seal rings and lids and do(es) not require custom ferrules. The number of juxtaposed fibers in this type of feedthrough is limited only by the available package lid seam length and the fiber diameter.
A still further advantage of the present invention resides in the fact that welding or soldering the lid onto the housing is combined with making the fiber feedthroughs. Until now, attaching the lid to the housing and providing the fiber feedthrough were two separate processes.
Within the framework of the present invention the term “solder preform” includes, amongst others, separate rings of solder (adapted to the shape of the lid and/or the shape of the opening in the package body or housing) and relatively thick coatings on the lid sealing surface and/or the housing sealing surface. Said coating can, for instance, be achieved by reflowing solder onto the sealing surface(s) using a proper flux.
The solder of which the preforms are made should have the property that it wets both the optical fiber or fibers and the sealing surfaces of the lid and the housing. Examples of suitable solders are indium, indium/silver, indium/tin, and glass solder (ex Gould).
In order to further simplify the process and make an improved seam it is preferred to use more than one solder preform. Thus, a stack can be obtained comprising at least five elements, e.g., the package body or housing, a first solder preform, the fiber or array of fibers, a second solder preform, and the lid. Since the solder which is to form the seam is present on both sides of the fiber or fibers, it is easier to ensure that it reflows completely around the fiber or fibers. Thus, the chance of obtaining an end product not suitable for sale is reduced.
Irrespective of the number of solder preforms, it is preferred that the (total) thickness of the preform or preforms exceeds the diameter of the stripped fiber, preferably at least by 20 percent. Thus, a hermetic seal with solder both below and above the fiber is ensured even when both the sealing surface of the lid and the sealing surface of the package body or housing are perfectly flat.
In a preferred embodiment, the optical fiber or fibers are solder coated prior to sealing the package. The solder used to precoat the fibers must have the property that it wets glass in a molten state. In this respect, suitable solders are, e.g., indium, indium/silver, and indium/tin solders. With solders of this type, the precoating of the fibers can be accomplished by simply dipping the stripped section of the optical fibers into the molten solder.
As mentioned, the package is sealed through the application of heat and pressure. During this sealing step all or nearly all the voids between the solder and the fiber or fibers are eliminated. In the process according to the invention, it is also possible to apply additional heat after the sealing step, for instance, to further melt the solder and improve the continuity of the solder seam between the package body, the lid, and the optical fiber or fibers.
Depending on the material used for the elements of the package, it can be advantageous to use an intermediate between the housing or the lid and the solder preform or the fibers. For instance, if the housing is made of a ceramic material, a metal sealing ring or brazing pad can be provided around the opening of the housing to enhance the adherence of the solder. The person skilled in the art will have no difficulty in selecting an appropriate combination of materials.
With the method according to the inven
Dilworth & Barrese LLP.
JDS Uniphase Photonics C.V.
Ryan Patrick
Stoner Kiley
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