Note: Descriptions are shown in the official language in which they were submitted.
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METHOD FOR MAKING A HERMETICALLY SEALED PACKAGE
COMPRISING AT LEAST ONE OPTICAL FIBRE FEEDTHROUGH
The invention pertains to a method for making a hermetically sealed
5 package, which package comprises a housing, a lid, and a feedthrough for
at least one stripped optical fibre.
Such a method is known from US 4,779,788. The method disclosed in this
publication involves feeding an optical fibre through a hole in one of the
10 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 fibre. 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
1~ the solder body also shrinks under the influence of surface tension. The
solder is drawn towards the hole and onto and around the glass fibre, thus
hermetically sealing in the fibre.
It is common practice to use an array of optical fibres instead of one single
20 fibre. An array of, e.g., seven optical fibres requires the making of at least
seven holes and seven hermetical feedthroughs, for instance by the
method disclosed in US 4,779,788. Accordingly, the process for
manufacturing fiat packages is complicated and expensive. Not
surprisingly, the hermetical feedthroughs are responsible for the greater
25 part of the total costs involved in said manufacturing process.
US 5,061,03~ concerns a method for making a hermetically sealed fibre
array comprising a bundle of optical fibres provided with a solderable metal
coating (preferably nickel and gold) to ensure adhesion to a solder. The
30 optical fibre bundle is placed in a supporting structure having a front face
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and a coatedinnersu~ace sothatthe end face ofthe opt~calfibre bundle i8
flush wi~ the front face of the supporting 4tluctute. The assembly is then
heated and fluxless solder ia applied to the end face and allowed to wick
' _h~cen the individual fibre~ and the inner !lurface of the support. Prefer~bly~
the solder is sucked in by means of a vacuum on the back face of the
supporting structure.
US 4,174,4g1 relates to a method in which a mehllised optical fibre is placed
in a groove in a mount substrate. A keeper subsbte which is alsc providsd
with a ~r~ove is placed on top of the rnount substrate 80 that the grooves
snusly ho~i the opticàl fibre. The groov~d substr~tes are weldeci to~ether by
means of a 301der coabng provided on their grooved ~rfaces. Subsequentty,
the i~lt~ r~.ace behNeen the optical fibre and the -~ubsllt.les is fllled up with a
low fusin~ solder.
DE 28 2g 548 dis~loses the deposition o~ a thin layer of indium and le~d on
both an upper and a lo~er substrate~s surf~ce. These layers are used to we!d
the said YU~ dl~ together. A~er this welding step, a 11~ is h~ etically
welded onto the ~pper subetrats, The assembly further comprise6 an optical
flbre which is ~ed throu~h ~rooYes in the upper and lower substrates. Anar
sealin~ of tho lid, the ~pace bet~r~e~n the optical fi~re on the one hand and the
upper and lower substrates on ~he other hand is fille~ up with a low fusing
solder.
US 5,412,748 discloses a similar Illcthod. A fibre re~Jll~ gh betvJ~en a cap
~nd a ~ubmount Is hermetlcally sealed in a separate process step after the
cap h~s been ~old~r~d to th~ submount. It is described how an auxiliary
solder is used for fflllng the ~pace beh~een the optical fibre a~ci the cap. This
auxiliary sealing solder ~an ~e formed by meltin~ a sclder havin~ a relatively
low meltin~ point and inJecting the solder into the sap using a capillaly
phenomenon.
~JIENDED SHEET
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It is an object o~ the prcsu~t inver,~ion to provide a method for hei,netically
~ealins pac~ s which involve~ comp~ratively few and simple p,o~er~
steps. This object is achieved by the following process steps in the method
describ!d in the first para~raph:
placing the optical fibre ~r ffbres and at least one solder preform
between the sealin~ surface of the lid and the sealin~ surhce of the
housing
- scaling the as~embly, at le~st around Ule optlcal flbre or fibres, by
applyin~ pre~ure and he~t ~o a8 to pre~s the fibre or nbr~s into the
solder.
Wth thi~ n~t~lod, all t~le optic~l fibre~ in an array can be ~e~led
simu~taneously in a very ef~icient and effective manner. The ~eam obtained
A~NDF~ S~EET
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with the method according to the invention is (practically) impervious to
moisture and other gases and easily meets the current standards.
Further, the hermetic fibre feedthrough(s) can be formed using
5 conventional package housings or seal rings and lids and do(es) not
require custom ferrules. The number of juxtaposed fibres in this type of
feedthrough is limited only by the available package lid seam length and
the fibre diameter.
10 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
fibre feedthroughs. Until now, attaching the lid to the housing and providing
the fibre feedthrough were two separate processes.
15 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
20 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 fibre or fibres and the sealing surfaces of the lid and
the housing. Examples of suitable solders are indium, indium/silver,
25 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
30 housing, a first solder preform, the fibre or array of hbres, a second solder
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preform, and the lid. Since the solder which is to form the seam is present
on both sides of the fibre or fibres, it is easier to ensure that it reflows
completely around the fibre or fibres. 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
fibre, preferably at least by 20 percent. Thus, a hermetic seal with solder
both below and above the fibre is ensured even when both the sealing
10 surface of the lid and the sealing surface of the package body or housing
are perfectly flat.
In a preferred embodiment, the optical fibre or fibres are solder coated prior
to sealing the package. The solder used to precoat the fibres must have
15 the property that it wets glass in a molten state. In this respect, suitable
solders are, e.g., indium, indium/silver, and indiumltin solders. With solders
of this type, the precoating of the fibres can be accomplished by simply
dipping the stripped section of the optical fibres into the molten solder.
20 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 fibre or fibres 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
25 solder seam between the package body, the lid, and the optical fibre or
fibres.
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
30 the solder preform or the fibres. For instance, if the housing is made of a
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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 diff~culty in selecting an
appropriate combination of materials.
With the method according to the invention it is also possible to place a
further solder preform on top of the fibre or fibres and a further layer
comprising at least one fibre on top of said further solder preform. Thus,
several layers containing at least one fibre each can be stacked one on top
10 of the other and sealed simultaneously.
The invention further pertains to a hermetically sealed package obtainable
by the method as described above, which package comprises a housing or
package body and a lid (preferably both having a fiat sealing surface)
15 sealed onto it by means of a continuous solder seam, through which solder
seam at least one fibre is fed.
This package allows very efficient manufacture, is inexpensive, and does
not require custom ferrules or the like. Also, the width of a fibre array
20 feedthrough can be made ver,v small (because ferrules or grooves are not
necessary), thus reducing the package size in the dimension of the array.
It should be noted that JP 63 085505 concerns a method for welding and
fixing optical fibres into V-grooves formed on a substrate. Heating elements
25 consisting of a high resistance material are deposited into the grooves and,
in turn, a "low melting metal" such as a solder is plated on the heating
elements. The optical fibres are coated with a welding metal such as gold
and placed in the grooves. During subsequent heating by means of the
heating elements, a plate is pressed onto the fibres and the "low melting
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metal" is melted. Thus, the optical fibres are adhered to the metal. A
hermetically sealed package is not disclosed.
Figure 1 shows schematically an assembly according to the invention just
5 prior to the application of heat and pressure. The assembly comprises a
package body or housing 1, a lid 2, two solder preforms 3, and an array 4
consisting of seven individually precoated optical fibres.
Figure 2 shows the assembly of Figure 1 after the application of heat and
10 pressure The solder preforms 3 have formed a continuous solder seam 5.
The invention will now be illustrated by way of a further, more detailed
example. As a matter of course, the invention is in no way restricted to this
example.
Examples
Hermetically sealed packages with up to 9 fibre feedthroughs were made
20 with the process described below. The packages had a single fibre
feedthrough at the input side and a fibre array feedthrough containing as
many as 8 fibres on the output side. All of the package housings described
hereinbelow were made of alumina and had gold over nickel plated Kovar
seal rings brazed onto gold plated pads on the package sealing surface.
25 The lids were also made of Kovar with gold over nickel plating. The optical
fibres were all Corning SMF-28. Pure indium was used as the solder to
make the seals. The fibre feedthrough fabrication and package sealing
process was comprised of the following steps:
30 1. Indium was applied to the top surface of the package seal ring and to the
sealing surface of the lid by first coating the sealing surfaces with a thin
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layer of flux. Indium rings, each 0.010" thick, were then pressed onto the
flux layers. The package bodies and lids were placed in a nitrogen
purged reflow oven and heated until the indium melted and completely
wetted the sealing surfaces. The package bodies and the lids were
5 allowed to cool to near room temperature in the oven. Flux residues
were removed from the coated sealing surfaces in an isopropanol bath.
2. Both the input single fibres and the output fibre arrays were also coated
with indium prior to package sealing. This was done by first stripping off
10 the acrylate coating from the sections of the fibres or fibre arrays that
would be sealed into the indium package/lid seam. The exposed silica
surfaces of the fibres were cleaned with isopropanol and lens tissue and
then blown dry. The cleaned portions were then slowly passed back and
forth through molten indium on a glass stage which was heated from the
15 bottom by a heated brass block and from the top by a heating element
dipped into the indium. For the fibre arrays, the fibres were maintained in
a linear array by external clamps. ~ stream of nitrogen was directed over
and around the molten indium. After being in the molten indium for about
30 seconds (about 2 or 3 passes), the fibres or hbre arrays were
20 removed and allowed to cool.
3. The fibre feedthroughs were formed by first positioning the indium
coated sections of the fibres or fibre arrays over the package seai ring at
the input and output locations and then holding them in place with
25 external clamps. Next, the lid was positioned directly above the package
seal ring and held in place by external guides. Heat and pressure were
then applied to the input and output ends of the lids until the indium on
the lid, then on the fibres, and hnally on the package seal ring melted
and flowed together to form a continuous indium seal between the seal
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ring and the lid and around the fibres. The heat was removed and the lid
was held in place until the indium had frozen.
4. Except for a short, less than 1 cm long, section, all of the remaining
5 sections of the packagellid seal were then formed using a similar
application of heat and pressure. The short section was left unsealed to
facilitate the bakeout of water and other volatiles from the package
interior.
10 5. Bakeout of the package assembly was carried out under vacuum at
100~C for 14 hours.
6. After bakeout, the package assembly was allowed to cool and was then
transported directly into a nitrogen purged dry box. The package sealing
15 was completed by applying heat and pressure to the remaining unsealed
section until the indium on the lid and on the package seal ring melted
and flowed together to form a continuous indium seal between the seal
ring and the lid.
20 Sectioning of 8-fibre feedthroughs sealed with the above process clearly
shows that the fibres are completely surrounded by indium and that the
indium completely fills the space between the package and the lid.
Packages having a 1-fibre input feedthrough and an 8-fibre output
feedthrough sealed with the above procedure have been helium leak tested
25 and then subjected to a number of environmental stress tests before being
leak tested again. The leak rates measured before and after the
environmental stress tests were all under the 1 x 10E-6 atm-cc/sec air
equivalent standard leak rate limit specified by MIL-STD-883 for hermetic
packages of the size sealed. These tests, and the associated package
30 counts, include:
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Low Temperature Storage (-40~C), 92û hours: Two packages.
High Temperature Storage (71~C), 600 hours: Two packages.
Temperature Cycling (-40 to 7Q~C, 20 cycles/day), 120 cycles: Three
packages.
Thermal Shock (0 to 100~C in water baths, 2 minutes per bath), 15 cycles:
Five packages.
RGA has also been carried out on similar packages after sealing and after
15 cycles of 0 to 100~C thermal shock in water baths. The measured water
content was under the 5000 ppm limit specified by MIL-STD-883 for
hermetic packages. These results clearly show that the process described
above simultaneously forms robust hermetic seals around the optical fibres
and between the package and the lid.
1~ Optical transmission and return loss measurements have been carried out
at 1.53 ~Lm on fibre feedthroughs made with the above process. The loss
per feedthrough was less than 0.05 dB. No return peaks above -70 dB
were observed in CDR scans. The fibre sealing process described above,
therefore, does not result in any cracking or microbending of the fibres.
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