Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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The present invention relates to a workpiece and a process for
making a workpiece suitable for use in a semiconductor device, and more part-
icularly a workpiece comprising a thermally and electrically conductive lead
and a refractory metal contact joined by a high temperature brazing process.
Passivated semiconductor devices generally inclucle a semiconductor
body composed substantially of silicon, a layer of passivating material such
as glass or plastic disposed about the semiconductor body, and at least one
metallic contact extending outwardly from the semiconductor body through the
passivating layer as an external contact for connection with the associated
~ 10 circuitry. More specifically, such devices require that the metallic contacts
be refractory in nature in order that the coefficients of thermal expansion Oe
the semiconductor body, the passivating layer and the metallic contaots be
reasonably matched to avoid breakage during thermal cycling. Molybdenum,
tungsten, tantalum and various special;alloys are typical of the refractory
metals used as such refractory metal contacts; however, since such materials
1 are both expensive and relatively poor conductors of both heat and electric
` current, the refractory metal contacts are generally joined to goQd convention-
- al con~uctors ( such as copper, silver or various special alloys) just beyond :
th-e passivating layer, the connection between the refractory contact and the
other circuitry elements being made by the conventional conductor. In the case ~ ;
of axial-lead-construction semiconductor rectifiers, the connection of the con-
ventional axial lead to the refractory metal contact is accomplished by one of
;. the following two procedures.
In the first procedure, the refractory material is initially plated
with a solderable metal such as silver before application of the passivating
layer. After the passivating operation, the axial leads are attached to the
refractory metal contacts using ~Isoft solder~' preforms with melting points
~`s typically less than 350 C. Devices containing such connections have the dis-
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advantages commonly associated with soft solder contacts. If extreme tempera-
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ture control is not exercised in soldering the axial lead to other circuit
components~ the axial lead may detach from the refractory metal contact as the
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soft solder heats up. In any case, it has been found that such soft solder ~ ~ ~
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joints are subject to thermal fatigue and a resultant short operating life.
In the other procedure, the refractory metal contact is joined to the
' axial lead by a special welding process known as butt or percussion welding~
-~ The joint and the axial lead must thereafter be exposed to all the chemical and
heat-treating processes subsequently required to (1) join the semiconductor body
to the refractory metal contact, (2) etch the subassembly, and (3~ apply and
fuse the passivating glass layer. The devices fashioned according to this pro-
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`~ cedure are not reliable in the first place because a true weld is not possible
'`! between the refractory metal of the contact and the conventional conductive
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~ metal of the axial lead. Furthermore, the processing operations required to
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complete the semiconductor device subsequent to formation of the refractory con-
tact/axial lead joint frequently result in a weak and porous joint which will
develop high electrical and thermal resistance in time or eventually even fail
~: mechanically and fall off. Experience has shown that butt welded joints have ;;
extremely high failure rates when exposed to conductions of high temperature and
- -high humidity, the failure rate rising as high as 50% at conditions of 85 C and
85% relative humidity. -
` Yet another disadvantage of the process of joining a refractory metal
contact to an axial lead by the butt or percussion welding technique is that
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~; certain details of the technique are proprietary information, not readily avail-
able on the open market. But even if the full details of this technique were
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i generally known, its very nature imposes undesirable constraints on the design
of the refractory metal contacts and the axial lea~suseful therein. In the
technique, the axial lead is actually in motion as the weld between the axial
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~; lead and the refractory metal contact is made, the parts being brought together
: to make the electrical contact by rapid advance of the axial lead once the
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refractory metal contact is in position. The technique requires that each
refractory metal contact be inserted into an individual clamp-designed welding
electrode in order to control the we]ding current. Thus, for the automatic
rapid positioning essential to mass production, it is necessary that the dim-
ensions of the refractory metal contact adhere to certain rigid specifications, `
and in particular that it be of a length sufficient to be grasped by the weld-
ing electrode clamp. To insure reasonable centering of the axial lead relative ~
to the end of the refractory metal contact, the end of the~axial lead must ~ -
generally be at least 0.5 mm. smaller in diameter than the end of the refract-
ory metal contact. Thus the technique requires a minimum cross-sectional area
.,
for the refractory met-al contact and additionally imposes~a maxi~um cross-
sectional area for the axial lead for a given refractory metal contact. These
; length and cross-sectional limitations restrict the degree of miniaturization
`~ which can be obtained in any semiconductor device ultimately embodying the
" axial lead/refractory metal contact subassembly.
Accordingly, it is an object of the present invention to provide a
~` process for attaching a thermally and electrically conductive lead to a refract- ~
ory metal contact using a high temperature brazing process. ~ ;
It is another object to provide such a process which avoids the afore-
; 20 mentioned disadvantages of 'Isoft solder" and "butt welded~ joints.
It is also an object to provide such a process wherein the resultant
joint is of higher physical strength and lower porosity than a butt-welded joint, ~;
withstands a higher temperature than a soft solder joint, and withstands condi~
tions of high temperature and high humidity better than a butt-welded joint.
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It is a further object to provide such a process which does not i ffl ~ ~ -
pose limitations on the minimum length or cross-section of the refractory metal
contact or on the maximum cross-section of the axial lead for a given refractory
metal contact and thus lends itself to unimpeded miniaturization of the semi-
conductor device embodying the axial lead/refractory metal contact subassembly.
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Yet another o'bject is to provide such a process which is adapted to
~' mass production techniques.
Still another object is to provide such a process which utilizes a
' special preform, is simple and economical to perform, is less expensive than
butt welding, and provides a joint superior to those produced by soft solder
or 'butt welding processes.
A final object is to provide a brazed lead electrode workpiece adapted
~ for use in a semiconductor device and comprising a lead member joined to a
l~ refractory metal contact by the aforesaid brazing process.
It has now 'been found that the above and related objects of the present ~ 5
; invention are obtained in a process for attaching a lead member to a cortact
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'~ member by initially providing a lead member formed substantially of a thermally
and electrically conductive metal and terminating at one end in a joining sur~
; faceg and an axially extending contact member formed substantially of a '~
'x refractory metal and terminating in a joining surface of such refractory metal~
The joining surfaces of the members are placed in contact with a silver/copper-
~'~ based brazing alloy having a wetting point of at least 450 C. The brazing
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alloy is then heated (preferably in an inert atmosphere at atmospheric pressure) ~ '"
- at least to its wetting point for a period of time sufficient to melt the
' 20 brazing alloy. Thereafter the molten brazing alloy is allowed to cool and sol-
'- idify in contact with the joining surfaces cf the members, thereby to join the
`~l contact member and the lead member into a unitary structure. '':~',,., :.
The refractory metal is selected from the group consisting of tungsten,
'` molybdenum, tantalum, and alloys thereof, and is preferably molybdenum. The
~'~ conductive metal is selected from the group consisting of sllver and copper and
''~3 alloys thereof, and is preferably copper. A preferred brazing alloy is, on a
weight basis, about 80-89% copper~ about 5-15% silver and about 4-6% phosphorus~preferably an 80/15/5 alloy. Preferably, the brazing alloy is provided as a
` preform with joining surfaces of substantially $imilar configuration and size
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a~utting the joining surfaces of the members.
The present invention provides a brazed lead electrode workpiece
adapted for use as a component of a semiconductor device. The workpiece com-
prises an axially extending contact member formed substantially of a refractory
metal and terminating at one end in a joining surface, a lead member formed
substantîally of a thermally and electrically conductive metal and terminating
at one end in a joining surface, and a brazing alloy melting point of at least
450 C disposed between and securing together the joining surfaces of the mem-
bers, thereby to join the members into a unitary structure.
Figure 1 is a fragmentary exploded plan view of a workpiece according
to the present invention.
Figure 2 is a fragmentary plan view of workpiece of Figure l; and
Figure 3 is a fragmentary plan view of a semiconductor device incorp-
orating the workpiece as a component thereof.
Referring now to the drawing, and in particular to Figure 2 thereof,
therein illustrated is a brazed lead electrode workpiece embodying the principles
of the present invention and generally designated by the numeral lO~the in-
dividual components thereof being illustrated in exploded view in Figure 1.
~ Generally speaking, the workpiece 10 comprises an axially extending contact
- 20 member generally designated by the numeral 20, a brazing alloy generally
designated by the numeral 30~ and a lead member generally designated by the ~;-
; numeral 40. The workpiece 10 is adopted for use as a component of a semi-
conductor device or subassembly S0 including a semiconductor body 52, as
illustrated in Figure 3.
More specifically7 the axially extending contact member 20, generally
referred to as a ~'slug~', includes joining surfaces 22 and 24 at opposite ends
thereof. The contact member 20 is formed substantially of a refractory metal
material and is preferably composed substantially of molybdenum, tungsten,
tantalum and alloys thereof. Whether the alloys be composed of two or more of
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the aforementioned refractory metals or of one or more of the refractory metals
with other materials, the alloys must~ of course, be selected according to their
Isnown coefficients of expansion to insure that the coefficients of expansion of
the se~iconductor body 52, the contact member 20 and any materials used to
passivate the semiconductor body 52 are compatible.
The brazing alloy preform 30 is of relatively planar configuration
and has opposed joining surfaces 32 and 34 at opposite ends thereof. The
~- brazing alloy is a coppellsilver-based alloy having a wetting point of at
least 450 C. The alloy may include, on a weight basis3 up to 25% of metals
other than copper and silver--for example, nickel, tin, phospho~s, etc.--so..
long as it is devoid of any high vapor pressure metals~(such as zinc or cadmium)which might adversely affect the semiconductor body 52 to which the workpiece
10 will ultimately be secured. Any available copper/silver-based brazing alloy
having a wetting point of at least 450 C may be used provided that the alloy is
capable of effectively wetting and bonding with both the refractory metal of -
contact member 20 and the conductive metal of lead member 40.
A preferred brazing alloy comprlses on a weight bases about 83-89%
copper, about 5-15% silver, and about 4-6% phosphorus, and may be a commerc-
ially available 80/15/5 high temperature brazing alloy marketed by Englehard
" Industries Division of Englehard Minerals and Chemicals Corp. (Murray Hill,
~ ~ New Jersey) under the trademark ~Silvaloy 15~~ and by Handy and Harmon, Inc.
i under the trademark "Silfos". This brazing alloy is characterized by a freez-
ing point of about 640 C and a wetting point of about 705 C. Because the ;~ -
i phosphorus acts as a flux, it requires neither an oxidizing nor a reducing
environment externally maintained during the brazing process. In this brazing
alloy composition~ the copper (melting point 1083 C) constitutes the main
brazing component, with the silver (melt m g point 960 C) and the phosphorus
(melting point 44 C) lowering the wetting point of the composition to a desir-
able temperatvre, and the phosphorus also providing the necessary flu7sing
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action to enable brazing in an inert atmosphere.
Other preferred brazing alloys include the 56-72/28-42 copper/silver_
based alloys such as a 72/2~ Ag/Cu eutectic commercially known as "BT braze"
(avai]able from Handy and Harmon, Inc. under the tradename "720 ALIOY"), a
71.5/28/0.5 Ag/Cu/Ni alloy ~qlS AIIOY"), a 60/30/10 Ag/Cu/Sn alloy ("603
AIIOYIl), a 56/42/2 Ag/Cu,~Ni alloy ("559 AILOY"), etc. These bra~ing alloys are
characterized by a freezing point of about 620-765C and a wetting point of
about 620- 850 C. As they contain no flux, these ~brazing alloys require a
slightly reducing environment externally maintained during the brazing process
(such as is conventionally provided by the presence of hydrogen gas)O Accord-
ingly, these brazing alloys are preferably used only when the lead members 40
are composed of materials which are not particularly subject to hydrogen em-
brittlement - e.g., oxygen-free copper. ;
The nail-headed axial lead member 40 has a joining surface 42 on the
exposed surface of the head 44 thereof, the tail or other end being available
~i; for connection to other circuitry membersO The lead member 40 is formed of a
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thermally and electrically conductive metal such as copper, silver, or alloys
thereof, the alloys of such metals by themselves or individually with other
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` materials being selected for their ability to braze well with the brazing alloy
of the preform 30. While it is preferred that the conductive metal of joining
surface 42 be formed substantially of the aforementioned copper, silver or
alloys thereof, a core or sheath is frequently used in connection with the Gon- :
tact member 40 to facilitate its function~ig as a heat sink for the semiconduct-
.
or body 52, to reduce the cost of the materials used in the lead member 40,
and/or to provide magnetic susceptibility for lead member 40. Suitable materials
for the lead member thus include oxygen-free copper (especially suited for use
in a reducing atmosphere), zirconium copper (for a stiffer lead~ and copper
clad iron (for economy).
The lead member 40 may be joined to the contact member 20 by placing
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the joining surface 42 of the lead member 40 and the joining surface 24 of the
contact member 2Q in contact with the joining surfaces 34 and 3~ respectively,
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of the preform 30. To facllitate this operation the various joining surfaces
24, 32, 34, 42 are preferably of the same size and configuration. If desired~
the lead member joining surface 42 m~y be greater in cross-section than the
contact member joining surface 24. The preform 30 is then rapidly heated (for
.
about 15 minutes) at least to its wetting point, held at that temperature for
a period of time sufficient to melt the alloy (generally about 5 minutes) and
; thereafter the molten alloy of the preform 30 is allowed to cool and solidify
: 10
n contact with the joining surfaces 24, 42 of the contact member 20 and lead
member 40, thereby joining the members 20, 40 into a unitary structure. Where
j the brazing alloy 30 contains a flux, this brazing operation may be performed
.~
under atmospheric conditions--i.e., in a reducing, inert~ or oxidizing envir-
onment; otherwise a slightly reducing atmosphere is required.
In a preferred embodiment of the process, the components of the
workpiece are assembled in a subassembly, as indicated in Figure 2, with the
~; joining surface 24 of the contact member 20 contacting the joining surface 32
of the preform 30, and the other joining surface 34 of the preform 30 contact-
,~
ing the joining surface 42 of the lead member 40. A brazing fixture or jig of
,! graphite~ stainless steel or other conventional material is suitably employed
to hold the components of the subassembly in proper orientation during passage
through the furnace. The entire subassembly is then placed on a conveyor belt ~ -
~ which carries it through a tunnel furnace which, if desired, may be provided
- with an inert atmosphere (such as dry nitrogen) at one atmosphere pressure.
m e maximum temperature within the tunnel furnace must be sufficient to wet or
~' melt the brazing preform 30, and is preferably about 10-15 C higher. Generally
' a dwell time of about 15-2~ minutes within the tunnel furna;ce is sufficient to
l cause brazing of the lead member 40 to the contact member 20. Preferably the
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~ entire subassembly 10 is heated over a 15 minute entry period to at least the
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wetting point of the brazing alloy, held there for about 5 minutes, and then
slowly cooled over a 15 minute period to below the wetting point of the brazing
alloy . The workpiece may then be further cooled and is then ready for sale or
incorporation into a se~iconductor device.
The process results in the workpiece components being integrated
into a unitary structure having bra3ed joints which withstand higher tempera-
tures than soft solder joints, are stronger and less porous than butt welded
` joints, and withstand thermal cycling to high temperatures and high humidity
conditions (such as 85C and 85% relative humidity) without failure or the
development of high electrical and thermal resistances. The solidity and
strength of the brazed joints thus formed permit the workpiece 10 to be further ~ ;
processed without resultant damage thereto.
Referring now to Figure 3g therein illustrated is a semiconductor ;~
device subassembly generally designated by the numeral 50 and incorporating
therein a pair of workpieces 10. The semiconductor generally designated by
the numeral 52 comprises a diffused silicon chip 54 and, at each end thereof,
a layer of evaporated aluminum 56 forming a joining surface for the semiconduc-
tor 52. The semiconductor body 54 is formed substantially of silicon, although
one or more portions thereof may have minute quantities of various conventional
."
dopants such as phosphorus, boron and the like~ as will be well recognized by
those skilled in the semiconductor art. For clarity of illustration the semi-
conductor 52 has been illustrated as a rectifier adapted for connection to only
two lead members~ although the workpieces 10 of the present invention may be
attached to any other semiconductors, such as the N-type, P-type, or combination
` type and those useful as junction, field effect, or other types of semiconductors.
The workpiece 10 of the present invention may be used regardless of whether the
semiconductor device comprises a single thin wafer-like diode (as shown) or a
relatively long stack of several chips joined in series and brazed together with
conventional materials (such as aluminum), with each of the various chips having
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a plurality of workpieces and/or other leads extending therefrom. The alumi-
num joining surfaces 56 may be applied to the silicon body 54 by conventional
techniques well knol1n in the semiconductor art, the preferred technique being
an evaporation deposition technique.
Each workpiece 10 is easily incorporated in the semiconductor device
50 as follows: The refractory contact member 20 is joined to the semiconductor
chip 52 by placing refractory joining surface 22 against aluminum joining sur-
face 56 and conventionally brazing the aluminum joining surface 56 to both of
the surfaces 52 and 22, to form a refractory metal/aluminum/silicon brazed joint.
The aluminum and silicon form a ~hard contactJJ eutectic having a melting point
of about 575C which joins extremely well with both the silicon body 54 and the
refractory metal joining surface 22. Due to the nature of the alumin~lm and
.`J silicon materials involved, it is essential that this joint be formed by brazing
~ in an inert atmosphere (i.e., one that is neither an oxidizing nor a reducing
; atmosphere), and is typically performed in a controlled environment of one
atmosphere, or slightly higher, of dry nitrogen, argon, or a similar inert gas.
~ - Thereafter, the exposed surface ofthe semiconductor body 54 is pre-
- ferably etched to remove contaminants (e.g., with a solution of nitric and
hydro M uoric acids), and a layer of passivating material 60 applied thereto to
prevent recontamination. The passivating material 60 is applied over the-ex-
posed surface of semiconductor body 54 and a portion of the contact member 20
between the joining surfaces 22~ 24 thereof to completely encapsulate the semi-
conductor device 52 and protect it from exposure to contamination. The passiv-
~` ating material 60 is typically plastic or glass which has been finely ground
~t into a slurry, applied by a conventional technique onto the exposed surface of
^ the semiconductor body 54, and finally heated in situ to a temperature suffi-
, .
cient to f`use the passivating material 60. For example, a tunnel furnace may
be maintained at a temperature of about 685 C to about 700 C to fuse a glass
passivating material 60 applied to the semiconductor device 52. Both the
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preform 30 and the eutectic at the joints may become fluid again at the tempera-
tures in the tunnel furnace; however, the surface tensions of the brazed joints
formed in the brazing processes are sufficient to maintain the molten joints
until cooling reoccurs as the subassembly leaves the tunnel furnace.
Because the passivating material 60, the semiconductor body 54 and
the refractory member all exhibit compatible coefficients of expansion, the
passivated semiconductor devices incorporating workpieces 10 of the present
invention exhibit a desirably long life under repeated thermal cycling.
^ It will be noted that brazed lead electrode or workpiece of the
10 present invention may be produced on standard industrial equipment and does not
require the use of the proprietary information or proprietary equipment in-
volved in butt or percussion welding. Furthermore, the process lends itself
well to the production of workpieces in mass production quantities as the re- ~
fractory metal contacts require neither insertion into nor retention by indiv- ~ ;
idual welding electrode clamps. As the joint between the axial lead and the
refractory metal contact is not formed while one of the parts is in motion,
there are`no minimum or maximum length or cross-sectional area requirements
for either the axial lead or the refractory metal contact. Indeed, according
to the present process~ the joining end of the axial lead may be larger in ~ ~
cross-section than the joining end of the refractory metal contact, so that ~ ;
- the entire joining end of the refractory metal contact member contacts the
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joining end of the wire lead, thus providing both greater strength for the
joint and better conduction between the members thereof. The absence of a
maximum cross-sectional area specification for the wire lead, relative to a
glven refractory metal contact, permits the use of axial leads of a given
diameter despite progressive miniaturi7ation of the refractory metal contact.
Finally the brazed lead electrode of the present invention is more reliable
than a comparable butt or percussion welded joint electrode because its lack
of porosity and its generally larger joint cross-section provide a joint of
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greater strength and thus a joint which is better able to resist the physical
processes encountered during incorporation of the brazed lead electrode into
the ultimate semiconductor device. Most importantly, semiconductor devices
incorporating the brazed lead electrodes of the present invention are able to
withstand high hw~idity and high temperature conditions with negligible fail-
; ures as opposed to similar devices incorporating butt or percussion welded lead
electrodes, the latter exhibiting significant failure rates under similar
conditions.
Now that the preferred embodiments of the present invention have been
shown and described, various modifications and improvements thereon will become
` readily apparent to those skilled in the art. For example, special high temp~r-
ature metallic compositions such as dumet (a nickel/iron core sheathed in
copper) or copper-clad iron may be considered as equivalent to a refractory
metal for the purposes of the present in~ention. As another example, the semi~
conductor body may bea diff`used silicon chip without any aluminim joining
surfaces thereon, and the refractory contact may be joined to an exposed sur-
~`~ face of the diffused silicon chip using an aluminum preform therebetween.
Accordingly, the spirit and scope of the present invention is to be understood
as being limited not by the foregoing disclosure, but only by the appended
claims.
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