U.S. patent application number 12/607456 was filed with the patent office on 2010-02-25 for transient liquid phase eutectic bonding.
This patent application is currently assigned to ROSEMOUNT AEROSPACE INC.. Invention is credited to Kimiko J. Childress, Odd Harald Steen Eriksen, Shuwen Guo.
Application Number | 20100047491 12/607456 |
Document ID | / |
Family ID | 36870299 |
Filed Date | 2010-02-25 |
United States Patent
Application |
20100047491 |
Kind Code |
A1 |
Eriksen; Odd Harald Steen ;
et al. |
February 25, 2010 |
TRANSIENT LIQUID PHASE EUTECTIC BONDING
Abstract
A structure including a first structural component, a second
structural component and a bonding structure bonding the first and
second structural components together, where the bonding structure
contains a hypoeutectic solid solution alloy. The hypoeutectic
solid solution alloy may be a gold-germanium solid solution alloy,
a gold-silicon solid solution alloy or a gold-tin solid solution
alloy.
Inventors: |
Eriksen; Odd Harald Steen;
(Brooklyn Park, MN) ; Guo; Shuwen; (Lakeville,
MN) ; Childress; Kimiko J.; (Farmington, MN) |
Correspondence
Address: |
THOMPSON HINE L.L.P.;Intellectual Property Group
P.O. BOX 8801
DAYTON
OH
45401-8801
US
|
Assignee: |
ROSEMOUNT AEROSPACE INC.
Burnsville
MN
|
Family ID: |
36870299 |
Appl. No.: |
12/607456 |
Filed: |
October 28, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
11120879 |
May 3, 2005 |
7628309 |
|
|
12607456 |
|
|
|
|
Current U.S.
Class: |
428/34.1 ;
428/450; 428/457; 428/469 |
Current CPC
Class: |
H01L 2924/01013
20130101; H01L 21/7624 20130101; H01L 2924/01322 20130101; H01L
2924/01024 20130101; H01L 2924/01027 20130101; H01L 2224/83805
20130101; H01L 24/83 20130101; H01L 2924/00011 20130101; H01L 24/26
20130101; H01L 2924/014 20130101; Y10T 428/13 20150115; H01L
2924/01042 20130101; H01L 2924/01074 20130101; H01L 2924/01032
20130101; H01L 2224/83141 20130101; H01L 2924/01078 20130101; H01L
2924/01033 20130101; H01L 2924/0104 20130101; H01L 2924/0105
20130101; H01L 2924/07802 20130101; H01L 2224/8385 20130101; H01L
2924/1461 20130101; H01L 2924/01005 20130101; H01L 2924/00011
20130101; Y10T 428/31678 20150401; H01L 2924/01006 20130101; H01L
2924/01079 20130101; H01L 2224/83825 20130101; H01L 2924/1461
20130101; H01L 2924/01073 20130101; H01L 2924/01327 20130101; H01L
2224/83205 20130101; H01L 2924/00 20130101 |
Class at
Publication: |
428/34.1 ;
428/457; 428/450; 428/469 |
International
Class: |
B32B 15/04 20060101
B32B015/04; B32B 9/00 20060101 B32B009/00 |
Claims
1. A structure comprising: a first structural component; a second
structural component; and a bonding structure bonding said first
and second structural components together, said bonding structure
comprising a hypoeutectic gold-germanium solid solution alloy, a
hypoeutectic gold-silicon solid solution alloy, or a hypoeutectic
gold-tin solid solution alloy.
2. The structure of claim 1 wherein said bonding structure has a
melting temperature of at least about 450.degree. C.
3. The structure of claim 1 wherein said bonding structure contains
at least about 97 atomic percent gold.
4. The structure of claim 1 wherein said first and second
components are silicon wafers or portions of silicon wafers.
5. The structure of claim 1 wherein at least one of said first and
second structural components is a ceramic material.
6. The structure of claim 1 wherein said bonding structure has a
generally uniform composition.
7. The structure of claim 1 wherein said bonding structure has a
melting temperature of at least about 1000.degree. C.
8. The structure of claim 1 further comprising a spacer located
between said first and second structural component, wherein said
spacer contacts both said first and second structural components to
limit the movement of said first and second components toward each
other.
9. The structure of claim 8 wherein said containment well is lined
with a dielectric material.
10. The structure of claim 1 wherein at least one of said first or
second components includes at least one containment well shaped and
configured to receive and retain a liquid therein.
11. A structure comprising: a first structural component; a second
structural component; and a bonding structure bonding said first
and second structural components together, said bonding structure
being a solid solution alloy having a eutectic point temperature of
less than about 400.degree. C. and a melting temperature of greater
than about 600.degree. C.
12. The structure of claim 11 wherein said solid solution alloy is
a gold-germanium alloy, or a gold-silicon alloy, or a gold-tin
alloy.
13. The structure of claim 11 wherein said bonding structure
contains at least about 97 atomic percent gold.
14. The structure of claim 11 wherein said first and second
components are silicon wafers or portions of silicon wafers.
15. The structure of claim 11 wherein said bonding structure has a
melting temperature of greater at least about 1000.degree. C.
16. The structure of claim 11 further comprising a spacer located
between said first and second structural component, wherein said
spacer contacts both said first and second structural components to
limit the movement of said first and second components toward each
other.
Description
CROSS REFERENCE TO RELATED APPLICATION
[0001] This application is a divisional of U.S. application Ser.
No. 11/120,879, filed May 3, 2005, the contents of which are hereby
incorporated by reference.
FIELD
[0002] The present invention is directed to a structure including
two components joined together by transient liquid phase eutectic
bonding.
BACKGROUND
[0003] In the manufacture of micromachined devices,
microelectromechanical systems ("MEMS"), microdevices,
microstructures and the like (collectively termed microstructures),
it is often desired to join various parts, wafers, portions of
wafers, components, substrates and the like together. The parts,
wafer, portions of wafers, components, substrates and the like that
are to be joined together may be relatively large components (i.e.
having, for example, a surface area of several inches) but may
carry a relatively high number of small microstructures thereon.
The joint that couples the two components must be sufficiently
robust to withstand subsequent processing steps and to withstand
environmental conditions to which the assembled structure may be
exposed (i.e., high temperatures, corrosive environments,
etc.).
[0004] When coupling two substrates together, each substrate
typically has a layer of bonding material located thereon. The
bonding material on one substrate can react with a bonding material
on the other substrate to couple the substrates together. Each
substrate may include an adhesion layer located below the bonding
material and on top of the substrate to secure the bonding
materials to the underlying substrate.
[0005] The adhesion and bonding materials are chosen based upon
various criteria including: 1) the materials of the adhesion layer
should react with or adhere to the associated substrate during the
deposition of the adhesion layer or during subsequent bonding of
the bonding materials; 2) the materials of the bonding layer should
react with or adhere to the associated adhesion layer, or take part
in the reaction of the adhesion layer with the substrate; 3) the
bonding material on one substrate should form a covalent or
chemical or alloying bond with the bonding material on the other
substrate; 4) the joint formed during bonding should be formed in
either the liquid state or in the solid state (i.e., through
chemical bonding or solid state diffusion); and 5) the bonding
materials should form reaction products or alloys that are stable
at service temperatures (which could be relatively high
temperatures) and should maintain their mechanical and electrical
properties after prolonged exposure to the service temperatures. It
is also desired to having bonding materials which can be bonded at
a relatively low temperature to provide a resultant alloy which has
a relatively high melting temperature.
SUMMARY
[0006] In one embodiment the present invention is a bonding method,
as well as a bonded structure, which meets the various criteria
outlined above. In particular, the bonding method provides a bond
which can be formed at relatively low temperatures yet can
withstand relatively high temperatures.
[0007] In particular, in one embodiment the invention is a method
for bonding two components together including the steps of
providing a first component, providing a second component, and
locating a first eutectic bonding material between the first and
second component. The first eutectic bonding material includes at
least one of germanium, tin, or silicon. The method further
includes the step of locating a second eutectic bonding material
between the first and second component and adjacent to the first
eutectic bonding material. The second eutectic bonding material
includes gold. The method further includes the step of heating the
first and second eutectic bonding materials to a temperature above
a eutectic temperature of an alloy of the first and second eutectic
bonding materials to allow a hypoeutectic alloy to form out of the
first and second eutectic bonding materials. The method includes
the further step of cooling the hypoeutectic alloy to form a solid
solution alloy bonding the first and second components
together.
[0008] Other objects and advantages of the present invention will
be apparent from the following description and the accompanying
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] FIG. 1 is a side cross section of a component having a
bonding layer located thereon;
[0010] FIG. 2 is a side cross section of the component of FIG. 1,
with another component positioned thereon;
[0011] FIG. 3 is a side cross section of the components of FIG. 2,
with capping layers located between the two components;
[0012] FIG. 4 is a detail side cross section of the components of
FIG. 3, after partial heating and melting;
[0013] FIG. 5 is a side cross section of the components of FIG. 4,
after further heating and melting;
[0014] FIG. 6 is a side cross section of the components of FIG. 5,
after even further heating and melting;
[0015] FIG. 7 is a side cross section of the structure of FIG. 6,
after initial solid formation;
[0016] FIG. 8 is a side cross section of the structure of FIG. 7,
after full solid formation;
[0017] FIG. 9 is a side cross section of the structure of FIG. 8,
after solid state diffusion;
[0018] FIG. 10 is a side cross section view of the components of
FIG. 3 after joining;
[0019] FIG. 11 is a side cross section of two components, with bond
contacts, to be coupled together; and
[0020] FIG. 12 is a eutectic diagram for germanium/gold alloys.
DETAILED DESCRIPTION
[0021] FIG. 1 illustrates a substrate 10, component or the like,
which is desired to be coupled to another substrate, component or
the like. The substrate 10 can be any of a wide variety of
materials, including but not limited to semiconductors (such as
silicon), ceramics, aluminum oxide, polysilicon, silicon carbide,
polyimide, sapphire, silicon nitride, glasses, a combination of
materials or nearly any other materials as desired to be joined.
When the substrate 10 is a ceramic material, this may include
materials such as aluminum nitride or any inorganic, nonmetallic
material that include clay, minerals, cement, and/or glass. The
substrate 10 is preferably a wafer or a portion of a wafer, and can
be or include a microstructure or transducer (such as a sensor or
actuator, not shown). The substrate 10 may be part of a
microstructure, and/or can include various micromachined mechanical
features, electrical circuitry, bond pads, etc. (not shown) located
thereon.
[0022] In the illustrated embodiment, the substrate 10 includes an
adhesion layer 12 located thereon, a diffusion barrier (also termed
an anti-diffusion layer) 14 located on the adhesion layer 12, and
bonding materials or a bonding layer 16 located on the diffusion
barrier 14. It should be understood that when a layer or component
is referred to as being located "on" or "above" another layer,
component or substrate, (such as the bonding layer 16 being located
on the diffusion barrier 14) this layer or component may not
necessarily be located directly on the other layer, component or
substrate, and intervening components, layers or materials could be
present. Furthermore, when a layer or component is referred to as
being located "on" or "above" another layer, component or
substrate, that layer or component may either fully or partially
cover the other layer, component or substrate.
[0023] The adhesion layer 12 adheres well to the substrate 10, as
well as to the diffusion barrier 14 and/or bonding layer 16. The
adhesion layer 12 also preferably adheres well to any subsequent
compounds formed within the various layers 10, 14, 16 during the
bonding process (as will be described below). The adhesion layer 12
preferably has an adhesion strength to a silicon substrate of at
least about 100 MPa as measured by a mechanical shear test. For
example adhesion strength can be determined by a die shear strength
test specified in Military Standard 883, Procedure 2019.5, or
methods similar thereto.
[0024] The adhesion layer 12 can be made of a variety of materials,
including tantalum, titanium, chromium, zirconium or any element
which will form a covalent or chemical compound with the substrate
10. In a preferred embodiment, the adhesion layer 12 has a
thickness between about 100 Angstroms and about 10,000 Angstroms,
and more preferably, between about 200 Angstroms and about 1,000
Angstroms, and most preferably, about 500 Angstroms (the drawings
of FIGS. 1-11 not necessarily being drawn to scale). The adhesion
layer 12 is deposited in a variety of manners, including by plasma
enhanced physical vapor deposition (also known as plasma
sputtering) or any other suitable deposition techniques known to
those skilled in the art.
[0025] Inward diffusion of materials (i.e. from the ambient
atmosphere or from the bonding layer 16) to the adhesion layer 12
and/or to the substrate 10 can cause chemical reactions that weaken
the adhesion layer 12 and/or substrate 10 or can otherwise
adversely affect their properties. Conversely, outward diffusion of
materials (i.e. outwardly from adhesion layer 12 and/or from
substrate 10 to the bonding layer 16) can weaken or otherwise
adversely affect the materials of the bonding layer 16.
Accordingly, the diffusion barrier 14 is provided to generally
block both inward and outward diffusion of materials.
[0026] For example, the diffusion barrier 14 generally blocks the
inward diffusion of the materials of the bonding layer 16 or the
diffusion of gases that may be present in the ambient atmosphere
such as oxygen, nitrogen, and the like. The diffusion barrier 14
also generally blocks the outward diffusion of the materials of the
substrate 10 and adhesion layer 12. Alternately, rather than
directly blocking diffusion itself the diffusion barrier 14
provides materials or combines with other materials to form
reactive byproducts that prevent inward and outward diffusion. Thus
the diffusion barrier 14 thereby prevents undesirable compounds
from forming throughout the various layers of the bonded
components.
[0027] The diffusion barrier 14 may include several sub-layers, but
is shown as a single layer herein for illustrative purposes. The
diffusion barrier 14 can be made of a variety of materials
including but not limited to tantalum silicide, platinum silicide,
tantalum, platinum, tungsten, molybdenum or a combination of these
materials. The diffusion barrier layer 14 has a variety of
thicknesses, for example, preferably between about 100 Angstroms
and about 10,000 Angstroms, and more preferably between about 1,000
Angstroms and about 10,000 Angstroms, and most preferably about
5,000 Angstroms.
[0028] The bonding layer 16 includes a material or materials which
preferably can form a eutectic or binary alloy with another
material under appropriate conditions. In the embodiment shown in
FIG. 1, the bonding layer 16 includes first 18 and second 20
bonding materials or layers that can form eutectics with each
other. For example, the first bonding material 18 is preferably
germanium, tin or silicon, but can be any element or material that
can form a eutectic alloy with the second bonding material 20. The
second bonding material 20 is preferably gold, but can be any
element or material that can form a eutectic alloy with the first
bonding material 18. In the illustrated embodiment the second
bonding material 20 is located between the first bonding material
18 and the diffusion barrier 14.
[0029] Both the first 18 and second 20 bonding materials may be
deposited on the substrate 10 by plasma sputtering or other
suitable deposition techniques known to those skilled in the art.
Further, the first 18 and second 20 bonding materials can be
deposited in a variety of thicknesses. As will be described in
greater detail below, the thickness of the bonding materials 18, 20
should be selected to provide the desired ratio between the first
18 and second 20 bonding materials in the end product bond.
[0030] FIG. 2 illustrates two substrates 10, 10' that are desired
to be coupled or bonded. Each of the substrates 10, 10' includes
its own corresponding adhesion layers 12, 12', diffusion barriers
14, 14', and bonding layers 16, 16' (including bonding materials
18, 20 and 18', 20' respectively). The substrates 10, 10' are
arranged such that the first bonding materials 18, 18' are adjacent
to each other or in contact each other.
[0031] FIG. 3 illustrates an alternate arrangement to the structure
of FIG. 2. In this case a capping layer 17, 17' is located on each
respective bonding material 18, 18' such that each bonding material
18, 18' is located between its respective capping layer 17, 17' and
its respective second bonding layer 20, 20' and substrate 10, 10'.
Each capping layer 17, 17' caps and protects the first bonding
materials 18, 18' to prevent oxidation of the first bonding
materials 18, 18' (i.e. when the substrates 10, 10' are not stacked
in the manner shown in FIG. 3). Each capping layer 17, 17' can be
any of a wide variety of materials which resist oxidation, such as
gold. Each capping layer 17, 17' is preferably the same material as
the second bonding layer 20, 20' so that the capping layers 17, 17'
participate in the eutectic joining between the first bonding layer
18, 18' and second bonding layer 20, 20'. Each capping layer 17,
17' is quite thin, preferably having a thickness of about 1000
Angstroms or less.
[0032] For the description below, it will be assumed that the first
bonding materials 18, 18' are germanium, and that the second
bonding materials 20, 20' and capping layers 17, 17' are gold to
allow discussion of the specific properties of the gold/germanium
eutectic alloy. In this example, FIG. 12 illustrates the phase
diagram for germanium/gold alloys. However, this discussion is for
illustrative purposes and it should be understood that various
other materials may be utilized as the first bonding materials 18,
18', second bonding materials, 20, 20' and capping materials 17,
17'.
[0033] In order to join the substrates 10, 10', the bonding layers
16, 16' are bonded in a transient liquid phase bonding process
which is well known in the art, but is outlined briefly below and
shown in FIGS. 4-9. To commence the transient liquid phase bonding
a light pressure (e.g. a few pounds) is applied to press the
substrates 10, 10' and bonding layers 16, 16' together. The bonding
layers 16, 16' are then exposed to a temperature at or above the
eutectic point or eutectic temperature of the bonding alloy, i.e. a
gold/germanium alloy. For example, as can be seen in FIG. 12, the
eutectic temperature of a gold/germanium alloy is about 361.degree.
C.
[0034] In the illustrative example the bonding layers 16, 16' are
exposed to a temperature of about 450.degree. C. (which is a
temperature that is 89.degree. over the eutectic temperature). For
many applications, the bonding layers 16, 16' are exposed to a
temperature between about 20.degree. C. and about 160.degree. C.
above the eutectic temperature, (89.degree. C. in the illustrated
embodiment.) However, the actual bonding temperatures will depend
upon the diffusion rate of the bonding materials 17, 17', 18, 18',
20, 20', the thickness of the bonding materials and the time
available to complete the diffusion such that a uniform solid
solution of the bonding alloy is achieved.
[0035] Once the materials at the gold/germanium interfaces reach
the eutectic temperature (i.e., 361.degree. C.), zones of melted or
liquid materials 30 are formed at each interface (see FIG. 4) due
to the melting of materials. In FIG. 4, the entire capping layers
17, 17' have melted (due to the thinness of those layers) to form
the central liquid zone 30, and portions of the first bonding
layers 18, 18' have melted to form the top and bottom liquid zones
30. Each zone of liquid material 30 has a composition that is at or
near the eutectic composition. In particular, as can be seen in
FIG. 12, the lowest melting point for gold-germanium alloys
(361.degree. C.) is provided by gold-germanium alloys having a
composition at point A (the eutectic point). Thus, the
initially-formed liquid zones 30 have the lowest possible melting
point for gold-germanium alloys and have about twenty-eight atomic
percent germanium and seventy-two atomic percent gold (12.5 weight
percent germanium and 87.5 weight percent gold).
[0036] As the bonding layers 16, 16' continue to heat up and
approach the ambient temperature (i.e., 450.degree. in the
illustrated example), the liquid zones 30 continue to grow and
expand until all the material of the germanium layers 18, 18' melt
and have been dissolved into the liquid zones 30. The separate
liquid zones 30 of FIG. 4 grow and ultimately combine to form a
single larger liquid zone 30 (FIG. 5). At the time shown in FIG. 5,
the last of the material of the germanium layers 18, 18' have been
dissolved, and the liquid zone 30 remains at composition A.
[0037] Next, the materials of the gold layers 20, 20' adjacent to
the liquid zone 30 continue to liquefy as the surrounding materials
approach the ambient temperature. As additional gold is melted and
added to the liquid zone 30, the germanium in the liquid zone 30 is
diluted and the percentage of germanium in the liquid zone 30 is
thereby reduced. Thus, the composition of the liquid zone 30 moves
up and to the left of point A along the liquidus line 37 of FIG.
12. As the melted gold continues to dilute the germanium, the
liquid composition ultimately reaches the composition at point B of
FIG. 12 when the liquid zone 30 reaches the ambient temperature of
450.degree. C.
[0038] FIG. 6 illustrates the bonding process wherein the liquid
zone 30 has grown and added gold such that the liquid zone 30 is at
composition B. At this stage the liquid zone 30 has reached the
ambient temperature of 450.degree. C., and has a composition of
about 24 atomic percent germanium and 76 atomic percent gold.
[0039] Once the composition of the liquid zone 30 reaches point B,
the germanium in the liquid zone 30 begins diffusing into the
remaining solid gold layers 20, 20' at the interface 31 of the
liquid zone 30 and the gold layers 20, 20'. As this occurs, the
concentration of germanium in the liquid zone 30 adjacent to the
interface 31 drops. Once the percentage of germanium at the
interface 31 drops sufficiently low (i.e., about 3 atomic percent
germanium or less), the liquid zone 30 at the interface 31 forms
into a solid solution phase 32 (see FIG. 7). The newly-formed
solids 32 have a composition indicated at point C on the graph of
FIG. 12. As can be seen in FIG. 12, the point C is located on the
solidus line 39, which indicates the percentage of germanium at
which solids will form for a given temperature. Thus the
newly-formed solids 32 have about 3 atomic percent germanium and 97
atomic percent gold.
[0040] The ambient temperature continues to be held at 450.degree.
C. and remaining germanium in the liquid zone 30 continues to
diffuse outwardly, into and through the newly-formed solids 32 and
into the predominantly gold layers 20, 20'. As the germanium in the
liquid zone 30 continues to diffuse outwardly, more germanium-poor
liquids at the interface 31 of the liquid zone 30 and the solids 32
are created and ultimately form into solids 32. In this manner the
solids 32 grow inwardly until the entire liquid zone 30 is consumed
(FIG. 8). At this point the solid 32 may be relatively
germanium-rich (i.e., 3 atomic percent germanium) and the
surrounding gold layers 20, 20' may be relatively germanium-poor
(i.e. less than 3 atomic percent germanium). In this case the
germanium continues to diffuse, through solid-state diffusion, from
the solid 32 into the gold layers 20, 20' until equilibrium is
reached and both the solid 32 and the gold layers 20, 20' all have
the same composition (shown as solid 32 in FIG. 9).
[0041] In one embodiment, rather than carrying out the bonding and
solid state diffusion at the same temperature, the bonding and
solid state diffusion can be carried out at different temperatures.
For example, the bonding may take place at, for example,
450.degree. C., and the solid-state diffusion shown in FIGS. 8 and
9 may take place at an elevated temperature such as 500.degree. C.
However, the particular bonding and solid-state diffusion
temperatures will depend upon the properties of the bonding
materials 16, 16'.
[0042] After all of the liquid 30 has been consumed, the structure
in FIGS. 9 and 10 is formed and the resultant solid solution 32
bonds the two substrates 10, 10' together. The solid 32 formed
after solid state diffusion is a gold/germanium alloy or solid
solution alloy that may have a composition at point C of FIG. 12
(i.e. about 3 atomic percent germanium).
[0043] As shown in FIG. 12, when the solid solution 32 has a
composition at point C, the solid solution 32 has an atomic
percentage of about 3 atomic percent germanium and a melting
temperature of 450.degree. C. However, the amount of available
germanium may be restricted such that the resultant solid 32 has a
composition of less than 3 atomic percent germanium (e.g., as low
as about 0.5 atomic percent germanium or even lower). With
reference to the phase diagram of FIG. 12, reducing the atomic
percentage of germanium to lower than 3 atomic percent provides a
solution located on the solidus line 39 above and to the left of
point C. Moving the composition to the left of point C provides a
solid solution 32 with a melting point above 450.degree. C., up to
a theoretical maximum of 1064.degree. C.
[0044] The initial thicknesses of the gold 20, 20' and germanium
layers 18, 18' determine the end composition of the solid solution
32. In particular, in one embodiment the initial germanium layers
18, 18' (i.e. of FIG. 3) each have a thickness of about 300
Angstroms, and the initial gold layers 20, 20' each have a
thickness of about 30,000 Angstroms (with, in this example, no or
negligible gold being provided by the capping layers 17, 17').
Thus, the relative thicknesses of the gold and germanium layers can
be selected and controlled to ensure that the resultant solid
solution 32 has the desired amount of germanium (i.e., in this
example about 1 atomic percent) upon complete diffusion of the
germanium in the gold.
[0045] In a further example, the initial germanium layers 18, 18'
each have a thickness of about 150 Angstroms and the initial gold
layers 20, 20' each have a thickness of about 30,000 Angstroms
(again, with no or negligible gold being provided by the capping
layers 17, 17'). In this example the end bond or solid solution
material 32 has 0.5 atomic percent germanium.
[0046] It may be desired to increase the thickness of the gold
layers 20, 20' to provide a solid solution 32 with an elevated
higher melting temperature. It may also be desired to reduce the
thickness of the germanium layer, but the germanium layers 18, 18'
should have a sufficient thickness to form an initial liquid
surface of sufficient volume to ensure the germanium/gold layers
18, 18'/20, 20' flow together in the liquid state and fill any gaps
to provide a complete bond. When materials other than gold and
germanium are used as the eutectic materials, the relative
thicknesses for these other materials would be adjusted based upon
their material properties and their phase diagram to provide the
desired composition for the end bond material.
[0047] In order to further reduce the amount of germanium in the
end product solid solution 32, a germanium-scavenging material may
be provided and located in the second bonding material 20, 20'. At
a sufficient temperature, the germanium-scavenging material will
react with the germanium 18, 18' when the germanium diffuses in the
solid solution. The germanium-scavenging material and germanium
will react to create finely dispersed intermetallic compounds. In
this manner the germanium scavenging material reduces the
concentration of germanium, thereby raising the melting point of
the solid solution 32 while not adversely affecting the strength of
the resultant bond 32.
[0048] The germanium-scavenging material can be a variety of
materials, including but not limited to platinum, nickel and
chromium. Of course, the scavenging materials will be made of
various other compositions when the material to be scavenged is a
material other than germanium (i.e. when tin or silicon are
utilized in place of germanium).
[0049] The scavenging material may take the form of a discrete
layer located between the second bonding layer 20 and
anti-diffusion layer 14. Alternately, the scavenging material may
be co-sputtered with the second bonding layer 20 (i.e. deposited at
the same time as the second bonding layer to form numerous discrete
layers or a gradient within the second bonding layer 20). The
germanium scavenging material is preferably concentrated on the
outer edges of the second bonding layer 20 (i.e. the edges of the
second bonding layer 20 facing the anti-diffusion layer 14) so that
germanium diffusing outwardly in the solid state is scavenged.
[0050] The scavenging material may also be a part or component of
the adhesion layer 12 and/or anti-diffusion layer 14, or integrated
into the adhesion layer 12 and/or anti-diffusion layer 14. For
example, if the anti-diffusion layer 14 is platinum, that
anti-diffusion layer 14 may also function as a germanium scavenging
material. Alternately, the germanium scavenging material may be
added to the adhesion layer 12 and/or anti-diffusion layer 14 as a
separate material.
[0051] The use of the germanium-scavenging material allows some
latitude in the strict thickness controls and allows for greater
flexibility in the manufacturing process. In particular, it can be
difficult and expensive to strictly control the thickness of the
germanium layers 18, 18' and gold layers 20, 20'. For example, it
can be difficult to limit the thickness of the initial germanium
layers 18, 18' to 300 Angstroms, and the thickness of the initial
gold layers 20, 20' to 30,000 Angstroms as in the example outlined
above. The use of a germanium-scavenging material allows for some
variance in the thickness controls, as extra germanium can be
scavenged away.
[0052] The transient liquid phase bonding method described above
preferably takes place at a temperature less than about 600.degree.
C., or further preferably less than about 500.degree. C. Due to the
relatively low eutectic temperature of gold/germanium eutectic 32
(as well as various other eutectic alloys), this bonding method can
be utilized to bond components having relatively
temperature-sensitive materials (i.e., such as aluminum bond pads,
which have a melting point of about 660.degree. C.) without
damaging or melting such temperature-sensitive materials. The
resultant bond has a melting temperature preferably greater than
about 600.degree. C., or more preferably greater than about
800.degree. C., or most preferably greater than about 1000.degree.
C.
[0053] Thus the transient liquid joining process of the present
invention allows the joining of two materials at a relatively low
temperature and results in a bond having a relatively high melting
temperature. The difference between the bonding temperature and the
melting temperature of the resultant alloy is preferably at least
about 400.degree. C., or further preferably at least about
500.degree. C., or most preferably at least about 640.degree.
C.
[0054] FIGS. 2 and 3 illustrate each of the substrates 10, 10' as
having a layer of gold 20 and germanium 18 located thereon.
However, if desired, one or both of the substrates 10, 10' may
include only gold or only germanium. In this case, the relative
thicknesses of the gold and germanium should be adjusted to provide
for the desired composition of the resultant solid solution
composition. Further, as noted above, gold/silicon or gold/tin
eutectic combinations may be utilized as the materials of the
bonding layers 16, 16'.
[0055] This bonding method can be used in bonding complete wafer
surfaces (as shown in FIGS. 1-10) in a manner similar to glass frit
bonding or oxide bonding. Each wafer or substrate 10 can be coated
with the bonding layers 16, and heat can be applied under load
until the heated bonding layers 16 merge and solidify. In addition,
because the materials of the bonding layers 16 are preferably
metallic, the resultant bond is electrically conductive which
allows bonding to electronic elements such as bond pads, contacts,
bond wires, etc.
[0056] Rather than coating the entire surface of each substrate 10,
10' with the adhesion layer 12, diffusion barrier 14 and/or bonding
layers 16, only selected portions of the substrate 10, 10' may be
coated with the adhesion layer 12, diffusion barrier 14, and/or
bonding layers 16. For example, FIG. 11 illustrates a pair of
substrates 38, 38', with each substrate 38, 38' having a pair of
bonding contacts 34 located thereon. Each bonding contact 34
includes an adhesion layer 12, a diffusion barrier 14 and a bonding
layer 16. The bonding contacts 34 of the substrates 38, 38' have
complementary sizes and shapes. In particular, the bonding contacts
34 are arranged such that when the substrate 38' is aligned with
and located on top of the substrate 38, each of the bonding
contacts 34 engage each other in the manner of a standard flip chip
bonding process.
[0057] After the bonding contacts 34 engage and contact each other,
the bonding layers 16 of each bonding contact 34 are then heated to
initiate the transient liquid phase bonding outlined above,
resulting in a solid solution 32 at each bonding contact 34 joining
the substrates 38, 38' together. In addition, because each of the
materials of the bonding contacts 34 are preferably electrically
conductive, besides providing structural joining the bonding
contacts 34 also allow for electrical interconnection between the
substrates 38, 38'.
[0058] Each substrate 38, 38' may have a plurality of
microstructures (not shown) located thereon, and the substrates 38,
38' may be joined as part of a batch fabrication process. After the
substrates 38, 38' are joined, the joined substrates can be
separated from each other, for example, by dicing through the
thickness of the joined substrates 38, 38' to thereby singulate
each microstructures from any other adjacent microstructures.
[0059] If desired, the spacing or gaps between the substrates 38,
38' can be controlled by locating spacers or standoffs 36 (made of,
for example, silicon oxide or other material that does not melt at
the bonding temperatures) between the two substrates 38, 38'. Upon
liquification of the bonding layers 16, the substrates 38, 38' will
move closer together until the standoffs 36 contact each other and
remain in a solid state to maintain the desired spacing between the
substrates 38, 38'.
[0060] The substrate 38 may include a plurality of containment
wells 40 formed therein and located adjacent to the bonding
contacts 34. The containment wells 40 are shaped and located such
that any liquid run-off from the bonding contacts 34 (i.e.
liquefied bonding materials 16) during the bonding process flow
into the containment wells 40 to contain the run-off. If the liquid
run-off were to spread, the run-off could react with or form
solutions with the material of the substrate 38, with other bonding
contacts 34, or with various other components located on the
substrate 38. The containment wells 40 are preferably lined with a
dielectric material (such as silicon dioxide) that is resistant to
reacting with or forming solutions with the liquid run-off.
[0061] The use of germanium, tin, or silicon in combination with
gold provides a relatively robust bond which has a high melting
point. Thus the bonding method of the present invention can be used
for conventional chip and wafer bonding, as well as any application
which is desired to join two components together. The present
invention allows joining at relatively low temperatures without the
use of solder, fluxes or braze materials, and can be particularly
useful where the bonded regions are inaccessible for adding solder
or braze materials. The bonding method of the present invention
does not require ultrasonic scrubbing in high heat which is
required for conventional eutectic die bonding and may be damaging
to the substrates.
[0062] Having described the invention in detail and by reference to
the preferred embodiments, it will be apparent that modifications
and variations thereof are possible without departing from the
scope of the invention.
* * * * *