Note: Descriptions are shown in the official language in which they were submitted.
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LEAD BONDING METHOD
This invention relates, in general, to a method for
bonding a semiconductor wafer to a thermally conductive
member, and more particularly, to a method for bonding
silicon to a metal using substantially pure lead as a
bonding material and titanium or similar refractory metal,
such as zirconium, hafnium, etc., a.s a wetting a~ent.
The formation of a satisactory bond between a semiconductor
device and a metal support therefor, involves several problems.
It is desirable to oktain both a low thermal resistivity
and a low electrical resistivity in the bond. Further,
where the coefficients of thermal expansion of the silicon
and the metal are dissimilar, it is necessary to provide a
bond which is not totally rigid, and which therefore
provides an interface which will yield rather than applying
shear or bending stress to the semiconductor wafer.
Similarly, it is desirable to provide a bond which will not be
` degraded by repetitive thermal cycling as would be expected to
occur as the device is heated and cooled. Lead-tin solder
compounds are especially susceptible to such recrystallization
under repetitive heating and cooling conditions.
It is an object of this invention to provide a method
for forming a silicon-to-metal bond which exhibits high
thermal and electrical conductivity, which is flexible and not
degraded by the effects of unequal coefficients of thermal
expansion between silicon and the particular metal utilized.
Briefly stated, and in accordance with one aspect
of this invention, a method for bonding a semiconductor to a
metal includes the steps of coating the surfaces to be bonded
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with titanium, or other similar refractory metal; providing
a source of substantially pure lead or other suitable bonding
material between the surfaces, and heating the bond to a
temperature sufficient to first melt the bonding ma~erial and
second, cause it to wet the surfaces of the semiconductor and
the metal.
The features of the invention which are believed to be
novel are pointed out with particularity in the appended claims.
The inven-tion itself, however, both as to its organization and
method of operation together with further objects and
advantages thereof may best be understood by reference to the
following description taken in connection with the accompanying
drawings in which:
FIGURE 1 shows in diagrammatic flow diagram form the
steps necessary for the formation of a semiconductor to
metal bond in accordance with one aspect of this invention.
FIGURE 2 shows in similar form the steps of another
embodiment of this invention.
FIGURE 3 shows an alternative method for the introduction
of bonding lead into the structure.
FIGURE 4 shows a structure to be utilized in the high
temperature stages of the process in accordance with this invention.
Referring now to FIGURE 1, there is shown in flow diagram
form the steps necessary to create a bond between a semiconductor
wafer 10 and a metallic support therefor 11. For purposes
of illustration we will assume that the semiconductor wafer is
a silicon wafer and that the metal is a refractory metal,
as for example, molybdenum. It is to be understood that this
invention is not so limited, however, and that other semiconductor
materials, as for example, germanium or Group III-V semiconductor
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materials, such as gallium arsenide, are suitable and similarly
that other metals, as for example, tungsten, tantalum and niobium
may be used. Also, other refractory metals may be utilized if
desired and therefore this invention is not intended to be
limited to the specific metals enumerated herein. It is to
be further understood that while for purposes of illustrating
the practice of this invention lead is chosen as a bonding
material, the invention is not limited solely to the use of
lead. For example, substantially pure tin, indium or bismuth
may be utilized as a substitute for lead. The important
characteristics to be considered in choosing a suitable bondiny
material are the melting point, and the ductility. It is
required that the material chosen be solid at the temperature of
operation of the device, and, additionally, it is desirable that
the melting point be as low as possible, a low melting point
being desirable insofar as the residual strain in the joint
is directly related to the temperature difference between
the ambient temperature and the melting point of the bonding
material. It is preferable that the bonding material chosen
have a melting point between 150 and 350C. It is pointed out
that cadmium while having an appropriate melting point and
ductility is not suitable insofar as it is highly
volatile and poisonous.
It is to be further understood that while titanium is
referred to as a wetting agent in the exemplary embodiment
of this invention described herein, the invention is not so
limited, and in fact, other refractory metals are suitable.
For example, zirconium and hafnium are also suitable to be
used in accordance with this invention.
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Before formation of the bond is begun it is necessary to
clean the surfaces of the semiconductor and the metal to be
bonded. Preferably the surfaces should be as free from grease
and/or surface oxides as possible. It has been found that a rinse
in an organic solvent such as acetone or methanol followed by an
etch including approximately equal amounts of nitric and hydro-
fluoric acids provides adequate cleaning capability for most semi-
conductors and metals. No particular method of cleaning is
required to practice this invention, however, and any o the well-
known methods for cleaning metals and/or semiconductors may
be used.
After the surfaces have been prepared as hereinabove des-
cribed, it is nacessary to provide thereon a thin coating of finely
dispersed titanium. It has been found that it is convenient to
utiliæed titanium hydride as a titanium source. Dispersed, finely-
divided titanium hydride in an amyl acetate carrier together with
a nitrocellulose binder, if desired, provides a source of titanium
easily applied, as for example by spraying or brushing onto the
surfaces of the semiconductor and metal. It is preferred that
in accordance with this invention whatever medium is selected to
provide the titanium coa~ing yield a layer of approximately 1 to
3 particles thick on each surface. It is emphasized that the
carrier and/or the binder are not critical to the invention and
merely provide a means for retaining the titanium at the surfaces
of the semiconductor and the metal to be bonded. Further, as
will be hereinbelow described, substantially pure titanium or
a combination of titanium and titanium hydride will provide the
necessary function.
Wafers 10 and 11 now having a titanium hydride coating 13
at the surfaces thereof which will form the bond, are
placed in a sandwich-like relationship with a source
of lead 15 therebetween. The resulting structure includes
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the semiconductor wafer 10, a thin layer of titanium hydride
13, a sheet of lead 15, a second thin layer of titanium hydride
13 and finally the metal wafer. The method for providing the
source of lead which will in fact ~orm the bond is not critical
insofar as the practice of this invention is concerned. The
placing of a thin lead sheet between the metal an~ the semi-
conductor to be bonded is exemplary only and, in fact, at least
one alternative thereto will be illustrated hereinbelow. Any
method ~or providing the source of lead, as Eor example those
methods herebefore practiced in any of a number of well-known
soldering techniques, is acceptable. It is desirable only that a
method for providing a source of bonding lead not introduce any
tendency to imperfection in the bond. For example, it would ob-
viously be detrimental to provide a source of lead which resulted
in trapped gas pockets or an otherwise nonuniform bond.
The sandwich-like structure formed as hereinabove described
is now subjected to an increase in temperature in an inert or
substantially inert atmosphere. For example, a vacuum chamber
17 may be used to surround the structure while the temperature is
raised. Alternately, an inert atmosphere, as for example argon or
nitrogen may be utilized to to somewhat reduce the requirements for
evacuation. It is to be understood that any method which substan-
tially prevents the formation of oxides during the high temperature
portions of the bonding process is suitable for use in accordance
with this invention. For example, nitrogen or argon provide
an acceptable atmosphere. It is to be understood however
that where the inert gas employed includes impurities, as is
usually the case, these impurities may degrade the character of
the bond formed therein. It is preferable therefore that where
an inert gas invironment is used that it be of the highest
quality. In this evacuated or inert environment the temperature
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is now increased to a level sufficient to melt the lead an~ form
the bond. As is well known lead melts at a temperature of ap-
proximately 32~C. Titanium hydride decomposes at a temperature
of approximately 400C, releasing hydrogen therefrom. Wetting
does not occur until the temperature reaches approximately
550C. It has been found therefore that it is preferable to
increase the temperature to approximately 600C to insure
that a satisfactor~ bond will be formed in all instances. It
is preferable in accordance with this invention to apply some
pressure to the semiconductor-metal str~cture during heating
to insure the formation of a satisfactory bond therebetween.
After wetting of the semiconductor and metal surfaces takes place
the structure should be cooled. It has been found that it is
desirable that cooling take place fairly rapidly in order to
minimize the residual strain in the structure. As is well known,
the residual strain increases as the solidification temperature
increases, and therefore to the extent that while the structure
remains at an ele~ated temperature lead and silicon may interact,
and further since a lead-silicon combination has a higher melting
point than that of pure lead, a relatively long cooling time will
produce a bond having a higher residual strain than a short cool-
ing time. An additional feature of this invention is that the
presence of a titanium film which substantially reduces the
interaction between lead and silicon will prevent the melting
point of the lead from being raised and will thereby produce a
bond having a substantially lower residual strain than was
possible heretofore.
Figure 2 illustrates in schematic flow diagram form another
method for forming a bond between a semiconductor and a metal
in accordance with this invention. As was hereinbefore
described in conjunction with FIGURE 1, it is necessary
before initiating the steps illustrated in FIGURE 2 -to provide
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a substantially clean surface on each of the elements to be
bonded. This step is assumed to have been performed before
the ~irst step illustrated at FIGURE 2. After the semiconductor
an~ metal surfaces to be bonded are clean, a thin layer of
titanium is sputtered onto those surfaces. Sputtering is
performed in low pressure aryon as illustrated h~rein by
vacuum chamber 17 into which semiconductor material 10 and
metallic wa~er 11 are placed. The two materials are disposed
with their nonbonding surfaces in contact with a substantially
nonreactive electrode 19 in a manner to provide good electrical
contact between each of the semiconductor and metal wafers and
the electrodes. A second electrode 21 i5 provided within th~
vacuum chamber and a voltage source 23 is connected between
the two electrodes. This voltage source is preferably a source
of high frequency electrical energy as, for example, a
13.5 Megahertz radio frequency source. Electrode 21 is
a titanium electrode which during the sputtering process
provides the source of titanium to be deposited on the
surfaces of the semiconductor and metal to be bonded. During
the sputtering process an inert gas, as for example argon,
is introduced into the vacuum chamber which will provide the
necessary energetic particles to dislodge minute quantities of
titanium from electrode 21 which will be accelerated by
voltage source 23 and deposited upon semiconductor and metal
wafers 10 and 11. The sputtering process creates thin layers
26 and 27 of titanium on the surfaces to be bonded~ It is
to be noted that this process provides substantially pure
titanium layers rather than the titanium hydride coatings
provided in the embodiment of this invention discussed herein-
above in conjunction with FIGURE 1.
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Both the use of finely dispersed titanium hydride
and sukstantially pure titanium layers on the surfaces to
be bonded provide desirable features. For example,
particulate titanium hydride provides some cleaning
as the hydrogen is released from the titanium hydride during
heating. Particulate titanium hydride does not, howev~r,
prevent interaction between silicons and lead, but does
dissolve oxygen which otherwise would contribute to the
formation of oxides at the surfaces to be bonded. Substantially
pure titanium substantially prevents interaction between
silicon and lead thus resulting in a bond having lower
residual strain, but does not provide any hydrogen and
therefore lacks the cleaning function of particulate
titanium hydride which may be desirable in certain instances.
Where the advantages of pure titanium and of titanium
hydride are desired, it is preferred in accordance with
this invention to provide a thin layer of substantially
pure titanium on the surfaces to be bonded and then ~o
expose these layers to a hydrogen atmosphere such that
some or all of the substantially pure titanium is changed
to titanium hydride. The concentration and length of
exposure to the hydrogen atmosphere will determine to
what extent the substantially pure titanium layer is changed
to titanium hydride. In this way, a uniform layer of titanium
hydride or of titanium hydride overlying pure titanium may
be formed.
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After the titanium layers 26 and 27 have been sputtered
onto the surfaces to ke bonded a source of lead, as for
example, relatively thin lead sheet 15 is interposed between
the semiconductor and metal wafers 10 and 11 having
titanium layers 26 and 27 on the surfaces thereof. A
sandwich-like structure is formed ~hereby having semiconductor
material 10, titanium layer 26, lead sheet 15, titanium layer
27 and metallic wafer 11 in that order.
The sandwich-like structure is now heated in an
evacuated or inert environment as hereinabove described in
conjunction with FIGURE 1 to a temperature of approximately
600, such temperature being sufficiently high to first melt
the lead and then permit the lead to wet the surfaces of the
titanium covered semiconductor and metal. It is to be pointed out
that the temperature to which the sandwich-like structure must be
elevated may vary somewhat from the figure suggested herein
depending upon the particular semiconductor and metal utilized.
Observation of the heating process will readily indicate
when wetting has occurred insofar as the contact angle of
the lead with respect to the semiconductor and the metal
materials will vary during heating until such time as
wetting occurs when it will flow onto the surface in a manner
similar to that well known in traditional soldering processes.
After wetting has occurred, a bonded structure should be
cooled as hereinbefore described.
FIGURE 3 shows an alternative method for providing
bonding lead as required in the processes in accordance with
this invention illustrated at FIGU~ES 1 and 2. It is assumed
that the employment of the method for providing bonding
lead as illustrated at FIGURE 3 follows the application of
the titanium or titanium hydride coatings by either
of the methods illustrated at FIGU~ES 1 and 2 or by
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any other method. A stand pipe 30 is inserted through an
orifice 32 in the metal wafer 11. A reservoir 33 of lead
is provided inside stand pipe 30. Metallic wafer 11 and
semiconductor material 10 are then placed in close proximity
one to the other and heated in an inert or evacuated
environment as hereinabove described. As the lead in the
reservoir 33 melts, lead flows from stand pipe 30 into the space
35 between metallic member 11 and semiconductor 10. This
method which may be utili~ed as hereinbefore described
in conjunction with the embodiment of this invention of
FIGURES 1 or 2 provides the aavantage that the introduction of
lead oxide into the gap between the semiconductor and the metal
is substantially reduced. It can be seen that as the lead melts
and begins to flow from the stand pipe to the space 35 between
the ssmiconductor and the metal, it leaves the lead oxide
behind in the stand pipe. In addition, only the top surface 37
of the reservoir of lead is exposed to the environment in
which the bond is formed. It will be appreciated therefore
that the large surface as would be present where the thin sheet
of binding lead 15 is provided between the semiconductor and
the metal, as for example as hereinbefore discussed in
conjunction with FIGURES 1 and 2, which surface would be
susceptible to the formation of oxides thereon, is eliminated
and a superior bond is formed.
FIGURE 4 illustrates a method in accordanc~ with this
invention for performing the heating step of FIGURES 1 and 2 in
an inert atmosphere, such as a conveyor belt furnace. A
graphite boat 40 is provided having a well 42 therein of
sufficient size to hold the particular semiconductor and metal
3~ sandwich-like structure desired to be bonded. The sandwich-like
structure is placed at the bottom of the well and a graphite
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holding member ~ is placed on top thereof to provide a
sufficient force to maintain the desired positional relationship
between the semiconductor and metal wafers. A thin sheet of
titanium 46 lines the inside of well 42 surroundlng the
sandwich-like structure 48 and graphite block 44, Boat 40
is conveniently heated in an oven which may, iE desired, be
segmented into compartments whereby the temperature may be
precisely controlled in stages. It is an advantage of
the structure illustrated at FIGURE 4 that where impurities,
as for example, mainly oxygen are present in the inert gas or
vacuum environment in which the bond is formed, these impurities
tend to react first with the graphite to form, for example,
volatile carbon dioxide which is carried away by tlle carrier
gas present in the oven and also with the titanium sheet ~6
which lines well 42 and which dissolves oxygen. In this way,
oxygen is substantially prevented from reaching the bond
as it is forming and a superior ~uality bond results.
The method ~or bonding a semiconductor to a metal
hereinabove described provides a bond of substantially improved
characteristics as compared to those hereinbefore obtainable.
~hile several specific techniques have been described for the
formation of a bond utilizing substantially pure lead along
with titanium in one of a number of forms as a setting agent,
it is to be understood that this invention is not limited to
2~ any particular form or process for forming a bond. For example,
ti-tanium hydride has been illustrated as one compound for
providing a source of titanium during the high temperature
portion of the bonding process. As has been pointed out,
the process is applicable to a large number of metals hereinhefore
enumerated but is not intended to be limited
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thereto insofar as is known any refractory metal, that is to
say, any metal which will not be degraded by -the heats necessary
to form the bond may be utilized. It is re~uired, of course,
that the metal be wetted by the lead in the presence of
titanium. Further, it has been found that æirconium hydride
is to some degree substitutable for titanium where desired.
Titanium, however, is to be preferred in that it produces a
superior quality bond. Two specific methods have been
described for providing a titanium coating, but the invention
is not intended to be one limited to any specific method
for providing the titanium coating and other methods, as for
example evaporation are equally suitable, the precise method
cnosen being determined by the availability of equipment for
performing the necessary processing steps. Whatever method
is chosen to provide a titanium or other coating before
bonding, it is preferred to obtain a layer from 300A-l mil
in thickness and in no case more than 10 mils. A thickness
of 5000 A produces good results.
'~hile the invention has been particularly shown and
described with reference to several preferred embodiments thereof,
it will be understood by those skilled in the art that various
changes in form and detail may be made therein without departing
from the true spirit and scope of the invention as defined by
the appended claims.
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