Note: Descriptions are shown in the official language in which they were submitted.
Vc a oll) ~ c~ ~o
MRT~IOD OF~ }~ SEMICONDUCTOR DEVICES
Technical Field
This invention relates to semiconductor devices
and involves making electrical contacts to semiconductor
devices by low temperature bonding.
Back~round of the Invention
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Many types of semiconductor devices require
electrical contacts and, accordingly, methods of making
electrical contactst that are both physically and
electrically reliable, to semiconductor devices are
extremely important in modern technology. In fact, the
making of reliable electrical contacts to such devices is
often a critical semiconductor device Eabrication step
and, consequently, numerous methods have been developed for
making such contacts. The semiconductor device must be
appropriately contacted by a method that perrnits the
desired connection to an external power source to be made
without adversely altering device or materials
characteristics.
One method that has been developed for making
contacts is commonly referred to by those skilled in the
art as thermocompression bonding. This method was
developed for external lead attachment to the electrical
terminals of semiconductor devices. The method is
presently implemented by pushing a metal lead wire into
firm physical contact with the desired metallized surface
using a heated tool. The forces that are applied are
typically high enough to produce appreciable plastic
deformation of the lead wire. See, for example, "9th
Annual Proceedings," ~ ~ , 1971, pp. 178-
186, English et al.
Another method that has been developed is
referred to by those skilled in the art as ultrasonic
bonding. ~s this method is typically implemented in
presen-t practice, an ultrasonic gel1erator provides high
frequency electrical power which is delivered to a
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transducer~ The transducer changes the electrical power
into mechanical vibrations which are ultimately coupled to
a bonding tool and the metal lead wire which transmits the
energy to the bond interface during the bonding process.
See, for example, Semiconductor Mea~surement
Microelectronic U~trasonic Bondinq, George G. Harman,
Editor, National sureau of Standards, January, 1974,
pp. 23-79.
Another bonding method, which is described in
UO S. Patent 4,005,523 issued on February 1, 1977 to
Samson Khaim Milshtein~ achieves bonding with low
mechanical stress. The method was developed for gold wire
bonding and produces localized high temperatures in the
contacted material.
Consideration of the above briefly described
processes leads one skilled in the art to the conclusion
that while they are perfectly ade~uate for making many
electrical bonds to semiconductor devices, they
unfortunately suffer several drawbacks which may
significantly and adversely effect device performance for
several reasons. For example, during thermocompression
bonding, the material is typically heated to an elevated
temperature which may be as high as 350 to 400 degrees C.
A temperature as high as 200 degrees C may be utilized in
ultrasonic bonding methods. The mechanical stresses that
are applied to the semiconductor device during the bonding
process can generate unwanted defects in the semiconductor
material which can result in, for example, catastrophic
failures during device operation. As there is no way to
accommodate the different thermal expansion coefficients of
the contact materials and the semiconductor rnaterials,
defects may be generated in these methods. The defects
generated by the lack of matching of thermal expansion
coefficients may result in catastrophic failures during
device operations when thermal cycling occurs. It should
also be noted that the effects of heating ancl mechanical
damage may be deleterious to both device performance and
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lGngevity.
a~ ~ Invention
In accordance with an aspect of the invention
there is provided in a method of manufacturing a semi-
conductor deYice, the method of making an electricalcontact to said device, said method Gomprising forming an
end portion of a wire, being used in making said electrical
contact, into an enlarged end portion configuration, adding
a contact material to said enlarged end portion of the wire
positioned by a guide; keeping said contact material at a
temperature near its mel~ing point; and touching said
contact ma-terial to a desired contact point, said touching
resulting in heating a localized area around said contact
point and in coupling of said enlarged end point con-
Eiguration of the wire and of said contact material so as
to orm an electrical connection.
In accordance with this invention, electricalcontacts may be made to semiconductor devices comprising
semiconductor materials or metallized bonding pads on semi-
conductor materials by heating the end of a wire which is,
for example, attached to a wire bond tip or another typeof guide; providing a contact material at the end of said
wire, and touching said end of said wire to the desired
contact point. The wire comprises an electrically con-
ducting material. The contact material is typically a
metal. In one preEerred embodiment, a ball comprising thewire material is formed and the contact metal is added to
the ball. The ball is advantageously formed by subjecting
-the wire to a burst from a torch such as a hydrogen flame.
The contact metal may have a single constituent or it may
be an alloy. In the latter case, additional metals may
also be added to the end of the wire to form an alloy,
iOeO, the metals may be separately contacted. The alloy
is typically an appropriate metal alloy having character-
istics, such as a low melting point and optimum metal-
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lurgica] characteristics, consistent with the requirements
of device operation, such as operating ternperature, etc.
The wire need not be heated initially as long as the con-
tact metal is maintained at a temperature near its melting
-temperature after it is added to the end of the wire. When
the ball or wire end contacts the surface, it does so under
its own weight and forms a bond with the surface. The
molten alloy heats a localized portion of the substrate;
the ball then cools and the alloy solidifies to form a
bond. The metals, forming the alloy, may be selectecl to
op-timize device electrical properties, mechanical strength,
and thermal expansion characteristics. ~hen the wire
bond tip is pulled away from the bond, either the tail
of the wire is utilized to form a second electrical
contact or the tail is left Eor other types of electrical
connections. The method of this invention is
advanta~eously employed in fabricating bonds to damaqe-
sensitive Group III-V semiconductor devices such as light-
emitting diodes, lasers, and photodetectorsO
I
FI~. 1 is a sectional view of the wire bond
apparatus after the wire has been heated to form a ball;
and
FIG. 2 is a sectional view of a bond made
according to our invention.
For reasons of clarity, the elements of the
device are not drawn to sca]e.
Detailed ~ n
Our invention is expeditiously implemented with a
conventional wire bonder, and one embodiment, as well as
several variations, will be discussed. Other variations
will be readily apparent to those skilled in the art. Such
wire bonders are well known to those skilled in the art and
need not be described in detail. The wire, which is
attached to, or carried by 7 a heated guide, and comprises
~old, aluminum, or another suitablé material, is heated to
a temperature slightly above the melting point of the
contact metal. The wire is then extended from the guide
tip and subjected to means which form a ball comprising the
wire metal at the end of the wire. Typical means comprise
a burst from the small torch, such as a hydrogen flame,
which is present in typical wire bonding apparatus. The
ball is supported by the wire~ and the ball is held away
from the guide tip.
The ball is now contacted to an appropriate
contact metal, which may have a single constituent or be
an alloy. The contact metal may be in solid or liquid
form. The metal or metal alloy is picked up by the metal
ball and either softens or alloys with the ball metal. The
contact metal is typically a metal alloy, i.e., it
comprises at least two metals, which have characteristics
consistent with the requirements of device operation. The
structure is depicted in EIG. 1 and com~rises cluide
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assembly 1, wire 3, ball 5, and alloy region 7.
Alternatively, the end of the heated wire may contact the
contact metal without a ball being formed.
The guide assembly carries the wire and alloyed
ball to the desired contact point, if it is not already at
that point, where the ball is lowered onto the contact
site on the semiconductor device. The resulting structure
is depicted in FIG. ~ with the hatched region 9 indicating
a bonding pad or semiconductor device. It should be noted
that the ball contacts the s~rface under its own weight and
th~s subjects the contacted structure to negligible heating
and minimal mechanical stress. The contact or bond is
typically Eormed by the alloy metal which is on the small
ball, and as the temperature is low, the amount of heat
transmitted to the device surface during the bonding
operation is small. The alloy solidifies in a short time,
e.g., 1 to 2 seconds. Consequently, the device surace
region is subjected only to a small thermal perturbation
and, in essence, the bonding is a low stress process. This
is advantageous because it minimizes diffusion of the
contact materials and impurities in the device within the
region of contact and thus does not alter the local
materials properties. This was demonstrated on InP-
Schottky barrier diode structures which are extremely
sensitive to contact formation induced damage.
The bond dimensions are limited primarily by the
size of the wire and the ball formed by the flame. The
smallest bond dimension easily obtainable appears to be the
wire diameter. The relevant parameters which control the
size of the ball are the diameter of the wire and -the
length of the wire exposed to the Elame, i.e., the amount
of material melted~ The minimum ball dimension is
approximately the wire diameter. In practice, the actual
contact area is somewhat smaller than the ball diameter. A
maximum ball diameter of approximately 6 to 8 wire
diameters can be EormedO The diameter oE the wire which is
heated is not critical and wires of essentially any size
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may be utilized provided that they are consistent with the
requirements of the device structure, the guide structure
and flame capabilities. In other wordst the desired site
of the bond is one of the parameters which cletermines the
appropriate wire size. For example, contact pads and
devices having dimensions of approximately 60 to 80 ~m may
be readily contacted with the method. Smaller contacts may
be made if desired. Furthermore, if still larger contact
areas or flat contact surfaces are desired, the ball may be
deformed by mechanical means. The deformation may be
performed by pressing the ball to a suitable nonbonding
surface with the wire ~uide tip. This step rnay be executed
either prior to, or after, contacting the metal alloy
material.
The positioning of the metal ball Eor bonding is
easily controlled by the guide apparatus and is thus
limited by the mechanical accuracy of the guide and the
operator and/or machine capabilities. Of course, this step
can be automated. It will be appreciated by those skilled
in the art that the limitation regarding operator
capabilities is similar to that present in the operation
of the standard wire bonding ap aratus rather than a
limitation unique to our method. The actual position of
the bond can in practice be controlled to a fraction of a
ball diameter.
Reproducibility of the contact characteristics is
limited by the consistency with which the desired length of
wire is exposed to the flame7 that is, the ball size; the
quantity of alloy metal picked up by the ball; and
precision of the alloyed bàll placement. In practice, the
length of exposed wire may be precisely controlled and the
ball dimensions may be similarly controlled. The quanti-ty
of the alloy metal picked up may be accurately controlled
by the use of metal preforms or a molten metal or metal
alloy bath.
Contacts may be formed on a variety of surfaces.
For example, the contacts may be forme~ on either bare
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semiconductor or metallized surfaces. Compound
semiconductors selected from the qroup consisting of
Group I[I-V semiconductors may be contac-ted by this method.
The quality of the electrical and physical characteristics
of the contacts fabricated by our method is predicated on
the interaction of the alloyed meta] and the contacted
material. Appropriate alloy systems which exist for
various semiconductors and contact metals are well known to
those skilled in the art and there is therefore essentially
no limitation on the materials which can be bonded. In
practice, alloys can be selected which have
characteristics, for example, thermal expansion
coefficients, that are similar to those of the contacted
surface. The ability to utilize various alloy systems
enab3es thermal expansion coefficients to be matched
between the contact alloy and the material contacted, e.g.,
semiconductor or semiconductor metallization, while the
alloy composition is also selected to minimize bonding
temperature thus again minimizing stresses in the device
structure.
The bond strength depends on the ~uality of the
semiconductor/alloy or metal/alloy interaction. It has
been found that mechanical strength of bonds made by this
method are sufficient to permit lifting by a force of
greater than 7 grams utilizing gold wires 25 ~m in diameter
and greater than 12 grams utilizing gold wires 50 ~m in
diameter.
Bond failure was observed to occur by two primary
mechanisms: first, by the wire parting either at the
3~ junction of the wire and bàll or in the main part of the
wire, i.e., ductile fracture; and second, by the contact
ball material parting from the alloy which is bonded to the
surface region. Adhesion is influenced by the chemical-
mechanical interaction of the contact alloy and the contact
material. Bond formation may be further enhanced by
utilizing fluxes in the contacting process. The flux may
be solid~ liquid, or gas. The flux may be used in an
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intermediate step between the contact alloy pickup and
contact formation and/or be-tween heating the wire and
contact alloy pickup.
It will be readily appreciated that many
materials may be used with our method. Any metal materia1s
which are bonded presently by thermocompression or
ultrasonic bonding techniques may be bonded with our
technique.
