U.S. patent application number 11/196049 was filed with the patent office on 2005-12-01 for integrated circuit assemblies and assembly methods.
This patent application is currently assigned to Micron Technology, Inc.. Invention is credited to Jiang, Tongbi, Yamashita, Tsuyoshi.
Application Number | 20050263888 11/196049 |
Document ID | / |
Family ID | 28040902 |
Filed Date | 2005-12-01 |
United States Patent
Application |
20050263888 |
Kind Code |
A1 |
Jiang, Tongbi ; et
al. |
December 1, 2005 |
Integrated circuit assemblies and assembly methods
Abstract
A method for assembling chips onto substrates includes applying
a flux-free, no-flow underfill material. In an embodiment, the
method includes removing oxide from interconnects without the use
of a flux and applying a flux-free, no-flow underfill. In an
embodiment, the method includes removing oxide from bumps, applying
no-flow underfill to a substrate, and fluxlessly connecting the
bumps to pads on the substrate. In an embodiment, oxide is removed
from the bumps by a plasma treatment. In an embodiment, oxide is
removed from the bumps by a subjecting the bumps to an oxide
reduction process. The assembly of the chips and substrate is free
from flux residue and/or flux cleaning solution residue.
Inventors: |
Jiang, Tongbi; (Boise,
ID) ; Yamashita, Tsuyoshi; (Wilmington, NC) |
Correspondence
Address: |
SCHWEGMAN, LUNDBERG, WOESSNER & KLUTH
1600 TCF TOWER
121 SOUTH EIGHT STREET
MINNEAPOLIS
MN
55402
US
|
Assignee: |
Micron Technology, Inc.
|
Family ID: |
28040902 |
Appl. No.: |
11/196049 |
Filed: |
August 3, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11196049 |
Aug 3, 2005 |
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10106009 |
Mar 25, 2002 |
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6926190 |
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Current U.S.
Class: |
257/738 ;
257/787; 257/E21.503 |
Current CPC
Class: |
H01L 2224/81805
20130101; H01L 2924/00 20130101; H01L 2924/00 20130101; H01L
2924/00 20130101; H01L 2924/351 20130101; H01L 21/563 20130101;
H01L 2224/73203 20130101; H01L 24/75 20130101; B23K 2101/40
20180801; H01L 2224/16 20130101; H01L 2924/12042 20130101; H01L
2924/14 20130101; H01L 2924/00011 20130101; H01L 2924/01029
20130101; H01L 2924/00011 20130101; H01L 2924/01077 20130101; H01L
24/81 20130101; H01L 2924/14 20130101; H01L 2924/01322 20130101;
H01L 2924/12042 20130101; H01L 2924/351 20130101 |
Class at
Publication: |
257/738 ;
257/787 |
International
Class: |
H01L 023/48 |
Claims
The invention claimed is:
1. An assembly, comprising: a die having a plurality of die
interconnects; a substrate having a plurality of substrate
interconnects that are joined to the die interconnects to form
flux-free joints; a void intermediate the joints, the die, and the
substrate; a flux-free, no-flow underfill in the void; and wherein
the void is free of flux cleaning residue.
2. The assembly of claim 1, wherein the joints have a height on the
order of about a nanometer.
3. The assembly of claim 1, wherein the plurality of die
interconnects are plasma cleaned solder bumps.
4. The assembly of claim 3, wherein the plurality of substrate
interconnects are pads.
5. The assembly of claim 1, wherein the plurality of substrate
interconnects are plasma cleaned solder bumps.
6. The assembly of claim 5, wherein the plurality of die
interconnects are pads.
7. An assembly, comprising: a die having a die surface and a
plurality of die interconnects on the die surface; a substrate
having a substrate surface and a plurality of substrate
interconnects on the substrate surface, the plurality of substrate
interconnects being joined to the die interconnects to form
flux-free joints; a void intermediate the joints, the die surface
and the substrate surface, wherein the void has a height of less
than about 100 microns; and a flux-free, no-flow underfill in the
void.
8. The assembly of claim 7, wherein the void is free of flux
cleaning residue.
9. The assembly of claim 7, wherein the height of the void is
greater than 75 microns.
10. The assembly of claim 7, wherein the void has a maximum height
of about 75 microns.
11. The assembly of claim 7, wherein the plurality of die
interconnects are plasma cleaned solder bumps.
12. The assembly of claim 11, wherein the plurality of substrate
interconnects are plasma cleaned solder bumps.
13. An electrical system, comprising: a user interface; and an
electrical circuit in communication with the user interface, the
electrical circuit including an assembly comprising: a die having a
plurality of die interconnects; a substrate having a plurality of
substrate interconnects that are joined to the die interconnects to
form flux-free joints; a void intermediate the joints, the die and
the substrate; a flux-free, no-flow underfill in the void; and
wherein the void is free of flux cleaning residue.
14. The electrical system of claim 13, wherein the user interface
includes at least one of keyboard, mouse, pointing device, tuning
dial, switch, pedal, and keypad.
15. The electrical system of claim 13, wherein the plurality of
substrate interconnects are plasma cleaned solder bumps.
16. The electrical system of claim 13, wherein the plurality of die
interconnects are plasma cleaned solder bumps.
17. An electrical system, comprising: a user interface; and an
electrical circuit in communication with the user interface, the
electrical circuit including an assembly comprising: a die having a
die surface and a plurality of die interconnects on the die
surface; a substrate having a substrate surface and a plurality of
substrate interconnects on the substrate surface, the plurality of
substrate interconnects being joined to the die interconnects to
form flux-free joints; a void intermediate the joints, the die
surface and the substrate surface, wherein the void has a height of
less than about 100 microns; and a flux-free, no-flow underfill in
the void.
18. The electrical system of claim 17, wherein the void is free of
flux cleaning residue.
19. The electrical system of claim 17, wherein the die includes a
processor.
20. The electrical system of claim 17, wherein the die includes a
memory device.
21. The electrical system of claim 20, wherein the memory device
includes a DRAM or an SRAM.
22. The electrical system of claim 17, wherein the substrate is a
surface mount device.
23. The electrical system of claim 17, wherein the flux-free,
no-flow underfill includes at least one of an epoxy resin, a
catalyst, and a hardener.
24. The electrical system of claim 17, wherein the flux-free,
no-flow underfill includes an organic acid-free underfill.
Description
[0001] This application is a Divisional of U.S. application Ser.
No. 10/106,009, filed on Mar. 25, 2002, which is incorporated
herein by reference.
FIELD OF THE INVENTION
[0002] The present invention generally relates to an integrated
circuit assembly and a method of integrated circuit device
assembly. More specifically, the present invention relates to chip
to substrate assembly without the use of flux.
BACKGROUND OF THE INVENTION
[0003] One aspect of manufacturing an integrated circuit chip or
die is mounting the integrated circuit chip to a substrate. In
today's technological environment, there is a continuously
increasing desire to increase integration of circuits onto a single
semiconductor chip. At the same time there is a requirement to
increase performance of the chip, whether it is a memory chip,
processor chip, telecommunication chip or other integrated circuit
chip. As more functions are integrated into a chip, the number of
interconnections to other chips and/or electrical devices
increases. Often, the goal is to provide the chip with as many
input/output ("I/O") terminals as possible. But the addition of I/O
terminals must be provided at a reasonable cost. Flip-chip bonding
is one of various surface mounting techniques which have been
developed in an effort to provide high density interconnections
between the chip and the substrate.
[0004] In the flip-chip bonding process, the die containing the
integrated circuit is mounted directly to the substrate. Generally,
the flip-chip process entails disposing a plurality of solder bumps
on the upper-surface of the die, flipping the die and mating these
solder bumps with corresponding bonding pads located on the
substrate, and then heating the die and the substrate so as to
reflow the solder bumps. Once reflowed each bump forms a bonded
joint between the die and the substrate, which functions as both an
electrical and physical contact. More specifically, the flip-chip
bonding process includes three basic steps: (1) pre-cleaning and
deoxidation of the surface oxides; (2) solder reflow and/or reflow
joining; and (3) post-soldering cleaning.
[0005] Known flip-chip bonding processes suffer from various
drawbacks. For example, the controlled collapse chip connection
("C4") flip-chip bonding process utilizes solder bumps comprising a
lead-tin alloy, which require an activating flux to reduce the
oxides which form on the surface of the bump. The oxides interfere
with the formation of an adequate bond due to their relatively high
melting points. Accordingly, oxides must be removed if an adequate
bond between the die and the substrate, i.e., the solder bumps and
bonding pads, is to be obtained. However, the use of flux leaves a
residue. The residue must be removed in order to provide an
adequate surface to which the underfill is adhered, prevent
voiding, prevent corrosion or reduce the potlife of the underfill
between the die and the substrate. Such removal or cleaning is a
difficult process to accomplish due to the small and ever
decreasing size of micro-electronic components. Additionally, there
are difficulties in removing the cleaning agents, flux, or residue
from the minute areas between components. Moreover, the cleaning
agents may damage some components and may pose an environmental
disposal problem. Some conventional residues include ionic (e.g.,
acidic or basic) substances. Some residues are corrosive. Some
residues hydrolyze into corrosive components in the presence of
moisture, which can lead to corrosion of the parts soldered or to
damage to the surrounding components. Accordingly, the cleaning
step adds both time and cost to the manufacturing process.
Furthermore, the use of the lead-tin alloy is often unsatisfactory
because of its tendency to fracture due to thermal stress. The C4
flip-chip bonding process also exhibits surface tension between the
solder bump and the bonding pad, which disadvantageously functions
to limit the minimum allowable distance between solder bumps.
[0006] Another drawback associated with the C4 process is that for
proper operation the minimum allowable size of the solder bumps
range from 3-5 mils. Notwithstanding the fact that the size of the
solder bumps cannot be further reduced (a further reduction would
allow more bumps per area), since the entire solder bump becomes
liquid during the bonding process, the substantial size of the bump
increases the probability of a bump forming a short circuit with
one or more adjacent bumps during the bonding process.
[0007] Another known flip-chip bonding process can be referred to
as the indium-bump process. In this process, the solder bump
including indium formed on the die is brought into contact with the
bonding pad on the substrate, and then heated so as to cause the
solder bump to reflow and form a solder joint. While, this process
does not rely on surface tension to assist in the alignment
process, the materials utilized for the solder bumps still require
the use of flux to remove the oxides prior to the reflow process.
As such, the indium-bump process suffers from the same drawbacks as
set forth above. Additionally, indium melts at 120 degrees C. and
thus uses a large portion of the chip's thermal budget.
Accordingly, once the reflow process is completed, further high
temperature operations are prohibited. Furthermore, the indium
joint, which bonds the die to the substrate, has a relatively low
strength. As such, the bond is susceptible to fractures resulting
from forces applied during subsequent processing techniques and/or
during normal operating conditions.
[0008] In addition to the aforementioned problems, some
conventional flip-chip bonding processes do not allow for the
formation of a hermetic seal surrounding the integrated circuit die
simultaneously with the bonding of the die to the substrate. This
results from the need to remove the flux from the bonds formed
during the flip-chip bonding process. However, while the C4 process
allows the generation of a hermetic seal, the seal formed is
undesirably large, typically on the order of 250 microns, with
reference to today's emphasis on reducing package size.
[0009] Various attempts at fluxless soldering have been made but
with limited success. For example, U.S. Pat. No. 4,921,157
discloses a fluxless soldering process for semiconductor devices.
In this process solder surface oxides are removed using a plasma
process. Solder having a surface oxide layer is deposited onto a
surface and flourine-containing plasma excitation is performed on
the solder. The solder is then reflowed. U.S. Pat. No. 5,407,121
discloses a method of soldering a copper layer without the use of
fluxing agents by exposing the copper layer to a
fluorine-containing plasma. Solder is then placed onto the surface
of the copper layer and reflowed. Reflow can take place at
relatively low temperatures, atmospheric pressure and in an inert
or oxidizing atmosphere using standard reflow equipment. Still
these attempts fall short of providing an adequate chip to
substrate packaging technique. Therefore, there remains, in view of
these prior attempts, a need in the art for an improved fluxless
chip to substrate assembly technique or process.
[0010] Flux residues cause further drawbacks when a capillary
underfill is used to fill the void between a chip and a substrate.
Flux residues interfere with the adhesion of the capillary
underfill to the surface of either the substrate or chip that has
the flux residue. Moreover, flux residue in the presence of
capillary underfill causes remnant voids between the chip and
substrate due to the flux residue interfering with the capillary
underfill completely filling the void between the chip and
substrate. The flux residue must be completely removed to prevent
these drawbacks. However, conventional residue cleaning techniques
limits the use of capillary underfills to large dimension for the
chip-substrate assembly. That is, adequately cleaning residue from
the chip and substrate becomes difficult in small dimension
chip-substrate assemblies. Moreover, cleaning small dimension
assemblies takes a significantly longer time. Thus, cleaning flux
residue from small dimension chip-substrate assemblies is not
practical from a manufacturing viewpoint.
SUMMARY OF THE INVENTION
[0011] The present invention includes a method for assembling chips
containing an integrated circuit onto substrates. The present
invention is directed to joining a chip to a substrate using a
no-flow underfill without the use of a flux. The oxides are removed
before joining. Therefore, there is no flux in the underfill and no
flux residue cleaning step is required before the underfill is
applied between the chip and substrate.
[0012] An embodiment of the invention includes applying a no-flow
underfill and not applying a flux to the interconnects of at least
one of the substrate and the die. In an embodiment, the method
includes removing oxide from bumps without the use of a flux,
applying no-flow underfill to a substrate, and fluxlessly
connecting the bumps to pads on the substrate. In an embodiment,
the no-flow underfill is applied to the substrate before the bumps
are connected to pads of the substrate. In an embodiment, the pads
are on the die and the bumps are on the substrate. In an
embodiment, oxide is removed from the bumps by a plasma treatment.
In an embodiment, oxide is removed from the bumps by a subjecting
the bumps to an oxide reduction process. The oxide reduction
process, in an embodiment, occurs in a reduction oven. In an
embodiment, the oxide reduction process is a thermal treatment. The
methods as described herein are, in some embodiments, particularly
suited for flip-chip assembly. An embodiment of the present
invention includes reflowing interconnects between a die and a
substrate at a temperature less than the temperature for curing a
no-flow underfill that is between the die and the substrate.
[0013] In an embodiment, the present invention includes an assembly
of a die and substrate with a no-flow underfill between the die and
substrate. An embodiment of the assembly includes joints between
the die and the substrate having a height of less than 100 microns.
In an embodiment, the joints have a height of greater than 75
microns. In an embodiment, the joints have a height of less than 75
microns. In an embodiment, the joints have a height on the order of
nanometers. In an embodiment, the void between the substrate and
the die is free of cleaning solution residue as such cleaning
solutions are not used to remove flux or oxides according to the
teachings of the present invention.
[0014] The present invention further includes equipment for
performing the methods as described herein and structures created
according to the teachings of the present invention. Further
embodiments of the invention include chip/substrate structures and
methods of varying scope, as well as apparatus, devices, modules
and systems making use of such chip/substrate structures and
methods. Such further embodiments will be apparent to one of skill
in the art upon reading and understanding the present
disclosure.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] FIG. 1 is a view of a substrate in a step according to the
present invention.
[0016] FIG. 2 is a view of a die and a substrate in a step
according to the present invention.
[0017] FIG. 3A is a view of a die/substrate assembly in a step
according to the present invention.
[0018] FIG. 3B is a partial cut-away, perspective view of a
die/substrate assembly according to the teachings of the present
invention.
[0019] FIG. 4 is a schematic view of an assembly chamber which can
be used to perform the process of the invention.
[0020] FIG. 5 is a block diagram of an assembly system according to
the teachings of the present invention.
[0021] FIG. 6 is a view of a circuit module according to the
teachings of the present invention.
[0022] FIG. 7 is view of a memory module according to the teachings
of the present invention.
[0023] FIG. 8 is a view of an electronic system according to the
teachings of the present invention.
[0024] FIG. 9 is a view of an embodiment of an electronic system
according to the teachings of the present invention.
[0025] FIG. 10 is a view of a computer system according to the
teachings of the present invention.
[0026] FIG. 11 is a flow chart of an embodiment of processing steps
according to the teachings of the present invention.
DETAILED DESCRIPTION
[0027] In the following detailed description of the invention,
reference is made to the accompanying drawings which form a part
hereof, and in which is shown, by way of illustration, specific
embodiments in which the invention may be practiced. In the
drawings, like numerals describe substantially similar components
throughout the several views. These embodiments are described in
sufficient detail to enable those skilled in the art to practice
the invention. Other embodiments may be utilized. Structural,
logical, and electrical changes may be made without departing from
the scope of the present invention.
[0028] The term wafer used in the following description includes
any structure having an exposed surface which is used to form the
integrated circuit (IC) structure of the invention. The term
substrate is understood to include semiconductor wafers. The term
substrate is also used to refer to semiconductor structures during
processing, and may include other layers that have been fabricated
thereupon. Both wafer and substrate include doped and undoped
semiconductors, epitaxial semiconductor layers supported by a base
semiconductor or insulator, as well as other semiconductor
structures well known to one skilled in the art. The term substrate
also refers to structures to which a die from a wafer, e.g., a
flip-chip, is connected. Examples of such substrates include
surface mount structures, printed circuit boards or other IC
support structures. The term conductor is understood to include
semiconductors. The term insulator is defined to include any
material that is less electrically conductive than the materials
referred to as conductors. The phrase no-flow underfill refers to
underfill material used in a process that does not rely on
capillary forces to fill a void between a die and a substrate. The
following detailed description is, therefore, not to be taken in a
limiting sense, and the scope of the present invention is defined
only by the appended claims, along with the full scope of
equivalents to which such claims are entitled.
[0029] Integrated circuit fabrication, such as flip-chip
fabrication, includes forming a plurality of integrated circuits on
a wafer. In some embodiments, the integrated circuits form a memory
device, such as a DRAM, SRAM, or RDRAM. By way of example, the
memory device is a memory device as described in U.S. Pat. Nos.
6,223,253; 6,219,283; 6,212,123; or 6,211,015, all assigned to
Micron Technology, Inc. and all incorporated herein for any
purpose. In an embodiment, solder bumps are formed on the wafer.
The wafer is diced into individual chips each having a plurality of
solder bumps. The solder bumps are then used as electrical
connections between the circuits of the die and external circuits
through a substrate.
[0030] FIGS. 1, 2, and 3A show steps in a no-flow underfill process
for connecting a die 10 to a substrate 12 according to the
teachings of the present invention. In an embodiment, substrate 12
is a surface mount technology device. In an embodiment, substrate
12 is a printed circuit board. In an embodiment, substrate 12 is a
ball grid array device. In an embodiment, substrate 12 is a chip
scale package. The substrate 12 includes at least one interconnect
14 for connecting the substrate to a die 10. In an embodiment,
substrate 12 includes a plurality of interconnects 14. The
interconnects 14, in an embodiment, are pads. In an embodiment, the
interconnects 14 are bumps or balls. In an embodiment, the
interconnects 14 are free of flux. An underfill dispenser 15
distributes a no-flow, flux-free underfill material 17 onto the
upper surface of the substrate 12. The no-flow, flux-free underfill
material 17, in an embodiment, completely fills the interstices 19
between the interconnects 14.
[0031] The die 10 includes at least one integrated circuit formed
therein. The integrated circuit, in an embodiment, includes a
memory device. In an embodiment, the integrated circuit includes a
processor or logic circuits. As shown in FIG. 2, die 10 further
includes at least one interconnect 21 that is electrically
connected to the integrated circuit. In an embodiment, die 10
includes a plurality of interconnects. In an embodiment, the
interconnects 21 are free of flux. Interconnects 21 provide inputs
to the integrated circuit and outputs from the integrated circuit.
A chip placement tool 25 grips the die 10 and aligns the die
interconnects 21 with the substrate interconnects 14. In an
embodiment, die interconnects 21 are formed on the top most layer
or level of the die 10. That is, the die interconnects 21 are one
of the last layers or elements fabricated on the die. The chip
placement tool 25, in this embodiment, grips the die 10, flips it,
and then aligns the die interconnects 21 with the substrate
interconnects 14.
[0032] As shown in FIG. 3A, the aligned die 10 and substrate 12 are
positioned in a processing chamber 26. As is recognized by one of
skill in the art, most oxide on interconnect 14 or 21 must be
removed before they can be joined into joints 27. The processing
chamber 26 includes a means for removing the oxide. The means for
removing the oxide does not include a flux. In an embodiment,
chamber 26 includes a plasma source that bathes the die
interconnects 21 in plasma, which removes the oxide coating from
the interconnects 21. In an embodiment, the plasma source bathes
the substrate interconnects 14 in plasma to remove the oxide from
the substrate interconnects 14. The chamber 26 then heats the
interconnects 14 and/or 21 to a reflow temperature. At this reflow
temperature, at least one of the die interconnects 21 and the
substrate interconnects 14 is reflowed to create joints 27. The
reflowing of the interconnect 14 or 21 includes heating the
material of the interconnect to a temperature causing the
interconnect 14 or 21 to flow. In an embodiment, tool 25 applies
pressure to the interconnects 14, 21 during the reflow. The joints
27 form an integral electrical and physical connection between the
die and the substrate. In an embodiment, joints 27 are solder
joints. In an embodiment, the solder joints include lead. In an
embodiment, the solder joints 27 include eutectic tin/lead solder
(melting point of 183 degrees C.). In an embodiment, the solder
joints 27 include lead/indium solder (melting point of 220 degrees
C.).
[0033] The plasma source in the chamber 26 removes oxide from at
least one of the interconnects 14 and 21. The plasma source
accordingly creates a plasma that reacts with the oxide to remove
it from the interconnects. In an embodiment, the plasma source
includes a plasma accelerator. In an embodiment, the plasma source
includes a plasma engine. In an embodiment, the plasma source
includes a plasma occillator. In an embodiment, the plasma source
includes a plasma torch. In an embodiment, the plasma source
includes a plasma gun. The plasma source produces a plasma having
sufficient energy to remove, i.e., clean, oxide off the
interconnects 14 or 21 while not harming the die 10 or substrate
12. In an embodiment, the plasma is a flourine plasma. The flourine
plasma may be created from SF.sub.6 or CF.sub.4. In an embodiment,
the plasma treatment of the interconnects 14, 21 is performed at
room temperature, e.g., 18 to 28 degrees Celsius. In an embodiment,
the plasma treatment occurs at a temperature less than the reflow
temperature of at least one of the interconnects. An example of a
temperature less than the reflow temperature is less than about 100
degrees Celsius.
[0034] In an embodiment, chamber 26 includes a thermal source that
transforms the chamber 26 into a reducing oven. The reducing oven
removes the oxide from at least one of the interconnects 14, 21. In
an embodiment, the reducing oven includes a reducing flame that has
excess fuel and is capable of a chemical reaction, such as removing
an oxide from a metallic oxide thereby leaving the metal. It will
be appreciated that the reducing oven must operate at a temperature
below the reflow temperature of the interconnects 14, 21 and must
not exceed the thermal budget of the chip or die 10.
[0035] Joints 27 are by themselves susceptible to breaking and
cracking due to the die 10 and substrate 12 having different
coefficients of thermal expansion. That is, temperature effects on
the assembly of the die 10 and substrate 12 can cause the joints to
electrically open or physically fail. Accordingly, the assembly of
the die 10 and substrate 12 includes no-flow, flux-free underfill
material 17. Underfill material 17 provides additional physical
support between the die and the substrate to prevent cracking of
the joints. In an embodiment, the underfill material insulates the
joints from each other and the die from the substrate. The
underfill material 17 is cured from a flowable material, which was
dispensed by underfill dispenser 15, into a solid, essentially
rigid material within the interstices 19 between the joints 27 and
opposed surfaces of the die 10 and the substrate 12. In an
embodiment, the underfill material 17 is hardened by the heating.
In an embodiment, the underfill material 17 is heated to
essentially the same temperature as the reflow temperature of the
interconnects 14 or 21. In an embodiment, the underfill material 17
is heated to a temperature greater than the reflow temperature of
the interconnects. An example of an underfill curing temperature is
about 165-170 degrees Celsius. Another example of an underfill
curing temperature is about 185 degrees Celsius. Another example of
an underfill curing temperature is about 190 degrees Celsius.
Another example of an underfill curing temperature is about 195
degrees Celsius. Still another example of an underfill curing
temperature is over about 200 degrees Celsius. The underfill
material 17, in an embodiment, includes a polymeric material. The
underfill material 17, in an embodiment, includes an epoxy resin.
In an embodiment, underfill material 17 includes at least one of a
catalyst and a hardener. The underfill 17, when cured, is an
insulator between the joints 27. Additionally, cured underfill 17
acts as a support mechanism between the die 10 and substrate 12 to
minimize the residual thermal stress resulted from the curing
process and subsequent temperature effects on the die/substrate
assembly.
[0036] In an embodiment, the no-flow, flux-free underfill material
17 is a film type underfill. The film type underfill is deposited
as a film on the either the surface of the chip or the surface of
the substrate. In an embodiment, the underfill material is cast
onto a release paper and then dried into a film. The underfill film
is cut into a proper shape, called a preform, and applied to the
chip or substrate. It will be understood that the underfill film,
in an embodiment, is applied to a wafer including a plurality of
chips or dies. Heating, with the application of pressure, causes
the underfill film to bond to the chip or substrate. In order to
apply the interconnects 21 to the underfill-coated chip, wafer or
substrate, openings are formed in the underfill film at each
interconnect location 14, 21. In one embodiment, the openings are
formed using laser machining techniques. Excimer lasers, for
example, are used to create openings in underfill films by a
photoablation process in which ultra-violet radiation causes the
long-chain polymers of the underfill film to break down into small
volatile by-products. Patterning is achieved using either a pattern
mask or a directed beam. Optical defraction grating patterning
methods are also used. Photoablation is used as it produces only a
minimum amount of heating and does not damage the chip, wafers or
substrate. The patterning process parameters are set so that
patterning stops when the layer below the underfill is exposed,
thereby making the process self-limiting. This is not always
necessary, however, since many materials, e.g. metals, are
resistant to laser ablation. Although UV lasers are described,
other lasers, such as infrared (IR) lasers can be used as well.
[0037] In an embodiment, the no-flow, flux-free underfill material
17 begins its curing process at a temperature less than the reflow
temperature of interconnect 14 or 21. The underfill material 17
begins to cure but a rate slow enough to allow the interconnects to
be joined prior to the underfill curing to such an extent that it
is not sufficiently flowable to allow creation of the joint. In
this embodiment, it is not necessary to heat the die/substrate
assembly 10, 12 above the reflow temperature. In an embodiment, the
underfill material 17 is held at a temperature above a minimum
curing temperature for a sufficient time to cure the underfill
material but not reflow the joint 27.
[0038] The no-flow, flux-free underfill of the present invention
includes several important criteria. In an embodiment, the no-flow
underfill has a minimal curing reaction below solder bump reflow
temperature (about 180-230 degrees Celsius). In an embodiment, the
no-flow underfill has a rapid curing reaction after the solder bump
reflow temperature is reached. In an embodiment, the no-flow
underfill has good adhesion to passivation layer, chip, substrate,
or solder mask. In an embodiment, the no-flow underfill has minimal
shrinkage during curing. In an embodiment, the no-flow underfill
include a low TCE. In an embodiment, the no-flow underfill includes
a self-fluxing ability. In an embodiment, the no-flow underfill
includes an adequate modulus to minimize the residual thermal
stress resulted from the curing process and sequence temperature
cycling condition.
[0039] The height of the joints 27 determine the void 29 that
exists in the die/substrate assembly 10, 12. Void 29 is
substantially filled by underfill 17. In an embodiment, void 29 is
completely filled by underfill 17. The height of the joints 27 and
hence the height of the void 29 is kept as small as possible to
reduce the size of the die/substrate assembly 10, 21. However, the
joints 27 have a minimal size to ensure that enough metal remains
on the interconnects 14 or 21 after the oxide, which forms on the
metal, is removed. The remaining metal must be sufficient to reflow
and create a stable mechanical and electrical bond between the
interconnects 14, 21. In an embodiment, the joints 27 have a height
of less than 100 microns. In an embodiment, the joints 27 have a
height of greater than 75 microns. In an embodiment, the joints 27
have a height of about 75 microns. In an embodiment, the heights of
the interconnects 14, 21 that form the joints have a combined
height of greater than the height of the joint 27. This allows for
removal of a portion of the metal with the oxide. This further
allows the interconnects to collapse during reflow. Accordingly,
with a sufficient collapse, the underfill material 17 applied onto
the substrate 12 does not need to extend above the substrate
interconnects 14 to provide an adequate support mechanism between
the die and substrate. In an embodiment, the height of at least one
of the interconnects 14, 21 is on the order of nanometers. The only
limitation on the size of the interconnects is the need to keep the
interconnects from being totally oxidized before the oxide removal
process, e.g., plasma treatment, occurs.
[0040] FIG. 3B shows a die/substrate assembly 30 according to the
teachings of the present invention. Assembly 30 includes a die 10
and a substrate 12. The die 10 includes a plurality of pads 40 on
which interconnects 21 are formed. Substrate 12 includes a
plurality of interconnects 14 facing the die 10. Cured no-flow,
flux-free underfill 17 fills the gaps between the joined substrate
interconnects 14 and chip interconnects 21. The cured underfill 17
provides mechanical support between the die 10 and substrate 12.
The substrate 12 further includes electrically conductive vias 42
that join the substrate interconnects 14 to pins 44. The pins 44
provide electrical connections to electrical circuits (not shown)
outside the assembly 30.
[0041] FIG. 4 shows a fabrication device 400 for joining the die 10
to the substrate 12. Fabrication device 400 includes a reaction
chamber 405 in which the substrate 12 and die 10 are positioned. In
an embodiment, chamber 405 is adapted to receive a plurality of
substrates 12 and dies 10. Device 400 includes a no-flow underfill
dispenser 415 which dispenses a flowable (e.g., dispensable)
no-flow, flux-free underfill 17 onto substrate 12. In another
embodiment no-flow dispenser 415 is an applicator for applying a
film type underfill 17 onto one of the die 10 and substrate 12. In
an embodiment, an oxide removal means 430 removes oxide from the
substrate interconnects 21 prior to dispensing the no-flow,
flux-free underfill 17 onto the substrate. In an embodiment,
dispenser 415 applies the no-flow, flux-free underfill 17 prior to
removing the oxide from the substrate interconnects 14. The oxide
removal means 430, in an embodiment, includes a plasma source 431.
The plasma source 431 removes oxide from the substrate
interconnects 14 in an embodiment. In an embodiment, plasma source
431 removes oxide from the chip interconnects 21. Plasma source
431, in an embodiment, has a gas source (not shown) and a
ionization device (not shown) for ionizing the gas supplied by the
gas source to create the plasma. The ionization device includes a
solid state radio frequency generator for ionizing the gas in an
embodiment. Plasma 431A contacts the die interconnects 21 and
reacts with the oxide on the interconnects 21 in an embodiment.
Plasma 431A removes the oxide from the die interconnects 21. In an
embodiment, plasma 431A contacts the substrate interconnects 14 and
reacts with the oxide on the interconnects 14 in an embodiment.
Plasma 431A removes the oxide from the substrate interconnects 14.
In an embodiment, the oxide removal means includes at least one
thermal source 432. The thermal source 432, in its oxide removing
capacity, increases the temperature in the chamber 405 thereby
creating a reducing oven that removes oxide from at least one of
the substrate interconnects 14 and chip interconnects 21. A chip
holder 425 holds chip 10 within the chamber 405. A substrate holder
427 supports the substrate 12 in the chamber 405. In an embodiment,
chip holder 425 is movable within the chamber 405. Chip holder 425
grips the chip 10, flips it, and aligns its interconnects 21 with
the substrate interconnects 14. Thereafter, chip holder 425 moves
the chip interconnects 21 into contact with substrate interconnects
14. Thermal source 432 raises the temperature of the chamber 405 to
a reflow temperature, which results in at least one of the chip
interconnects 21 and substrate interconnects 14 to reflow and to
form joints 27. In an embodiment, thermal source 432 holds the
temperature in the chamber 405 at a minimum temperature to cause
the no-flow underfill to cure. Thermal source 432, in an
embodiment, holds the minimum temperature for a period of time so
that the no-flux, flux-free underfill 17 is cured. Chamber 405
further includes an exhaust 435, which removes by-product gases
from the underfill curing process, removes heated ambient gases
from the chamber, or removes the plasma gases from the chamber.
[0042] FIG. 5 shows a fabrication system 500 according to the
teachings of the present invention. System 500 is adapted to join
die 10 to substrate 12 to create a die/substrate assembly 10, 12.
System 500 includes a fabrication device 510 that joins the die 10
to the substrate 12 and a control unit 560. In an embodiment,
device 510 is the same as fabrication device 400 as described
herein. Device 510 communicates through lines 562 with control unit
560 such that control instructions are passed from the control unit
560 to fabrication device 510 and feedback regarding the status of
fabrication unit is sent to the control unit 560. It is noted that
communication lines 562 may be only one physical line, in which
communications are bidirectional.
[0043] The control system 560 may include, integrally or
separatable therefrom, a machine readable media 570 which contains
instructions for performing the present invention. Media 570 may be
an electrical, magnetic, optical, mechanical, etc. storage device
that stores instructions that are read by control unit 560. Such
storage devices include magnetic disks and tape, optical disks,
computer memory, etc. Control unit 560 may also include a processor
(not shown) for issuing instructions to control fabrication device
510 based upon instructions read from machine readable media
570.
[0044] Circuit Modules
[0045] FIG. 6 shows a circuit module 600 that includes at least one
die/substrate assembly 610 according to the teachings of the
present invention. Assembly 610 includes the die 10 and substrate
12 as described herein. In an embodiment, assembly 610 includes a
protective casing, which protects the die, substrate and joints
from environmental contaminants. In an embodiment, circuit module
600 includes a plurality of assemblies 610 to enhance or extend the
functionality of an individual assembly 610. Circuit module 600 may
be a combination of assemblies 610 representing a variety of
functions, or a combination of assemblies 610 containing the same
functionality. One or more assemblies 610 of circuit module 600
contain at least one assembly in accordance with the invention.
[0046] Some examples of a circuit module include memory modules,
device drivers, power modules, communication modems, processor
modules and application-specific modules, and may include
multilayer, multichip modules. Circuit module 600 may be a
subcomponent of a variety of electronic systems, such as a clock, a
television, a cell phone, a personal computer, an automobile, an
industrial control system, an aircraft and others. Circuit module
600 will have a variety of leads 611 extending therefrom and
coupled to the assemblies 610 providing unilateral or bilateral
communication and control.
[0047] FIG. 7 shows one embodiment of a circuit module as memory
module 700. Memory module 700 contains multiple memory devices 710
contained on support 715, the number generally depending upon the
desired bus width and the desire for parity. Memory module 700
accepts a command signal from an external controller (not shown) on
a command link 720 and provides for data input and data output on
data links 730. The command link 720 and data links 730 are
connected to leads 740 extending from the support 715. Leads 740
are shown for conceptual purposes and are not limited to the
positions shown in FIG. 7. At least one of the memory devices 710
contains an assembly according to the teachings of the present
invention.
[0048] Electronic Systems
[0049] FIG. 8 shows one embodiment of an electronic system 800
containing one or more circuit modules 600. Electronic system 800
generally contains a user interface 810. User interface 810
provides a user of the electronic system 800 with some form of
control or observation of the results of the electronic system 800.
Some examples of user interface 810 include the keyboard, pointing
device, monitor or printer of a personal computer; the tuning dial,
display or speakers of a radio; control of a music playing device;
the ignition switch, gauges or gas pedal of an automobile; and the
card reader, keypad, display or currency dispenser of an automated
teller machine. User interface 810 may further describe access
ports provided to electronic system 800. Access ports are used to
connect an electronic system to the more tangible user interface
components previously exemplified. One or more of the circuit
modules 600 may be a processor providing some form of manipulation,
control or direction of inputs from or outputs to user interface
810, or of other information either preprogrammed into, or
otherwise provided to, electronic system 800. As will be apparent
from the lists of examples previously given, electronic system 800
will often be associated with certain mechanical components (not
shown) in addition to circuit modules 600 and user interface 810.
It will be appreciated that the one or more circuit modules 600 in
electronic system 800 can be replaced by a single integrated
circuit. Furthermore, electronic system 800 may be a subcomponent
of a larger electronic system. It will also be appreciated that at
least one of the circuit modules 600 contains an assembly according
to the teachings of the present invention.
[0050] FIG. 9 shows one embodiment of an electronic system as
memory system 900. Memory system 900 contains one or more memory
modules 700 and a memory controller 910. The memory modules 700
each contain one or more memory devices 710. At least one of memory
devices 710 contains an assembly according to the teachings of the
present invention. Memory controller 910 provides and controls a
bidirectional interface between memory system 900 and an external
system bus 920. Memory system 900 accepts a command signal from the
external bus 920 and relays it to the one or more memory modules
700 on a command link 90. Memory system 900 provides for data input
and data output between the one or more memory modules 700 and
external system bus 920 on data links 940. It will also be
appreciated that at least one of the memory modules 700 contains an
assembly according to the present invention.
[0051] FIG. 10 shows a further embodiment of an electronic system
as a computer system 1000. Computer system 1000 contains a
processor 1010 and a memory system 900 housed in a computer unit
1005. Computer system 1000 is but one example of an electronic
system containing another electronic system, i.e., memory system
900, as a subcomponent. Computer system 1000 optionally contains
user interface components. Depicted in FIG. 10 are a keyboard 1020,
a pointing device 1030, a monitor 1040, a printer 1050 and a bulk
storage device 1060. It will be appreciated that other components
are often associated with computer system 1000 such as modems,
device driver cards, additional storage devices, etc. It will
further be appreciated that the processor 1010 and memory system
900 of computer system 1000 can be incorporated on a single
integrated circuit. Such single package processing units reduce the
communication time between the processor and the memory circuit. It
will be appreciated that at least one of the processor 1010 and
memory system 900 contain an assembly according to the teachings of
the present invention.
[0052] FIG. 11 shows a flow chart 1100 according to the teachings
of the present invention. A die containing an integrated circuit is
fabricated on a wafer, step 1101. The die fabrication step includes
fabricating integrated circuits, forming interconnects on the
wafer, and dicing the wafer. In an embodiment, the die is
fabricated as a flip-chip including interconnects, such as solder
bumps, on its top level. The die is prepared for bonding, step
1103. The oxides are removed from the die interconnects, step 1105.
The removal of the oxide, in an embodiment, includes plasma
treating the interconnects to remove surface oxides from the
interconnects. In an embodiment, die preparation includes placing
the die, or the wafer before dicing, in a reducing oven to remove
the surface oxide. However, flux is not applied to interconnects on
the die. A substrate is fabricated in step 1107. The substrate
fabrication includes forming interconnects on the substrate. Other
steps involved with forming the substrate are understood by one of
skill in the art. In an embodiment, the interconnects on the
substrate are prepared for bonding, step 1109. However, flux is not
applied to interconnects on the die. In an embodiment, both the die
and the substrate are prepared for bonding according to the
teachings of the present invention. Oxide on at least one of the
die interconnects or the substrate interconnects is removed, step
1111. Removal of the oxide is necessary to provide a good wetting
of the interconnect material to create a stable joint. Oxide on the
interconnects raises the reflow temperature and prevents formation
of a stable joint within the thermal budget of the chip. Using a
flux on the interconnects is a conventional method for removing
oxide, but it introduces many drawbacks. In an embodiment of the
present invention, removing the oxide includes plasma treating the
die interconnects. In an embodiment, removing the oxide includes
plasma treating the substrate interconnects. In an embodiment,
removing the oxide includes placing one of the die and the
substrate in a reducing oven to use thermal treatment to remove the
oxide. The substrate receives a non-flow, no-flux underfill
material, step 1113. The interconnects on the die are joined to the
interconnects on the substrate, step 1115, without application of a
flux. An adequate joint is produced without flux as the oxides are
removed from the interconnects of the substrate and the die.
Joining the interconnects includes aligning the respective
interconnects and reflowing at least one of the interconnects to
create a joint that include both the die interconnects and the
substrate interconnects. In an embodiment, the aligned
interconnects are pressed together during the heating thereof to a
reflow temperature. The flux-free, no-flow underfill material is
cured, step 1117. Accordingly, the assembly of the die and the
substrate is complete.
[0053] It will be appreciated that some of the above steps as
described with respect to FIG. 11 may be simultaneously performed
or performed in a different order than recited above. For example,
it is within the scope of the present invention to first fabricate
the substrate and thereafter fabricate the die.
Conclusion
[0054] Fluxes that are used in removing oxides from interconnects
to join chips to substrates introduce additional steps in
fabrication, e.g., packaging, processes. The present invention
provides a method for joining chips to substrates without a fluxing
step. It is desirable to eliminate the flux from the process to
reduce the steps involved with fabrication. It is desirable to
eliminate the flux from the process to not leave flux residue that
interferes with adhesion and complete void filling by capillary
underfills. It is further desirable to join chips to substrates
without the use of flux because the flux reduces the potlife of
no-flow underfill and the glass transition temperature of the
underfill. Both of these drawbacks of using flux reduce the number
of materials that are used as underfill. Removal of the flux will
expand the number of materials that can be used as an underfill
when joining a chip to a substrate.
[0055] An embodiment of the present invention includes using a
no-flow, i.e., non-capillary, underfill. When compared to the
standard assembly process using capillary underfilling processes,
the present invention does not require a cleaning step to remove
all of the flux residue after joining the chip to the substrate.
The cleaning step and the capillary underfill step of the
conventional process requires that the space between the chip and
substrate remain large enough to insert cleaning solution, remove
the cleaning solution and provide the needed flow of the underfill
between the chip and substrate. Accordingly, the present invention
allows for smaller ball or interconnect sizes than are believed
presently used. Moreover, the present invention removes the
cleaning solution related steps all together. Thus, the present
invention allows the void between the chip and substrate to be
scaled to a smaller size than conventional processes.
[0056] The present underfill material does not require a very
active acid group to provide a flux effect to adhere to the
substrate or chip. The oxides are removed, for example, by plasma
treatment from the substrate and the chip. Accordingly, the
underfill may include a less active acid group. Accordingly, the
number of materials for the underfill and the formulations thereof
are increased.
[0057] The present invention also does not require cleaning of the
flux from the assembly prior to the application of the underfill.
The chip/substrate assembly is thus cleaner due to no flux residue
and no cleaning solution residue. The underfill thus is not
degraded by the presence of residues during its curing.
* * * * *