U.S. patent application number 09/881466 was filed with the patent office on 2001-10-11 for method and structure for preventing adhesive bleed onto surfaces.
This patent application is currently assigned to International Business Machines Corporation. Invention is credited to Appelt, Bernd K., Johansson, Gary A., Jones, Gerald W., Matienzo, Luis J., Nguyen, Yenloan H., Papathomas, Konstantinos I..
Application Number | 20010028117 09/881466 |
Document ID | / |
Family ID | 24144825 |
Filed Date | 2001-10-11 |
United States Patent
Application |
20010028117 |
Kind Code |
A1 |
Appelt, Bernd K. ; et
al. |
October 11, 2001 |
Method and structure for preventing adhesive bleed onto
surfaces
Abstract
A method and structure is provided for preventing wetting or
bleed of an adhesive, such as an epoxy, onto noble metal wire bond
pads on the surface of a dielectric substrate when attaching an I/C
chip to the substrate. The method includes treating the wire bond
pads with a chemical composition which prevents bleeding onto the
surfaces of the wire bond pads by a component of the epoxy. The
chemical composition is a chemical which will provide
"Self-Assembled Monolayers" (SAMs) on the surface of the gold.
These compositions are characterized by a molecule having at least
one group, such as a mercaptan or disulfide, connected to a
hydrocarbon moiety, such as a (CH.sub.2).sub.x chain. The affinity
of the thiol or sulfur-containing portion of the molecule
chemically bonding with the noble metal provides a relatively
strong attachment of the molecule to the metal surface. The
hydrocarbon segment presents a surface on the noble metal that has
the characteristics of the hydrocarbon portion of the molecule
which has a low surface tension, and, thus, prevents wetting of the
noble metal by an epoxy adhesive component. The SAMs, once they
provide protection from the bleed of the die attach adhesives, self
desorb during the wire bonding or soldering temperatures.
Inventors: |
Appelt, Bernd K.; (Brackney,
PA) ; Johansson, Gary A.; (Hockessin, DE) ;
Jones, Gerald W.; (Apalachin, NY) ; Matienzo, Luis
J.; (Endicott, NY) ; Nguyen, Yenloan H.;
(Binghamton, NY) ; Papathomas, Konstantinos I.;
(Endicott, NY) |
Correspondence
Address: |
DRIGGS, LUCAS BRUBAKER & HOGG CO. L.P.A.
DEPT. IEN
8522 EAST AVENUE
MENTOR
OH
44060
US
|
Assignee: |
International Business Machines
Corporation
Armonk
NY
|
Family ID: |
24144825 |
Appl. No.: |
09/881466 |
Filed: |
June 14, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09881466 |
Jun 14, 2001 |
|
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09537959 |
Mar 28, 2000 |
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6252307 |
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Current U.S.
Class: |
257/784 ;
257/E21.518; 257/E23.077 |
Current CPC
Class: |
H01L 23/49894 20130101;
H01L 2924/01029 20130101; H01L 2924/01046 20130101; H01L 2924/01047
20130101; H01L 2924/014 20130101; H01L 2924/01087 20130101; B82Y
30/00 20130101; H01L 2924/01033 20130101; H01L 2924/14 20130101;
H01L 24/80 20130101; H01L 21/4864 20130101; H01L 2924/01078
20130101; H01L 2924/01079 20130101; B82Y 10/00 20130101; H01L
2924/01006 20130101 |
Class at
Publication: |
257/784 |
International
Class: |
H01L 029/40 |
Claims
What is claimed is:
1. A method of preventing the bleeding of an adhesive component
onto noble metal of a wire bond surface on a dielectric substrate
when bonding an electrical component thereto comprising the steps
of: contacting the noble metal surface with a chemical composition
that will provide a Self-Assembled Monolayer (SAM) on said wire
bond surface, wherein said chemical composition is an alkyl thiol
or disulfide that terminates in a non-polar group; and thereafter
bonding said component to said substrate with an adhesive having a
higher surface tension than the resulting SAM.
2. The invention as defined in claim 1 wherein said chemical
composition has the formula HS--R or R--S--S-R', wherein R and R'
are the same or different hydrocarbon moieties selected from the
group of (CH.sub.2).sub.n and substituted (CH.sub.2).sub.2
terminated with a methyl or fluorinated methyl group.
3. The invention as defined in claim 2 wherein said chemical
composition is selected from the group (HS(CH.sub.2).sub.nX)
wherein n is an integer and X is a methyl, ethylene, or fluorinated
methyl terminated group.
4. The invention as defined in claim 3 wherein said integer is from
1 to 21.
5. The invention as defined in claim 4 wherein said chemical
composition is [HS--(CH.sub.2).sub.17--CH.sub.3].
6. The invention as defined in claim 1 wherein said noble metal
surface is a wire bond surface on a chip carrier.
7. The invention of claim 6 further characterized by wire bonding a
wire to said wire bond surface.
8. The invention as defined in claim 1 wherein said electrical
component is an I/C chip.
9. The invention as defined in claim 8 wherein said dielectric
substrate is an organic material.
10. The invention as defined in claim 9 wherein said substrate has
a cavity therein in which said I/C chip is bonded.
11. The invention as defined in claim 6 wherein said wire bond
surface is selected from the group voltage ring, ground ring, and
wire bond pad.
12. The invention as defined in claim 9 wherein said dielectric
substrate is free of SAM.
13. The invention as defined in claim 1 wherein the adhesive is an
epoxy.
14. A component mounting device comprising a noble metal wire bond
surface on a dielectric substrate, a structure on said substrate
for mounting an electrical component thereon; said noble metal
surface having a SAM thereon formed from a chemical composition
that is an alkyl thiol or disulfide that terminates in a non-polar
group, and an electrical component bonded to said substrate with an
adhesive having a higher surface tension than the SAM.
15. The invention as defined in claim 14 wherein said chemical
composition has the formula HS--R or R--S--S--R', wherein R and R'
are the same or different hydrocarbon moieties selected from the
group of (CH.sub.2).sub.n and substituted (CH.sub.2).sub.n
terminated with a methyl or fluorinated methyl group.
16. The invention as defined in claim 15 wherein said chemical
composition is selected from the group (HS(CH.sub.2).sub.nX)
wherein n is an integer and X is a methyl, ethylene, or fluorinated
methyl terminated group.
17. The invention as defined in claim 16 wherein said integer is
from 1 to 21.
18. The invention as defined in claim 17 wherein said chemical
composition is [HS--(CH.sub.2).sub.17--CH.sub.3].
19. The invention as defined in claim 14 wherein said noble metal
surfaces are wire bond surfaces on a chip carrier.
20. The invention of claim 19 further characterized by a wire
bonded to said wire bond surface.
21. The invention as defined in claim 14 wherein said electrical
component is an I/C chip.
22. The invention as defined in claim 21 wherein said dielectric
substrate is an organic material.
23. The invention as defined in claim 22 wherein said substrate has
a cavity therein in which said I/C chip is bonded.
24. The invention as defined in claim 19 wherein said wire bond
surface Is selected from the group voltage ring, ground ring, and
wire bond pad.
25. The invention as defined in claim 22 wherein said substrate is
free of SAMs.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] This invention relates generally to the prevention of
bleeding of organic material onto a metal substrate and, more
particularly, to preventing bleeding of adhesive or components
thereof onto a noble metal substrate. In even more particular
aspects, this invention relates to the prevention of the bleeding
of any components of a die attach adhesive onto noble metal
circuitry on a dielectric substrate and, specifically, onto gold
wire bond surfaces on a chip carrier or other noble metal circuitry
on an I/C chip mounting substrate.
[0003] 2. Background Information
[0004] In the mounting of integrated circuit (I/C) chips on
substrates having electrical connections, such as wire bond pads
thereon, one technique of mounting utilizes a two-part epoxy
adhesive to bond the chip either to gold plating on a planar
surface of a substrate, or onto copper or some dielectric material
in a cavity formed in an electrical substrate, which substrate also
has electrical contacts or wire bond surfaces on the surface
thereof. One particular problem encountered when using such bond
techniques is the tendency of the components of the epoxy adhesive
to "bleed" and spread onto the wire bond surfaces, such as the wire
bond pads or other noble metal surfaces, such as ground rings or
voltage rings, and adhere thereto. This "bleed" causes the surface
of the bond pads or other metal surfaces to be unreceptive to the
bonding of the wire bonds necessary to electrically connect the
wire bond surfaces to the contacts on the I/C chip. This, of
course, results in unsatisfactory electrical contacts and, thus,
contributes to an unsatisfactory chip package.
[0005] One prior art technique for solving this problem is to treat
the part with a CF.sub.4 plasma. The plasma reacts with the organic
portion of the substrate, and the fluorine containing moieties
settle on the gold surface of the wire bond pads and other gold
surfaces and make them less susceptible to being wetted by bleed
from the adhesive. However, this is not a completely satisfactory
treatment in all instances in that the fluorine containing moieties
are not strongly bonded to the gold but are only lightly adhered
thereto. Therefore, they can be easily removed with organic
solvents during processing before attachment of the I/C chip takes
place, but not easily removed with aqueous systems during
processing. Moreover, the CF.sub.4 plasma tends, in many cases, to
adversely affect the dielectric material for subsequent application
of encapsulating material.
[0006] It is, therefore, an object of the present invention to
provide a method and structure for substantially reducing or
eliminating the wettability of metal surfaces, particularly noble
metal surfaces, and specifically gold or other noble metal
surfaces, to organic materials, particularly any components of an
adhesive, such as an epoxy used for attaching an I/C chip to a
substrate, and which I/C chip is subsequently bonded thereto.
SUMMARY OF THE INVENTION
[0007] According to the present invention, a method and structure
for preventing wetting or bleed of an adhesive, such as an epoxy,
onto a metal surface and particularly wire bond surfaces on the
surface of a dielectric substrate is provided. This invention is
particularly useful in preventing bleed from any components of an
adhesive for attaching an I/C chip or a die to a substrate, and
more particularly when a two-part epoxy adhesive is used. The
method includes treating the surfaces of the wire bond pads with a
chemical composition that prevents wetting of, or bleed onto, the
wire bond surfaces by a component of the epoxy used to attach the
die to the substrate surface. The chemical composition is a
chemical which will provide "Self-Assembled Monolayers" (SAMs) on
the surface of the gold. These compositions are characterized by a
molecule having at least one group, such as a mercaptan or
disulfide, connected to a hydrocarbon or substituted hydrocarbon
moiety, such as (CH.sub.2).sub.n chain. A particular molecule that
is useful for this is 1-octadecanethiol
[HS--(CH.sub.2).sub.17--CH.sub.3]. The thiol or sulfur-containing
portion of the molecule has an affinity for gold or other noble
metals, such as palladium, platinum and silver, and chemically
bonds to the metal surface, leaving the hydrocarbon segment
oriented away from the surface. The affinity of the thiol or
sulfur-containing portion of the molecule chemically bonding with
the metals provides a relatively strong attachment of the molecule
to the metal surface. The hydrocarbon segment oriented away from
the surface presents a surface on the metal which has the
characteristics of the hydrocarbon portion of the molecule which
has a low surface tension, and, thus, prevents wetting of the gold
or other noble metal by an epoxy adhesive component. Other chemical
moieties, such as disulfides having a general formula of
R--S--S--R', where R and R' are the same or different hydrocarbon
moieties, such as (CH.sub.2).sub.x or substituted chains, could be
used to form the SAMs.
[0008] The SAMs, once they provide protection from the bleed of the
die attach adhesives, self desorb or are otherwise removed from the
gold surface during the wire bonding temperatures, thus rendering
the gold surface free from any organic layer which would interfere
with wire bonding process. The composition is applied as a coating,
preferably in a solvent such as butylcarbitol.
DESCRIPTION OF THE DRAWINGS
[0009] FIG. 1 is a top plan view of a planar chip carrier after
coating the gold surfaces thereon to form a SAM according to this
invention and before the attachment of an I/C chip;
[0010] FIG. 2 is a sectional view taken substantially along the
plane designated by the line 2-2 of FIG. 1;
[0011] FIG. 3 is a view similar to FIG. 1 with an I/C chip
attached;
[0012] FIG. 4 is a top plan view of a chip carrier with a cavity
configuration for the reception of an I/C chip after treatment of
the gold surfaces to form a SAM according to this invention;
[0013] FIG. 5 is a sectional view taken substantially along the
plane designated by the line 55 of FIG. 4; and
[0014] FIG. 6 is a view similar to FIG. 4 with an I/C chip attached
thereto.
DESCRIPTION OF THE PREFERRED EMBODIMENTS(S)
[0015] Referring now to the drawings, and, for the present, to FIG.
1, a planar chip carrier for use in the present invention is shown
The chip carrier includes an organic dielectric substrate 10 which
has formed thereon a central die attach pad 12 which is connected
to a ground ring 14 by fingers 16. A voltage or power ring 18 is
also provided, as well as wire bond pads 20 on the surface of the
substrate 10. All of the central die pad 12, ground ring 14,
fingers 16, voltage or power ring 18, and wire bond pads 20,
preferably are formed of gold, although other noble metals, such as
palladium, platinum or silver, may be used in some instances.
However, gold is the preferred material for these structures.
[0016] To attach an I/C chip or die 24 (FIG. 3), an adhesive (not
shown) is applied to the surface of the central die attach pad 12,
and the die 24 is then mounted to the central die attach pad 12. A
particularly useful adhesive for attaching the die 24 is a two part
epoxy adhesive, such as Ablestick 965-IL, manufactured by Ablestick
Corp. During this mounting process, there is a tendency for the
adhesive to bleed during the attach period and to spread out from
the attach surface 12 onto the fingers 16, the ground ring 14.
Ground ring 14 is the first wire bondable gold surface that the die
attach adhesive comes in contact with but, depending on how the
substrate is configured, gold surfaces 18 and 20 may be affected.
This is because the surfaces of uncontaminated gold have a surface
tension which is extremely high, i.e. in the range of 1,200 to
1,500 dynes/cm. This is much higher than the surface tension of the
epoxy and, thus, the epoxy, or components thereof, would bleed
thereon, thereby wetting this surface. Therefore, during subsequent
processing of attaching wires to the wire bond surface by
compressive wire bonding, a good bond of the wire to the surface
could not be assured.
[0017] To overcome this problem, the surface of the central die
attach 12, the ground ring 14, fingers 16, the voltage ring 18, and
the wire bond pads 20, are treated with a chemical composition
which will produce on the gold surface a "Self-Assembled Monolayer"
(SAM) 26 (FIG. 2). The chemical composition is generally
characterized by a group that has an affinity to bond to gold or
other noble metal, and is connected to one or more hydrocarbon
groups, which terminate in hydrophobic and oleophobic groups.
Preferably, the groups that bond to the gold are thiol or other
sulfur containing groups, such as disulfides. These compositions
have the general formulae of HS--R or R--S--S--R', where R and R'
may be the same or different, and are hydrocarbon moieties such as
(CH.sub.2).sub.n or substituted (CH.sub.2).sub.n chains which may
be terminated with a methyl or fluorinated methyl group, which have
hydrophobic and oleophobic characteristics. In the case of the
HS--R molecule, the HS group attaches to the gold. In the case of a
disulfide molecule, the bond between the sulfur atoms is believed
to cleave, leaving two molecules, both of which can attach through
the sulfur to the gold depending upon the resulting
(CH.sub.2).sub.n or substituted (CH.sub.2).sub.n chain lengths as
to which will predominate. The gold surface then takes on the
characteristics of the hydrocarbon surface, which can have a
surface tension as low as about 18 dynes/cm, which is lower than
the surface tension of the epoxy used. A particularly useful class
of compounds are those in the classes (HS(CH.sub.2).sub.nX) wherein
n is an integer preferably from 1 to 21, and x is methyl, ethylene
or fluorinated methyl terminated group. Such compositions are
described in the publication "Thin Films, Self Assembled Monolayers
of Thiols", edited by Abraham Ulman, Vol. 24, at pages 14-17, which
is incorporated herein by reference. The hydrocarbon groups listed
on page 15 under low-energy surfaces produce such low-energy
surfaces. Alkyl thiols and disulfides that terminate in nonpolar
groups absorb onto gold and other noble metal surfaces and have
hydrophobic and oleophobic characteristics. Thus, these are the
preferred compositions. A particularly useful thiol is
1-octadecanethiol [HS--(CH.sub.2).sub.17--CH.sub.3]. (Mercaptans
tend to oxidize to disulfides if left standing for a period of
time.) This chemical composition preferably is applied either by
spraying or dipping or otherwise coating the entire surface of the
substrate 10 with the chemical composition that forms the SAM 26.
The SAM 26 on the gold surfaces, as shown in FIG. 2, will protect
the gold surfaces from any bleed from the adhesive wetting the gold
surfaces when an I/C chip 24 is attached, as shown in FIG. 3. SAMs
will not form on the organic substrate material 10.
[0018] When it is time to perform the wire bonding, which is at a
temperature of at least about 120.degree. C. and higher for
compressive wire bonding, the SAM material 26 will desorb from the
various gold surfaces and allow the wire bond to take place, as
shown in FIG. 3, one of which wires is shown at 27 extending from
pad 28 on I/C chip 24 to a wire bond pad 20. It is to be understood
that wire bonding can take place between the pads 28 on the chip 24
and either or both the ground ring 14 or voltage ring 18.
[0019] Referring now to FIGS. 4 through 6, another embodiment of a
laminate chip carrier is shown. This chip carrier is of the cavity
type with the chip being mounted in the cavity. The chip carrier
includes a substrate 30 which includes a stiffener 32 having an
adhesive 34 which attaches a polyimide dielectric sheet of material
36 thereto. A second layer of adhesive 38 attaches a soldermask 40
thereto, and a dielectric laminate 42 is attached to the soldermask
40. The stiffener 32 is preferably formed of copper or nickel clad
copper. The laminate preferably is an FR-4 material comprised of a
glass filled epoxy, as is well known in the art. A cavity 44
extends through the laminate 42, the soldermask 40, the adhesive
38, the polyirnide 36, the adhesive 34, to the stiffener 32, on
which the die or I/C chip 24 will be mounted, as will be described
presently. The laminate 42 has a gold band 46 formed on interior
surface 48 thereof, which surrounds the upper portion of the cavity
44. Laminate 42 also has a top surface 50 on which is formed a
ground ring 52, a voltage ring 54 and wire bond pads 56, which are
separated by soldermask 58. The ground ring 52, the voltage ring 54
and wire bond pads 56, which are wire bond surfaces, all are metal,
preferably gold.
[0020] As can be seen in FIGS. 5 and 6, an I/C chip 24 is bonded to
the top surface 62 of stiffener 32, (i.e. the surface is exposed in
the cavity 44 of the substrate 30) by an epoxy 64. The epoxy 64
preferably is a two component epoxy, such as Ablestick 965-1L, the
same as the epoxy described with respect to FIG. 3. As previously
described, in order to prevent bleed of the epoxy 64, all of the
surfaces of the gold band 46, the ground ring 52, voltage ring 54
and gold wire bond pads 56, are treated with a chemical composition
which will form a SAM 26 (FIG. 5) in the same manner and with the
same composition as described with respect to FIGS. 1-3. This will
prevent bleed from the epoxy 64 from wetting the surface of the
various gold or other noble metal surfaces since the sulfur or
thiol moieties will adhere to the surface with a chemical bond,
thus presenting an outward low surface tension surface of
hydrocarbon to the surrounding space.
[0021] As described previously, where bonding takes place, the SAM
26 will desorb from the gold surfaces and allow the wire bond to
take place, as shown in FIG. 6, one of which wires is shown at 27
extending from pad 28 on the I/C chip 24 to wire bond pad 56.
[0022] It is to be understood that various modifications of the
above-described embodiments can be made, for example, rather than
relying on the processing temperatures during wire bonding.
* * * * *