Note: Descriptions are shown in the official language in which they were submitted.
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PRESSURE-SENSITIVE ADHESTVE TAPES
FOR ELECTRONICS APPLICATIONS
Background of the Invention
Field of the Invention
The invention relates to antistatic removable
microparticulate pressure-sensitive adhesive tapes useful
in applications requiring electrical conductivity or
elimination of electrostatic charge, especially in large
scale robotic printed circuit board and chip plants.
These adhesive tapes provide a remarkable capacity for
preventing static charge build-up while having low
adhesion to allow accurate slicing of wafers and easy
removal of individual chips without transfer of adhesive
to the chip.
Description of the Art
Antistatic adhesive compositions are useful for
attaching surface-mount components at points on printed
circuit boards where they are to be conductively attached
e.g., by soldering. Antistatic adhesives, when coated on
selected substrates and suitably converted, provide
antistatic, pressure-sensitive adhesive tapes. The tapes
are useful for various applications in the electronic
industry.
One application for antistatic tapes which is very
important to the semiconductor fabrication industry is
the attachment of newly manufactured wafers through the
die slicing step where the individual chips are sliced
with a diamond saw through 60-800 of the wafer thickness,
and continued attachment through a testing process to
determine if any chips are defective. The good chips
then need to be cleanly snapped from the wafer and
removed from the tape backing.
A problem with antistatic tapes already available is
that either the antistatic properties are not
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sufficiently strong, i.e., too much charge is present
when the chips are removed, causing damage, or the
adhesion is not strong enough to hold the chips during
processing.
Another problem is that the adhesion of wafer to
tape must be low enough to allow the severed chips to be
both completely separated from the wafer and removed from
the adhesive tape by manual or mechanical means without
leaving adhesive residue, but must be strong enough to
hold the wafer in place during the sawing process. If
the wafer moves, the slicing will not be clean, and
damaged chips will again result.
Finally, the chips are usually removed from the tape
by bending the portion of tape or applying pressure to
the opposite surface of the tape to that surface holding
a chip until the chip "pops" from the tape. To be
useful, the tape backing must be flexible enough so that
it does not split during this process, and the adhesion
between the backing and the adhesive must be greater than
the adhesion between the adhesive and the chip. Failure
of either of these properties will result in portions of
adhesive or tape remaining on the chip, which will cause
slowdown or stoppage of a manufacturing line while such
is remedied, or the chip cannot be properly affixed to an
electronic device.
A number of methods are known for preparing
antistatic adhesive compositions. One common method is
the addition of conductive moieties to conventional
adhesive formulations. Antistatic species may be
introduced as conductive materials such as electrically
conductive metal or carbon particles. Compositions of
this type are disclosed in prior art references including
E 0276691A, E 0518722A, U.S. 4,606,962, E 0422919A, U.S.
3,104,985, U.S. 4,749,612 and U.S. 4,548,862.
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The addition of ionic materials to reduce generation
of static charge is also known. Suitable materials of
this type include ion conducting species such as those
disclosed in Japanese patents JP 61,272,279 and JP
63, 012, 681.
U.S. 4,098,945 discloses a conductive composition
which comprises a polymeric binder system, a plurality of
insoluble spherical domains dispersed in the system, and
at least one electrically conductive filler dispersed in
the binder which provides conductive pathways through the
composition. The spherical domains are preferably
adhesive microspheres, the use of which lessens the
amount of conductive filler used.
Yet another type of antistatic tape material is
provided using a metal foil tape backing. one example of
this, disclosed in U.S. 3,497,383, provides embossed foil
tapes where contact points of metal project from the
surface of the adhesive.
No tapes available today provide all of the
properties required for good performance; those having
low adhesion tend to have less effective antistatic
properties because filler is used to reduce the adhesion
which tends to change conductivity properties, and those
which provide good antistatic properties may have higher
adhesion than desirable.
Surprisingly, the present invention provides a
removable adhesive tape which holds the wafer in place
and yet provides easy removablility for each severed
chip, when desired. Further, the tapes of the invention
provide exceptional antistatic properties, thus
exhibiting significantly lower charges when the chip is
~ removed from the tape than any product currently
available.
The unique properties of materials of the present
invention are provided by the use of polymeric
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microparticles having complexes of polymer electrolytes
on the surface of each microparticle.
Complexes of polyethylene oxide (PEO) and lithium
salts have been shown to be promising materials as solid
state polymer electrolytes. The use of these materials
in the development of high energy lithium batteries is
considered by Gilmour et al in Proc. Electrochemical
Society, 89-94, (1989). Lithium salts, like those
disclosed in WO 8,303,322, U.S. 4,471,037 and FR
2,568,574, are most commonly used with PEO in polymer
electrolytes. Other metal salts such as alkaline earth
salts may also enhance electrolytic properties as
described in U.S. 5,162,174. Applications for polymer
electrolytes have expanded from a focus on energy storage
batteries to their use in other areas such as
electrochromic displays and addition to molding resins in
the production of conductive molded articles.
When coated on suitable substrates and converted
into tape format, adhesives of the invention provide
antistatic tapes which are extremely effective in
dissipating electrostatic charge and have adhesion levels
sufficient to hold the chips onto the tape during
processing, but not so high as to preclude removal. They
also exhibit excellent adhesion between the backing and
the adhesive which reduces adhesive transfer to a chip
during removal therefrom.
Particulate adhesives are known in the art, and have
been coated on a variety of substrates and used primarily
in applications requiring a low level of adhesion, e.g.,
removability. Such spheres and their use in aerosol
adhesive systems having repositionable properties are
disclosed in U.S. Pat. No. 3,691,140 (Silver). These .
microparticles are prepared by aqueous suspension
polymerization of alkyl acrylate monomers and ionic
comonomer, e.g., sodium methacrylate, in the presence of
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an emulsifier. The use of a water-soluble, substantially
oil-insoluble ionic comonomer is critical to preventing
coagulation or agglomeration of the microparticles.
However, particulate adhesives disclosed in the prior art
have all been useful as repositionable adhesives for such
applications as Post-Its"' brand notes, and other removable
items. However, pressure-sensitive tapes made with this
type of adhesive have not been considered suitable for
use as antistatic tapes due to their ease of removal.
Further, such adhesives have been water-dispersible,
and thus have not been able to withstand the water
washing step in the wafer dicing operations of robotic
printed circuit board manufacture.
However, it has now been discovered that it is
possible to provide a surface conductive polymer particle
adhesive which possesses sufficient adhesion to adhere
during electronic processes, and still retains low
tribocharging characteristics.
Surprisingly, the adhesives and tapes of the
invention also can withstand water washing without
swelling or crazing, while retaining respositionability
and low tribocharging characteristics.
Summary of the Invention
The invention provides a water-resistant,
antistatic, removable pressure-sensitive adhesive tape
comprising a flexible polymeric film support bearing on
at least one major surface thereof a non-tribocharging,
microparticulate adhesive formed from conductive,
polymeric, inherently tacky, solvent-insoluble, solvent-
dispersible, elastomeric microparticles. The tape is
especially useful in semiconductor fabrication where low
adhesion and flexibility of backing are required, along
with resistance to high humidity and water.
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Useful removable microparticulate pressure-
sensitive adhesive tapes comprise acrylate or modified
acry:Late particles having a surface comprised of chains of
an ionically conducting polymer electrolyte, preferably
polyethylene oxide. The microparticles may be solid or
hollow, as desired.
More specifically, the invention provides a water-
resi:~tant, anti-static pressure-sensitive adhesive tape
suitable for use as a wafer dicing tape comprising a
flexible substrate having opposing surfaces, at least one
surface bearing thereon a removable, aziridine-crosslinked
microparticulate adhesive comprised of microparticles having
a surface bearing thereon an ionic conductive material
formed from a polymer electrolyte base polymer, and at least
one .ionic salt selected from the group consisting of salts
of alkali metals and salts of alkaline earth metals, wherein
said microparticles have an average diameter of at least
1 mi~~rometer, said adhesive having an adhesion to steel of
from 0.5 Newtons/100 mm (N/100 mm) to 10 N/100 mm.
According to one aspect of the present invention,
there is provided a water-resistant, anti-static pressure-
sensitive adhesive tape comprising a flexible substrate
having opposing surfaces, at least one surface bearing
thereon a removable, aziridine-crosslinked microparticulate
adhesive comprised of microparticles comprising a polymer of
monomers comprising at least one alkyl (meth)acrylate or
vinyl ester, said aziridine cross linking agent being
present in an amount of from about 0.1 part to 2 parts per
100 :part monomer, said microparticles having a surface
bearing thereon an ionic conductive material formed from a
polymer electrolyte base polymer selected from the group
consisting of polyethylene oxide, polyphenylene oxide,
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polyphenylene sulfide, polyethylene sulfide,
polyethyleneimine, polypropylene oxide, polybutylene oxide,
polybutylene sulfide, and polybutylene imine, and at least
one ~_onic salt selected from the group consisting of salts
of a=_kali metals and salts of alkaline earth metals, wherein
said microparticles have an average diameter of at least 1
micrometer, and a pH from 2.5 to 7Ø
Preferred pressure-sensitive adhesives useful in
taper of the invention comprise conductive, polymeric,
inherently tacky, solvent-insoluble, solvent-dispersible,
elasi~omeric microparticles comprising 100 parts monomers,
comprising:
a) from 70 to 99 of at least one monomer selected
from alkyl (meth)acrylate esters and vinyl esters; and
b) up to 15 parts by weight of at least one polar
monomer,
c) from 0.1 part to 10 parts of a polymer
electrolyte;
d) from 0.05 part to 3.0 parts of at least one
ionic salt selected from the group consisting of
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salts of alkali metals and salts of alkaline earth
metals, and
e) from 0.01 to 2.0 parts of an aziridine
crosslinker.
Preferred microparticles use polyethylene oxide as
the polymer electrolyte base polymer to form the surface
polyelectrolyte complex.
As used herein, these terms have the following
meanings.
1. The term "polymer electrolyte" means a
polymeric species containing electron donating atoms
which may be associated with acceptor atoms.
2. The term "polymer electrolyte base polymer"
means a polymer which is capable of forming a polymer
electrolyte during formation of the microparticle.
3. The term "polymer electrolyte functional unit"
means the group containing the electron donating species.
4. The term "microparticle" means a particle
having a diameter of from 1 micrometer to 250
micrometers.
5. The term "tribocharging" means electrostatic
charge generation associated with friction between
separable surfaces.
6. The term "droplet" means the liquid stage of
the microparticles prior to the completion of
polymerization.
7. The term "cavity" means a space within the
walls of a droplet or microparticle when still in the
suspension or dispersion medium prior to drying, and thus
containing whatever medium was used.
8. The term "void" means an empty space completely
.: within the walls of a polymerized microparticle.
9. The term "hollow" means containing at least one
void or cavity.
10. The term "solid" means voids or cavity-free.
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11. The term "alkyl (meth)acrylate" means an alkyl
acrylate or alkyl methacrylate.
12. The term "modified surface" means a surface
which has been subjected to a priming, coating or
treatment such as chemical or radiation treatment such
that the original properties of the surface have been
changed.
13. The term "wafer" means a large disc consisting
of many integrated circuits.
14. The term "chip" means an individual integrated
circuit.
As used herein, all parts, percents, and ratios are
by weight, unless specifically stated otherwise.
Detailed Description of the Invention
Tapes of the invention are suitable for use in a
variety of applications where transport of electrical
current or prevention of electrostatic charge is
important. However, tapes of the invention are
especially useful in the printed circuit board industry,
in an application commonly called "wafer dicing". This
process places a wafer on a surface and then slices the
wafer into individual integrated circuits or "chips".
These chips are then individually useful in electronic
components. The chips must be held onto the substrate
with sufficient force that they do not loosen and fall to
the floor when being moved, either manually, or by a
robotic arm. However, the adhesion to the substrate must
be sufficiently low that they can be removed-when
desired, and no adhesive buildup occurs on the lower
surface. The removal is typically achieved by bending '
the substrate and "popping" the chip from it, and the
adhesive tape therefor must be flexible enough that the
chip can be easily popped from it.
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Particulate adhesive compositions can be formulated
which show remarkably little susceptibility to
tr;ibocharging. In the form of adhesive tapes, these
compositions are eminently suitable for use in wafer
dicing operations and other similar applications where
protection of sensitive electronic components is
essential along with ease of removal and integrity of
adhesive to tape backing.
Useful microparticulate adhesives have adhesion
values to steel of from 0.5 N/100 mm to 10 N/100 mm,
preferably from 1 N/100 mm to 5 N/100 mm.
Useful microparticles comprise alkyl acrylate or
meahacrylate monomers, especially monofunctional
unsaturated acrylate or methacrylic esters of non-
tertiary alkyl alcohols, the alkyl groups of which have
from 4 to about 14 carbon atoms. Such acrylates are
o:Leophilic, water emulsifiable, have limited water
solubility, and as homopolymers, generally have glass
transition temperatures below about -20°C. Included
within this class of monomers are, for example, isooctyl
acrylate, 4-methyl-2-pentyl acrylate, 2-methylbutyl
acrylate, isoamyl acrylate, sec-butyl acrylate, n-butyl
acrylate, 2-ethylhexyl acrylate, isodecyl methacrylate,
isononyl acrylate, isodecyl acrylate, and the like,
singly or in mixtures.
Preferred acrylates include isooctyl acrylate,
isononyl acrylate, isoamyl acrylate, isodecyl acrylate,
2-ethylhexyl acrylate, butyl acrylate (such as n-butyl
acrylate and sec-butyl acrylate) and mixtures thereof.
Preferred methacrylates include isooctyl methacrylate,
2-ethylhexyl methacrylate, isononyl methacrylate, isoamyl
methacrylate, isodecyl methacrylate, butyl methacrylate
(such as n-butyl acrylate and sec-butyl acrylate) and
mixtures thereof. Acrylate or methacrylate or other vinyl
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monorners which, as homopolymers, have glass transition
temperatures higher than about -20°C, e.g., tert-butyl
acrylate, vinyl acetate, and the like, may be utilized in
conjunction with one or more of the acrylate or methacrylate
monomers provided that the glass
. .. ..
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transition temperature of the resultant polymer is below
about -20°C.
Useful vinyl ester monomers are those which form
homopolymers having glass transition temperatures below
about 10°C. Such esters comprise 1 to 14 carbon atoms,
and includes such monomers as vinyl 2-ethylhexanoate,
vinyl caproate, vinyl laurate, vinyl pelargonate, vinyl
hexanoate, vinyl propionate, vinyl decanoate, vin yi
octanoate, and the like.
Useful polar monomers include moderately polar
monomers such as N-vinyl-2-pyrrolidone, N-vinyl
caprolactam, acrylonitrile, vinyl acrylate, and diallyl
phthalate, as well as strongly polar monomers such as
acrylic acid, methacrylic acid, itaconic acid,
hydroxyalkyl acrylates, cyanoalkyl acrylates,
acrylamides, substituted acrylamides. When more than one
polar monomer is used, mixtures may include monomers
having similar or unlike polarities, e.g., one moderately
polar and one strongly polar monomer or two monomers from
2 0 o;ne group .
The conductive microparticles and the pressure
sensitive adhesives made therefrom comprise at Least 70
parts by weight of at least one alkyl (meth)acrylate
esker or vinyl ester and correspondingly, up to 30 parts
by weight of one or more polar monomers.
Polymer electrolyte base polymers suitable for use
i.n the invention include polyethylene oxide,
polyphenylene oxide, polyphenylene sulfide, polyethylene
:>ulfide, polyethyleneimine, polypropylene oxide,
polybutylene oxide, polybutylene sulfide, polybutylene
imine, and the like. Polyethylene oxide is preferred.
useful amounts of the polymer electrolyte base polymer in
microparticles of the invention range from 0.1 part to 20
parts, preferably from 1 part to 5 parts, based on 100
parts monomer weight.
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The conductive properties of the polymeric
microparticles are further enhanced by the addition of
ionic salts to adhesive compositions which contain the
mi.croparticles. It is believed that the ionic salts
become associated with the electron donating groups
present in the amorphous polymer domains. The adhesive
contains from 0.01 moles to 10 moles of at least one salt
oi: an alkali metal or alkaline earth metal per mole of
polymer electrolyte base unit, or from 0.05 part to 3.0
parts per 100 parts monomer.
' Salts used for this purpose include salts of alkali
metals, and alkaline earth metals, including but not
limited to, NaI, NaSCN, BaCF3S03, NaBr, NaC104, LiCl,
LiN03, LiC:F3S03, LiS04, LiN (SOZCF3) 2, LiOH and KOH. Lithium
salts are preferred for the present invention, especially
lithium nitrate.
In order to exhibit the necessary water-resistance,
the composition also contains an aziridine crosslinking
agent. Useful aziridines include pentaerythritol-tris-
(~i-(N-aziridinyl)propionate), and trimethylolpropane-
t:ris-(~-(N-aziridinyl)propionate), both available as 10$
:solutions in iPrOH under the trade name "XAMA", i.e.,
XAMA-2 and XAMP.-7, from B:F. Goodrich, Specialty
Chemicals, and trimethylolpropane-tris-(~-(N-
methylaziridinyl)propionate), available as "CX-100" from
Sieneca Resins, and mixtures of one or more of the above.
Surprisingly, the use of such aziridines provides the
grater-resistance necessary while maintaining the required
balance of adhesion and removability. The aziridines axe
present in an amount of from 0.01 part to 2 parts per 100
parts monomer.
Further crosslinking agents may also be included,
such as a multifunctional (meth)acrylate, e.g.,
butanediol diacrylate or hexanediol diacrylate, or others
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multifunctional crosslinker such as divinylbenzene. When
used, the crosslinker(s) is(are) added at a level of up
to 1 percent, preferably up to 0.5 percent, of the total
polymerizable composition.
Tapes of the present invention display dramatically
different tribocharging properties than continuous
adhesive layers of similar chemical components. For
example, when coated on a film substrate, an acrylate-
based emulsion adhesive produces a continuous film with a
planar surface. Upon application and removal from a
planar surface, this adhesive tape will cause generation
of charged species on the surface of the adhesive and on
the planar surface to which it was attached. The
residual charge has a magnitude of up to several thousand
volts. However, adhesive tape samples of the current
invention, under similar conditions generate almost no
charge upon removal from the planar surface.
Without wishing to be bound by theory, it is
believed that the improved electrical properties of the
adhesive are due to two aspects of its particulate
natures firstly, the particulate prevents full area
contact of the adhesive layer with the planar surface.
The reduced area of attachment results in a reduction of
area of separation when the tape is removed from the
planar surface, and thus there is less tendency for
charged species to be generated. Secondly, there is a
surface layer of conductive species available on each
microparticle. The surface layer is provided by
materials which facilitate conduction of electrical
charge. Provision of the host polymer in spherical form
allows increased availability of electron donating
polymer chains.
Also, it is possible to exert better control over
the length of the chains so as to increase the relative
number of amorphous domains. This provides a larger
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network of conductive sites which allows more efficient
conduction of electric current.
Electrical characteristics of pressure-sensitive
a
adhesives of the invention vary from somewhat resistive
to significantly conductive materials.
The microparticulate adhesives and an emulsion
containing the microparticles may be prepared by various
emulsification processes, which are varied depending on
whether hollow or solid microparticles are desired.
Aqueous suspensions of hollow microparticles may be
prepared by a "two-step" emulsification process which
first involves forming a water-in-oil emulsion of an
aqueous solution of polar monomers) in oil phase
monomer, i.e., at least one (meth)acrylate or vinyl ester
monomer, with a polymer electrolyte base polymer, using
an emulsifier having a low hydrophilic-lipophilic balance
(HLB) value. Suitable emulsifiers are those having an
HLB value below about 7, preferably in the range of 2 to
7. Examples of such emulsifiers include sorbitan
monooleate, sorbitan trioleate, and ethoxylated oleyl
alcohol such as Brij~ 93, available from Atlas Chemical
Industries, Inc.
Thus, in this first step, oil phase monomer(s),
polymer electrolyte base polymer, emulsifier, a free
radical initiator, and a crosslinking monomer or monomers
are combined, and an aqueous solution of all or a portion
of the polar monomers) is agitated and poured into the
oil phase mixture to form a water-in-oil emulsion. The
polymer electrolyte base polymer may be added to either
the oil phase or the water phase. A thickening agent
,
e.g., methyl cellulose may also be included in the
aqueous phase of the water-in-oil emulsion. In the
second step, a water-in-oil-in-water emulsion is formed
by dispersing the water-in-oil emulsion of the first step
into an aqueous phase containing an emulsifier having an
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HLB value above 6. The aqueous phase may also contain
any portion of the polar monomers) which was not added
in step one. Examples of such emulsifiers include
ethoxylated sorbitan monooleate, ethoxylated lauryl
alcohol, and alkyl sulfates. In both steps, when an y
emulsifier is utilized, its concentration should be
greater than its critical micelle concentration, which is
herein defined as the minimum concentration of emulsifier
necessary for the formation of micelles, i.e.,
submicroscopic aggregations of emulsifier molecules.
Critical micelle concentration is slightly different for
each emulsifier, usable concentrations ranging from 1.0 x
10-4~to 3.0 moles/liter. Additional detail concerning the
preparation of water-in-oil-in-water emulsions, i.e.,
multiple emulsions, may be found in various literature
references, e.g., Surfactant Systems: Their Chemistry,
Pharmacy, & Biology, (D. Attwood and A.T. Florence,
Chapman & Hall Limited, New York, New York, 1983).
The final process step of this method involves the
application of heat or radiation to initiate
polymerization of the monomers. Useful initiators are
those which are normally suitable for free radical
polymerization of acrylate or vinyl ester monomers and
which are oil-soluble and of very low solubility in
water. However, when the polar monomer is N-vinyl
pyrrolidone, the use of benzoyl peroxide as the initiator
is recommended.
Examples of such initiators include azo compounds,
hydroperoxides, peroxides, and the like, and
photoinitiators such as benzophenone, benzoin ethyl
ether, and 2,2dimethoxy-2-phenyl acetophenone.
Use of a water-soluble polymerization initiator
causes formation of substantial amounts of latex. The
extremely small particle size of latex particles renders
any significant formation of latex undesirable. The
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initiator is generally used in an amount ranging from
0.01 percent up to 10 percent by weight of the total
polymerizable composition, preferably up to 5 percent.
Aqueous suspensions of hollow conductive
microparticles may also by prepared by a "one-step"
emulsification process comprising aqueous suspension
polymerization of at least one alkyl (meth)acrylate ester
monomer or vinyl ester monomer and at least one polar
monomer and a polymer electrolyte base polymer in the
presence of at least one emulsifier capable of producing
a water-in-oil emulsion inside the droplets which is
substantially stable during emulsification and
polymerization. As in the two-step emulsification
process, the emulsifier is utilized in concentrations
greater than its critical micelle concentration. In
general, high HLB emulsifiers are required, i.e.,
emulsifiers having an HLB value of at least 25, will
produce stable cavity-containing droplets during the
polymerization, and are suitable for use in this one-step
process. Examples of such emulsifiers include
alkylarylether sulfates such as sodium alkylarylether
sulfate, e.g., Tritons'' W/30, available from Rohm and
Haas, alkylarylpolyether sulfates such as
alkylarylpoly(ethylene oxide) sulfates, preferably those
having up to about 4 ethyleneoxy repeat units, and alkyl
sulfates such as sodium lauryl sulfate, ammonium lauryl
sulfate, triethanolamine lauryl sulfate, and sodium
hexadecyl sulfate, alkyl ether sulfates such as ammonium
lauryl ether sulfate, and alkylpolyether sulfates such as
alkyl polyethylene oxide) sulfates, preferably those
having up to about 4 ethyleneoxy units. Alkyl sulfates,
alkyl ether sulfates, alkylarylether sulfates and
mixtures thereof are preferred as they provide a maximum
void volume per microparticle for a minimum amount of
surfactant. Nonionic emulsifiers, e.g., Siponic~' Y-500-
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70 (ethoxylated oleyl alcohol), commercially available
from Alcolac, Inc, and Pluronic~"' P103 (a block copolymer
of polypropylene oxide and polyethylene oxide
r
commercially from BASF Corporation) can be utilized alone
or in conjunction with anionic emulsifiers. Polymeric
stabilizers may also be present but are not necessary.
Solid microparticles useful in adhesive tapes of the
invention may be made by a similar one-step process
comprising aqueous suspension polymerization of at least
one alkyl (meth)acrylate ester monomer or vinyl ester
monomer, at least one polar monomer and a polymer
electrolyte base polymer in the presence of a suspension
stabilizer. It is not necessary to use a high HLB
emulsifier because the droplets formed need not be
cavity-containing droplets. Examples of such useful
lower HLB emulsifiers include ammonium lauryl sulfate
such as Standapol~ A, available from Hercules and other
steric or electrosteric polymeric stabilizers such as
polyvinyl alcohol, polyacrylic acid, polymethacrylic
acid, polyacrylamide, polyvinyl pyrrolidone, polyvinyl
methylether, and the like.
Preparation of microspheres may be modified by
withholding the addition of all or part of the polymer
electrolyte base polymer, and polar monomers until after
polymerization of the oil phase is initiated; however,
the components must be added to the polymerizing mixture
prior to 100 polymer conversion.
Discrete conductive polymeric microparticles may
also be prepared via suspension polymerizations disclosed
in U.S. Pat. Nos. 3, 691, 140, 4, 166, 152, 4, 636, 432,
0
4,656,218, and 5,045,569, for preparing adhesive
compositions.
The conductive microparticles are normally tacky,
elastomeric, insoluble but swellable in organic solvents,
and small, typically having diameters of at least 1
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micrometer, preferably in the range of 1 to about 250
micrometers, more preferably from 1 to 50 micrometers.
They may be solid or contain a single void, or multiple
voids.
Following polymerization, an aqueous suspension of
them microparticles is obtained which is stable to
agglomeration or coagulation under room temperature
conditions. The suspension may have non-volatile solids
contents of from 10 to 50 percent by weight. Upon
prolonged standing, the suspension separates into two
phases, one phase being aqueous and substantially free of
polymer, the other phase being an aqueous suspension of
conductive microparticles. Where high HLB emulsifiers
are used the droplets have one or more cavities which,
upon drying, become voids. Both phases may contain a
minor portion of small latex particles. Decantation of
the microparticle-rich phase provides an aqueous
suspension having a nonvolatile solids content on the
order of about 40-50 percent which, if shaken with water,
will readily redisperse.
If desired, the aqueous suspension of conductive
microparticles may be utilized immediately following
polymerization to provide inherently tacky pressure-
sensitive adhesive coatings having low tribocharging
characteristics, or "antistatic" adhesives.
Tapes of the invention especially useful in wafer
dicing operations may be produced by coating
microparticle containing compositions of the invention
onto a flexible substrate which will allow sufficient
flex to "pop" a chip without causing delamination of the
adhesive or tearing of the backing. Suitable substrates
include polymeric films such as (poly)vinylidiene
chloride, polyesters, polyethylene terephthalate,
polyphenylene sulfide, polypropylene, polyethylene,
polyetherimide, and polyethersulfone.
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The coating may be carried out by conventional
methods such as knife coating, Meyer bar coating, knurled
roll, and other conventional means known in the art for
r~
coating adhesives such as use of an extrusion die.
Primer or binders may be used, but they are not
required. Preferred embodiments comprise a binder to
ensure that the adhesion between the backing and the
adhesive exceeds the adhesion between the adhesive and
the electronic component to which it will be attached.
Useful primers include phenolic resins, acrylic resins,
rubbery components, block copolyers, and mixtures
thereof.
Where high-temperature properties are required, a
useful primer will comprise at least one phenolic resin
and at least one rubbery component. Useful rubbery
components include natural rubbers such as butyl rubbers,
and various synthetic compounds, including but not
limited to, acrylonitrile-butadiene, acrylonitrile-
butadiene-styrene copolymers, styrene-butadiene-styrene,
styrene-ethylene butylene-styrene, polychloroprene,
polybutadiene, polyisoprene, styrene-isoprene-styrene,
and mixtures thereof. Preferred primers contain mixtures
of two or more rubbery compounds, such as acrylonitrile-
butadiene, and polychloroprene.
Useful phenolic resins, include but are not limited
to, phenol formaldehyde resin, available commercially
from Union Carbide under the trade names UCAR BKR-2620,
and UCAR CK-1635, novolak resins and the like, and
mixtures thereof. Preferred primers contain from 40 to
120, preferably from 40 to 100 parts of phenolic resin
per 100 parts of rubbery compound.
When used, a primer may further comprise additives
such as tackifying agents, antioxidants, colorants,
viscosity adjusting agents, solvents and other
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conventional additives, which may be used in such amounts
as are known in the art.
The tape may be commercialized in roll form, or may
be divided into segments for sale, such as strips or
labels. Additionally, the adhesive may be provided
between two substrates, e.g., the adhesive is coated onto
a paper substrate, which can be used as a label, which
may be provided on a low adhesion backsize or other
easily removable surface for individual use.
The adhesion properties of the adhesives may be
altered by addition of tackifying resin and/or
plasticizer. Preferred tackifiers for use herein include
hydrogenated rosin esters commercially available from
companies such as Hercules Inc., under such trade names
as Foral~ 65, Foral~ 85, and Foral~ 105. Other useful
tackifiers include those based on t-butyl styrene.
Useful plasticizers include dioctyl phthalate, 2-ethyl
hexyl phosphate, tricresyl phosphate, and the like.
It is also within the scope of this invention to
include various other components to tapes of the
invention, such as pigments, fillers, including
additional conductive fillers, stabilizers, or various
polymeric additives.
These and other aspects of the invention are
illustrated by the following examples which should not be
viewed as limiting in scope.
Test Methods
Resistivity Measurements of Antistatic Coatings
Resistivity is a measure of the intrinsic ability of
1
a material to conduct electrons. It is a property which
is independent of the dimensions of the material sample.
The surface resistivity of coatings of the invention
was measured by connecting a Keithley 616 digital
electrometer (Keithley 6105 resistivity adapter) to a 500
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volt power supply, and attaching to an electrometer.
Individual samples were measured using standard
procedures according to-ASTM D-257.
Peel Adhesion
Peel adhesion is the force required to remove a
coated flexible sheet material from a test panel measured
at a specific angle and rate of removal. In the
examples, this force is expressed in grams per centimeter
(cm) width of coated sheet. The procedure followed is:
A strip 1.27 cm in width of the coated sheet is
applied to the horizontal surface of a clean glass test
plate with at least 12.7 lineal cm in firm contact. A 2
kg hard rubber roller is used to apply the strip. The
free end of the coated strip is doubled back nearly
touching itself so the angle of removal will be 180°.
The free end is attached to the adhesion tester scale.
The steel test plate is clamped in the jaws of a tensile
testing machine which is capable of moving the plate away
from the scale at a constant rate of 2.3 meters per
minute. The scale reading in grams is recorded as the
tape is peeled from the steel surface. The data is
reported as the average of the range of numbers observed
during the test.
Wafer Dicing Tape Performance
A useful wafer dicing tape will survive a series of
processing steps during which individual integrated
circuit chips are separated from the main wafer. The
steps include wafer mounting, sawing, washing, drying and
die picking.
Wafer Mounting
This process combines the steps of attaching an
integrated circuit wafer to the adhesive tape, attaching
the tape to a circular support frame and trimming excess
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tape from around the outer circumference of the support
frame.
Wafer Sawing
The wafer sawing unit contains a computer-controlled
diamond saw that scores the wafer along lines to define
individual chips or dies. Water jets provide cooling and
cleaning during the cutting operation, requiring the
adhesive to resist water attack and retain a firm grip on
the wafer.
Wafer Washing
Following transfer from the wafer sawing unit to the
wafer washing station, high power jets of water wash over
the wafer and its associated support structure. This
cleans the wafer by removing residual saw-dust.
Thereafter the washed assembly is dried before die
picking.
Die Picking
Computer controlled equipment directs force behind a
selected chip formed in the wafer. The force causes the
wafer to crack around the chip or die and releases the
chip from the main body of the wafer sufficiently to be
cleanly picked from the wafer and released by the wafer
dicing tape.
Key to Abbreviations
IOA Isooctyl Acrylate
iPrOH Isopropyl Alcohol
AA Acrylic Acid
' PEO Polyethylene oxide Acrylate
PEO (750) Acrylate terminated PEO having a
750 MW
BPER 70 o Benzoyl Peroxide, Lucidoh"' 70
PEODMA Polyethylene Oxide Dimethacrylate
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[(PEO)9DMAJ
1,6 HDDA 1,6 Hexanediol Diacrylate
ALS Ammonium Lauryl Sulfate,
Standapoh' A Ammonium Lauryl Sulfate from
Hercules
Key to Materials
Lithium Nitrate is provided as a 20$, solution in
distilled water.
Benzotriazole, anti-corrosion agent, is provided as a 10~
solution in 50/50 iPrOH/Water
XAMA-7 - Pentaerythritol-tris-(~i-(N-aziridinyl)
propionate) crosslinker as a 10$ solution in iPrOH
XAMA-2 - Trimethylolpropane-tris-(~i-(N-
aziridinyl)propionate) crosslinker as a 10$ solution
in iPrOH.
Both crosslinkers are available from B.F. Goodrich,
Specialty Chemicals, Performance Resins and
Emulsions Division.
CX-100 - Trimethylolpropane-tris-(~i-(N-
aziridinyl)propionate) provided as a 10~ solution in
iPrOH, available from Zeneca Resins.
Key to Film Substrates
Blue PVC film from Ross & Roberts
Scotchcah'' Film (Plasticized PVC) from 3M Company
Clear PVC film from American Mirrex
Biaxially Oriented Polypropylene (BOPP) film
Phenolic Resin BKR-2620-Phenol-Formaldehyde Resin,
designated BKR-2620 UCAR, by Union Carbide.
TM
SantivarA-Antioxidant di-tertiary amyl hydroquinone
TM
Piccolyte S115-Polyterpene resin (tackifier)
7M
Zirex-Zinc Resinate (tackifier)
Phenolic Resin (CK-1635)-Phenol-Formaldehyde Resin, by
Union Carbide, designated CK-1635 UCAR.
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Examples
Preparations of Microparticles
Example 1
Acrylic acid (5.4 g), polyethylene oxide acrylate
' 5 (PEO 750) (13.5 g), PEODMA (0.15 g) and 70~. benzoyl
peroxide (0.99 g) were dissolved in isooctyl acrylate
(223.2 g). This solution was added to an aqueous
solution of surfactant. The surfactant solution
comprised Standapoh'' A, available from Hercules, (8.4 g)
dissolved in de-ionized water (360 g) in a glass-lined
vessel. An emulsion of the isooctyl acrylate solution in
the aqueous solution was produced by high shear mixing
using an Omni mixer at setting 5. Mixing was continued
until the average particle size of the oily droplets was
approximately 3 um. Size was determined using an optical
microscope.
The resulting oil-in-water emulsion was charged to a
1 liter glass resin reactor equipped with four baffles, a
paddle stirrer and a suitable heat source, such as a
heating mantle. With continuous stirring at a rate of
400 rpm, the reactor and contents were heated to 60C.
At this point the reactor was degassed with
nitrogen. A reaction proceeded in the absence of oxygen.
This was allowed to continue for a period of 22 hours
while both temperature and stirring rate were maintained.
The resulting aqueous suspension contained insoluble
particles of approximately 5 um in diameter.
To 100 parts of this particulate adhesive, was added
a combination of lithium salts to increase ionic
conductivity, ammonium hydroxide for pH adjustment,
benzotriazole for corrosion inhibition and a crosslinker
to improve water resistance of the coating. Each of the
additional ingredients was slowly stirred into the
adhesive composition and thoroughly mixed just prior to
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coating. Use of the coating composition within one hour
of crosslinker addition provided optimum coating results.
Table 1 c'
Adhesive Compositions
Composition Sample 1 Sample 2 Sample 3 '
iAdhesive 100 g 100 g 100 g
I
20$ LiN03 2.5 g 2.5 g 2.5 g
10~ benzo 0.5 g 0.5 g 0.5 g
triazole
10~ CX-100 0.06 g - -
10~ XAMA-2 - 0.06 g -
10~ XAMA-7 - - 0.03 g
Water - 50 g 102 g
Table 1 (font. )
Adhesive Compositions
Sample 4 Sample 5 Sample 6 Sample 7
Adhesive 100 g 100 g 100 g 100 g
20~ LiNOs 2.5 g 2.5 g 2.5 g 2.5 g
10~ benzo 0.5 g 0.5 g 0.5 g 0.5 g
triazol
10~ XAMA-7 0.06 g 0.06 g 0.06 g 0.06 g
NH90H - 0.3 g 0.44 g 0.50 g
Water 50 g 50 g 50 g 50 g
pH 2.7 7.2 8.0 8.9
n i i i i i
Samples 4 and 5 showed good water resistance. Preferred
compositions have a pH from 2.5 to 7.0; most preferred
compositions from 3.0 to 7Ø At higher pHs such as 8 or
more, poor resistance to water was seen (Samples 6 and
7) .
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Preparation of Primer Solution
wt ~ t a ,.. ; ~ '1
Butadiene/Acrylonitrile 75.00 parts 5.755
Neoprene W. 25.00 parts 1.918
Phenolic Resin BKR-2620 19.90 parts 1.527
Santivar A 3.95 parts 0.3038
Piccolyte S115 49.67 parts 3.811
Zirex 49.67 parts 3.811
Phenolic Resin 69.43 parts 5.328$
Methyl Ethyl Ketone 329.57 parts 25.290$
iso-Propanol 60.00 parts 4.604
Toluene 621.00 parts 47.652
The resins, tackifiers and antioxidant indicated
above are dissolved in a mixed solvent comprising methyl
ethyl ketone, iso-propanol and toluene to provide a
primer coating for film supports. Conventional churns,
equipped with stirrers, or similar equipment may be used
for primer solution preparation. The solution is
inspected for clarity and filtered if necessary.
Film Priming
One part of the primer composition was diluted,
prior to coating, with two parts of a mixed solvent of
2:1 toluene:MEK. Both the PVC and Scotchcah' films
received a coat of primer using a #4 Meyer bar.
Evaporation of the primer solvent at 77°C, for about 3
minutes, yielded a dried primed film suitable for coating
with adhesive. The dried primer layer was approximately
7.9x10-6 mm to 1.2x10-5 mm thick. The primer was direct
coated onto both films.
(An alternative means of primer application uses a
gravure roll coater equipped with a knurled roll of about
10 lines/mm with drying at 75°C.)
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Adhesive Coatin
Two methods, for applying the adhesive to the primed
backing, gave suitable product for application as wafer
t
dicing tapes. The ScotchCal~ was directly coated with
adhesive, i.e., the adhesive applied directly to the
primed substrate using a #12 Meyer bar then dried at 66°C
for 6 minutes. The PVC was coated by a transfer method,
i.e., a knurled-roll coating was used to prepare an
adhesive layer, 1.2x10-4mm thick, on a release liner.
Thereafter transfer of the adhesive layer to the primed
surface of the film produced the adhesive tape of the
present invention.
An alternative means of adhesive application uses a
gravure roll water having a knurled roll of about 3.5
lines/mm.
Table 2
Adhesive Tape Properties
The following table demonstrates the properties of
wafer dicing tape of the invention as well as two
comparative tape constructions. The first comparative
tape, called C1, is a commercially available antistatic
tape, available from Nitto Corporation. The second
comparative, called C2 is a non-water resistant adhesive
similar to those of the invention, but having a pH of
9.2.
Sample Volts Tribocharge Resistance AdhesionWater
Ohms oz/inch resistance
On Steel On Silicon
C1 Nitto 1840 1183 5.2 x 101 2.8 Good ..
Tape
Sample 3 6 2 8.5 x 10' 3.0 Good
on
Blue PVC
Sample 3 13 1 4.0 x 10' 3.5 Good
on
Scotchcal
C2 on Blue 6 2 5.2 x 10' 2.5 Swells
PVC
