Fabrication of coatings based on epoxy resins for environmental protection of microelectronics

Article abstract of DOI:10.1134/S1070427213070094

Modified coating for protection of microelectronics from external effects, including radiation, was created on the basis of epoxy resins. The coating was produced with a modifier, thioalkaphene B, containing polysulfide derivatives of ortho-tert-butyl phenol. The mechanism of its action was explained. The technology for coating deposition was tested on an industrial automated line; the thus protected semiconductor devices successfully passed tests in extreme working conditions.

Full text of DOI:10.1134/S1070427213070094

ISSN 1070-4272, Russian Journal of Applied Chemistry, 2013, Vol. 86, No. 7, pp. 997−1000. © Pleiades Publishing, Ltd., 2013.
Original Russian Text © A.P. Krysin, O.I. Yakovlev, 2013, published in Zhurnal Prikladnoi Khimii, 2013, Vol. 86, No. 7, pp. 1064−1068.
ELECTROCHEMISTRY AND OTHER
PROCESSES OF CHEMICAL TECHNOLOGY
Fabrication of Coatings Based on Epoxy Resins
for Environmental Protection of Microelectronics
A. P. Krysina and O. I. Yakovlevb
aVorozhtsov Institute of Organic Chemistry in Novosibirsk, Siberian Branch,
Russian Academy of Sciences, Novosibirsk, Russia
bNovosibirsk Plant of Semiconductor Devices with an R & D Office, Novosibirsk, Russia
e-mail: krysin_ap@mail.ru
Received August 29, 2012
Abstract—Modified coating for protection of microelectronics from external effects, including radiation, was
created on the basis of epoxy resins. The coating was produced with a modifier, thioalkaphene B, containing
polysulfide derivatives of ortho-tert-butyl phenol. The mechanism of its action was explained. The technology
for coating deposition was tested on an industrial automated line; the thus protected semiconductor devices suc-
cessfully passed tests in extreme working conditions.
DOI: 10.1134/S1070427213070094
Thequalityandpropertiesofmodernmicroelectronics
are determined by the forward and reverse voltages
and forward and reverse currents. The irradiation and
external factors are the main reason for the increase in
the reverse current of a p–n junction, which reduces the
intensity of electromagnetic signals passing through
a device. To diminish the extent of these and other
negative external factors, semiconductor devices are
protected by coatings.
wall creates conditions for an ordered redistribution of
the energy flux and reduces its energy via acceleration
of electron motion in the rings of aromatic compounds
[3].
It was found that a part of shortcomings inherent
in coatings based on epoxy resins can be eliminated
by raising the adhesion of epoxy compounds to metals
from which current leads and places at which thee
are soldered to devices: tin, copper, silver, aluminum,
and alloys of these. A good adhesion to these metals
is provided by introduction of compounds containing
a chain of six and more sulfur atoms into the epoxy
resin formulation, followed by heating of the composite
[4]. The good moisture resistance of the coatings
formed on semiconductor devices provides stability of
electromagnetic signals passing through the devices and
makes longer their stable operation.
Coatings based on an epoxy resin, distinguished by
mechanical strength and simplicity of deposition, have
found mass practical application [1]. In addition to
providing necessary mechanical properties of a coating,
it is necessary to guarantee that requirements to devices
working under extreme conditions should be satisfied: a
device must be stable against moisture and temperature
drops, be chemically inert, and leave unchanged
parameters of electromagnetic signals passing through
the device under increased radiation conditions.
A set of additives is suggested, which enables a
reliable protection of modern semiconductor devices
from external effects when used to fabricate a
protective coating [5]. For example, three additives are
introduced into the epoxy resin prepared on the basis
of dihydroxybiphenyl: (i) HMP8 phenolic antioxidant
(methylphenolsulfides),(ii)radiationprotectionadditive,
The problem of the radiation protection of
microelectronics was solved in one of patents by using
epoxy resins containing a large number of aromatic rings
[2]. In this case, a wall of interacting electrons, ordered
by aromatic rings, appears on the way of radiation. The
997

998
KRYSIN, YAKOVLEV
and (iii) morpholine disulfide (Actor R) capable of
providing, under certain formulation heating conditions,
adhesion of a protective coating to the conductor metal.
A thermal polymerization of a formulation of this kind
yields a compound that is suitable for protection of
the semiconductor and solder and is distinguished by
high moisture resistance, good adhesion to metals, and
stability against heating.
the same temperature range.
For example, thermolysis in the course of coating
fabrication leads to an oxidative burst of the TAB
components, as shown in Scheme 1. This process is
also typical of other phenolic modifiers and ultimately
converts these into antioxidants. If a polymer is in the
environment of the radical species formed in this process,
the species react with the polymer and chemically
modify its properties. This gives a new polymer
having improved physical-mechanical properties and
containing a fragment of a phenolic antioxidant [9].
A domestic product was used to reliably protect
semiconductors in stead of three imported compounds.
It simultaneously contained aromatic rings, and
a polysulfide chain, and a fragment of a phenolic
antioxidant capable of interaction with epoxy resins.
All these three requirements are satisfied for he
thioalkaphene B stabilizer (TAB, the abbreviation
suggested by Khimpolymer Research Institute for
polymer stabilizers). The stabilizer is obtained
by interaction of 2-tert-butylphenol with sulfur
monochloride in the presence of dimethylformamide.
The composition of the resulting products has been
thoroughly studied [6]. Its main component (70%) was
a mixture of di-and polysulfides:
However, there is another, also highly important
case, when a metal consists the environment of the
“exploded” modifier. Its fragment can be “linked” to the
metal, with an adherence layer thereby formed [4].
TheTABstabilizer1interactswithED-20epoxyresin
6 or ED-24 (the resins are interchangeable) by various
schemes, depending on the nature of a catalyst and on
the extent to which the formulation is heated. In the first
stage, in the presence of the catalyst (boron oxide), the
reaction of esterification of the phenolic group having
only a single tert-butyl group in its environment readily
occurs at a temperature of 120°C to give ether 2.
Further cleavage of the disulfide bridge in compound
2 is observed in the range 160–18°C to give radical-con-
taining compounds 3 and 4, which presumably interact
with the conductor metals and form a solid layer adhering
to the metal and solder to the device, to give product 5
with a probable structure (ArS)n–M, where M is the metal
that closes the gap between the protection polymer and
the conductor and thereby precludes penetration of mois-
ture into the gap during operation of the device.
HO
Sn
OH
n = 1, 20%
n = 2, 50%
n = 3, 10%
n = 4, 10%
(1)
In addition to the polysulfides of specified structure,
two groups of compounds (10% each) containing one or
two polysulfide chains were found in TAB.
These reactions occur in a thin layer on the surface of
chip, with area not exceeding 2–4 mm2. In the first stage,
at a temperature of 120°C, a solid oligomeric compound
2 is formed. It is easily detachable from the substrate to
form a free space between the protective layer and the
device, because no firm adherence layer is formed under
these conditions. No adhesion could be observed at this
temperature even upon a very prolonged keeping. The
reason is that the process in which radicals necessary
for obtaining an adherence layer is slow. The radicals
are fully “caught” by the TAB stabilizer components.
Therefore, it is necessary to take the stabilizer in an
amount of 2.4%, which substantially exceeds that
recommended (0.05–0.5%) for polymers in industrial
practice, and perform the protection fabrication process
itself at a considerably higher temperature (180°C).
It has been found previously in a study of the thermal-
kinetic properties of the components of TAB that the
monosulfide it contains [compound (1), n = 1, 20% in
the mixture] is an ordinary antioxidant additive. The rest
of the components [di- and polysulfides, compounds (1)
with n = 2, 3, 4] belong to a newly discovered group of
phenolic polymer modifiers [7]. Compounds of this kind
behave at room temperature as ordinary antioxidants,
but heating results in reaching the temperature of their
decomposition into radical species acting as oxidizing
agents. The thermal decomposition point of the disulfide
[compound (1) with n = 2, the main component of TAB]
is 160–180°C [8].Anoticeable decomposition of tri- and
polysulfides [compound (1) with n = 3, 4] is observed in
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 86 No. 7 2013

FABRICATION OF COATINGS BASED ON EPOXY RESINS
999
Scheme.
OH
OH
S
120oC
O
O
O
O
+
B2O3
S
S
S
HO
O
HO
6
1
O
Ar
2
180oC
S
S
OH
OH
O
Ar
O
M _ Al
Sn
M
Ag
Cu
3
4
M
(ArS)n
5
The formation of an adhesion layer solves one of the
most important problems of microelectronics, its
moisture protection. The above transformations have
no effect on the initial electrical characteristics of a
semiconductor device, with its negative characteristic,
reverse current, reduced by a factor of 1.7.
diodes (1700 pieces) was divided into two parts before
being protected. The firs part was protected and then
encapsulated with EKLB epoxy compound containing
ED-20 epoxy resin and a polymerization catalyst, boron
oxide).To these was added (in a 100 : 2.4 mass ratio)TAB
modifier.Thesecondpartwasprotectedandencapsulated
with EKLB epoxy compound without modifier. The
protected articles were simultaneous subjected to hot-
curing polymerization in a drying oven in the following
mode: 1st stage, temperature 120°C, keeping for 6 h;
EXPERIMENTAL
A batch of high-voltage rectifier piles of KTs113A-1
Properties of the protection of a p–n junction of control and test groups of semiconductors in their manufacture on an automated
production line
Median leakage current Jrev, 10–9
A
After storage for nine months
Content of TAB in
ED-20 epoxy resin,
%
Median
enlargement
ratio
Percentage of
rejects upon
irradiation, %
Median
Percentage of
leakage current
rejects, %
before
after
irradiation
irradiation
Jrev, 10–9
Å
0
4.45
min: 2.7
max: 19.7
5.7
7.5
min: 3.0
max: 16.0
8.5
1.68
0
6.4
14
min: 3.2
max:74
5.45
2.4
1.49
0
0.5
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 86 No. 7 2013

1000
KRYSIN, YAKOVLEV
2nd stage, temperature 180°C and keeping for 14 h. After
being protected the articles were potted with the same
compound and subjected to polymerization in a drying
oven in the mode: 1st stage, temperature 85°C and time
4 h; 2nd stage, 120°C and 7 h; and 3rd stage, 180°C
and 15 h. Formation of the protective layer formed in
this way provides a high reliability of microelectronic
devices in extreme working conditions. The finished
articles were irradiated with 2.4 MeV electrons (electron
beam current 0.3 A) for 3 h 30 min.
microelectronic devices have been successfully per-
formed at Novosibirsk plant of semiconductor devices
on an automated production line [10]. The working
capacity of these devices was tested under prolonged-
storage conditions with the ordinary package containing
a moisture absorber and in extreme working conditions.
It should be noted that the chemical modification
considered here is promising not only for microelectron-
ics, but also for production of TAB-modified polymers
for articles insensitive to solvents and water. These poly-
mers also better withstand mechanical treatments [11].
After a control measurement of their electrical
parameters, the articles were stored at room temperature
for nine months, after which the parameters were again
measured. The measurement results are listed in the
table.The reverse currents in these devices are noticeably
lower than those in the control group. In the articles
produced with TAB modifier, the reject percentage
decreased from 14 to 0.5%. The reverse current Jrev
in the modified samples decreased, compared with the
control group of devices, by approximately a factor of
1.7 on average (from 0.012 to 0.007 μA).
CONCLUSIONS
(1) A thermoreactive-resin formulation containing
an epoxy resin, polymerization catalyst, and modifier
(thioalkaphene B) was suggested for producing a
coating for protection of microelectronic devices from
the environmental exposure.
(2) Technological conditions were found for thermal
polymerization of the formulation for producing a
protective coating for semiconductor devices, with the
subsequent formation of an adhesion layer.
No rejects were found in devices with a modified
coating after their storage for 12 months. The above
transformations were simulated in parallel in cuvettes
made of various metals. No adhesion transformations
were observed at 120°C: the polymeric compound (2)
being formed was easily detached from the mold. In
the subsequent stage performed at 180°C with TAB-
containing compounds, the adhesion of the new polymer
(5) being formed to the cuvette material was so high that
the cuvettes had to be eliminated by abrasion to extract
their contents.
REFERENCES
1. Kremnevye planetarnye tranzistory (Planar Silicon
Transistors), Fedorov, Ya.A., Ed., Moscow, 1973.
2. Grassie, N. and Scott, G., Polymer Degradation and
Stabilisation, New York: Cambridge Univ. Press, 1985.
3. WO Patent 120993.
4. Japan. Patent 168730.
Thus, a number of problems could be solved: (i) the
reverse current in the semiconductor could be reduced,
which improves its quality, and the effect of irradiation
could be substantially diminished; (ii) the solid adhesion
layer on the semiconductor surface precludes penetration
of moisture into microcracks of the protective coating,
which leads to a stable and prolonged operation of the
device; and (iii) the initial properties of semiconductor
devices are stabilized in their storage and operation
and the commonly used moisture protection of devices
becomes unnecessary.
5. Japan. Patent 08 2666.
6. Krysin, A.P., Egorova, T.G., Komarova, N.I., et al., Zh.
Prikl. Khim., 2009, vol. 82, no. 11, pp. 1817–1821.
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9. USSR Inventor’s Certificate no. 1387361.
10. RF Patent 2391361.
Industrial tests of the new method for protection of
11. USSR Inventor’s Certificate no. 1657517.
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