Postpolymerization of functional organosiloxanes: An Efficient strategy for preparation of low-k material with enhanced thermostability and mechanical properties

Article abstract of DOI:10.1021/ma501263c

A novel functional oligomer (Si-TFVE) with a siloxane backbone and thermally cross-linkable trifluorovinyl ether groups (-OCF=CF₂) is reported here. When postpolymerized at high temperature, Si-TFVE converts to an amorphous cross-linked network (Si-PFCB), which shows a dielectric constant of 2.33 and dielectric loss below 2.1 × 10^-3 at 30 MHz. Si-PFCB also shows excellent film uniformity with the surface roughness less than 5.79 nm over a 1 μm square area. Moreover, Si-PFCB shows high thermostability with a 5 wt % loss temperature of 472°C and no obvious T_g below 350°C. In regard to the mechanical properties, Si-PFCB has Young's modulus, hardness, and bonding strength with silicon wafer surface of 10.06 GPa, 0.392, and 4.93 GPa, respectively. These results suggest that such oligomer is suitable for utilization in ultralarge scale integration circuits. In addition, this contribution provides a new route to prepare cross-linked organosiloxanes only by heating instead of using catalysts or initiators in the traditional procedures. (Chemical Equation Presented).

Full text of DOI:10.1021/ma501263c

Article
pubs.acs.org/Macromolecules
Postpolymerization of Functional Organosiloxanes: An Efficient
Strategy for Preparation of Low‑k Material with Enhanced
Thermostability and Mechanical Properties
Chao Yuan,† Jiajia Wang,† Kaikai Jin, Shen Diao, Jing Sun, Jiawei Tong, and Qiang Fang*
Key Laboratory of Synthetic and Self-Assembly Chemistry for Organic Functional Molecules, Shanghai Institute of Organic
Chemistry, Chinese Academy of Sciences, 345 Lingling Road, Shanghai 200032, People’s Republic of China
S
* Supporting Information
ABSTRACT: A novel functional oligomer (Si-TFVE) with a
siloxane backbone and thermally cross-linkable trifluorovinyl
ether groups (−OCFCF2) is reported here. When
postpolymerized at high temperature, Si-TFVE converts to
an amorphous cross-linked network (Si-PFCB), which shows
a dielectric constant of 2.33 and dielectric loss below 2.1 ×
10−3 at 30 MHz. Si-PFCB also shows excellent film uniformity
with the surface roughness less than 5.79 nm over a 1 μm
square area. Moreover, Si-PFCB shows high thermostability
with a 5 wt % loss temperature of 472 °C and no obvious Tg
below 350 °C. In regard to the mechanical properties, Si-PFCB has Young’s modulus, hardness, and bonding strength with
silicon wafer surface of 10.06 GPa, 0.392, and 4.93 GPa, respectively. These results suggest that such oligomer is suitable for
utilization in ultralarge scale integration circuits. In addition, this contribution provides a new route to prepare cross-linked
organosiloxanes only by heating instead of using catalysts or initiators in the traditional procedures.
cross-linkable trifluorovinylether (−OCFCF2) side chains
INTRODUCTION
■
(Scheme 1). Si-TFVE showed good film forming capacity, and
In the microelectronic industry, low dielectric constant (low-k)
materials are crucially needed to reduce the resistance-
capacitance (RC) delay and cross-talk in integrated circuits
the film prepared from spin-coating was treated at high
temperature to produce a cured network, which exhibited a k
(IC).1−6 Therefore, great efforts have been made in search of
novel low-k materials in the past decades. Inorganic and organic
siloxanes7,8 as well as organic polymers9−15 have been
recognized as the preferable candidates for interlayer dielectric
(ILD) materials. However, only few of them are compatible
with the back-end-of-the-line (BEOL) production proce-
dures.2,4 For example, the insulators derived from silicon
dioxide exhibit nonideal dielectric performance, while organic
polymers usually show disqualified thermostability, anisotropy
of dielectric, inferior adhesion strength, and relatively low glass
transition temperature (Tg).9−15 Thus, to improve the above-
mentioned situation two strategies have been developed. One is
to introduce pores with favorable size and distribution,4,16,17
and the other is to use the least polarizable bonds such as C−F
bond in molecular design. Nevertheless, precise control of
pores’ generation and efficient molecular design still remain a
big challenge.16−18 To improve the thermostability and
mechanical properties, dense cross-linkable groups are
introduced into the side chains of the polymers.19 However,
in many cases, the modified polymers show unsatisfactory k
values.
value of 2.33 at 30 MHz with a dielectric loss less than 2.1 ×
10−3. Moreover, the cured Si-TFVE showed high thermo-
stability and good mechanical properties, suggesting that it
could meet the requirements of Cu damascene metallization. It
is worth emphasizing that the cross-linking of organosiloxanes
usually requires catalysts or initiators (platinum or peroxides),20
while in our case the cross-linking occurs by only heating,
indicating that this contribution provides a new route to form
cross-linked organosiloxanes and is of considerable industrial
importance.
EXPERIMENTAL SECTION
■
Reagents and Instruments. All starting materials were purchased
from Aldrich and Acros companies and used without further
purification. All solvents were dehydrated and distilled under an
inert atmosphere before use. 1H and 13C NMR spectra were recorded
on Bruker DRX 400 or Varian mercury 300 spectrometer using CDCl3
as solvent and TMS as internal standard.
Synthesis. Synthesis of compounds 1, 2, and 3 (see Scheme 1):
these compounds were prepared according to the previously reported
procedures.21−23
On the basis of the high thermostability and good adhesion
properties of siloxanes and the low-k values of fluoropolymers,
we synthesized a new oligomer (trifluorovinylether-function-
alized siloxanes (Si-TFVE)) with a siloxane backbone and
Received: June 18, 2014
Revised: August 20, 2014
Published: September 4, 2014
© 2014 American Chemical Society
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Scheme 1. Procedures for the Synthesis of Si-TFVE and Si-PFCB
Synthesis of oligomer Si-TFVE: a solution of 3 (18.38 g, 60 mmol)
in acetic acid (36 g, 0.6 mol) was refluxed with vigorous stirring for 24
h. After being cooled to room temperature, the mixture was
evaporated under vacuum. Si-TFVE was obtained in a yield of 99%.
state after spin-coating. Accordingly, the flattening of the
surface was easily achieved, and the microgaps could be
conveniently filled.
Thermal cross-linking (or curing) behavior of Si-TFVE was
evaluated by differential scanning calorimetry (DSC). As
depicted in Figure 1, the curing started at about 155 °C and
1
Mn = 2700 and polydispersity (PDI) = 1.86. H NMR (400 MHz,
CDCl3), δ (TMS, ppm): 7.74−7.14 (m, 2H), 7.13−6.71 (d, 2H),
0.57−0.18 (m, 3H). 19F NMR (376 MHz, CDCl3), (TMS, ppm):
−134.1∼−134.3 (m, 1F), −126.2∼−126.8 (m, 1F), −119.3∼−119.8
(m, 1F).
Devices Fabrication and Measurement of Dielectric Properties.
Preparation of Si-PFCB film and measurement of dielectric properties:
a solution of Si-TFVE in toluene was spin-coated on an aluminum-
backed silicon wafer (heavily doped single crystal silicon with a
resistivity of 3.5 × 10−3 Ω.cm) to form a sticky film.
The wafer was quickly placed on a hot-plate, which was preheated
to 350 °C. After being maintained at the temperature for 60 s, the
wafer was moved to an oven and heated at 250 °C for an additional 4
h under nitrogen. Thus, the fully cured Si-PFCB film was obtained.
The thickness of Si-PFCB film was measured by field emission
scanning electron microscopy (FE-SEM). For measurement of the
dielectric properties of the film, aluminum was then deposited on the
surface of the film via vacuum evaporation as a top electrode (the
diameter = 3.54 mm). A parallel plate capacitor was thereby fabricated.
The dielectric properties of the film were measured from an Agilent
4294A precision impedance analyzer at room temperature, and the
data were processed according to the previously reported method.9
Figure 1. DSC traces of Si-TFVE under N2 with a heating rate of 10
°C/min.
the peak was at 241 °C. Such curing behavior of Si-TFVE was
similar to that of the polymers containing −OCFCF2
groups,25,26 indicating that the thermal cross-linking of Si-
TFVE was only motivated by the [2 + 2] cyclodimerization
reaction of −OCFCF2 rather than the reaction between
silanols.
RESULTS AND DISCUSSION
■
As shown in Scheme 1, Si-TFVE was obtained in a quantitive
yield by hydrolysis of precursor 3 in acetic acid.24 Analysis by
GPC (polystyrene standards) showed that Si-TFVE had
number-average molecular weight (Mn) and polydispersity
index (PDI) values of 2700 and 1.86, respectively. According to
the Mn, the number of the repeating unit of Si-TFVE was
estimated at about 12. It was noted that the hydrolysis of
siloxanes in the presence of acetic acid usually gave a mixture of
linear and cyclic products.24 Such a phenomenon was also
observed in our case, whereas the linear products were
dominant. As can be seen from a curve of GPC of Si-TFVE
(see Supporting Information, Figure S1), the ratio between Si-
TFVE and the cyclic products was estimated as about 93:7. The
molecular weight of Si-TFVE was also confirmed by matrix-
assisted laser desorption/ionization time-of-flight mass spec-
trometry (MALDI-TOF-MS), as shown in Supporting
Information (Figure S2).
According to the DSC traces, we tried to prepare the cured
Si-TFVE (called as Si-PFCB) film for the measurement of
dielectric and mechanical properties. It was noted that most of
the thermosetting resins such as epoxy and phenolics resins
tended to shrink during curing, which were unbeneficial to
prepare films of high quality. In our case, irregular shrinking
was also observed when the curing ran for long time at 160 and
250 °C, respectively (near its onset and peak exothermic
temperatures in DSC, see Figure 1). Through many trials, we
finally found that the curing at a much higher temperature (350
°C) within a short period (60 s) can efficiently restrain the
shrinking and extremely smooth film was thus obtained.
A comparison of Si-TFVE with Si-PFCB in FT-IR spectra
was depicted in Figure. 2. After curing, the −OCFCF2
stretching vibration peak at 1739 cm−1 disappeared in Si-
TFVE, and a new perfluorocyclobutyl peak appeared at 962
cm−1 in Si-PFCB.26b This meant that Si-TFVE had completely
converted to Si-PFCB.
Si-TFVE was easily soluble in common solvents such as
chloroform, toluene, and tetrahydrofuran. It was interesting that
the film on the wafer was in a sticky liquid rather than a solid
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Figure 4. A field emission scanning electron microscopy image of a Si-
PFCB film.
Figure 2. FT-IR spectra of Si-TFVE (up) and Si-PFCB (down).
In order to well understand the formation of Si-PFCB, a
schematic description was made. As shown in Figure 3, the
thermoplastic Si-TFVE turned into thermoset Si-PFCB upon
heating without any additives.
The dielectric constants of Si-PFCB films were measured by
the capacitance method,9 and the detailed procedures are
described in the Experimental Section. The thickness of the
film was determined by field emission scanning electron
microscopy (FE-SEM). Figure 4 shows an FE-SEM micrograph
of the section plane of a Si-PFCB film on a silicon wafer.
Obviously, the Si-PFCB film possesses good surface uniformity.
The dielectric constant (k) and dielectric loss (tan δ) of the
film are depicted Figure 5. As expected, the average k value of
Si-PFCB film keeps below 2.45 with a dielectric loss below 2.1
× 10−3 in a range of frequencies varying from 1 to 30 MHz.
Moreover, the dielectric value is only 2.33 at 30 MHz, which is
much lower than those of most widely used low-k materials,7−18
such as polyimides, polycyanate esters, and SiLK resins.
This is mainly due to the existence of fluorous groups
showing large free volume and low capacity to be polarized.2
On the other hand, X-ray diffraction (XRD) patterns (Figure 6)
demonstrate that the Si-PFCB film is essentially amorphous
and molecular stacking is avoided. The peak at 2θ = 13.8° (d =
0.64 nm) is assigned to the interlayer distance of Si-PFCB.
Such large interlayer distance implies that Si-PFCB emerges as
an untight stacking structure in the solid state, resulting in low
density. On account of the above-mentioned characteristics, Si-
PFCB thereby exhibits low-k value.
Figure 5. Dielectric constant and dielectric loss of a Si-PFCB film at
different frequencies at room temperature.
Figure 6. XRD patterns of Si-PFCB film.
range of temperatures from 50 to 350 °C (Figure 7, see
attached chart), demonstrating the good thermostability of Si-
PFCB. Such high thermostability was also supported by TGA
results (Figure 7) in which Si-PFCB showed the 5 wt % loss
temperature of 472 °C. From the view of application, the heat
The thermostability of Si-PFCB was evaluated by differential
scanning calorimetry (DSC) and thermal gravimetric analysis
(TGA). DSC trace indicated no evidence of glass transition in a
Figure 3. A schematic description for the formation of Si-PFCB via heating Si-TFVE.
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Figure 7. TGA curve of Si-PFCB under N2 with a heating rate of 10
°C/min. The attached chart is DSC trace of Si-PFCB.
resistance of Si-PFCB could meet the requirements for the
fabrication of Cu metallization during the damascene
processes.2,4
The hydrophobicity of Si-PFCB films was evaluated by water
contact angle test. On the surface of the film, water contact
o
angle was measured as 105 (Figure 8), exhibiting good
Figure 9. Results from nanoindentation (a) and nanoscratch (b) tests
for a Si-PFCB film.
hydrophobicity of the film. Thus, moisture absorbance, which
results in deterioration of the dielectric properties, could be
effectively prevented.
Figure 8. Contact angle of water on the surface of a Si-PFCB film.
The mechanical properties of Si-PFCB film were determined
from a nanoindentation/scratch system. The results showed
that the film had an average hardness of 0.392 GPa and a
Young’s modulus of 10.06 GPa (Figure 9). The bonding
strength between the film and a silicon wafer was measured as
4.93 GPa, which also satisfied the BEOL procedures.2,4
Film uniformity is crucial to the production of a low-k
material as well. As can be seen from Figure 4, the cross-section
picture from the FE-SEM manifests the Si-PFCB film spin-
coated on a silicon wafer has a smooth surface. Furthermore,
this is verified by atomic force microscopy (AFM) with both a
o
planar graph and a stereogram from an angle of 45 (Figure
10). Over a 1 μm square area, the surface roughness was less
than 5.79 nm, which satisfied the demand of thickness lower
than 10% within the wafer.10
Figure 10. AFM images of a Si-PFCB film (thickness of 170.2 nm) on
a silicon wafer. (a) Planar graph and (b) stereogram (45°).
CONCLUSIONS
■
In summary, based on the high heat resistance of organo-
siloxanes and the good dielectric properties of fluoropolymers,
we successfully prepared a novel oligomer with a siloxane
backbone and thermocurable trifluorovinylether (−OCF
CF2) side chains. Upon postpolymerization, the oligomer
converted to a cross-linked network (Si-PFCB), which showed
high thermostability and good dielectric properties with an
average dielectric constant below 2.45 and a dielectric loss less
than 2.1 × 10−3 in a range of frequencies varying from 1 to 30
MHz. In addition, the Si-PFCB film exhibited excellent surface
smoothness and good hydrophobicity. The mechanical proper-
ties of Si-PFCB were also satisfactory with Young’s modulus,
hardness, and bonding strength with silicon wafer surface of
10.06 GPa, 0.392, and 4.93 GPa, respectively. These data
indicated that the oligomer was suitable for utilization in
ultralarge scale integration circuits. Moreover, this article also
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ASSOCIATED CONTENT
■
S
* Supporting Information
The GPC curve and MALDI-TOF-MS Spectrometry of the
polymer, NMR Spectra of precursor 3, and the polymer. This
material is available free of charge via the Internet at http://
pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: qiangfang@mail.sioc.ac.cn. Fax: +86 21 5492 5337.
Tel: +86 21 5492 5337.
Author Contributions
†C.Y. and J.W. contributed equally.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Financial supports from Ministry of Science and Technology of
China (2011ZX02703 and 2015CB931900) and the Natural
Science Foundation of China (NSFC, No. 21374131) are
gratefully acknowledged. Q.F. thanks Professor Ping Yao in
Fudan University for her kind help in the measurement of
MALDI-TOF-MS Spectrometry.
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