Incompletely condensed POSS-based spin-on-glass networks for impeccable ultra low-k integration

Article abstract of DOI:10.1039/c5tc02683k

Poly(methyl)silsesquioxane-based spin-on-glass resins incorporating both a cyclic precursor, 1,3,5,7-tetramethyl 1,3,5,7-tetrahydroxyl cyclosiloxane, and incompletely condensed methyl-substituted POSS were synthesized and their thermal, mechanical, and el

Full text of DOI:10.1039/c5tc02683k

Journal of
Materials Chemistry C
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PAPER
Incompletely condensed POSS-based
spin-on-glass networks for impeccable ultra
low-k integration†
Cite this: J. Mater. Chem. C, 2015,
3, 11605
Albert S. Lee,a Seung-Sock Choi,a Sung Yeoun Oh,a He Seung Lee,a Bomin Kim,a
Seung Sang Hwangab and Kyung-Youl Baek*ab
Poly(methyl)silsesquioxane-based spin-on-glass resins incorporating both a cyclic precursor, 1,3,5,7-
tetramethyl 1,3,5,7-tetrahydroxyl cyclosiloxane, and incompletely condensed methyl-substituted POSS
were synthesized and their thermal, mechanical, and electrical properties were investigated as a function
of the POSS loading content. By introducing incompletely condensed methyl-substituted POSS
compounds at the molecular level as sol–gel precursors, exceptional thermal stability (4700 1C), good
mechanical properties (elastic modulus 44.0 GPa), and an ultra-low dielectric constant (k = 1.8) were
obtained. In addition to providing a new route towards fully poly(methyl)silsesquioxane-based spin-on-glass
resins with the above properties, the realization of the application of integration circuits was evaluated to
show that the ultra low-k, mechanically robust spin-on-glass materials were able to withstand harsh wet
chemical and dry etching, as well as chemical mechanical planarization (CMP) processing.
Received 28th August 2015,
Accepted 12th October 2015
DOI: 10.1039/c5tc02683k
www.rsc.org/MaterialsC
traits with regard to spin-on-glass low-dielectric materials.
However, many of these favorable traits rely greatly on the organic
Introduction
Over the past several decades, the search for low-dielectric functional group –R, with the methyl –CH3 group substituted
constant materials for semiconductor chip fabrication has been poly(methyl)silsesquioxanes (PMSSQs) exhibiting the best balance
relentless to fulfill the need for smaller feature nodes, higher between thermal stability (4450 1C), low-dielectric constant
numbers of transistors per unit area, and thus faster processors, (k B 2.7), and mechanical properties (E 4 3.0 GPa).7
according to Moore’s Law.1,2 However, low-dielectric constant
There have been many methods to lower the dielectric
materials require not only low-dielectric constants, but exception- constant of spin-on-glass polysilsesquioxane-based materials
ally good mechanical properties, chemical resistance, and excel- for interlayer applications. Numerous studies have investigated
lent processability, which invariably limits the range of materials the effect of the structure of the internal Si–O–Si bonds on the
available for actual integration into semiconductor chips.3,4 Of the dielectric constant and mechanical properties of poly(methyl)-
available materials, spin-on-glass materials have received the most silsesquioxanes7,8 with varying degrees of success. However, the
attention due to their thermal stability, mechanical properties, dielectric constant of PMSSQ-based spin-on-glass materials
and solution processability.
(k B 2.7) is still too high for next generation low-dielectric
Polysilsesquioxanes, with chemical formula [RSiO1.5]n, are constant materials, and the search for integration-level feasible
inorganic–organic hybrid materials obtained through hydrolysis– ultra low-k materials (k o 2.0) is ongoing today. And while the
polycondensation, or sol–gel processing of tri-functional silane use of sacrificial porogens has been extensively investigated to
monomers.5,6 Through networking of the Si–O–Si bonds, proper- utilize the low permittivity of free space (e = 1) formed through
ties such as thermal stability, mechanical robustness, chemical pyrolysis,9 the subsequent degree of depreciation of mechan-
stability, and dielectric properties have been endorsed as favorable ical properties, as well as the fabrication of smooth interfaces
with metal gates, metal leaching, and residual ash have been
reported as areas of critical concern.10
a Materials Architecturing Research Center, Korea Institute of Science and
Technology, Hwarangno 14-gil 5, Seong-Buk Gu, Seoul 136-791, Republic of Korea.
E-mail: baek@kist.re.kr
b Nanomaterials Science and Engineering, University of Science and Technology,
217 Gajungro, 176 Gajung-dong, Yuseong-Gu, Daejeon 305-333, Republic of Korea
† Electronic supplementary information (ESI) available: FT-IR and 29Si NMR
characterization of T8-methyl POSS and incompletely condensed methyl-
substituted POSS (IC-methyl-POSS). See DOI: 10.1039/c5tc02683k
Within the polysilsesquioxane family, polyhedral oligomeric
silsesquioxanes, or more commonly known as POSS, have been
widely studied due to their 3-dimensional Si–O–Si inorganic
polyhedra with organic functional groups radially stemming
outwards from the vertices.11,12 The well-defined inorganic
inner core provides the smallest particle of silica known so far,
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with sizes ranging from 1 nm to 3 nm depending on the poly- 2950 under N2. Nanoindentation measurements were conducted
hedron size, with the most common POSS being the octahedron, on a Hysitron Inc. TriboIndenter equipped with a Berkovich
T8-POSS.11 While numerous studies have detailed the solution diamond tip. Measurements of modulus were performed as
processability of POSS-based materials13 and their potential use a continuous stiffness measurement on samples coated on
as low-dielectric constant porous materials, methyl-substituted silicon wafers at a thickness of 500 nm. Dielectric constants
POSS compounds are notorious for their low solubility in organic were measured at 1 kHz using the metal–insulator–metal (MIM)
solvents,14 thus limiting their application as low-dielectric constant method with thermally evaporated aluminum as metal with PSSQ
spin-on-glass interlayer materials in their fully condensed resins spin coated with 25 wt% propylene glycol monomethyl
cubic form.
ether acetate (PGMEA) slurries with a thickness of B500 nm.
In our previous study, we examined the effect of a methyl- The reported dielectric constant values were the average of
substituted cyclic siloxane precursor, MT4-OH, as a comonomer 10 measurements. All integration tests were conducted at the
for the sol–gel synthesis of PMSSQs, which revealed that the MT4- KAIST Nanofab Center.
OH groups substantially improved the mechanical properties.15
However, the obtained dielectric constants for these materials
(k B 2.7) were representative of conventional PMSSQs.
Synthesis of 1,3,5,7-tetramethyl 1,3,5,7-tetrahydroxyl
cyclosiloxane (MT4-OH)
In this study, we hydrolyzed a methyl-substituted POSS
to obtain incompletely condensed methyl-substituted POSS
materials, and utilized them as sol–gel precursors along with
MT4-OH. Through sol–gel processing, we introduced the POSS
moieties at the molecular level, which we hypothesized would
increase the free-volume within the spin-on-glass PMSSQ
matrix network to effectively lower the dielectric constant, while
the MT4-OH monomers would function to maintain exceptional
mechanical robustness. Moreover, our multifunctional spin-
on-glass PMSSQ resins were selected for integration reliability
tests, as the above concerns over metal leaching and residual
ash derived from porogens can be ignored, as both MT4-OH and
incompletely condensed methyl-substituted POSS are hybrid
siloxane-based materials.
Hydrolysis of the hydride, 1,3,5,7-tetramethyl-2,4,6,8-tetrahydrido
cyclosiloxane (MD4-H) (19 ml, 0.089 mol) was carried out using
water (6.4 ml, 0.36 mol) and 10% activated Pd/C (0.8 g) for 2 h in
THF (220 ml) at 10 1C, according to Scheme 1. Excess water and
Pd/C were removed by dispersing the reaction solution in MgSO4
and Cellite, followed by filtering and slowly evaporating excess
THF at temperatures below 20 1C. The obtained MT4-OH was
stored as a powder at À15 1C prior to use.
Synthesis of T8-methyl POSS
The synthesis of T8-methyl POSS followed a literature procedure.16
To an ice-cooled mixture solution of H2O (90 ml), acetone (40 ml),
acetonitrile (10 ml), and triethylamine (10 ml, 72 mmol), formic
acid (0.5 ml) and concentrated HCl (0.5 ml) were added. After
removing the ice-bath and letting the solution temperature
increase to room temperature, MTES (178.3 g, 1 mol) was added
dropwise. Then the solution was equipped with a condenser
and refluxed for 2 days. Afterwards, the solution was cooled to
room temperature and the excess liquid was decanted, and the
Experimental
Materials
Methyltriethoxysilane (MTES) (Shin Etsu, 99%), 1,3,5,7-tetra-
methyl-2,4,6,8-tetrahydrido cyclosiloxane (MD4-H) (Gelest, 98%),
and tetrahydrofuran (J. T. Baker, HPLC grade) were vacuum
distilled prior to use. Pd/C 10% (Acros, 98%) was used as
received. Deionized water was used for hydrolysis of MD4-H
and sol–gel reaction with MT4-OH and MTES. All other solvents
and reagents were used as received.
Characterization
Weight averaged molecular weight (Mw) and molecular weight
distributions (Mw/Mn) of the polymers were measured using
a JASCO PU-2080 plus SEC system equipped with a refractive
index detector (RI-2031 plus), a UV detector (l = 254 nm,
UV-2075 plus), and a Viscotek SLS apparatus using THF as
the mobile phase at 40 1C with a flow rate of 1 mL minÀ1. The
samples were separated through four columns (Shodex-GPC
KF-802, KF-803, KF-804, and KF-805). 1H-NMR and 29Si NMR
spectra were recorded in CDCl3 at 25 1C on a Varian Unity
INOVA (1H: 300 MHz, 29Si: 59.6 MHz). FT-IR spectra were
recorded using a Perkin-Elmer FT-IR system (Spectrum-GX)
using solvent cast films on KBr pellets. Thermogravimetric
analysis (TGA) was performed using a TA Instrument TGA
Scheme 1 Synthesis of (a) MT4-OH through hydrolysis of MD4-H,
(b) incompletely condensed methyl-substituted POSS (IC-methyl-POSS),
and (c) MT4-MPOSS spin-on-glass resins.
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powders were precipitated in copious amounts of water. Filtering the hydride, 1,3,5,7-tetramethyl-2,4,6,8-tetrahydrido cyclosiloxane
and drying under vacuum at 40 1C yielded T8-methyl POSS with (MD4-H) under aqueous conditions as shown in Scheme 1a.18,19
good yield (59.1 g, yield 88%).
This cyclic monomer functioned to give not only low-dielectric
constant resins due to the high Si–C content, but also functioned
to provide exceptional mechanical robustness, due to the highly
condensed cyclic structure. In addition to the MT4-OH monomer,
incompletely condensed methyl-substituted POSS (IC-methyl-
POSS) was synthesized as shown in Scheme 1b. Through hydro-
lysis of the siloxane Si–O–Si bonds, a mixture of di-silanol POSS
and tri-silanol POSS compounds was obtained.17 FT-IR and 29Si
NMR spectra for both the T8-methyl POSS and IC-methyl-POSS
compounds are given in ESI,† Fig. S1 and S2. In comparison to the
fully condensed T8-methyl POSS, the mixture of IC-methyl-
POSS compounds exhibited vastly improved solubility in organic
solvents for use as sol–gel monomers.17 In addition to the
improved solubility, the uncondensed silanol groups functioned
as a sol–gel condensation point for the introduction of POSS
moieties at the molecular level. It is worth noting that while
some incompletely condensed POSS compounds are commer-
cially available, the methyl-substituted compound was not and
isolation of the di-silanol POSS and trisilanol POSS has been
previously reported as immensely tedious due to the inability to
form single crystals.17 Thus, we directly utilized these mixture
compounds without isolation of the structural isomers as sol–gel
monomers for the synthesis of MT4-MPOSS spin-on-glass resins.
Sol–gel reactions carried out using MT4-OH and IC-methyl-
POSS involved a fixed ratio between MTES and MT4-OH at 7 : 3
to ensure impeccable solubility of the PMSSQ matrix, and
IC-methyl-POSS was added at 3, 5, and 10 mol%, with the
resulting PMSSQ matrix named MT4-MPOSS3, MT4-MPOSS5,
and MT4-MPOSS10 according to the various IC-methyl-POSS
loading contents. The sol–gel reaction conditions followed a
modified previous method,8 and the obtained MT4-MPOSS
spin-on-glass resins were obtained in excellent yields (490%).
The obtained MT4-MPOSS spin-on-glass resins were first
Synthesis of incompletely condensed methyl-substituted POSS
(IC-Methyl-POSS)
The synthesis of incompletely condensed methyl-substituted
POSS (IC-methyl-POSS) followed
a
literature procedure.17
A 250 ml round bottom flask was charged with T8-methyl POSS
(4.94 g, 9.2 mmol) dispersed in THF (100 ml). Then, a 35%
aqueous solution of tetramethylammonium hydroxide (4 ml,
9.2 mmol) was added quickly. After equipping the round bottom
flask with a condenser, the reaction mixture was stirred vigorously
under reflux for 2 h. After cooling to room temperature, the
solution was neutralized with dilute HCl. THF was evaporated
under reduced pressure, and the resinous material dissolved in
diethylether and extracted with water several times, followed by
drying the organic layers with anhydrous magnesium sulfate.
After filtering, diethylether was evaporated under reduced pressure
to yield a mixture of incompletely condensed methyl POSS com-
pounds (2.1 g, yield 35%).
Synthesis of IC-methyl-POSS containing PMSSQs (MT4-MPOSS)
In a 3-necked jacketed flask set at 25 1C, a reaction solution
of H2O (2.4 g, 0.13 mol), THF (20 ml), and concentrated HCl
(1.23 g) was first prepared. To this reaction mixture, a monomer
mixture of MTES (5.0 g, 0.28 mol), MT4-OH (3.65 g, 12 mmol),
and various mol% of IC-methyl-POSS was added dropwise over
10 minutes. The MTES : MT4-OH mol ratios were fixed at 7 : 3
and the IC-methyl-POSS additive percentages were varied as:
3 mol%, 5 mol%, and 10 mol%. After addition of the silane
monomers, the reaction flask was sealed and the temperature
was increased to 60 1C, and then the reaction mixture was magne-
tically stirred for 18 h. After completion of the hydrolysis–
polycondensation reactions, the reaction mixture was dissolved
in dichloromethane and extracted with water several times to
remove HCl. By drying over MgSO4, filtering, and evaporating
dichloromethane at temperatures below 20 1C, MT4-MPOSS
resins were obtained as white powders, which were stored at
À15 1C. PMSSQ resins were named: MT4-MPOSS3 for resins
synthesized with 3 mol% of IC-methyl-POSS.
1
characterized by H NMR. As shown in Fig. 1, the MT4-MPOSS
spin-on-glass resins were completely hydrolyzed as indicated by
the lack of ethoxy protons attributed to MTES at 3.8 ppm.
Impeccable hydrolysis of alkoxy groups is a critical factor for
spin-on-glass polysilsesquioxane resins, as unhydrolyzed alkoxy
Processing of MT4-MPOSS spin-on-glass resins
MT4-MPOSS dissolved in propylene glycol methyl ether acetate
(PGMEA) at 25 wt% was syringe-filtered prior to spin coating at
1500 rpm for 60 s on ozone treated Si wafers. The spin-coated
wafers were then thermally cured at 400 1C for 2 h under
nitrogen to fully cure the MT4-MPOSS spin-on-glass resins.
Results and discussion
Synthesis and characterization of MT4-MPOSS spin-on-glass
resins
The synthesis and characterization of MT4-OH were carried out
using our previous method through quantitative hydrolysis of
Fig. 1 (a) 1H NMR spectra of MT4-MPOSS spin-on-glass resins in DMSO-d6.
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groups contribute to a higher degree of film shrinkage due to
secondary condensations between Si–OH/Si–alkoxy groups over
Si–OH/Si–OH group pairs.20 Moreover, the Si-methyl peaks
centered at 0 ppm were very broad, indicative of the polymeric
nature of the PMSSQ matrix with high Si-methyl content. Also,
some uncondensed Si–OH groups were found at 6.8 ppm;
however, the relative amounts of the silanol groups were difficult
to quantify from 1H NMR, due to substantial hydrogen-bonding.
29Si NMR spectra of the MT4-MPOSS spin-on-glass resins
were obtained to investigate the siloxane structure. As shown in
Fig. 2, all of the MT4-MPOSS spin-on-glass resins exhibited two
general peaks centered at À68 ppm and À58 ppm, attributed
to the alkyl-Si(OSi–)3 T3 and alkyl-Si(OSi–)2OH T2 peaks, respec-
tively.21 Interestingly, within the broad T3 and T2 peaks, sharp
protruding resonance peaks were observed. These sharp peaks
were attributed to the introduction of the IC-methyl-POSS
compounds at the molecular level, and their characteristic
sharp peaks were retained even after sol–gel reactions with
MT4-OH and MTES. Moreover, the degrees of condensation
(DoC), as described by Bae and others,21–23 were shown. In the
MT4-MPOSS spin-on-glass resins obtained in this study, we
found that the degrees of condensation (DoC) values ranged
between 73 and 87%. These high degrees of condensation
(DoCs) were attributed to both the MT4-OH and IC-methyl-
POSS materials, as these monomers are already very highly
condensed, with MT4-OH consisting of wholly T2 structures and
IC-methyl-POSS having a minimum of 75% T3 even when
considering that all IC-methyl-POSS were of the tri-silanol type.
Thus through 29Si NMR analyses, we were able to elucidate the
high DoC values, and provide spectroscopic evidence of the
introduction of POSS moieties at the molecular level.
Fig. 3 FTIR spectra of (a) MT4-MPOSS5 before and after curing, and (b) all
of the MT4-MPOSS spin-on-glass resins after curing.
amorphous PMSSQ network matrix.5 After curing, we observed
that all of the uncondensed Si–OH groups disappeared, as
well as a change in the characteristic Si–O–Si peaks at around
1000–1200 cmÀ1, indicating the efficacy of the curing process.
Moreover, as shown in Fig. 3b, as the IC-methyl-POSS loading
levels increased from 3 mol% to 10 mol%, we observed that all the
Si–OH groups were completely cured after thermal treatment.
FT-IR analysis of MT4-MPOSS spin-on-glass resins was carried
out to investigate both the curing conditions as well as the
siloxane structure. As shown in Fig. 3a, MT4-MPOSS5 exhibited
Si–O–Si characteristic a, b peaks at 1150 cmÀ1 and 1050 cmÀ1
,
Thermal, electrical, and mechanical properties
in addition to the uncondensed Si–OH groups c at around As thermal stability is one of the most critical prerequisites for
3500 cmÀ1. While characteristic Si–O–Si peaks for T8-methyl integrated low dielectric constant materials, we examined the
POSS at 1100 cmÀ1 were not found,11,13 this was probably thermal properties of the obtained MT4-MPOSS spin-on-glass
attributed to the networking of the Si–O–Si bonds, and the resins using thermogravimetric analysis (TGA). As shown in
relatively low loading levels of IC-methyl POSS compared to the Fig. 4, all of the MT4-MPOSS spin-on-glass resins exhibited
superior thermal stability. According to the increase in the
IC-methyl-POSS loading content, the initial 15–20 wt% thermal
degradation at 150 1C of the MT4-MPOSS spin-on-glass resins
increased, as indicated by the lower residual weight percentage.
This trend was consistent with the degree of condensation
(DoC) values calculated using 29Si NMR in Fig. 2, revealing that
the initial degradation at 150 1C can be attributed to the
secondary condensations between silanol groups. Moreover,
after 300 1C, negligible weight loss was observed, as the weight
percent remained relatively constant until 700 1C. These results
are representative of the exceptional thermal stability of the
Si-methyl groups, and indicate that no other monomeric or
oligomeric groups are lost during the thermal treatments
required for chip fabrication.8,20
Fig. 2 29Si NMR spectra of the MT4-MPOSS spin-on-glass resins in
DMSO-d6.
The electrical properties of the MT4-MPOSS spin-on-glass
resins were evaluated as well. In Fig. 5, the dielectric constant
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sacrificial porogens,9,10 the subsequent concern over residual ash,
and depreciation in mechanical properties were all but eliminated
for our ultra low-k IC-methyl-POSS incorporated MT4-MPOSS
spin-on-glass resins, which we stipulated may in turn allow for
impeccable integration with various semiconductor processing.
In order to evaluate our above hypothesis, the mechanical
properties of the MT4-MPOSS spin-on-glass resins were evalu-
ated through a nanoindentation method described elsewhere.24
Fig. 6 illustrates the elastic modulus and hardness values of the
various MT4-MPOSS spin-on-glass resins before and after curing.
As shown, the mechanical properties improved substantially after
curing, and both elastic moduli and hardness values were
relatively similar regardless of the IC-methyl-POSS content,
with the absolute values on par with other reports of cross-
linked spin-on-glass resins.25 As the moduli values for all of
the MT4-MPOSS spin-on-glass materials were around 4 GPa, the
mechanical properties are also summarized in Table 1, where
the brittleness indices25,26 are also shown. Before curing, the
brittleness indices of the MT4-MPOSS spin-on-glass resins
remained relatively similar B0.12, even as the IC-methyl-
POSS content increased from 3 mol% to 10 mol%. However,
after curing, the brittleness indices increased almost three-fold
to 0.3 for MT4-MPOSS3, and a pronounced decrease in the
brittleness index was observed as the IC-methyl-POSS content
increased to 10 mol% to 0.19. This is more evidence to suggest
that the IC-methyl-POSS provides internal free-volume, as the
brittleness index is highly dependent on the hardness value,
which in turn is governed by the chain–chain interactions,26
even as the absolute elastic moduli and hardness values are
relatively similar. Thus, the incorporation of IC-methyl-POSS
Fig. 4 TGA thermograms of MT4-MPOSS spin-on-glass resins.
Fig. 5 Dielectric constant and breakdown voltage values for MT4-MPOSS
spin-on-glass resins after curing.
and breakdown voltage values for the MT4-MPOSS spin-on-glass
resins are shown. While the breakdown voltages remained rela-
tively constant around 4.5 MV cmÀ1, regardless of the IC-methyl-
POSS loading content, the dielectric constant values decreased
substantially as the IC-methyl-POSS loading content increased.
This phenomenon was attributed to the fact that the increase in
IC-methyl-POSS does not change the Si–C : Si–O ratio which
invariably affects the BV of the matrix, but may introduce internal
free-volume into the PMSSQ matrix through the molecular-level
introduction of IC-methyl-POSS, with an octahedron-core size of
approximately 1–3 nm.11 This size effect, along with the fact that
the POSS octahedral core contributes to steric bulkiness through
repulsion,12 may contribute to the ultra low-k properties of the
obtained MT4-MPOSS spin-on-glass resins. When considering that
ultra low-k materials are commonly obtained through pyrolysis of
Fig. 6 Elastic moduli and hardness values of various MT4-MPOSS spin-
on-glass resins before and after curing.
Table 1 Summary of mechanical properties for MT4-MSSQ spin-on-glass resins
Elastic modulus, E, GPa
(before curing/after curing)
Hardness, H, GPa
(before curing/after curing)
Brittleness index, H/E
(before curing/after curing)
Sample
MT4-MPOSS3
MT4-MPOSS5
MT4-MPOSS10
2.5/4.0
2.3/3.8
2.1/3.5
0.3/1.2
0.27/0.8
0.22/0.65
0.12/0.3
0.12/0.21
0.10/0.19
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only functioned favourably to greatly decrease the dielectric coatability was confirmed through the micron-scaled 3D-confocal
constant to ultra low-k levels, while the thermal properties, image with exceptionally low surface roughness (Ra = 1.1 nm).
breakdown voltage, and mechanical properties were shown to This excellent processability was also confirmed through spin-
be exceptional, as the inherent properties of polysilsesquioxane- coating MT4-MPOSS5 on patterned substrates, followed by metal
based hybrid spin-on-glass materials were well-preserved.
gate deposition (Fig. 7c and d). As shown, the large-scale coating
layer was impeccable, and the metal–insulator interface showed
no ill effects of delamination, cracks, or metal leaching into the
insulator coating layer.
Integration tests with MT4-MPOSS
To evaluate the integration feasibility of the IC-methyl-POSS
containing MT4-MPOSS spin-on-glass resins, we chose MT4-
MPOSS5 as our sample for integration, as it provided the best
balance between an ultra-low dielectric constant (2.1) and
mechanical properties (E = 3.8 GPa). Fig. 7a showcases the
excellent processability of the MT4-MPOSS5 resin, as a smooth
and clean interface between the coating layer and the Si-wafer
substrate was obtained. Moreover, the large-scale dimension
Moreover, in order to confirm the chemical stability and
mechanical robustness to withstand chemical mechanical
planarization (CMP) processes, we etched the MT4-MPOSS5
coating layer using both wet-chemical etching and dry etching.
Fig. 8a shows the MT4-MPOSS5 coating layer after a buffered
oxide etch (BOE) in a 5 wt% HF aqueous solution for 20 min.
As shown, the chemical etching process did not severely
dissolve the coating layer and only a 15 nm loss in coat
thickness was observed after the CMP process (Fig. 8c). More-
over, dry etching under conditions of C4F8 (10 sccm), Ar
(180 sccm), CH2F2 (12 sccm), and O2 (14 sccm) at 60 mtorr
revealed that the coating layer was also intact, and only a 59 nm
decrease in coating thickness was observed. Under these harsh
conditions, the MT4-MPOSS5 resin was able to maintain its
structure and withstand the planarization process of CMP very
well. Also, after etching, gap-fillable patterns were fabricated
with large dimensions (tens of microns). These patterns show
the impeccable reliability of MT4-MPOSS5, having both the
chemical and mechanical robustness to withstand basic semi-
conductor processing.
Fig. 7 (a) SEM image of MT4-POSS5 coated on Si-wafer, (b) 3D confocal
microscope image of MT4-MPOSS coated on Si-wafer, (c) MT4-MPOSS5
coated on a patterned substrate with a Pt-line, and (d) cross-sectional SEM
image of MT4-MPOSS5 coated on a patterned substrate with a Pt-line.
Fig. 8 SEM image of (a) MT4-MPOSS5 after wet-chemical etching,
(b) MT4-MPOSS5 after dry etching, (c) MT4-MPOSS5 after CMP, and
(d) MT4-MPOSS5 etched gap-fillable patterns.
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Conclusion
In conclusion, a series of fully poly(methyl)silsesquioxane (PMSSQ)-
based spin-on-glass resins were synthesized using both a cyclic
siloxane precursor and an incompletely condensed methyl-
substituted POSS compound as sol–gel precursors. The cyclic
precursor provided mechanical robustness, and the incorpora-
tion of incompletely condensed methyl-substituted POSS
compounds imbued ultra low-k properties. Through the incor-
poration of incompletely condensed methyl-substituted POSS
compounds at the molecular level through sol–gel hydrolysis–
polycondensation reaction of the silanol groups, the free-
volume formed through the octahedral siloxane core of POSS
lowered the dielectric constant of the obtained spin-on-glass
resins all the way down to ultra low-k levels (k B 1.8), while
maintaining mechanical robustness (E 4 4.0 GPa) for practical
applications in integrated circuits. The ultra low-k and mechanical
properties revealed that these materials were able to withstand
severe etching conditions and CMP processing, and may ulti-
mately prove useful for next generation ultra low-k integrated
circuits.
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This work was financially supported by a grant from the
Fundamental R&D Program for Core Technology of Materials
funded by the Ministry of Knowledge, Economy, Republic of
Korea and partially by the International Collaborative R&D
Program (N0000678) funded by the Ministry of Trade, Industry,
and Energy, (MOTIE, Korea).
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