A method to derivatize surface silanol groups to Si-alkyl groups in carbon-doped silicon oxides

Article abstract of DOI:10.1039/c6ra20355h

A carbon-doped silicon oxide (CDO) finds use as a material with a low dielectric constant (k) for copper interconnects in multilayered integrated circuits (ICs). Hydrophilic silanol groups (Si-OH) on its surface, however, can attract moisture, thereby causing an undesirable increase in the dielectric constant. Modification of the exposed hydrophilic surface to a hydrophobic functional group provides one solution to this problem. We report here a strategy for converting surface Si-OH to hydrophobic silicon hydride (Si-H) without affecting the internal oxide network in CDO. The approach involves esterification of the exposed silanol to its triflate (silyltrifluoromethanesulfonate, Si-O-Tf), followed by reduction to Si-H with diisobutylaluminum hydride. Si-H is further modified by a photochemical reaction with an alkene (1-octadecene) to yield Si-R (R = -C₁₈H₃₇) to provide a more moisture resistant, and less polar Si-C as opposed to the Si-O backbone. Fourier transform infrared (FT-IR) spectroscopy, X-ray photoelectron spectrometry (XPS), and measurements of the contact angle with water substantiated the successful conversion. The reaction scheme is versatile, transforming silanol groups to silicon hydride in widely varying chemical sites, from small molecules to the surfaces of silica gels, SiO_x and CDO wafers. A comparison with the films (self-assembled monolayers) derivatized with the octadecyltrichlorosilane indicated that the new method leads to a thicker (≈5 nm) but more loosely packed hydrocarbon film with slightly lower contact angles.

Full text of DOI:10.1039/c6ra20355h

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A method to derivatize surface silanol groups to Si-alkyl groups in
carbon-doped silicon oxides
Received 00th January 20xx,
Accepted 00th January 20xx
Srikar Rao Darmakkollaa, Hoang Trana, Atul Guptaa, James Blackwellb, Shankar B. Rananavarea*
DOI: 10.1039/x0xx00000x
A Carbon-doped silicon oxide (CDO) finds use as a material with a low dielectric constant (k) for copper interconnects in
multilayered integrated circuits (ICs). Hydrophilic silanol groups (Si-OH) on its surface, however, can attract moisture,
thereby causing an undesirable increase in the dielectric constant. Modification of the exposed hydrophilic surface to a
hydrophobic functional group provides one solution to this problem. We report here a strategy for converting surface Si-
OH to the hydrophobic silicon hydride (Si-H) without affecting the internal oxide network in CDO. The approach involves
esterification of the exposed silanol to its triflate (silyltrifluoromethanesulfonate, Si-O-Tf), followed by reduction to Si-H
with diisobutylaluminum hydride. Si-H is further modified by a photochemical reaction with an alkene (1-octadecene) to
yield Si-R (R=-C18H37) to provide a more moisture resistant, and less polar Si-C as opposed to Si-O backbone. Fourier
transform infrared (FT-IR) spectroscopy, X-ray photoelectron spectrometry (XPS), and measurements of the contact angle
with water substantiated the successful conversion. The reaction scheme is versatile, transforming silanol groups to silicon
hydride in widely varying chemical sites, from small molecules to the surfaces of silica gels, SiOx and CDO wafers. A
comparison with the films (self-assembled monolayers) derivatized with the octadecyltrichlorosilane indicated that the
new method leads to thicker (≈ 5nm) but more loosely packed hydrocarbon film with slightly lower contact angles.
www.rsc.org/
become
interconnected.
Similarly,
thermomechanical
1. INTRODUCTION
properties such as thermal stress and its temperature-
dependent slope decrease continuously with increasing
porosity.
Advances in ultra-large scale integrated devices require
replacement of traditional silicon oxide (SiOx), which has a
dielectric constant (k) that is relatively high (k ≈ 4), with low-k
Fabrication of copper interconnects in modern ICs employs
a well-known dual Damascene process to pattern CDO using
materials for high-frequency operation (f ̴ 1/k). Contemporary
optical lithography and copper deposition by electroplating11
,
low k materials provide an insulating sheathe, the so called
inner layer dielectrics (ILD)s, between the copper metal
interconnect lines in modern multilayered integrated circuits
(ICs)1, 2. Carbon-doped silicon oxide (CDO) is an important low
k (≈ 2.5) material, crafted from organosilanes by plasma-
enhanced chemical vapor deposition (PECVD)3-8. The resulting
nanoporous CDO is composed of Si, C, O and H (SiCOH) with
extensive cross-linking of Si-Si, Si-CH2-Si, Si-O-Si, Si-CH2-O-Si
groups, creating a delicate porous network9. The porosity/void
fraction directly affects the dielectric constant and
thermomechanical properties of the insulating CDO films10. As
the porosity (φ) increases from 0% to 50%, both k and thermal
conductivity decrease continuously without significant
discontinuity at the percolation threshold where the pores
which are critical steps in the so-called back end of line
processing (BEOL). Subsequently, oxygen plasma (ashing)
removes the remaining sacrificial organic photoresist film
deposited during lithographic patterning12. An undesirable
feature of O2 plasma is the generation of oxygen radicals which
react not only with the photoresist film but also with the
functional groups of the CDO film, breaking Si-CH3 and Si-H
bonds, and creating dangling bonds of silicon. These dangling
bonds can easily react with hydroxyl ions in the plasma
environment to form Si-OH groups13. The resulting increase in
the number of hydrophilic Si-OH groups makes the CDO films
more polar and hydrophilic and allows the uptake of moisture.
Because kH2O = ~80, absorbed moisture increases the value of
kCDO. Current carried by protonic conduction14 leaks across the
CDO, compromising its insulating quality leading to so “leaky”
capacitors15 and contributes to high k values due to Maxwell-
Wagner-Sillars polarization effects16.
Several approaches have explored the use of plasma
processing to minimize the damage to films of porous CDO
with ultra-low k. One method deposits a thin inert SiNx
passivation layer to protect the CDO film from damage after
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the oxygen plasma. Ammonia (NH3) plasma exposure of a thin high optical transmission, high specific surface area, low
layer of hybrid-organic-siloxane-polymer (HOSP) produces the dielectric constant, low refractive index and low sound
SiNx film17. This inert layer can decrease the uptake of velocity. Its moisture sensitivity, however, limits its practical
26
moisture by the HOSP film and increase its resistance to attack use25,
.
Various
derivatization
schemes
use
by O2 plasma. Another method entails treatment with helium methyltrimethoxysilane
(MTMS),
hexamethyldisilazane
(He) plasma after CDO deposition18. Repair of the damage (HMDS), dimethylchlorosilane (DMCS), trimethylethoxysilane
resulting from the O2 plasma by reducing silanol to silicon (TMES), and hexadecyltrimethoxysilane (HDTMS) to alter the
hydride has also been explored using hydrogen (H2) plasma19. wetting properties of the surface26. These methods replace
Overall the harsh reactive environment, during plasma hydrogen from the hydroxyl group (SiO-H)23, 27-36 with a polar
processing, causes a thinning of the CDO films and irreversible siloxy group (SiO-R) to render the surface hydrophobic. The
modification of the Si-C-OH network.
method developed here instead replaces hydroxyl (-OH)
An alternative to these plasma-based methods is a surface groups with a nonpolar Si-C group bearing long alkyl chains (Si-
modifying chemical approach. For example, treatment with R, R=CxH2x+1 or another hydrocarbon) which may be
hexamethyldisilazane (HMDS) converts silicon-hydroxyl (Si-OH) advantageous in certain types of chromatographic separations.
functional groups on the damaged surface of the CDO to Following is a brief outline of the paper. We first present a
hydrophobic Si-O-Si(CH3)320. The resulting terminal methyl synthetic approach to transforming small molecular silanol
group, however, is insufficient to prevent further attack by groups to silicon hydride to determine optimal reactions
moisture. Silanization with a longer alkyl is an obvious remedy, conditions for derivatization of solid surfaces of silica gel, SiOx
but it introduces undesirable polar Si-O functional groups that and CDO coated wafers. We then combine our hydrogen
elevate k values of the film.
termination method with the classic hydrosilylation route
This report presents a gentle and spatially selective using octadecene to protect ILD better against the moisture
approach for modifying surface-exposed, hydrophilic silicon- infiltration. Next appears a comparison of our method with the
hydroxyl (Si-OH) functional groups to hydrophobic hydrogen- classic silanization approach as revealed through electrical and
terminated silicon (Si-H). The reaction scheme provided below optical properties of the corresponding films. The paper
further enhances the surface hydrophobicity by hydrosilylation concludes with a brief discussion of the relevance of the
of the Si-H with a long alkyl chain (Si-R, R= C18H37)21, 22 and the chemistry for the BEOL processing.
resulting Si-C bond is far less polar and offers a lower k
alternative.
2. EXPERIMENTAL SECTION
2.1 Materials
Tert-butyldimethylsilanol
butyldimethylsilyl
((CH3)3CSi(CH3)2OH),
tert-
trifluoromethanesulfonate
(CF3SO3Si(CH3)2C(CH3)3),
trimethylaluminum
(Al(CH3)3),
≥99%),
trifluoromethanesulfonicanhydride
((CF3SO2)2O,
ethoxytriphenylsilanol (C20H20OSi), chloroform (CHCl3, ≥99%),
anhydrous toluene (C6H5CH3, 99.8%)
Reaction Scheme 1. Developed chemical pathway to derivatize
surface-exposed silanol to silicon hydride followed by alkyl
termination.
diisobutylaluminumhydride ((CH3)3CHCH2)2AlH), 1-octadecene
(C18H36), and octadecyltrichlorosilane (C18H38SiCl3) were all
reagent grade, purchased from Sigma-Aldrich and used as
received. Sigma-Aldrich was the source of amorphous silica
gel (high-purity grade (Davisil Grade 62), SiO2, 60 – 200 mesh,
Chromatography grade). Intel Corporation provided CDO and
SiOx coated silicon wafers.
Compared to methyl groups, the long chain alkyl groups
form a thicker, close-packed hydrophobic film that effectively
blocks the uptake of moisture23. The significant advantage of
this approach is that the selective derivatization occurs only on
the exposed silicon-hydroxyl (Si-OH) groups of the CDO film
without damaging the rest of the nano-porous cage. Also, the
organic layer blocks the ionic migration through the exposed
inner layer dielectric (ILD) surface. Furthermore, derivatization
with different reactants with silicon hydride formed during the
intermediate stage provides new surface functionalities24. For
example, it provides a convenient route for insertion of
additional functional groups such as terminal amines in the
alkene structure thereby enabling a rational way to interface
with other semiconductors such as carbon nanotubes or
members of III-VI systems.
2.2 Substrate preparation for heterogeneous chemical
reactions
2.2.1 Silica gel: Above chromatography-grade silica gel, was
reported by the manufacturer to have particle sizes ranging
from 75 to 250 µm, 1.15 × 10-6 m3/g, and surface area of 300
m2/g, with a pore size of 15 nm. Before derivatization, the
silica gel was dried by heating at 400o C for 4 hours and
allowed to cool under modest vacuum (100 mTorr).
2.2.2 SiOx and CDO Wafers: 100 nm thick CVD deposited oxide
wafers and CDO on silicon wafers, prepared using the plasma-
enhanced chemical vapor deposition (PECVD) process, were a
gift of Intel Corporation. Note that these oxide layers were
deposited from vapor phase as opposed to thermally grown;
the latter can have surface roughness below 0.1 nm when
The method extends easily to other applications. For
instance, the hydrophilic surfaces of commonly used silica gel
in chromatography or silica aerogels may be transformed to a
hydrophobic surface.
The aerogels exhibit exceptional
properties such as low thermal conductivity, high porosity,
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atomically smooth silicon wafers are used. Using standard CDO, gave the layer thickness and refractive indices as a
RCA-1 cleaning solution (NH4OH, H2O2, H2O 1:1:5 v:v:v), CDO function of wavelength. Squaring the measured refractive
wafer coupons (1 x 2 cm in size) were cleaned and then dried index yielded optical dielectric constants. Additionally,
under a stream of dry N2 37. Following the cleaning step, the Gaertner Ellipsometer (He-Ne laser light source, λ = 632.8 nm,
wafers were washed several times in a dry N2 glove box with fixed incidence angle of 70o) supplanted the measurements of
anhydrous toluene to remove any traces of adsorbed moisture film thicknesses.
before reaction.
2.3.6 Contact angle measurements: A CCD camera (Kodak
MDS 100) connected to a desktop computer acquired droplet
2.3 Characterization
2.3.1 FT-IR Spectroscopy
:
FT-IR spectra, collected in a images on the surface of CDO and SiOx wafers. The images
transmission mode on a Thermoscientific, NicoletTM iS TM5 FT- were analyzed using the program ImageJ (Freeware available
IR Spectrometer from 400 to 4000 cm-1 at a resolution of from NIH) to calculate the contact angle. Typical uncertainty in
4 cm-1, enabled qualitative estimation of functional groups. CA measurements was about 3-4○
.
For collecting FT-IR spectra from samples of silica gel, small
amounts of a sample (≈ 5 wt%) were mixed with KBr salt and 2.4 Common synthetic procedure for silica gel, SiOx and CDO
pressed into thin pellets. CDO as well as SiOx wafers (1 x 2 cm) wafers
were placed directly in the path of the IR beam. After each The first step in reaction scheme 1 employed 0.1 M triflic
stage of reactions, few samples were removed from a glove- anhydride ((CF3SO2)2O) in anhydrous toluene at 50-60o C for 48
box and their IR spectra collected in the ambient air (relative hours to esterify the Si-OH functional group to yield
humidity 40-60 %) which may account for partial hydrolysis or silyltrifluoromethane sulfonate (Si-O-Tf, where Tf indicates the
readsorption of ambient moisture that affected OH stretch SO2CF3 group). In the second step, 0.1 M DIBAL-H (di-
vibration peaks in 3200-3600 cm-1. However, the samples, at isobutylaluminum hydride, (CH3)2CHCH2)2AlH) reagent at 50-
the intermediate stages of derivatization, once removed from 60o C for 48 hours reduced the triflate intermediate (Si-O-
glove box were discarded after collecting their IR spectra. (SO2)CF3) to silicon hydride. Because hydrogen-terminated
Thus, in all cases, the final IR spectra came from samples silicon is unstable, Si-H was further treated with 1-octadecene
maintained in the glove box throughout all the stages of (CH3(CH2)15CH=CH2) in the presence of UV light (254 nm) for 2
derivatization, i.e., these samples were never exposed to hours to produce Si-R (Si-C18H37)38. All reactions were carried
ordinary laboratory conditions till they had undergone all the out in dry N2 within a glovebox. Between each set of
steps in the derivatization scheme.
reactions, silica gel and CDO wafers were thoroughly washed
2.3.2 NMR Spectroscopy: For NMR studies, reaction mixtures with anhydrous toluene.
were dissolved in C6D6 or DMSO-d6 or CDCl3. Typical
1
acquisition conditions for H- and 19F-NMR spectra, on a 400 2.5 Octadecyltrichlorosilane(OTS) derivatization of CDO and
MHz Bruker NMR instrument, used average of 256 scans. In all SiOx wafers
reactions discussed here, percentage yields were calculated Nanoporous CDO and SiOx wafers (1x2 cm in size) were
1
from integrated peak intensities of H- or 19F-NMR spectra of cleaned using RCA-1 solution and washed thoroughly with DI
crude mixtures.
water. Then the wafers were cleaned with Acetone, CHCl3, and
2.3.3 Gas Chromatography-Mass Spectrometry (GC-MS): GC- anhydrous Toluene solvents and followed by drying under the
MS of test organic compounds and derivatives were collected flow of dry N2 gas. As cleaned wafers were immersed in 50
in Agilent 7890 GC using 5975 Mass Selective Detector (MSD) mM solution of OTS in anhydrous Toluene (99.99%) at room
operated in an electron-impact mode.
temperature and incubated for 24 hours. All the above steps
2.3.4 X-ray Photoelectron Spectroscopy (XPS): Derivatized and were carried out in a dry nitrogen filled glovebox (moisture
underivatized CDO and oxide wafer coupons (three samples concentration <50 ppm). After the reaction, OTS derivatized
each and three separate locations on a given coupon) were wafers were cleaned with anhydrous Toluene, CHCl3, acetone
investigated using XPS. The instrument used was a PHI solvents by ultra-sonication and followed drying under the
VersaProbe II Scanning XPS equipped with an Argon (Ar)/C60 flow of N2 gas.
sputtering gun and dual-charge neutralizer. Data acquisition
conditions for low-resolution survey spectra were: 187.85 eV 2.6 Synthesis of tert-butyldimethylsilyltrifluoro-methane-
pass energy, 1 eV/step, 20 ms per step, five cycles, and five sulfonate (TBDMS-OTf)
sweeps. High-resolution spectra from Si (2p) region employed
One mmol of tert-butyldimethylsilanol and two mmol of
23.5 eV pass energy, 0.1 eV step, 20 ms per step, five cycles, triflic anhydride was dissolved in 1 ml of anhydrous toluene
and five sweeps. Typical uncertainty in the estimated and mixed at 50-60o C for up to 72 hours. Percentage yields,
percentages for Si-X bond concentration varied between 3-5 calculated from the 1H-NMR of the crude reaction mixture
%.
(Figure S1), were 30, 60, 98 and 100 % for reactions lasting 2,
2.3.5 Film thickness measurements: Filmetrics f20 24, 48 and 72 hours, respectively.
spectrometer, operating in reflectance mode over the 400 nm- 1H-NMR (C6D6, 400 MHz) δ 0.60 (s, 9H) and δ -0.00 (s, 6H); 19F-
1000 nm wavelength range, was employed for thin film NMR (CDCl3, 400 MHz) δ -76.84 (s, 3F); MS parent peak
characterization. Non-linear least squares fits of reflectance calculated for CF3SO3Si(CH3)2C(CH3)3 was 264.34 m/z, found at
spectra, collected from silicon wafers bearing silicon oxide and
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265.0 m/z (Figure S2); IR (KBr) υmax 3027, 2926, 2857, 1228, was monitored by the 1H-NMR of the reaction mixture over 72
1193, 1039 cm-1.
hours. The reaction reached completion (≈ 98 %) within 48
hours based on proton NMR spectral analysis (Figure S1).
2.7 Synthesis of tert-butyldimethylsilane
One mmol of tert-butyldimethylsilyltrifluromethane sulfonate
and two mmol of DIBAL-H reagents were dissolved in 1 ml of
anhydrous toluene and mixed at 50-60o C for up to 72 hours.
Percentage yields, calculated from 1H- and 19F-NMR of the
crude reaction mixture, were 0, 61, 89 and 91% for 2, 24, 48
and 72 hours, respectively (Figures S3, S4).
1H-NMR (CDCl3, 400 MHz) δ 3.64 (m, 1H), δ 0.88 (s, 9H), δ 0.02
(d, J = 3.1 Hz, 6H); MS parent peak calculated for H-
Si(CH3)2C(CH3)3 was 116.28 m/z, found at 116.10 m/z (Figure
S5); IR (KBr) υmax 3027, 2926, 2857, 2105 cm-1.
2.8 Synthesis of tert-butyltrimethylsilane
One mmol of tert-butyldimethyltrifluoromethane sulfonate
and two mmol of trimethylaluminum reagents were dissolved
in anhydrous toluene and mixed at 25o C for 24 hours.
1H-NMR (C6D6, 400 MHz) δ 0.93 (s, 9H), δ 0.01 (s, 9H) (Figure
S6); the MS parent peak calculated for (CH3)3SiC(CH3)3 was 130
m/z, found at 130.1 m/z (Figure S7).
Figure 1. Schemes for synthesis of: a) TBDMS Triflate and tert-
butyldimethylsilane;
triphenylsilane.
b)
tert-butyltrimethylsilane;
c)
2.9 Synthesis of triphenylsilane
In the second step, the reduction of TBDMS-triflate to tert-
butyldimethylsilane (Figure 1a) was achieved by treatment
with DIBAL-H. After 24 hours of reaction, the emergence of a
new heptet caused by Si-H at δ 3.64 (m, 1H) suggested the
formation of the hydrogen-terminated product (Figures S3).
The yield achieved at 48 hours was over 95 %
One mmol of ethoxytriphenylsilane and two mmol of DIBAL-H
reagent were dissolved in anhydrous toluene and mixed at 50-
60o C for 48 hours. Percentage yield calculated from 1H-NMR of
the crude reaction mixture was 48%.
1H-NMR (DMSO-d6, 400 MHz) δ 7.58 (m, 15H), δ 5.7 (s, 1H)
(Figure S8); IR (KBr) υmax 3067, 2119, 1427, 1113, 802, 730, 697
cm-1.
Treatment of the TBDMS-triflate with trimethylaluminum
at 25o C produced terminal methyl groups and synthesized t-
butyl methyl silane (Figures 1b and S6 and S7). Thus, DIBAL-H
and trimethylaluminum reagents successfully reduced the
triflate ester of a silanol to hydride and methyl groups,
respectively. In addition to tert-butyldimethylsilanol, we have
confirmed the hydrogen termination on ethoxytriphenylsilanol
(Figures 1c and S8).
3. RESULTS & DISCUSSION
Small organic molecules bearing silanol groups provided
suitable model systems to optimize reactions conditions, such
as temperature, duration and reactant concentrations, for
reactions which are shown in Scheme 1. The optimized
reaction conditions were applied to derivatize silica gel which
presents a model solid surface bearing high concentration Si-
OH groups. Simple transmissive IR spectroscopy allowed
characterization of functional groups generated during
derivatization reactions. Finally, the method was adopted to
SiOx and CDOcoated silicon wafers which have a relatively low
concentration of surface accessible Si-OH groups. XPS, FT-IR
and contact angle measurements indicated successful
derivatization of these surfaces. Electrical and optical
characterization of CDO and SiOx layers included
measurements of low and high-frequency k values, and film
thicknesses, respectively
3.2 Reaction on Silica gel
Having established the feasibility of the scheme 1, we
explored its potential for derivatizing silica gel, which is
structurally similar to CDO. Silica gel, with its a highly porous
structure and a large number of air-exposed hydroxyl groups39,
provided a useful control heterogeneous substrate. The FT-IR
allowed monitoring the progress of the reaction (Figure 2).
The IR spectrum (the black trace in Figure 2) from the silica gel
exhibited
a
broad peak at 3439 cm-1 caused by the
superimposed stretching modes of Si-OH groups and the
hydroxyl groups of physically adsorbed water. Peaks at 1626
cm-1, 1480 cm-1, were assigned to the bending vibrations of
water and silanol, respectively. Peaks at 1102 cm-1 and 969 cm-
3.1 Reactions of Model Organic Compounds
tert-Butyldimethylsilanol ((CH3)3CSi(CH3)2OH) served as a
test molecule for the reaction scheme (Figure 1a).
Transformation of the molecule to triflate and hydride
molecules can be readily tracked by NMR, IR and GC-MS
(Figures S1-S5), enabling optimization of the reaction
conditions and yield. The kinetics of the triflation reaction
1
reflected the Si-O-Si stretch vibration modes of the silica
gel23, 27, 29, 31, 32, 34
.
After reaction with triflic anhydride, FT-IR of silica gel
(Figure 2, the blue trace) exhibited new peaks at 1033 cm-1
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Therefore, a control experiment was carried out in the
glovebox to assess the effect of adventitious moisture by
stirring triflic anhydride solution in anhydrous toluene for 24
and 48 hours at 60 °C. The corresponding 19F NMR spectra of
the reaction mixtures yielded (triflic acid) :( triflic anhydride)
ratios of 1:148 and 1:80 for the shorter and more prolonged
stirring, respectively.
With 0.2 g of silica gel, the
corresponding ratios after 24 and 48 hours of triflation
reaction at 60 °C increased to 1:9 and 1:2, respectively. These
data indicated that the moisture present in the glovebox
contributed negligibly to the triflic acid from reaction 2. Based
on these 19F-NMR data, the esterification reaction reached 44
% completion after 24 hours, which subsequently increased to
above 90% after 48 hours.
Reaction with the reducing reagent, DIBAL-H with silica gel
yielded a new set of peaks in FT-IR at 2259 cm-1 and 888 cm-1,
which correspond to Si-H stretching and bending vibrations,
respectively27 (Figure 2, the pink curve). The Si-H stretching
frequency depends on the polarity of the other three bonds
attached to the silicon, increasing from 2109 to 2250 cm-1 in
going from C3Si-H to O3Si-H41.
Figure 2. FT-IR spectra of silica gel at various stages of the
reaction: pure Si gel (black); after treatment with triflic
anhydride (blue); DIBAL-H reduced (pink); and after treatment
with 1-octadecene (red).
Because of Si-O bond polarization, the attached Si-H bond
tends to hydrolyze upon exposure to atmospheric moisture. A
typical lifetime of the SiH group of the silica surface in the air
as determined from IR studies was less than half an hour. We,
and 1257 cm-1, corresponding to S=O asymmetric and
symmetric stretches, and an 1177 cm-1 peak ascribed to C-F
stretching corresponding to the formation of triflate (Si-O-
(SO2)CF3) groups. Because some OH groups on silica gel are
inaccessible to triflic anhydride, the triflation reaction did not
completely remove the broad OH peak. Other possibility
includes readsorption of adventitious moisture from the air.
Notably, the peak intensity increased after triflation perhaps
due to incomplete removal of the triflic acid generated during
the reaction, (1). The precise molecular mechanism for the
observed enhancement in O-H peak intensities is unknown at
this time.
therefore, used
a well-known hydrocarbon-derivatization
scheme for its protection38. Silicon hydride groups (Si-H) were
hydrosilylated (Si-R) by treating the powder with 1-octadecene
in the presence of UV light (254 nm) for 2 hours. Samples were
washed with anhydrous chloroform (CHCl3) to remove excess
adsorbed 1-octadecene and then air dried 38, 42. New IR peaks
(Figure 2, the red curve) at 2856 cm-1 and 2927 cm-1,
corresponding to -CH2 symmetric and asymmetric stretches,
and a shoulder peak at 2974 cm-1 corresponding to -CH3
confirmed the alkyl-chain derivatization of the silica gel
surface23, 29, 31, 32, 34, 35. Previous studies of the effect of surface
curvature on the formation of SAMs indicated that higher
curvature led to less ordered liquid like monolayers43. It was
noted that SAM of a C12-silanes on 9 nm radius of curvature
silica particles caused to a shift of FT-IR peak due C-H
antisymmetric stretch from 2925 cm-1 for a planar surface to
2929 cm-1 for the 9 nm curved surface. In light of 15 nm pore
size of the silica particles used in our study, the observed shift
is consistent with these previous finding. The same study
established that a decreased packing density (i.e. decreased
extent of derivatization ) could also blue-shift the IR peak in
the direction observed. An elegant approach to quantify the
extent of silicon derivatization using solid-state NMR CP-MAS
(Cross-Polarization and Magic Angle Spinning) technique was
established which exploited quantitative analysis of the peaks
corresponding to derivatized and underivatized Si atoms44.
In the FT-IR spectrum of alkyl-derivatized silica gel, the
disappearance of peaks corresponding to alkene C-H stretch
(C=C-H) at 3083 cm-1 and the C=C stretch at 1644 cm-1
indicated the absence of 1-octadecene physically adsorbed to
the surface of the silica gel. To gauge further the effect of
adsorbed alkenes, as opposed to chemically bonded alkyl
Surface concentrations of silanol in a typical silica gel is 2.8
mmol/g with 4.6 SiOH/nm2 40. The percentage conversion of
the surface silicon-hydroxyl to silyltrifluoromethanesulfonate
ester was determined by 19F NMR spectroscopy of the reaction
mixture based on the reaction between triflic anhydride and
Si-OH on the surface of silica particles:
19F NMR spectra exhibited chemical shifts of methyl fluorines
in triflic anhydride (reactant) and triflic acid (product)
remaining in the reacting solution at -72.6 ppm and -77.3 ppm,
respectively (Figure S10). The integrated peak intensities
determined the extent of the triflation reaction, assuming that
the only source of triflic acid was from the silanol esterification
reaction (Eqn. 1). Moisture, however, if present, can also
hydrolyze the triflic anhydride to give two moles of triflic acid:
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groups, we conducted another control experiment. The dried proportionately smaller. After DIBAL-H reduction the Si-H
silica gel was immersed in 1-octadecene, and exposed to UV peak at 2259 cm-1 was not observed presumably because of
light under identical conditions. The FT-IR spectrum (Figure the lower total content of SiOH groups on the SiOx film surface
S11) of silica gel after these steps showed no significant peaks compared to silica gel (cf. pink curves in Figures 2 and 3; also
corresponding to alkyl chains, indicating negligible adsorption see below). Nevertheless, silica gel Si-H peaks disappeared
of 1-octadecene on the silica surface.
upon exposure to air within half an hour. Similar limited
stability of the Si-H groups in the air has been previously
noted45 when the remaining three group bound to silicon are
3.3 Reactions on SiOx wafer
Derivatization studies of SiOx coated wafer provided a highly polar, such as Si-O.
suitable comparison with silica gel (vide supra) and low k
We proceeded with hydrosilylation of the hydrogen-
dielectric, CDO (vide infra). These studies utilized silicon wafers terminated surface with long-chain terminal alkene without
decorated with 100 nm thick silicon oxide treated identical exposing the sample to atmospheric moisture. Upon
reaction conditions. The surface of the CVD oxide had an derivatization, we observed a decreased surface roughness
average roughness of 0.41 nm as determined by AFM (see (0.34 nm vs. 0.41 nm, Table S1) along with hints of island type
Table S1 in ESI). A clean SiOx wafer before the reaction showed structures (Figure S18 a).
the peaks (the black trace in Figure 3) corresponding to the
After the photochemical reaction, the presence of peaks at
stretching (3376 cm-1 ) and bending vibrations (890 cm-1) of the 2853 and 2924 cm-1, corresponding to CH2 symmetric and
silanol (Si-OH); the symmetric (890 cm-1) and asymmetric asymmetric stretches, and a shoulder peak at 2976 cm-1 (red
stretching (1105 cm-1) of cage Si-O-Si; and the bending curve and red inset in Figure 3) confirmed the hydrosilylation.
vibration (610 cm-1) of Si-O-Si9. Lower peak intensities Compared to silica gel (2927 cm-1), the lower value of C-H
associated with O-H stretch vibrations (at 3500cm-1) when asymmetric stretch vibration is consistent with a more ordered
compared to Si-O-Si peaks (at 1100 cm-1), indicated a lower net hydrocarbon layer on a smoother surface as discussed before.
content of surface Si-OH groups on SiOx wafer films than silica Nevertheless, the chains are not packed in crystalline state43, 46
gel (cf. black curves in Figures 2 and 3).
which would have resulted in the blue shift of the peak
position to 2917cm-1. The ratio of C-H (2800-2950 cm-1) to Si-
O-Si (1100 cm-1) peak intensities was higher than silica gel (cf.
Figures 2 and 3) and could arise from a greater percentage of
inaccessible surface SiOH groups in silica gel. The absence of
peaks at 3083 and 1004 cm-1 corresponding to alkene C
=C-H
stretching and bending vibrations, and the C=C stretch at 1644
cm-1 of 1-octadecene, suggested that 1-octadecene was
chemically bonded to the silicon surface rather than physically
adsorbed. Contact angles before and after derivatization of
SiOx surfaces were 10o and 91o respectively.
3.4 Reaction on CDO (Carbon Doped Oxide) wafer
FT-IR, X-ray photoelectron spectroscopy (XPS) and water
contact angles (CA) techniques were employed to monitor the
progress of the reaction, and the appearance of new
functional groups on CDO wafer surfaces. When compared to
SiOx wafer the water contact angle on CDO after RCA clean,
was significantly higher (35o vs. 10o, see Table 1, below )
consistent with a more hydrophobic surface due to its higher
carbon content and a lower density of SiOH group (cf. black
curves in Figures 3 and 4).
Figure 3. FT-IR spectra of SiOx wafer at various stages of the
reaction: clean SiOx film (black); after treatment with triflic
anhydride (blue); DIBAL-H reduced (pink); and after treatment
with 1-octadecene (red)
Morphological AFM studies, (Figure S17 and Table S1)
revealed the nanoporous nature of CDO. An average
roughness of 0.37 nm was comparable to the CVD SiOx surface
discussed earlier. Line scans revealed valleys as deep as 2 nm
and the lateral features of 20 nm or smaller broadened by
AFM tip effects47. Thus studies presented in this paper differ
The Si-OH absorption peak intensity significantly reduced
upon triflation consistent with the surface accessibility of most
Si-OH groups in the SiOx film (cf. black and blue traces). After
the triflation reaction, new peaks appeared at 1039, 1228 cm-1
and 1193 cm-1 corresponding to symmetric and asymmetric
stretches of S=O bond, and C-F stretch, respectively, as in the
case of silica gel. However, their relative peak intensities were
from atomically smooth silicon or silicon oxide surfaces48
.
Figure 4 shows the transmission FT-IR spectra of a CDO wafer
at different stages of derivatization. Major peaks
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intensity and disappearance of alkyl shoulder-peaks (the green
trace, Figure 4).
A control experiment to further test the hypothesis of
DIBAL-H binding on the CDO surface, entailed treating clean
CDO, SiOx wafers, and silica gel sample with a DIBAL-H reagent
(see Figure S12) in anhydrous toluene. After 24 hours, samples
were washed with dry toluene and dried. Their FT-IR spectra
revealed the alkyl peaks only for the CDO confirming
preferential DIBAL-H trapping/adsorption on the surface/pores
of a slightly hydrophobic CDO surface (vide supra) as opposed
to hydrophilic surfaces of silica gel or SiOx wafer.
Like SiOx wafer, Si-H stretching peaks were not detected
presumably either because of low surface concentration or
42
rapid degradation in the air
during FT-IR data acquisition.
Furthermore, observed weaker IR peak intensities (blue inset
in Figure 4) for the triflate groups on CDO and SiOx wafer
surface compared to silica gel (blue curves in Figures 2, 3 and
4) supported the above interpretation. FT-IR spectrum after
hydrosilylation was similar to SiOx wafer results (cf. red curves
in Figures
3 and 4). A control experiment involving
hydrosilylation of underivatized CDO surface revealed a lack
adsorption of octadecene or its photochemical reaction
product (Figure S13). The observation of DIBAL-H
adsorption/trapping (Figure S12) but not of octadecene on
Figure 4. FT-IR spectra of CDO wafer in various stages of
reaction; Clean CDO (black), after reaction with triflic
anhydride (blue), after reaction with DIBAL-H (pink), after
glacial acetic acid washing (green) and after octadecene
derivatization (red). Stars indicated in the DIBAL-H treated
spectrum correspond to alkyl groups arising from the surface
bound, excess DIBAL-H reaction products. Insets expand
selected spectral regions of triflic anhydride Tf2O and
octadecene-derivatized silica in the IR spectra.
clean CDO surface (Figure S13) implied that
a slightly
hydrophilic CDO pores were unable to retain nonpolar
octadecene, pointing to the amphipathic nature of the pore
wall. This weaker pore-octadecene interaction would result in
immediate removal of octadecene, but not DIBAL-H, during
the CDO wafer rinsing/drying stage, before collecting
respective IR spectra. In theory, following DIBAL-H reduction of
pore surface triflate groups, octadecene could enter CDO
pores and derivatize the hydrophobic silicon hydride pore
surfaces (red curve Figure 4). Alternatively, the steric effects
such as nanopore size could preclude entry of larger
octadecane while allowing smaller DIBAL-H molecule. Future
studies using smaller size alkenes are planned, however, with
the data at hand, we are unable to verify whether the internal
pore surface is derivatized or not.
observed correlated well with the SiOx wafer IR peaks
illustrated in Figure 3. However, significantly lower peak
intensity in the OH vibrational frequency range ( ≈ 3300 cm-1)
compared to silica gel or SiOx wafer (Figures 2, 3), implied a
lower net content of OH on the CDO surface. Accordingly,
after triflation, intensities of peaks associated with S=O and C-
F stretches, (shown Figure 4, the blue curve (inset)), were
smaller than the corresponding peaks observed in the triflated
SiOx films (blue curve, Figure 3).
Asymmetric stretching vibration due to C-H group
occurred, as in the case of SiOx wafer, at 2924 cm-1. Also,
within a factor of two, the C-H vibrational peak intensities
were also comparable to those observed in derivatized SiOx
wafer. These findings suggested that the hydrocarbon layers in
both CDO and SiOx wafer surfaces were fluid and ordered
analogous to lipid-chain packing in a lamellar liquid crystals50-
53. AFM studies revealed that the surface roughness on CDO
surface upon octadecene derivatization decreased from 0.37
nm to 0.22 nm (Table S1) indicating the formation of a smooth
hydrocarbon layer and possibly multilayer islands (Figure S18b,
Table S1). Thus, the growth of these hydrocarbon layers was
rather complex somewhat analogous to Stranski–Krastanov
model54.
Reduction of triflate ester on the CDO surface using DIBAL-
H proceeded as in the case of SiOx 49. After reduction with
DIBAL-H several new peaks, not observed in silica gel or SiOx
surfaces, appeared in the range from 2800-3000 cm-1,
attributed to alkyl C-H vibrations. These peaks could originate
from traces of the degraded DIBAL-H reagent (e.g. diisobutyl
groups), physically adsorbed or trapped in pores of CDO
surface (pink trace, Figure 4). Also, supporting the above
hypothesis was an observed increase in the peak intensity near
the OH region (3300 cm-1) due to the formation of Al-OH group
a byproduct of the reduction reaction. Since an aluminum
oxide or its hydroxide dissolves readily in an acidic medium,
we treated the wafer with 25% glacial acetic acid in anhydrous
toluene for five minutes to remove trapped/ degraded DIBAL-
H reagent from the CDO surface. The FT-IR spectra collected
after the acid treatment displayed a reduction in OH peak
A
complementary technique of X-ray photoelectron
spectroscopy (XPS) further supported for derivatization on
CDO wafers. Survey spectra obtained from a CDO surface
before (Figure 5a) and after (Figure 5c) reaction revealed Si, C,
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and O elements. After derivatization, the intensity of the about 90o (Figure 6b, Table 1). The water contact angles on
carbon (1s) peak increased, consistent with the derivatization derivatized surfaces were stable for more than a year (See
of surface groups with alkyl chains. In deconvoluted high- Figure S14) as were the corresponding IR spectra (Figure S15).
resolution Si (2p) spectra of clean CDO wafers before the
b)
reaction (Figure 5b), three strong peaks at 101.6, 102.3 and
103.1 eV corresponding to Si-C, -Si-O-C- and -O-Si-O-,
respectively were observed. The XPS spectrum after the
reaction (Figure 5d) had the same three strong peaks at similar
positions but with different intensities. The area of the -O-Si-
O- peak decreased from 26 % to 10 % of the Si (2p) peak area,
and the relative area of -Si-C peak increased from 11 % to 22
%.
Figure 6. Water contact angles measured on (a) un-derivatized
and (b) octadecene derivatized CDO wafers
3.5 Comparison with octadecyl trichlorosilane (OTS)
derivatization
OTS, a commonly used silanization agent, readily forms a
self-assembled monolayer on SiOx film transforming it to from
hydrophilic to a hydrophobic surface by inserting an alkoxy
ether (Si-O-R) group bearing a C18 alkyl chain. Hence, we
compared the results of octadecene-based vs. the OTS-based
derivatization on CDO and SiOx surfaces. FT-IR spectra shown
in Figure 7 ( for full spectra, see Figure S16), indicated that the
antisymmetric stretch was essentially constant at 2924 cm-1
regardless of the substrate or the derivatization method used.
Both the oxide and CDO surface are nanoporous, which could
prevent the formation of planar crystalline monolayers. This
invariance of the peak position rules out any significant
difference in hydrocarbon ordering between derivatization
modes, i.e., OTS vs. Octadecene. Nevertheless, it is hard to
quantitatively disentangle the competing effects of curvature
and packing density as discussed before. Curiously, the
intensity of the IR peaks was consistently higher for
octadecene derivatization than the OTS SAMs (blue vs. red
traces). One potential explanation for this could be due to
Figure 5. XPS spectra of CDO wafer. Survey spectrum, a, and Si
(2p) high-resolution spectrum, b, collected before the reaction
and c, d are the corresponding spectra collected after the
reaction.
55
formation of multilayers46, (vide infra). AFM studies (Table
S1) revealed
a reduction in surface roughness upon
This finding validates conversion of -O-Si-OH to –O-Si-R (R=
octadecyl). No significant change was observedin the areas for
the –Si-O-C- group (from 57% before derivatization to 54%
derivatization and multilayer island formation in the case of
CDO surface.
The silanol peak intensities, around 3300cm-1, were greater
in the OTS derivatized surfaces. An increase in OH peak
intensity could come from the free Si-OH groups on the
hydrolyzed OTS molecule as schematically illustrated in Figure
7 (inset). These free O-H groups could provide interfacial
hydrophilic sites for water adsorption underneath the alkyl
chains of the OTS film. Thus, an insertion of Si-OR group which
has a higher dipole moment than Si-R group and new free –OH
groups would be expected to increase the k values of
CDO/Oxide films derivatized using OTS.
after derivatization), suggesting that
g
no structural
modification in the cage structure of the CDO film took place.
More surface chemical studies are needed to confirm this
hypothesis.
Derivatization of the CDO surface with octadecene should
change its hydrophobicity. The contact angle of water droplets
deposited on the surface confirmed that shift (Figure 6). Clean
CDO wafers (Figure 6a) had a low contact angle 36o caused by
the presence of the hydrophilic hydroxyl groups (Si-OH).
Derivatization with octadecene increased the contact angle to
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1. Best-fit refractive index values for CDO films were lower
than SiOx film commensurate with the porous structure of CDO
that can trap either water or air, both of which have lower
refractive indices than SiO2. Furthermore, the effective
refractive index values of OTS derivatized films were
consistently lower than the octadecene derivatized implying
perhaps their larger adsorbed moisture content. In either SiOx
or CDO films the refractive index of DIBAL-H-treated films was
greater than the OTS-treated films corresponding to its thicker
hydrocarbon film of the greater refractive index (1.4439) than
refractive indices of either water (1.33) or air (1.0).
To estimate film porosity of CDO crudely, the refractive
index of these composite films (ηexp) was decomposed as a
sum of volume fraction (φ) weighted refractive indices of the
constituents60:
Figure 7. Comparison CDO and SiOx wafer derivatized by
DIBAL-H assisted chemistry (blue) vs. OTS chemistry (red).
Black curves depict IR spectra of the respective wafers before
derivatization.
η
= ∑η φ
i i
exp
(3)
i
where index i denotes a specific component and ϕi its volume
fraction. Assuming that the CDO matrix is made up of SiO2
(ηSiO2 = 1.473) and pores are filled completely with either air
(ηSiO2 = 1) or water (ηH2O = 1.333) gave approximate the
effective porosity of 3 or 11 percent respectively. Within limits
of optical measurements of the film thickness using filmetrics,
the hydrocarbon layer thickness in octadecene derivatization
scheme were comparable (≈ 5nm) for both CDO and SiOx
(Table 1). These values were significantly higher than expected
for a densely packed upright monolayer of C18 chain (2 nm).
3.6
Effects of derivatization on dielectric constant and
contact angle
Effects of Maxwell-Wagner polarization (MWP) hampered the
low frequency (0.1-1 MHz) electrical measurements of k values
due to adsorption of moisture in nanopores of CDO. Partial
dissociation of trapped/adsorbed water molecules imparts the
film a finite electrical conductivity creating electrically leaky
(resistive) as opposed to insulating films14. Such samples56
have higher dielectric contribution arising from distortion and
dipolar polarization which raises their effective k57. To
disentangle MWP and other polarization effects require
measurements of the film porosity, frequency dependent
dielectric constants as a function of water saturation58 (i.e.,
humidity). Since these data were unavailable, we provide a
qualitative description of our k (1MHz) measurements (Figure
S19) that revealed following ordering in the magnitude of kOTS
>kCDO > kDIBAL-H, consistent with the expected interfacial
polarity of these samples (vide supra). One way around this
problem is to reduce surface SiOH group concentration by
high-temperature baking in an inert environment, and such
studies are currently underway (vide infra).
Ellipsometric measurements (Table1) also gave
a higher
thickness of hydrocarbon layer in octadecene derivatized
samples. The estimated hydrocarbon thicknesses were much
smaller than the wavelength of light, suffered from higher
uncertainties since they represented a difference of thickness
before and after derivatization. Further the formation of island
structures as indicated in the AFM studies (Figure S18)
contributed to the increased uncertainty. AFM studies
presented in Table S1 also indicated island thickness of 4-6 nm,
corresponding to bi-tri layer formation. Overall, these three
independent techniques (AFM, refractive index and
ellipsometry) qualitatively suggested the formation of
multilayers upon octadecene derivatization which was further
supported by higher IR peak intensities for the C-H stretch
vibrations (Figure 7) when compared to OTS derivatized
monolayer films. The origin of multilayer formation could be
justified by partial cross-linking/polymerization of octadecene
Another approach to access k is through measurements of
film refractive index; noting that high frequency (optical) k
values simply depend on the electronic polarization and scale
as the square of refractive index59. Note, k values at optical
frequencies do not contain dielectric contributions from MWP
and dipole polarizations57 (see below). Analysis of reflectance
spectra (Filmetrics) collected from these wafers yielded best-fit
values of refractive index and film thickness; tabulated in Table
during the hydrosilylation step55
.
Table 1. Contact angle, film thicknesses of CDO and SiOx
wafers.
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oxide surfaces, these oxide films were treated essentially as
smooth surfaces. So the Cassie equation could be inverted to
extract f the extent of surface derivatization/packing density
of chains at the interface.
Sample
Refracti kopt
ve index
(η)
=
Overlayer
Thickness
(nm)
Overlayer
Thickness
(nm)
Hydrocarbon
layer thickness
(nm)
CA
(deg.)
η2
Filmetrics Ellipsometry
Filmꢀ
metrics
Ellipso
metry
CDO clean
1.457
1.447
2.12
101.2 ± 0.6
103.1 ± 2.4
102.9 ± 0.6
123 ± 4
0
0
36 ± 3˚
87 ± 3˚
Now ascribing region 1 to the hydrocarbon monolayer (θ1
= 107o) and region 2 to the underivatized SiOx(θ2 = 11o, from
Table 1), estimated f values were 0.97 for OTS and 0.92 for
Octadecene derivatized SiOx surfaces. By contrast, the CDO
surface (θ1 = 87 and 92o and θ2 = 37o) had a thicker and
perhaps even more loosely packed hydrocarbon layer due to
the lower density of surface OH groups. The corresponding f
values on the CDO surfaces were, as expected, were smaller;
0.75 (OTS) 0.69 (Octadecene) respectively. The results also
illustrated that the hydrocarbon layer thickness, which tended
to be higher for the octadecene derivatization (Table 1 and
Figure 7), did not affect the contact angle, but the areal
fraction of derivatized (i.e., f) surface did. One significant
limitation of above analysis of the extent of derivatization is
that it approximates θ2, contact angles for the clean surface, to
be equal to the contact angle of smooth, nonporous CDO and
SiOx surfaces because we did not have access to equivalent
smooth surfaces65.
1ꢀOctadecene
deriv. CDO
through DIBALꢀH
2.09
1.99
2 ± 3
10 ± 5
OTS deriv. CDO
1.412
106.4 ± 0.3
107.2 ± 0.9
5 ± 1
4 ± 2
91 ± 4˚
SiOx clean
1.473
1.448
2.17
2.09
107 ± 2
115.1 ± 0.3
117.0± 2.4
0
0
11 ± 2˚
1ꢀOctadecene
deriv. SiOx through
DIBALꢀH
111.2 ± 0.9
4 ± 3
2 ± 3
101 ± 4˚
OTS deriv. SiOx
1.435
2.06
118.9 ± 0.6
117.0 ± 0.9
2 ± 3
2 ± 1
105 ± 3˚
The contact angle measurements revealed similar trends. The
contact angles for both derivatization schemes depended on
the chemical nature of oxide layer, i.e., CDO vs. SiOx and not on
the chemical route of derivatization, i.e., silanization vs.
hydrosilylation. The CDO surface, which had a higher contact
angle before derivatization (36o (CDO) vs. 11o (SiOx)),exhibited
a lower contact angle after derivatizations (87o (Octadecene)
vs. 91o (OTS) ) compared to the CVD SiOx surface( (101o
(Octadecene) vs. 104o (OTS) ). For CVD SiOx surface the
observed contact angles after derivation were indicative of a
more loosely packed fluid monolayer compared to the almost
crystalline OTS monolayers assembled on SiOX grown
thermally on smooth silicon surfacee61 that are known to
exhibit 112 o water contact angle.
The reaction of hydrofluoric acid (HF) can also enhance the
hydrophobicity of the silicon surface by creating a hydrogen-
terminated surface that may be hydrosilylated, but it would
produce undesirable etching of the CDO film. Recently, we
have used sacrificial silanes to prevent the etching of SiOx by
HF. The chemistry, however, is carried out in an aqueous
medium of low pH, which would likely reoxidize Si-H groups66
and hence undesirable for this protecting the inner layer
dielectrics from moisture.
Although the use of Cassie-Baxter equation has been
controversial, we justify its use following Marmur et al62 who
summarized “The condition under which the Wenzel or Cassie
equation correctly estimates the most stable contact angle is
reiterated and demonstrated: these equations do hold when
the drop size is sufficiently large compared with the
wavelength of roughness or chemical heterogeneity.’’ Note, in
present study, surfaces of CDO/SiOx exhibited nanoscopic
topological features (Table S1) and the contact angle
measurements employed macroscopic droplets. Thus, applying
the Cassie equation for a measured contact angle (θm ) on a
surface made up of two types of regions (labeled with
superscripts 1 and 2) with differing contact angles, θ1 and θ2,
3.7 General Discussion
In modern interconnect fabrication, low k materials have
become increasingly important, and several reviews
emphasizing processing and chemical aspects have appeared
in literature67. Besides low k, the key requirements of these
ILD insulators are film hardness, processability at temperatures
as high as 400o C, and low coefficient of thermal expansion.
The success of CDO based dielectrics over the last two decades
stems from their ability to meet these challenging demands.
However, the path forward is now more demanding as simply
increasing the porosity of films to reduce k further is not
practical as it leads to undesirable features such as
unacceptable line roughness, lower hardness and film
cracking. The latter feature is especially problematic for
chemical mechanical polishing (CMP) steps used in the BEOL
processing. The introduction of carbon in SiOx backbone gives
film toughness but at the cost of lower hardness. Thus,
inserting hydrocarbons at the porous surface seem to be one
avenue for optimizing low k materials in terms pore sealing,
preserving hydrophobicity and reducing interfacial dipoles. At
current GHz clock speeds, primary contribution to dielectric
constant comes from dipolar, electronic and distortion
polarizations as predicted from the classic Debye equation:
gives63, 64
:
cos
θ
= f (cosθ − cosθ ) + cosθ2
(4)
1 2
m
where f represents a fraction of the surface of type 1. Crudely,
identifying region 1 as SiOx (θ1 = 11o) and region 2 as pores
(i.e., air with θ2 = 180o) on CDO surface, the estimated upper
limit for f = 0.91 implying a 9 percent surface porosity. Should
the lower OH density on the CDO surface is accounted for by
setting (θ1 > 11o) the measured porosity would decrease. For
example, if θ1 =30o for the nonporous CDO surface, then the
calculated pore fraction would be 3 percent, similar to the
estimates from the refractive index measurements (Vide
Supra). Since we did not have a nonporous CDO sample, we
did not have a value for θ1. Given these low areal fractions of
pores and sub-nanometer roughnesses for the CDO and CVD
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Intel Corporation provided graduate research assistantship
to HT, wafers, and chemicals used in this research. We also
thank Jon L. Adams, Ross Burlingame, and David Engle for their
help in experimental work. Support of Drs. James Chen (Four
Dimensions Inc.) and Bart Gordon (Materials Development
2



k −1
k + 2
4
π
N
ꢀ
(5)
=
α
+
α
+
Distort

e
3
3k T



B

where αe and αDistort are electronic (the only contribution at Corporation), who provided CV measurements at no cost, are
optical frequencies) and distortion polarizations respectively; μ gratefully acknowledged. We thank Professors Stephen Hall
is the dipole moment, and N is its number density. Clearly, the and Andres La Rosa for many productive conversations and
dipole moment contribution is quite significant since it affects constructive suggestions.
k quadratically. The dipole moments for Si-O68 and Si-C69
bonds are 3.08 D and 0.6 D respectively, so to lower k, it is REFERENCES:
advantageous to insert more Si-C bonds. However, bond 1.
enthalpy of Si-O bond is 450 kJ/mole vs. 290 kJ/mole for Si-C
bond making films containing Si-O bonds harder than Si-C
bonds. Nevertheless, these considerations are not relevant for 2.
the dangling Si-OH groups that are not part of the network. So
their conversion to Si-C bonds as shown in this paper may 3.
provide an alternate solution than a silanization approach that
introduces polar Si-O bonds. One of the key shortcomings of
our method is susceptibility of normal alkanes to oxidize
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Ar ) can prevent oxidative degradation and already there is
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