Effect of silylation hardening on the electrical characteristics of mesoporous pure silica zeolite film

Article abstract of DOI:10.1149/1.3021041

A pure silica zeolite film was formed by use of hydrothermally crystallized zeolite nanoparticles in a porous silica precursor. The effects of silylation hardening by tetramethylcyclotetrasiloxane (TMCTS) vapor treatment on the electrical characteristics

Full text of DOI:10.1149/1.3021041

H98
Journal of The Electrochemical Society, 156 ͑2͒ H98-H105 ͑2009͒
0013-4651/2008/156͑2͒/H98/8/$23.00 © The Electrochemical Society
Effect of Silylation Hardening on the Electrical Characteristics
of Mesoporous Pure Silica Zeolite Film
T. Seo,a,z T. Yoshino,b N. Ohnuki,b Y. Seino,b Y. Cho,a N. Hata,b and
T. Kikkawaa,b,
*
aResearch Institute for Nanodevice and Bio Systems, Hiroshima University, Higashi-Hiroshima, Hiroshima
739-8527, Japan
bAdvanced Semiconductor Research Center, National Institute of Advanced Industrial Science and
Technology, Tsukuba, 305-8569, Japan
A pure silica zeolite film was formed by use of hydrothermally crystallized zeolite nanoparticles in a porous silica precursor. The
effects of silylation hardening by tetramethylcyclotetrasiloxane ͑TMCTS͒ vapor treatment on the electrical characteristics of pure
silica zeolite films were investigated. The results from Fourier-transform-IR spectroscopy indicated that the O–H bond decreased
by zeolite formation, resulting in the decrease of the leakage current by 1/10. Silylation hardening by TMCTS vapor treatment
could reduce the leakage current by 4 orders of magnitude due to the reduction of Si–OH and O–H bonds. The elastic modulus of
5.18 GPa and the dielectric constant of 1.96 were achieved simultaneously by silylation hardening. Consequently, the electrical
and mechanical characteristics of the pure silica zeolite film as well as the time dependent dielectric breakdown lifetime were
improved.
© 2008 The Electrochemical Society. ͓DOI: 10.1149/1.3021041͔ All rights reserved.
Manuscript submitted August 19, 2008; revised manuscript received October 7, 2008. Published November 25, 2008.
With the shrinking of interconnect feature sizes of silicon
during synthesis to prevent agglutination of silica. The suspension
prepared by the hydrothermal crystallization method contained zeo-
lite nanoparticles. For comparison, the precursor solution without
the hydrothermal synthesis was prepared for porous silica formation.
The porosity of the zeolite was approximately 40%.6 To reduce
the k-value, a surfactant was introduced. The concentration of the
surfactant to the suspension was controlled from 0 to 9.31 wt %.
Butanol and a surfactant of ethylene oxide propylene oxide ethylene
oxide triblock copolymer, ͑EO͒ ͑PO͒ ͑EO͒13, were added to the
suspension while stirring, so that mesoscopic size pores of several
nanometers diameter were formed to reduce the k-value of the zeo-
lite film. The zeolite silica film was formed on a Si wafer by spin
coating using 2000 rpm for 30 s at room temperature. The film
ultralarge-scale integrated circuits,1 the signal delay time increases
due to the increase of interconnect resistance and parasitic capaci-
tance. To overcome this problem, low-dielectric-constant interlayer
dielectric films are needed. Mesoporous silica with pore sizes rang-
ing from 2 to 50 nm have been studied as a potential candidate for
ultralow-k materials.2 However, the mechanical strength of the
low-k film is degraded when mesopores are introduced into the skel-
etal materials to reduce the film density. Chemical mechanical pol-
ishing may cause damage to the porous low-k material. Moreover, it
is known that the low-k film to which mechanical strength is weak
receives damage in the packaging, so both low-k and high mechani-
cal strength are required. Pure silica zeolite is a promising candidate
as an advanced low-k material.3,4 The Young’s modulus of the zeo-
lite is 110 GPa,3 which is larger than that of SiO2 ͑73 GPa͒. This is
because the silica network in the zeolite has a three-dimensional
crystal structure. The zeolite also has low-k because the film density
is lower than that of quartz due to the micropores in the crystalline
silica. It has a higher elastic modulus, thermal conductivity, and
hydrophobicity than SiO2.4-10 The mobile 11 ͑MEL͒-type zeolite
was adopted in this work.11,12 The zeolite was made by a hydrother-
mal crystallization method. MEL-type zeolite has a 10-membered
ring with pore diameters of 0.53 and 0.54 nm as shown in Fig. 1. In
this paper, the effects of silylation on the film properties of pure
silica zeolite dielectric films are investigated.
13
20
Silica-zeolite
Pore
4.5 nm
Experimental
Substrate
The formation process of MEL-type zeolite is shown in Fig. 2. A
few nanometers thick native oxide was formed on the surface of Si
wafer. First, the precursor solution of tetrabutyl ammonium hydrox-
ide ͑TBAOH͒, tetraethyl orthosilicate ͑TEOS͒, and ethyl alcohol
͑EtOH͒ were mixed and stirred for 24 h at room temperature. Hy-
drolysis of TEOS was caused by EtOH. TBAOH was purified by
filtering with the ion exchange resin. Both TEOS and EtOH were of
semiconductor grade. The hydrothermal crystallization method was
a synthesis method of zeolite crystal with high temperature and high
pressure in water existence. The precursor was heated in an auto-
clave for 110 h at 100°C. The precursor was then cooled down to
room temperature, and heated up again for 10 h at 100°C. The two-
stage synthesis is believed to improve the yield for zeolite nanocrys-
tal without changing the particle size.13,14 The precursor was stirred
Si
0.53 nm
O
0.54 nm
Figure 1. ͑Color online͒ A schematic diagram of mesoporous silica low-k
film and zeolite crystal.
*
Electrochemical Society Active Member.
z E-mail: adgsfh1234@yahoo.co.jp
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Journal of The Electrochemical Society, 156 ͑2͒ H98-H105 ͑2009͒
H99
tetra-butylammonium hydroxide (TBAOH)
tetraethylorthosilicate (TEOS)
ethylalcohol (EtOH)
Precursor formation
(a)
Hydrothermal crystallization
Surfactant addition
100℃ 110h,
V
100℃ 10h
ethylene oxide propylene oxide ethylene oxide
(EO)13(PO)20(EO)13
Spin coating 2000rpm 30sec
Prebake 90℃ 1h
Calcination 400℃ 5h air
tetramethylcyclotetrasiloxine (TMCTS) treatment
Aluminum electrode deposition
Figure 2. ͑Color online͒ Process flow of pure silica zeolite film formation.
thickness could be controlled by the rotation speed of a spinner.
After the film was prebaked on a hot plate for 1 h at 90°C, it was
calcined at 400°C in air atmosphere. The temperature was 1°C/min.
The film was annealed at 400°C for 5 h before being cooled down
in air.
The film was treated in the tetramethylcyclotetrasiloxane ͑TM-
CTS͒ atmosphere at 400°C and annealed in the N2 atmosphere. The
gas flow of N2 was 10 sccm; pressure was 0.8 atm. By this treat-
ment, the pore wall surface was silylated by TMCTS molecules that
reacted with Si–OH groups on the pore wall surface as shown in
Fig. 3a, and formed the polymer cross-linking network as shown in
Fig. 3b.15
The effect of silylation was confirmed by electrical, mechanical,
and spectroscopic measurements. To measure the dielectric constant
of the film, aluminum electrodes with a diameter of 2.0 mm and
thickness of 1 ␮m were deposited on the silica film using a vacuum
evaporation system through a stencil mask. The leakage current of
pure silica zeolite and porous silica film was measured as shown in
Fig. 4a. The dielectric constants of the pure silica zeolite and porous
(b)
C-V meter
HP 4284A
Probe
Wafer
N2
Heater
Figure 4. ͑Color online͒ Measurement setup for I-V and capacitance-voltage
characteristics. ͑a͒ Wafer-lead measurement. ͑b͒ Schematic circuit diagram.
silica film were calculated by measuring the capacitances of metal-
insulator-semiconductor structures at 1 MHz as shown in Fig. 4b.
The range of the voltage of cyclic voltammetry sweep is 10 V. The
zeolite film was baked at 300°C for 2 h in a nitrogen-filled glove
box to get rid of adsorbed water. The Si substrate used here was a
low-resistance substrate of the p type ͑100͒, 0.1 ⍀ cm. The mea-
surement was carried out at a resolution of 1.0% of relative humid-
ity. The current-voltage ͑I-V͒ test was measured in the accumulation
region. Pore size distribution was calculated by small angle X-ray
scattering ͑SAXS͒. Measurements of elastic modulus were carried
out using nanoindentation.
(a)
CH3
Si
CH3
Si
O
H
H
Si
Si
H
O
H
O
CH3
Si
CH3
O
O
O
H3C
O
H3C
O
O
Si
Si CH3
Si
Si CH3
O
H
H
O
Film Properties
Si
Si
Si
Si
Si
O
Si
O
O
O
O
O
O
O
O
Chemical bonding of pure silica zeolite films was measured by
transmission Fourier transform IR ͑FTIR͒ spectroscopy. Figure 5
shows a comparison of the FTIR absorbance spectra between pure
silica zeolite and porous silica films in vacuum. The existence of
zeolite was confirmed by the absorption peak around 560 cm−1
which is associated with an asymmetric stretching mode of five-
membered ring blocks in MEL-type zeolite skeletal.16 This absorp-
tion band was absent in porous silica.
O
O
O
O
O
O
O
O
O
O
The film thickness and refractive index were measured by spec-
troscopic ellipsometry. The pure silica zeolite film and porous silica
film thicknesses ranged from 210 to 260 nm. The differences of re-
fractive index and porosity on the surfactant concentration of the
zeolite film and porous silica film were examined. Figures 6a and b
show the dependence of refractive index and porosity on surfactant
concentration, respectively. The porosity x was calculated by using
Lorentz–Lorenz’s expression
CH3
Si
H
H3C
H3C
Si
O
CH3
O
H
H3C
Si
(b)
Si
O
O
Si
O
Si
O
O
O
CH3
CH3
O
O
O
O
O
H3C Si
O
O
H3C
O
O
Si
Si
Si
O
Si
Si
H3C
Si
H3C
H3C
O
Si
O
O
Si
Si
Si
Si
Si
O
O
O
O
O
O
O
x = 1 − ͓͑n2 − 1͒/͑n2 + 2͔͒/͓͑nS2iO − 1͒/͑n2SiO + 2͔͒
͓1͔
O
O
O
O
O
O
O
O
O
O
2
2
where n and nSiO are refractive indexes of dielectric film and SiO2,
respectively. The2porosities of zeolite and porous silica increased as
the surfactant concentration increased. The porosity increased
greatly by 4.66 wt % of surfactant concentration, and it saturated in
Figure 3. ͑Color online͒ Schematic illustration of silylation. ͑a͒ Adsorption
of TMCTS. ͑b͒ Its cross-linking.
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H100
Journal of The Electrochemical Society, 156 ͑2͒ H98-H105 ͑2009͒
Figure 7. ͑Color online͒ Pore size distribution of zeolite films which is
9.31% surfactant concentration with and without silylation.
Figure 5. ͑Color online͒ FTIR spectra of pure silica zeolite and porous silica
films.
9.31 wt % of it. The porosity increased up to 52.6% with increasing
the surfactant, while it decreased by 10% with TMCTS treatment.
Furthermore, the porosity of zeolite is twice as large as that of po-
rous silica.
The pore size distribution was calculated from SAXS measure-
ments. The pore distribution with 9.31% surfactant concentration
was confirmed at about 4 nm. However, zeolite micropores around
0.5 nm could not be identified by SAXS. The average pore size
increased with increasing surfactant concentration. The increase of
the surfactant concentration increased porosity and pore sizes. The
decrement of porosity by the TMCTS treatment was understood
from porosity and pore size distribution as shown in Fig. 6 and 7.
The porosity by the TMCTS treatment was decreased from
4.512 to 4.399 nm, Fig. 7. This is because pore surfaces have cross-
linked TMCTS, which practically fill the pores.
FTIR absorbance spectra of zeolite and porous silica films with
and without silylation are shown in Fig. 8. The measurements were
performed in vacuum to remove the physically adsorbed moisture. A
broad absorption in zeolite around 3513 cm−1, which is associated
with the O–H group, was less than that of porous silica film. Both
the isolated Si–OH at 3745 cm−1 and the O–H group absorptions
were dramatically decreased by the silylation treatment as shown in
Fig. 8a. Furthermore, the presence of C–H, SiHCH3O2, and SiHO3
bonds were exhibited in Fig. 8b, indicating that the film became
hydrophobic. Figures 9a and b show dependences of O–H groups
and Si–OH groups of zeolite films on surfactant concentrations
without silylation and with silylation, respectively. The broad O–H
area decreased as the surfactant concentration increased as shown in
Fig. 10. Furthermore, the O–H area and the Si–OH area decreased
approximately 1/5 by silylation with the TMCTS.
Figure 11 shows that the dielectric constant decreased with in-
creasing surfactant concentration because the O–H area decreased as
the surfactant concentration increases, shown in Fig. 10. Pure silica
zeolite films show lower dielectric constants than porous silica films.
It is thought that the O–H area of the zeolite is smaller than that of
porous silica from Fig. 8a. Furthermore, the Si–OH and O–H area
decreased dramatically by silylation as shown in Fig. 10, so the
dielectric constant decreased from 2.206 to 1.962. Therefore, dielec-
tric constant depends on Si–OH and O–H area.
It is understood that the dielectric constant decreases by the TM-
CTS treatment, and the elastic modulus and hardness of the pure
silica zeolite film increased as shown in Fig. 12. The dielectric con-
stant of 1.96 was achieved with the elastic modulus of 5.18 GPa for
TMCTS silylated porous zeolite film. The silylation treatment pos-
sibly passivates mechanically weak defects on the pore wall surfaces
Figure 6. ͑Color online͒ Dependence of surfactant concentration for pure
silica zeolite and porous silica films with and without silylation. ͑a͒ Refrac-
tive index vs surfactant concentration. ͑b͒ Porosity vs surfactant concentra-
tion.
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Journal of The Electrochemical Society, 156 ͑2͒ H98-H105 ͑2009͒
H101
Figure 9. ͑Color online͒ FTIR spectrum of pure silica zeolite film. ͑a͒
Si–OH formation without TMCTS silylation. ͑b͒ Si–OH formation with TM-
CTS silylation.
Figure 8. ͑Color online͒ FTIR spectra. ͑a͒ Pure silica zeolite and porous
silica films with and without silylation around 3000 cm−1. ͑b͒ Pure silica
zeolite film with TMCTS silylation in an expansion scale around 2200 cm−1
.
2
ͱ
JS = CRDT exp͕͑␤S E − q␾ ͒/kBT͖
͓2͔
S
where CRD is the Richardson–Dushman constant, T is the absolute
temperature, E is the electric field, q is the charge of electron, ␾ is
by forming a polymerized network as shown in Fig. 3. The bridged
structure was formed by the TMCTS treatment so that the mechani-
cal strength of the film increased.15
S
Electrical Current Characteristic
The leakage current of the pure silica zeolite films was lower
than that of porous silica film by 1 order of magnitude and decreased
with increasing the surfactant concentration as shown in Fig. 13.
The effect on the reduction of leakage current is improved by sily-
lation with TMCTS. The leakage current decreased 4 orders of mag-
nitude at 2 MV/cm. The decrement of the leakage current is attrib-
uted to the reduction of OH-related absorption, resulting in a
dramatic decrease in the ionic current component.2 The dielectric
constant and leakage current of pure silica zeolite films were lower
than those of porous silica films under the present process condi-
tions.
The temperature dependence of leakage current vs electric field
was examined in Fig. 14. Figures 14a-d show the temperature de-
pendency of the leakage current vs the electric field with and with-
out silylation for the surfactant concentrations of 9.31, 4.66, 2.33,
and 0 wt %. JS is expressed in the Schottky emission ͑SE͒ model as
Figure 10. ͑Color online͒ Dependence Si–OH area and OH area on surfac-
tant concentration with and without TMCTS silylation.
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Journal of The Electrochemical Society, 156 ͑2͒ H98-H105 ͑2009͒
Figure 11. ͑Color online͒ Dielectric constants vs surfactant concentration for
pure silica zeolite and porous silica films with and without TMCTS silyla-
tion.
3
ͱ
the barrier height, kB is the Boltzmann constant, ␤S is q /4␲␧ ␧r,
o
where ␧ is the dielectric constant of air and ␧ is relative dielectric
o
r
constant. The leakage current is dominated by the Poole–Frenkel
͑PF͒ mechanism, JP, which is described as
2
ͱ
JP = CtE exp͕͑␤P E − q␾ ͒/kBT͖
͓3͔
P
where Ct is a trap density-related constant, ␾ is the barrier height,
P
3
ͱ
and ␤P is q /␲␧ ␧r. Figures 15a-d show the SE plot for the surfac-
o
tant concentration of pure silica zeolite with and without silylation,
respectively. The slopes of the SE plots are ␤S/kBT. The correspond-
ing PF plots are shown in Fig. 16, and the slopes are determined by
␤P/kBT. A linear slope appeared in the region where these mecha-
nisms are applicable. The linear slope changed around 1.0 MV/cm
in the zeolite films without TMCTS treatment as shown in Fig. 15
and 16. The calculated values of ␧ from SE and PF plots at the
r
electric field of 1.0 MV/cm were 1.49–3.39 and 7.27–53.8, respec-
tively. From spectroscopic ellipsometry, the square of the refractive
index n2 of the zeolite films ranged from 1.44 to 1.65, which was
Figure 13. ͑Color online͒ Leakage current vs surfactant concentration for
pure silica zeolite and porous silica films. ͑a͒ With TMCTS silylation. ͑b͒
Without TMCTS silylation.
close to the ␧ of SE, 1.49–3.39. Consequently, the conduction
r
mechanism of the silica zeolite film surpassed 1.0 MV/cm, follow-
ing the SE model rather than the PF model. The zeolite film with
silylation by TMCTS treatment showed neither SE- nor PF-type
conductions below 1.5 MV/cm. The slope changed around
1.6 MV/cm in the zeolite films with TMCTS treatment. The zeolite
film with silylation by TMCTS treatment generates an SE-type leak-
age current when the electric field surpasses 1.6 MV/cm. The SE
model was confirmed for the TMCTS-treated film only at high elec-
tric field. Figure 17 shows an Arrhenius plot of Schottky leakage
current as a parameter of electric field. In the low temperature range,
the leakage current did not depend on temperature very much so the
Schottky mechanism was not dominant. The Schottky barrier height
␾
in the temperature range of 110–230°C varied from
S
0.65 to 1.25 MV/cm as shown in Fig. 18. The Schottky barrier
height increased with increasing the surfactant concentration. The
O–H area decreases as the surfactant concentration increases, so that
the defect density in the zeolite crystal decreases and the Schottky
barrier height increases. The increase in the leakage current is attrib-
uted to the Schottky barrier height lowering because of increasing
O–H area.
Figure 19 shows a Weibull plot for the lifetime of time dependent
dielectric breakdown ͑TDDB͒ at 3.6 MV/cm. The TDDB lifetime of
zeolite film with silylation was improved in comparison with the
porous silica film with silylation by a factor of approximately 2.5.
Figure 12. ͑Color online͒ Elastic modulus and hardness vs dielectric con-
stant for pure silica zeolite and porous silica films with and without silyla-
tion.
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Journal of The Electrochemical Society, 156 ͑2͒ H98-H105 ͑2009͒
H103
Figure 14. ͑Color online͒ Dependence of
leakage current vs electric field on tem-
perature. ͑a͒ Surfactant concentration
9.31 wt %. ͑b͒ Surfactant concentration
4.66 wt %. ͑c͒ Surfactant concentration
2.33 wt %. ͑d͒ Surfactant concentration
0 wt %.
Figure 15. ͑Color online͒ Schottky plot of
silica zeolite films without silylation. ͑a͒
Surfactant concentration 9.31 wt %. ͑b͒
Surfactant concentration 4.66 wt %. ͑c͒
Surfactant concentration 2.33 wt %. ͑d͒
Surfactant concentration 0 wt %.
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Journal of The Electrochemical Society, 156 ͑2͒ H98-H105 ͑2009͒
Figure 16. ͑Color online͒ PF plot of silica
zeolite films without silylation. ͑a͒ Surfac-
tant concentration 9.31 wt %. ͑b͒ Surfac-
tant concentration 4.66 wt %. ͑c͒ Surfac-
tant concentration 2.33 wt %. ͑d͒ Surfac-
tant concentration 0 wt %.
Figure 17. Arrhenius plot of Schottky
leakage current. ͑a͒ Surfactant concentra-
tion 9.31 wt %. ͑b͒ Surfactant concentra-
tion 4.66 wt %. ͑c͒ Surfactant concentra-
tion 2.33 wt %. ͑d͒ Surfactant concentra-
tion 0 wt %.
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Journal of The Electrochemical Society, 156 ͑2͒ H98-H105 ͑2009͒
Conclusions
H105
The dielectric constant and leakage current of pure silica zeolite
film confirming hydrothermally crystallized zeolite nanoparticles
were lower than those of a conventional porous silica film. A film
containing pure silica zeolite has a dielectric constant of 1.96, an
elastic modulus 5.18 GPa, and a leakage current less than 1
ϫ 10−8 A/cm2 after silylation with TMCTS. The leakage current
mechanism of the mesoporous pure silica zeolite film was found to
be Schottky emission and the Schottky barrier height increased with
increasing the surfactant concentration, resulting in the decrease of
the leakage current. The TDDB lifetime of pure silica zeolite film
was improved by forming the zeolite crystal in the film. The TM-
CTS treatment is effective not only for reducing leakage current but
also for increasing the mechanical strength of porous zeolite by
forming the polymerized network which eliminates water adsorption
and passivates defects on the pore wall surfaces.
Hiroshima University assisted in meeting the publication costs of this
article.
Figure 18. Schottky barrier height vs surfactant concentration for pure silica
zeolite films without silylation.
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Figure 19. Weibull plot of TDDB lifetime at 3.6 MV/cm in 200°C for silica
zeolite and porous silica films with silylation.
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