Properties of liquid-phase deposited silica films for low-k dielectric applications

Article abstract of DOI:10.1111/j.1551-2916.2009.03186.x

Both the electrical and mechanical properties of silica thin films deposited by liquid phase deposition (LPD) have been evaluated in this study. Silica thin films have been prepared on glass surface by immersing it in a supersaturated Hexafluorosilicic ac

Full text of DOI:10.1111/j.1551-2916.2009.03186.x

J. Am. Ceram. Soc., 92 [10] 2388–2391 (2009)
DOI: 10.1111/j.1551-2916.2009.03186.x
r 2009 The American Ceramic Society
ournal
J
Properties of Liquid-Phase Deposited Silica Films for Low-k Dielectric Applications
Liwei Wang, Shijun Yu, and Junghyun Chow
Department of Mechanical Engineering & Materials Science and Engineering Program, State University of New York
(SUNY) at Binghamton, Binghamton, New York 13902-6000
Both the electrical and mechanical properties of silica thin films
deposited by liquid phase deposition (LPD) have been evaluated
in this study. Silica thin films have been prepared on glass sur-
face by immersing it in a supersaturated Hexafluorosilicic acid
(H2SiF6)-based solution at a low temperature of 501C. The as-
deposited LPD silica films exhibit a low dielectric constant (k)
that varies from 1.7 to 2.7 depending on the film morphology and
fluorine content of the film. Young’s modulus of these films was
measured in the range of 18.9–24.5 GPa by a nanoindentation
technique. The combination of extremely low k and fairly high
modulus made this low-temperature-processed LPD silica films
a very promising candidate for an interlayer dielectric film for
the next-generation semiconductor devices.
current, higher dielectric breakdown strength, and lower k than
the CVD silica films, and tested and explored in processing of ICs,
thin film transistors, waveguides, and solar cells.14–19 Furthermore,
the simplicity to process and low-temperature use in the processing
make LPD silica very attractive not only for next-generation
interlayer dielectric films, but also applications that cannot ac-
commodate high-temperature processing and high manufacturing
cost.
II. Experimental Procedure
The precursor was prepared by adding silicic acid powder
(SiO2 Á xH2O) into a 3.2M H2SiF6 solution and dissolved by
stirring overnight at 251C. The residual powder was then re-
moved by centrifugal separation. The films were prepared on the
commercially available F-doped tin oxide (FTO)-coated glass
(Hartford Glass Co. Inc., Hartford City, IN). Before immersing
the substrate, the solution was diluted with DI water to desired
concentration, and 0.005–0.01M boric acid (H3BO3) was added
to facilitate the saturation of SiO2 particle in the solution. Three
samples with the different ratio of the H2SiF6 concentration to
the H3BO3 concentration were prepared. Table I lists the details
of the acid and their concentration ratio used for each sample in
the order of an increasing deposition rate. All the deposition
temperature was set at 501C. In order to avoid the settlement of
precipitated particles and their aggregates, the substrates were
positioned vertically, and the deposition was conducted in a
stepwise fashion, i.e., after every 0.5–1 h of heating, the precur-
sor solution was drained and fresh precursor solution was filled.
The thickness of the film was measured by a profilometer
(Dektac 8 Advanced Development Profiler, Veeco Instruments
Inc., Plainview, NY). The morphology of the film was observed
through scanning electron microscope (SEM) (Zeiss Supra 55,
Jena, Germany). All the SEM images were taken with an accel-
erating voltage of 1–2 kV and a working distance around 5 mm.
The SEM cross-section analyses were also conducted to further
confirm the film thickness and structure. In addition, atomic force
microscope (AFM) was used to observe surface roughness.
The mechanical properties of the as-deposited films were
measured by a nanoindentation system (Hysitron TriboInden-
ter, Minneapolis, MN) with a standard Berkovich diamond ind-
enter tip. In particular, Young’s modulus was measured by the
nanodynamic mechanical analysis (nanoDMA) method, which
provides the mechanical properties as a function of time or dis-
placement in a single indentation. The quasi-static load was set
to increase in discrete steps up to a maximum value of 5000 mN
to ensure the displacement into film could be significant enough
to observe the substrate effect. The frequency of the superim-
posed sinusoidal loading was set at 50 Hz, and a dynamic load
of 10 mN was used for all the LPD silica samples.
I. Introduction
With the increasing demands on reducing the feature size in mi-
croelectronics, problems including propagation delay, cross-talk
noise, and power dissipation become significant and seriously
degrade the performance of electronic devices.1–3 Therefore, re-
placing the conventional silica that has a dielectric constant (k)
value of 4.0 with low-k materials as dielectric interlayer in ultra
large-scale integrated circuit (IC) is very critical and inevitable.3–6
Several approaches have been developed for lowering the k of
silica films. Basically, the k can be reduced through a reduction in
electronic polarization or through the introduction of porosity.7
The introduction of porosity is widely studied due to the con-
trollability of pore size and distribution in some deposition pro-
cess, like sol–gel.8–12 However, the high-porosity silica-based film
is at the cost of mechanical strength, which makes the integration
process more difficult and device performance less reliable.
Therefore, introducing the fluorine ion (F), which has the least
polarizability, into the silica film becomes a very promising means
of achieving low-k silica. A variety of deposition techniques
have been used to deposit silica films with F doping. Plasma-en-
hanced chemical vapor deposition introduced F into the
silica film using a fluorine gas source, such as CF4 and SiF4.13
In the sol–gel method, the F ions were introduced by using
hydrofluoric acid (HF) as a catalyst.9,11 The resultant F-doped
silica films showed an obvious decrease in the k from both
processing methods.
In our study, the silica films were prepared by the liquid phase
deposition (LPD) technique14–16 and the fluorine ion was intro-
duced into the films through Hexafluorosilicic acid (H2SiF6). Both
the electrical and mechanical properties of deposited LPD silica
films were characterized. LPD-processed silica films have been re-
ported to possess better electrical properties, such as lower leakage
M. Menon—contributing editor
The capacitance and k of the films were obtained using metal
insulator conductor (Ag/SiO2/FTO-Glass) test structure. Silver
dots with an area around 2 mm2 were patterned on the top sur-
face of the film; the bottom side of the film was in contact with
Manuscript No. 25944. Received March 2, 2009; approved April 27, 2009.
wAuthor to whom correspondence should be addressed. e-mail: jcho@binghamton.edu
2388

October 2009
Communications of the American Ceramic Society
2389
Table I. Precursor Details and the Resultant Film Thickness
and Roughness for Each Prepared Sample
the FTO coating, which served as the bottom electrode. The
capacitance of the aforementioned metal–insulator–conductor
structure was measured by using a HP 4284A LCR meter
(Hewlett Packard, Santa Clara, CA) at a frequency of 1 kHz
and 1 MHz. The k was then calculated from the capacitance, the
film thickness, and the area of the metal electrode.
Deposition
rate
Film
thickness
(nm)
Surface
roughness
(nm)
Chemical concentration
Sample
H2SiF6 (M)/H3BO3 (M)
(nm/h)
LPD_S1
H2SiF6 (0.5)/
H3BO3 (0.01)
H2SiF6 (2.0)/
H3BO3 (0.005)
H2SiF6 (1.0)/
H3BO3 (0.005)
43
82
30076 52.271.3
575720 59.771.8
830730 64.374.1
LPD_S2
LPD_S3
III. Results and Discussion
Figure 1 shows the SEM micrographs for the LPD-silica film
grown on the FTO-coated glass substrate. It was found that a
layer of dense silica film was obtained in all three deposition
cases. Different morphologies, however, were developed due to
the differences in the processing condition; more specifically, a
different ratio of the H2SiF6 to the H3BO3 was added into the
precursor. With 2.0M of H2SiF6 and 0.005M of H3BO3, uni-
formly distributed grain structure was obtained with few parti-
cles adsorbed onto the growing films (Fig. 1(b)). By decreasing
the H2SiF6 concentration to 1.0M at the same H3BO3 concen-
tration, the deposition rate increased, which may be attributed
to the less etching effect from HF on the growing SiO2 film
during deposition. The grain structure was, however, much less
uniform, got more aggregated and rougher, and had more num-
ber of particles adsorbed from bulk precipitation in precursor
solution (Fig. 1(c)). AFM characterization of each film surface
confirms this aspect, as listed in Table I.
118
(a)
By further reducing the H2SiF6 concentration to 0.5M, the
deposition rate was, however, dramatically reduced due to more
contribution from bulk precipitation, and so very slow deposi-
tion occurred with the same H3BO3 concentration. Therefore,
the H3BO3 concentration was raised to 0.01M, and the film de-
position occurred with very fine grain structure (Fig. 1(a)). In
addition, much smaller particles obtained from bulk precipita-
tion were adsorbed onto the growing film. This observation in-
dicates that H3BO3 increases supersaturation of the precursor
solution, and therefore promotes more homogeneous nucleation
of SiO2 particles in solution by scavenging the HF and gener-
ating water.14 On the other hand, the H2SiF6 concentration
controls more on surface reaction and grain structure through
the number of nuclei formed. It is therefore critical to optimize
the concentration ratio of H2SiF6 to H3BO3 to minimize aggre-
gated particles on the growing film surface.
1.0 µm
(b)
Figure 2 shows examples of indentation (reduced) modulus as
a function of displacement from the nanoindentation tests con-
ducted for S1, S2, and S3 samples. As shown in the plot, for all
these samples, there is a gradual increase in indentation mod-
ulus with displacement, implying the substrate effect. In par-
ticular, the thinnest film S1 exhibited the highest modulus and
the largest scattering of data, both of which decreased with the
1.0 µm
(c)
100
90
80
70
60
50
40
LPD_S1_Test 1
LPD_S1_Test 2
30
LPD_S2_Test 1
LPD_S2_Test 2
20
10
0
LPD_S3_Test 1
LPD_S3_Test 2
0
50
100
150
200
250
300
1.0 µm
Displacement (nm)
Fig. 1. Scanning electron micrographs of liquid phase deposited silica
sample: (a) S1, (b) S2, and (c) S3.
Fig. 2. Plot of reduced modulus as a function of displacement for liquid
phase deposited (LPD) silica samples (two examples for each sample).

2390
Communications of the American Ceramic Society
Vol. 92, No. 10
alyzed to determine the modulus of each sample. The results are
shown in Fig. 3(a), where all three displayed similar elastic
moduli around 18.9–24.5 GPa although the S1 sample exhibited
the highest Young’s modulus within the data error probably
because of the highly dense, fine grain structure as shown in
Fig. 1.
(a)
50
40
30
20
10
0
24.5
Figure 3(b) shows the k’s measured at 100 kHz and 1 MHz
with a precision LCR meter. All of the studied LPD silica films
presented a relatively low k compared with most of the silica
films fabricated by other techniques. As illustrated in the column
chart, among all the LPD SiO2 films, sample S2 exhibited the
lowest k value of 1.7, which is very likely an outcome of the F
content in the silica film because the H2SiF6 concentration for
S2 was the highest among all the samples. Even though sample
S3 was prepared with a higher H2SiF6 concentration than sam-
ple S1, the k of S3 was much larger than that of S1. The plau-
sible explanation for this result is that sample S3 was prepared
with the highest deposition rate, and so had little time to etch the
Si–OH network with HF, resulting in less amount of F content
incorporated into the film. This sample also has more data scat-
tering due to rougher surface.
20.8
18.9
LPD_S1
LPD_S2
LPD_S3
(b)
Table II summarizes the k and modulus of SiO2 prepared by
various methods from the literature,21–24 compared with the
values reported in this paper. The low k (ko2.3) along with
sufficient mechanical properties (E418 GPa) for S1 and S2
films makes the LPD-SiO2 films suitable for a low-k film for
such applications as in microelectronics, OLEDs, and flexible
electronics. In addition, this SiO2 film is generated at near
room temperature without incorporating significant void struc-
ture. The dense film structure will be beneficial to provide a
good thermal conduction path to dissipate the generated heat
more efficiently.
4.00
3.50
3.00
2.50
2.00
1.50
1.00
0.50
0.00
100 kHz
1 MHz
2.7
2.4
1.9
1.8
1.7
1.7
IV. Conclusions
Both the electrical and mechanical properties of SiO2 films
deposited by the LPD method have been investigated in this
study. By adjusting the chemical concentrations in the
LPD precursor, film properties can be tuned accordingly. The
as-deposited LPD silica films generally exhibit a low k from
1.7 to 2.7 depending on the processing condition and film
morphology. An ultra low k value of 1.7 obtained is likely
related to the high level of F content in the film. Young’s
modulus of these films varied from 18.9 to 24.5 GPa depending
on the film roughness and structure. The remarkably low k
and fairly high modulus LPD silica films achieved by this low-
temperature process make it into a very promising candidate
for low-k dielectric materials for semiconductor devices and
flexible electronics.
LPD_S1
LPD_S2
LPD_S3
Fig. 3. Column chart presents the Young’s modulus (a) and dielectric
constant (b) for all the studied samples. Note that in part (b), the results
from two frequencies (100 kHz and 1 MHz) are compared.
film thickness. Therefore, direct data reading was not feasible to
obtain the ‘‘true’’ film modulus. One indentation standard
(bISO/DIS 14577-4)20 was adopted to obtain true Young’s
modulus from the film by extrapolating the indentation data
to an infinite thickness. At least, four measurements were an-
Table II. Comparison of Dielectric Constant and Modulus of Liquid Phase Deposition (LPD) Silica Films from this Work with
Literature Values
References
Dielectric constant
Deposition method
Modulus/measurement technique
Homma and Murao15
Zhang and Boyd8
Yu et al.12
3.7–3.9
1.7
2.0
LPD
Photo-induced sol–gel processing
Sol–gel processing
Sol–gel method with catalyst HF
Dip-coating process
Evaporation-induced self-assembly (EISA)
Sol–gel method and spin coating technique
Sputtering
PECVD
PECVD
Evaporation-induced self assembly (EISA)
NA
NA
NA
NA
NA
NA
NA
He et al.9
1.65
Shen et al.10
Farrell et al.22
Zhen et al.11
Maruo et al.21
Shen et al.23
Wang et al.7
Jung et al.24
B2.5
2.3
1.67
1.5–1.7
3.09
B2.4
2.55
1.7–2.7
NA
13.98 GPa/nanoindentation
B4 GPa/nanoindentation
13–14.4 GPa/nanoindentation
This paper
LPD with different ‘‘F’’ content and microstructure 18.9–24.5 GPa/nanoindentation
PECVD, plasma-enhanced chemical vapor deposition.

October 2009
Communications of the American Ceramic Society
2391
13S. J. Ding, D. W. Zhang, J. T. Wang, and W. W. Lee, ‘‘Low Dielectric Con-
stant SiO2: C,F Films Prepared from Si(OC2H5)4/C4F8/Ar by Plasma-Enhanced
CVD,’’ Chem. Vap. Deposition, 7 [4] 142–6 (2001).
References
1M. E. Thomas, ‘‘The Future of Interconnects,’’ Solid State Technol., 41 [2] 44
14H. Nagayama, H. Honda, and H. Kawahara, ‘‘A New Process for Silica
Coating,’’ J. Electrochem. Soc., 135 [8] 2013–6 (1988).
(1998).
2Y. Y. Jin, K. Kim, and G. S. Lee, ‘‘Preparation of Low Dielectric Constant
Silicon Containing Fluorocarbon Films by Plasma Enhanced Chemical Vapor
Deposition,’’ J. Vacuum Sci. Technol. B, 19 [1] 314–6 (2001).
15T. Homma and Y. Murao, ‘‘Properties of Liquid-Phase-Deposited SiO2 Films
for Interlayer Dielectrics in Ultralarge-Scale Integrated Circuit Multilevel Inter-
connections,’’ Thin Solid Films, 249 [1] 15–21 (1994).
3K. Maex, M. R. Baklanov, D. Shamiryan, F. Lacopi, S. H. Brongersma, and
Z. S. Yanovitskaya, ‘‘Low Dielectric Constant Materials for Microelectronics,’’
J. Appl. Phys., 93 [11] 8793–841 (2003).
16J.-S. Chou and S.-C. Lee, ‘‘Improved Process for Liquid Phase Deposition of
Silicon Dioxide,’’ Appl. Phys. Lett., 64 [15] 1971–3 (1994).
4M. Vasilopoulou, A. M. Douvas, D. Kouvatsos, P. Argitis, and D. Davazoglou,
‘‘Characterization of Various Insulators for Possible Use as Low-k Dielectrics De-
posited at Temperatures below 2001C,’’ Microelectron. Reliab., 45 [5–6] 990–3 (2005).
5M. Morgen, E. T. Ryan, J. H. Zhao, C. Hu, T. H. Cho, and P. S. Ho, ‘‘Low
Dielectric Constant Materials for ULSI Interconnects,’’ Annu. Rev. Mater. Sci.,
30, 645–80 (2000).
17C. F. Yeh and C. L. Chen, ‘‘Controlling Fluorine Concentration and Thermal
Annealing Effect on Liquid-Phase Deposited Sio2–xFx Films,’’ J. Electrochem.
Soc., 142 [10] 3579–83 (1995).
18Y. Sakai, H. Norimatsu, Y. Saito, H. Inomata, and T. Mizuno, ‘‘Silica Coat-
ing on Plastics by Liquid Phase Deposition (LPD) Method,’’ Thin Solid Films, 392
[2] 294–8 (2001).
6S. J Martin, J. P Godschalx, M. E Mills, E. O Shaffer, and P. H Townsend,
‘‘Development of a Low-Dielectric-Constant Polymer for the Fabrication of In-
tegrated Circuit Interconnect,’’ Adv. Mater., 12 [23] 1769–78 (2000).
7Y. H. Wang, M. R. Moitreyee, R. Kumar, S. Y. Wu, J. L. Xie, P. Yew,
B. Subramanian, L. Shen, and K. Y. Zeng, ‘‘The Mechanical Properties of Ultra-
Low-Dielectric-Constant Films,’’ Thin Solid Films, 462–463, 227–30 (2004).
8J.-Y. Zhang and I. W. Boyd, ‘‘Low Dielectric Constant Porous Silica Films
Formed by Photo-Induced Sol–Gel Processing,’’ Mater. Sci. Semicond. Process., 3
[5–6] 345–9 (2000).
19T. P. Niesen and M. R. De Guire, ‘‘Review: Deposition of Ceramic Thin Films
at Low Temperatures from Aqueous Solutions,’’ Solid State Ionics, 151 [1–4] 61–8
(2002).
20ISO/DIS 14577-4, ‘‘Metallic Materials—Instrumented Indentation Test for
Hardness and Materials Parameters (Part 4: Test Method for Metallic and Non-
Metallic Coatings),’’ International Organization for Standardization, 2005.
21T. Maruo, S. Tanaka, H. W. Hillhouse, N. Nishiyama, Y. Egashira, and K.
Ueyama, ‘‘Disordered Mesoporous Silica Low-k Thin Films Prepared by Vapor
Deposition Into a Triblock Copolymer Template Film,’’ Thin Solid Films, 516,
4771–6 (2008).
9Z. W. He, X. Q. Liu, D. Y. Xu, and Y. Y. Wang, ‘‘Effect of Annealing on the
Properties of Low-k Nanoporous SiO2 Films Prepared by Sol–Gel Method with
Catalyst HF,’’ Microelectron. Reliab., 46 [12] 2062–6 (2006).
22R. A. Farrell, K. Cherkaoui, N. Petkov, H. Amenitsch, J. D. Holmes, P. K.
Hurley, and M. A. Morris, ‘‘Physical and Electrical Properties of Low Dielectric
Constant Self-Assembled Mesoporous Silica Thin Films,’’ Microelectron. Reliab.,
47 [4–5] 759–63 (2007).
10J. Shen, A. Luo, L. Yao, X. Lin, B. Zhou, G. Wu, and X. Ni, ‘‘Low Dielectric
Constant Silica Films with Ordered Nanoporous Structure,’’ Mater. Sci. Eng. C,
27 [5–8] 1145–8 (2007).
23L. Shen, K. Zeng, Y. Wang, B. Narayanan, and R. Kumar, ‘‘Determination
of the Hardness and Elastic Modulus of Low-k Thin Films and Their Barrier
Layer for Microelectronic Applications,’’ Microelectron. Eng., 70 [1] 115–24
(2003).
11C. M. Zhen, J. J. Zhang, Y. J. Zhang, C. X. Liu, C. F. Pan, and D. L. Hou,
‘‘Fluorinated Nanoporous SiO2 Films with Ultra-Low Dielectric Constant,’’
J. NonCryst. Solids, 354 [32] 3867–70 (2008).
12S. Yu, T. K. S. Wong, X. Hu, and K. Pita, ‘‘Sol–Gel Derived Mesoporous
Silica Films Used as Low Dielectric Constant Materials,’’ Thin Solid Films, 462–
463, 311–5 (2004).
24S.-B. Jung, C.-K. Han, and H.-H. Park, ‘‘Electrical and Mechanical Properties
of Surfactant-Templated Mesoporous Silica Thin Films Using Brig-76 Surfact-
ant,’’ Appl. Surf. Sci., 244, 47–50 (2005).
&