MEL-type Pure-Silica Zeolite Nanocrystals Prepared by an Evaporation-Assisted Two-Stage Synthesis Method as Ultra-Low-k Materials

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DOI: 10.1002/adfm.200701134

MEL-type Pure-Silica Zeolite Nanocrystals Prepared byan Evaporation-Assisted Two-Stage Synthesis Method asUltra-Low-k Materials**

By Yan Liu, Minwei Sun, Christopher M. Lew, Junlan Wang, and Yushan Yan*

MEL-type pure-silica zeolite (PSZ), prepared by spin-on of nanoparticle suspensions, has been shown to be a promising

ltra-low-dielectric-constant (k) material because of its high mechanical strength, hydrophobicity, and chemical stability. In our

revious works, a two-stage synthesis method was used to synthesize a MEL-zeolite nanoparticle suspension, in which both

anocrystal yield and particle size of the zeolite suspension increased with increasing synthesis time. For instance, at a crystal yield of

3%, the particle size is 80 nm, which has proved to be too large because it introduces a number of problems for the spin-on films,

ncluding large surface roughness, surface striations, and large mesopores. In the current study, the two-stage synthesis method is

odified into an evaporation-assisted two-stage method by adding a solvent-evaporation process between the two thermal-

reatment steps. The modified method can yield much smaller particle sizes (e.g., 14 vs. 80 nm) while maintaining the same

anocrystal yields as the two-stage synthesis. Furthermore, the nanoparticle suspensions from the evaporation-assisted two-stage

ynthesis show a bimodal particle size distribution. The primary nanoparticles are around 14 nm in size and are stable in the final

uspension with 60% solvent evaporation. The factors that affect nanocrystal synthesis are discussed, including the concentration,

H value, and viscosity. Spin-on films prepared by using suspensions synthesized this way have no striations and improved elastic

odulus (9.67� 1.48GPa vs. 7.82� 1.30GPa), as well as a similar k value (1.91� 0.09 vs. 1.89� 0.08) to the previous two-stage

ynthesized films.

1. Introduction

For application of the next-generation microprocessors,

ultra-low-dielectric-constant (ultra-low-k) materials are

required to reduce signal delays, power consumption, and

crosstalk noise between copper wires in the interconnects.[1]

The needs and the projected timeline for implementation of

low-k materials to reduce parasitic capacitance, enable faster

switching speeds, and lower dissipation have been described in

the International Technology Roadmap for Semiconductors.[2]

However, the development of low-k materials has been falling

behind the predicted schedule because of the stringent

requirements of electrical properties and other necessary

characteristics, such as thermal conductivity, mechanical

strength, and hydrophobicity.[3,4] The low-k material currently

[�] Prof. Y. S. Yan, Y. Liu, M. W. Sun, C. M. LewDepartment of Chemical and Environmental EngineeringUniversity of California, RiversideRiverside, CA 92521-0403 (USA)E-mail: [email protected]

Prof. J. L. WangDepartment of Mechanical EngineeringUniversity of California, RiversideRiverside, CA 92521-0403 (USA)

[��] Y. Liu and M. W. Sun contributed equally to this work. The authorsthank the NSF (Grant CTS-0404376), Advanced Micro Devices Inc.,Intel Corporation, and a UC-Discovery Grant for financial support; JieLian is thanked for help with themechanical propertymeasurements;and Mr. Steven McDaniel for assistance in obtaining TEM images.

� 2008 WILEY-VCH Verlag GmbH &

used in chips is carbon-doped silica, and it can have a k value as

low as 3.3, which is lower than that of traditional dense silica

(i.e., k� 3.9).[5,6] While the semiconductor industry is already

able to produce low-k materials with k values between 2.3 and

2.7,[2,7] the solutions to ultra-low-k materials (i.e., k< 2.1) are

still unknown to date. Therefore, ultra-low-k materials have

been a bottleneck for the development of next-generation

chips in the semiconductor industry.

Already investigated as possible ultra-low-k materials,[8–11]

pure-silica zeolites (PSZs) have advantages of uniform

microporosity, high thermal conductivity, superior mechanical

strength,[12] and high hydrophobicity over other low-k material

candidates (e.g., sol–gel silica, mesoporous silica, and poly-

mers[13,14]). In our previous work, we developed PSZ MFI and

PSZ MEL ultra-low-k thin films,[12,15–19] and two film-

deposition processes in situ crystallization and spin-on of the

zeolite nanoparticle suspension. PSZ MEL outperforms

MFI-type PSZ because it has a lower framework density

(17.4 vs. 18.4) and thereby larger microporosity and a lower k

value.[15–19] PSZs with a lower framework density than MEL

zeolites have also been explored for potential low-k applica-

tions. For example, PSZ BEA films were studied by Mintova

et al.[20] More recently, Corma et al. synthesized PSZ LTA,[21]

which offers the exciting possibility to further reduce the

k value. However, a reliable and commercially viable process

to prepare these two PSZs into thin films is still unavailable.

The k value and mechanical strength of zeolite materials as

low-k materials have been demonstrated and one critical need

Co. KGaA, Weinheim Adv. Funct. Mater. 2008, 18, 1732–1738

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Y. Liu et al. / Silica Zeolite Nanocrystals as Ultra-Low-k Materials

Figure 1.Nanocrystal size (Dm) and yield (Y) of the nanoparticle suspen-sions versus the weight percentage of solvent evaporated (Evap) duringthe evaporation process.

for this technology to move forward is a way to produce a

spin-on film that is small and free of striations. The main goal of

the present work is to address this technology challenge via

precise manipulation of the crystallization process of the

nanoparticle suspension so that small particles can be produced

with a high crystal yield. Insights into the crystallization process

are also obtained that may help future efforts in engineering

the yield–particle size relationship for other zeolites such as

BEA and LTA. The crystal yield and crystal size in the zeolite

nanoparticle suspension are both critical parameters affecting

the spin-on film properties. Generally, a higher crystal yield

corresponds to better mechanical strength, lower dielectric

constant, and higher thermal conductivity of the film.

However, larger crystal size corresponds to greater surface

roughness, more serious surface striations, and larger meso-

pores. With the traditional one-stage hydrothermal method,

higher nanocrystal yield is normally achieved by increasing

synthesis time or temperature (i.e., 150–180 8C), but unfortu-

nately this is always accompanied with micrometer or

sub-micrometer crystal size, which is too large for spin-coating.

Previously, a two-stage method was employed to replace the

traditional one-stage method to obtain smaller crystal size and

higher crystal yield.[18,19,22] With this method, a nanocrystal

yield as high as 63% was obtained. However, the nanocrystal

size corresponding to this yield (ca. 80 nm) was considered too

large for producing high-quality spin-on films because it causes

unavoidable radial striation in the film. Moreover, etching very

small features (e.g., <60 nm) on a film composed of 80 nm

particles is a serious challenge.

In this study, a new synthesis protocol for preparing smaller

MEL nanocrystals (i.e., evaporation-assisted two-stage method)

is introduced. An evaporation process is added between the two

thermal-treatment stages, which helps to produce smaller

nanoparticles while holding the nanocrystal yield high. The

mechanism of nanocrystal growth is discussed by investigating

the nanoparticle size distribution. Bimodal distribution is

observed and the primary 14 nm nanocrystals are preserved

in the final suspension with a yield of 63%. To our knowledge,

this is the first time that the synthesis of such small PSZ

nanoparticles with high yield has been reported. Compared to

the previous two-stage synthesis, the new method produces

films with no striations, slightly improved elastic modulus and

hardness, and a similar dielectric constant at around 1.91� 0.09.

Note that in previous work by Hsu et al., an evaporation process

was incorporated into the first-stage of PSZ MFI synthesis to

accelerate crystallization and obtain a smaller particle size.[23]

Our work is different in that the evaporation is added between

the two thermal-treatment stages. Moreover, we were able to

obtain a smaller particle size while maintaining the same

crystallinity as that in the previous two-stage method.

2. Results and Discussion

The nanocrystal size and nanoparticle yield from suspension

are two critical factors influencing the films’ quality for

ultra-low-k applications. Figure 1 shows the intensity-weighted

Adv. Funct. Mater. 2008,18,1732–1738 � 2008 WILEY-VCH Verl

mean particle size, analyzed by dynamic light scattering (DLS)

measurements, and the nanocrystal yield of different samples

as a function of the evaporation amount. The mean particle size

initially increases with a small amount of solvent evaporation,

from 77 nm (E-0) to 88 nm (E-15). It decreases sharply, from

88 nm (E-15) to 61 nm (E-60), when the solvent evaporation is

higher than 15 wt%. For MEL E-60 (see Experimental for

explanation of sample designation MEL E-xx), the mean

particle size is 61 nm, which is much smaller than E-0 (77 nm).

The yield of MEL nanocrystals is maintained at around 63%

regardless of how much solvent is evaporated.

In order to understand the nanocrystal growth mechanism

during synthesis, the nanoparticle size distributions of

as-synthesized suspensions are analyzed by DLS (Fig. 2).

Particle of a selected diameter, their relative intensity, and

cumulative intensity (horizontal line) are shown below the

plots. Relative intensity is relative to the highest intensity, and

cumulative intensity is the cumulative intensity up to the

particle diameter. When the evaporation amount is less than

40 wt%, MEL nanoparticle suspensions have a monomodal

distribution both in intensity-weighted distribution and

number-weighted distribution (Fig. 2a). Once the evaporation

amount is greater than 40 wt%, the nanoparticle size has a

bimodal distribution. The bimodal distribution is shown in two

formats: intensity-weighted and number-weighted profiles.

Figure 2b is the intensity-weighted distribution of E-60. Since

in the intensity-weighted distribution, the population is

proportional to the particle diameter to the power of 4, larger

particles hold higher weight than smaller ones, and as a result

large particles of 60 nm dominate in the distribution. By

contrast, the number-weighted distribution provides the same

weight to different sizes, as shown in Figure 2c. The majority

(98.4%) of the nanoparticles have a size around 14 nm, and

1.6% of particles are about 60 nm.

The particle size and distribution of E-60 suspension from

DLS analysis are also confirmed by transmission electron

microscopy (TEM) images, as shown in Figure 3. In Figure 3a,

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Figure 2. DLS data of the nanoparticle suspensions from the evaporation-assisted two-stage synthesis. a) Number-weighted particle size distributionof MEL E-0, b) intensity-weighted particle size distribution of E-60, andc) number-weighted particle size distribution of E-60.

Figure 3.TEM images of MEL E-60 nanoparticles with scale bars:a) 100 nm, b) 20 nm.

1734 www.afm-journal.de � 2008 WILEY-VCH Verlag GmbH

most crystals are smaller than 20 nm, while a few agglomerates

are around 60 nm. The magnified image (Fig. 3b) shows that

the nanocrystals do not have a regular shape, and the lattices

have different orientations, which is characteristic of the

crystalline structure of these 20 nm particles.

X-ray diffraction (XRD) was also employed to characterize

the crystalline structure of the MEL nanoparticle powder. In

Figure 4a, the XRD patterns verify that the nanocrystals from

different batches all have the MEL structure, regardless of the

amount of solvent evaporated.[24] The Scherrer formula is used

& Co. KGaA, Weinheim Adv. Funct. Mater. 2008, 18, 1732–1738

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Figure 4. XRD patterns and calculated crystal sizes (Dc). a) XRD patternsof powder samples from evaporation-assisted two-stage synthesis sus-pensions, and b) crystal sizes (Dc) calculated from the (501) peak.

here to estimate the mean primary nanocrystal size from the

characteristic (501) peak of the XRD patterns:[25]

L ¼ Kla1

ðbm � b0Þ cos u(1)

where L is the particle size of the sample, K is a constant

(usually K¼ 0.9), la1 is the X-ray wavelength, equal to 1.54060

A for Cu Ka1, bm is the measured full width at half-height of

the peak positioned at 2u, and b0 is the broadening peak due to

the XRD machine itself.[26] As shown in Figure 4b, all the mean

primary nanocrystal sizes fall within a small range of

approximately 12.8–14.5 nm. The particle size first increases

and then decreases with evaporation amount.

The mean primary nanocrystal sizes calculated from the

XRD patterns are in good agreement with those taken from

the DLS and TEM analyses. It is reasonable to conclude that in

the as-synthesized MEL suspension, the primary nanocrystals


are small (e.g., 14 nm) and there are different degrees of

agglomeration in different batches. For the E-0 suspension, all

of the primary particles (about 13.1 nm) agglomerate into

secondary particles (about 77 nm). For the E-60 suspension,

most primary particles (98.4%) do not agglomerate and are

preserved in the final synthesized suspension, although there

are still less than 2% of agglomerated particles. The

agglomerates have a size around 60 nm, which is slightly

smaller than the secondary particle sizes in the E-0 suspension

(77 nm) because of the smaller primary particle size of E-60, as

shown in Figure 4b, which shows that the primary particle size

first increases and then decreases with evaporation amount. All

these results are believed to rise from the favorable changes in

several parameters, including the concentration, pH value, and

viscosity, during the second-stage synthesis.

During the evaporation process, the concentrations of silica

species, structure-directing agent (SDA), and hydroxide anions

all increase. According to a recently published crystallization

mechanism of PSZ with tetraethylorthosilicate (TEOS) as the

silica precursor,[27] the nucleation process starts with core

(silica)/shell (SDA) amorphous nanoparticles (fresh nanopar-

ticles), goes through a series of intermediate phases (mature

nanoparticles) that gradually approach the zeolite-like struc-

tures, and eventually ends up with the perfect zeolite structure

(nuclei). Throughout this process, while their shape and size

remain the same, the nanoparticles are subjected to structure

and chemical composition adjustments via incorporation of

surrounding SDA. The crystal growth is proposed to be the

oriented aggregation of nuclei and attachment of mature

nanoparticles to growing crystals.

The evaporation process between the two thermal-

treatment stages in our method increases the concentration

of the species in the suspension. This would facilitate crystal

growth. On the other hand, more nuclei could also be formed

because of the evaporation-induced supersaturation. These

two processes compete for the mature nanoparticles in

solution. Obviously, the increase in crystal growth rate will

boost the mean particle size in the as-synthesized suspension,

while the increase in nucleation rate will reduce the mean

particle size. The change in particle size reveals that the

aggregation process of mature nanoparticles growing into

crystals dominates when evaporation is less than 30% and the

transformation into nuclei prevails for further evaporation

amounts. When the evaporation amount is less than 30%, the

mature nanoparticles tend to attach to growing crystals during

the second-stage synthesis, and thereby the primary crystal size

increases, as shown in Figure 4a. When the evaporation

amount is greater than 30%, the mature nanoparticles are

likely to transform into nuclei (nucleation reaction) instead.

Although Mintova et al. already observed that nucleation

occurs throughout the entire synthesis of aluminosilicate

zeolite Y,[28] we report this phenomenon for the first time for a

pure-silica-zeolite synthesis.

Another factor that reduces the nanocrystal size is the

change in solution pH value. Figure 5 shows that the pH values

of the solutions at the beginning of the second-stage synthesis


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Figure 5. Solution pH value and viscosity (m) with weight percentage ofthe solvent evaporated (Evap).

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with different amounts of evaporation increase from 11.4 to

12.5. The pH value affects both the repulsive force among

nanoparticles and the reaction for crystal growth in the

solution. Nikolakis[29] studied zeta potential in the tetraalk-

ylammonium silicate solution system with TEOS as the silica

precursor. When the pH value changes from 10 to 12, the zeta

potential decreases from �40 to �60 mV, and as a result, the

particles are highly charged, and the repulsive forces between

nanoparticles increase significantly with pH value. Usually the

nanocrystals of around 14 nm are not stable in the suspension

because of the high surface energies, and instead, they tend to

agglomerate into secondary particles; hence, the mean primary

crystal size of E-0 sample is only 13 nm while the average

particle size in the suspension measured by DLS is 77 nm. For

MEL E-60, the increase in pH value results in a repulsive force

between nanoparticles so high that it is difficult for the particles

to get close to each other, and therefore, the particles are stable

in the final suspension. The mean primary nanocrystal size

estimated by XRD is consistent with the measured values by

DLS and TEM. Bringing the results together, most primary

crystals of 14 nm are preserved in the E-60 as-synthesized

suspension and only a small amount of agglomerated particles

(less than 2%) have sizes around 60 nm.

Furthermore, it is difficult for crystals to grow in high pH

suspension. The reaction formula for the silicon–oxygen–

silicon connectivity is described as:

R� Si� O� þ R0 � Si� OH

, R� Si� O� Si� R0 þ OH� (2)

Obviously, at high pH value, the reaction for crystal growth

is not favored. According to the results in Figure 4, the pH

factor likely plays a major role when the evaporation amount is

greater than 30 wt%.

Finally, the increase in suspension viscosity (Fig. 5) is also

responsible for the particle size decrease. When the suspension

viscosity increases from 20 to 100 cP (1 cP¼ 10�3 Pa s), the

www.afm-journal.de � 2008 WILEY-VCH Verlag GmbH

movement of particles and all the other species will be

increasingly restricted, and the resistance of both oriented

aggregation for crystal growth and agglomeration into

secondary particles should become higher.

According to the discussion above, the dramatic decrease in

particle size is the combined result of the changes in solution

concentration, pH value, and viscosity. When the amount of

solvent evaporated is less than 30 wt%, crystal growth

dominates, and when it is greater than 30 wt%, nucleation

prevails, which is helpful in reducing the particle size. The

increase in pH value from 11.5 to 12.5 results in a higher charge

on the particle surface according to previous work,[29] making

the repulsive force stronger and the nanoparticles more stable.

The reaction pathway also indicates that the crystal growth is

not preferred when pH values are increased. Finally, increasing

the solution viscosity by 80 cP is helpful in increasing the

resistance to agglomeration and crystal growth.

For ultra-low-k applications, two spin-on thin films, one from

the two-stage synthesis (E-0) and the other from the evaporation-

assisted two-stage synthesis (E-60) MEL suspensions, were

prepared. The k values of the E-0 and E-60 films are 1.89� 0.08

and 1.91� 0.09, respectively; essentially the same within the

measurement uncertainty. Therefore, although the synthesis

processes are different, the two films have similar k values

because of the similar yield of nanocrystals.

In terms of other engineering requirements for ultra-low-k

applications, the spin-on film from the evaporation-assisted

two-stage synthesis greatly outperforms that from the previous

two-stage synthesis. According to our experience, smaller

particles are helpful in suppressing or even eliminating

significant issues, such as surface roughness, surface striations,

and large mesopores. Figure 6 shows the optical microscopy

images of an E-60 film compared with a striated E-0 film after

thermal treatment to remove the SDA. With the primary

particle size of 14 nm and only less than 2% of agglomerated

60 nm secondary particles, the surface of the E-60 film is much

smoother than that of the E-0 film. Minimal striation-type

surface texture is observed in the E-60 film. Furthermore, this

particle size distribution eliminates the problem of groove

etching at 65 nm, which is required by industry nowadays.

Another critical requirement for ultra-low-k materials is the

mechanical strength. Low-k materials have to withstand

significant stresses developed during the packaging process

and also have to survive chemical–mechanical polishing during

chip processing.[30] The semiconductor industry has set an

unofficial minimum threshold value of the elastic modulus at

6 GPa.[17] Since the crystal sizes in the MEL E-60 film vary

from 14 nm to 60 nm, much smaller than that of the E-0 film

(77 nm), the nanocrystals are packed closer so that the E-60

film density is higher, which results in a slightly larger elastic

modulus and hardness. The E-60 film has a reduced elastic

modulus of 9.67� 1.48 GPa as compared to 7.82� 1.30 GPa for

the E-0 film, and the respective hardness values are 0.34� 0.05

and 0.30� 0.03 GPa. Compared with the E-0 film, the E-60 film

has 23.7% and 14% improvements in reduced elastic modulus

and hardness, respectively.


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Figure 6. Optical microscopy images of the spin-on films: a) MEL E-0calcined film, b) MEL E-60 calcined film.

3. Conclusions

In this study, we have developed an evaporation-assisted

two-stage synthesis method to prepare an MEL nanoparticle

suspension that can produce small MEL particles (e.g., 14 nm)

at high yield (e.g., 63%). Ultra-low-k films prepared by the

previous two-stage synthesis (E-0) suspension and the

evaporation-assisted two-stage synthesis (E-60) suspension

were obtained and compared. The E-60 film shows better film

surface quality and higher reduced elastic modulus and

hardness than the E-0 film while having a similar dielectric

constant of 1.9. The mechanisms of the nanocrystal growth

during the synthesis and the three factors (i.e., concentration,

pH value, and viscosity) that help to reduce the nanocrystal size

are discussed. The evaporation-assisted two-stage synthesis is a

promising method to prepare MEL-zeolite nanoparticle

suspensions for ultra-low-k film applications, and may be a

general technique for the future efforts to engineer synthesis of

other PSZs for low-k and other applications.


4. Experimental

An MEL-PSZ nanocrystal suspension was synthesized in thefollowing way: 9.15 g of tetrabutylammonium hydroxide (TBAOH,40% aqueous solution, Sachem) and 4.67 g of double deionized (DDI)water were added into 10 g of tetraethylorthosilicate (TEOS, 98%,Aldrich). The mixture was stirred in a sealed plastic bottle for 24 h atroom temperature, and finally a clear homogeneous solution wasformed with the molar composition of 0.3TBAOH:1SiO2:4EtOH:10H2O. The solution was then thermally treated at 80 8C for2 days with constant stirring in an oil bath (noted as the first stage).Afterwards, a specific amount (varying from 10 to 60 wt%) of solventwas evaporated off by a house vacuum at room temperature withstirring. This solution was subsequently transferred to a Teflon-linedautoclave and kept in a convection oven preheated at 114 8C for 24 h(noted as the second stage). The entire procedure was conducted in anair atmosphere. This synthesis approach is hereby referred to as anevaporation-assisted two-stage synthesis method. For convenience, inthis Full Paper, E-xx is used to designate a sample prepared via theevaporation-assisted two-stage synthesis method with xx wt% solventevaporated off (e.g., E-15 means 15 wt% solvent was evaporated). Ifthere is no evaporation process, it is called the two-stage synthesismethod and the resulting MEL suspension is noted as E-0.

To quantify the yield of the nanoparticle suspension, the followingprotocol was devised. The as-synthesized MEL suspension wasdiluted 1:5 (in volume) in DDI water and subject to centrifugationat 20 000 rpm (45 700 g) for 1 h. The separated nanocrystals andsupernatant were dried in an 80 8C oven overnight and calcined at400 8C in air for 2 h to remove the organic structure-directing agent(SDA). The calcined crystal and leftover were weighed (noted as Wc

and Wa, respectively). The yield of the nanocrystals was defined as Wc/(WcþWa)�100%.

Particle size and distribution were measured by dispersing 0.05 mLof as-synthesized suspension in 4 mL of DDI water and analyzed bydynamic light scattering (DLS) with a zeta potential analyzer(ZetaPALS, Brookhaven Instruments, NY). The mean particle sizewas the intensity-weighted average. Both the intensity-weighted distri-bution and the number-weighted distribution of the as-synthesizedsuspension were monitored.

Particle size and crystallinity were observed with both transmissionelectron microscopy (TEM, Philips Tecnai12) with an acceleratingvoltage of 120 kV and powder X-ray diffraction (XRD) (Bruker D8Advanced diffractometer) with Cu Ka radiation. To prepare samplesfor TEM imaging, 1 mL of the E-60 suspension was dispersed into40 mL of ethanol and subject to ultrasonication. Then a drop of theresultant suspension was deposited onto Cu grids coated with a thin(5 nm thickness) holey carbon support film.

To prepare the ultra-low-k thin films, the E-0 suspension was mixedwith an equal mass of butanol and spun onto low-resistance siliconwafers at 3300 rpm for 20 s with an acceleration of 1275 rpm at roomtemperature on a Laurell spin-coater (WS-400A-6NPP/LITE). Inorder to dilute the E-60 suspension, 0.71 g of ethanol, 0.75 g of water,and 2.46 g of butanol were added to every gram of E-60 suspension.After 30 min of ultrasonication, a semi-transparent suspension wasobtained and the uniform suspension was subsequently spun onto asilicon wafer at 3300 rpm for 20 s. As-prepared films were baked in anoven at 80 8C for 8 h in air. Then the films underwent calcination byheating to 400 8C in air at a rate of 1 8C min�1 and were annealed at thistemperature for 2 h before being cooled to room temperature at a rateof 1 8C min�1. The optical microscopy images of these two calcinedfilms were taken with a Nikon Eclipse ME 600 microscope.

Before measuring the k value of the films, silylation was performedto make the films hydrophobic, as described elsewhere [15]. Aluminumdots with a diameter of 1.62 mm were then deposited onto the low-kfilms using evaporation deposition (PAC-1 Pelco Advanced Coater9500) through a contact mask. The backside of the samples wasdeposited with a layer of aluminum by evaporation as well. Thedielectric constants of the films were calculated by measuring


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1738

the capacitance of the aforementioned metal–insulator–metal struc-ture using an Agilent 4285A precision LCR meter combined with aSignatone S-1160 probe station. The thickness of the calcined films wasmeasured on a Jobin Yvon Uvisel spectroscopic ex-situ, phase-modulated ellipsometer (model M200) with a spectral range of240–850 nm and confirmed by scanning electron microscopy (SEM,Philips XL30-FEG). The mechanical properties of the films weremeasured with a Hysitron Ubi1 nanomechanical test instrument with adiamond Berkovich tip.

Received: October 2, 2007Revised: January 9, 2008

Published online: June 6, 2008

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