Fabrication of high aspect ratio ferroelectric microtubes by vacuum infiltration using macroporous silicon templates

Article abstract of DOI:10.1111/j.1551-2916.2006.01123.x

Ordered arrays of high aspect ratio (>10:1) ferroelectric Pb(Zr _0.52Ti_0.48)O₃ (PZT) tube structures were fabricated by vacuum infiltration of macroporous silicon (Si) templates. Improved phase purity was achieved when PZT microtubes were pyrolyzed at 300°C and partially released from the Si template to prevent a chemical reaction between the Pb and the Si during subsequent high-temperature crystallization. The free-standing microtubes were crystallized by rapid thermal annealing at 750°C for 1-3 min. Perovskite phase formation was confirmed by X-ray diffraction and transmission electron microscopy methods. Coaxial structures comprised of metallic LaNiO₃, PZT, and Pd layers were also processed to enable future electrical characterization of the ferroelectric microtubes.

Full text of DOI:10.1111/j.1551-2916.2006.01123.x

J. Am. Ceram. Soc., 89 [9] 2695–2701 (2006)
DOI: 10.1111/j.1551-2916.2006.01123.x
r 2006 The American Ceramic Society
ournal
J
Fabrication of High Aspect Ratio Ferroelectric Microtubes by Vacuum
Infiltration using Macroporous Silicon Templates
S. S. N. Bharadwaja,w M. Olszta, and S. Trolier-McKinstry
Materials Research Institute, The Pennsylvania State University, University Park, Pennsylvania 16802
X. Li and T. S. Mayer
Electrical Engineering Department, The Pennsylvania State University, University Park, Pennsylvania 16802
F. Roozeboom
Philips Research Laboratories, Eindhoven, the Netherlands
Ordered arrays of high aspect ratio (410:1) ferroelectric
Pb(Zr0.52Ti0.48)O3 (PZT) tube structures were fabricated by
vacuum infiltration of macroporous silicon (Si) templates. Im-
proved phase purity was achieved when PZT microtubes were
pyrolyzed at 3001C and partially released from the Si template
to prevent a chemical reaction between the Pb and the Si during
subsequent high-temperature crystallization. The free-standing
microtubes were crystallized by rapid thermal annealing at
7501C for 1–3 min. Perovskite phase formation was confirmed
by X-ray diffraction and transmission electron microscopy
methods. Coaxial structures comprised of metallic LaNiO3,
PZT, and Pd layers were also processed to enable future elec-
trical characterization of the ferroelectric microtubes.
top-down approach, precise structures can be processed using
conventional lithography, combined with microfabrication tech-
niques. Pb(Zr0.52Ti0.48)O3 (PZT) nanostructures have been pre-
pared using electron beam and focused ion beam techniques.13,14
However, the walls of these structures were exposed to high-en-
ergy ion bombardment during processing that could induce cry-
stallographic damage or non-stoichiometry. In the bottom-up
approach, many synthesis schemes have been reported for
processing ferroelectric structures. Recently, Suyal et al.15 and
Liu et al.16 reported processing of KNbO3 nanorods using a
hydrothermal process. Urban et al.17 reported processing of sin-
gle crystalline BaTiO3 nanowires using the decomposition of a
barium titanium isopropoxide (BaTi[OCH(CH3)2]6) precursor.
The major disadvantage of this approach is the random posi-
tioning of the particles created.
I. Introduction
The third technique involves replicating the pores of micro-
porous and mesoporous templates such as polymer resists, Si, or
anodized alumina to form high aspect ratio ferroelectric tubes or
rods. The advantage of this synthetic route is that well-aligned
and ordered multicomponent oxide structures can be processed.
Aoki et al.18 reported fabrication of (Pb,La)(Zr,Ti)O3 pillars
0.6–2 mm in lateral dimension and up to 1 mm tall using a sol–gel
infiltration of an e-beam patterned photoresist. Fabrication of
ferroelectric nanotubes utilizing anodized alumina and anodized
titania membranes has also been reported.19 Mishina et al.20 and
Luo et al.21 molded ferroelectric microtubes by infiltrating the
pores of Si templates with PZT solution under room-tempera-
ture and pressure conditions. Recently, Morrison et al.22 report-
ed processing of bismuth layer structure ferroelectric microtubes
by liquid source-misted chemical deposition using Si templates.
Chemical reactions between the ferroelectric and the template
walls, especially at high temperatures, can lead to the formation
of secondary phases that play a crucial role in determining the
final properties of these structures. In particular, many samples
prepared in Si templates showed some residual amorphous
phase.19,20 This paper outlines a process for optimizing phase
purity in crystalline and ordered arrays of high aspect ratio
ferroelectric PZT microtubes using vacuum infiltration of Si
templates. In addition, the processing of potential electrodes,
including LaNiO3 and Pd, as well as coaxial multilayer tube
structures, such as LaNiO3/PZT/LaNiO3/Pd, is described.
HE ability to make piezoelectric devices with complex de-
signed structures enables one to tailor stress and electric
T
field profiles to optimize response.1–3 While fabrication tech-
niques for such solid pillar structures are available for milli- to
micrometer-scale devices,4,5 the processing of micrometer-scale
piezoelectric tube arrays is less well described in the literature.
This paper details the processing of 1–3 microtube air/piezoe-
lectric composites with large aspect ratios (length to diameter
410). Arrays of high aspect ratio piezoelectric structures are
anticipated to offer advantages in a variety of piezoelectric sen-
sors and actuators,6 high-resolution biomedical ultrasound sys-
tems, and tunable photonic applications.7
In addition to device applications, such free-standing micro-
tubes offer the opportunity to study the properties of piezoelectric
thin films without the influence of substrate clamping. Most thin
films are under hundreds of MPa to GPa of in-plane stress as a
consequence of intrinsic growth stresses, lattice misfits in unre-
laxed films, and/or the thermal expansion coefficient mismatch
between the film and the substrate. These stresses can shift the
transition temperature, alter the equilibrium domain structure,
change the order of the phase transition, affect the domain wall
mobility, and shift the phase transition sequence as a function of
temperature.8–11 Eliminating the substrate should greatly reduce
such effects12 and allow investigations of the role of substrate
clamping on factors such as the mobility of domain walls.
Current manufacturing methods to process high aspect ratio
structures at the micrometer scale can be classified into (1) top-
down, (2) bottom-up, and (3) template replication routes. In the
II. Experimental Procedure
M. Kuwabara—contributing editor
(1) Materials
Macroporous Si templates (Philips Research Laboratories,
Eindhoven, the Netherlands, or Norcoda Inc., Edmonton, Can-
ada) consisting of hexagonal two-dimensional (2D) pore arrays
were created by deep reactive ion etching (DRIE). These tem-
Manuscript No. 21252. Received December 14, 2005; approved April 7, 2006.
wAuthor to whom correspondence should be addressed. e-mail: bss11@psu.edu
2695

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Journal of the American Ceramic Society—Bharadwaja et al.
Vol. 89, No. 9
plates have pore diameters and an inter-pore separation of 1.8–
2.0 and 1.5 mm, respectively.
Infiltrate template in PZT
Chemical solution deposition was used to prepare all of the
microtube structures described in this paper. A 2-methoxy-
ethanol-based 0.70–0.75M PZT solution was used to process
the PZT microtubes. The precursors used to prepare the PZT
solution were lead acetate trihydrate (99.99%), zirconium (IV)
propoxide (99.999%), titanium (IV) isopropoxide (99.999%),
and 2-methoxyethanol (199.9%). Details of the PZT solution
processing are given elsewhere.23 A 0.3M LaNiO3 solution was
prepared by mixing a stoichiometric ratio of lanthanum nitrate
hexahydrate (99.99%) and nickel (II) acetate tetrahydrate
(99.99%) in 2-methoxyethanol and refluxing at 1051–1101C
for 3 h. The Pd solution was prepared by mixing poly (D,L-lac-
tide) and palladium (III) acetate (199.9%) in a 1:1 ratio. These
were then dissolved in chloroform (99.8% A.C.S. spectropho-
tometric grade).24 All the chemicals were purchased from Si-
gma-Aldrich (St. Louis, MO).
solution under vacuum
(Until gas bubbling stops)
Remove surface gel layer
Pyrolyze:
(150°C + 300 °C for 1 min.)
Complex oxide structures
in templates
(2) Template Cleaning
Before infiltrating the chemical solutions, the templates were
cleaned to eliminate moisture, and layers of organics and silicon
dioxide (SiO2) present on the sidewalls of the pores. For this, the
templates were first exposed to an oxygen plasma operated at
100 mTorr with an RF power of 80 W for 10 min (Plasma Tech-
nology Inc, Concord, MA) to remove any hydrocarbon polymer
residue that passivates the pore sidewall during Si DRIE. Sub-
sequently, the templates were immersed with a 10:1 buffered
oxide etch (BOE) solution (from J. T. Baker) for 10 min. to re-
move the SiO2 and expose a bare Si surface. A slight vacuum
was drawn over the surface of the BOE solution to remove gas
trapped in the pores and facilitate uniform infiltration of the
etchant. After this step, the templates were taken out of the BOE
solution and rinsed thoroughly with deionized water. Water in-
filtration into the pores was achieved via vacuum infiltration.
Reactive ion etch
((CF4+ O2) for 2 min to remove
surface oxide layer)
Remove template
(XeF2 RIE for ~25 min)
Crystallize
(750 °C 1-2 min. in an RTA)
Fig. 1. Flow chart for fabricating Pb(Zr0.52Ti0.48)O3 (PZT) microtubes
by vacuum infiltration of chemical solutions into the pores of a Si
template.
(3) Processing Technique and Characterization
Figure 1 shows a flow chart of the processing steps for making
high aspect ratio tube structures. Cleaned and dried Si templates
were immersed in a sealed container of PZT solution. The pres-
sure above the solution was reduced to approximately 20 kPa
below atmospheric pressure using a dry vacuum pump to facil-
itate uniform infiltration of the solution into the pores. This
minimized the entrapment of air in the pores that otherwise led
to incomplete infiltration (see Fig. 2).
Bubbling of the solution because of removal of the trapped
gas in the Si pores was observed during vacuum infiltration, and
completion of the infiltration was identified by termination of
bubble formation in the PZT solution. This typically took about
5 min per infiltration. Such bubble formation was not observed
when the above process was repeated with an unpatterned
Pt-coated silicon substrate. After each infiltration, the surface
of the template was cleaned using a cotton swab dipped in 2-
methoxyethanol to remove the excess solution from the surface
of the template. The samples were then pyrolyzed at 3001–3501C
for 1 min in air. This process was repeated several times to build
up the PZT layer in the templates to the desired wall thickness.
Typically, a 300 nm PZT wall thickness was achieved in 17 in-
filtrations.
XeF2 RIE operated at a pressure of 1 Torr with a pulse cycle of
1 min was used to partially release the tubes (Xetch e0; series,
Xactix Inc., Pittsburgh, PA). In most cases, B27 mm of Si was
removed to produce an ordered array of free-standing PZT mi-
crotubes anchored at the bottom. The samples were heat treated
in 99.98% oxygen using a rapid thermal annealer to examine
After PZT infiltration, the microtubes were partially released
from the silicon matrix using one of two methods:
(i) Wet chemical etch: The PZT layer was first heat treated
in a rapid thermal annealer in O2 for 2 min to crystallize it. For
release, a 30 wt% KOH in a water solution maintained at 701C
was used as an etch solution. Templates with crystallized PZT
tubes were immersed in the etchant until the tubes were released
from the surface. The released tubes were rinsed thoroughly with
deionized water several times.
(ii) Reactive ion etching: A brief plasma etch in CF4:O2 was
used to remove any residual gel as well as the native SiO2 layer
from the top surface of the Si template. An isotropic-pulsed
Fig. 2. Scanning electron microscope photograph of dome-shaped
Pb(Zr0.52Ti0.48)O3 (PZT) caps on the silicon pores formed during infil-
tration of PZT at atmospheric pressure. Incomplete and non-uniform
infiltration resulted during this process.

September 2006
Fabrication of High Aspect Ratio Ferroelectric Microtubes
2697
perovskite phase formation as a function of the annealing tem-
perature.
Similar infiltration procedures were used to process LaNiO3
and Pd microtubes to optimize the processing conditions indi-
vidually. The surface of the Si template was cleaned after each
infiltration using cotton swabs dipped in 2-methoxyethanol
(LaNiO3 solutions) and/or chloroform (for Pd solution) to
avoid blocking the pores. Pyrolysis of LaNiO3 layers in the sil-
icon template was performed at 1501C for 2 min, followed by
3001C for 2 min. These tubes were crystallized in oxygen at
temperatures between 6001 and 7501C for 1–2 min using a rapid
thermal annealer (RTA). LaNiO3 was typically crystallized
within the Si template to enable subsequent infiltrations (e.g.,
with PZT). For the Pd microtubes, a three-step solvent removal/
pyrolysis process was used: 1001C for 5 min, followed by 2501C
for 10 min, and finally at 4001C for 5 min. The Pd structures
were crystallized at 5501–7001C for 1–2 min in a nitrogen am-
bient using an RTA. In the case of Pd tubes, the residual Pd gel
layer on the surface of the template was removed using an Ar
ion beam etch after the crystallization.
Fig. 3. Scanning electron microscope photograph of an ordered array
of Pb(Zr0.52Ti0.48)O3 microtubes.
Coaxial microtube structures consisting of LaNiO3/PZT/
LaNiO3/Pd layers were also fabricated using vacuum infiltra-
tion to enable outer and inner electrical contacts to the ferro-
electric PZT tubes. LaNiO3 (pseudo cubic perovskite structure
with a lattice parameter B0.386 nm) was chosen as an outer
shell layer because it is conductive and can be used to integrate
Pb-based ferroelectric thin layers directly on silicon subst-
rates.25,26 The use of this layer thus enables each of the mate-
rials to be crystallized individually while still in the Si template,
which facilitates building concentric electrode/ferroelectric/elec-
trode structures. In this case, the Si templates were first infil-
trated with a 2-methoxyethanol-based 0.3M LaNiO3 solution
following similar processing steps as those for PZT. The LaNiO3
structures were pyrolyzed at 3501–4001C for 5 min, followed by
the heat treatments mentioned above. The PZT solution was
subsequently infiltrated using the same procedure and heat treat-
ments mentioned above. After crystallization of the PZT (lattice
parameter B0.408 nm), another layer of LaNiO3 was processed
in the same manner. After this, an inner Pd layer was prepared.
This layer can act as an etch stop during patterning of the con-
tacts for electrical measurements. After completing build up of
the co-axial multilayer microtube structures inside the silicon
molds, the template surface was cleaned using an argon ion-mill-
ing step, followed by RIE and XeF2 etching as detailed earlier.
The surface morphology of the microtubes was examined us-
ing scanning electron microscopy (SEM; Model: Hitachi S-3500
N, Pleasanton, CA). For tube wall thickness measurements, a
field emission SEM (Model: JEOL 6700F, Peabody, MA) was
used. Crystallization was confirmed via X-ray diffraction (XRD;
Scintag, Sunnyvale, CA) using CuKa radiation for 2y from 201
to 651 with a 0.0251 step size and a 60 s exposure time/step. The
samples were examined using a Philips 420 TEM (Briarcliff
Manor, NY) at an accelerating voltage of 120 keV. To prepare
TEM samples, B180 nm thick tubes were initially dispersed in
ethanol and subsequently a drop of the solution was placed on a
lacy carbon grid.
the tubes produced by vacuum infiltration, the wall thickness
was measured along the length of the tube. This was done by
partially releasing samples, and then breaking off the projecting
tubes to expose the cross section. Controlled released depths
were achieved by removing B1 mm of Si per XeF2 gas pulse. The
layer thickness was measured by field emission SEM. Figure 5
shows the uniformity in the wall thickness of PZT, LaNiO3, and
Pd tubes along the tube length. It can be seen that along the
majority of the length, the tube walls produced are nearly con-
stant in cross section. The thickness at the base is difficult to
measure by this technique. Optical microscopy suggests that
there is an increase in wall thickness near the closed end of the
tube. From the wall thickness measurements, the estimated layer
thickness for PZT, LaNiO3, and Pd tubes per infiltration was
18.170.6 nm, 10 nm, and 20 nm, respectively.
III. Results and Discussion
Using the method of vacuum infiltration in silicon templates,
ordered arrays of ferroelectric structures could be achieved by
annealing partially released structures at elevated temperatures
as shown in Fig. 3. It was found that the use of KOH to release
the PZT microtubes led to pitted sidewalls, probably because of
chemical attack of the PZT by the strong base. To eliminate this
type of damage, samples were released using a XeF2 gas phase
removal of the Si.
Vacuum infiltration allowed the chemical solutions to deposit
evenly at the bottom of the Si template pores. Figure 4 shows
SEM images of PZT and Pd microtubes that were released from
the template by RIE and sonication. It is clear that the bases of
the tubes are closed. In order to characterize the uniformity of
Fig. 4. Scanning electron microscope images of released (a)
Pb(Zr0.52Ti0.48)O3 and (b) Pd microtubes. The closed ends of the tubes
confirm infiltration of solution along the pore length using vacuum in-
filtration. The rippled sidewalls replicate the original pore geometry.

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Journal of the American Ceramic Society—Bharadwaja et al.
Vol. 89, No. 9
Fig. 5. (a) Uniformity of wall thickness as a function of depth for tubes
produced using the vacuum infiltration method. (b) Pb(Zr0.52Ti0.48)O3
(PZT) tube wall thickness as a function of the number of infiltrations.
Figure 6 shows a cross-sectional SEM image of a PZT/Pd
concentric bi-layer structure. For this purpose, 150 nm thick
PZT and 100 nm thick Pd layers were processed using the vac-
uum infiltration method. For cross-sectional SEM images, the
samples were prepared as mentioned above. As can be seen, the
inner Pd layer has large grains compared with the outer PZT
layer. Progressive sectioning of the infiltrated tubes shows the
same features all along the tube length. Thus, well-defined
co-axial structures were achieved. The use of inner and outer
electrodes in tube structures should enable much lower driving
voltages than can be achieved for structures electroded on the
tube ends. In addition, the higher achievable capacitance should
be very useful for electrical impedance matching.
XRD patterns of PZT and LaNiO3 tubes annealed at various
temperatures are shown in Fig. 7. The LaNiO3 tubes were crys-
tallized in the silicon molds before release. Crystallization oc-
curred between 6001 and 6751C in LaNiO3 tube structures.
Complete perovskite phase formation was achieved at 7501C
in an RTA for 1 min in an oxygen ambient. The PZT tubes were
released from the Si template along the majority of their length
to minimize reaction with the Si before annealing them at var-
ious temperatures. As has typically been reported for the
processing of planar films, on heat treatment, a pyrochlore or
fluorite phase develops first.27 Perovskite peaks appeared in PZT
tubes at 7001C. The pyrochlore phase was detected in PZT tubes
annealed below 7501C. As the thermal budget was increased to
7501C for 1 min, all the perovskite structure peaks appeared
along with a second phase peak at 2yB35.91. No such second-
Fig. 6. X-ray diffraction of (a) LaNiO3 and (b) Pb(Zr0.52Ti0.48)O3
(PZT) tubes microtubes as a function of annealing temperature and
(c) PZT tubes deposited after three infiltrations of a LaNiO3 buffer lay-
ers. These bilayers were annealed at 7501C for 1 min.
ary phase was noticed in the X-ray pattern of LaNiO3/PZT
bilayer tubes, suggesting that LaNiO3 served as a barrier layer,
preventing reaction between lead in PZT with the silicon matrix
(Fig. 7(c)). This would be consistent with previous reports for
flat films with LaNiO3 buffer layers.28

September 2006
Fabrication of High Aspect Ratio Ferroelectric Microtubes
2699
Fig. 7. Cross-sectional scanning electron microscopy image of a coaxial
bi-layer Pb(Zr0.52Ti0.48)O3 (PZT)/Pd tube structure. The inner Pd layer
has larger grains compared with the outer PZT layer.
Fig. 9. Selective electron diffraction pattern of Pb(Zr0.52Ti0.48)O3 (PZT)
microtubes from the area in Fig. 8(c). The single crystalline pattern
originates from the large single crystals of PZT, whereas the polycrys-
talline ring is from the smaller pyrochlore phase.
It is interesting to note that the thermal budget required to
crystallize the released tubes was higher than that required to
process well-crystallized planar films.29 Thus, a well-crystallized
sol–gel-deposited planar PZT thin film on a LaNiO3-coated Si
substrate was achieved at 7001C for 1 min using RTA. The dif-
ference in the crystallization temperatures between tubes and
thin films could be a consequence of poor thermal contact be-
tween the released tubes and the underlying substrate.
Bright-field transmission electron microscope (TEM) images
of the microstructures of PZT tubes are shown in Fig. 8. As can
be seen, the microtubes are comprised of two different poly-
crystalline phases (Figs. 8(a) and 8(b)); the average grain size of
the perovskite phase was estimated to be 100–150 nm, whereas
the pyrochlore grains were approximately 5–10 nm in size. The
majority of the tube, as seen in Fig. 8(a), is comprised of the
large perovskite crystals, but the pyrochlore phase is still present
in small amounts throughout the tube. The Moire fringes ob-
served in the high-resolution TEM (Fig. 8(c)) are a result of
small pyrochlore grains overlapping the large perovskite crys-
tallite. A selected area diffraction (SAD) pattern from the PZT
tube in Fig. 8(c) is shown in Fig. 9. The single crystalline dif-
fraction pattern corresponds to the perovskite structure of PZT,
while the polycrystalline ring corresponds to the face-centered
cubic (FCC) structure of the pyrochlore.
Previous research on preparation of high aspect ratio struc-
tures using a Si template utilized a procedure in which the amor-
phous ferroelectric was first annealed at high temperatures and
then released from the template.18–20 However, the reaction be-
tween Si and PZT is detrimental to perovskite phase formation
in PZT, as is shown in Fig. 10. The resulting degradation in
phase purity will certainly affect the properties. In the present
studies, second phase formation in PZT tubes prepared using Si
templates could only be achieved by avoiding direct contact with
the Si during heat treatments. LaNiO3 served as an appropriate
barrier for this purpose.
Fig. 8. Bright-field transmission electron microscopy (TEM) images of (a) a Pb(Zr0.52Ti0.48)O3 microtube and (b) a magnified view of the highlighted
area in (a). These tubes were annealed at 7501C for 1 min after releasing from the silicon mold (c). High-resolution TEM image of the interface between a
perovskite crystallite and an area of pyrocholore phase. The Moire fringes in the image are a result of the small 5–10 nm pyrocholore crystals overlapping
the large perovskite single crystals.

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Journal of the American Ceramic Society—Bharadwaja et al.
Vol. 89, No. 9
tubes. A thin layer of LaNiO3 served as a barrier layer between
the PZT and the Si template for in-mold crystallization, mini-
mizing the development of secondary phases in PZT tubes. Use
of RIE, followed by a XeF2 release, reduced surface damage to
the tubes relative to release in KOH. The perovskite phase for-
mation temperatures of these tubes were found to be signifi-
cantly higher than those for planar thin films. In addition, the
processing of coaxial multilayer tube structures, such as La-
NiO3/PZT/LaNiO3/Pd, is described, along with a method to
expose the inner and outer contacts.
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Figure 11 shows an SEM image of LaNiO3/PZT/LaNiO3/Pd
concentric structures with an exposed inner Pd contact. For this
purpose, following the optimal processing conditions for indi-
vidual LaNiO3, PZT, and Pd layers, concentric multilayer
LaNiO3/PZT/LaNiO3/Pd microtubes were processed. Later,
these tubes were partially released from the Si template using
gas-phase etching. The released parts of the tubes were exposed
to BOE solution for 5 min. to remove the oxide layers and ex-
pose the inner Pd layer.30 Subsequently, the rest of the Si tem-
plate was removed using gas-phase etching as described in earlier
sections. It can be seen that in this way access to inner and outer
electrodes can be achieved. A future paper will report on the
electrical and electromechanical properties of these structures.
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IV. Summary
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In summary, the vacuum infiltration method for fabricating
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September 2006
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2701
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&