Applied Physics Letters, 89(2): pp. 3.Piezoelectric microcantilever sensors (PEMSs) consisting of a piezoelectric layer bonded to a nonpiezoelectric layer offer the advantages of electrical self-actuation and self-detection. Here we report PEMSs 60-300 mu m in length fabricated from 1.5-mu m-thick sol-gel PbZr0.53Ti0.47O3 (PZT). films with a 2 mu m grain size, a dielectric constant of 1600, and a saturation polarization of 55 +/- 5 mu C/cm(2). The PEMSs exhibited up to four resonance peaks with quality factors Q ranging from 120 to 320. In humidity sensing tests, a PEMS with a 60 x 25 mu m PZT/SiO2 section and a 24 x 20 mu m SiO2 extension exhibited 1 x 10(-15) g/Hz mass sensitivity, two orders of magnitude better than the sensitivity of the current PZT PEMS

Shen, Zuyan

Shih, Wan Y.

Shih, Wei-Heng

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APPLIED PHYSICS LETTERS 89, 023506 2006Self-exciting, self-sensing PbZr0.53Ti0.47O3/SiO2 piezoelectric
microcantilevers with femtogram/Hertz sensitivity
Zuyan Shen, Wan Y. Shih,a and Wei-Heng Shih
Department of Materials Science and Engineering, Drexel University, Philadelphia, Pennsylvania 19104
Received 27 March 2006; accepted 22 May 2006; published online 12 July 2006
Piezoelectric microcantilever sensors PEMSs consisting of a piezoelectric layer bonded to a
nonpiezoelectric layer offer the advantages of electrical self-actuation and self-detection. Here we
report PEMSs 60–300 m in length fabricated from 1.5-m-thick sol-gel PbZr0.53Ti0.47O3 PZT
films with a 2 m grain size, a dielectric constant of 1600, and a saturation polarization of
55±5 C/cm2. The PEMSs exhibited up to four resonance peaks with quality factors Q ranging
from 120 to 320. In humidity sensing tests, a PEMS with a 6025 m PZT/SiO2 section and a
2420 m SiO2 extension exhibited 110−15 g /Hz mass sensitivity, two orders of magnitude
better than the sensitivity of the current PZT PEMS. © 2006 American Institute of Physics.
DOI: 10.1063/1.2219994Microcantilever sensors have attracted considerable in-
terest because of their capability for label-free biological
detection.1–12 As a sensor, receptors are immobilized on the
cantilever surface. Binding of target biological or chemical
agents to the receptors on the cantilever surface forces the
cantilever bend1 and its resonance frequency shift.2–12 Detec-
tion of target analytes can be achieved by either measuring
the cantilever tip displacement1 or its resonance frequency
shift.2–12 Piezoelectric microcantilever sensors PEMSs are
microcantilevers consisting of a piezoelectric layer, e.g., lead
zirconate titanate PZT bonded to a nonpiezoelectric layer,
e.g., SiO2. Unlike silicon-based microcantilevers that require
an external actuator for excitation2,3 and an external optical
system for detection,1–3 PEMSs can electrically self-excite
and self-detect. Applying an alternating current ac voltage
across the piezoelectric layer causes the PEMSs to vibrate,
which generates a measurable piezoelectric voltage across
the piezoelectric layer that can be used to monitor PEMS
resonance frequency shift.4,5,7–12 Commercial ZnO-based
PEMSs have been used for in-air vapor detection13–16 by
either monitoring bending13,14 or by monitoring the reso-
nance frequency shift.14–16 PZT PEMSs due to their higher
piezoelectric coefficients have been demonstrated for in situ
biological detections4,5,7,10,11 in addition to in-air chemical12
and ex situ protein detection.8,9 Current 300503 m3
ZnO PEMS and 150502 m3 PZT/Si3N4 PEMS exhib-
ited a mass sensitivity of 10−11–10−12 g /Hz for in-air trini-
trotoluene TNT vapor detection14,15 and 10−13 g /Hz for in-
air ex situ protein detection,8,9 respectively.
The objective of this study is to further increase the mass
sensitivity of PEMSs to the level of femtogram/hertz by re-
ducing the size of PEMSs through microfabrication and ma-
terials synthesis. Smaller cantilevers are known to exhibit
better mass sensitivities: A 3.5 m long silicon nanocantile-
ver exhibited 10−21 g /Hz mass sensitivity in vacuum.17 Fur-
ther studies showed that the mass sensitivity f /m of a
cantilever of length L and width w was related to L and w
as
18 f /mi2 / L3w, where m and f denote the mass
change and the corresponding resonance frequency shift, re-
spectively, and i
2 the dimensionless ith-mode eigenvalue.
This indicates that reducing a cantilever’s dimensions pro-
aElectronic mail: shihwy@drexel.edu
0003-6951/2006/892/023506/3/$23.00 89, 02350
Downloaded 25 Jul 2006 to 129.25.131.237. Redistribution subject toportionally by a factor of  increases its mass sensitivity by
a factor of −4 and that use of a higher-mode resonance peak
increases the mass sensitivity. The difficulty with a reduced
cantilever’s size is that the resonance peak heights quickly
diminish, rendering the cantilever unsuitable for sensing.
Strategies to improve a PEMS resonance peak height include
amplifying the piezoelectric voltage and reducing the noise
with a bridge circuit19 or incorporating on the silicon sub-
strate a piezoelectric patch of the same material and dimen-
sion as the PEMS,20 and/or replacing the ZnO that offers
piezoelectric coefficients in the range of d31=−4 pm/V and
d33=12.4 pm/V,21 with PZT. Most PZT films exhibit piezo-
electric coefficients ranging from d31=−58 pm/V Ref. 20
to d33=190–250 pm/V.22,23 Although these values are much
higher than those of ZnO, they are only about 20%–40% of
those of the bulk commercial PZT,24 due to interfacial diffu-
sion and substrate pinning. As a result, it is challenging to
achieve small PEMSs that can both self-excite and self-
sense. A PZT layer with improved piezoelectric properties
may help reduce the size of self-exciting and self-sensing
PEMSs.
An earlier study25 has shown that using a sol-gel proce-
dure with ethylene glycol as solvent, one could achieve
1.5-m-thick fully dense PZT films that exhibited an aver-
age grain size of 2 m, a dielectric constant of 1600, and a
saturation polarization of 55±5 C/cm2 with a maximum
applied electric field Emax=600 kV/cm, better than the typi-
cal dielectric constant 800–1000 and saturation polarization
20–40 C/cm2 of most PZT films.
In this study, we fabricated self-exciting and self-sensing
PZT/SiO2 PEMSs as small as 60 m in length with a 24
20 m2 SiO2 extension using PZT films obtained with the
above sol-gel procedure. First, a 2-m-thick low-stress SiO2
layer was deposited on a 3 in. silicon wafer by steam oxida-
tion at 850 °C. A 1500-Å-thick platinum bottom electrode
with a 400-Å-thick TiO2 bonding layer was then deposited
by Radiant Technologies Albuquerque, NM. A
1.5-m-thick PZT layer was then deposited. On the PZT
layer, 400 Å titanium followed by 1000-Å-thick platinum
and 5000-Å-thick nickel was deposited by e-gun evaporation
Semicore Equipment, Livermore, CA and patterned by a
photolithographic lift-off process. The nickel layer was used
as the hard mask in the dry etching process. The PZT layer
© 2006 American Institute of Physics6-1
 AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp
023506-2 Shen, Shih, and Shih Appl. Phys. Lett. 89, 023506 2006and the TiO2/Pt bottom electrode were patterned by induc-
tively coupled plasma ICP dry etching Applied Materials
Cluster Tool, Santa Clara, CA. The underside of the silicon
wafer was removed by backside KOH wet etching. The SiO2
membrane was etched by reactive ion etching RIE P5000
MERIE, Applied Materials, Santa Clara, CA to release the
PEMSs. All PEMSs had a 1.5-m-thick PZT layer and a
2.0-m-thick SiO2 layer. Figures 1a and 1b respectively
show the scanning electron microscopy SEM LEO 440,
Carl Zeiss Worldwide, Germany micrograph of a 300
70 m PZT/SiO2 PEMS PEMS-1 and that of a 60
25 m PZT/SiO2 PEMS with a 2420 m SiO2 exten-
sion PEMS-2. Clearly, the PZT layer was dense with a
smooth surface. Figure 1c shows a SEM micrograph of an
ICP-etched PZT surface revealing that the PZT layer had an
average grain size of about 2 m, larger than the PZT thick-
ness. This indicates that the PZT layer was well sintered,
consistent with the earlier results that the PZT film could
withstand an electric field of 600 kV/cm.
Both PEMS-1 and PEMS-2 exhibited a relative dielec-
tric constant of 1500 at 10 kHz as measured by an HP 4275A
multifrequency LCR meter, consistent with the pre-
microfabrication value, 1600, and indicated that the micro-
fabrication process had little damage to the PZT layers. The
PEMSs were “poled” at room temperature by a dc bias volt-
age of 15 V 100 kV/cm for 30 min during the impedance
spectrum scan 4294A, Agilent, Palo Alto, CA. In what fol-
lows, all resonance spectra were taken with a 15 V dc bias.
The resonance frequency spectrum of PEMS-1 is shown in
Fig. 2. As can be seen, PEMS-1 exhibited four resonance
peaks at 17.9, 106.2, 305.7, and 608.6 kHz, corresponding to
the first four flexural vibration modes. The calculated first
four flexural resonance frequencies using a method for
PEMSs of uniform thickness18 were 17.8, 110.4, 307.9, and
612.3 kHz, as marked by the vertical dashed lines. For
PEMS-2, we observed only the first peak at 408 kHz, as
shown in Fig. 3a by the dashed-dotted line at 31.2% rela-
tive humidity, which was in agreement with the theoretical
FIG. 1. a A SEM photograph of a 30070 m PZT/SiO2 PEMS PEMS-
1, b that of a 6025 m PZT/SiO2 PEMS with 2420 m SiO2 ex-
tension PEMS-2, and c that of the top surface of an ICP-etched PZT
layer.
FIG. 2. Resonance spectrum of a 30070 m PZT/SiO2 PEMS PEMS-1.
The vertical dashed lines indicate the theoretical resonance frequencies.
Downloaded 25 Jul 2006 to 129.25.131.237. Redistribution subject tofirst-peak resonance frequency, 412 kHz, obtained using a
model for a two-sectioned PEMS Ref. 30 and marked by
the vertical dashed line. Note that all the resonance peaks
exhibited high quality factors Q ranging from 120 to 320,
where Q was defined as the ratio of the resonance frequency
relative to the resonance peak width at half the peak height.
For mass sensitivity determination, the PEMSs were
placed in a closed probe station Micromanipulator, Carson
City, NV with an initial relative humidity of 68.3% and
temperature of 21.6 °C. The humidity level inside the cham-
ber was reduced by flowing dry nitrogen. Different humidity
levels were achieved by controlling the N2 flow rate. After
adjusting the flow rate, we monitored for 10 min to allow the
humidity level, the temperature, and the resonance frequency
to stabilize. The resonance spectra of PEMS-2 at various
relative humidity levels are shown in Fig. 3a. Clearly, the
resonance peak shifted to a higher lower frequency when
the relative humidity was decreased increased due to the
adsorption desorption of water molecules on the SiO2 sur-
face. In all experiments, the temperature was constant with
less than 0.1 °C variations. In Fig. 3b, we plot the reso-
nance frequency of the third resonance peak of PEMS-1 and
that of the first resonance peak of PEMS-2 versus the humid-
ity where the full open symbols indicate humidity-
increasing decreasing cycles. As can be seen the resonance
frequency shift versus relative humidity was reversible and
the data from both humidity-increasing and humidity-
decreasing cycles collapsed in one single curve for each
PEMS. For the relative humidity change from 60% to 30%,
we obtained a −2400 Hz frequency shift with the third reso-
nance peak of PEMS-1 and a −4600 Hz frequency shift with
the first resonance peak of PEMS-2, corresponding to a sen-
sitivity of 80 and 150 Hz frequency shift per percent humid-
ity change for PEMS-1 and PEMS-2, respectively. To quan-
tify the mass per unit area change due to the humidity
change, a 10 MHz quartz crystal microbalance QCM26
FIG. 3. a The first-mode resonance spectra of PEMS-2 at various humidity
levels and b resonance frequency vs relative humidity. Open filled tri-
angles and open filled circles denote the humidity downsweep upsweep
for the third-mode resonance peak of PEMS-1 and the first-mode resonance
peak of PEMS-2, respectively.Fortiming Co., Marlborough, MA was also placed in the
 AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp
023506-3 Shen, Shih, and Shih Appl. Phys. Lett. 89, 023506 2006humidity chamber. The present PEMS had a platinum sur-
face on one side and a SiO2 surface on the other. For close
comparison, we sputtered Denton II, Denton, Moorestown,
NJ platinum on one electrode of the QCM and coated sili-
con oxide on the other by spin coating 3-mercaptopropol-
silane MPS Alfa Aesar, Ward Hill, MA followed by
cross-linking at pH=11.27 For the same humidity change
from 60% H to 30% H, the 10 MHz QCM exhibited a reso-
nance frequency shift fQCM=50 Hz. Note that the 4600 Hz
shift obtained with PEMS-2 was about two orders of magni-
tude larger than that obtained by the QCM and was also
more than 20 times larger than the 170 Hz shift obtained by
a ZnO PEMS with a humidity shift from the ambient humid-
ity to saturation.16 The mass change per unit area, , can be
deduced from fQCM using the Sauerbrey equation,26
=−fQCMqq /2f2, where f is the natural resonance fre-
quency of the QCM, q=2.9471011 dyn/cm2 and
=2.648 g/cm3 are the shear modulus and density of the
quartz, respectively. The Sauerbrey equation was valid in the
present humidity range in that neither resonance peak inten-
sity change nor deviation in the slope of the resonance fre-
quency versus relative humidity in the QCM was observed in
the humidity range of 30%–60%, indicating no appreciable
viscosity effects for the QCM in this humidity range.28,29
With fQCM=50 Hz =2.210−7 g /cm2 was obtained.
With m /f =Lw /f , we obtained m /f =210−14 and
110−15 g /Hz for the third resonance peak of PEMS-1 and
the first resonance peak of PEMS-2, respectively, about four
to five orders of magnitude more sensitive than the m /f
=810−10 g /Hz for the 10 MHz QCM, three to four orders
of magnitude more sensitive than ZnO-based PEMS,14,15 and
100 times more sensitive than the existing PZT/Si3N4 PEMS
Refs. 8 and 9 for in-air detection applications. The im-
proved sensitivity was in part attributed to the deduced can-
tilever size that contributed a factor of about 50 to the mass
sensitivity improvement of PEMS-2 over that of the
PZT/Si3N4 PEMS Refs. 7–9 and an additional enhance-
ment factor of about 1.5 due to the SiO2 extension design of
PEMS-2.30
In general, both mass changes and spring-constant
changes can contribute to a cantilever’s resonance frequency
shift. For a uniformly distributed mass change m, over the
entire cantilever surface, the mass sensitivity can be ex-
pressed as18 m /fmass=2M / f where M and f are the
mass and resonance frequency of the PEMS, and the sub-
script “mass” denotes the mass-change effect only. With M
=3.310−7 g for PEMS-1 and M =2.510−8 g for PEMS-2,
we obtained m /fmass=2.010−12 g /Hz for the third
resonance peak of PEMS-1 and 1.210−13 g /Hz for the first
resonance peak of PEMS-2. Both were about 100 times less
sensitive than the experimentally obtained values, indicating
that mechanisms other than mass change were operating at
this length scale. Similar 100-times mass sensitivity en-
hancements have also been observed in other PZT/Si3N4
PEMS,7–9 and were attributed to the spring-constant change
stresses.7–9 It is of interest to find out at what cantilever size
the crossover from the mass-loading regime to the
adsorption-stress regime occurs.Downloaded 25 Jul 2006 to 129.25.131.237. Redistribution subject toThis work is supported in part by the National Institute
of Health NIH under Grant No. 1 R01 EB000720 and the
Environmental Protection Agency EPA under Grant No.
R82960401. This publication was supported by the Pennsyl-
vania State University Nanofabrication Facility and the Na-
tional Science Foundation Cooperative Agreement No.
0335765, National Nanotechnology Infrastructure Network,
with Cornell University.
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Self-exciting, self-sensing PbZr0.53Ti0.47O3 /SiO2 piezoelectric microcantilevers with femtogram/Hertz sensitivity

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Self-exciting, self-sensing PbZr0.53Ti0.47O3 /SiO2 piezoelectric microcantilevers with femtogram/Hertz sensitivity

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