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Sensors and Actuators B 134 (2008) 390–395
Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical
journal homepage: www.elsevier.com/locate/snb
Moisture measurement using porous aluminum oxide coated microcantilevers
Prathima Kapa a , Liu Pan a , Arogydeepika Bandhanadham a,b , Ji Fang a ,
Koday Varahramyan a , Wes Davis b , Hai-Feng Ji a,∗
a
b
Institute for Micromanufacturing, Louisiana Tech University, Ruston, LA 71272, USA
Sensidyne Inc. Clearwater, FL 33760, USA
a r t i c l e
i n f o
Article history:
Received 4 December 2007
Received in revised form 13 May 2008
Accepted 13 May 2008
Available online 21 May 2008
Keywords:
Microcantilever
Sensor
Porous aluminum oxide
Surface stress
Resonance frequency
a b s t r a c t
Two types of aluminum oxide (Al2 O3 ) modified microcantilevers (MCLs) were tested for their sensitivity and reproducibility for detection of low levels of moisture. Aluminum was sputtered on the MCLs and
oxidized to Al2 O3 through thermal oxidation (Method I) and anodization method (Method II). Both deflection and frequency change of the MCLs were investigated. Thermally treated MCLs showed a 16 ± 2 nm
deflection and a 25 Hz change in resonant frequency, i.e., a mass change of 1.85 × 10−11 g, to a 200 ppm
level of moisture. MCLs from Method II showed a 95 ± 5 nm deflection and a 340 Hz change in resonant
frequency, i.e., a mass change of 41.76 × 10−11 g, to the 200 ppm level of moisture. The MCLs’ response time
to moisture was less than 3 min using the deflection mode, and less than 25 s using the frequency method.
The MCLs prepared by Method II demonstrated a higher sensitivity toward moisture measurement due
to the larger surface area of the porous Al2 O3 coating. The comparison also suggests that the frequency
responses of microcantilever sensors to moistures are much faster than the deflection responses. These
sensors were stable for months stored under ambient conditions. The bending amplitudes and frequency
change were proportional to the moisture level, and the detection of moisture was not affected by alcohols
in the environment.
© 2008 Elsevier B.V. All rights reserved.
1. Introduction
Current techniques to measure low-level water vapor content
include cooled (chilled) mirrors, electrolytic cells, oscillating crystals, infrared absorption, metal oxide or polymer capacitive films,
etc. [1,2]. The prices range from a couple of thousand to tens of
thousands US dollars. Besides these devices, length-of-stain tubes
occupy the low end of technology. These detector tubes are quick,
cost-effective, and convenient to operate, but the trade-off is its low
accuracy.
Each device has their advantages and disadvantages. The optical devices suffer from higher cost and interference from alcohols
although they are the only devices that can handle high corrosive
gases since these devices do not directly contact with gases. In general, other devices are less sensitive than optical devices, but more
cost-effective. The disadvantages of all these practical moisture
measurement systems demand a sensor which withstands corrosive and contaminating gases, a sensor which is sensitive enough
to sudden and drastic changes of moisture, and a sensor which has
a higher calibrated life period.
∗ Corresponding author. Present address: Department of Chemistry, Drexel University, Philadelphia, PA 19104, USA. Tel.: +1 318 257 5125; fax: +1 318 257 5104.
E-mail address: hji@chem.latech.edu (H.-F. Ji).
0925-4005/$ – see front matter © 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.snb.2008.05.014
Advances in the field of micro-electro-mechanical systems
(MEMS) now offer unique opportunities to design sensitive
and cost-effective analytical methods. Recently, microcantilevers
(MCLs) have been proven to be an attractive platform for sensors with on-chip electronic circuitry and extreme sensitivity [3,4].
Because the micromechanical aspects of the MCL can be integrated
with on-chip electronic circuitry, it provided an outstanding platform for chemical [5,6] and biological sensors [7,8]. Extremely
sensitive chemical vapor sensors based on MCLs have been demonstrated using selective coatings on the MCLs.
Two characteristics of a microcantilever, the deflection and resonance frequency, can be used to detect chemicals.
MCLs operating in the dynamic mode, i.e., resonance frequency
mode, are essentially mechanical oscillators. From the simple classical models such as simple harmonic motion, the frequency of an
oscillating MCL, f, can be given in equation
1
f =
×
2
k
,
m∗
(1)
where k is the spring constant and m* is the effective mass of the
cantilever [9,10].
The three main factors that affect the change of resonating
frequency of the cantilever are adsorption-induced mass loading,
viscosity of the media induced damping, and environment-induced
P. Kapa et al. / Sensors and Actuators B 134 (2008) 390–395
391
elasticity changes in MCL materials. In many sensing applications, the frequency changes due to viscosity of the medium, and
elasticity changes of the material of the MCLs are insignificant
when operated in air. Hence, the analyte mass binding to the
MCL can be related to a shift in the MCL resonance frequency
from initial frequency, f0 , to final frequency, f1 , as expressed in
equation.
m =
k
42
1
f12
−
1
f0
2
,
(2)
where m is the additional mass.
The static mode, i.e., the deflection mode, was widely used in
studies of MCL sensors. MCLs undergo bending due to molecular
adsorption by confining the adsorption to one side of the cantilever.
Adsorption or intercalation of the analyte will markedly change the
surface characteristics of the MCL, and results in the bending of the
MCL. Using Stoney’s formula [11], the deflection at the end of a MCL,
Z, due to adsorption can be written as:
Z = 3(1 − )
L2
Et 2
␦s,
(3)
where and E are Poisson’s ratio and Young’s modulus for the substrate, respectively, L is the length of the cantilever, t is the thickness
of the MCL, and ␦s is the surface stress.
MCL-based moisture sensors have been developed using SiO2 ,
Si3 N4 , and polymer coatings [12–15]. However, these sensors are
not sensitive enough for ppm level moisture detection. Aluminum
oxide (Al2 O3 ), on the other hand, has been demonstrated highly
selective for moisture measurement [16–18] and an excellent material for measurement of moisture in most industrial gases. Recently,
we reported that Al2 O3 film modified MCLs can be used for sensitive
detection of moisture [19]. The Al2 O3 film was prepared by thermal
oxidation of Al on the MCLs. The adsorption of water molecules on
the thin film resulted in the surface stress on the Al2 O3 film that
deflected the MCL. The sensitivity, temperature effects, and selectivity of the Al2 O3 -modified MCLs for low-level moisture detection
have been discussed. The moisture detection limit was 10 ppm
using the deflection mode, but the frequency change of the MCLs
was not investigated. The sensitivity could be further improved by
fine tuning of the coatings.
It was now well-known that the surface characteristic of the
coatings is critical for the MCL sensing performance. Increasing surface area is a practical approach for enhancing sensitivity of MCL
sensors. For moisture measurement, it is anticipated the porous
Al2 O3 -modified MCLs will have better sensitivity than our previous reported ones. The nano-porous film was developed by an
electroplating method so called anodization process. Aluminum
when anodized in basic or acidic solution with a suitable applied
potential and time would be converted into porous aluminum
oxide [20]. In this paper, we compare the sensitivity for moisture measurement of MCLs modified by the anodized porous
aluminum oxide and the aluminum oxide prepared by thermal
oxidation of aluminum. Both dynamic and static modes were investigated.
2. Experimental
2.1. Materials
In our experiments, we used commercially available silicon
MCLs (Veeco Instruments, Santa Barbara, CA). The dimensions of
the V-shaped silicon MCLs were 180 ␮m in length, 25 ␮m in leg
width, and 1 ␮m in thickness. Both sides of these MCLs were originally covered with a thin film of chromium (3 nm) and followed
Fig. 1. SEM image of (A) thermal oxided Al2 O3 and (B) porous Al2 O3 on MCLs.
by a 20-nm layer of gold, both deposited by e-beam evaporation.
One side of MCL was then deposited by a layer of 500-nm-thick aluminum (Al). The Al films were oxidized to Al2 O3 using two different
methods. For Method I, the Al film was oxidized by oxygen in a high
vacuum chamber while oxygen gas was flowing through at 100 ◦ C.
A SEM picture of the thermal oxided Al2 O3 coating was shown in
Fig. 1A. For Method II, aluminum was converted to nano-porous
aluminum oxide by an anodization process [21]. The aluminum
on the MCLs was anodized in a 0.1 M oxalic acid solution with
a voltage of 40 V and a current density of 0.5 mA/cm2 for 8 min.
This condition was applied to generate pores at smaller diameter for larger adsorption capacity. The Al-coated MCL was used as
the anode and a Pt wire was used as the cathode. The aluminum
films with thickness varied from 100 to 500 nm were investigated
to study the thickness effects on sensor sensitivity. However, only
aluminum with a 500 nm thickness was found effective in producing pores, and no porous Al2 O3 were formed from the aluminum
films with the thickness of 100, 200, and 300 nm. Fig. 1 is a SEM
image of the porous Al2 O3 film with 500 nm in thickness on a MCL
after the anodization process. After anodization, the thickness of
the Al2 O3 decreased to 125 nm. The pores were clearly seen on the
aluminum grains. The average diameter of the pores was approximately 7 nm. Because of the Al2 O3 layer was relativity thin, we
used the same Young’s modulus and spring constant of the silicon cantilevers for a simplified calculation although errors are
expected.
392
P. Kapa et al. / Sensors and Actuators B 134 (2008) 390–395
Fig. 2. Deflection of Al2 O3 -modified MCLs prepared by the two methods versus time at various ppm of moisture in nitrogen at 25 ◦ C. For comparison, the Y-axis ranges in
both figures are from −30 to 60 nm.
2.2. Gas system
3. Results and discussions
Dry nitrogen was used as the carrier gas for sensing validation.
Dry nitrogen was passed through a gas bubbler containing distilled
water used to generate wet gas. Dual stage gas regulators for wet
and dry gases controlled the gas flow into a gas mixing setup. The
desired moisture level was obtained by controlled mixing of the
dry and wet gases. The magnetic heater and thermometer as well
as a water bath provided the temperature control of the vapor generation system at 25 ◦ C. The moisture level of the final mixture was
measured using a Meeco Waterboy moisture meter (Warrington,
PA) with a range of 1–5000 ppm and an accuracy of ±5%. A Cole
Parmer model 00122QA pressure gauge was used for continuous
pressure monitoring of gas flow to maintain the pressure of the gas
at a desired level of 40 psig. The flow rate of the gas inside the cell
was 100 mL/min. The volume of the sample glass cell including the
plumbing was 0.5 cm3 , thus ensuring fast exchange of gases. Typically 10–20 min will be needed to stabilize the MCL to reach a stable
baseline prior to the measurement.
Both static and dynamic modes of MCLs were used to compare
the sensing performance of MCLs prepared from by Method I and
Method II for measurement of low-level moisture.
2.3. Deflection and frequency measurement
A MCL was placed in a flow-through glass cell (Veeco Instruments, Santa Barbara, CA) and dry nitrogen gas was passed through
the cell at a constant 100 mL/min flow rate during each experiment. When the stable baseline was reached the moisture gas was
switched in for testing.
The bending of the MCL was measured by monitoring the position of a laser beam reflected from the gold-coated side of the MCL
onto a four-quadrant atomic force microscope (AFM) photodiode.
We define bending toward the gold side as “upward bending”;
“downward bending” refers to bending toward the Al2 O3 side.
When the adsorption occurs on the Al2 O3 surface, in general, the
upward bending is caused by repulsion or expansion of molecules
on the Al2 O3 surface, which is so called compressive stress.
Frequency tests were made using a reported method that
improved the amplitude of cantilever vibration and Q factor value
by using a mixed positive signal feedback mechanism [22]. A
custom built electronic amplifier was used to amplify the signal detected by the position sensitive photodiode from different
quadrants and to compute the vertical displacement. A resonant
frequency peak was obtained when the phase of the driving signal
and the vertical displacement signal from position sensitive diode
matched and measured using a dynamic spectrum analyzer.
3.1. Deflection of the MCLs
Fig. 2 compares the bending responses of MCLs prepared by
Method I and Method II to 200 ppm moisture in nitrogen at a
100 mL/min flow rate. The moisture gas was switched in at the
marked time. The MCLs underwent upward bending. After approximately 5 min, the dry nitrogen was switched back through the
fluid cell, and the MCL bent downward back to their original positions. The response time to reach equilibrium was approximately
200 s. Fig. 3 shows the MCLs maximum deflection amplitude vs.
the moisture concentration. The maximum deflection amplitudes
of the MCLs were proportional to the concentrations of moisture.
The lowest detectable concentration with Method II was approximately 0.5 ppm, which was improved over the MCLs prepared using
Method I with a detection limit at approximately 30 ppm. This sen-
Fig. 3. Deflection amplitude of Al2 O3 -modified MCLs prepared by the two methods
versus the concentration of moisture in nitrogen.
P. Kapa et al. / Sensors and Actuators B 134 (2008) 390–395
sitivity is as competitive as other mature techniques in the market.
The detection limit of moisture is of the 100 ppb to 1 ppm level with
electrolytic technique [23] or optical techniques such as FTIR spectroscopy [24] or semiconductors [25]. It is noteworthy that in our
previous studies using the method I, the sensitivity we obtained
was 10 ppm when relatively thinner Al2 O3 film (100 nm in thickness) was used. For direct comparison with the MCLs by Method
II, the aluminum film in MCLs from Method I in this study were
500 nm in thickness.
The larger MCL deflection suggests larger surface stress change
on the MCL surface with porous Al2 O3 coating, which may due to
the larger surface area of the porous structure. For a 96 nm maximum deflection corresponding to 200 ppm of moisture, the surface
stress change was 0.25 N/m according to Eq. (1). The lifetime tests
were conducted on MCLs with 6 months storage under ambient
condition. The deflection of these MCLs showed a similar profile
and bending amplitude to those in Fig. 2.
The effect of alcohols is a major concern of many moisture
meters for low-level moisture detection since the alcohols generally
interfere with moisture detection and cause errors. Our previous
work showed that the Al2 O3 -modified MCLs prepared by Method
I were not affected by alcohols, which qualifies them for accurate
moisture detection without calibration when alcohols exist in the
environment. The potential interference of alcohols on the porous
Al2 O3 -modified MCLs was also evaluated in this study. No deflection of porous Al2 O3 -modified MCLs was observed upon exposure
to 200 ppm alcohol. This result is consistent with our previous
observations, i.e., the MCL response to moisture was not interfered
by alcohols.
3.2. Frequency change of the MCLs
MCL frequency vs. time under different moisture levels was
also investigated. The MCLs prepared by the two methods were
compared. For each method, five MCLs were tested. The frequency
of MCLs prepared from Method I and Method II in dry nitrogen
were 28,870 ± 25 and 23,500 ± 50 Hz, respectively. This initially
lower frequency of MCLs by Method II might be a result of the
lower spring constant of the porous Al2 O3 film prepared by Method
II. MCLs prepared by the same method had a ±3% difference
in their resonance frequencies. The MCLs used in these experiments were triangle silicon MCLs that have a surface area of
1.12 × 10−2 mm2 .
393
Fig. 4 shows the frequency of the MCLs vs. time on exposure
to 200 ppm moisture. The moisture gas was switched in at the
marked time. The response profiles of the two MCLs prepared by
the two methods were quite similar. The frequencies of both MCLs
decreased quickly on exposure to moisture and returned fully to
their initial frequencies when N2 was switched in to replace moisture. Both MCLs showed reproducible response to moisture.
The two MCLs were different in their frequency change percentages and response times. The frequency change of the MCL by
Method I was 25 Hz, or 0.0866% frequency change, corresponding
to 1.85 × 10−11 g of water adsorption on the MCL surface according
to Eq. (2). The MCL prepared by Method II, however, showed a significant 300 Hz frequency decrease, or 1.27% of frequency change,
corresponding to 41.76 × 10−11 g of water adsorption on the MCL
surface. These results demonstrated the enhanced adsorbing capacity of the porous aluminum oxide film due to its larger surface
areas.
The frequency decreases on exposure to moisture reached their
equilibrium in 10 and 15 s for MCLs prepared by Method I and
Method II, respectively. The frequency recovery took approximately
15 and 30 s for MCLs by Method I and Method II, respectively,
when N2 was switched in to replace moisture. These slightly longer
response and recovery time of MCLs prepared by Method II was a
result of longer adsorption and desorption time related to porous
structures of the aluminum oxide film. Although relatively slower,
the response of the MCL sensors prepared by Method II has an
advantage in response time over other methods such as an electrochemistry method, which took approximately 2 min to reach
equilibrium.
A control experiment performed with an Al/Au/Si/Au MCL to
moisture showed no change in resonance frequency.
Fig. 5 shows the MCLs resonance frequency change vs. the vapor
concentration. The frequency changes of the MCLs were proportional to the concentrations of moisture. The frequency decreased
as the concentration of moisture increased. The lowest detectable
concentration by Method II was approximately 1 ppm, which was
more sensitive than those prepared using Method I with a detection
limit at approximately 10 ppm.
Lifetime experiments conducted on MCLs stored for 6 months
under ambient condition showed a similar frequency change profile
to those in Fig. 4.
It is not very clear why it took longer time for the deflection of
the MCLs to reach equilibrium than the resonance frequency. One
Fig. 4. Resonance frequency of Al2 O3 -modified MCLs versus time upon exposure to 200 ppm of moisture in nitrogen. MCLs prepared from the two methods were compared.
For comparison, the frequency ranges in both figures are 600 Hz.
394
P. Kapa et al. / Sensors and Actuators B 134 (2008) 390–395
Fig. 5. Resonance frequency of Al2 O3 -modified MCLs prepared by the two methods versus the concentration of moisture in nitrogen. For comparison, the frequency ranges
in both figures are 400 Hz.
possible explanation is that the water molecules adsorbed on the
MCL surface in the initial stage did not have effective contribution
to the MCL bending since the scattered molecules did not interact with others to generate the surface stress. The water molecules
adsorbed on the MCLs surface in the later stage, although in a
relatively smaller amount, contributed significantly to the surface
stress that bent the MCLs. It took longer time for the molecules in
the later stage to be adsorbed on the surface due to steric effect.
4. Conclusion
MCLs coated by aluminum oxide prepared by thermal oxidation
and anodization process were compared for low levels of moisture
measurement. Both the deflection and frequency change showed
the MCLs prepared by the anodization process were more sensitive than those by thermal oxidation. Porous aluminum oxide
films by the anodization process have a larger surface area and a
higher moisture absorption capacity than that of plain aluminum
oxide films. Comparing to the current moisture detection systems, the microcantilever sensor has a relatively fast response.
Other characteristics including the long-term stability and especially non-interference from alcohol make the cantilever approach
very competitive. Our results showed that porous Al2 O3 -coated
MCLs are excellent sensors for low-level moisture detection and
may be used for moisture monitoring in low-level moisture environment.
Acknowledgements
This work was supported by NSF under SGER ECCS-0643193,
NSF MRI grant 0618291, and Board of Regent Industrial Ties and
Research subprogram under contract number LEQSF(2005-04)-RDB-19.
References
[1] M.J. McDonough Jr., Device for water vapor & hydrocarbon dew point determination, Proc. Int. School Hydrocarbon Meas. 75 (2000) 512–515.
[2] (a) B.J. Mychajliw, Determination of hydrocarbon dew point in natural gas, Proc.
Int. School Hydrocarbon Meas. 79 (2004) 111–113;
(b) B.J. Mychajliw, Devices for water vapor and hydrocarbon dew point determination in Natural Gas, Proc. Int. School Hydrocarbon Meas. 78 (2003) 501–503.
[3] H.P. Lang, M.K. Baller, R. Berger, Ch. Gerber, J.K. Gimzewski, F.M. Battiston, P.
Fornaro, J.P. Ramseyer, E. Meyer, H.J. Guntherodt, An artificial nose based on a
micromechanical cantilever array, Anal. Chim. Acta 393 (1999) 59–65.
[4] T. Thundat, E.A. Wachter, S.L. Sharp, R.J. Warmack, Detection of mercury vapor
using resonating cantilevers, Appl. Phys. Lett. 66 (1995) 1695–1697.
[5] H.-F. Ji, R. Dabestani, E. Finot, T. Thundat, G.M. Brown, P.F. Britt, A novel selfassembled monolayer coated microcantilever for low level cesium detection,
Chem. Commun. (2000) 457–458.
[6] Z.G. Davis, G. Abadal, O. Kuhn, O. Hansen, F. Grey, A. Biosen, Fabrication and characterization of nano-resonating devices for mass detection, J. Vac. Sci. Technol.
B 18 (2000) 612–616.
[7] M. Sepaniak, P. Datskos, N. Lavrik, C. Tipple, Microcantilever transducers: a new
approach in sensor technology, Anal. Chem. 74 (2002) 568A–575A.
[8] R. Raiteri, H.-J. Butt, Measuring electrochemically induced surface stress with
an atomic force microscope, J. Phys. Chem. 99 (1995) 15728–15732.
[9] H.F. Ji, K.M. Hansen, Z. Hu, An approach for detection of pH using various microcantilevers, Sens. Actuators B. Chem. 3641 (2001) 1–6.
[10] P.G. Datskos, Remote infrared radiation detection using piezoresistive microcantilevers, Appl. Phys. Lett. 69 (1996) 2986–2988.
[11] P.R. Muller, R. Kern, About the measurement of absolute isotropic surface stress
of crystals, Surf. Sci. 301 (1994) 386–398.
[12] H.-J. Butt, A sensitive method to measure changes in the surface stress of solids,
J. Collioid Interface Sci. 180 (1996) 251–260.
[13] A.C. Stephan, E.L. Finot, H.-F. Ji, T.G. Thundat, L.F. Miller, R.J. Warmack, Micromechanical measurement of active sites on silicon nitride using free surface
energy, Ultramicroscopy 91 (2002) 1–8.
[14] B.S. Berry, W.C. Pritchet, Bending-cantilever method for the study of moisture
swelling in polymers, IBM J. Res. Dev. 28 (1984) 662–667.
[15] C.Y. Lee, G.B. Lee, Micromachine-based humidity sensors with integrated temperature sensors for signal drift compensation, J. Micromech. Microeng. 13
(2003) 620–627.
[16] L. Westcott, G. Rogers, Humidity sensitive MIS structure, J. Phys. E. Sci. Instrum.
18 (1985) 577–586.
[17] R.S. Jachowitz, Electronics to microelectronics, 4th European Conf. Electrotechnics – EUROCON’80, 1980.
[18] M.G. Kovac, D. Chleck, P. Goodman, A new moisture sensor for in-situ monitoring of sealed packages, Solid State Technol. 21 (1978) 35–39.
[19] X. Shi, Q. Chen, J. Fang, K. Varahramyan, H.-F. Ji, Al2 O3 coated microcantilevers
for detection of moisture at ppm level, Sen. Actuators B 129 (2007) 241–245.
[20] M. Viviani, M.T. Buscaglia, L.P. Nanni, Barium perovskites as humidity sensing
materials, J. Eur. Ceram. Soc. 21 (2001) 1981–1984.
[21] G.Q. Ding, M.J. Zheng, W.L. Wu, W.Z. Shen, Fabrication of controllable freestanding ultrathin porous alumina membrane, Nanotechnology 16 (2005)
1285–1289.
[22] A. Mehta, S. Cherian, D. Hedden, T. Thundat, Manipulation, controlled amplification of Brownian motion of microcantilever sensors, Appl. Phys. Lett. 78
(2001) 1637–1639.
[23] P.J. Eng, T.P. Traintor, G.E. Brown Jr., G.A. Waychunas, M. Newville, S.R. Sutton,
M.L. Rivers, Structure of the hydrated alpha-Al2 O3 (0001) surface, Science 288
(2000) 1029–1033.
[24] B.R. Astallard, L.H. Espinoza, R.K. Rowe, Trace water-vapor detection in nitrogen and corrosive-gases by FTIR spectroscopy, J. Electrochem. Soc. 142 (1995)
2777–2782.
[25] K.F.S. Boumsellek, Instrument design for sub-ppb oxygenated contaminants
detection semiconductor processing, J. Inst. Environ. Sci. 40 (1997) 17–21.
Biographies
Prathima Kapa completed her Bachelors in chemical engineering from Dr. B.V.
Raju Institute of Technology, (JNTU), India, in 2004. She received her Master’s in
molecular sciences and nanotechnology from Louisiana Tech University in 2007.
During Master study, she worked as a graduate research assistant under Dr. HaiFeng Ji and her research interests include sensors, surface modification techniques,
microfabrication methods. Currently, she is working as a process engineer in a
semiconductor-based manufacturing company.
Liu Pan received her BS degree from University of Science and Technology Beijing,
China, in 1998. She is currently a PhD candidate at Institute for Micromanufacturing,
P. Kapa et al. / Sensors and Actuators B 134 (2008) 390–395
Louisiana Tech University. Her research interests focus on microfabrication, MEMS
devices, and microsensor characterization.
Arogydeepika Bandhanadham received her BS degree from Jawaharlal Nehru Technological University, India, in 2006. She is currently a master candidate at Institute
for Micromanufacturing, Louisiana Tech University. Her research interests focus on
developing electrical circuit for microsensors.
Ji Fang received BS degree of electrical engineering in 1965 from Tianjing University,
China. Since 2002, he is a senior research engineer at Institute for micromanufacturing in Louisiana Tech University. His research interests include micro fluidic devices
and system, sensor, microreactor, and total analysis system on chip, optical lens
system and micro/nano fabrication technologies.
Koday Varahramyan received his PhD in electrical engineering, Rensselaer
Polytechnic Institute, in 1983. He has been the Director of the Institute for Micro-
395
manufacturing, Louisiana Tech University and the entergy professor of electrical
engineering. He is currently a professor at Indiana Univeristy-Purdue University
Indianapolis (IUPUI). His research is focused on micro/nano scale processes, materials, devices and systems.
Wes Davis received his BS from Georgia Institute and completed his course work
and requirement for a PhD from Emory University. He is currently the manager of
engineering at Sensidyne, Florida. He has accomplished skills in electronic design,
software and mechanical design.
Hai-Feng Ji received his PhD degree of chemistry from Chinese Academy of Science,
China, in 1996. He is currently an associate professor at Institute for Micromanufacturing, Louisiana Tech University. His research interests focus on MEMS devices,
surface modification, and nanoassembly. He will move to Department of Chemistry,
Drexel University after September 2008.