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Page 1: Moisture measurement using porous aluminum oxide coated microcantilevers

Sensors and Actuators B 134 (2008) 390–395

Contents lists available at ScienceDirect

Sensors and Actuators B: Chemical

journa l homepage: www.e lsev ier .com/ locate /snb

Moisture measurement using porous aluminum oxide coated microcantilevers

Prathima Kapaa, Liu Pana, Arogydeepika Bandhanadhama,b, Ji Fanga,Koday Varahramyana, Wes Davisb, Hai-Feng Ji a,∗

a Institute for Micromanufacturing, Louisiana Tech University, Ruston, LA 71272, USAb Sensidyne Inc. Clearwater, FL 33760, USA

a r t i c l e i n f o

Article history:Received 4 December 2007Received in revised form 13 May 2008Accepted 13 May 2008Available online 21 May 2008

Keywords:MicrocantileverSensorPorous aluminum oxideSurface stressResonance frequency

a b s t r a c t

Two types of aluminum oxide (Al2O3) modified microcantilevers (MCLs) were tested for their sensitiv-ity and reproducibility for detection of low levels of moisture. Aluminum was sputtered on the MCLs andoxidized to Al2O3 through thermal oxidation (Method I) and anodization method (Method II). Both deflec-tion and frequency change of the MCLs were investigated. Thermally treated MCLs showed a 16 ± 2 nmdeflection and a 25 Hz change in resonant frequency, i.e., a mass change of 1.85 × 10−11 g, to a 200 ppmlevel of moisture. MCLs from Method II showed a 95 ± 5 nm deflection and a 340 Hz change in resonantfrequency, i.e., a mass change of 41.76 × 10−11 g, to the 200 ppm level of moisture. The MCLs’ response timeto 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 dueto the larger surface area of the porous Al2O3 coating. The comparison also suggests that the frequencyresponses 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

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. Introduction

Current techniques to measure low-level water vapor contentnclude cooled (chilled) mirrors, electrolytic cells, oscillating crys-als, infrared absorption, metal oxide or polymer capacitive films,tc. [1,2]. The prices range from a couple of thousand to tens ofhousands US dollars. Besides these devices, length-of-stain tubesccupy the low end of technology. These detector tubes are quick,ost-effective, and convenient to operate, but the trade-off is its lowccuracy.

Each device has their advantages and disadvantages. The opti-al devices suffer from higher cost and interference from alcoholslthough they are the only devices that can handle high corrosiveases since these devices do not directly contact with gases. In gen-ral, other devices are less sensitive than optical devices, but moreost-effective. The disadvantages of all these practical moisture

easurement systems demand a sensor which withstands corro-

ive and contaminating gases, a sensor which is sensitive enougho sudden and drastic changes of moisture, and a sensor which hashigher calibrated life period.

∗ Corresponding author. Present address: Department of Chemistry, Drexel Uni-ersity, Philadelphia, PA 19104, USA. Tel.: +1 318 257 5125; fax: +1 318 257 5104.

E-mail address: [email protected] (H.-F. Ji).

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moisture level, and the detection of moisture was not affected by alcohols

© 2008 Elsevier B.V. All rights reserved.

Advances in the field of micro-electro-mechanical systemsMEMS) now offer unique opportunities to design sensitivend cost-effective analytical methods. Recently, microcantileversMCLs) have been proven to be an attractive platform for sen-ors with on-chip electronic circuitry and extreme sensitivity [3,4].ecause the micromechanical aspects of the MCL can be integratedith on-chip electronic circuitry, it provided an outstanding plat-

orm for chemical [5,6] and biological sensors [7,8]. Extremelyensitive chemical vapor sensors based on MCLs have been demon-trated using selective coatings on the MCLs.

Two characteristics of a microcantilever, the deflection and res-nance frequency, can be used to detect chemicals.

MCLs operating in the dynamic mode, i.e., resonance frequencyode, are essentially mechanical oscillators. From the simple clas-

ical models such as simple harmonic motion, the frequency of anscillating MCL, f, can be given in equation

= 12�

×√

k

m∗ , (1)

here k is the spring constant and m* is the effective mass of theantilever [9,10].

The three main factors that affect the change of resonatingrequency of the cantilever are adsorption-induced mass loading,iscosity of the media induced damping, and environment-induced

Page 2: Moisture measurement using porous aluminum oxide coated microcantilevers

Actuators B 134 (2008) 390–395 391

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lasticity changes in MCL materials. In many sensing applica-ions, the frequency changes due to viscosity of the medium, andlasticity changes of the material of the MCLs are insignificanthen operated in air. Hence, the analyte mass binding to theCL can be related to a shift in the MCL resonance frequency

rom initial frequency, f0, to final frequency, f1, as expressed inquation.

m = k

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here �m is the additional mass.The static mode, i.e., the deflection mode, was widely used in

tudies of MCL sensors. MCLs undergo bending due to moleculardsorption by confining the adsorption to one side of the cantilever.dsorption or intercalation of the analyte will markedly change theurface characteristics of the MCL, and results in the bending of theCL. 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

Et2

]�s, (3)

here � and E are Poisson’s ratio and Young’s modulus for the sub-trate, respectively, L is the length of the cantilever, t is the thicknessf the MCL, and �s is the surface stress.

MCL-based moisture sensors have been developed using SiO2,i3N4, and polymer coatings [12–15]. However, these sensors areot sensitive enough for ppm level moisture detection. Aluminumxide (Al2O3), on the other hand, has been demonstrated highlyelective for moisture measurement [16–18] and an excellent mate-ial for measurement of moisture in most industrial gases. Recently,e reported that Al2O3 film modified MCLs can be used for sensitiveetection of moisture [19]. The Al2O3 film was prepared by thermalxidation of Al on the MCLs. The adsorption of water molecules onhe thin film resulted in the surface stress on the Al2O3 film thateflected the MCL. The sensitivity, temperature effects, and selec-ivity of the Al2O3-modified MCLs for low-level moisture detectionave been discussed. The moisture detection limit was 10 ppmsing the deflection mode, but the frequency change of the MCLsas not investigated. The sensitivity could be further improved byne tuning of the coatings.

It was now well-known that the surface characteristic of theoatings is critical for the MCL sensing performance. Increasing sur-ace area is a practical approach for enhancing sensitivity of MCLensors. For moisture measurement, it is anticipated the porousl2O3-modified MCLs will have better sensitivity than our pre-ious reported ones. The nano-porous film was developed by anlectroplating method so called anodization process. Aluminumhen anodized in basic or acidic solution with a suitable appliedotential and time would be converted into porous aluminumxide [20]. In this paper, we compare the sensitivity for mois-ure measurement of MCLs modified by the anodized porousluminum oxide and the aluminum oxide prepared by thermalxidation of aluminum. Both dynamic and static modes were inves-igated.

. Experimental

.1. Materials

In our experiments, we used commercially available siliconCLs (Veeco Instruments, Santa Barbara, CA). The dimensions of

he V-shaped silicon MCLs were 180 �m in length, 25 �m in legidth, and 1 �m in thickness. Both sides of these MCLs were orig-

nally covered with a thin film of chromium (3 nm) and followed

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ig. 1. SEM image of (A) thermal oxided Al2O3 and (B) porous Al2O3 on MCLs.

y a 20-nm layer of gold, both deposited by e-beam evaporation.ne side of MCL was then deposited by a layer of 500-nm-thick alu-inum (Al). The Al films were oxidized to Al2O3 using two differentethods. For Method I, the Al film was oxidized by oxygen in a high

acuum chamber while oxygen gas was flowing through at 100 ◦C.SEM picture of the thermal oxided Al2O3 coating was shown in

ig. 1A. For Method II, aluminum was converted to nano-porousluminum oxide by an anodization process [21]. The aluminumn the MCLs was anodized in a 0.1 M oxalic acid solution withvoltage of 40 V and a current density of 0.5 mA/cm2 for 8 min.

his condition was applied to generate pores at smaller diame-er for larger adsorption capacity. The Al-coated MCL was used ashe anode and a Pt wire was used as the cathode. The aluminumlms with thickness varied from 100 to 500 nm were investigatedo study the thickness effects on sensor sensitivity. However, onlyluminum with a 500 nm thickness was found effective in produc-ng pores, and no porous Al2O3 were formed from the aluminumlms with the thickness of 100, 200, and 300 nm. Fig. 1 is a SEM

mage of the porous Al2O3 film with 500 nm in thickness on a MCLfter the anodization process. After anodization, the thickness ofhe Al2O3 decreased to 125 nm. The pores were clearly seen on the

luminum grains. The average diameter of the pores was approx-mately 7 nm. Because of the Al2O3 layer was relativity thin, wesed the same Young’s modulus and spring constant of the sil-

con cantilevers for a simplified calculation although errors arexpected.

Page 3: Moisture measurement using porous aluminum oxide coated microcantilevers

392 P. Kapa et al. / Sensors and Actuators B 134 (2008) 390–395

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M1miflt2the moisture concentration. The maximum deflection amplitudesof the MCLs were proportional to the concentrations of moisture.The lowest detectable concentration with Method II was approxi-mately 0.5 ppm, which was improved over the MCLs prepared usingMethod I with a detection limit at approximately 30 ppm. This sen-

ig. 2. Deflection of Al2O3-modified MCLs prepared by the two methods versus timoth figures are from −30 to 60 nm.

.2. Gas system

Dry nitrogen was used as the carrier gas for sensing validation.ry nitrogen was passed through a gas bubbler containing distilledater used to generate wet gas. Dual stage gas regulators for wet

nd dry gases controlled the gas flow into a gas mixing setup. Theesired moisture level was obtained by controlled mixing of thery and wet gases. The magnetic heater and thermometer as wells a water bath provided the temperature control of the vapor gen-ration system at 25 ◦C. The moisture level of the final mixture waseasured using a Meeco Waterboy moisture meter (Warrington,

A) with a range of 1–5000 ppm and an accuracy of ±5%. A Colearmer model 00122QA pressure gauge was used for continuousressure monitoring of gas flow to maintain the pressure of the gast a desired level of 40 psig. The flow rate of the gas inside the cellas 100 mL/min. The volume of the sample glass cell including thelumbing was 0.5 cm3, thus ensuring fast exchange of gases. Typi-ally 10–20 min will be needed to stabilize the MCL to reach a stableaseline prior to the measurement.

.3. Deflection and frequency measurement

A MCL was placed in a flow-through glass cell (Veeco Instru-ents, Santa Barbara, CA) and dry nitrogen gas was passed through

he cell at a constant 100 mL/min flow rate during each experi-ent. When the stable baseline was reached the moisture gas was

witched in for testing.The bending of the MCL was measured by monitoring the posi-

ion of a laser beam reflected from the gold-coated side of the MCLnto a four-quadrant atomic force microscope (AFM) photodiode.e define bending toward the gold side as “upward bending”;

downward bending” refers to bending toward the Al2O3 side.hen the adsorption occurs on the Al2O3 surface, in general, the

pward bending is caused by repulsion or expansion of moleculesn the Al2O3 surface, which is so called compressive stress.

Frequency tests were made using a reported method thatmproved the amplitude of cantilever vibration and Q factor valuey using a mixed positive signal feedback mechanism [22]. Austom built electronic amplifier was used to amplify the sig-

al detected by the position sensitive photodiode from differentuadrants and to compute the vertical displacement. A resonantrequency peak was obtained when the phase of the driving signalnd the vertical displacement signal from position sensitive diodeatched and measured using a dynamic spectrum analyzer.

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various ppm of moisture in nitrogen at 25 ◦C. For comparison, the Y-axis ranges in

. Results and discussions

Both static and dynamic modes of MCLs were used to comparehe sensing performance of MCLs prepared from by Method I and

ethod II for measurement of low-level moisture.

.1. Deflection of the MCLs

Fig. 2 compares the bending responses of MCLs prepared byethod I and Method II to 200 ppm moisture in nitrogen at a

00 mL/min flow rate. The moisture gas was switched in at thearked time. The MCLs underwent upward bending. After approx-

mately 5 min, the dry nitrogen was switched back through theuid cell, and the MCL bent downward back to their original posi-ions. The response time to reach equilibrium was approximately00 s. Fig. 3 shows the MCLs maximum deflection amplitude vs.

ig. 3. Deflection amplitude of Al2O3-modified MCLs prepared by the two methodsersus the concentration of moisture in nitrogen.

Page 4: Moisture measurement using porous aluminum oxide coated microcantilevers

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P. Kapa et al. / Sensors and

itivity is as competitive as other mature techniques in the market.he detection limit of moisture is of the 100 ppb to 1 ppm level withlectrolytic technique [23] or optical techniques such as FTIR spec-roscopy [24] or semiconductors [25]. It is noteworthy that in ourrevious studies using the method I, the sensitivity we obtainedas 10 ppm when relatively thinner Al2O3 film (100 nm in thick-ess) was used. For direct comparison with the MCLs by Method

I, the aluminum film in MCLs from Method I in this study were00 nm in thickness.

The larger MCL deflection suggests larger surface stress changen the MCL surface with porous Al2O3 coating, which may due tohe larger surface area of the porous structure. For a 96 nm maxi-

um deflection corresponding to 200 ppm of moisture, the surfacetress change was 0.25 N/m according to Eq. (1). The lifetime testsere conducted on MCLs with 6 months storage under ambient

ondition. The deflection of these MCLs showed a similar profilend bending amplitude to those in Fig. 2.

The effect of alcohols is a major concern of many moistureeters for low-level moisture detection since the alcohols generally

nterfere with moisture detection and cause errors. Our previousork showed that the Al2O3-modified MCLs prepared by Methodwere not affected by alcohols, which qualifies them for accurateoisture detection without calibration when alcohols exist in the

nvironment. The potential interference of alcohols on the porousl2O3-modified MCLs was also evaluated in this study. No deflec-

ion of porous Al2O3-modified MCLs was observed upon exposureo 200 ppm alcohol. This result is consistent with our previousbservations, i.e., the MCL response to moisture was not interferedy alcohols.

.2. Frequency change of the MCLs

MCL frequency vs. time under different moisture levels waslso investigated. The MCLs prepared by the two methods wereompared. For each method, five MCLs were tested. The frequencyf MCLs prepared from Method I and Method II in dry nitrogenere 28,870 ± 25 and 23,500 ± 50 Hz, respectively. This initially

ower frequency of MCLs by Method II might be a result of the

ower spring constant of the porous Al2O3 film prepared by MethodI. MCLs prepared by the same method had a ±3% differencen their resonance frequencies. The MCLs used in these exper-ments were triangle silicon MCLs that have a surface area of.12 × 10−2 mm2.

ut

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ig. 4. Resonance frequency of Al2O3-modified MCLs versus time upon exposure to 200 por comparison, the frequency ranges in both figures are 600 Hz.

ors B 134 (2008) 390–395 393

Fig. 4 shows the frequency of the MCLs vs. time on exposureo 200 ppm moisture. The moisture gas was switched in at the

arked time. The response profiles of the two MCLs prepared byhe two methods were quite similar. The frequencies of both MCLsecreased quickly on exposure to moisture and returned fully toheir initial frequencies when N2 was switched in to replace mois-ure. Both MCLs showed reproducible response to moisture.

The two MCLs were different in their frequency change per-entages and response times. The frequency change of the MCL byethod I was 25 Hz, or 0.0866% frequency change, corresponding

o 1.85 × 10−11 g of water adsorption on the MCL surface accordingo Eq. (2). The MCL prepared by Method II, however, showed a sig-ificant 300 Hz frequency decrease, or 1.27% of frequency change,orresponding to 41.76 × 10−11 g of water adsorption on the MCLurface. These results demonstrated the enhanced adsorbing capac-ty of the porous aluminum oxide film due to its larger surfacereas.

The frequency decreases on exposure to moisture reached theirquilibrium in 10 and 15 s for MCLs prepared by Method I andethod II, respectively. The frequency recovery took approximately

5 and 30 s for MCLs by Method I and Method II, respectively,hen N2 was switched in to replace moisture. These slightly longer

esponse and recovery time of MCLs prepared by Method II was aesult of longer adsorption and desorption time related to poroustructures of the aluminum oxide film. Although relatively slower,he response of the MCL sensors prepared by Method II has andvantage in response time over other methods such as an elec-rochemistry method, which took approximately 2 min to reachquilibrium.

A control experiment performed with an Al/Au/Si/Au MCL tooisture showed no change in resonance frequency.Fig. 5 shows the MCLs resonance frequency change vs. the vapor

oncentration. The frequency changes of the MCLs were propor-ional to the concentrations of moisture. The frequency decreaseds the concentration of moisture increased. The lowest detectableoncentration by Method II was approximately 1 ppm, which wasore sensitive than those prepared using Method I with a detection

imit at approximately 10 ppm.

Lifetime experiments conducted on MCLs stored for 6 months

nder ambient condition showed a similar frequency change profileo those in Fig. 4.

It is not very clear why it took longer time for the deflection ofhe MCLs to reach equilibrium than the resonance frequency. One

pm of moisture in nitrogen. MCLs prepared from the two methods were compared.

Page 5: Moisture measurement using porous aluminum oxide coated microcantilevers

394 P. Kapa et al. / Sensors and Actuators B 134 (2008) 390–395

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ig. 5. Resonance frequency of Al2O3-modified MCLs prepared by the two methodsn both figures are 400 Hz.

ossible explanation is that the water molecules adsorbed on theCL surface in the initial stage did not have effective contribution

o the MCL bending since the scattered molecules did not inter-ct with others to generate the surface stress. The water moleculesdsorbed on the MCLs surface in the later stage, although in aelatively smaller amount, contributed significantly to the surfacetress that bent the MCLs. It took longer time for the molecules inhe later stage to be adsorbed on the surface due to steric effect.

. Conclusion

MCLs coated by aluminum oxide prepared by thermal oxidationnd anodization process were compared for low levels of moistureeasurement. Both the deflection and frequency change showed

he MCLs prepared by the anodization process were more sen-itive than those by thermal oxidation. Porous aluminum oxidelms by the anodization process have a larger surface area and aigher moisture absorption capacity than that of plain aluminumxide films. Comparing to the current moisture detection sys-ems, the microcantilever sensor has a relatively fast response.ther characteristics including the long-term stability and espe-ially non-interference from alcohol make the cantilever approachery competitive. Our results showed that porous Al2O3-coatedCLs are excellent sensors for low-level moisture detection anday be used for moisture monitoring in low-level moisture envi-

onment.

cknowledgements

This work was supported by NSF under SGER ECCS-0643193,SF MRI grant 0618291, and Board of Regent Industrial Ties andesearch subprogram under contract number LEQSF(2005-04)-RD--19.

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iographies

rathima Kapa completed her Bachelors in chemical engineering from Dr. B.V.aju Institute of Technology, (JNTU), India, in 2004. She received her Master’s inolecular sciences and nanotechnology from Louisiana Tech University in 2007.uring Master study, she worked as a graduate research assistant under Dr. Hai-

eng Ji and her research interests include sensors, surface modification techniques,icrofabrication methods. Currently, she is working as a process engineer in a

emiconductor-based manufacturing company.

iu Pan received her BS degree from University of Science and Technology Beijing,hina, in 1998. She is currently a PhD candidate at Institute for Micromanufacturing,

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P. Kapa et al. / Sensors and

ouisiana Tech University. Her research interests focus on microfabrication, MEMSevices, and microsensor characterization.

rogydeepika Bandhanadham received her BS degree from Jawaharlal Nehru Tech-ological University, India, in 2006. She is currently a master candidate at Institute

or Micromanufacturing, Louisiana Tech University. Her research interests focus oneveloping electrical circuit for microsensors.

i Fang received BS degree of electrical engineering in 1965 from Tianjing University,hina. Since 2002, he is a senior research engineer at Institute for micromanufactur-

ng in Louisiana Tech University. His research interests include micro fluidic devicesnd system, sensor, microreactor, and total analysis system on chip, optical lensystem and micro/nano fabrication technologies.

oday Varahramyan received his PhD in electrical engineering, Rensselaerolytechnic Institute, in 1983. He has been the Director of the Institute for Micro-

HCfsD

ors B 134 (2008) 390–395 395

anufacturing, Louisiana Tech University and the entergy professor of electricalngineering. He is currently a professor at Indiana Univeristy-Purdue Universityndianapolis (IUPUI). His research is focused on micro/nano scale processes, mate-ials, devices and systems.

es Davis received his BS from Georgia Institute and completed his course worknd requirement for a PhD from Emory University. He is currently the manager ofngineering at Sensidyne, Florida. He has accomplished skills in electronic design,oftware and mechanical design.

ai-Feng Ji received his PhD degree of chemistry from Chinese Academy of Science,hina, in 1996. He is currently an associate professor at Institute for Micromanu-

acturing, Louisiana Tech University. His research interests focus on MEMS devices,urface modification, and nanoassembly. He will move to Department of Chemistry,rexel University after September 2008.