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Sensors and Actuators B 123 (2007) 896–901

A nanowire WO3 humidity sensor integrated with micro-heaterand inverting amplifier circuit on chip manufactured

using CMOS-MEMS technique

Ching-Liang Dai a,∗, Mao-Chen Liu a, Fu-Song Chen a,Chyan-Chyi Wu b, Ming-Wei Chang c

a Department of Mechanical Engineering, National Chung Hsing University, Taichung 402, Taiwan, ROCb Center for Measurement Standards, Industrial Technology Research Institute, Hsinchu 300, Taiwan, ROC

c Center for RFID Technology, Industrial Technology Research Institute, Hsinchu 310, Taiwan, ROC

Received 4 August 2006; received in revised form 25 October 2006; accepted 26 October 2006Available online 29 November 2006

bstract

The fabrication of a nanowire WO3 humidity sensor integrated with an inverting amplifier circuit and a micro-heater on a chip using theommercial 0.35 �m complementary metal oxide semiconductor (CMOS) process and a post-process have been implemented. The humidityensor is composed of a sensing resistor and a humidity sensing film. Tungsten trioxide prepared by a sol–gel method is adopted as the humidityensing film. The fabrication of the humidity sensor requires a post-process to etch the sacrificial layers and to expose the sensing resistor, and

hen the humidity sensing film is coated over the sensing resistor. The humidity sensor, which is a resistive type, changes the resistance whenhe sensing film adsorbs or desorbs water vapor. An inverting amplifier circuit is utilized to convert the resistance of the humidity sensor into theoltage output. The micro-heater is utilized to provide a super-ambient working temperature to the humidity sensor, which can avoid the humidityensor to generate the signal drift. Experimental results show that the sensitivity of the humidity sensor is about 4.5 mV/% RH at 60 ◦C.

2006 Elsevier B.V. All rights reserved.

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eywords: Humidity sensor; CMOS; Micro-heater; Inverting amplifier circuit

. Introduction

Humidity sensors that are utilized to detect humidity andoisture are important devices in many industrial and biomed-

cal applications. For instance, Tetelin et al. [1] developed aapacitive humidity sensor to equip a medical microsystem foriagnosis of pulmonary diseases. Boisen et al. [2] proposedhumidity sensor for monitoring environment. Dokmeci andajafi [3] reported a high sensitivity humidity sensor for moni-

oring hermetic micropackages. Bokmann et al. [4] presented a

umidity sensor for industrial humidity measurement.

Various microsensors have been manufactured using micro-lectromechanical systems (MEMS) technology [5]. Humidityensors fabricated by MEMS technology have the advantage of

∗ Corresponding author. Tel.: +886 4 22840433; fax: +886 4 22877170.E-mail address: cldai@dragon.nchu.edu.tw (C.-L. Dai).

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925-4005/$ – see front matter © 2006 Elsevier B.V. All rights reserved.oi:10.1016/j.snb.2006.10.055

mall size, low weight, high performance, easy mass-productionnd low cost [6]. Several studies have recently used MEMSechnology to fabricate micro-humidity sensors. For instance,’Halloran et al. [7] employed bulk micromachining technique

o fabricate a capacitive porous silicon humidity sensor. Rit-ersma et al. [8] fabricated a capacitive porous silicon humidityensor using a surface micromachining technique. Park et al. [9]resented a thin-film surface micromachining technique to makeumidity sensors. Boltshauser et al. [10] utilized an industrialMOS process to make a resonant humidity sensor, which wasased on silicon-oxide resonators coated with vapor-absorbentolyimide thin films achieving a sensitivity of 270 Hz/% RH. Aapacitive humidity sensor with an anodized aluminum oxideAl2O3) thin film, proposed by Nahar [11], had a sensitivity of00 pF/% RH. Story et al. [12] developed a polymetric humid-

ty sensor, and its sensitivity was 10 k�/% RH. A surface andulk micromachining process, reported by Lee et al. [13], wasmployed to fabricate a capacitive humidity sensor with sus-ending structures, which had a sensitivity of 2 nF/% RH.

Actuators B 123 (2007) 896–901 897

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fitvtitrioxide film and the sensing resistor. The sensing resistor is2 �m wide, 0.4 �m thick and 4000 �m long.

Fig. 3 shows the energy band diagram of the humidity sen-sor. The tungsten trioxide is an n-type semiconductor, and the

C.-L. Dai et al. / Sensors and

The CMOS-MEMS technique [14] uses the commercialMOS process, usually employed to produce integrated circuits

IC), to fabricate MEMS devices. Many microdevices such asunable resonators [15], micromirror arrays [16] and microwavewitches [17] fabricated by the CMOS-MEMS technique requirepost-process to release the suspended structures or coat the

unctional films. The advantage of micromachined devices fab-icated by the CMOS-MEMS technique is its ability to integrateith circuits as a system on a chip (SOC) due to their compati-ility with the CMOS process.

In this study, we employ the CMOS-MEMS technique toabricate a nanowire WO3 humidity sensor integrated with aicro-heater and an inverting amplifier circuit on a chip, which

an enhance the performance and reduce the volume and cost.he area of the integrated humidity sensor chip is about 1 mm2.

. Structure of the integrated humidity sensor

Fig. 1 illustrates the humidity sensor integrated with a micro-eater and an inverting amplifier circuit. The humidity sensor,hich is composed of a humidity sensing film and a sensing

esistor, is a resistive type. The sensing resistor is a polysili-on winding line. The humidity sensing film is tungsten trioxidehich is prepared by a sol–gel method and coated on the sensing

esistor. The micro-heater is used to provide a super-ambientorking temperature, which can avoid the humidity sensor toenerate the signal drift. The material of the micro-heater is the

olysilicon layer of the CMOS process. The sensing resistor,hich is connected to the inverting amplifier circuit, changes its

esistance as the sensing film adsorbs or desorbs water vapor. Thenverting amplifier circuit that contains an operational ampli-

ig. 1. Schematic structure of the humidity sensor integrated with heater andircuit.

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Fig. 2. Cross-section of the integrated humidity sensor.

er and resistances is employed to convert the resistance ofhe humidity sensor into the voltage output. The cross-sectionaliew of the integrated humidity sensor is shown in Fig. 2. Theungsten trioxide film is located at the surface layer of the humid-ty sensor. A silicon dioxide layer is located between the tungsten

ig. 3. Energy band diagram of the sensor for (a) a low humidity and (b) a highumidity. EF is the Fermi level, Ec-WO3 is the conduction band of WO3, Ev-WO3

s the valence band of WO3, EFi-WO3 is the intrinsic Fermi level of WO3, Ec-poly

s the conduction band of polysilicon, Ev-poly is the valence band of polysilicon,nd EFi-poly is the intrinsic Fermi of polysilicon.

8 Actuators B 123 (2007) 896–901

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cMpsflspity sensor, micro-heater and circuit. In the humidity sensor, themetal and via layers are adopted as the sacrificial layers, and

98 C.-L. Dai et al. / Sensors and

olysilicon is p-type. When the surface of WO3 is exposed toater vapor, electrons are produced at the surface of WO3 due

o the electron-donating adsorption [18]. As shown in Fig. 3(a),n accumulation of electrons is formed at the surface of WO3hen H2O(gas) interacts with WO3, so that the conduction andalence band edges of WO3 bend downward, leading to generatenegative surface potential, φs [19]. An accumulation of holes

t the oxide–polysilicon interface is formed by the negative sur-ace potential φs, which causes the conduction and valence banddges of polysilicon to bend upward as shown in Fig. 3(a) andesults in the production of the potential barrier, Vs. The conduc-ance of polysilicon, G, is determined by the potential barrier,s, as described by the following relationship [20]:

= gqμsNd exp

(−qVs

kT

)(1)

here q represents the electronic charge, g is a constant deter-ined by the semiconductor geometry, μs is the mobility of

lectrons, Nd is the density of donors, k is the Boltzmann con-tant, and T is the absolute temperature. According to Eq. (1),he conductance of the polysilicon reduces as the potential bar-ier Vs increases, which leads to an increase in resistance of theolysilicon. Fig. 3(b) illustrates the energy band diagram of theumidity sensor in a highly humid environment. The surfaceotential φs increases when the WO3 film of the humidity sen-or interacts with more water vapor, resulting in an increment ofhe potential barrier Vs and a decrement of the polysilicon con-uctance G. Therefore, the resistance of polysilicon increases ashe humidity sensed by the WO3 increases.

The inverting amplifier circuit shown in Fig. 1 is employedo convert the resistance variation of the humidity sensor intovoltage output. If the operational amplifier is ideal, then the

utput voltage of the circuit can be expressed as

o = −Rs

R1Vin (2)

here Rs represents the resistance of the sensing resistor in theumidity sensor, R1 is a constant resistance (10 k� in the design)

nd Vin is the input voltage. Fig. 4 shows the design of the oper-tional amplifier circuit, where VDD represents a voltage powerupply and VSS is the ground. Professional circuit simulationoftware, HSPICE, is used to analyze the operational ampli-

Fig. 4. Design of the operational amplifier circuit.

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Fig. 5. Frequency response of the operational amplifier.

er and inverting amplifier circuit. Fig. 5 depicts the simulatedesults of the frequency response for the operational amplifier.he dc open loop gain of the operational amplifier is approxi-ately 82 dB, and the phase margin of the operational amplifier

s about 53◦. Fig. 6 reveals the simulated results of the invertingmplifier circuit. In this simulation, the input voltage Vin is 1 V,he resistance R1 is given with 10 k� and the resistance of theumidity sensor Rs varies from 12 to 45 k�. The output voltagef the inverting amplifier circuit changes from 1.07 to 1.67 V ashe resistance of the humidity sensor varies from 12 to 45 k�.

. Fabrication of the integrated humidity sensor

The integrated humidity sensor chip is manufactured by theommercial 0.35 �m CMOS process of Taiwan Semiconductoranufacturing Company (TSMC). The chip requires a post-

rocess to etch the sacrificial layers and to coat the humidityensing film after completion of the CMOS process. The processow of the integrated chip is illustrated in Fig. 7. The cross-ection of the integrated chip after completion of the CMOSrocess is shown in Fig. 7(a). The chip consists of the humid-

he polysilicon layer is used as the sensing resistor and micro-eater. The materials of the metal and via layers are aluminum

Fig. 6. Simulated results of the inverting amplifier circuit.

C.-L. Dai et al. / Sensors and Actuators B 123 (2007) 896–901 899

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ig. 7. Process flow of the humidity sensor: (a) after the CMOS process, (b)tching sacrificial layers, and (c) coating the sensing film.

Al) and tungsten (W), respectively. Fig. 7(b) shows that thehip is immersed in two etchants: one is an Al etchant withhosphoric acid, nitric acid, acetic acid and deionized water in

ratio of 14:1:2:3. The other is a W etchant with sulfuric acid

nd hydrogen peroxide in a ratio of 2:1. The sacrificial layers areemoved from the humidity sensor, exposing the sensing resis-or. Fig. 8 shows the photograph of an integrated humidity sensor

ig. 8. A photograph of the integrated humidity sensor chip after the wet etchingrocess.

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Fig. 9. A SEM image of tungsten trioxide film.

hip after the wet etching process. Then, the chip is put in anven at 140 ◦C for 2 h, so that the polysilicon surface grows a thinilicon dioxide layer. Finally, the tungsten trioxide is coated onhe sensing resistor as shown in Fig. 7(c). The tungsten trioxide,hich is made by a sol–gel method [21], is prepared according

o the following steps: (1) 2.02 g Na2WO4·2H2O is mixed with00 ml deionized water and stirred until a homogenous solutions obtained; (2) the solution of 1 ml H2SO4 is added into thea2WO4·2H2O solution with stirring until the mixing solutionecomes homogenous, producing a primrose yellow slurry; (3)he product is aged at room temperature for 240 h; (4) the result-ng product is filtered, dropped on the humidity sensor chip,ollowed by calcination in air at 150 ◦C for 3 h. Fig. 9 showsscanning electron microscopy (SEM) image of the tungsten

rioxide film. The sensing film, which is constructed by manyanowires of tungsten trioxide as shown in Fig. 9, is porous andas a large surface area.

. Results and discussion

A power supply, a function generator, an oscilloscope and aest chamber (GTH-099-40-1P, Giant Force Instruments Enter-rise Co.) were employed to measure the performance of thentegrated humidity sensor chip. The sensor chip was set in theest chamber. The power supply and function generator providedbias voltage of 3.3 V and an input voltage of 1 V, respectively,

o the inverting amplifier circuit in the sensor chip. The oscillo-cope was utilized to record the output signal of the sensor toumidity changes. The test chamber was capable of providing aemperature range of 0–100 ◦C and a humidity range of 25–95%H. In the test chamber, the temperature and humidity could be

uned separately and maintained at constant levels.The test chamber supplies different humidity levels, and the

umidity sensor changes its resistance as the humidity rises orrops. The output voltage of the inverting amplifier circuit is

easured by the oscilloscope. Fig. 10 shows the output voltage

hanges of the inverting amplifier circuit. In this investigation,he temperature was maintained constant at 75 ◦C, while theumidity was increased from 25% RH to 85% RH in 35 min and

900 C.-L. Dai et al. / Sensors and Actuators B 123 (2007) 896–901

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Fig. 10. Measured results of the humidity sensor at 75 ◦C.

hen dehumidified to 25% RH at the same rate. The experimentsevealed that the humidity sensor exhibited a small degree ofumidity hysteresis. With the same procedure, the output voltagef the humidity sensor was measured in the humidity range from5% RH to 85% RH at constant temperatures of 25, 30, 45 and0 ◦C. Fig. 11 displays the measured results of the humidityensor at different temperatures. The humidity sensitivity, whichould be obtained by the linear fitting to the data in Fig. 11, was.5 mV/% RH at 60 ◦C and 4.8 mV/% RH at 75 ◦C. The resultshowed that the humidity sensitivity increased as the temperatureose. Therefore, the humidity sensor has the disadvantage of theariation in humidity sensitivity with ambient temperature.

The micro-heater in the chip was adopted to provide a super-mbient working temperature to the humidity sensor in ordero overcome the influence of ambient temperature. A dc powerupply provided different voltages of 2, 4, 6, 8 and 10 V to theicro-heater, and the test chamber was maintained at a constant

emperature of 25 ◦C. In this experiment, the humidity sensoras heated by the micro-heater, and was not heated by the test

hamber. Fig. 12 displays the measured results of the humid-ty sensor with the micro-heater under different voltages. Theesults showed that the micro-heater could be used to provide the

orking temperature of the humidity sensor. The humidity sen-

or should not produce any signal drift if the ambient temperatures lower than the working temperature from the micro-heater.

ig. 11. Measured results of the humidity sensor at different temperatures.

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ig. 12. Measured results of the humidity sensor with the micro-heater underifferent voltages.

. Conclusion

The nanowire tungsten trioxide humidity sensors integratedith an inverting amplifier circuit and a micro-heater were suc-

essfully implemented using the commercial 0.35 �m CMOSrocess and the post-process. The humidity sensor contained aensing film and a sensing resistor. The sensing film, which wasrepared by a sol–gel method, was the structure of nanowireungsten trioxide. The humidity sensor, which was a resistiveype, changed its resistance when the sensitive film adsorbed oresorbed water vapor. The inverting amplifier circuit was usedo convert the change in resistance of the humidity sensor intovoltage output. The post-process utilized etchants to etch the

acrificial layers to expose the sensing resistor, and then tungstenrioxide was coated on the sensing resistor. Experimental resultshowed that the sensitivity of the humidity sensor was about.5 mV/% RH at 60 ◦C and 4.8 mV/% RH at 75 ◦C. The humid-ty sensor had a variation in humidity sensitivity with ambientemperature. In order to solve the influence of ambient temper-ture, the micro-heater was adopted to provide a super-ambientorking temperature to the humidity sensor, which could avoid

he humidity sensor to generate the signal drift.

cknowledgements

The authors would like to thank National Center for High-erformance Computing (NCHC) for chip simulation, Nationalhip Implementation Center (CIC) for chip fabrication and

he National Science Council of the Republic of China fornancially supporting this research under Contract No. NSC5-2221-E-005-043-MY2.

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iographies

hing-Liang Dai received the MS degree in the applied mechanics fromational Taiwan University, Taiwan, in 1993, and the PhD degree in the mechan-

cal engineering from Nation Taiwan University, in 1997. He is currently anssociate professor at the Department of Mechanical Engineering, Nationalhung Hsing University, Taiwan. His research interests are CMOS-MEMS,

ntegrated microsensors and RF MEMS.

ao-Chen Liu received the MS degree in the precision engineering fromational Chung Hsing University, Taiwan, in 2004. He is currently a candi-ate for doctor’s degree at the Department of Mechanical Engineering, Nationalhung Hsing University, Taiwan. His research interests are semiconductor sen-

ors, analog circuits and radio frequency circuits.

u-Song Chen received the BS degree in the mechanical engineering fromational Chung Hsing University, Taiwan, in 2004, and the MS degree in the pre-

ision engineering from National Chung Hsing University, in 2006. His researchnterests are gas sensors and micro-actuators.

hyan-Chyi Wu received the PhD degree in the mechanical engineering fromational Taiwan University, Taiwan, in 2001. He is currently a researcher

t Industrial Technology Research Institute, Taiwan. His research interestsre optical physics, biophysics, optical interferometry and near-filed/far-fieldicroscopy.

ing-Wei Chang received the BS degree in the mechanical engineering from

uafan University, Taiwan, in 2000 and the MS degree in the mechatronics engi-eering from Huafan University, Taiwan, in 2002. He is currently an engineeringesearcher and an advanced project leader at the RFID center, Industrial Tech-ology Research Institute, Taiwan. His research interests are micromachinedransducers and Bio-MEMS.