Microsyst Technol (2006) 12: 893–898 DOI 10.1007/s00542-006-0119-x
T E C H N I C A L P A PE R
Chia-Yen Lee Æ Ping-Ru Hsieh Æ Che-Hsin Lin Po-Cheng Chou Æ Lung-Ming Fu Æ Che-Ming Chiang
MEMS-based formaldehyde gas sensor integrated with a micro-hotplate
Received: 1 August 2005 / Accepted: 28 October 2005 / Published online: 29 March 2006 Springer-Verlag 2006
Abstract This paper presents a novel micro-fabricated formaldehyde gas sensor with enhanced sensitivity and detection resolution capabilities. The device comprises a quartz substrate with Pt heaters as a micro-hotplate and deposited formaldehyde-sensing layer on it. A sputtered NiO thin film is used as the formaldehyde-sensing layer. A specific orientation of NiO becomes more apparent as the substrate temperature increases in the sputtering process, which helps the formation of NiO material with a correct stoichiometric ratio. The gas sensor incorporates Pt heating resistors integrated with a micro-hotplate to provide a heating function and utilizes Au inter-digitated electrodes. When formaldehyde is present in the atmosphere, oxydation happens near the sensing layer with a high temperature caused by the micro-hotplate and causes a change in the electrical conductivity of the NiO film. Therefore, the measured resistance between the inter-digitated electrodes changes C.-Y. Lee Æ P.-R. Hsieh Department of Mechanical and Automation Engineering, Da-Yeh University, 515 Changhua, Taiwan, R.O.C E-mail:
[email protected] E-mail:
[email protected] C.-H. Lin Department of Mechanical and Electro-mechanical Engineering, National Sun Yat-sen University, 804 Kaohsiung, Taiwan, R.O.C E-mail:
[email protected] P.-C. Chou Department of Interior Design, Shu-Te University of Science and Technology, 824 Kaohsiung, Taiwan, R.O.C E-mail:
[email protected] L.-M. Fu Graduate Institute of Materials Engineering, National Pingtung University of Science and Technology, 912 Pingtung, Taiwan, R.O.C E-mail:
[email protected] C.-M. Chiang (&) Department of Architecture, National Cheng-Kung University, 700 Taiwan, Taiwan, R.O.C E-mail:
[email protected] Tel.: +886-6-2757575
correspondingly. The application of a voltage to the Pt heaters causes the temperature of the micro-hotplate to increase, which in turn enhances the sensitivity of the sensor. The nanometer scale grain size of the sputtered oxide thin film is conducive to improving the sensitivity of the gas sensor. The experimental results indicate that the developed device has a high stability (0.23%), a low hysteresis value (0.18%), a quick response time (13.0 s), a high degree of sensitivity (0.14 X ppm1), and a detection capability of less than 1.2 ppm.
1 Introduction Formaldehyde (HCHO) is a highly important commercial chemical due to its chemical activity, high purity and relatively low cost. However, various risk factors have been associated with this chemical recently (Korpan et al. 2000; Kataky et al. 2002). It has been shown that throat and nose irritation can occur at formaldehyde levels as low as 0.08 ppm (WHO Regional Office for Europe 2001). The NIOSH (National Institute for Occupational Safety and Health, USA) has established a permissible long-term exposure limit of 1 ppm (Dirksen et al. 2001). Historically, the detection of formaldehyde has been achieved using a stringent semi-batch air-sampling procedure followed by a batch analysis of the sample (GC-MS). This procedure is clearly incapable of providing immediate formaldehyde exposure information. Hence, several researchers have developed optical sensors for formaldehyde quantification applications (Mine et al. 1997; Friedfeld and Fraser 2000). However, the associated optical arrangements are bulky and elaborate. Recently, emerging MEMS technologies and micro-machining techniques have contributed significantly to the miniaturization of sensors and have permitted the development of a new generation of micro-scale sensing instrumentation. Recently, Dirksen et al. (2001)
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fabricated NiO thin-film formaldehyde gas sensors using a dipping process with a nickel acetylacetonate solution for an alumina substrate. A 0.5 lm NiO thin film was formed. The conductivity of this film was found to change as the formaldehyde concentration was varied for temperatures ranging from 400 to 600C. At approximately 3 lm, the sintered grain size was rather large and was hence expected to decrease the sensitivity of the device. However, a linear formaldehyde sensitivity of 0.825 mV ppm1 was attained at 600C. Although the sensor demonstrated a high sensitivity, its detection limit was 50 ppm, which significantly exceeds the prescribed ‘‘long-term exposure’’ limit of 1 ppm. Furthermore, the proposed sensor lacked any form of heating device. Lee et al. (2002) described the use of Pt resistors as reliable micro-heaters in MEMS-based temperature control systems. The current study develops a new process for the fabrication of a MEMS-based formaldehyde-sensing device comprising micro-hotplates with Pt resistance heaters, a sputtered NiO layer, and Au IDEs. The experimental data indicate a high stability (0.23%), a low hysteresis value (0.18%), a quick response time (13.0 s), a high degree of sensitivity (0.14 X ppm1), a low detection limit (1.2 ppm), and a simple arrangement with no requirement for any form of external heating device.
2 Experiments 2.1 Catalysis A catalyst is an agent which causes reactions which are thermodynamically favoured but kinetically slow to take place at a faster rate. Both metal clusters and oxides are used as catalyst for oxidation reactions. Two oxidation reactions are of interest in the oxidation of formaldehyde, i.e. HCHOðgÞ + 1/2 O2ðgÞ ! HCOOHðgÞ and HCHOðgÞ + O2ðgÞ ! H2 OðgÞ + CO2ðgÞ : The former reaction produces formic acid, while the second produces water and CO2. Dirksen et al. (2001) listed all the catalysts used for the oxidation of formaldehyde, and reported that the most active catalytic oxide appeared to be NiO. Oxygen partial pressure in the atmosphere plays an important role in determining the electrical conductivity of these oxides. This effect can be exploited to sense the presence of a gas by causing the catalytic oxidation of this gas at the surface of an oxide. The resulting reactions decrease the partial pressure of the oxygen at the oxide surface and hence alter the electrical conductivity of the oxide. The change in electrical conductivity is then detected in the form of an electrical signal, whose magnitude is dependent upon the rate of catalytic oxidation of the gas (Hotovy et al. 2002).
When formaldehyde is present in the atmosphere, it is adsorbed into, and subsequently reacts with, the sensing layer. Consequently, the surface coverage of atomic oxygen is decreased. Since the electrical conductivity of the sensing layer is proportional to the nickel cation concentration of NiO (Matsumiya et al. 2002), its conductivity is increased through the adsorption and reaction of formaldehyde. This conductivity increase leads to a reduction in the measured resistance. Furthermore, increasing the partial pressure of formic acid in the atmosphere, i.e. the product of the formaldehyde oxidation reaction, will also decrease the surface coverage of the atomic oxygen, hence increasing the electrical conductivity (Dirksen et al. 2001). 2.2 Thin film deposition In this study, the NiO film was prepared using an RF magnetron sputtering system with a NiO target of 99.98% purity (Jiang et al. 2002). The oxide was sputtered on quartz substrates measuring 7.6·2.6·0.1 cm (length · width · thickness), which were placed at a distance of 11.4 cm from the target. Sputtering deposition was performed at a gas pressure of 0.01 torr and the target was maintained at a constant RF power of 200 W. A mixture of argon (50%) and pure oxygen gas (50%) was used as the reactive sputter gas. The effects of substrate temperature during the sputtering process on the properties of NiOx films have been discussed previously (Lu et al. 2002). The investigated range of substrate temperatures in the present study extended from 350 to 450C. Prior to deposition, the chamber was pumped to a background pressure of 106 torr for 1 h and a presputtering process was performed for 10 min to clean the target surface and to remove all possible traces of contamination. The NiO films deposited in this study all had a thickness of approximately 2 lm. The phase of the deposited films was studied using the X-ray diffraction (XRD) technique. 2.3 Micro-hotplate The developed gas sensor operates on the principle that changes in the coverage of adsorbed or chemisorbed gas species on the sensing film causes a detectable change in the electrical properties of this film, specifically its conductance. Gas-sensing devices typically operate at elevated temperatures in order to activate the reactions which produce a sensor response and to reduce humidity effects (Cavicchi and Suehle 1995). A drawback of these devices is that the observed response may actually be the result of the simultaneous presence of more than one gas. To enhance the ‘‘selectivity’’ of a device, its operating temperature must be optimized in order to reduce the effects of competing reactions. In the present study, Pt micro-heaters are fabricated on the quartz plate to
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heat the formaldehyde-sensing layer. When a voltage is applied to these heaters, the temperature of the microhotplate increases and subsequently attains a constant elevated temperature for a constant applied voltage. Application of a voltage to the heaters generates a simultaneous heating effect in the thin film-sensing element, which is deposited directly on the hotplate. The optimized operation temperature can be determined by varying the temperature of the hotplate and comparing the detection sensitivity of different competing reactions at different temperatures. 2.4 Design As shown in Fig. 1, the present study integrates the sensing layer, the heating device, and the IDEs on the same substrate. Compared with previous studies, the current integrated micro-hotplates permit an instantaneous and precise temperature control capability. Furthermore, the IDEs enable the direct electrical measurement of the conductivity change of the sensing layer. The sputtered NiO grain size is very small, hence enhancing both the sensitivity and the sensing limit of the device. 2.5 Micro-fabrication As stated above, the present study used quartz substrates. Figure 2 presents a schematic illustration of the simplified fabrication process. Initially, a 2-lm NiO-sensing layer was sputter deposited on the substrate. Then, a thin layer of Cr (0.01 lm) was deposited on the NiO layer to serve as an adhesion layer for the subsequent 0.3-lm Pt layer, which was deposited using an electron-beam evaporation process. A standard lift-off process was employed to pattern Pt resistors to serve as heaters on the micro-hotplate. The resistance of the heater was designed to be 30 X. Finally, Au IDEs (0.4 lm) were formed using the same metallization process.
Fig. 1 Schematic illustration of formaldehyde sensors with integrated micro-heaters
Fig. 2 Simplified fabrication process of formaldehyde gas sensor with NiO thin films
Figure 3 shows the photo of the present formaldehyde gas sensor integrated with a micro-hotplate. Please note that Au IDES were also deposited and integrated on the same quartz substrate. Figure 4 presents the SEM photo of grain size of the sputtered NiO thin film and the grain size was measured to be less 100 nm in the SEM photo, which means nanometer scale grain sizes can be formed by the proposed fabrication process in the study. The nanometer scale grain sizes of the sensing layer help to enhance the characteristics of the proposed sensors in the next section.
Fig. 3 Photo of the present formaldehyde gas sensor integrated with a micro-hotplate
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Therefore, it is important to grow NiO films with a preferred crystallographic orientation when the film is to be used as a sensing layer. The dependence of the crystallinity on the substrate temperature can be reasoned as follows. During the sputtering process, atoms must travel a certain distance before they impact on the sample surface to form a deposited film. At higher substrate temperatures, the sputtered atoms not only possess kinetic energy, but also an additional thermal energy provided by the substrate when attaining their equilibrium positions. Therefore, there is greater probability of a more perfect crystalline structure being formed at higher substrate temperatures. Fig. 4 SEM photo of grain size of the sputtered NiO thin film
3 Results and discussion 3.1 Effects of substrate temperature on the NiO films Figure 5 shows the diffraction patterns of samples prepared at different substrate temperatures. In the 2h range under current investigation, the (200) diffraction peak was observed. Diffraction peaks corresponding to Ni, NiOOH, Ni2O3, and Ni(OH)2 are not evident at any substrate temperature. The XRD spectra results indicate that the micro-structure of the sputtered NiO film depends on the substrate temperature. It can be seen that NiO films with good crystallinity and the (200) preferred orientation are obtained at a substrate temperature of 350C and above. NiO films with (200) preferred orientations represent suitable candidates for the sensing layers of oriented oxide films since they are chemically stable, similar in symmetry to the oxygen ion lattice, and have lattice constants between those of NiO films and oriented oxide films (Fujii et al. 1996).
Fig. 5 XRD diffraction patterns of NiO films deposited at different substrate temperatures—350, 400 and 450C (target, 99.98% NiO; RF power of target, 200 W; argon flow rate, 50%; oxygen flow rate, 50%; working pressure, 0.01 torr)
3.2 Effects of applied voltages on the micro-hotplates Figure 6 indicates the heating characteristics of the micro-heaters on the micro-hotplates. It can be seen that the temperature rise depends strongly on the applied voltage. An applied voltage of 14.2 W is found to yield a constant temperature of 280C, which is taken as the optimized temperature for the formaldehyde-sensing test presented below. 3.3 Stability of the formaldehyde gas sensor The experimental results indicate that the variation in device stability of the formaldehyde gas sensor is small. The variation is 0.23% at 5.0 ppm of formaldehyde concentration. 3.4 Signal hysterisis of the formaldehyde gas sensor Ideally, a gas sensor should follow the same resistance path as the formaldehyde concentration is increased or decreased. Inevitably, some gas sensors exhibit a small degree of hysteresis, i.e. the resistance paths for increasing and decreasing concentration differ to some extent. Figure 7 presents an analysis of the concentration hysteresis of the proposed gas sensor. In this investigation, the formaldehyde concentration was increased from 0 to 14.35 ppm in 30 min, and then decreased to 0 ppm at the same rate. Note the Fig. 7
Fig. 6 Micro-hotplate temperature for micro-heaters at different applied voltages
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Fig. 7 Hysteresis curve of the formaldehyde gas sensor
plots the increasing resistance using a solid line, and the return path using a dashed line. The results show that the formaldehyde gas sensor has a low hysteresis of 0.18%, which means the proposed sensor shows good repeatability while the formaldehyde concentration is increased and decreased. 3.5 Time response The analysis time of formaldehyde concentration measurement takes from hours to days by conventional instruments, and it is of great importance to utilize a real-time gas sensor. Figure 8 presents the time response of the gas sensor considered in the present study. The average time constant of the proposed formaldehyde gas sensor is determined to be 13.0 s in the formaldehyde concentration range of 4.0–8.0 ppm. 3.6 Formaldehyde concentration sensitivity As shown in Fig. 9, a linear dependency is observed between the resistance difference and the formaldehyde concentration at different sensor working temperatures
Fig. 8 Time response of the formaldehyde gas sensor from 4.0 to 8.0 ppm
Fig. 9 Experimenal ressults of formaldehyde concentration sensitivity at different micro-hotplate temperatures: 280C (diamond), 215C (square) and 150C (triangle)
(micro-hotplate temperatures). The slopes of the plotted lines represent the sensitivity of the device and are found to be 0.14 X ppm1 at 280C, 0.12 X ppm1 at 215C and 0.10 X ppm1 at 150C, respectively. The lowest detection limit of the device is determined to be 1.2 ppm at 280C, 2.4 ppm at 215C and 3.6 ppm at 150C, which is significantly lower than the detection limit capabilities of the devices presented in previous studies. 3.7 Thermal effect on formaldehyde concentration sensitivity and lowest detection limit Figure 10 shows the relationship between the concentration sensitivity and the working temperature of the gas sensor. The sensitivity of the sensor linearly increases as the working temperature increases. Figure 11 indicates the relationship between the lowest detection limits of the gas sensor. The detection limit of the sensor also linearly decrease as the working temperature increases. The linear thermal effect on both of the two characteristics of the formaldehyde gas sensor provides the convenience to
Fig. 10 Effect of sensor working temperature on formaldehyde concentration sensitivity
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sensitivity of the gas sensor (0.14 X ppm1) and improves its detection limit capability (1.2 ppm). The integrated micro-hotplate simplifies the experimental setup and can be realized using a simple fabrication process. The present micro-fabricated formaldehyde gas sensor with a self-heating NiO thin film is suitable not only for industrial process monitoring, but also for the detection of formaldehyde concentrations in buildings in order to safeguard human health. Acknowledgement The authors would like to thank the financial supports provided by the National Science Council in Taiwan (NSC 94-2211-E-212-009 and NSC 94-2218-E-006-045). Fig. 11 Effect of sensor working temperature on lowest detection limit of formaldehyde concentration
predict the sensor performance at different working temperatures for different measurement ranges. 3.8 Discussion The micro-fabricated formaldehyde gas sensors presented in this study demonstrate a high sensitivity and a low detection limit capability. Moreover, the integrated micro-hotplate provides a simultaneous heating function without the requirement for any form of external airheating device. However, a systematic investigation requires the sensor selectivity to be considered for the case where organic gases other than formaldehyde are also present in the air. Additionally, the optimized temperature for formaldehyde sensing must be discussed in a future study.
4 Conclusions This study has successfully demonstrated a novel microfabricated formaldehyde gas sensor with an integrated micro-hotplate. A new fabrication process has been developed for the formaldehyde gas sensor with a selfheating NiO thin film. The NiO thin film is deposited on the micro-structure, and Pt metal resistors are deposited as micro-heaters. Au IDEs are formed to measure the conductivity change caused by formaldehyde oxidation at the oxide surface. Not only can the high stability, the low hysteresis value and a short response time be attained for the proposed MEMS-based sensor, but decreasing the grain size of the oxide sensor material in the sputtering process also significantly increases the
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