MEMS-based microelectrode system incorporating carbon nanotubes for ionization gas sensing

Available online at www.sciencedirect.com

Sensors and Actuators B 127 (2007) 637–648

MEMS-based microelectrode system incorporating carbon nanotubes for ionization gas sensing Zhongyu Hou ∗ , Hai Liu, Xing Wei, Jiahao Wu, Weimin Zhou, Yafei Zhang, Dong Xu, Bingchu Cai The National Key Laboratory of Micro/Nano Fabrication Technology, Key Laboratory for Thin Film and Microfabrication of Ministry of Education, Research Institute of Micro and Nano Science and Technology, Shanghai Jiaotong University, Shanghai 200030, China Received 6 September 2006; received in revised form 20 May 2007; accepted 21 May 2007 Available online 26 May 2007

Abstract A novel microelectrode system incorporating CNTs with some short gap sizes (S = 6, 7, 8, 10, 12 ␮m) that can generate non-thermal plasmas without high voltage operation and additional ionization sources is introduced in this paper. The characteristic current–voltage (I–V) and current–time (I–t) in the discharge process exhibit some self-protecting behaviors from the thermal plasma generation, which is similar to the dielectric barrier discharges. The threshold effects, which are sensitive to the gap size and gas species, definitely exist in the micro discharges in the electrode with a micrometer gap size and one-dimensional materials. As gas sensors that can monitor gas species and concentration at the atmospheric pressure, the sensitivity, selectivity and stability issues are tested. The results show the significantly improved performance, including the safe operation voltage (around 36 V), higher accuracy and selectivity, over the conventional device operated by the same principle. Furthermore, the device is facile to be realized using the microelectromechanical system (MEMS) fabrication technology, thanks to its chip-based nature. Additionally, the underlying physics are also under scrutiny in this paper in light of the fluid model of the discharge. © 2007 Elsevier B.V. All rights reserved. Keywords: Micro-gas sensors; MEMS; Carbon nanotubes; Micro discharges; Microelectrode systems

1. Introduction Electric magnetic field (EMF) induced ionization of gases in a given electrode system can produce conductive bridging plasmas with definite electric characteristics, which are defined only by the gas property and the nature of the applied field. Besides, under certain conditions and careful control of operation parameters, the gas–plasma transition can be a rapidly reversible (the order of 10−9 s) and highly reproducible process. Implementing these features as operation principles, an ionization gas sensor that fingerprints the electric thresholds in such processes of different gases can be instrumented to detect and monitor the gaseous environments. Comparing to their absorption-type counterparts [1–4], operated by the sensitivity of electric property of solid-state material to the changes in gas species and concentration, they are less limited by the electrophilicity or

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Corresponding author. Tel.: +86 21 2846 7002; fax: +86 21 6282 5555. E-mail addresses: [email protected], [email protected] (Z. Hou).

0925-4005/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2007.05.026

absorption energy of atoms and molecules; instead, they are sensitive to the ionization energy and drift property of molecules of gases. Such an ionization gas sensor owes its origin to Townsend’s postulations in 1910 [5] of the gaseous breakdown criterion, which was needed to formulate the underlying physics in the device operation and the theoretical foundation for the device instrumentation. Since then, academic investigations were kept in almost every aspect related with this device (e.g., read books of Meek and Craggs [6] and Loeb [7]). However, the innovations were still hindered by the bulky external apparatus, and the risky high voltage operation, until recently, some novel attempts of incorporation of low dimensional materials into the conventional capacity-type of electrode system has been reported, including the film of multiwalled CNTS (MWCNTs) [8–11], diamond tips [12] and single ‘ultra-sharp (the level of nanometer radii)’ metal tip [13]. In these reports, the effect of lowering the operation voltage, i.e., the breakdown threshold is appreciable, comparatively, but still too high for a reasonable portable device. The other branch of studies relating to lowering the threshold of dis-

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charges was conveyed by the reports on the short gap discharges on the scale of several micrometers [14–16], and even shorter [9,13]. These works demonstrated that the breakdown criterion decreased with the decreasing of gap size through experiments or numerical simulations. Sensitivity tests of the novel design of CNTs incorporated sensors were reported by Zhang et al., in which, attractive ppm level of sensitivity in gas mixtures of the ionization gas sensors were demonstrated [10]. In fact, this work together with other works aiming to fabricate novel pressure sensors under the principle of Paschen’s law [17] showed an inherent advantage of using plasma-generation type of sensors in the dilute gaseous environment where the hot plasma, devastating to the hard architecture of the device, was not easily generated. However, to this end, one may ask two questions in this context: (1) towards the innovation of a miniaturized and safely operated device, how could the possible solutions of incorporating low-dimensional materials and short gaps be integrated by certain microdevice compatible fabrication technology? (2) What about the performance of such a device, especially the sensitivity at atmospheric pressure? By the demonstration of the design, fabrication and test of a micro/nano electrode surface system, it is the aim of this paper to answer above questions at the same time. Firstly, this paper demonstrates the fabrication of microelectrode systems with short gap spacing of 6–12 ␮m, incorporating CNTs, using MEMS-based technology. Granted that MEMS technology is capable of realizing the three-dimensional structures, such as a gas-gap capacitance (herein referred to as ‘GGC’) structure with CNT films (CNTFs) reported in current literatures [8–11], great process efforts and the performance of the functional material may be consumed. Furthermore, it is difficult to precisely control the gap spacing on the scale of several micrometers. Consequently, this paper introduces an MEMSbased hollow slot electrode system with CNT sidewalls (herein referred to as ‘HSEN’), the geometry feature of which can be easily defined in two-dimensional structure by microlithography technology. Secondly, the performance, including the sensitivity, selectivity and stability tests of the device at atmospheric gases is reported, and compared with that of a metallic and a CNTs-to-CNTs (without micro-hollow) electrode system with the similar structure to the HSEN. 2. Device design Illustrations of Fig. 1 schematically demonstrate the layout of the HSEN structure in Fig. 1a, and the cross-sectional view of the geometry of the HSEN in Fig. 1b. The advantages of the HSEN structure over the GGC structure are as follows. Firstly, the key parameters of the electrode geometry, especially the gap spacing, are defined in two-dimensional structure, which can be realized through microelectronic fabrication technology featured by submicrometer patterning capability. Secondly, a sensor array with more than one breakdown criterion to identify certain gas can be constructed by the single elements with different gap sizes, and realized easily through micro pattern transfer technology, based on the fact that electric characteristics of gas discharges are sensitive to the gap size.

Fig. 1. (a) Planform of the schematic view of the sensor system, where different gaps of SAB , SBC , SCD and SDE define different sensor units in an array. (b) Broken section view of the HSEN electrode geometry.

To investigate the effect of CNTs and the microcavity structure in the microelectrodes, the other two samples with similar geometry dimensions defined by the same mask in photolithography process were fabricated for comparison. A metallic micro slot electrode was fabricated to shed light on the role of CNTs in gaseous discharge. The micro electroforming fabrication method is described in supporting material. A ‘CNTs-to-CNTs’ micro slot electrode system without microcavity structure, which can significantly impact on the fluidic nature of the discharge, was fabricated to study the influence of the geometry factor in the microplasma generation process. The fabrication details of these microelectrodes are out of the scope of this paper and can be found in the reference [18]. 3. Experimental 3.1. Materials The slurry-based CNTs are versatile in the CNTF patterning technology [19,20] and capable of low cost and large area applications. The slurry generally consists of the organic solution and polymer matrices, except for the CNT powder and other functional additives [21,22]. The liquidity of the CNT slurry kept by the organic solution can guarantee the film formation capability through different technologies, such as spin-coating and printing. After the heat treatment (the parameters depend on the composition), the solution and part of the matrices can

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Fig. 2. Processes of the fabrication flow.

be removed and result in a composite film with controllable purity of CNTs. In this paper, turpentine and ethylic cellulose were used as the organic solution and the polymer matrices, respectively; the ratio was 100:3 in weight. The CNT powder and the organic component were mixed around by steel ball milling for 20 min. The powder bought from NTP Co. Ltd. consists of ∼99% purity, highly dispersed multiwalled CNTs of ∼40–50 nm in diameter and ∼10–15 ␮m in length. The square resistivity is 400–600 ± 20 /, which is measured by a four-probe method; this means that the CNTF electrode itself can act as an efficient current-limiting resist in the test loop. 3.2. Fabrication methods 3.2.1. Process flow The fabrication of HSEN can be realized in two steps of microlithography process, as shown in Fig. 2, after the multilayered film stacking processes. The first step defined the metal lead layer pattern that is identical to the CNTF electrode pattern, and argon plasma etching was used to patterning the Cr/Au layer. The Cr/Au film was 300 nm in thickness and sputter-deposited. The second step generated the CNTF pattern, with the same mask defining the metal layer, after the deposition of the CNTF. The processes are explicated in detail as follows. 3.2.2. CNTF deposition method A conventional screen-printing method is capable of selectively patterning of CNTs slurry. However, the pattern quality is very poor, e.g., the rough edge of the film; besides, the alignment among multiple-films in the micro pattern transfer technology is also quite limited. Consequently, an efficient CNTF microprinting method is introduced into the HSEN device fabrication in order to improve the pattern quality [23]. This method is based on micro-photo resist molding and screen-printing; it can significantly improve the process alignment capability. Fig. 3 is the schematic diagram of the process flow. First, a micro photolithography method was used to pattern the photoresist mold layer on a Cr/Au film with certain electrode patterns as indicated in Fig. 3a. Second, the CNTs slurry was screen-printed (200 mesh) on the surface of the whole wafer, where the mold can be filled with the slurry as indicated in Fig. 3b. Third, the wafer was annealed at 90 ◦ C for 6 h; this could cause the shrinking of the slurry film and its rupture along the edge of the mold as indicated in Fig. 3c. Fourth, the mold was removed through developing in the acetone; then, after annealing at 350 ◦ C for 40 min, organic solutions were partially removed and an array of CNTFs of 1.5 ␮m in thickness was formed.

Fig. 3. Schematic of the process flow of the CNTF micropatterning. The inset of (c) is the optical micrograph of a sample after annealing process.

3.2.3. CNTF micropatterning Single-mask micro pattern transfer process is needed for the fabrication of the sensor array. As indicated in Fig. 2b, after the photolithography process using SU-8 photoresist, plasma patterning of CNTFs was performed in a RIE system (Nextral NE100) for 40 min with 50 sccm O2 as an etchant. Because the photoresist was difficult to be etched by the oxygen plasma, the plasma lateral flow undercut the CNTFs and resulted in a HSEN structure demonstrated by the FESEM image of Fig. 6b. 3.3. Discharge tests The discharge behavior of the HSEN microelectrode systems in five atmospheric gases has been carefully examined. The method and apparatus being used to establish certain gas environment are schematically shown in Fig. 4. In detail, the species and concentration of the tested gases were controlled using a partial pressure method in a vacuum chamber at 280–300 K [24]. In the case of obtaining a high purity (>99.9%) gas, e.g., He or CO2 , the chamber was pumped to 3 × 10−3 Pa, then He or CO2 was introduced to 1 atm. To increase the purity, the ‘pumpingintroducing’ procedure was repeated three times. In the other case of obtaining gas mixtures with certain concentration, e.g., 1% or 10% of He or CO2 mixed with air, the vacuum chamber filled with high purity gases was pumped to 0.01 or 0.1 atm, and air was introduced until reaching 1 atm. The maximum concentration deviation (3%) of the gas samples was determined by the precision of the digital gauge, if the leakage of the chamber was not considered. The external current (I) as a function of the applied voltage (V, 0–40 V) and the sampling time (t) was examined using an Agilent 4156C. The sampling point number was

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mode device works at two statues, where the applied bias (Vsb ), which is lower than the breakdown criterion of the air (Vst ), is equal to the breakdown threshold for the target gas mixtures (Vtg ). • Statue I: ‘common’ state, Iext in the external loop is in the range of environmental noises (Inoise ) and the system just stands by, with little power consumption. • Statue II: the target gas present, Iext is in the predefined range of the target gas mixtures (Itg ) and the system launch further re-examination procedures for alarming.

Fig. 4. Schematic view of the test set-up.

set to 850–1000, which indicated a 31–47 mV step increment and a 15–50 ms time interval. 3.4. System design In the range of low pressure less than 1 atm, where the cold plasmas can be reliably operated, the ionization sensor can be used to distinguish the pure gases and monitor the gaseous concentration corresponding to the pressure changes. If the HSEN sensor is capable of monitoring the changes in gas species and concentration at atmospheric pressure, under the condition that it can be operated safely and reliably, its application range will greatly be broadened. To such a gas monitor, e.g., an alarm of poisoning gases, there should be two operation modes, i.e., the sweeping mode and the ‘stand-by’ mode, for two kinds of gas mixtures, respectively. The ‘stand-by’ mode is used to deal with the gases that are mixed with air so that the breakdown voltage of the air is decreased. As shown in Fig. 5a schematically, the ‘stand-by’

The sweeping mode is to deal with the gaseous species, when mixed with air, which can increase the breakdown voltage of the gas mixture over the air. The ‘stand-by’ mode will fail in such a circumstance that the Vsb will always be higher than the Vst and keep the device in the state of discharging, which consumes the power and erodes the electrode materials, fruitlessly. As shown in Fig. 5b, at the sweeping mode, a bias that is higher than Vst but lower than the breakdown voltage of the target gas mixture (Vtg ) is applied across the gap, in the pulsating mode, which can save the power consumption in the dutycycles and limit the possible erosive temporal-spatial propagation of the discharge. In the common state, Iext is in the range of the discharge current of the air (Ist ); while the target gas presence, Iext is reduced to the noise level due to Vsw , which is larger than Vst . After certain re-examination process was launched, the signal can be processed via, say, a voltage comparator, then, the system alarms. The electric parameters of the operation modes are determined experimentally and stored in the circuit memory as the references to distinguish the target gases; and the understanding of the character of the current versus voltage (I–V) and current versus time (I–t) relationships in the discharge process are fundamentally important to the sweeping mode and the ‘stand-by’ mode, respectively. This will be dealt with in Section 4.2.

Fig. 5. Diagram of the system design. (a) ‘Stand-by’ mode: the system for the target gas with a lower breakdown voltage than that of air. (b) Sweeping mode: the system for the target gas with a higher breakdown voltage than that of air. Vst , Ist , Vtg and Itg are the breakdown voltage and current for air and the target gas, respectively; Vsb , Vsw are the applied biases in the two modes; Inoise is the measurement noise; Iext is the measurement current.

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Fig. 6. FESEM micrographs of (a) the device structure, (b) the microcavity, (c) film stacking of a beam and (d) the surface morphology of CNTF. The inset of (a) is the FESEM image of the device layout.

Fig. 7. FESEM micrographs of a ‘CNTs-to-CNTs’ referenced microelectrodes without microcavity.

4. Results and discussion 4.1. Device characterizations Fig. 6 shows the field emission scanning electron microscopy (FESEM) images of the device. In Fig. 6a, the spacing region between every two microelectrodes, shown schematically in Fig. 1a, forms the microslot gaps on the scale of several micrometers. The FESEM observation reveals that the undercut effect in the O2 reactive ion etching (RIE) process of the CNTFs generates the microcavity geometry of the device, which can be observed in Fig. 6b. To investigate the states of CNTs in the device, parts of the SU-8 photo resist (PR) film on the CNTF were flaked off

and exposed for observation. Fig. 6c is the FESEM image of the in situ surface morphology of the intersection region between the CNTF and SU-8 PR film. Fig. 6d is the magnified micrograph of the surface morphology, in which, one can find that the CNTs are blended with the matrix of the SU-8 polymer, but the density is still very high even at the interface region. Furthermore, this method can help to evaluate the undercut depth, i.e., the depth of the microcavity. The FESEM images of the ‘CNTs-to-CNTs’ microslot electrode structure with lower density of CNTs than the HSEN device are demonstrated in Fig. 7, where an open structure without cavity can be evidently observed. In Fig. 8, the appearances of the fabricated device chip after dicing, welding of the external leads and packaging are demonstrated.

Fig. 8. The photographs of the fabricated chip after (a) dicing, (b) welding the leads and (c) packaging. The scale bars in the images are 8 mm long.

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Fig. 9. (a), (b) and (c) are the plots of voltage sweep results of a HSEN device with a gap spacing of 8 ␮m, from which three groups of data ranging in 18–22 V have been plotted at the same time in (d).

4.2. Basic electric characteristics 4.2.1. I–V and onset characteristics The atmospheric air electric discharge I–V characteristic curves of different sweep times of the device with a gap spacing of 8 ␮m are demonstrated in Fig. 9. The devices with other gap spacings behave similarly. Fig. 9 indicates that the threshold voltage of the discharge in air in this electrode system is only around 20 V, which is below the 36 V safety criterion and can be powered by handful batteries. Comparing with recent literatures [8–10], the threshold voltage of the HSEN device can be over 10 times lower than that of the conventional designs. This result may be attributed to the factors inherent in the CNT electrodes, including the large electric field enhancement factor (β) [25,26] and fine field emission property [27], which can strongly change the nature of the electric field distribution. Moreover, the short gap spacing can further lower the necessary bias to generate an intense electric field. A numerical study will be given in Section 4.4.1. Besides, the breakdown current density is on the order of 101 –102 mA/cm2 , which can guarantee an output on the order of microampere, and result in an acceptable signal to noise ratio (SNR). However, as shown in Fig. 9d, one can find that the good reproducibility in those breakdown bursts, which indicates that the largest deviation of the breakdown voltage (voltage corresponding to the first burst peak) is less than 0.25 V and the largest relative deviation of breakdown current from the averaged value of 50 sets of results is less than 8.5%. Based on the analysis of the I–V characteristics, it is reasonable to define the breakdown voltage criterion according to the voltage that corresponds to the peak of the first burst, which is highly reproducible, compar-

atively. Besides, the pulse width of the discharge current may not be resolved by the 4156C, i.e., the poor reproducibility may be partially caused by the apparatus being used and the float of the breakdown burst measurements may partially impute to the same instrumental source. Devastating sparking current (80–100 mA) without a current limit resistor occurs at about 190 ± 30 V for a device with a gap spacing of 8 ␮m and it is not reproducible. This cannot be measured by Agilent 4156C because of the protection compensation setup in the apparatus. The sparking current is on the order of 103 –104 A/cm2 . By introducing a nickel microelectrode with similar structure to the HSEN device, one can estimate the contribution of CNTs to the effect of lowering the threshold voltage. The referenced nickel microelectrode is realized simply through micro-electroforming technology, a standard MEMS fabrication process (see reference [18]); its geometry parameters are (1) S (gap spacing) ≈ 4.5, 6, 8, 10, 12, 14 ␮m; (2) d (height of the electrode film) = 6 ␮m. Except for electrode height, the other geometry parameters are identical to that of the HSEN’s, because the very same mask was used in the microlithography process of their fabrication. Table 1 demonstrates the air breakdown threshold voltages and currents of the two devices. Except for the nickel electrodes with S ≈ 4.5 ␮m (Vt = 170–200 ± 35 V), the discharges of others cannot be monitored using Agilent 4156C because the discharge breakdown voltage in those devices is higher than its measurement criteria (200 V). One can see that, if the CNTs are not incorporated, the breakdown voltage is apparently higher as indicated in Table 1; this result is within the expectation of the publications about gas dis-

Table 1 Comparison of the discharge thresholds in air between the devices with or without CNTs Gap size (␮m) 6

With CNTs Without CNTs a

Thresholds.

8

10

12

Vt (V)a

It (␮A)a

Vt (V)a

It (␮A)a

Vt (V)a

It (␮A)a

Vt (V)a

It (␮A)a

14 ± 0.3 359 ± 20

0.44 ± 0.07 0.5 ± 0.15

20 ± 0.3 392 ± 20

0.68 ± 0.1 0.5 ± 0.15

25 ± 0.3 413 ± 20

0.91 ± 0.2 0.7 ± 0.2

37 ± 0.3 427 ± 20

0.85 ± 0.14 0.8 ± 0.2

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Fig. 10. Current vs. time sampling results at applied voltages of (a) 8 and 18 V, (b) 19 V, (c) 30 V and (d) 20 V.

charge experiments of metallic electrode systems [6,7,15,16]. Besides, in the case of nickel electrode, the discharge is always in the form of electric sparking, which causes the electrode damages. 4.2.2. I–t characteristics Fig. 10 demonstrates the I–t curves of the devices with a gap spacing of 8 ␮m. The devices with other gap spacings behave similarly, regardless of the magnitudes. Phenomenally, the I–t curves at some definite applied voltages from 2 to 35 V in 1–2 V steps can be ranked into three groups, possibly reflecting three discharge evolution phases: (1) 1–100 pA corresponding to the range of 0–6 V; (2) 10–50 nA corresponding to the range of 8–19 V; (3) 0.1–2.5 ␮A corresponding to the range of 20–40 V. As shown in Fig. 10, in contrast to the random pulses in phase III, in phases I and II (the signal curve in phase I is not plotted because they reflect more likely the environmental noises), no pulsating-like patterns have ever been recorded except for the cases of 18 and 19 V that are close to the transition threshold. Fig. 10 also indicates that, in phase II, I increases with t; this will be discussed in Section 4.4.2.

4.3. Sensing performance 4.3.1. Sensitivity As shown in Fig. 11, which shows ionization thresholds in different gases, the device can be operated based on distinguishing gases with their relevant breakdown thresholds, including the threshold current (It ) and the threshold voltage (Vt ). Generally, the average deviation of It is in the range of 8–12%, which is greater than that of Vt (4–6%). This is because It is determined by the number of bridging channels, which are less reproducible comparing with Vt that is determined by the electrode geometry and the gas property. Moreover, another interesting result of Fig. 11 is that mixing a small amount of (10%) CO2 or (1%) He in air can produce appreciable changes in Vt . Such responses of the device are also within the expectations of the ionization model induced by electron-dominated collision. As expected by the model, the addition of CO2 can increase the breakdown threshold because the electronegative CO2 molecules consume electrons via impacts in the discharge and result in higher breakdown energy provided by the electric field. The reason why 1% volume proportion of helium molecules added in air can produce observable differences in threshold values is possibly because

Fig. 11. The breakdown voltage (Vt ) in different gases of the HSEN devices with different gap sizes (SAB = 6 ␮m, SBC = 7 ␮m, SCD = 8 ␮m, SDE = 10 ␮m and SEF = 12 ␮m). From (a) to (e), the plots demonstrate the sensitivity and selectivity of the array to certain gas; from (f) to (j), the plots demonstrate the sensitivity of certain device to different gases. To show the results more precisely, different scales are used from (f) to (j).

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Table 2 Comparison of the breakdown voltage (Vt ) and current (It ) in different gases between the microelectrode systems with and without microcavity S = 8 ␮m

S = 10 ␮m

With cavity

He Air CO2

Without cavity

With cavity

Without cavity

Vt

It

Vt

It

Vt

It

Vt

It

4.91 19.85 23.75

1.32 0.68 0.78

6 35 57

0.80 0.40 0.55

9.24 24.77 31.24

1.54 0.91 0.90

8 50 73

0.45 0.37 0.24

the helium molecules can ‘dilute’ the gaseous environment due to their much smaller impact cross-section than those of nitrogen and oxygen molecules. This leads to the circumstance that the electrons can acquire more energy; thanks to the increased mean free path. This result reflects that the threshold behavior of discharge in the gas spacing on the order of several micrometers is still sensitive to the gas species, although the collisions are relatively less intense. Table 2 demonstrates the comparison of the microelectrodes incorporating CNTs with and without microcavity geometry. The results show that, at the same gap spacing, the criterion voltages in the tested gases are greatly lower in the micro-hollowed structure than that of the open structure, while the criterion current is higher. This may partially (other reasons will be discussed in Section 4.4.2) be resulted from a 10 M resistor in series in the test loop of the open structure device, which is necessary to prevent from the thermal plasma generation. Consequently, Table 2 exhibits that the HSEN type of devices are more competitive in the aspects of lowering the operation feeding bias and the higher SNR. Sensitivity tests of the referenced metallic microelectrode were not performed, due to its high operation voltage, poor reproducibility and high probability of hot plasma generation. 4.3.2. Selectivity-array’s behavior The other fundamental information indicated in Fig. 11 is that the thresholds are very sensitive to the gap size; this could be used to increase the identification accuracy because more ionization characteristics could be utilized as the references for a gas. Technically, this could be realized easily through fabrication of more slot patterns. This feature may be considered as a high performance array behavior of the HSEN device. 4.3.3. Stability The evaluation of the stability of the gas sensor probably concerns four aspects, i.e., the sensitivity poisoning, the irreversible material damaging, the device life-span and the signal environmental interfering. Firstly, the poisoning of sensors means that the effect that the sensitivity depends on cripples the sensitivity in turn. Almost all of the absorption type of gas sensors suffers poisoning problems, which may exhibit, functionally, as long recover time, or the necessity of some additional treatments. Two factors inherent in gaseous discharge process, i.e., new molecules generation and residual charges may cause possible poisoning effects on the sensitivity of the ionization gas sensor. The first effect that

changes original gas species can be limited by the optimization of the design of electrode structure, where the gap can be drafty for gas flow. Another method is to keep the discharge in the nonthermal plasma status to limit the chemical reactions. For the second effect, although it is transient, the discharge-generated charged particles may residue in the vicinity of the CNTFs. This problem can be eased by applying a pretreatment neutralization bias to the electrodes, with an inverse polarity to the former time. Secondly, thermal plasma, e.g., sparks and arcs, can cause irreversible damages to the electrode materials and result in completely disabling the sensitivity. Both of the referenced electrodes suffered from sparking greatly. It is a simple method to connect a resistor in series with the sensor to prevent thermal plasma generation in high-pressure gases, but this can significantly reduce the output, as well as the SNR of the sensor, e.g., the case of ‘CNTs-to-CNTs’ side-wall electrodes. Another commonly used method, under the name of ‘dielectric barrier discharge (DBD)’, is to deposit a dielectric layer on the surface of the electrodes to prevent the direct contact between the plasma and the electrode materials. This can result in the accumulation of charges with the inverse polarity to the electrode on the surface of the dielectric layer and lead to the quenching of the discharge before thermal plasma formation, due to the decrease of the electric field in the gap so that the discharge cannot sustain. In the I–V curves of the HSEN device, shown in Table 2, at the applied bias of 40 V that is two times of the breakdown threshold criterion, the discharge is still limited in the non-thermal domain; this should be considered as an inherent advantage of the HSEN device to limit the thermal plasma damages. Thirdly, the device was tested for about 150 times in maximum, but this cannot determine the life-span of the HSEN device. Generally speaking, it is mainly the thermal plasma generation that damages the electrodes and shortens their ‘lifespan’. As discussed above, the HSEN devices can be free from such a damage source. Besides, the materials used in this design were chemically stable and there were proper heat treatment process after every film stacking process, which could cure the residual stress in the film interfaces. Consequently, it is reasonable to believe this novel device may have a moderate life-span. Fourthly, some publications believe that the environmental factors, e.g., humidity and temperature, only moderately affect the discharge process and the threshold values [6–8]. The scrutiny of the effects of those factors on the device performance is necessary for further researches.

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4.4. Discussion and modeling 4.4.1. A numerical study of the electric field distribution of CNT electrodes Consider the uniform electric field in the parallel plate electrodes with a gap spacing of 8 ␮m and an applied voltage of 20 V, about 2.5 V/␮m, one can find that the field is not intense enough to initiate an electrical breakdown (say, for atmospheric air, Vt = 330 V, based on Paschen’s law). Consequently, CNTs must play an essential role in the electric field distribution of the HSEN electrodes, and this section focuses on its quantitative characterization to investigate these questions: (1) the intensity and the range of the CNTs’ impact on the field distribution, and (2) the impact of the configuration of the CNTs on the field distribution. Granted that the electric field enhancement effect can be treated through handful methods [25,28,29], the evaluation of the macroscopic character of a vast number of CNTs, e.g., β of the CNTFs, or the spatial distribution of the modified field, strongly relies on the experimental and numerical method. In this study, the numerical calculation method is preferred, because it allows picturing the spatial distribution of the electric field with moderate details and precision. We solve the Laplacian equation, the electric field model in classical field theory, using the finite element method (FEM) in two- and three-dimension. Two models are calculated: one concerns the condition that two CNTs confront each other ‘tip-to-tip’, and the other ‘sidewallto-sidewall’. These two models are the ‘ideal’ configurations that approximate the extremes of the practical statues of every CNT, randomly distributed in a massive stacking pile—a printed CNTF. That is to say, every two CNTs in the opposite side of the HSEN electrodes may be approximated as some intermediate condition of these two models. This allows a quantitative evaluation of the actual condition of field distribution, e.g., the maximum intensity and the effective volume of β of single CNT and the CNT array. The results of the calculation along the sampling line where the field intensity reaching the maximum are plotted in Fig. 12. The insets of left and right hand sides of every figure are the magnified plots of the vicinity of central point and the CNTs, respectively. As shown in Fig. 12, one can find that, firstly, the ‘sidewallto-sidewall’ configuration can generate higher field intensity, as much as 28.67%. This indicates that the intensity of the enhancement effect of the CNTs depends on their spatial configuration. In other words, statistically, in a randomly distributed CNT array electrode system, the electric field in the vicinity of the CNT electrode has a non-uniform distribution; if the diameter and tip geometry were considered, the non-uniformity would be more complex. Secondly, the field enhancement effect of the electrodes incorporating CNTs is evident (β = 14.4 in ‘tipto-tip’ model and β = 20.4 in ‘sidewall-to-sidewall’ model), but the effective domain is so limited that the electric field can be roughly considered as uniform in most of the gap space. In order to quantitatively evaluate the volume of the space where the field intensity is larger than certain critical values, a 3D simulation is performed for ‘sidewall-to-sidewall’ models, and the results are listed in Table 3. Under the condition that corona dis-

Fig. 12. Illustration of the electric field distributions of the devices, calculated using the finite element method: (a) and (b) are of different configurations. The left and right of the inset plots are of different positions of the modeling domain.

charge model applies, two critical values, i.e., the onset voltage and the sparking breakdown voltage of the whole gap concern with the characterization of the discharge process, but here it is of interest to consider only the first value because sparking is evitable in the range of the operation voltage of the HSEN device. The value, on the order of 2.6 V/␮m for atmospheric air, is determined by the electric field at the distance at which α (first Townsend coefficient) = η (recombination coefficient), which limits the active avalanche zone, where ionization of neutrals takes place and ionized particles only drift along the field stream line outside this region. The reason why different values are given is that the concrete value cannot be given because its derivation needs to calculate Raether criteria equation [7], which involves in the determination of three ionization parameters, i.e., α, γ (secondary ionization coefficient) and η, which Table 3 Volume integration of the elements with field intensity larger than 1.3–10.4 V/␮m3

1.3 V/␮m3 2.6 V/␮m3 5.2 V/␮m3 10.4 V/␮m3

V=4V

V = 10 V

V = 20 V

V = 30 V

V = 40 V

326.28 222.04 96.05 21.80

375.73 346.03 261.70 134.61

382.65 375.73 346.03 261.70

383.49 381.10 367.69 318.92

383.76 382.65 375.73 346.03

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depend on the electric field intensity and must be given by simulation or experiments. Such works are out of the scope of this paper. Thirdly, by Paschen’s law, the breakdown voltage of a gas in uniform electric field is determined by the gas pressure (P) and the gap spacing (d). In the HSEN device with the 8 ␮m gap size, under atmospheric condition, Pd = 0.608 Torr cm; this yields about 330 V by Paschen’s curve and 42.5 V/␮m electric field strength, which is much larger than the calculated strength in the uniform electric field region (see the left insets of Fig. 12). That is to say, although the field enhancement effect of CNTs only impacts quite limited space, the discharge cannot be considered as a field-induced plasma generation process in a uniform field, which can be described by Paschen’s law. 4.4.2. A staged ionization model In the context of understanding the device operation principles and the underlying mechanisms, the electric characterization of the discharge phenomenology implies several distinct properties of the discharge deserving further investigation: • Vt in atmospheric air of the HSEN device is 10–20 times lower than the referenced metallic electrodes; • Vt in atmospheric He, air and CO2 of the HSEN device is lower than the referenced electrode with CNTs but without microcavity geometry, as indicated in Table 2; • evident self-protecting behavior in voltage sweeping; • I increases with the time at a fixed applied voltage, far lower than the breakdown criteria. We propose a staged-ionization-based hydrodynamic model of discharge, trying to elucidate the above-mentioned phenomena. This model is based on three postulations: first, ionization can be initiated in the vicinity of CNT anode and cathode region above certain criteria voltage (Vt0 ), lower than the breakdown voltage of the gap (Vt ) and lead to a space charge region of positive ions and negative ions (in the case of the electronnegative gases) resulted from discharge. Second, the microcavity geometry confines the electro-hydrodynamic flow of the ionsdrift-driven, so that the flow speed outward the cavity is slow, comparing to the case of the open geometry. Third, the space charge region can act as virtual electrodes, extending from the CNTs and buffering the bridging plasma streamer channels, at an applied voltage higher than Vt . Consider the I–t experimental results. These hypotheses imply a three-staged discharge model: • Stage I (V < Vt0 ): The gaseous discharge in an electric field properly begins with an avalanche, which is initiated only when the field exceeds certain criteria, Et0 , corresponding to Vt0 in a given electrode geometry; at an applied voltage lower than Vt0 , no ionization avalanche takes place. In the external circuit, comparing to the state of no applied voltage, no evident changes in current can be detected. • Stage II (Vt0 < V < Vt ): In a highly inhomogeneous field, avalanches develop near those regions where the field is greatest, i.e., where the radii of curvature of the electrodes is smallest, in the HSEN device, near the CNTs. Within the

space in the vicinity of the CNTs, where E > Et0 , the avalanche becomes self-sustaining only when the volume of this space exceeds a criteria, Ωt0 , corresponding to the criteria number of electrons for self-sustaining avalanche [7] and the inward (cathode) and outward (anode) development of the avalanche properly leads to space charge region formation outside the ionization region. The space charge region with the same polarity as the corresponding electrodes can increase the potential difference and the electric field across the gap. In other words, the space charge regions behave like movable virtual electrodes, analogously to the effect of decreasing the gap size. Because the field enhancement effect of CNTs is limited in space (see Table 3), Ωt0 and consequently Vt0 are sensitive to the amount of CNTs. The secondary process in the ionization region relies on the photon and ions-induced ionization resulted from the first process. In the external circuit, comparing to the state of stage I, an evident increase in current that is induced by the space charge drift in electric field can be detected, e.g., see Fig. 10a and b. • Stage III (V > Vt ): The breakdown streamer initiates when the applied voltage exceeds the criteria, Vt , at which plasma channels establish. However, they bridge the space charge regions instead of the solid electrodes, due to the confinement effect of the cavity geometry to the electro-hydrodynamic of the partially ionized gases in the vicinity of the CNTs. This can greatly increases the recombination rate of bipolar charges. As a result, the space charge region will shrink and greatly weaken the electric field in the gap so that the streamers cannot sustain. Consequently, instead of developing into a devastating spark, as in the case of the electrodes without microgravity, breakdown streamers cease their spatial propagation and prevent from damaging the solid electrodes, electrically similar to DBD. This leads to the current in the external circuit of stage III oscillating from the intense transient streamer state to the steady space charge-swarming state (stage II). Based on the model, the following statement can reasonably elucidate some of the electric phenomena observed in the experiment. First, given the fact that the field is converged in the vicinity of CNT tips, although the stressed space of every CNT is very small, the large number of the CNTs in a screen-printed CNTF can make the gross volume large enough to produce the critical number of electrons for a self-sustained avalanche or a breakdown streamer. Consequently, when CNTs are incorporated, Vt becomes smaller and Vt of the HSEN device is lower than that of the referenced CNT electrodes because the density is much smaller in the latter case, as shown in Fig. 7. Second, the self-protective-like behavior in the post-breakdown voltage range just reflects the discharge process in stage III, electrically. Third, assuming that the number density of the space charge (N) increase with the time, one can explain the phenomenon shown in Figs. 10a and b, i.e., the current that is a function of N increases with time. Another proof to this is that, as shown in Fig. 10c, at 19 V, very close to Vt , after about 55 s of accumulation, the enhancement of the potential drop induced by the space charge can compensate the external bias to meet the criteria for initiating a transient breakdown streamer. The third proof concerns

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5. Conclusions The chip-based HSEN ionization gas sensors, realized by MEMS toolkits, exhibit significant improvement in almost all key aspects of performance comparing to conventional designs, for example: (1) the operation voltage is in the range below 36 V, which is the human safety criterion, (2) the selectivity and accuracy are improved, thanks to the simplicity in producing a sensor array on a chip, (3) the self-protective behavior prevents the device from sparking damage and makes it suitable for atmospheric operation and (4) it possesses good integration compatibility as a chip-based device. Two referenced microelectrodes have been fabricated for the performance comparison and the mechanism investigation. The operation method, fundamental to the circuit level and system level design, is also discussed. Based on the experimental study of the discharge process and the numerical study of the electric field distribution of CNT-to-CNT electrodes, a three-staged discharge model is proposed and succeeds in elucidating the experimental results, qualitatively. The performance of this design makes it a competitive candidate for developing smart MEMS-based gas sensor systems. Acknowledgement This work was supported by National Basic Research Program of China (No. 2006CB300406). Fig. 13. The I–t relationship expressed by linear curve fitting: (a) is the method of the fitting method for the current sampling data containing current bursts. In (b), P1, P2 and P3 represent the fitted line in stage II; Q1, Q2, Q3, Q4 represent the fitted line in stage III. In both (a) and (b), Sm-n is the fitted No. n line in short time at the same voltage as the Qm . The unit of applied voltage is in volts.

the dynamic changes of I–t relationships in different phases and is shown in Fig. 13, where several sheets of current sampling results in stage II (P1 , P2 and P3 ) and stage III (Q1 , Q2 , Q3 , Q4 , S2-1 , S2-16 , S4-1 and S4-11 ) have been plotted, in a way of linear curve fit. The method of plotting the linear fit of stage III, which ignores the portion of breakdown bursts, is schematically illustrated in Fig. 13a because the breakdown bursts can disturb the distinction of space charge drift induced currents. If the postulation of N increase with t is valid, the slope of the plots should reflect the speed of the temporal development of N. Furthermore, if it is valid, in Fig. 13, one should find that (1) the slope is positively proportional to the applied voltage; this is true because the higher the applied voltage (stage III) is, the greater speeds N can develop in, because higher energy can be provided by the background field. (2) The base-line current in stage III, i.e., the non-burst current is larger than that in stage II; this is because the velocity of the space charge flow is larger in the higher electric field at stage III. In addition, the breakdown bursts resulting in the shrinking of N can slow the speed of temporal development of N down in the long run (Q lines), comparing to its speed in short term (S lines). (3) Because the breakdown bursts are stochastic in nature, the slope of the I–t curves after linear fitting cannot reflect the relationship between the temporal development speed of N and the applied voltage in stage III, just as the case of Q lines in Fig. 13b. This is in contrast to the condition of stage II.

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Biographies Zhongyu Hou received the BS degree (with honor) in metal casting engineering and MS degree in laser processing engineering from Jilin Institute of Technology in 2001. In 2007, he received his PhD degree in electronic science and technology in Shanghai Jiaotong University. His research interests include micro plasma and nuclear physics, nano-scale science and technology, microelectronic fabrication technology and MEMS-based electronic devices. Hai Liu is a MS candidate in electronic science and technology in Shanghai Jiaotong University. His research interests include MEMS-based gas sensors. Xing Wei received the BS degree in materials science from School of Materials Science and Engineering, University of Science and Technology, Beijing in 2004. She received a MS degree in Shanghai Jiaotong University, in 2007. Her research interests include micro/nano systems, especially micro gas sensors. Jiahao Wu received the BS degree in physics from Shanghai Jiaotong University in 2005. He is a MS candidate in microelectronic now. His research interests include MEMS-based gas sensors. Weimin Zhou received the PhD degree in microelectronics and solid-state electronics from Shanghai Jiaotong University in 2007. Currently, he is a researcher at Shanghai Nanotechnology Promotion Center, China, and is centering on the development and applications of nanoimprint lithography (NIL) techniques to the fabrication of nanostructures and nanodevices. His previous research work includes synthesis of nanowires (Si, SiC, Ga2 O3 ) and nanowire-based sensors and electronic devices. Yafei Zhang is a professor in Shanghai Jiaotong University. His research interest includes nanoscale science and technology. Dong Xu is a professor in Shanghai Jiaotong University. Her research interest includes micro/nano electromechanical systems. Bingchu Cai is a professor in Shanghai Jiaotong University. His research interest includes electric thin film material and MEMS devices.