new perspectives of gas sensor technology

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Please cite this article in press as: N. Yamazoe, K. Shimanoe, New perspectives of gas sensor technology, Sens. Actuators B: Chem. (2009), doi:10.1016/j.snb.2009.01.023 ARTICLE IN PRESS G Model SNB-11236; No. of Pages8 Sensors and Actuators B xxx (2009) xxx–xxx Contents lists available at ScienceDirect Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb New perspectives of gas sensor technology Noboru Yamazoe, Kengo Shimanoe ∗ Faculty of Engineering Sciences, Kyushu University, 6-1 Kasuga-koen, Kasuga, Fukuoka 816-8580, Japan article info Article history: Received 15 August 2008 Received in revised form 11 November 2008 Accepted 5 January 2009 Available online xxx Paper presented at the International Meet- ing of ChemicalSensors 2008 (IMCS-12), July 13–16, 2008, Columbus, OH, USA. Keywords: Semiconductor Gas sensor Oxide Receptor Depletion MEMS abstract Two recent topics important for advancing gas sensor technology are introduced. Semiconductor gas sensors have been developed so far on empirical bases but now a fundamental theory has been made available for further developments. The theory reveals the roles of physical properties of semiconductors and chemical properties of gases in the receptor function. MEMS techniques have been applied to fabrication of micro-platforms for use in gas sensors. The micro-platforms appear to provide gas sensors with new innovative function. © 2009 Elsevier B.V. All rights reserved. 1. Introduction Looking back its history, gas sensor technology was inaugurated when three kinds of pioneering gas sensors were put in prac- tice in Japan, i.e., oxide semiconductor gas sensors for gas leakage alarms [1], solid electrolyte oxygen sensors for car emission control systems [2], and ceramic humidity sensors for automatic cooking ovens [3]. These sensors demonstrated dramatically how important it was to monitor a specific gas species in situ, in real time and continuously for ensuring safety from gas hazards, protecting environments, or making home appliances intelligent or friendly to users. Importance of such emerging technology was well recognized world wide when the first International Meeting on Chemical Sensors (IMCS) was held at Fukuoka, Japan, in 1983, under the lead- ership of the late Professor Seiyama [4]. Research and development was triggered off all over the world to seek new and/or better gas sensors. Currently various kinds of gases including reducing ones (methane, propane, carbon monoxide, ammonia, hydrogen sulfide, etc.) and adsorptive ones (oxygen, nitrogen dioxide, ozone, etc.) have been made detectable with gas sensors using semiconductors, electrolytes or catalytic combustion. Yet there are various new demands to gas sensors ranging from detecting VOCs (Volatile ∗ Corresponding author. Tel.: +81 92 583 7876; fax: +81 92 583 7538. E-mail address: simanoe@mm.kyushu-u.ac.jp (K. Shimanoe). Organic Compounds) at very low concentrations (ppb levels) to constructing sensor network systems. Needless to say, even the established gas sensors are demanded for innovations towards better sensing performances, lower power consumption and more compact device structures. To meet these demands, semiconductor gas sensors are considered to be best suited because they have advantageous features such as simplicity in device structure and circuitry, high sensitivity, versatility and robustness. It is pointed out that most of the sensors in this group have been developed empirically from a lack of theoretical understandings. Fortunately, we recently succeeded in deriving theoretical equations to describe the response of these sensors to adsorptive or reducing gases quantitatively [5–7]. We expect that the new theory will provide useful guidelines on how to elaborate selection and processing of sensing materials and additives used as well as device structure and fabrication techniques to be used. From such a standpoint, theory of semiconductor gas sensors is introduced here as a main topic. Another topic, MEMS-assisted gas sensors, is selected because those are likely to give rise to a new breakthrough in gas sensor technology. 2. Theory of semiconductor gas sensors 2.1. Overview of empirical information and problems Semiconductor gas sensors detect a specific target gas from a change in the electric resistance of a sensing body which is a 0925-4005/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2009.01.023 Please cite this article in press as: N. Yamazoe, K. Shimanoe, New perspectives of gas sensor technology, Sens. Actuators B: Chem. (2009), doi:10.1016/j.snb.2009.01.023 ARTICLE IN PRESS G Model SNB-11236; No. of Pages8 2 N. Yamazoe, K. Shimanoe / Sensors and Actuators B xxx (2009) xxx–xxx Fig. 1. Three basic factors controlling semiconductor gas sensors. porous assembly of tiny crystals (particles) of oxide semiconductors such as SnO 2 . Gas sensing properties have been accounted for by three basic factors, namely, receptor function, transducer function and utility factor, as schematically shown in Fig. 1 [6–10]. The first one is concerned with how each crystal responds to the stimulant gas in problem, while the second does how the response of each crystal is transduced into device resistance. The third one describes how the device response (resistance change) is attenuated in an actual porous sensing body due to a consumption of the stimulant gas during its diffusion inside. Among these factors, only the last one has been clarified theoretically [9,10]. As for the receptor function, it is known that oxygen is adsorbed on the crystals in air, presumably as O − ,overthetem- perature range of interest [11] to form a depletion layer in them. Upon contact to a stimulant (H 2 ), the oxygen adsorbates are consumed more or less, causing the depletion layer to decrease. The transducer function can also be easily understood qualitatively based on the double Schottky barrier model so far assumed popu- larly. It is remarked, however, that the double Schottky barrier model is nothing but estimated as an extension from other semiconductor devices. In fact, some sensor devices fabricated with wet-coating techniques have been found to exhibit temperature- independent resistance in air in disagreement with the model, and tunneling transport of electrons between adjacent crystals has b een strongly suggested instead [8]. The scheme of receptor function above is also too qualitative, failing to account for many pieces of experimental information including grain size effects on sensitivity. Definitely quantitative approaches to the receptor function are badly neede d. This is a main concern here in the first topic. 2.2. How to formulate receptor function 2.2.1. Scope of formulation On each semiconductor crystal, adsorption and/or reactions of gases take place to capture or release electrons, while a corresponding redistribution of electrons takes place inside to achieve an electrostatic equilibrium. The surface chemical affair and the subsurface physical one are not independent but united together, and this provides a base on which the receptor function is formulated as follows. The chemical affair can be formulated easily. In an oxygen atmosphere, for instance, oxygen adsorption is expressed as follows: O 2 + 2e = 2O − (R1) (K O2 P O2 ) 1/2 [e] s = [O − ] (1) Here K O2 and P O2 are adsorption constant and partial pressure of oxygen, respectively, and [e] s and [O − ] are surface densities of free electrons and O − , respectively. In the presence of a reducing gas (H 2 ), O − is consumed by the reaction: H 2 + O − = H 2 O + e (R2) By coupling with (R1), the following equation results in the steady state: K O2 P O2 [e] s 2 = [O − ] 2 + cP H2 [O − ] (2) P H2 is partial pressure of H 2 , and c is a constant defined by c = k 2 /k −1 , where k 2 and k −1 are rate constants of (R2) and reverse reaction of (R1), respectively. Eqs. (1) and (2) combine between [e] s and [O − ] in the oxygen atmosphere in the absence and presence of H 2 , respectively. Since the electrostatic equilibrium condition gives rise to another interrelation between [e] s and [O − ] as stated later, the two variables can be determined uniquely if P O2 , P H2 and physical parameters of semiconductor crystals are fixed. 2.2.2. Electrostatic equilibrium for large crystals Let us consider electrostatic equilibrium inside a semiconductor crystal. In case the crystal is large enough, depletion is limited in the shallow region from the surface. We can assume a flat surface for which the electrostatic equilibrium has been discussed well by using an energy band diagram as shown in Fig. 2. A location in the crystal is expressed by a depth from the surface, x. Under simplifying conditions of complete ionization of donors, no tailing of electron distribution, and no surface states other than O − , Poisson’s equation Fig. 2. Potential energy diagram for depletion in a large semiconductor crystal. Here qV(x) is the potential energy of electron, qV s surface potential energy of electron, x distance from the surface, w depletion depth, E c conduction band edge, E v valence band edge, and E F is the Fermi level. Please cite this article in press as: N. Yamazoe, K. Shimanoe, New perspectives of gas sensor technology, Sens. Actuators B: Chem. (2009), doi:10.1016/j.snb.2009.01.023 ARTICLE IN PRESS G Model SNB-11236; No. of Pages8 N. Yamazoe, K. Shimanoe / Sensors and Actuators B xxx (2009) xxx–xxx 3 can be solved easily to give the following equations [5]: qV kT =  1 2  x − w L D  2 (3) qV s kT =  1 2  w L D  2 (4) [e] s = N d exp  −qV s kT  (5) [O − ] = −Q sc q = N d w (6) Here, q is the elementary charge of electron, kT thermal energy, V and V s electric potentials at depth x and surface, respectively, w depletion depth, N d donor density, L D Debye length defined by L D =(q 2 N d /εkT) −1/2 (ε, permittivity), and Q sc is the surface charge density. [e] s and [O − ] are seen to be correlated implicitly through w by Eqs. (4) through (6). By coupling these equations with (1) or (2), [e] s and [O − ] can be solved for given P O2 and P H2 . 2.2.3. Electrostatic equilibrium for small crystals In case the crystal is small, we encounter two kinds of uncon- ventional phenomena. First, depletion extends to cover the whole crystal with increasing P O2 , and what would happen thereafter? Second, the solutions of Poisson’s equation become dependent on the shape of crystal. These phenomena have important meanings for actual semiconductor gas sensors, as discusse d later. The first phenomenon is illustrated schematically in Fig. 3, where three thin plates with thicknesses of l, l/2 and l/4 are exposed to stepwise increasing partial pressures of oxygen, P O2 (I), P O2 (II) and P O2 (III). When depletion extends to l/4 from both surfaces for the thickest plate at P O2 (I), depletion just covers the whole region for the l/2 thick plate, while the thinnest plate is put into a new type of depletion. When depletion covers the whole region of the thickest plate at P O2 (II), the thinner plates are both in the new type depletion. The new type depletion is seen to show up more easily (at lower P O2 ) as the thickness is reduced or P O2 is increased. The new type depletion and the conventional type one are called here volume depletion and regional depletion, respectively, and the border state is called boundary depletion. Nature of volume depletion is easily understood from the dia- grams of potential energy and electron distribution shown in Fig. 4, where the one-dimensional coordinate, x, is redefined as a distance from a center of the crystal (thickness 2a). The crystal is in the flat band state at P O2 = 0 as assume d. In the presence of oxygen (P O2 (I)), surface state (O − ) is formed and depletion takes place from both Fig. 3. Stages of depletion in thin semiconductor plates with three different thicknesses placed under three different partial pressures of oxygen. Fig. 4. Potential energy diagram (a) and electron distribution diagram (b) for depletion in a thin semiconductor plate. sides of the plate to a depth at which an electronic equilibrium is reached between bulk and the surface state (O − ) (regional depletion). Free electrons have been transferred from a shallow region as shown in the electron distribution diagram, where a tailing effect of distribution is taken into account. With increasing P O2 , the surface state (O − ) shifts to lower energy and so depletion depth w also increases until it reaches finally the center of the plate (w = a) at P O2 (II) (boundary depletion). For a further increase in P O2 to P O2 (III), however, there is no room to extend w further. Instead the location of Fermi l evel shifts down by an adequate quantity (pkT), while keeping the potential energy profile the same as that for P O2 (II), to satisfy the new equilibrium (volume depletion). At this stage, free electrons are squeezed out of the whole plate, as seen from the electron distribution diagram. This is a reason why it is named volume depletion. It is inferred that volume depletion corresponds to Region III reported by Rothschild and Komen [12,13]. The potential energy of electrons inside the plate is formulated for each type of depletion as follows [6]: qV r (x) kT =  1 2  x − (a − w) L D  2 (7) qV b (x) kT = (1/2L 2 D )x 2 (8) qV v (x) kT = (1/2L 2 D )x 2 + p (9) Please cite this article in press as: N. Yamazoe, K. Shimanoe, New perspectives of gas sensor technology, Sens. Actuators B: Chem. (2009), doi:10.1016/j.snb.2009.01.023 ARTICLE IN PRESS G Model SNB-11236; No. of Pages8 4 N. Yamazoe, K. Shimanoe / Sensors and Actuators B xxx (2009) xxx–xxx Fig. 5. Potential energy diagram drawn relative to the flat band state. Suffices r, b and v stand for regional, boundary and volume depletion, respectively. Once the potential energy is known, [e] s can be derived from (5). On the simple abrupt model (Fig. 2), [O − ]isgiven by (6). For more precise treatment, it should be modified to [O − ] = N d w{1 − A(a, w)} (10) Here A(a, w) is a quantity to be corrected for non-ideal behavior such as the tailing effect. Thus the two variables, [e] s and [O − ], are correlated implicitly by (9) and (10), and hence they are determined uniquely by coupling these equations with (1) or (2). The meaning of Fermi level shift, pkT, is worth being discussed. Fig. 5 is a potential energy diagram redrawn relative to the flat band state. On going from boundary depletion to volume one, the potential energy shifts up by pkT. The shift causes the surface potential energy to increase and so causes [e] s to decrease correspondingly. On the other hand, oxygen adsorption equilibrium (1) indicates that an increase in P O2 should be met by a change in [O − ]/[e] s ratio. In the stage of volume depletion, [O − ] cannot increase so much since most of conduction electrons available have been exhausted already. That is, oxygen adsorption is controlled by a supply of electrons. This means that the change in the above ratio is achieved mainly by that in [e] s . Thus it can be stated that the Fermi level shift plays a role to connect between the surface chemical equilibrium and the subsurface electrostatic equilibrium in the stage of volume depletion. Now we consider the second phenomenon, dependence on the shape of crystals. Fig. 6 illustrates the coordinates systems conveniently selected for three shapes of crystals, i.e., one-dimensional coordinate (plate), three-dimensional spherical coordinates (sphere) and three-dimensional columnar coordinates (column). By choosing such coordinates systems, Poisson’s equation can be simplified as follows: d 2 V dx 2 = −qN d ε (plate) (11)  1 r 2  d dr  r 2 dV dr  =−q N d ε (sphere) (12)  1 r  d dr  r dV dr  =−q N d ε (column) (13) The solution of (11) has already been shown as (7)–(9) for plates. Similarly (12) and (13) can b e solved mathematically under the same boundary conditions [6]. Thus [e] s and [O − ] can also be determined uniquely for spheres and columns. 2.3. Response to oxygen and other stimulants As so far mentioned, we can formulate receptor function theoretically. In order to visualize it, however, we need to transduce it into device resistance (R). For this purpose, it is assumed that R is proportional to [e] s , and that it follows the following equation: R R 0 = [e] s N d = exp  −qV s kT  (14) R 0 is device resistance at P O2 = 0 (flat band state). Although R is influenced by many factors other than the receptor function, those factors are cancelled out for R/R 0 , which is called reduced resistance hereafter. For devices consisting of large crystals, R/R 0 is correlated with P O2 and P H2 by the following equations. In pure oxygen, X = m exp  m 2 2  , R R 0 = exp  m 2 2  (15) In oxygen mixed with H 2 , X =  1 +  Y m  1/2 exp  m 2 2  , R R 0 = exp  m 2 2  (16) X is reduced adsorptive strength of oxygen defined as X =(K O2 P O2 ) 1/2 /L D , Y is reduced reactivity of H 2 defined as Y = cP H2 /N d L D (see (2) for c) and m is reduced depth of depletion defined as m = w/L D . The correlations, obtained by numerical calculations, explain the power laws governing the response to O 2 and H 2 , but no other important information relevant to sensing properties can be drawn. Fig. 6. Coordinates systems selected for plates, spheres and columns. Please cite this article in press as: N. Yamazoe, K. Shimanoe, New perspectives of gas sensor technology, Sens. Actuators B: Chem. (2009), doi:10.1016/j.snb.2009.01.023 ARTICLE IN PRESS G Model SNB-11236; No. of Pages8 N. Yamazoe, K. Shimanoe / Sensors and Actuators B xxx (2009) xxx–xxx 5 Fig. 7. Correlations between reduced resistance (R/R 0 ) and reduced adsorptive strength of oxygen (X) for semiconductor spheres different in reduced size (n)as drawn on logarithmic scales. Situation becomes utterly different for devices using small crystals. When the crystals are spheres of radius a, for example, the following equations are derived for the response to oxygen. For regional depletion, X =   n 3   1 −  (n − m) n  3  − A S (n, m)  exp  m 2 6   1 + 2 (n − m) n   R R 0 = exp  m 2 6   1 + 2 (n − m) n   (17) For volume depletion, R R 0 =  3 n  X +  3 n  A S (n, n)exp  n 2 6  =  3 a  (K O2 P O2 ) 1/2 + C s (n)(18) Here n is reduced radius defined as n = a/L D and m is reduced depth of depletion. A S (n, m) is a correction term for non-ideal behavior such as the tailing effect. C s (n) is a constant fairly close to unity for small n (<4). The correlation between R/R 0 and X shown by (17) is implicit and non-linear, while R/R 0 is a linear function of X or P O2 as indicated by (18). The correlations given by these equations are drawn on logarithmic scales in Fig. 7, where n is varied between 1 and 10. For large n (=10), regional depletion prevails over a wide range of X up to about 10 8 , while its range deteriorates rather sharply, being replaced by volume depletion, with decreasing n, and it becomes practically invisible at n = 2. The meanings of the correlations are made clearer in Fig. 8, where the same correlations are drawn for a small range of X up to 150 on linear scales. Linear correlations are obtained between R/R 0 and X for volume depletion, with their slopes (sensitivity) being equal to 3/n, although regional depletion is also visible for n > 3. Evidently this accounts for the grain size effect on the response to oxygen. Similar discus- sion can be made on the crystals of other shapes. For brevity, only the correlations for volume depletion are compared below: R R 0 =  1 a  (K O2 P O2 ) 1/2 + C p (n) (plate) (19) R R 0 =  2 a  (K O2 P O2 ) 1/2 + C c (n) (column) (20) R R 0 =  3 a  (K O2 P O2 ) 1/2 + C s (n) (sphere) (18) It is understood that the linearity constant (sensitivity) increases to two or three times, on going from plates to columns or spheres. This is nothing but indication of a shape effect on sensitivity. It is worth stating that the linearity constants are in coincidence with the surface area/volume ratios of the crystals of respective shapes. The response to H 2 in air can be treated similarly. Under the condition that the rate of reaction (R2) is much faster than the rate of desorption of O − , reduced resistance for a spherical crystal-based device is given by R g R 0 = N 1/2 d (ac/3K O2 P O2 (air)) −1/2 P −1/2 H2 (21) Here R g is resistance under exposure to H 2 , P O2 (air) is P O2 in air, and c has already been defined for (2). Conventionally defined response, R a /R g (R a , resistance in air), is expressed as R a R g =  Sc aN d  1/2 P 1/2 H2 (22) S is shape factor and is allotted values of 1, 2 and 3 for plates, columns and spheres, respectively. R a /R g is linear to the square root of P H2 (power law). Its slope (sensitivity) is determined by the sev- eral physical and chemical parameters included in the parentheses. To achieve high sensitivity, spherical crystals (S = 3) are best suited, and a and N d should be reduced as much as possible. It is also understood that selectivity among reducing gases is determined by their reactivity (c) under these conditions. The response to adsorptive gases can also be formulated, as exemplified here for the case of NO 2 . Its adsorption is expressed as NO 2 + e = NO 2 − (R3) K NO2 P NO2 [e] s = [NO 2 − ] (23) K NO2 and P NO2 are adsorption constant and partial pressure of NO 2 , respectively, and [NO 2 − ] is surface density of NO 2 − .NO 2 molecules compete with O 2 to capture electrons so that formulation of the response to NO 2 is fairly complex for the stage of regional depletion. For volume depletion, in contrast, it is quite simple, reduced Fig. 8. Redrawing of the same correlations in Fig. 7 on linear scales. Please cite this article in press as: N. Yamazoe, K. Shimanoe, New perspectives of gas sensor technology, Sens. Actuators B: Chem. (2009), doi:10.1016/j.snb.2009.01.023 ARTICLE IN PRESS G Model SNB-11236; No. of Pages8 6 N. Yamazoe, K. Shimanoe / Sensors and Actuators B xxx (2009) xxx–xxx resistance being expressed as R R 0 =  S a  K NO2 P NO2 +  S a  (K O2 P O2 ) 1/2 + C(n)(24) S is shape factor, being 1, 2 and 3 for plates, columns and spheres, respectively. C(n) is a small constant close to unity for small n. Adsorption of NO 2 and O 2 is seen to contribute to reduced resistance linearly as well as additively. Conventionally defined response is given as R g R a =  S a  R a R 0  −1 K NO2 P NO2 + 1 (25) R a is device resistance in air in the absence of NO 2 . It is noted that R a /R 0 cannot be derived from (24) since the condition of volume depletion is not always satisfied in the absence of NO 2 . The response is seen to be inversely proportional to a. 2.4. Applicability to experimental data The theory developed above has been shown to be well consistent with experimental data, though available data are still limited at present. Here two examples are introduced. Fig. 9 is a reproduction of the data showing grain size ef fects observed with SnO 2 -based devices [14]. In the region of smaller grain sizes, device resistances in air (R a ) as well as the responses to H 2 or CO (R a /R g ) tend to increase sharply with decreasing size. Previously we estimated that such behavior could be associated with depletion of the whole region of each constituent grain. In the light of the present theory, however, this estimation should be corrected largely, though not totally wrong. First of all, it is shown that R a should be independent of grain sizes if volume depletion is assumed to prevail. The sharp increase of R a observed can be attributed instead to an increase in the number of grains without donors (insulating grains). This can happen when the grain size (diameter) is made smaller than an average separation between neighboring donors under the condition of a fixed donor density (N d ). From the grain size value at which R a begins to increase, N d is estimated to be 5.6 × 10 18 cm −3 . This in turn leads to L D = 2.4 nm at 600 K, showing that reduced sizes, n = a/L D , of the grains tested are between 5.4 (largest) and 0.9 (smallest). This range of n is consistent with the assumption of volume depletion. From (22), the response to a fixed Fig. 9. Dependence of device resistance in air (R a ) and response to H 2 or CO in air (R a /R g ) on grain sizes of SnO 2 (diameter d) [16]. Fig. 10. Response data to H 2 or CO as correlated with a −1/2 . concentration of H 2 or CO should be proportional to a −1/2 : R a R g = (3cP gas /N d ) 1/2 a −1/2 (gas; H 2 or CO) (26) The response data are plotted against a −1/2 in Fig. 10. It is seen that three data on the larger grains fall on a straight line passing through origin in either cases of H 2 (A) and CO (B) in agreement with (26), while the remaining data on the smaller grains deviate upward probably through improvements of utility factor due to the appearance of insulating grains. The ratio of the slopes of straight lines, A/B, gives the ratio of reaction rate constants (k 2 )ofH 2 and CO; k 2 of H 2 is analyzed to be 14 times as large as that of CO. Sim- ilar H 2 response data obtained with Al-doped SnO 2 [15] are also plotted in the same figure (C). The slope ratio, C/A, gives the ratio of donor density (N d ), which is analyzed to be 1/29. In this way, the influences of a, k 2 and N d on the response can be accounted for quantitatively. Sensing behavior to NO 2 in air can be explained satisfactorily as well. Sensitivity to NO 2 has been shown to be enhanced greatly with decreasing grain size or lamella thickness for WO 3 -based devices, as shown in Fig. 11 [16]. This is consistent quite well with (25), which predicts sensitivity inversely proportional to grain size or lamella thickness. As another typical feature, response behavior to NO 2 is strongly dependent on operating temperature. As shown in Fig. 12, the response is linear to concentrations of NO 2 down to small concentration levels at a low temperature (200 ◦ C). At higher temperatures (300 and 400 ◦ C), however, besides a decrease in sensitivity, the response becomes non-linear in the region of smaller concentrations, and the non-linear region grows more conspicuous with increasing temperature. Notably even in these cases linearity is maintained for the larger concentrations of NO 2 . Such behavior is consistent with what is expected from the theory. At a low temperature where adsorption constant (K NO2 ) is large enough to fulfill the condition of volume depletion down to a small concentration of NO 2 , the linearity holds seemingly over the whole range of NO 2 concentrations according to (25). With increasing temperature, K NO2 is lowered to cause regional depletion to appear in a range of smaller concentrations of NO 2 , while volume depletion dominates at larger concentrations. This explains why the response exhibits non-linear behavior followed by linear one at the higher temperatures as observed as well as why linearity constant (sensitivity) of the linear region decreases with increasing temperature. It is worth noting that the dependence of response behavior on temperature in Fig. 12 is very similar to that on reduced size (n)inFig. 8. Please cite this article in press as: N. Yamazoe, K. Shimanoe, New perspectives of gas sensor technology, Sens. Actuators B: Chem. (2009), doi:10.1016/j.snb.2009.01.023 ARTICLE IN PRESS G Model SNB-11236; No. of Pages8 N. Yamazoe, K. Shimanoe / Sensors and Actuators B xxx (2009) xxx–xxx 7 Fig. 11. Response data to NO 2 (1 ppm) in air at 200 and 300 ◦ CforWO 3 -based devices as correlated with grain size (diameter d) or lamella thickness (l). In fact this similarity is rationalized because a decrease in K NO2 is equivalent to an increase in n. 3. MEMS-assisted gas sensors An important technology called MEMS (Micro-Electro- Mechanical System) was born about two decades ago. Since then, it has been developed greatly for realizing various types of physical Fig. 12. Response of a lamellar WO 3 -based device at three operating temperatures as a function of NO 2 concentration. Fig. 13. Structure and heating and cooling characteristic of a micro-platform [17]. sensors and actuators. Although gas sensors have no electro- mechanical parts, micro-fabrication techniques elaborated for MEMS can be transferred to gas sensors [17,18]. In recent years, MEMS techniques have been gathering a strong focus in the field of gas sensors, especially for the purpose of realizing a micro- platform, suspended in a cavity of silicon for thermal insulation, on which sensing materials are coated. One of the greatest difficulties associated has been how to attain stably an operating temperature high enough to secure gas sensing (up to 450–500 ◦ C). After various efforts, such micro-platforms have been realized. An example is shown in Fig. 13 [19]. The platform, a square of about 100 ␮min width, is attached with a micro-heater and a pair of comb type electrodes. On heating in a pulse mode of 100 ms on and 100 ms off, device temperature changes rapidly between 450 ◦ C and room temperature, the high temperature being stabilized in as short as 30 ms after switching on, as shown. Such an excellent characteristic is granted by elaborating the platform design with regard to heater conductance, heat capacitance, insulation of heat, and so on. Micro-platforms are expected to provide gas sensors with various benefits. First, power consumption can be reduced drastically, especially through a pulse-mode heating operation with a small duty ratio. By saving power consumption, sensor devices can be made drivable with a battery, which is beneficial to cordless or portable gas sensors. For example, realization of battery-driven gas sensors durable for 5 years without charge is planned for installation in houses and vehicles. Second, sensor devices can be miniaturized drastically. Coupled with the small power consumption, this feature makes the devices easier to install in a small space and so more appropriate to apply for various new ubiquitous sensor systems. Third, the excellent heating and cooling characteristics can provide gas sensors with new functions. For example, it is possible to scan operating temperature stepwise in a short time. Probably response data at 10 different temperatures will be acquired within a few seconds, and the resulting response vs. temperature profile will be a useful tool to identify the gaseous component(s) in problem. False alarming will thus be eliminated Please cite this article in press as: N. Yamazoe, K. Shimanoe, New perspectives of gas sensor technology, Sens. Actuators B: Chem. (2009), doi:10.1016/j.snb.2009.01.023 ARTICLE IN PRESS G Model SNB-11236; No. of Pages8 8 N. Yamazoe, K. Shimanoe / Sensors and Actuators B xxx (2009) xxx–xxx Fig. 14. Structure of adsorption–combustion type VOC sensor (a) and sensing capability to toluene gas achieved (b). effectively. The same characteristics will facilitate to introduce useful analytical techniques such as condensation and extraction into gas sensing. This is exemplified well by the adsorption–combustion type VOC sensor developed recently [20]. A catalytic combustion type sensor fabricated on twin micro-platforms (Fig. 14) is oper- ated in a mode of pulse heating (for 0.4 s) and off or low power heating (for 9.6 s). VOCs are adsorbed and accumulated on the catalyst layer during the off- or low power heating-period. Those adsorbates are subjected to catalytic combustion during the pulse heating period. If the sensor response is continuously monitored, the concentration of VOCs surrounding the sensor can be estimated from the response peak height. It is recognized that the key of this sensor is possessed by the condensation of VOCs through adsorption. By optimizing the temperature of adsorption, the sensor can detect as low as 10 ppb toluene in air, as shown. As mentioned above, gas sensors are expected to be innovated greatly by the use of micro-platforms. However, there are great tasks to be cleared before the innovated sensors are com- mercialized. One of such tasks is to establish micro-fabrication techniques to deposit a well-qualified sensing layer efficiently on micro-platforms. Micro-characterization methods should also be established. 4. Conclusions Gas sensors will ever continue to b e requested for further advancements i n the future as a key to solve or control various problems associated with gases. In this regard, particularly semiconductor gas sensors which have pioneered gas sensor technology have to be innovated further. The sensors of this group have been developed successfully mostly on empirical bases thanks to the excellence of the sensing properties oxide semiconductors intrinsi- cally possess, but further developments seem to be difficult without theoretical supports. Fortunately, a theory dealing with the receptor function of these sensors was proposed recently. The theory was derived by combining together the chemical affairs taking place on the surface of individual semiconductor crystals and the physical affairs inside. It reveals the important roles played by the chemical parameters of gases (adsorption constants and reaction rate constants) as well as the physical parameters of semiconductor crystals (grain size, grain shape and donor density). The theory is expected not only to facilitate fundamental understandings but also to give rise to a breakthrough in the design and processing of materials toward innovations of gas sensors. Micro-platforms fabricated with MEMS techniques are almost ready to be used in gas sensors. Excellent heating and cooling characteristics of them are expected to provide gas sensors with new innovative functions toward realization of various next-generation sensors. References [1] H. Ihokura, DENKIKAGAKU 50 (1982) 99. [2] T. Takeuchi, K. Saji, H. Kondo, I. Igarashi, The Electrochemical Society Extended Abstracts, 78-1, Pittsburgh, PA, October 15–20, 1978, 196 pp. [3] T. Nitta, Z. Terada, S. Hayakawa, Humidity-sensitive electrical conduction of MgCr 2 O 4 –TiO 2 porous ceramics, J. Am. Cer. Soc. 63 (1980) 259–300. [4] T. Seiyama, K. Fueki, J. Shiokawa, S. Suzuki (Eds.), Proceedings of International Meeting on Chemical Sensors, Fukuoka, Japan, September 19–22, Kodansha Ltd., Tokyo, 1983. [5] N. Yamazoe, K. Shimanoe, Theory of power laws for semiconductor gas sensors, Sens. Actuators B: Chem. 128 (2008) 566–573. [6] N. Yamazoe, K. Shimanoe, Roles of shape and size of component crystals in semiconductor gas sensor. (1) Response to oxygen, J. Electrochem. Soc. 155 (4) (2008) J85–J92. [7] N. Yamazoe, K. Shimanoe, Roles of shape and size of component crystals in semiconductor gas sensors. (2) Response to NO 2 and H 2 , J. Electrochem. Soc. 155 (4) (2008) J93–J98. [8] N. Yamazoe, K. Shimanoe, C. Sawada, Thin Solid Films 515 (2007) 8302–8309. [9] G. Sakai, N. Matsunaga, K. Shimanoe, N. Yamazoe, Sens. Actuators B: Chem. 80 (2001) 125–131. [10] N. Matsunaga, G. Sakai, K. Shimanoe, N. Yamazoe, Sens. Actuators B: Chem. 83 (2002) 216–221. [11] N. Yamazoe, J. Fuchigami, M. Kishikawa, T. Seiyama, Interactions of tin oxide surface with O 2 ,H 2 O and H 2 , Surf. Sci. 86 (1979) 335–344. [12] A. Rothschild, Y. Komen, On the relationship between the grain size and gas- sensitivity of chemo-resistive metal-oxide gas sensors with nanosized grains, J. Electroceram. 13 (2004) 697–701. [13] A. Rothschild, Y. Komen, The effect of grain size on the sensitivity of nanocrys- talline metal-oxide gas sensors, J. Appl. Phys. 95 (2004) 6374–6380. [14] C. Xu, J. Tamaki, N. Miura, N. Yamazoe, Grain size effects on gas sensitivity of porous SnO 2 -based elements, Sens. Actuators B: Chem. 3 (1991) 147–157. [15] C. Xu, J. Tamaki, N. Miura, N. Yamazoe, Promotion of tin oxide gas sensor by aluminium doping, Talanta 38 (10) (1991) 1169–1175. [16] Y G. Choi, G. Sakai, K. Shimanoe, N. Miura, N. Yamazoe, Wet process-prepared thick films of WO 3 for NO 2 sensing, Sens. Actuators B: Chem. 95 (2003) 258–265. [17] J.S. Suehle, R.E. Cavicchi, M. Gaitan, S. Semancik, Tin oxide gas sensor fabricated using CMOS micro-hotplates and in-situ processing, IEEE Electron Dev. Lett. 14 (1993) 118–120. [18] G. Müller, A. Friedberger, P. Kreisl, S. Ahlers, O. Schulz, T. Becker, A MEMS toolkit for metal-oxide-based gas sensing systems, Thin Solid Films 436 (1) (2003) 34–45. [19] K. Yoshioka, T. Tanihira, K. Shinnishi, K. Kaneyasu, Development of extremely small semiconductor gas sensor, Chem. Sens. 23 (Suppl. B) (2007) 16–18. [20] M. Egashira, T. Sasahara, Adsorption–combustion type micro-gas sensor, Mater. Integrat. 21 (5) (2008) 91–96. Biographies Noboru Yamazoe has been a professor at Kyushu University since 1981 until he retired in 2004. He received his BE degree in applied chemistry in 1963 and PhD in engineering in 1969 from Kyushu University. His research interests include the development and application of the functional inorganic materials. Kengo Shimanoe has been a professor at Kyushu University since 2005. He received his BE degree in applied chemistry in 1983 and ME degree in 1985 from Kagoshima University and Kyushu University, respectively. He joined the advanced materials and technology laboratory in Nippon Steel Corp. and studied the electronic characterization on semiconductor surface and the electrochemical reaction on materials to 1995. He received PhD in engineering in 1993 from Kyushu University. His cur- rent research interests include the development of gas sensors and other functional devices. . new breakthrough in gas sensor technology. 2. Theory of semiconductor gas sensors 2.1. Overview of empirical information and problems Semiconductor gas sensors detect. history, gas sensor technology was inaugurated when three kinds of pioneering gas sensors were put in prac- tice in Japan, i.e., oxide semiconductor gas sensors

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