Anal. Chem. 2004, 76, 2470-2477
Tuning Sensitivity and Selectivity of Complementary Metal Oxide Semiconductor-Based Capacitive Chemical Microsensors Adrian M. Kummer, Andreas Hierlemann,* and Henry Baltes
ETH Zu¨rich, Physical Electronics Laboratory PEL, HPT H4.2, ETH Ho¨nggerberg, 8093 Zu¨rich, Switzerland
New details on selectivity and sensitivity of fully integrated CMOS-based capacitive chemical microsensor systems are revealed. These microsystems have been developed to detect volatile organics in ambient air and rely on polymeric sensitive layers. The sensitivity and selectivity changes induced by thickness variation of the sensitive polymer layer allow for tuning of the layer parameters to achieve desired sensor features. Cross-sensitivity to interfering agents can be drastically reduced, as is shown for two important cases: (a) rendering the capacitive sensor insensitive to a low-dielectric-constant analyte (lower than that of the polymer) and (b) reducing the influence of a high-dielectric-constant analyte, such as water, on the sensor response. The second case is of vital importance for capacitive sensors, since water is omnipresent and evokes large capacitive sensor signals. The thickness-induced selectivity is explained as a combination of dielectric constant change and swelling and has been confirmed by measurements. Experimentally determined sensitivities qualitatively and quantitatively coincide with the calculated values implying understanding of the sensing mechanism. Gas sensors are widely used for applications such as process control, quality control, and environmental monitoring. For potential commercialization, low-cost production is focused on, for example, by miniaturization and integration, leading to the field of microsensors. A variety of microsensors relying on different transducing principles has been implemented with circuitry in complementary metal oxide semiconductor (CMOS) technology.1-6 Chemocapacitors (dielectrometers) rely on changes in the dielectric properties of a sensing material upon analyte exposure. CMOS capacitive microsensors with sensitive polymeric coatings are promising devices for chemical sensing, because no micro* To whom correspondence should be addressed. www.iqe.ethz.ch/pel/. (1) Kakerow, R.; Manoli, Y.; Mokwa, W.; Rospert, M.; Meyer, H.; Drewer, H.; Krause, J.; Cammann, K. Sens. Actuators, A 1994, 43, 296-301. (2) Cane, C.; Go¨tz, A.; Merlos, A.; Gracia, I.; Errachid, A.; Losantos, P.; LoraTamayo, E. Sens. Actuators, B 1996, 35, 136-140. (3) Qiu, Y. Y.; Azeredo-Leme, C.; Alcacer, L. R.; Franca, J. E. Sens. Actuators, A) 2001, 92, 80-87. Koll, A. Ph.D. Thesis, ETH Zu ¨ rich, Switzerland, 1999. (4) Hierlemann, A.; Baltes, H. Analyst 2003, 128, 15-28. (5) Hagleitner, C.; Hierlemann, A.; Brand, O.; Baltes, H. Sensors Update; Baltes, H., Korvink, J., Fedder, G., Eds.; Wiley VCH: Weinheim, New York, 2002; Vol. 11, pp 101-155. (6) Hagleitner, C.; Hierlemann, A.; Lange, D.; Kummer, A.; Kerness, N.; Brand, O.; Baltes, H. Nature 2001, 414, 293-296.
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mechanical parts, such as membranes or cantilevers, are needed, rendering the chip more rugged and reducing the number of postprocessing steps and, hence, costs. Moreover, the low power consumption favors usage in hand-held devices. Interdigitated structures analogous to the room temperature chemoresistors are predominantly used.7-10 Plate-capacitor-type structures with the sensitive layer sandwiched between a porous thin metal film (permeable to the analyte) and an electrode patterned on a silicon support have also been developed.11,12 A variant exhibits polyimide columns sandwiched between metal electrodes.13 The fabrication of capacitors integrated with CMOS circuitry components is described in several references.8,10,14-19 The capacitances usually are measured at an AC frequency of a few up to 500 kHz. Initial application areas of chemocapacitors have been in humidity sensing using polyimide films,7-11,18,19 because water has a high dielectric constant of 76.6 (liquid state, 303 K), leading to large capacitance changes. Capacitive humidity sensors are commercially available from, for example, Sensirion, Vaisala, and Humirel.20 More recent applications include the detection of organic volatiles in the gas phase using polymeric layers14-17,21-26 or liquid crystals.27 (7) Sheppard, N. F.; Day, D. R.; Lee, H. L.; Senturia, S. D. Sens. Actuators 1982, 2, 263-274. (8) Senturia, S. D. Tech. Dig. - Transducers 1985; 198-201. (9) Glenn, M. C.; Schuetz, J. A. Tech. Dig. - Transducers 1985; 217-219. (10) Denton, D. D.; Senturia, S. D.; Anolick, E. S.; Scheider, D. Tech. Dig. Transducers 1985; 202-205. (11) Delapierre, G.; Grange, H.; Chambaz, B.; Destannes, L. Sens. Actuators 1983, 4, 97-104. (12) Shibata, H.; Ito, M.; Asakursa, M.; Watanabe, K. IEEE Trans. Instrum. Meas. 1996, 45, 564-569. (13) Kang, U.; Wise, K. D. Tech. Dig. - Solid State Sens. Actuator Workshop; Hilton Head 1998, 183-186. (14) Cornila, C.; Hierlemann, A.; Lenggenhager, R.; Malcovati, P.; Baltes, H.; Noetzel, G.; Weimar, U.; Go ¨pel, W. Sens. Actuators, B 1995, B25, 357361. (15) Steiner, F. P.; Hierlemann, A.; Cornila, C.; Noetzel, G.; Bachtold, M.; Korvink, J. G.; Go ¨pel, W.; Baltes, H. Tech. Dig. - Transducers 1995, 2, 814-817. (16) Hagleitner, C.; Koll, A.; Vogt, R.; Brand, O.; Baltes, H. Tech. Dig. Transducers 1999, 2, 1012-1015. (17) Koll, A.; Kummer, A.; Brand, O.; Baltes, H. Proc. SPIE 1999, 3673, 308317. (18) Boltshauser, T.; Baltes, H. Sens. Actuators, A 1991, 26, 509-512. (19) Boltshauser, T.; Chandran, L.; Baltes, H.; Bose, F.; Steiner, D. Sens. Actuators, B 1991, 5, 161-164. (20) http://www.sensirion.com/. http://www.vaisala.com/. http://www.humirel. com/ (21) Endres, H. E.; Drost, S. Sens. Actuators, B 1991, 4, 95-98. (22) Casalini, R.; Kilitziraki, M.; Wood, D.; Petty, M. C. Sens. Actuators, B 1999, 56, 37-44. (23) Dickert, F. L.; Achatz, P.; Bulst, W. E.; Greibl, W.; Hayden, O.; Ping, L.; Sikorski, R.; Wolff, U. Proc. SPIE-Int. Soc. Opt. Eng. 1999, 3857, 116-23. 10.1021/ac0352272 CCC: $27.50
© 2004 American Chemical Society Published on Web 03/26/2004
As with all chemical sensors, selectivity, speed of response, and reversibility depend on the thermodynamics and kinetics of sensitive material/analyte interactions. Therefore, it is necessary to compromise between high selectivity, typically associated with strong interactions, and perfect reversibility, requiring weak interactions.28 For example, to ensure reversibility, sensor coatings consisting of organic polymers that exhibit only partial selectivity to most VOCs are commonly used.6,22,29-34 Selectivity toward an analyte can be achieved with certain specific sensitive layers.35,36 However, as stated above, the selectivity of a sensitive layer is inversely correlated to the reversibility of the absorption, and since reversibility is considered highly important here, low partial selectivities have to be accepted. Hence, other strategies are necessary to achieve the required selectivity, the most common of which is the use of multiple different sensors combined with mathematical algorithms for data evaluation.37,38 The ability of such algorithms to predict analyte concentrations strongly depends on variables such as the selectivity pattern produced by the different analytes, the cross-sensitivity toward interfering agents, the concentration of interfering agents, and the sensor noise and drift. Humidity is a relevant interfering agent because it is omnipresent and the concentration is high with respect to the saturation vapor pressure. For capacitive chemical sensors, humidity is a particularly challenging interfering agent due to the high dielectric constant of water. Therefore, it is necessary to develop techniques that enable the detection of low analyte concentrations, even in the presence of humidity, that is, to find ways to recover expectedly small analyte signals on the background of large water signals. In this paper, we present three methods to enhance the physical selectivity of capacitive microsensors, all of which are independent of the absorption characteristics of the polymer coating. The common basis of these selectivity enhancements is to drastically reduce the cross-sensitivity toward a specific interfering agent by taking advantage of the fact that two physical effects govern the sensor response of capacitive sensors: (i) change of the dielectric constant and (ii) polymer swelling upon analyte (24) Rehacek, V.; Novotny, I.; Tvarozek, V.; Riepl, M.; Hirsch, T.; Mass, M.; Schweiss, R.; Mirsky, V. M.; Wolfbeis, O. S. ASDAM ’98, Conf. Proc., Second Int. Conf. Adv. Semicond. Devices Microsyst. 1998, 172, 255-258. (25) Josse, F.; Lukas, R.; Zhou, R.; Schneider, S.; Everhart, D. Sens. Actuators, B 1996, 36, 363-369. (26) Domansky, K.; Liu, J.; Wang, L. Q.; Engelhard, M. H.; Baskaran, S. J. Mat. Res. 2001, 16 (10), 2810-2816. (27) Dickert, F. L.; Zwissler, G. K.; Obermeier, E. Ber. Bunsen-Ges. Phys. Chem. 1993, 97, 184-188. (28) Grate, J. W.; Abraham, M. H. Sens. Actuators, B 1991, 3, 85-111. (29) Hierlemann, A. Ph.D. Thesis, University of Tu ¨ bingen, Germany, 1996. (30) Zee, F.; Judy, J. W. Sens. Actuators, B 2001, B72, 120-128. (31) Endres, H. E.; Hartinger, R.; Schwaiger, M.; Gmelch, G.; Roth, M. Sens. Actuators, B 1999, B57, 83-87. (32) Hughes, R. C.; Casalnuovo, S. A.; Wessendorf, K. O.; Savignon, D. J.; Hietala, S. L.; Patel, S. V.; Heller, E. J. Proc. SPIE 2000, 4038 (1-2), 519-529. (33) Maute, M.; Raible, S.; Prins, F. E.; Kern, D. P.; Ulmer, H.; Weimar, U.; Go ¨pel, W. Sens. Actuators, B 1999, B58, 505-511. (34) Battiston, F. M.; Ramseyer, J. P.; Lang, H. P.; Baller, M. K.; Gerber, C.; Gimzewski, J. K.; Meyer, E.; Guntherodt, H. J. Sens. Actuators, B 2001, B77, 122-131. (35) Atkinson, J.; Cranny, A.; Simonis de Cloke, C. Sens. Actuators, B 1998, B47, 171-180. (36) Zellers, E. T.; Batterman, S. A.; Han, M. W.; Patrash, S. J. Anal. Chem. 1995, 67, 1092-1106. (37) Gutierrez-Osuna, R. IEEE Sens. J. 2002, 2, 189-202. (38) Go ¨pel, W.; Hesse, J.; Zemel, J. N. Sensors a Comprehensive Survey; Verlagsgesellschaft mbH: Weinheim, Germany; Vol. 2, 1991.
Figure 1. Schematic of sensing principle showing analyte absorption and the two relevant effects changing the sensor capacitance: change of the dielectric constant and swelling. The interdigitated electrodes (+, -) on the substrate (black) are coated with a polymer layer (gray). Big and small globes represent analyte and air molecules, respectively. Analyte molecules are polarized (v) in the electric field (solid lines). Analyte-induced polymer swelling is indicated with the dashed lines (right side).
sorption. The respective contribution of these two effects to the overall sensor response is strongly depending on the layer thickness, as will be shown. By varying the layer thickness, it is, therefore, possible to also modulate the selectivity pattern of a capacitive sensor. The first two methods result in insensitivity to interfering agents with lower dielectric constants than that of the polymer, whereas the third method reduces the cross-sensitivity toward interfering agents with high dielectric constants, such as water. Additionally, the last-mentioned method also drastically reduces sensor drift, thus further enhancing the quality of the measurement data for subsequent data processing. The basics of the sensing principle are detailed in the in the second part of the introduction. The additional selectivity through the layer thickness is explained and verified in the first section of the results. The sensor device is characterized by measurements and by simulations, confirming the theory section. Successionally, the three methods for cross-sensitivity reduction are presented. Transduction Principle and Sensing Characteristics. The sensing principle is shown in Figure 1 and described in detail below. Polymer films deposited on the interdigitated electrodes of the capacitive sensor serve as sensitive films for the detection of volatile organic compounds in air. The investigations were restricted to polymeric films for which physisorption and bulk dissolution of the analyte within the polymer volume are the predominant mechanisms.28 Upon analyte absorption, the physical properties of the polymer layer change as a result of the incorporation of the analyte molecules into the polymer matrix. Changes in two physical properties influence the sensor capacitance: (a) volume (swelling) and (b) dielectric constant.14,15 The resulting capacitance change is detected by the read-out electronics. The partition coefficient, KC, characterizes the absorption behavior of a polymer with regard to a specific analyte. It is a chemical equilibrium constant and is defined as the ratio between A the analyte concentration in the polymer phase, cpoly , and that in A the gas phase, cgas , given by
KC )
cApoly cAgas
(1)
For a specific analyte, KC is inversely proportional to the saturation vapor pressure of this analyte and, therefore, strongly Analytical Chemistry, Vol. 76, No. 9, May 1, 2004
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temperature-dependent. Furthermore, KC depends on the analyte/ polymer interaction, responsible for the partial selectivity. More information about the absorption mechanism and methods to determine the partition coefficient are described in detail in refs 28 and 39. The analyte concentrations in both phases can be replaced by the partial pressure, pA, of the analyte and by the volume fraction of the analyte in the sensitive layer, φA, both used in the rest of this paper. Equation 1 can be rewritten as
pA M K φA ) RT C F
(2)
where M and F denote the molar mass and the density of the analyte in liquid state, and R and T the universal gas constant and the absolute temperature. The two effects changing the capacitance are swelling and change of the dielectric constant. For low analyte concentrations, the swelling is linear in the amount of absorbed analyte, expressed by eq 3.
heff ) h(1 + QφA)
(3)
Here, h and heff denote the initial polymer thickness and the resulting effective thickness after analyte absorption, respectively; Q is a dimensionless nonideality factor of the swelling; and φA is the volume fraction of the analyte in the polymer. φA is proportional to the concentration of the analyte in the gas phase, assuming the validity of Henry’s law. The proportionality factor has to be determined experimentally for every polymer/analyte combination or can be estimated with solubility parameters or linear solvation energy relationships.28 Q ) 1 represents ideal swelling; that is, the total volume is given by the addition of the volume of the absorbed analyte in its liquid state to that of the polymer. Nonideal swelling might have two reasons. The first is an intrinsic nonideality of the swelling process of a free polymer volume, that is, in the absence of any mechanical stress. This effect depends on the combination of analyte and polymer but should be negligible, according to ref 40. Second, swelling might be hindered by mechanical stress generated through the swelling process itself. For this effect, Q is
