Capacitive humidity sensors

Boltshauser, T. and Baltes, H. 1991. Capacitive humidity sensors in SACMOS technology with moisture absorbing photosensitive polyimide. Sensors and Actuators, Ser. A, 25-27 509-512. 

The absorption of water by polymers is especially suited to the development of a capacitive humidity sensor based on the high dielectric constant of water, since capacitance depends on the area, the thickness, and the dielectric constant of a dielectric. Uptake of water causes an increase in the dielectric constant followed by an increase in capacitance. Polyimide is a suitable sensitive material, but so are certain inorganic ceramics such as AI2O3 [142] and low-density Ta205, which change their capacitance with the formation of water dipoles. Low-density Ta20s layers can be formed by anodic oxidation of sputtered tantalum films, whereas polyimide films are deposited by spin coating and subsequent polymerization. 

Ahn H, Park J-H, Kim S-B, Jee SH, Yoon YS, Kim DJ (2010) Vertically aligned ZnO nanorod sensor on flexible substrate for ethanol gas monitoring. Electrochem Solid-State Lett 13(11) J125-J128 Arena A, Donato B, Saitta G (2009) Capacitive humidity sensors based on MWCNTs/polyelectrolyte interfaces deposited on flexible substrates. Microelectron J 40 887-890... 

Zampetti E, Pantalei S, Pecora A, Valletta A, Maiolo L, Minotti A, Macagnano A, Fortunato G, Bearzotti A (2009) Design and optimization of an ultra thin flexible capacitive humidity sensor. Sens Actuators B 143 302-397... 

Dai CL (2007) A capacitive humidity sensor integrated with micro heater and ring oscillator circuit fabricated by CMOS-MEMS technique. Sens Actuators B 122 375-380... 

Chandran L, Baltes H, Korvink J (1991) Three-dimensional modeling of capacitive humidity sensors. Sens Actuators A... 

Chen W-P, Zhao Z-G, Liu X-W, Zhang Z-X, Suo C-G (2009a) A capacitive humidity sensor based on multi-wall carbon nanotubes (MWCNTs). Sensors 9 7431-7444... 

Cornila C, Hierlemann A, Lenggenhager R, Malcovati P, Baltes, Hierlemann H, Noetzel G, Weimar U, Gopel W (1995) Capacitive sensors in CMOS technology with polymer coating. Sens Actuators B 25-27 357-361 Dai C-L (2007) A capacitive humidity sensor integrated with micro heater and ring oscillator circuit fabricated by CMOS-MEMS technique. Sens Actuators B 122 375-380... 

Delapierre G, Grange H, Chambaz B, Destannes L (1983) Polymer based capacitive humidity sensor—characteristics and experimental results. Sens Actuators A 4 97-104... 

Kang U, Wise K (2000) A high speed capacitive humidity sensor with on-chip thermal reset. IEEE Trans Electron Devices 47(4) 702-710... 

Oprea A, Barsan N, Weimar U, Bauersfeld ML, Ebling D, Wbllenstein J (2008) Capacitive humidity sensors on flexible RFID labels. Sens Actuators B 132 404-410... 

Fig. 18.6 Operating characteristics of a-Al203-based capacitance humidity sensors during long-term stability test (Data from Chen and Jin 1992)...

Anchisini R, Faglia G, GaUazzi MC, Sbervegheri G, Zerbi G (1996) Polyphosphazene membrane as a very sensitive resistive and capacitive humidity sensor. Sens Actuators B 35(36) 99-102 Ando M, Kobayashi T, Haruta M (1996) Humidity-sensitive optical absorption of CO3O, film. Sens Actuators B 32 157-160... 

Qayton WA, Preud PJ, Baxter RD (1985) Contamination resistant capacitive humidity sensor. In Proceedings of the conference on humidity and moisture, Washington, DC, pp 535-544 Colomban P (ed) (1992) Proton conductors. Cambridge University Press, New York... 

Dey D, Munshi S (2008) An artificial neural network based system for measurement of humidity and temperature using capacitive humidity sensor and thermistor. Sens Transducers J 97(10) 1-10 Di Francia G, Noce MD, Ferrara VL, LanceUotti L, MorvUlo P, Quercia L (2002) Nanostractured porous silicon for gas sensor application. Mater Sci Technol 18 767-771... 

Islam T, Pramanik C, Saha H (2005) Modeling, simulation and temperature compensation of porous polysfiicon capacitive humidity sensor using ANN technique. Microelectron Reliab 45 697-703... 

Kang U, Wise KD (1999) A robust high-speed capacitive humidity sensor integrated on a polysUicon heater. In Proceedings of Transducers 99, Sendai, Japan, pp 1674-1677 Kang U, Wise KD (2000) A high-speed capacitive humidity sensor with on-chip thermal reset. IEEE Trans Electron Dev 47(4) 702-710... 

Tao B, Zhang J, Miao F, Li H, Wan L, Wang Y (2009) Capacitive humidity sensors based on Ni/SiNWs nanocomposites. Sens Actuators B 136 144-150... 

Visscher GJW, Komet JG (1994) Long-term tests of capacitive humidity sensors. Meas Sci Technol 5 1294-1302 Viviani M, Buscaglia MT, Buscaglia V, Leoni M, Nanni P (2001) Barium perovskites as humidity sensing materials. J Eur Ceram Soc 21(10-11) 1981-1984... 

SCHEME 15.4 Advantages of resistive and capacitive humidity sensors. (Adapted from Sakai Yohko et al. Sensor Actual. B. 35 no. 1-3 (1996) 85-90.)... 

Figure 6. Schematic cross-sectional views of two capacitive humidity sensors (a) Design A High-speed structure (b) Design B Conventional structure[42],...

Kim JH, Hong SM, Moon BM, Kim K (2010) High-performance capacitive humidity sensor with novel electrode and pol3dmide layer based on MEMS technology,Microsyst Technol 16 2017-2021... 

Oprea, A., Barsan, N., Weimar, U., et al., 2007. Capacitive humidity sensors on flexible RFID labels. IEEE Transducers 2007, 2039-2042. 

Figure 5.8. Capacitive humidity sensor with porous 8-alumina dielectric. Right equivalent circuit...

Capacitive humidity sensors commonly contain layers of hydrophilic inorganic oxides which act as a dielectric. Absorption of polar water molecules has a strong effect on the dielectric constant of the material. The magnitude of this effect increases with a large inner surface which can accept large amounts of water. An example of this type of dielectric is porous j8-alumina. Colloidal ferric oxide, certain semiconductors, perowskites and certain polymers have also been used. /1-alumina is characterized by ionic conductance. Materials of this type can be characterized by a complex resistance composed of real (ohmic) as well as capacitive terms. The behaviour of such solids can be symbolized by a model and an associated equivalent circuit as given in Fig. 5.8. 

Lazarus, N., Fedder, G.K., 2011. Integrated vertical parallel-plate capacitive humidity sensor. J. Micromech. Microeng. 21 (6), 065028. 

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