Capacitance device sensor

Figure 6. Scheme of a capacitance device sensor with an inter-digitated structure. 

MJ. Schoning, M.H. Abouzar, S. Ingebrandt, J. Platen, A. Offenhauser, and A. Poghossian, Towards label-free detection of charged macromolecules using field-effect-based structures scaling down from capacitive EIS sensor over ISFET to nano-scale devices, in Mater. Res. Soc. Symp. Proc. 915, 0915-R05-04 (2006). 

The position of the sample is adjusted by the piezoelectric translator. Piezoelectric crystals show creep and hysteresis, which affect the accuracy of the distance determination [142]. One possibility to overcome this problem is to use piezoelectric translators with integrated capacitative position sensors, which are commercially available [143]. In the same setup, another deficit of commercial AFM s was overcome. Standard fluid cells of commercial AFMs are small and manually difficult to access. In addition, they consist of different materials (glass, steel, silicon, etc.) that are difficult to clean. In self-made devices the... 

The miniaturized capacitive arrays play a critical role in the development of microsystems in biomedical applications since it can provide much higher sensitivity compared to the single-element capacitive sensor. Each capacitor of capacitance-based membrane sensor array is composed of a stretchable electrode. Satyanarayana et al. introduce a 3 x 3 array of individual sensor unit cells with an area of 1 cm [2]. Tsouti et al. reported a capacitive membrane-based sensor array made up of 256 elements with an area 1.44 cm [3]. Despite the large number of elements, only 32 contacting pad are used to address all the elements. The sensing units of the array have already been described in the capacitive membrane sensor section. The stretchable electrodes of the device are deposited on a silicon membrane and the counter electrode is fabricated... 

In particular, Connolly et al. (2005) designed NH capacitive sensor with 500-nm-thick porous SiC film. The response in humidity was very low for RH<50 %, which was attributed to the porous dimensions. The exact sensing mechanism is still not clear, but NH levels as low as-0.5 ppm were detected. Porous alumina (AI2O3) has also been examined as a sensing material for capacitive gas sensors and in particular for humidity measurements (Nahar and Khanna 1982 Timar-Horvath et al. 2008). The Al Og-based humidity sensor was a volume-effect device based on physical adsorption. At low humidity, the walls of the pores are lined with one-molecular-thickness liquid layer. As the humidity increases, after saturating the walls, due to a capillary condensation effect, the water starts condensing in the pores (Boucher 1976 Neimark and Ravikovitch 2001). It was established that the water molecules, even at a partial pressure higher than the saturated vapor pressure tend to condense in capillary pores with a radius below the Kelvin radius r, which is defined as function (1) (Boucher 1976) ... 

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

Capacitive Sensors. This device usually consists of a capacitor which is formed either from two concentric cylinders or from a pair of parallel plates. The solid sample to be analyzed for moisture content is passed between these plates. Since w has a large dielectric constant, the w content of the sample causes a significant change in the dielectric constant of the solid, which is measured using bridge or frequency techniques. 

Grain boundaries have a significant effect upon the electrical properties of a polycrystalline solid, used to good effect in a number of devices, described below. In insulating materials, grain boundaries act so as to change the capacitance of the ceramic. This effect is often sensitive to water vapor or other gaseous components in the air because they can alter the capacitance when they are absorbed onto the ceramic. Measurement of the capacitance allows such materials to be used as a humidity or gas sensor. 

Microhotplates, however, are not only used for metal-oxide-based gas sensor applications. In all cases, in which elevated temperatures are required, or thermal decoupling from the bulk substrate is necessary, microhotplate-like structures can be used with various materials and detector configurations [25]. Examples include polymer-based capacitive sensors [26], pellistors [27-29], GasFETs [30,31], sensors based on changes in thermal conductivity [32], or devices that rely on metal films [33,34]. Only microhotplates for chemoresistive metal-oxide materials will be further detailed here. The relevant design considerations will be addressed. 

---