Interdigital capacitors

Bulk and surface imprinting strategies are straightforward tools to generate artificial antibodies. Combined with transducers such as QCM (quartz crystal microbalance), SAW (surface acoustic wave resonator), IDC (interdigital capacitor) or SPR (surface plasmon resonator) they yield powerful chemical sensors for a very broad range of analytes. 

Many vendors have come up with interdigited capacitors (or IDCs). These extra low ESL capacitors are based on the same principle with which I built my monolithic 5V/50A Flyback described in Chapter 5, titled Maximizing the Effectiveness of the Ground Plane. See Figure 4-17. As you can see, they must have been a pain to lay out inside the chip, but wait until you try to connect them on your PCB without defeating the very purpose they... 

Since the adsorption of a gas is able to modify the dielectric constant of zeolites, chemical sensors based on interdigital capacitors (IDCs) using zeolites layers as sensitive coatings offer a wide field of applications depending on the type, modification, and working temperature of the coated IDC sensor. 

Thus, zeolite-coated IDCs have been tested for sensing n-butane [317] and also, NH3, NO, and CO [318,319] on Na-Y and NaPtY zeolite-based sensors at temperatures high enough to where chemical reactions may also occur (above 200°C). The response time is of the order of seconds and the cross-sensitivity to water is small at high temperatures, at which no water condensation occurs in the zeolite-pore system. Under certain conditions, selectivity of these reactive chemical sensors is remarkable. Thus, the detection of 10 ppm of n-butane with a NaPtY interdigitated capacitor with no response to CO and H2 has been reported [318]. Similarly, Moos et al. [320] described a ZSM-5 based capacitor sensor with on-chip heating for temperatures up to 450°C capable of detecting NH3 with no cross-sensitivity to CO, hydrocarbons, and O2. 

Zeolite membranes and films have been employed to modify the surface of conventional chemical electrodes, or to conform different types of zeolite-based physical sensors [49]. In quartz crystal microbalances, zeolites are used to sense ethanol, NO, SO2 and water. Cantilever-based sensors can also be fabricated with zeolites as humidity sensors. The modification of the dielectric constant of zeolites by gas adsorption is also used in zeolite-coated interdigitaled capacitors for sensing n-butane, NH3, NO and CO. Finally, zeolite films can be used as barriers (for ethanol, alkanes,...) for increasing the selectivity of both semiconductor gas sensors (e.g. to CO, NO2, H2) and optical chemical sensors. 

Alberti K etal 1991 Zeolite coaled interdigital capacitors as a new type of gas sensor Catal. Today 8 509-13... 

For applications requiring a large sensing surface area, such as those measuring permittivity and conductivity of tissue, an interdigital capacitor was introduced as the capacitive element in an LC circuit. The change in capacitance resulted in a variation of the impedance and resonant frequency of the LC sensor, which can be interpreted as alterations in the permittivity and conductivity of tissue. The sensor was implanted into the phantom tissue and a saturation depth was achieved such that data were only collected from the tissue layer of interest (Yvanoff and Venkataraman 2009). 

Urbiztondo MA, Pellejero I, Rodriguez A, Pina M.P, Santamarfa J. Zeolite-coated interdigital capacitors for humidity sensing. Sens Actuators B Chem 2011 157 450-459. 

Figure 18.12(a) shows the roughness of the epi-poly layer after deposition and Figure 18.12(b) shows a detail of the interdigital capacitors of the drive structure after poly-Si CMP, DRIB and vapour-phase HF release etch of the sacrificial oxide layer. Only the smooth surface after CMP allows the precision etch of the capacitor spaces <1 pm. 

Two categories of capacitors are available in ceramic technology. The planar or interdigital capacitor (Figure 9.13) can be printed on the surface or inside a ceramic package. The second type is the plate capacitor that consists of two or more electrodes with dielectrics in between (Figure 9.14). 

Printed interdigital capacitor with 50-pm lines and spaces. (Courtesy of Micro Systems Engineering GmbH Co., Germany.)... 

Capacitance density for interdigital capacitors in Fodel -Technology. 

Interdigital capacitors offer high-quality factors because dielectric losses of the substrate and the surrounding air (if printed on the surface) are very low. Conductor losses mainly contribute to the overall loss. 

Typical interdigital capacitors are designed with equal spacings and line widths. They can be calculated using Equation 9.9 [20] ... 

Domansky K, Liu J, Wang LQ, Engelhard MH, Baskaran S (2001) Chemical sensors based on dielectric response of functionaUzed mesoporous silica films. J Mater Res 16 2810-2816 Endres HE, Drost S (1991) Optimization of the geometry of gas sensitive interdigital capacitors. Sens Actuators B 4 95-98... 

Lin J, MiUler S, Obermeier E (1991) Two-dimensional and three-dimensional as basic elements for chemical sensors interdigital capacitors. Sens Actuators B 5 223-226... 

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