Insulating dielectrics layers

Figure 9. Propagating brush discharges, (a) Insulating dielectric layer of dielectric permittivity e thickness dlayer, and surface charge ey with conductive backing, (b) Example of improper use of plastic (PVC) pipe where surface charge builds up leading to risk of propagating brush discharge.

Another principle on which sensor development is based is capacitance change. If the insulating, dielectric layer between two electroconductive plates changes, eg, through pressure, a variation in capacitance can be measured. 

Passivation The process in which an insulating dielectric layer is formed over the surface of the die. Passivation is normally achieved by thermal oxidation of the siHcon and a thin layer of siHcon dioxide is obtained in this manner (a combination of PECVD oxide and PECVD nitride deposited at lower temperature (below 450° C)). Other passivation dielectric coatings may also be applied (used), such as siHcon glass (siHcon oxynitride). 

Oxides are often chosen as insulation materials, for example as sheaths for resistive heaters, due to their low electrical and thermal conductivity. In MOS transitors (MOS = Metal-Oxide-Semi-conductor), a thin Si02 layer is deposited between a doped silicon substrate and the metallic gate to control the channel conductivity. In more complicated electronic devices, with several integration levels, Si02 is also used to make insulating dielectric layers. 

Substrate material Price per unit area (arb. units) Dielectric permittivity of insulator Maximum working temperature (K) Resistivity of dielectric layer (fl-cm) Density (g/cm3) Linear expansion coefficient x i[Pg.490]

A cross-sectional schematic of a monolithic gas sensor system featuring a microhotplate is shown in Fig. 2.2. Its fabrication relies on an industrial CMOS-process with subsequent micromachining steps. Diverse thin-film layers, which can be used for electrical insulation and passivation, are available in the CMOS-process. They are denoted dielectric layers and include several silicon-oxide layers such as the thermal field oxide, the contact oxide and the intermetal oxide as well as a silicon-nitride layer that serves as passivation. All these materials exhibit a characteristically low thermal conductivity, so that a membrane, which consists of only the dielectric layers, provides excellent thermal insulation between the bulk-silicon chip and a heated area. The heated area features a resistive heater, a temperature sensor, and the electrodes that contact the deposited sensitive metal oxide. An additional temperature sensor is integrated close to the circuitry on the bulk chip to monitor the overall chip temperature. The membrane is released by etching away the silicon underneath the dielectric layers. Depending on the micromachining procedure, it is possible to leave a silicon island underneath the heated area. Such an island can serve as a heat spreader and also mechanically stabihzes the membrane. The fabrication process will be explained in more detail in Chap 4. 

A schematic view of the microhotplate with transistor heater is shown in Fig. 4.17 [125]. In order to ensure a good thermal insulation, only the dielectric layers of the CMOS process form the membrane. The inner section of the membrane includes an... 

Conduction electrons in a metal are nearly free to move within the metal in response to an applied electric field. A surface plasma wave, also called a surface plasmon, is an electromagnetic wave that propagates along the boundary between a metal and a dielectric (an electrical insulator). The electromagnetic field decreases exponentially into both layers but is concentrated in the dielectric layer. 

A four-electrode capacitively coupled (contactless) detector has been integrated on a Pyrex glass chip for detection of peptides (1 mM) and cations (5 mM K+, Na+, Li+). The A1 electrode (500 nm Al/100 nm Ti) was deposited in a 600-nm-deep trench and was covered with a thin dielectric layer (30-nm SiC). The other parts of the channel were covered and insulated with Si3 N4 (160 nm). To avoid gas bubble formation after dielectric breakdown, the electric field for separation was limited to 50 V/cm [145]. This four-electrode configuration allows for sensitive detection at different background conductivities without the need of adjusting the measurement frequency [328]. 

The drain current Id is proportional to the charge-carrier mobility the transistor dimensions, where W and L are the channel width and length respectively, the applied voltage, where Vgs and VT are the gate-source and threshold voltage, and the insulator capacitance Cj (Eq. 1). Thus, the impact of the dielectric material on transistor performance is given by the dielectric capacitance which results from a geometrical quotient of area A and distance d (distance is the film thickness of the dielectric layer) and a material factor where s0 and er are the dielectric constant in a vacuum and of the material, respectively (Eq. 2). As consequences of these correla-... 

Besides the general insulating properties of insulating materials, chemical and thermal stability is required and excellent film-forming properties and methods for patterning the insulating layer. Therefore, the most common polymers (e.g. polyethylene, polypropylene, polyvinylchloride etc.) have not yet been used as gate-dielectric layers. 

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