Barrier layer capacitors

Barriers of the Schottky type control the behaviour of voltage-dependent resistors (VDRs), PTC resistors and barrier-layer capacitors. Their behaviour is by no means as well understood as that occurring in semiconductors such as silicon but, where appropriate in the text, simplified models will be presented to indicate the principles involved. 

Barrier-layer capacitors are based on the limited reoxidation of a reduced composition. This results, in the simplest case, in a surface layer of high resistivity and a central portion of conductive material so that the effective dielectric thickness is twice the thickness hQ of a single reoxidized layer and there is an apparent gain in permittivity over that of a fully oxidized unit by a factor of h/2ha, where h is the overall dielectric thickness (Fig. 5.51). Alternatively each... 

Fig. 5.52 Schematic diagram of a section through an internal-barrier-layer capacitor.

There are two types of the barrier layer capacitors. In the surface capacitors thin insulat layers are present at the surface of semiconducting ceramics these surface layers determine dielectric properties, and are formed by using a reduction/reoxidation process. In the sect type, ihe so-calied intergranular capacitor, the insulating layers have been formed at the gr boundaries (see e.g. Wernicke, 1978). 

Any theory which includes an infinite barrier to metal electrons at the interface will make the reciprocal capacitance too large because it makes the effective interplanar spacing of the inner-layer capacitor too large.76 This is why Rice s early5 results (see below) were so incorrect. The fact that the electron tail penetrates a region of higher dielectric constant further reduces the calculated inverse capacity.30,77... 

As mentioned previously, we include reports and discussions of CVD processes for metallic electrodes and diffusion barrier layers. The diffusion barrier layer is interposed between the electrode and poly-Si plug to prevent chemical reaction between them. It becomes quite clear that all these CVD processes should be developed simultaneously because of the structural complexity of capacitors and process integration requirements. 

TiCl has been extensively used for the deposition of TiN thin films as the top electrode for Ta O dielectric capacitors as well as for barriers layers in A1 or Cu metalization because it has sufficient vapor pressure and is non-viscous liquid at room temperature. However, thermal decomposition of this precursor and elimination of Cl contamination in the film usually requires high deposition temperatures (> 600°C). As discussed later, the BST CVD process requires a low deposition temperature, less than roughly 450°C, due to issues of process integration and conformal deposition which render the, TiCl useless for this case. We need a metal-organic precursor. 

The main commercial use of solid SiO is as a vapor-deposition material for the production of SiOx thin films for optical or electronic applications (antireflective coatings, interference filters, beam-splitters, decorative coatings, dielectric layers, isolation layers, electrodes, thin-film capacitors, thin-film transistors, etc.), for diffusion barrier layers on polymer foils or for surface protection layers.Other uses for SiO have been proposed, such as the substitution of elemental silicon in the Muller-Rochow process for the production of organosilicon halides, because solid SiO can be produced at lower temperatures than elemental silicon. 

For the first time, there was a mathematical expression for the impedance dispersion corresponding to the circular arc found experimentally. The equation introduced a new parameter the somewhat enigmatic constant a. He interpreted a as a measure of molecular interactions, with no interactions a = 1 (ideal capacitor). Comparison was made with the impedance of a semiconductor diode junction (selenium barrier layer photocell). 

In electrical engineering, it is common to classify dielectrics in three main classes. Actually, dielectric materials are identified and classified in the electrical industry according to the temperature coefficient of the capacitance. Two basic groups (Class I and Class II) are used in the manufacture of ceramic chip capacitors, while a third group (Class III) identifies the barium-titanate solid-structure-type barrier-layer formulations used in the production of disc capacitors. 

Anodizing is also employed in the fabrication of electrolytic capadlors, although the properties of the oxide film and the process conditions used contrast markedly with those described above. The oxide layer should be relatively thin for an electrolytic capacitor, but non porous and very compact so as to act as an effective barrier to electron transfer, i.e. barrier layers are required. 

Anodized coatings are typically 2 to 25 pm thick, and consist of a thin nonporous barrier layer next to the metal with a porous outer layer that can be sealed by hydrothermal treatment in steam or hot water for several minutes. The resultant oxide is also nonconductive. This particular property of the anodic oxide is useful in the production of electrolytic capacitors using a special bath of boric and/or tartaric acids. 

The continuous improvement of PEDOTrPSS, or poly(3,4-ethylenedioxythio-phene) poly(styrenesulfonate), pol5uner dispersions over the last decade has made the application of these dispersions for polymer capacitors feasible. Waterborne PEDOT PSS dispersions were developed for the formation of the outer polymer layer first. The requirement on conductivity is much lower for this application than for the inner solid electrolyte because the electrical current passes perpendicular to the 5 to 50 microns thick outer polymer layer. Filmforming properties, adhesion to the anode body and edge, and comer coverage, which are critical to guarantee good barrier layer properties, are adjusted by appropriate formulations of PEDOTPSS. In Figure 10.10 a dense outer layer made of a PEDOTPSS dispersion on a tantalum capacitor is shown. 

Potential barriers can be commonly seen in semi-conductor ceramics. The applications include the use of SiC, thus insulated, as substrate (see section 11.6.1), grain boundary layer capacitors (section 11.6.2), PTC thermistors (section 11.6.6), varistors (section 11.6.7), ferrites (section 11.6.8) and gas detectors [WOL 91]. 

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