Dielectrics polymers

Polymer electronics on foils require mechanically flexible gate dielectric layers. Unfortunately, inorganic insulating films suffer from high deposition temperatures and a lack of mechanical elasticity. In a first step the inorganic gate dielectric is substituted by a polymer film, still using a silicon substrate because of their smooth and well-known surface. 

Initially, the experiments apply a commercially available polyimide coating, which is well known from many microelectronic applications. The polyimide (PI 2545, HD Microsystems GmbH) is a high-temperature coating that can be patterned by a positive photoresist. It is dissolved in the same process step as the exposed resist using an alkaline photoresist developer [6], but by different etching rates. 

Beeause of the high euring temperature of the introdueed polyimide film, it is inapplieable to polymer substrates. Therefore, a eommereially available eoating varnish Beetron (based on modified alkyd ehemistry), favoured beeause of the low euring temperature of about 80 °C, is spin eoated onto a silieon substrate. It is eured in a eonveetion oven at 80 °C for 30 min, resulting in a 1 pm to 1.5 pm thiek film. In eontrast to the OFETs mentioned in this ehapter so far, the transistor on the Beetron varnish uses bottom gate and titanium top drain and source contacts, structured by a shadow mask. The latter is due to the lack of chemical resistance of the varnish towards solvents. 

Because of the low process temperatures the resist can be easily adapted on plastic films as the substrate material. Here OFETs are first integrated on a 23 pm thick polyester foil that is cut to a diameter of 100 mm, cleaned in acetone and deionised water, dried and put on a silicon wafer in order to be processed by the equipment of the silicon semiconductor technology. A 150 nm thick aluminium layer serves as the common gate electrode for the electrical analysis of the devices. 

In this section the characteristics of transistors using different gate dielectrics will be shown. For the interpretation of the results, the way of determining the transistor characteristics is introduced first. 

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The change of capacitance in relation to frequency is a matter of the polarizability of the dielectric. This change is very large for ceramics and large for most polymer dielectrics, but very small for Si3N4 and Si02. 

The dissipation factor of capacitors at high frequencies is determined by the series resistance. For low frequencies there may be losses caused by leakage currents as well as by slow components in the polarizability, especially of high e ceramics and polymer dielectrics. The dissipation factor of the SIKO at room temperature is below 10-4. At 200 °C it is still very low (2X10-4). 

Power in Vivo and Biologically Derived 4472 Conformal, Ultrathin Polymer Dielectrics ... 

The 10 000-fold increased currents at >4.5 V dc are not due to irreversible breakdown of the polymer dielectric, however, because an ohmic response is again obtained when Fappi < 4.5 V dc. It appears that a reversible ion or atom migration, derived from the soft contact electrode, may be occurring. This possibility has recently been invoked to explain phenomena reported for certain molecular electronics... 

For chips mounted face up, heat is transferred to the substrate by conduction through the interconnection layers, and because the polymer dielectric has poor thermal conductivity, heat conduction is often promoted by an array of metallized vias through the interconnection layers (Figure 16) (100). For face-down-mounted chips, the heat may be removed from the back side by using pistons (as in the TCM) or conductive fluids, or heat may be conducted through the solder bonds to the interconnection substrate (98). 

Comparison of Approaches. The additive processes can achieve conductor features with a large aspect ratio, although selective plating can create a negative side wall angle in the conductor that reduces the spacing between conductor lines and that is is difficult to coat with polymer dielectrics. Elec-... 

Typical values of electrical properties of the homopolymer without additives and treatments are in the Table 3.8. The values can be substantially changed by the type of cooling and post-treatments, which determine the morphological state of the polymer. Dielectric constants as high as 17 have been measured on oriented samples that have been subjected to high electrical fields (poled) under various conditions to orient polar crystalline form.74... 

The Infineon group reported a pure polymer dielectric which has also been shown to improve pentacene performance, both as an unmodified polymer, and with a silane treatment performed on it [7b]. In addition, Samsung SDI has recently reported a proprietary polymer dielectric which enables them to achieve high mobility in pentacene TFTs [12]. Table 2.2 emphasizes some of the reports of increasing pentacene mobility. 

H. E. Katz, C. Kloc, Z. Bao, J. Zaumseil, and V. Sundae, Field-effect transistors made from macroscopic single crystals of tetracene and related semiconductors on polymer dielectrics , Journal of Materials Research 19, 1995 (2004). 

A detailed consideration of more advanced theoretical treatments clarifies the role played by the polymer dielectric constant. In the absence of intermo-lecular electrostatic interactions, one would desire the lowest possible dielectric constant, e.g., PMMA would be a better host matrix than polycarbonate. This is because the dielectric constant of the polymer host would act to attenuate the poling field felt by the chromophores. On the other hand, in the presence of intermolecular electrostatic interactions, optimum electro-optic activity will be achieved for polymer hosts of intermediate dielectric constant. The dielectric constant of the host acts not only to attenuate the externally applied poling field, but also fields associated with intermolecular electrostatic interactions. 

Fig. 6.22. (a) Chemical structure of the polymer dielectrics used by Nunes in his experiments, (b) Measured dielectric permittivities of the five styrenic polymers from 40 Hz to 1 MHz [162],... 

Just as polymers may be used to form printable semiconductors, so they may be used to form dielectrics as well. Indeed, polymer dielectrics are in widespread use in conventional microelectronics as well. For printed electronics applications, polymer dielectrics are therefore a natural choice for use in printed transistors. Several families of polymer dielectrics have been studied and used in printed transistors. These include various polyimides and other polymer dielectrics such as pol)rvinylphenol (PVP). In general, these dielectrics are characterized by the following properties ... 

Frequency characteristics — Polymer dielectrics typically show signification frequency dependence in their electrical characteristics. For example, many polymers show significant roll-off in their... 

Several polymer conductors are commercially available, and have been used in the demonstration of printed transistors. These include PEDOT PSS, which is a commercially available polymer conductor, as well as various versions of polyaniline. The latter is typically doped with an acid or salt to increase conductivity. Both of these material systems are water soluble and easily printable. They also typically form good interfaces to organic semiconductors, making them attractive for use in printed transistors. As with polymer dielectrics, however, it is important to note that their usability with inorganic semiconductors is questionable, of course. 

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