Electrically functional inks

Silver microtracks with an excellent electrical functionality could be produced, created by electrohydrodynamic jet printing using a commercial metaUo-organic ink (29). The printing process was performed in a fuUy voltage-controlled fashion. With a 20 /rm nozzle and a reduced printing distance of 50 pm, metallic tracks with a line width below 100 pm could be formed on silicon substrates. 

The LEC structure that involves the addition of ionic dopants and surfactants to the printable inks enables the ability to print a top electrode without restriction by the work function of the metal. Silver, nickel, or carbon particle-based pastes are generally the preferred printable electron injecting electrodes however, the shape and size of the particles combined with the softening properties of the solvent can create electrical shorts throughout the device when printed over a thin polymer layer that is only several hundred nanometers thick. For optimal performance, the commercially available pastes must be optimized for printing onto soluble thin films to make a fully screen-printed polymer EL display. 

It has been mentioned before (Section 3.4.4.3) that with an electric field applied, carbon nanotubes are able to emit electrons from their tips. This effect is termed field emission and may be employed to various appHcations. The production of field emission displays is one of the most attractive among them. These devices can be designed much brighter and more efficient in energy consumption when using carbon nanotubes. Just recently, a fuUy functional picture screen has been presented (Figure 3.107). Its production became feasible thanks to a new, ink-jet related method of generating structured nanotube patterns. Other techniques... 

Colloidal inks containing 5-7 nm particles of gold and silver in an organic solvent, i.e., a-terpmeol, cf.. Figure 4.5, can be used to build electrically and mechanically functional metallic structures. After sintering at 300°C the resistivity of printed silver structures was found to about twice that of bulk silver (33). 

Electrical test. An automatic, computer-driven electrical test system then checks the functionality of each chip on the wafer. Chips that do not pass the test are marked with red ink for rejection. 

Thick-film copper conductor inks are not pure copper. The ink consists of a functional material of copper, a solvent, a temporary binder, and a permanent binder. The permanent binder tailors the CTE to that of the substrate. It also aids in the adhesion of either the substrate or the dielectric. The thick-film firing process in nitrogen bums out the solvent and temporary binders. This leaves just the copper and the permanent binder. In addition to tailoring the CTE, the permanent binder significantly reduces both the electrical and thermal conductivities of the conductor. A typical fired conductor thickness is 15-18 pm. If it were pure copper, it would have a sheet resistivity of 0.94-1.13 mQ/Q. The 9924 thick-film copper conductor material from El DuPont Electronics specifies a sheet resistivity of 1.9-4.8 mD/ for the same thickness. This results in the published resistivity of this thick-film material being only 23% of the resistivity of pure copper. 

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