Introduction Processing of Thick-

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4.2 Fabrication Materials

The ingredients for fabricating thick-film free-standing micro-generators consist of PZT pastes, carbon pastes, conductor pastes and base substrate. General purpose thick-film pastes are available commercially for fabricating passive circuit elements such as conductors, dielectrics and resistors. However, special purpose thick-film pastes for fabricating micro-generators are not available commercially at the present time; therefore customized pastes have to be made in-house. Carbon pastes were also formulated in-house, similar to that described by Birol et al [96] for low temperature co-fired ceramic LTCC technology. These were used as the sacrificial layer for fabricating the free-standing structures. There are a range of electrode pastes available commercially. Typically used thick-film electrode pastes are gold, silver, and silverpalladium pastes. However, not all of the electrode pastes are suitable for high temperature co-firing with PZT, since problems such as electro-migration can occur which can degrade the piezoelectric activity in the PZT materials. A range of substrates can also be used to fabricate free-standing devices. As the free- standing cantilever structures do not need physical support for bending mode operation, the substrate material is therefore not critical in determining the properties of the devices. However, the substrate has to be able to withstand the high temperatures used in the processing of thick-film materials.

4.2.1 Lead Zirconate Titanate PZT Pastes

The main ingredient for thick-film piezoceramic pastes are PZT powder, high temperature permanent binder, low temperature temporary binder and solvent. These special formulised pastes using PZT as the functional material have been reported in [97, 98]. Typically, the PZT powders sinter at a temperature higher than 800 C in order to produce high piezoelectric activity material. Lead borosilicate glass is used as the 85 permanent binder. It is often available in the form of powder, also known as glass frit. During the co-firing process the glass melts and binds the PZT particles together and later forms solid composite films once cooled down to a lower temperature. These films adhere firmly to the substrate. The presence of glass modifies the mechanical properties as well as the piezoelectric properties of the film. Therefore, it is important to mix PZT powder and glass frit in correct proportions. If the percentage of glass frit is more than necessary, hence reducing the PZT powder loading, then this will result in a lower piezoelectric activity. Polymer binders are also used as the permanent binder for fabricating flexible structures [99], which are generally cured at lower temperature typically 100 – 200 C with an infra-red dryer. Temporary binders such as organic polymers together with solvents such as pine oil or terpineol are used to make thixotropic pastes, which can easily pass through the printing screen. They also serve to hold the paste together during the drying process, and are eventually evaporated off during the firing stage. Excessive solvent, however, will result in a smeared print and reduced definition of the printed geometry. The thickness that can be produced for PZT thick-films ranges from a few microns to hundreds of microns. The minimum film thickness is governed by the particle size of PZT, which is typically 0.8 – 2 m. There is no definite upper limit of thickness that can be produced, but films with thickness greater than 200 m is suitable to be fabricated with bulk piezoelectric materials for higher piezoelectric activity.

4.2.2 Carbon Pastes

A carbon paste, similar to that described by Birol et al [100] for the purpose of producing low temperature co-fired ceramic LTCC technology, was used as the sacrificial layer for the free-standing structure. Graphite was chosen because it can be fully burnt out in air at a temperature above 800 C Figure 4-1 and is therefore compatible to the piezoceramic sintering temperature.