89 A sequence of printing and drying is repeated for each layer of the films to make a
multi-layer composite structure. The resultant film was strongly bonded to the substrate and was not easily pulled off during a standard tape peel test.
For composite films of thickness greater than the printing paste can achieve 50 µm, especially for electrode layer, it is necessary to use a brush to smear the pastes across
the area where the step is to ensure that the electrode is properly connected to the free- standing structure.
Figure 4-2: A photograph of AgPd films printed on carbon sacrificial layers.
4.4 Three-Dimensional Co-Firing Technique
Conventionally, each layer of thick-film in a composite structure is printed, dried and fired individually before another layer of film is printed on them, and usually this
process is carried out in an air environment.
This process, however, is not possible for fabricating a 3-Dimensional structure. This is because once the carbon sacrificial layers are burnt out in air, the thick-films would be
released as free-standing structures. These structures are too brittle and fragile to be printed on with another layer. One solution for this issue is to fire the thick-films in a
nitrogen environment to retain the carbon sacrificial layer while the process of printing, drying and firing is repeated for fabricating a multilayer structure, similar to that
described by Stecher [83].
Co-firing is a technique whereby multiple layers are printed and dried before being fired once as a complete structure, but for devices containing PZT, each successive firing
Alumina Substrate Carbon Sacrificial
Layer AgPd Layer
Potential Free- standing
Structure Base
90 results in lead evaporation, altering the chemical composition away from the
stoichiometric optimum and leading to a reduction in piezoelectric activity [101]. Therefore, this suggests that multilayers of composite thick-films printed on carbon
sacrificial layers can be co-fired together in an air environment, without the need to fire each layer separately in a nitrogen environment. This one-step co-firing method not
only improved the piezoelectric activity in the material but also reduced the complexity of the process and hence reduced the cost of the fabrication.
Typical co-firing profile temperatures for thick-film layers on silicon as described by Glynne-Jones
et al
[102] are in the range of 750 C to 1000
C. Films at a low co-firing temperature of 750
C exhibited poor sintering, whilst at temperatures above 800 C the
films show acceptable adhesion and sintering. However, co-firing at higher temperature 900
C is undesirable because it may causes free-standing structures to be more brittle and prompt cracking.
In order to completely burn out the carbon sacrificial layer, co-firing temperatures have to be set above 800 °C. This temperature is conducive to the curing temperature of PZT
films. The quality of a piezoelectric thick-film can be compared by measuring its piezoelectric charge constant,
d
33
. A study by Torah
et al
[20] showed that the values of
d
33
for samples co-fired at peak temperature of 800 C were not much different from
those co-fired at peak temperature of 1000 C. At 800 °C, the value of
d
33
was measured at about 110 pCN whilst at 1000
C it increased a little to 169 pCN.
Due to the differences in coefficients of thermal expansion of PZT and AgPd, pre-stress will be induced in these layers [103]. AgPd material has a higher thermal expansion
coefficient and therefore expands with a faster rate compared to PZT film when they are co-fired, and contracts faster when they are allowed to cool to room temperature at the
end of the fabrication process, which leads to stress gradients. The effect of the pre- stress is essential in forming a free-standing structure by extending and bending the
material from the anchor area where the base and the free-standing structure meet. The adverse effects of the process are the formation of cracks and warping on the structures.
However, these issues can be rectified by techniques which will be discussed in this chapter.
