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.
91
4.5 Co-firing Process
A multi-zone furnace is used to set the desirable co-firing profile for fabricating the devices. The multi-zone furnace consists of 8 zones with heating coils which can be
controlled to set desirable temperatures in each zone. The furnace is also fitted with 5 air curtains which control the air flow vertically downward for maintaining the
temperature while the fabrication process is running. Fabrication samples are placed on a conveyor belt with controllable speed, which is important in setting co-firing profiles.
In this study, three co-firing profiles were used with similar total co-firing process time of 45 minutes but different peak temperatures of 550 °C denoted as 550 Profile, 850
°C 850 Profile and 950 °C 950 Profile as shown in Figure 4-3. The objective is to identify the best co-firing profile for fabricating robust free-standing cantilevers and
with high piezoelectric performance, which can be compared by the piezoelectric constant,
d
33
.
The air flow was set to 50
l
min, 40
l
min, 5
l
min, 40
l
min, and 50
l
min in five sequential zones respectively. The higher air flows at both ends of the furnace act as a
curtain to prevent drastic change of temperature and also provide uniform air circulation inside the furnace. The air flow in the middle zone of the furnace was set to an
appropriate level for burning carbon and co-firing process.
Figure 4-3: Three different co-firing profiles for fabricating free-standing structure.
92
4.6 Experiment Results and Discussion
A few experiments have been carried out to investigate the structure of thick-film free- standing cantilever as a result of different co-firing profiles and fabrication sequence of
PZT-AgPd. Another experiment where AgPd was printed in an IDT pattern exploiting the piezoelectric effect of
d
33
, was used as a mean to investigate the role of the AgPd material in supporting free-standing structure. Finally, multilayer composite structures
of PZT-AgPd were fabricated with improvements to produce robust and flat cantilevers.
4.6.1 Effect of PZT-AgPd Fabrication Sequence
There are a few problems faced by piezoceramic free-standing structures. One of which is thermal shock, which may result in structures cracking as an effect of rapid
temperature change during the co-firing process. For a thick-film printed directly on a substrate, the thermal shock can be reduced as the expansion and contraction of the film
is prohibited as it is rigidly clamped to the substrate. PZT films are not able to be free- standing by themselves as shown in Figure 4-4 a, where the films broke off from the
base after the carbon sacrificial layer burnt out.
Figure 4-4 b shows that AgPd films were able cope with rapid temperature change in holding together the film as part of a free-standing structure but the rates of expansion
and contraction of the materials are relatively fast therefore they collapse and adhere to the alumina substrate after the carbon film burnt out forming a wave-like structure.
These experiments conclude that none of the materials is able to be free-standing by itself.
Composite free-standing structures consisted of sandwich layers of piezoceramics and AgPd conductors were investigated. Because the structure consists of two different
materials with two different coefficients of thermal expansion, increasing or decreasing the processing temperature will produce a surface stress on the structure and thus create
a pronounced bending. The direction of bending depends on the arrangement of the layers between ceramics and conductors.
