99 Therefore, this can be concluded that ceramics are playing two roles, one of which is to
protect the conductor layers from burning in high temperature and the other role is to have a flattening effect on the free-standing structures. The ceramics, however, are
brittle and not strong enough to withstand the thermal shock which will result in cracking. In order to prevent this issue, a layer of AgPd was printed prior to the IDT
electrodes as shown in Figure 4-13. This metal layer acts as a mechanical support platform for the brittle PZT cermet structure.
Figure 4-12: Enhanced structures with a layer of ceramic printed over AgPd IDT conductors.
Figure 4-13: A layer of AgPd as supporting layer can prevent the cermet from cracking after co-firing.
Crack free at free-standing rising area
Supporting Layer AgPd IDT Conductor
Sign of crack at the free- standing rising area
Flatter Surface PZT Ceramic
PZT Ceramic AgPd IDT Conductor
100
4.6.4 Multilayer PZT-AgPd
At a co-firing profile with peak temperature of 850 °C, the upper and lower layer of AgPd conductors suffer warping effects and peeled off from the surface of the ceramics
as shown in Figure 4-14. This is because the lower electrodes tend to pull the structures down while the upper layers pulled the structure the opposite way as an effect of
different thermal expansion coefficient between ceramics and electrodes.
Figure 4-14: Upper and lower electrodes peeling off from ceramic layers.
The warping effect was minimised by covering the bottom AgPd conductor with a layer of PZT, and the resultant of fabrication is shown in Figure 4-15. However, the
structure is not flat; they can be seen to be curved side-ways to form a “U” shaped
structure and pulled the structures off the substrate.
The structures can be further improved by covering the upper AgPd electrode with another layer of PZT. The resultant structures are flatter and adhered firmly to the
substrate as shown in Figure 4-16. This experiment established that PZT ceramic film is effectively acting as a protective layer to prevent AgPd conductors from suffering
warping effects at high temperatures.
AgPd layers are peeling off from ceramic layers
AgPd conductor
AgPd conductor PZT Ceramic
101
Figure 4-15: Composite films of ceramic-electrode-ceramic-electrode which curve side-way and pull off from substrate.
Figure 4-16: Flatter free-standing structures as a result of protective films of ceramic printed on both upper and lower side of the structures.
AgPd Electrode PZT Ceramic
Carbon Sacrificial Layer
Ceramic is adhered firmly to the substrate
Flat Beam Structure S
-Beam Structure Ceramic pulling off
from substrate
Curving side-way AgPd Electrode
PZT Ceramic Carbon Sacrificial Layer
