Effect of Air-Flow and Co-Firing Profile

96 Figure 4-7: A photograph of failure free-standing structure fabricated with reduced air flow. In another experiment with lower co-firing temperature at 550 °C Figure 4-3, the samples were found to be free-standing before the carbon sacrificial layer completely burnt off. This resulted in a free-standing structure as shown in Figure 4-8. The films were released from the substrate to form free-standing structures because the polymer binder of the sacrificial layer was burnt out at 550 °C but the thick-films were not properly cured, therefore the structures were fragile and easily broken. Figure 4-8: Thick-film co-fired with 550 Profile. Samples co-fired with 850 Profile were found to be more robust as shown in Figure 4-9 a. As the temperature of the co-firing was increased to 950 °C, a sign of electro- migration from AgPd to PZT layer can be observed as the structures turned to a darker colour as shown in Figure 4-9 b. It also shows signs of cracks especially near to the base of the structure which was a result of stress caused by the surface tension after the fabrication process. Carbon Residual Adhered to substrate 97 Figure 4-9: Thick-film co-fired with: a 850 and b 950 Profile. In another experiment, the printed films were arranged in an upside-down manner and the fabrication results showed no significant difference from the right-side-up arrangement. This meant that the pre-stress introduced by the thermal expansion is greater than the gravitational effect, which is not a significant factor in influencing the gap height of the structure. The height of the free-standing structure from the base is dependent on the gap between the two alumina substrates as shown in Figure 4-10. Three small alumina substrates of thickness 0.6 mm are stacked together to make a total gap height of about 2 mm includes air gaps between alumina substrates. The experiment results also showed that, at the end of co-firing process, the films did not adhere to the covering substrate, but left some trace of glass binder on its surface. Figure 4-10: Schematic diagram of an arrangement of alumina substrates with a gap of 2 mm. 2 mm 0.6 mm Base Free-Standing Structure Alumina Substrate a b 98

4.6.3 Investigation on the Structure Support Role of AgPd Using

Interdigitated Electrode IDE The AgPd ESL 9633B pastes were used to print electrodes as well as support layers for the fragile ceramics. A layer of IDT patterned AgPd electrode was printed over PZT ceramic layers as shown in Figure 4-11 a. The films were then co-fired together at 850 °C. Figure 4- b shows the result of the co-firing process, where the lower layer of ceramics broke off and adhered firmly to the substrate. The free-standing structures were seen to be only supported by the IDT electrodes. The free-standing structures were curved side-ways, because of different thermal expansion between conductors and ceramics. As the temperature cooled to the room temperature at the end of the co-firing process, the conductors contracted faster than the ceramics and pulled them together to make a ‘U’ curved free-standing structure. Figure 4-11: IDT patterned electrode on ceramic-conductor composite structure: a schematic diagram of a conductive layer printed on seven layers of ceramic; b fabrication results. A layer of ceramics printed over the IDT conductors was able to enhance the structure as shown in Figure 4-12. An S -beam and flat beam structures were clearly formed, but there were cracks in the ceramics at the anchor area, which connected to the base. AgPd IDT Electrode PZT ceramic Carbon Sacrificial Layer a b PZT ceramic is broken at the free- standing rising area, connected only by IDT electrodes PZT ceramic is firmly adhered to alumina substrate PZT ceramic layer makes a ‘U’ curve at width-length 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