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
102
4.7 Final Fabricated Samples and Polarisation
The composite free-standing structures with different length were fabricated. Longer structures formed distinctive features of
S
-beam and flat beam, which are raised to an angle of about 45
from the base to form flat cantilever beams at a height of 2 mm from the substrate as shown in Figure 4-17. There are signs of warping effects on the exposed
AgPd layer, peeling off from the substrate near to the solder pads. This problem can be solved by covering with a layer of PZT to protect them from exposure to high
processing temperatures.
Figure 4-17: Photograph of fabricated samples of free-standing cantilever with different lengths
with a gap height of 2 mm.
Due to the high thermal expansion coefficient of AgPd material, a shrinkage of 10 from the original design size was noticed, as shown in Figure 4-18. A few samples with
no additional layer of PZT covering on the upper electrode were also fabricated to investigate their mechanical and electrical properties. These samples appear to be
slightly indented at the spine of the cantilever making a ‘U’ shaped free-standing cantilever as shown in Figure 4-19.
Exposed AgPd electrode films
suffer from warping effect
4.5 mm 6.75 mm
9 mm 11.25 mm
13.5 mm
20 mm 2 mm
103
Figure 4-18: Photographs of free-standing structures: a original designed model, with length,
l
and width,
w
; b samples of fabrication outcome.
Figure 4-19: Sample A1 with no additional PZT covers on the upper electrode.
Polarisation is the final fabrication step, which is needed to induce remnant polarisation in piezoceramic materials before they are piezoelectric. An electric field up to several
MVm typically 2-5 MVm is applied to the upper and lower electrodes of the sandwich structure, at an elevated temperature typically 80-150
C. The electric field is applied for around 30 minutes, and the sample is allowed to cool down to room
temperature for another 30 minutes before the electric field is removed. This is to prevent the sample from depolarising below the Curie temperature. The influence of
poling conditions such as poling temperature, poling electric field and poling duration on piezoelectric properties of thick-film PZT had been studied by Dargie
et al
[104].
Top View Side View
Potential Free-Standing
Structure Length,
l
b
Width, w
b
Solder Pad Sacrificial
Layer
a b
