87 The melting temperature for the AgPd system can be modified by adjusting the ratio
atomic of the metals in the system of the solid-solution formation. The solidus and liquidus increasing in temperature monotonically from Ag to Pd T
m Ag
= 692 C, T
m Pd
= 1552
C [94]. For low firing process at around 1000 C, a solution with 85 of Ag and
15 of Pd is used to formulate the paste. Usually the AgPd thick-film conductors are fritted with borosilicate or similar glass phases which are used to bond the metal
particles to the surface of the alumina substrate on firing.
The other alternative electrode material is gold Au. Apart from its relatively high cost, gold can be made as an excellent electrode paste. It exhibits better wire bondability and
migration resistance compared to Ag. Gold is usually added with Pt or Pd to form alloys for thick-film applications to improve solderability with SnPb solder. The properties of
the electrode materials are summarised in Table 4-1.
Although Au is better than AgPd, the trade off between the cost of fabrication and performance makes AgPd preferable as the electrode material. Conversely, due to the
use of high temperature solders to connect the thick-film terminals, Au is a better candidate for the soldering pad material.
Table 4-1: Comparison of material properties for silver, palladium and gold [94].
Metal Density
gcm
3
Melting Temperature
C Electrical
Resistivity 10
-8
m, 298 K
Thermal Expansion
Coefficient 10
-6
K Thermal
Conductivity WmK, 300
K Young’s
Modulus GPa
Silver, Ag 10.5
961 1.59
19.2 429
76 Palladium,
Pd 12
1825 10.8
11.2 71.8
112 Gold, Au
19.3 1063
2.35 14.2
317 80
88
4.2.4 Substrate Materials
The substrate is the essential material acting as a base on which films or layers of thick- film material are deposited and processed to build a whole device. Some bulk ceramics
need to be adhered to the substrates to make them function as a complete device. The substrates for bulk ceramic devices are usually rough, big and strong enough to provide
support to thick piezoceramic ranging from hundreds of microns to millimetres thick.
The choice of a substrate for thick and thin-film devices is critically dependent on the process of fabrication. There are a few substrates that are suitable for thick-film devices
such as alumina, silicon, stainless steel, polymer and glass.
In this study, alumina is used as the substrate for processing PZT thick-film. Alumina is used because it can withstand the high temperatures used for thick-film processing,
which can reach up to 1000 C. It has a thermal expansion coefficient that is comparable
to most thick-film pastes. Besides that, it offers good adhesion for printed layers and is rigid enough to withstand the tensile stress of shrinking thick-film pastes after the
curing process. It is also known as a hermetic material, where it can prevent moisture seeping into it, which can reduce the quality of the thick-film layers during firing.
Compared to other substrates, it is relatively low-cost and can be used for mass production.
4.3 Thick-Film Printing Process
One layer of sacrificial carbon is printed first on an alumina substrate. The film was then dried in an infra-red dryer at 150 °C for 10 minutes. A second layer of film which
can be either AgPd or PZT is then printed over the sacrificial layer with part of the film covering the alumina substrate as shown in Figure 4-2. This creates a step between
the sacrificial layer and the upper film layer, with a height equal to the thickness of the carbon layer. Therefore the sacrificial layer is preferred to be as thin as possible to
ensure the film above the sacrificial layer is properly connected between the base and the potential free-standing structure.
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
