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Materials for Microelectromechanical Systems 2-23

PZT can be deposited by a wide variety of methods including cosputtering, CVD, and sol–gel process-
ing. Sol–gel processing has been receiving attention lately due to its being able to control the composition
and the homogeneity of the deposited material over large surface areas. The sol–gel process uses a liquid
precursor containing Pb, Ti, Zr, and O to create PZT solutions [Lee et al., 1996]. The solution is then
deposited on the substrate using a spin-coating process. The substrates in this example consist of a Si wafer
with a Pt/Ti/SiO thin-film multilayer on its surface. The deposition process produces a PZT film in multi-
2
layer fashion, with each layer consisting of a spin-coated layer that is dried at 110°C for 5 min and then
heat treated at 600°C for 20min. After building up the PZT layer to the desired thickness, the multilayer was
heated at 600°C for up to 6 hr. Prior to this anneal, a PbO top layer was deposited on the PZT surface. A
Au/Cr electrode was then sputter-deposited on the surface of the piezoelectric stack. This process was used
to fabricate a PZT-based force sensor. Like Si, PZT films can be patterned using dry etch techniques based
on chlorine chemistries, such as Cl /CCl , as well as ion beam milling using inert gases such as Ar.
4
2
2.12 Conclusions

The early development of MEMS can be attributed to the recognition of silicon as a mechanical material.
The rapid expansion of MEMS over the last decade is due in part to the inclusion of new structural mate-
rials that have expanded the functionality of microfabricated devices beyond what is achievable in silicon.
As shown by the examples in this chapter, the materials science of MEMS is not only about the structural
layers, but also about the associated sacrificial and masking layers and how these layers interact during
the fabrication process in order to realize the final device. In simple terms, MEMS is about material sys-
tems; therefore, analysis of what makes MEMS devices work (or, in many cases, not work) relies on a
thorough understanding of this fact.

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