PhD Thesis Arne Lüker final version V4 - Cranfield University

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162 Appendix A: PZT actuated Diamond Cantilever Technology Appendix A: PZT actuated Diamond Cantilever Technology Diamond cantilever actuators show high resonance frequencies but need also high actuation forces, pointing towards piezoelectric actuation by a PZT/diamond unimorph. In this study, which was a calloboration between Cranfield University and the University of Ulm/Germany, lead zirconate titanate (Pb(Zr,Ti)O3, PZT) layers have been deposited onto nanocrystalline diamond films by sol-gel deposition to realize high-speed MEMS actuators. The fabrication technology is based on self-aligned patterning and on optical lithography. A mechanical resonance frequency of 3.9 MHz has been obtained for 30 µm cantilever length dominated by the nanodiamond Young’s modulus of diamond of approximately 1000 GPa. A.1 Introduction Switches are an essential passive part of any electrical circuit. Used on-chip they need to be manufactured using microtechnology. However, in integrated circuits they are realized by switch transistors in many cases like for example in transmit/receive (TR) modules. But the conductivity limit in semiconductors will cause noticeable losses, even if heterostructure field effect transistors with high electron mobility channels are used. In some cases like phase shift antenna structures signal levels may be extremely low and transistor structures are difficult to integrate. An alternative concept is of course to use mechanical switches, which can be integrated into the semiconductor circuitry by direct monolithic integration or hybrid integration. Especially in the last case new materials may be implemented. However, although switching transistors have a high insertion loss, their signal delays may be in the ps-range, whereas most mechanical structures can only switch with µs-speed. In this investigation nanodiamond has been investigated as base material for cantilever switches operating at high switching speeds up to the ns-range. As figure of merit, the mechanical resonance frequency was chosen, which should then reach into the MHz-range.
Sol-Gel derived Ferroelectric Thin Films for Voltage Tunable Applications Nanodiamond should provide a high resonance frequency of the basic cantilever, because of its high stiffness and low mass density. In respect to the driving principles, which may be considered, these are the electrostatic, electrothermal, and piezoelectric drives [1]. Each one has specific advantages and drawbacks. The electrostatic drive is fast but needs high driving voltages in combination with stiff materials, which may then interfere with the signal path. The thermoelectric bi-metal driving principle can provide high bending moments, but switching may be limited by thermal time constants. The piezoelectric actuation can generate high bending moments at low voltages, if a material with high piezo-coefficients like lead zirconate titanate (Pb(Zr,Ti)O3, PZT) can be used and if high driving fields can be generated without losses. It may therefore represent an optimum compromise, if the resonance of the diamond cantilever is not degraded by the mass density of the PZT piezoceramic. In this investigation a diamond/PZT unimorph has been investigated, using PZT deposited by a sol-gel technique as described previously [2]. This bimorph structure was then analysed theoretically, manufactured using a self- aligned patterning technique and dynamically tested. A.2 Design A.2.1 Modes of operation Typically, two different modes can be used in the design of a piezoelectrically actuated unimorph. This is firstly the longitudinal mode (d33-mode; Fig. 1a), where the polarisation of the piezo ceramic is laterally developed in the deposited film, which is in direction of the cantilever length. Both associated electrodes may be placed on top of the film. An applied electrical field stimulates elongation or contraction (εx,1) depending on the polarization of the stimulating bias. Normally contraction is limited by the materials stability and therefore very limited in use. Elongation results in downward bending of the actuator, if the piezoelectric film is deposited on top of the beam. In the transversal mode (d31-mode; Fig. 1b) piezoelectrically induced stress is in the vertical direction and the film is either vertically stretched and contracted (εx,2), with stretching being the more stable regime. In this case the beam is lifted up. 163
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Intensity [arb. Units] (g) (f) (e)
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PhD Thesis Arne Lüker final version V4 - Cranfield University

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