Figure 9. Development of the ferroelectric properties of the deposited 1 μm thick P(VDF–TrFE) films annealed at 125 ◦ C throughout the outlined process flow given in figure 7.

with the definition of the top electrode (step F) and closing with the released device (step L). Failure to protect the P(VDF– TrFE) layer during device release (that means including step J) becomes clearly apparent in the properties of the released device.

The developed process flow for PVDF-based microsystems has now been applied to the fabrication of a common proof mass device. Effectively, the preliminary backside etching of the silicon wafer is analogously performed but leaves out a cylindrical proof mass as discussed in the introduction. The subsequently defined P(VDF–TrFE) stacks now bear this proof mass with a thickness of about 450 μm after device release. An optical microscopy image of a complete device is shown in figure 10.

The structural integrity of the released device can be demonstrated by mechanically inducing a resonant vibration via a piezoelectric shaker. Monitoring the vibration characteristics of this device under different base displacements (shown in figure 11) results in a hysteretic response characteristic for a Duffing oscillator with a

Figure 11. Nonlinear frequency response of the device shown in figure 10 for different base displacements and sweeps of the excitation frequencies. The device operates in a nonlinear stretching-determined regime and the maximum induced stretching can be approximated to be about 1.5 .

pronounced nonlinearity. The maximum induced strain state can be approximated to be about 1.5 and was at this point limited by device damping effects and the resolution of the applied vibrometer.

In total, the strongly elastic nature of the P(VDF–TrFE) in combination with the ductile properties of the metallization layers easily allow a strain state to be approached during device operation that is already beyond the brink of the device failure which has been experienced for similarly prepared ceramicbased microsystems.

4. Conclusions

In summary, lithographical pattern transfer to P(VDF– TrFE) films were found to be eligible if certain restrictions are accounted for which is largely unusual for polymeric substances. With respect to this, a thorough investigation of the damage potential of the exposure to chemicals during

lithography as well as of the applied reactive ion etching steps has been performed and resulted in a corresponding microsystem fabrication process. Hence, the inherent material advantages could be transferred to a functional device rendering novel strongly strain-based device designs as in piezoelectric microgenerators feasible. The prepared devices were indeed found to be highly insusceptible to stretching induced material failure and featured a nonlinear resonance as expected. Points that should still be addressed include fostering understanding as to why lithographical pattern transfer is altogether possible for this polymeric system and how the fabrication process can be improved. Several process variables that seem to contribute to these questions have been identified in this work. Furthermore, the remaining metal electrodes could be exchanged for polymeric electrodes in a concluding step to fully tap the potential of a strain-based microsystem design and evade the possible impact of intrinsic material stresses. Finally, future research should theoretically assess whether exchanging higher strain levels for lower piezoelectric coupling coefficients can lead to improved device performance. However, no static comparison may be performed but the possibility to obtain unequaled strain levels during operation has to be taken into account which may imply a complete redesign of the respective devices.

Acknowledgments

This work was supported by the Hans. L Merkle Foundation for the Association for the Promotion of Science and Humanities in Germany (Grant T 113/16679/07).

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