in some ceramic oxides, the usage of PVDF may be especially tantalizing in piezoelectric microgenerators. Indeed, PVDF as a part-crystalline polymer is able to withstand extraordinary elongations on the order of 50–100 at room temperature [17, 18] and may therefore enable the design of interesting new stretching-determined microsystems. A first investigation of its aptitude for piezoelectric energy harvesting has been provided [19], but a comprehensive evaluation considering its enormous elasticity is nonexistent. This is mainly due to additional problems arising from the chemically sensitive nature of this material with respect to the microsystem integration processes, especially in thin-film form. In the literature, several complex patterning processes have emerged that comprise direct x-ray-etching [20], stencil masking [21], micro-imprinting [22], soft lithography [23] or dip-pen processing [24] among others. Evidently, all these methods are not satisfactory for microsystem designs that rely on several consecutive structuring steps. It is common belief that due to the incredibly strong sensitivity of most polymeric substances to a multitude of different solvents, pattern transfer to PVDF films by lithographical means is not possible [21]. This is largely true, especially for exposure to acetone which constitutes a classical choice for most resist lift-off processes. However, in this paper it will be shown that when properly performed, lithographical patterning is possible. If this is the case, the integration of P(VDF–TrFE) thin films into heavily strain-based piezoelectric energy harvesting devices may be viable.

2. Experimental procedure

All processing steps were conducted in a standard clean room. First, the active stack has been processed concluding with the deposition of the piezoelectric layer. Standard (100)- oriented silicon wafers were wet oxidized up to an oxide thickness of 450 nm. Applicable electrode materials for subsequent PVDF processing comprise for instance platinum electrodes, gold electrodes or aluminum electrodes. Due to fabrication considerations, bottom electrodes were formed by sputter depositing 100 nm thick (111)-oriented platinum films on top of the silicon dioxide with a 15 nm thin titanium oxide adhesion layer in between. Subsequently, a pre-anneal at 700 ◦C has been performed for every platinum bottom electrode before the P(VDF–TrFE) film was deposited to obtain a defined stress state [25]. Thin films of P(VDF– TrFE) were now cast from solution via spin-coating using two different solvents as referred to below. The P(VDF–TrFE) compound utilized in this work has been provided by Solvay under the trademark Solvene 190 and was of the composition P(VDF0.76-TrFE0.24).

Second, deposition of the gold top electrodes has been performed via thermal evaporation into a pre-defined pattern. Again, the top electrode thickness was fixed to be 100 nm. However, the surface had been pre-structured in two different ways. For reference purposes, evaporation has been conducted via a shadow mask. This will be compared to a lithographical pattern transfer that used the following parameters. UV exposure has been performed using SUSS Microtec MA6

contact lithography with a mercury lamp that features a radiation peak at 365 nm (i-line). A Microchemicals AZ 5214E negative photoresist has been applied in two different concentrations (resultant thickness optionally 1.4 or 3 μm) along with a 90 ◦C pre-bake and a 115 ◦C post-exposure bake followed by development using AZ 826 mif. Lift-off after gold evaporation and removal of the resist were performed using propylene glycol monomethyl ether acetate (PGMEA) as discussed below. Samples obtained in this manner will be electrically characterized in this work.

Third, to fabricate actual microsystems, reactive ion etching processes have been later on used for structuring and releasing the designed microsystems. The etch patterns were defined by a consecutive resist deposition as discussed above. The micromachining was conducted using Roth & Rau Microsys equipment with an electron cyclotron resonance microwave coupled plasma source and wafer backside cooling. An additional RF electric field is used to direct the reactive ions toward the wafer. Different etching agents are utilized depending on the layer in question. Regarding this, platinum and titanium oxide layers were exclusively etched by physical impact (i.e. argon ions). Silicon has been etched using an SF6/O2 plasma using power and bias settings such that it is highly selective toward silicon dioxide. Finally, the silicon dioxide as well as the P(VDF–TrFE) itself have been etched at a substrate temperature of −5 ◦C using 25 sccm CF4 and 25 sccm CHF3 at a working pressure of 5 μbar. The chosen RF settings resulted in etch rates of about 60 nm min−1 for SiO2 and 40 nm min−1 for the P(VDF–TrFE) layer.

The characterization of the crystalline structure of all thin films presented in this contribution has been accomplished using a PANalytical X’Pert Pro diffractometer in glancing

ferroelectric test equipment (aixACCT laboratories) under the application of the denoted electric fields at a frequency of 100 Hz and a triangular waveform if not otherwise specified. The same equipment in combination with a double-beam laser interferometer [26] has been applied to the detection of piezoelectric activity in the respective thin films. Measurements were performed in large-signal (ferroelectric polarization and strain response) and small-signal operation (dielectric permittivity and d33 evaluation).

Finally, microsystems actuated by a dynamic base excitation as in piezoelectric microgenerators have been characterized with respect to their structural integrity during vibration. Deflection measurements have been performed using a Polytec OFV 512 fiber interferometer connected to an OFV 3001 vibrometer controller. Base excitation was provided by a piezoelectric shaker sweeping through different excitation frequencies at a constant base displacement level.

3. Results and discussion

3.1. Characterization of deposited P(VDF–TrFE) films

While several methods have been demonstrated to prepare thin films of P(VDF–TrFE), deposition from solution is one of

Figure 1. Resultant film thicknesses for different P(VDF–TrFE) powder concentrations in two different solvents after spin-coating for 30 s at 2000 rpm.

the most simple. Here, methyl ethyl ketone (MEK) as well as diethyl carbonate (DEC) have been found to be especially appropriate solvents to prepare high quality P(VDF–TrFE) films. Figure 1 displays the obtained film thickness for different powder concentrations in solution if spin-coating had been performed at 2000 rpm for 30 s. As smooth and homogeneous films are of special importance for the following processing steps the potential film thickness is at some point limited by the solubility of the P(VDF–TrFE). MEK-based films further feature a streak pattern for higher concentrated coating solutions. Thus, MEK may be applied for thin films whereas DEC can also be used to obtain films up to 1 μm. Interestingly, the choice of solvent strongly influences the obtained crystallinity as is shown for different annealing temperatures in figure 2. Annealing has been performed for 90 min at the respective temperature. Corresponding to literature reports, the emerging peaks can be attributed to contributions from the non-polar2 α- and the polar β-phase3 of PVDF [27, 28], while a slight shift due to the contained TrFE fraction is found. Peak splitting is observed in films derived from DEC mostly for annealing temperatures higher than 130 ◦C which is consistent to the phase diagram [16].

With regard to the electronic properties of the deposited films, both solvents are largely comparable such that DEC has been applied for further experiments. However, the choice of the annealing temperature is of particular importance. If defined too low, inferior ferroelectric and dielectric properties are obtained as is evident in figure 3. On the other hand, if determined too high, partial or total recrystallization will seriously reduce the fraction of the polar phase in the material. Therefore, the shift from the β-phase to the α-phase as has been found to commence at about 130 ◦C will lead to increasing coercive fields even if no abrupt loss of obtainable polarization occurs. This may be attributed to the poling effect of the measurement field itself. Nevertheless, a certain

2International Center for Diffraction Data, Data Set 42-1650.

3International Center for Diffraction Data, Data Set 42-1649.

Figure 2. Characteristic x-ray diffraction patterns of 200 nm thick P(VDF–TrFE) films for different annealing temperatures in glancing angle geometry (θ = 1◦). The left pattern has been derived from films cast from methyl ethyl ketone (MEK) and accordingly the right pattern stems from diethyl carbonate-based films (DEC).

temperature regime is eligible which will be important in the following.

Apart from the polar nature of the P(VDF–TrFE) films prepared from solution, the electromechanical coupling is the second figure of merit for the deposited films. Figure 4 illustrates the large-signal strain as well as the small-signal piezoelectric coefficient detected from electrical actuation while performing laser interferometry. Strong piezoelectricity is found for films annealed between 120 and 135 ◦C. A piezoelectric coefficient of 25–30 pm V−1 is observed which is roughly one third of PZT thin films [29]. Hence, the investigated temperature range is altogether appropriate for the fabrication of high quality P(VDF–TrFE) films. In contrast to this, samples annealed at lower temperatures of 100 ◦C already feature piezoelectric activity but with not well-defined material properties. This can be deduced from a blurred transition in the strain response where a sharp switching related point of inflection should be located upon reaching the coercive field. However, a proper discussion of the involved mechanisms is beyond the scope of this contribution.

3.2. Patterning of integrated P(VDF–TrFE) layers

Notwithstanding the adequate material properties found so far, pattern transfer to P(VDF–TrFE) layers constitutes a severe challenge for the integration of P(VDF–TrFE) films into functional piezoelectric microsystems. With respect to this, P(VDF–TrFE) layers were found to be highly resistant toward the application of the solvent propylene glycol monomethyl ether acetate (PGMEA). Indeed, instead of outright dissolution PGMEA mainly swells the P(VDF–TrFE) layer which may lead to delamination if not carefully controlled. Interestingly, besides being a solvent for AZ 5214E-based resists used during photolithography this solvent also represents a viable alternative to acetone for the lift-off of the very same resists.