The use of sonication in preparing solution-based deposition baths ensures that the polymer is uniformly dispersed and completely dissolved in the solvent, producing smoother and thinner films than obtained without sonication. The 0.26 m film listed in Table III (row 2) was deposited from a sonicated solution of methyl ethyl ketone (MEK) with a uniform and defect-free appearance, while a film formed from an unsonicated solution of MEK was mugh thicker and suffered from cracks and tears. Even though thick (1.5 m) films without defects or cracks have been produced from unsonicated solutions (Table III, row 3), the surface roughness of this film was much higher than from sonicated suspensions. For this reason sonication is recommended.

Film Conformality

Figure 11 is an SEM showing a P(VDF-TrFE) film electrophoretically deposited from a sonicated acetone solution over a high-aspect-ratio silicon fin (30 m high by 6m wide) formed by deep reactive-ion-etching. Note that sidewall coverage and deposition conformality are reasonably good.

Ferroelectric Testing

Figure 10 shows a ferroelectric hysteresis loop measured on a 0.260 m thick P(VDF-TrFE) film formed by the EPD solution approach. The ferroelectric properties of such solution-based EPD films have so far not measured up to the properties of EPD films formed by the suspension approach (as in Figure 8). The largest remanent polarization obtained for solution-based EPD films has been Pr = 2.2 C/cm2, less than the 4.1 C/cm2 obtained for the suspension-approach EPD film, or the literature value of 5 - 10 C/cm2 (16). The reason for the lower remanent polarization of solution-deposited EPD films has not been determined.

SUMMARY TABLES AND COMPARISON OF METHODS

Qualitative observations and measured values appear in Tables IV and V, from which comparisons of the suspension and solution approaches to electrophoretic deposition can be made.

Electrophoretic deposition of P(VDF-TrFE) is the first example in the literature of electrodeposition of piezoelectric polymer from the liquid-phase. On a related note, electro-spray (ESP) deposition of PVDF is an example of electrodeposition of piezoelectric polymer from the vapor-phase (17).

CONCLUSIONS

P(VDF-TrFE) films have been formed by electrophoretic deposition. Approaches for forming both thick (10-100 m) and thin (0.1-1 m) P(VDF-TrFE) films have been given. The conformality of these films hold promise for enabling the fabrication of high- aspect-ratio piezoelectric polymer microactuators.

ACKNOWLEDGEMENTS

The authors would like to thank Prof. L. DeJonghe for originally introducing one of the authors to the technique of electrophoretic deposition, and for making laboratory equipment available, Dr. M. Thompson of Measurement Specialties for sharing his background on piezoelectric polymers and supplying piezoelectric polymer powder, Mr. R. Wilson for taking the SEMs, and Prof. T. Sands, Prof. J. Newman, and Mr. A. Leming for helpful discussions. One of the authors (J.F.) would also like to acknowledge the support of fellowships from the Department of Defense and the William and Flora Hewlett Foundation.

REFERENCES

1.P. Sarkar and P. S. Nicholson, J. Am. Ceram. Soc., 79 [8] 1987 (1996).

2.J. Mizuguchi, K. Sumi, and T. Muchi, J. Electrochem. Soc., 130, 1819 (1983).

3.W. Machu, Handbook of Electropainting Technology, Electrochemical Publications, Glasgow (1978).

4.F. Beck, Progress in Organic Coatings, 4, 1 (1976).

5.D. Merricks, in Special Polymers for Electronics & Optoelectronics, J. A. Chilton and M. T. Goosey, Editors, p. 37, Chapman & Hall, London (1995).

6.M. Shimbo, K. Tanzawa, M. Miyakawa, and T. Emoto, J. Electrochem. Soc., 132, 393 (1985).

7.C. G. Fink and M. Feinleib, J. Electrochem. Soc., 94, 309 (1948).

8.S. Nakamura, K. Iida, and G. Sawa, in Metal/Nonmetal Microsystems: Physics, Technology, and Applications, Proc. SPIE, 2780, p.72 (1996).

9.S. Sugiyama, A. Takagi, and K. Tsuzuki, Jpn. J. Appl. Phys. 30 [9B] 2170 (1991).

10.T. Sweeney and R. W. Whatmore, in Proc. Tenth IEEE Int’l Symp.on Appl. Ferroelectrics, B. M. Kulwicki et al., Editors, p. 193, IEEE, NY (1996).

11.J. Van Tassel and C. A. Randall, in Conference on Smart Materials Technologies, Proc. SPIE, 3324, 14 (1998).

12.H. Kawai, Jpn. J. Appl. Phys., 8, 975 (1969).

13.Y. Higashihata, J. Sako, T. Yagi, Ferroelectrics, 32, 85 (1981).

14.L. F. Brown, R. L. Carlson, and J. M. Sempsrott, in IEEE Ultrasonics Symposium Proceedings, S. C. Schneider, M. Levy, and B. R. McAvoy, Editors, p.1725, IEEE, NY (1997).

15.G. M. Garner and K. J. Humphrey, in Special Polymers for Electronics & Optoelectronics, J. A. Chilton and M. T. Goosey, Editors, p. 37, Chapman & Hall, London (1995).

16.T. T. Wang, J. M. Herbert, and A. M. Glass, Editors, The Applications of Ferroelectric Polymers, Blackie, Glasgow (1988).

17.J. Sakata and M. Mochizuki, Thin Solid Films, 195, 175 (1991).

Table I. Assumed values of constants for the film thickness plots in Figure 4.

The particle mobility of 1.77 (µm/s)/(V/cm) is based on a relative dielectric permittivity εr = 20 for the dispersion media, viscosity η = 10-3 Pa*s for the dispersion media, and zeta potential = 100 mV for the dispersed particles.

Table II. P(VDF-TrFE) film thickness and deposition rate for suspension-based EPD.

Note for Table II: Thickness calculated from weight gain of samples. Uncertainty in weight gain measurements is about 0.003 g (i.e. 12% uncertainty for 6th sample). Same suspension used for all samples, re-sonicated after each deposition. Deposition rate tends to decrease with additional depositions from the same suspension.

Table III. P(VDF-TrFE) film thicknesses for solution-based EPD.

Crack through film to substrate

Figure 7. Optical microscope images of cracks in two P(VDF-TrFE) films after annealing. The thick (roughly 80 m) film shown at the left has pronounced cracks widely separated, while the thinner (roughly 30 m) film at the right has less pronounced cracks more closely spaced. This suggests that thinner films may be desirable in order to avoid cracking.

Electric Field (V/ m)

Figure 8. Ferroelectric hysteresis loops measured on a 12 m thick P(VDF-TrFE) film formed by suspension-based EPD using multiple deposition/anneal cycles. Top electrode area is 0.63 mm2, top and bottom electrodes are evaporated aluminum, and the substrate is silicon. Three amplitudes for the hysteresis loop are shown. Since the hysteresis loop is not saturated for the 500 V amplitude shown, values of remanent polarization and coercive field for this electrophoretically-deposited film are higher than shown here.