
- •BACKGROUND
- •ANALYSIS OF DEPOSITION CONFORMALITY
- •EXPERIMENTAL – SUSPENSION APPROACH TO EPD
- •Procedure
- •Process Details
- •Deposition Rates
- •Annealing Problems
- •Multiple Deposit/Anneal Cycles and Ferroelectric Testing
- •EXPERIMENTAL – SOLUTION APPROACH TO EPD
- •Procedure
- •Process Details
- •Deposition Rates
- •Film Conformality
- •Ferroelectric Testing
- •SUMMARY TABLES AND COMPARISON OF METHODS
- •CONCLUSIONS
- •ACKNOWLEDGEMENTS
- •REFERENCES
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.
ratio of film resistivity to suspension resistivity |
resistivity ratio = 100 or 104 |
suspension concentration |
C = 5 g/l |
|
|
electrode separation |
L = either 1 cm or 10 cm |
|
|
applied potential |
Φ = 100 V |
|
|
film density |
ρ = 3 g/cm3 |
particle mobility |
µ = 1.77 (µm/s)/(V/cm) |
|
|
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.
Sample |
Deposition |
Deposited |
Avg. Thickness |
Deposition |
Order |
Time |
Mass |
of Dense Film* |
Rate |
|
(min) |
(g) |
(microns) |
(microns/min) |
1st |
10 |
0.0681 |
29.0 |
2.9 |
2nd |
10 |
0.0779 |
33.1 |
3.3 |
3rd |
10.25 |
0.0558 |
23.7 |
2.3 |
4th |
10.33 |
0.0282 |
12.0 |
1.2 |
5th |
10 |
0.0325 |
13.8 |
1.4 |
6th |
10.33 |
0.0261 |
11.1 |
1.1 |
|
Average Deposition Rate (microns/min): |
2.0 |
||
Assumptions: |
1.88 g/cm3 film density, 12.5 cm2 electrode area* |
|
||
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(*considering both frontside and backside of electrode) |
|
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.
Deposition System |
P(VDF-TrFE) Film |
|
Deposition Conditions |
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Thickness |
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(Row #). polymer in |
deposition |
counter |
dep. |
voltage range, current, |
||
solvent |
electrode |
electrode |
time |
field range, current density |
||
1. |
P(VDF-TrFE) in DMF |
~ 0 nm |
~ 0 nm |
4 min, |
25-30 V, ~200 µA const. current, |
|
|
|
|
|
40 s |
31-38 V/cm, 32 µA/cm2 |
|
2. |
P(VDF-TrFE) in MEK, |
260 nm |
~100 nm |
8 min |
168-235 |
V, constant 515 µA, |
sonicated |
|
|
|
258-362 |
V/cm, 82 µA/cm2 |
|
3. |
P(VDF-TrFE) in |
1.5 µm |
~90 nm |
5 min |
128-218 |
V, constant 506 µA, |
acetone, unsonicated |
|
|
|
197-335 |
V/cm, 81 µA/cm2 |
Table IV. Qualitative summary table for electrophoretic deposition.
|
Suspension Deposited |
Solution Deposited |
|
|
Films |
Films |
|
|
|
|
|
|
Suspension of P(VDF-TrFE) |
Solution of P(VDF-TrFE) |
|
Deposition bath |
polymer strands in acetone |
||
particles in isopropanol |
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or MEK |
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Bath stability |
< 1 hour |
indefinite |
|
Sonication |
required |
highly recommended |
|
Current density |
8 – 16 A/cm2 |
32 – 160 A/cm2 |
|
Voltage |
50 – 200 V |
100 – 300 V |
|
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|
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Resistance increase during |
low (~15%) |
moderate (~50%) |
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deposition |
|||
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Mechanical behavior of the |
brittle |
ductile |
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as-deposited film |
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Appearance of the as- |
opaque and white |
transparent |
|
deposited film |
|||
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Film thickness |
thick (8 – 50 m) |
thin (0.25 – 1.5 m) |
|
Conformality |
moderate |
good |
|
Porosity (as-deposited) |
high (~50%) |
low (unknown%) |
|
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|
|
|
|
|
|
Effect of annealing on film |
produces cracks and/or |
does not cause cracks, |
|
pinholes |
may cause pinholes |
||
|
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Effect of multiple |
eliminates cracks and |
eliminates occasional |
|
pinholes with preferential |
|||
deposition/annealing cycles |
pinholes |
||
deposition |
|||
|
|
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Multiple deposition/ annealing |
yes, otherwise film is |
no, pinhole density can be |
|
low enough to allow testing |
|||
cycles required for electrical |
shorted through a crack or |
||
after a single deposition |
|||
testing of films? |
pinhole |
||
and anneal |
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Surface roughness of films |
high |
low |
|
with single dep/anneal |
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Surface roughness with |
high |
moderate |
|
multiple dep/anneal |
|||
|
|
Table V. Quantitative summary table for P(VDF-TrFE) films formed by EPD.
|
Suspension Deposited |
Solution Deposited |
|
Film (Fig. 8) |
Film (Fig. 10) |
|
|
|
Coercive Field, Ec |
30.1 V/ m |
45 V/ m |
Remanent Polarization, Pr |
4.1 C/cm2 |
2.2 C/cm2 |
Breakdown Field, Ebd |
> 44 V/ m |
112 V/ m |
Note for Table V: Ferroelectric hysteresis loops were not saturated due to power supply limitations (suspension film) and electrical breakdown (solution film), so values measured under saturated conditions would be higher than those reported here.

power supply |
+ |
- |
deposition time: |
(80 V) |
|
V |
(5 min) |
substrate |
|
- |
|
- |
- |
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(silicon die) |
- |
- |
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- |
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- |
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counter- |
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- |
- |
- |
|
- |
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electrode |
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- |
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- |
- |
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- |
- |
- |
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- |
(silicon die) |
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- |
- |
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- |
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- |
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- |
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- |
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particle |
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- |
- |
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- |
- |
- |
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- |
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dispersion |
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- |
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(0.2 m dia. |
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- |
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- |
- |
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- |
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- |
media |
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- |
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P(VDF-TrFE) |
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(isopropanol) |
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polymer)
Figure 1. Schematic of electrophoretic deposition setup. (Typical values shown in parentheses for suspension approach.) Solution approach differs in that “particles” are solvated polymer strands.
NONCONFORMAL |
CONFORMAL |
|
suspension nonuniform |
suspension |
uniform |
film |
|
film |
location a |
a |
exterior |
|
|
corner |
location b |
b |
interior |
|
|
|
substrate |
substrate |
corner |
|
Figure 2. Modes of film deposition over substrate surface topography. The film shown on the left coats the substrate nonconformally (film thickness is nonuniform). The film shown at the right coats the substrate trench conformally (film thickness is uniform over all parts of the substrate). Differences in electric field strength and depletion of film particles in trenches can lead to nonuniformities (in the diagram on the left, the film thickness at (b) is much less than at (a)), but our goal is to produce conformal films.
Figure 3. One-dimensional deposition model. Differences in electric field strength between different locations in actual deposition (similar to two-dimensional model, Figure 2) are represented by differences in electrode separation.
high-field location a
counter electrode
1 cm
susp.
film |
substrate electrode
low-field location b
counter electrode
10 cm
susp.
film |
substrate electrode

1:1 Resistivity Ratio
|
100.0 |
|
|
|
|
(microns) |
90.0 |
high-field |
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|
80.0 |
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location |
a |
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||
70.0 |
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60.0 |
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Thickness |
50.0 |
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40.0 |
|
low-field |
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30.0 |
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20.0 |
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location |
b |
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Film |
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10.0 |
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0.0 |
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0 |
5 |
10 |
15 |
20 |
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Deposition Time (minutes) |
|
|
104:1 Resistivity Ratio |
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||||
|
50.0 |
|
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(microns) |
45.0 |
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40.0 |
high-field |
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|||
35.0 |
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||||
location |
a |
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30.0 |
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Thickness |
25.0 |
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20.0 |
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15.0 |
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low-field |
b |
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Film |
10.0 |
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location |
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5.0 |
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0.0 |
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0 |
20 |
40 |
60 |
80 |
100 |
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Deposition Time (mintues) |
|
Figure 4. Film thickness at locations (a) and (b) for a resistivity ratio of film to suspension of 1:1 and for a resistivity ratio of film to suspension of 104:1. On the left, deposition rates are constant with time, and deposition is always enhanced at high field locations. On the right, deposition is “self-limiting” due to the resistive nature of the film, so that the film thickness approaches similar values regardless of position on the substrate. In conclusion, higher resistivity films deposit more conformally (also, films from lower resistivity suspensions deposit more conformally).
Figure 5. SEM showing 0.2 m diameter P(VDF-TrFE) particles. These particles were dispersed in isopropanol, then electrophoretically deposited onto this silicon substrate. Deposition was halted before a complete layer could form, to be able to see individual particles more distinctly. Particle agglomerates are evident. Better suspension stability would prevent agglomerate formation.

100 m thick P(VDF-TrFE) film
Si substrate
90 m thick P(VDF-TrFE) film
Figure 6. Two SEMs of a thick, porous P(VDF-TrFE) film electrophoretically deposited on a silicon substrate from a suspension of P(VDF-TrFE) particles in isopropanol. Film thickness is ~100 m on frontside of substrate and ~90 m on backside of substrate. This film shows good conformality, since the thicknesses on the frontside and backside are almost the same. (Thinner films from the same deposition bath would not be as conformal, however.)

Crack through film to substrate
Thick film |
1 mm |
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Thin film |
1 mm |
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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.
|
5 |
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4 |
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) |
3 |
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2 |
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( C/cm |
2 |
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1 |
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Polarization |
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0 |
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-1 |
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Amplitude of |
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Electric |
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-2 |
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Hysteresis Loop |
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-3 |
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300 V |
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400 V |
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-4 |
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500 V |
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-5 |
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-60 |
-40 |
-20 |
0 |
20 |
40 |
60 |
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.

PVDF |
Si |
PVDF Si
PVDF
Si30fin: m high
30Si finm high
Si fin: 6 m wide
Figure 9. SEM with schematic showing thin (~0.5 m thick) P(VDF-TrFE) film electrophoretically deposited from solution over a high-aspect-ratio silicon fin. Conformality and sidewall coverage are reasonably good, although occasional film defects and thickness variations do exist. Nearest neighbor fin is 100 m away.
|
2.5 |
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2.0 |
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) |
1.5 |
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2 |
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( C/cm |
1.0 |
|
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|
0.5 |
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Polarization |
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0.0 |
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-0.5 |
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Amplitude of |
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Hysteresis Loop |
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Electric |
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-1.0 |
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15 V |
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-1.5 |
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22 V |
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-2.0 |
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27 V |
|
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-2.5 |
|
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|
-150 |
-100 |
-50 |
0 |
50 |
100 |
150 |
Electric Field (V/ m)
Figure 10. Ferroelectric hysteresis loop of 0.260 m thick P(VDF-TrFE) film electrophoretically deposited from MEK solution and annealed at 190°C for 5 min. The top electrode is evaporated Al, of 0.54 mm2 area; the bottom electrode is the Si substrate. The P(VDF-TrFE) film was not poled, instead ferroelectric hysteresis loops of gradually increasing amplitude were applied until film breakdown occurred during a hysteresis loop of 29 V amplitude (103 V/ m field). Coercive field and remanent polarization from the hysteresis loop of 27 V amplitude (not saturated) are listed in Table V.