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Статьи на перевод PVDF_P(VDF-TrFE) / ELECTROPHORETIC DEPOSITION OF THE PIEZOELECTRIC P(VDF-TrFE).pdf
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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*

 

 

(*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

Thickness

 

 

 

 

 

 

(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

 

or MEK

 

 

 

 

 

 

 

 

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

 

 

 

Resistance increase during

low (~15%)

moderate (~50%)

deposition

 

 

Mechanical behavior of the

brittle

ductile

as-deposited film

 

 

Appearance of the as-

opaque and white

transparent

deposited film

 

 

 

 

 

Film thickness

thick (8 – 50 m)

thin (0.25 – 1.5 m)

Conformality

moderate

good

Porosity (as-deposited)

high (~50%)

low (unknown%)

 

 

 

 

 

 

Effect of annealing on film

produces cracks and/or

does not cause cracks,

pinholes

may cause pinholes

 

Effect of multiple

eliminates cracks and

eliminates occasional

pinholes with preferential

deposition/annealing cycles

pinholes

deposition

 

 

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

 

 

 

 

 

Surface roughness of films

high

low

with single dep/anneal

 

 

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

 

-

 

-

-

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

(silicon die)

-

-

 

-

 

 

 

-

 

counter-

 

-

-

-

 

-

 

 

electrode

 

 

-

 

 

 

 

 

 

-

-

 

 

 

 

 

 

 

-

-

-

 

 

 

 

-

(silicon die)

 

-

-

 

-

 

 

 

 

 

-

 

-

 

 

-

 

 

particle

 

-

-

 

 

 

 

 

 

-

-

-

 

-

 

 

 

dispersion

 

-

 

 

 

 

 

 

(0.2 m dia.

 

-

 

-

-

 

-

 

-

media

 

 

-

 

 

 

P(VDF-TrFE)

 

 

 

 

 

 

 

(isopropanol)

 

 

 

 

 

 

 

 

 

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

 

 

80.0

 

 

location

a

 

 

70.0

 

 

 

 

 

 

60.0

 

 

 

 

Thickness

50.0

 

 

 

 

40.0

 

low-field

 

 

30.0

 

 

 

20.0

 

location

b

 

Film

 

 

10.0

 

 

 

 

 

 

 

 

 

 

0.0

 

 

 

 

 

0

5

10

15

20

 

 

Deposition Time (minutes)

 

 

104:1 Resistivity Ratio

 

 

50.0

 

 

 

 

 

(microns)

45.0

 

 

 

 

 

40.0

high-field

 

 

35.0

 

 

location

a

 

 

30.0

 

 

Thickness

25.0

 

 

 

 

 

20.0

 

 

 

 

 

15.0

 

 

low-field

b

 

Film

10.0

 

 

location

 

5.0

 

 

 

 

 

 

 

 

 

 

 

 

0.0

 

 

 

 

 

 

0

20

40

60

80

100

 

 

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

 

 

 

 

 

 

Thin film

1 mm

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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

 

 

 

 

 

 

 

4

 

 

 

 

 

 

)

3

 

 

 

 

 

 

2

 

 

 

 

 

 

 

( C/cm

2

 

 

 

 

 

 

1

 

 

 

 

 

 

Polarization

 

 

 

 

 

 

0

 

 

 

 

 

 

-1

 

 

 

 

Amplitude of

 

 

 

 

 

 

 

Electric

 

 

 

 

 

 

-2

 

 

 

 

Hysteresis Loop

 

-3

 

 

 

 

300 V

 

 

 

 

 

 

 

 

 

 

 

 

400 V

 

 

 

 

 

 

 

 

 

-4

 

 

 

 

500 V

 

 

 

 

 

 

 

 

 

-5

 

 

 

 

 

 

 

-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

 

 

 

 

 

 

 

2.0

 

 

 

 

 

 

)

1.5

 

 

 

 

 

 

2

 

 

 

 

 

 

 

( C/cm

1.0

 

 

 

 

 

 

0.5

 

 

 

 

 

 

Polarization

 

 

 

 

 

 

0.0

 

 

 

 

 

 

-0.5

 

 

 

 

Amplitude of

 

 

 

 

 

 

Hysteresis Loop

 

Electric

 

 

 

 

 

 

-1.0

 

 

 

 

15 V

 

 

 

 

 

 

 

-1.5

 

 

 

 

22 V

 

 

 

 

 

 

 

 

-2.0

 

 

 

 

27 V

 

 

-2.5

 

 

 

 

 

 

 

-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.