ELECTROPHORETIC DEPOSITION OF THE PIEZOELECTRIC

POLYMER P(VDF-TrFE)

Jonathan D. Foster and Richard M. White

Berkeley Sensor & Actuator Center

University of California, 497 Cory Hall,

Berkeley, CA 94720-1770, USA

Deposition of the piezoelectric copolymer poly(vinylidene fluoride – trifluoroethylene), P(VDF-TrFE), has been carried out by electrophoretic deposition (EPD). This is the first example of piezoelectric polymer electrodeposition from the liquid-phase available in the literature. Thick (~8-50 m) and thin (~0.25-1.5 m) P(VDF-TrFE) films have been electrophoretically deposited from copolymer suspensions and solutions, respectively. Hysteresis loop measurements indicate that P(VDF-TrFE) deposited from suspension is ferroelectric and therefore also piezoelectric. The good conformality obtained with electrophoretically deposited films holds promise for enabling the fabrication of high-aspect-ratio piezoelectric microactuators.

BACKGROUND

As shown in Figure 1, electrophoretic deposition (EPD) is a technique in which films are formed by the migration of charged particles under an applied electric field (electrophoresis) towards an electrode on which they adhere (deposition). This process produces a porous film, which may be subsequently annealed to form a dense film.

Capabilities of EPD. EPD is capable of forming films of a wide variety of materials, including ceramics (1-2), polymers (3-5), metals, semiconductors, and glasses

(6). Thin (submicron) and thick (>20 m) films have been formed. Electrophoretic deposition is conformal (deposits film conformally over substrate topography) and selective (deposits film on conductors but not on dielectrics or photoresist masks).

The EPD of polymeric materials has been carried out by two approaches. Deposition baths have been made up by dispersing solid polymer particles in liquids to form suspensions (7), and polymer has also been dissolved in solvents to form solutions (3-5, 8). The solution approach to EPD tends to produce better conformality than the suspension approach due to the higher resistance of the film during electrodeposition.

The use of EPD for piezoelectrics has been limited to the deposition of ceramic piezoelectrics, such as lead zirconate titanate (PZT) (9-11). However, PZT interacts with silicon, making integration of PZT on silicon difficult. Since the EPD of PZT is a high temperature process compared to conventional deposition techniques like chemical vapor deposition or sputtering, avoiding deleterious interactions with the EPD technique would likely be even more difficult than with those conventional techniques. On the other hand, P(VDF-TrFE) does not interact with silicon (piezoelectric properties are not disrupted by silicon) and is annealed at only 150ºC – 200ºC, making P(VDF-TrFE) much more silicon compatible.

P(VDF-TrFE) Background. Poly(vinylidene fluoride – trifluoroethylene) (P(VDF-TrFE)) is a piezoelectric copolymer. Piezoelectricity was discovered in poly(vinylidene fluoride) (PVDF) homopolymer by Kawai in 1969 (12). PVDF is polymerized from vinylidene fluoride (CF2=CH2) monomer. The fluorine and hydrogen bonds form dipoles along the length of the polymer chain, and these dipoles are what leads to ferroelectric and piezoelectric behavior. For PVDF to be piezoelectric, it must be in the proper crystalline phase (the β-phase), which may be obtained by stretching the polymer 3-5 times its original length. Poling is then carried out to orient the dipoles. PVDF polymer cannot be deposited on silicon, because stretching is not possible after deposition.

The copolymer of vinylidene fluoride (VDF) with TrFE (trifluoroethylene) P(VDF-TrFE) (13), assumes the piezoelectric β-phase without stretching. Thus P(VDFTrFE) may be spin coated or otherwise deposited. The bulky extra fluorine atom from TrFE stabilizes the β-crystalline phase and discourages the α-crystalline phase from forming.

Importance of Conformality. Conventional piezoelectric deposition techniques are not capable of depositing piezoelectric polymers conformally. While chemical vapor deposition and sputtering are capable of depositing the ceramic piezoelectric lead zirconate titanate (PZT) with some degree of conformality, these techniques are not applicable to P(VDF-TrFE). P(VDF-TrFE) is typically deposited by spin coating (14), which tends to planarize surface topography rather than coat it uniformly. The increasing prevalence in the MEMS field of high-aspect-ratio DRIE-plasma-etched features makes deposition techniques that can coat such features uniformly of increasing interest. At UC Berkeley we are developing high-aspect-ratio microactuators that utilize sidewall piezoelectric films. We have investigated electrophoretic deposition as a means to deposit P(VDF-TrFE) on those sidewalls.

ANALYSIS OF DEPOSITION CONFORMALITY

Electrophoretic deposition (EPD) is capable of covering substrates with a relatively uniform film thickness, even when those substrates have rough or otherwise nonplanar surfaces. The reason for the good conformality (also called throwing power) of electrophoretic deposition is that under certain circumstances the deposit tends to be self-limiting, so that it becomes difficult to deposit more than a certain film thickness in any one area. This area is then “shielded” from subsequent deposition, so that deposition may switch over to the hard-to-reach areas, such as deep trenches or blind channels. The conditions under which EPD provides good deposition conformality are expressed here in terms of the resistivity of the deposit compared to the resistivity of the suspension.

Equation [1] provides the deposition rate for electrophoretic deposition. The rate of mass deposition per unit area dm/dt (g/cm2/s) is equal to the mean particle velocity v (cm/s) multiplied by the particle concentration C (g/cm3). The particle velocity v is related to the applied electric field E (V/cm) by the particle mobility µ ((cm/s)/(V/cm)) according to v = µE. The deposition rate may be expressed in terms of film thickness x rather than mass per unit area by the equation dm/dt = ρ dx/dt, where ρ is the density of the film (g/cm3). Equation [1] is then obtained by combining dm/dt = vC, v = µE, and dm/dt = ρ dx/dt.