C/cm2. Typical literature values are Ec ~ 50 V/ m and Pr ~ 5 – 10 C/cm2 for most ferroelectric polymers (16). Since the hysteresis loop of 530 V amplitude was not yet saturated (Ec and Pr were still increasing with increasing amplitude), Ec = 30.1 V/ m and Pr = 4.1 C/cm2 are minimum estimates of the true coercive field and remanent polarization values. That the film showed ferroelectric behavior in the hysteresis loop measurement also indicates that the film was piezoelectric, because all ferroelectric materials are piezoelectric. Thus electrophoretic deposition is capable of depositing P(VDF-TrFE) in piezoelectric form.

EXPERIMENTAL – SOLUTION APPROACH TO EPD

Electrophoretic deposition of P(VDF-TrFE) by the suspension approach is capable of conformal deposition, but only for thick films (Figure 6). Electrodeposited paint (3,4) and electrodeposited photoresist (5) are two examples of polymer EPD which exhibit better conformality than suspension-based P(VDF-TrFE) deposition. One of the differences between these techniques and suspension-based P(VDF-TrFE) is that for depositing paint and photoresist the polymer is not in the form of solid polymer particles, but is at least partly soluble in the dispersion media. In order to form thinner conformal piezoelectric polymer films, a related approach is taken with P(VDF-TrFE).

In this section we will describe this solution approach to EPD, in which the charged particles in the EPD deposition bath consist of individual P(VDF-TrFE) polymer strands, or groupings of P(VDF-TrFE) polymer strands, where the polymer strands are solvated by the liquid media.

For deposition of P(VDF-TrFE) piezoelectric polymer, the characteristics of suspension-based EPD and solution-based EPD are quite different. Films from solutionbased EPD are thinner, more conformal, and less selective to deposition on conductors than films from suspension-based EPD. The problems with cracking during the anneal that thick suspension-based EPD films experience do not occur for thin solution-based EPD films. Solution-based EPD films also tend to have less surface roughness than suspension-based films. All these differences from suspension-based EPD are due to the higher density of the as-deposited film for solution-based EPD.

Procedure

As in the suspension approach, silicon die substrates and 0.2 m diameter P(VDF-TrFE) particles were used for solution-based EPD. The difference is that the polymer particles are dissolved, not dispersed, so that they exist in the deposition bath as individual polymer strands or associations of polymer strands (micelles) rather than as 0.2 m polymer particles. The polymer strands are then the basic “particle” for solutionbased electrophoretic deposition. The procedure for solution-based EPD is as follows.

1. Clean substrate. Piranha, HF, and DI water rinses. (Same as suspension approach.)

2. Prepare solution. Standard concentration of P(VDF-TrFE) powder in acetone solvent is 20 g/l. (Higher concentration than suspension approach.)

3. Sonicate solution. Unlike suspensions, which are metastable states, solutions are stable once formed. For this reason sonication is not required for the solution approach as it is for the suspension approach. However, sonication is recommended

because it reduces the time required to dissolve the polymer from about 1 hour to about 2 minutes, and improves solution uniformity by dispersing the powder before it dissolves.

4. Deposit P(VDF-TrFE). Same electrode position as for suspension-based electrophoretic deposition. Deposit at 500 A constant current (80 A/cm2 current density). Voltage varies during deposition, but is typically in the range from 100 – 250 V. (Current density for solution-based deposition is 5-10 times higher than for suspension-based deposition.)

5. Post-deposition procedure. Remove substrate from solution soon after voltage turned off, as film could dissolve back into the acetone if left too long. Allow film to dry in air. Unlike for the suspension approach, a post-deposition immersion is not carried out for the solution approach.

6. Anneal P(VDF-TrFE). Oven anneal 150-200°C for 1-10 min. (Same as suspension approach.)

This procedure was used to deposit some of the solution-based EPD films described in this section. Important deviations from this procedure are noted where they occur. In addition to these six steps, two more should be carried out to make P(VDFTrFE) films piezoelectric, namely a crystallization anneal and electrical poling.

Process Details

In order to achieve a sufficient deposition rate, current densities for EPD from solution are typically 5-10 times higher than for EPD from suspension. For another solution-based EPD technique, the electrodeposition of paint, Beck (4) and Machu (3) list current densities of 1-5 mA/cm2 and 2-10 mA/cm2, respectively, so it is not surprising that high current densities should be required for solution deposition of P(VDF-TrFE). We have used from 32 A/cm2 to 160 A/cm2 for solution-based EPD of P(VDF-TrFE). Higher current densities may be desirable, but have not been used due to high resistivity of the deposition solutions and the 300 V maximum voltage of the electrodeposition power supply.

Additional processing details can be obtained by contacting the authors.

Deposition Rates

Table III lists thicknesses for EPD P(VDF-TrFE) films deposited from solution. Film thicknesses are usually in the submicron range. In comparision to Table II, note that film thicknesses are much lower for solution-deposited EPD films than for suspensiondeposited EPD films. This is for two reasons; one, the solution-deposited film is more insulating than the suspension-deposited film, because the flexible polymer strands depositing in the solution approach are able to pack more tightly than the rigid spherical particles depositing in the suspension approach, and two, the film is soluble in the dispersion media, so that for every “two steps forward” of deposition, there is “a step back” due to film dissolution.

We expect that the net deposition rate is determined by the balance between electrophoretic deposition and film dissolution. For very strong solvents, such as dimethyl formamide (DMF), the balance is shifted in favor of dissolution, and no film forms (Table III, row 1), while for moderately strong solvents, such as acetone and MEK, films exist but are thin (row 2). Further decreasing the solvent strength may allow thicker solution-deposited EPD films to be formed.