**3.3. Direct UV-patterning**

by lift-off process using thick photoresist [57], hydrophobic self-assembly-monolayer, or thin ZnO film [58] as a sacrificial layer was already proved. However, the feature size was mostly limited above 50 μm, and also PZT films exhibited random crystalline structure, large leakage current, and rather poor ferroelectric properties. Recently, Tue et al. demonstrated sub-5 μm pattern of sol-gel-derived PZT films with a thickness of 80–390 nm by a novel lift-off process using solution-processed amorphous metal oxides as a sacrificial layer (**Figure 13**) [59]. The process includes three steps as follows: (1) deposition and patterning of the sacrificial lift-off layer (In-Zn-O), (2) PZT spin coating, and (3) etching of the sacrificial layer for PZT lift-off. It was found that the amorphous In-Zn-O layer acted as a good barrier between the Pt substrate and PZT film, inhibiting the crystallization of PZT film. In addition, the In-Zn-O film can be easily removed by a wet etching leading to a clean and smooth surface. As a result, the lift-off PZT film exhibited better ferroelectric properties, higher breakdown endurance, and more

**Figure 13.** AFM image of fine PZT pattern: (a) 2D morphology, (b) 3D morphology, and (c) local surface morphology [59].

**Figure 12.** Hysteresis loop of the 1-μm-thick PZT film patterned by a wet-etch process [39].

102 Ferroelectrics and Their Applications

well-defined shape compared with the wet-etched ones.

A general scheme for patterning of PZT thin film by a UV light is shown in **Figure 14**, which is similar to a photoresist patterning process. An UV-sensitive PZT sol is first synthesized, and then spin-coated on a substrate without thermal drying step. After that, the gel film is irradiated under the UV light through a mask for photolysis step. The pattern on the mask will be transferred to PZT film according to the exposed and unexposed area. After the photolysis, the PZT film is placed in a nonionic surfactant solution to remove the unexposed area and is sintered for crystallization.

Many studies have reported the use of UV light for direct patterning PZT thin films using photosensitive PZT sol solutions [60–64]. Calzada et al. synthesized photo-sensitive PbTiO3 solutions, which have a maximum of absorption in UV between 200 and 300 nm [60]. Weihua et al obtained an UV photosensitive PZT sol using chemical modification in acetylacetone [61]. Marson et al. developed a highly concentrated solution for producing photo-patternable layers of PZT by dissolving an amorphous PZT powder into acrylic acid [62]. Although ferroelectric/piezoelectric properties of PZT films patterned by the UV-light are comparable to those of conventional PZT thin films, they normally require complicated modification of the precursor solution, and also feature sizes of PZT patterns are relatively large (in the order of tens of micrometers).
