**3.4. Direct nanoimprinting lithography (NIL)**

Since the first development in 1995, the nanoimprint lithography (NIL) has become one of the advanced patterning methods for nanofabrication. The idea of NIL is to transfer patterns by pressing a designed master mold into resist [65]. NIL overwhelms other lithographic processes by its low cost, high throughput, and high resolution. Various kinds of functional materials can be textured by NIL, and functional devices are obtained accordingly.

Li et al. reported pattern transfer of nanoscale ferroelectric PZT gratings on a platinized substrate by a reversal NIL without any chemical etch processes [66]. PZT sol was spin coated onto

use of a sharp-tip mold combined with an underlying soft gel film may overcome issues of the conventional NIL. This approach is more favorable because of its simplicity and reduced temperature as well as imprinting pressure. Besides, imprinting on the gel film instead of on the bulk film makes the NIL much easily to be carried out. It was also successfully demonstrated that the NIL on the metal/ferroelectric bilayer structure would help to overcome the flattened problem of pattern in a gel film. Moreover, the metal film prevents the mold from directly contacting with the PZT gel. Therefore, no residual or contaminant will be observed

Lead Zirconium Titanate Films and Devices Made by a Low-Temperature Solution-Based Process

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Thanks to multifunctional properties, PZT films can be utilized for various applications in sensor, actuator, nonvolatile memory, and energy harvesting as well. Once PZT films can be deposited at a low temperature, which is compatible with Si technology, direct integration of PZT films to other active control element, i.e., Si-CMOS, becomes possible. Further lowering of PZT process temperature to be compatible with flexible substrates enables flexible PZTbased electronics. This part briefly summarizes emerging applications of low-temperature

A diaphragm-type piezo actuator was demonstrated using a low-temperature (450°C) solutionprocessed PZT film [15]. Cross-sectional structure and SEM image of the fabricated actuator is given in **Figure 16(a)** and **(b)**. The observed experimental data agreed well with simulation result in which the actuator displacement linearly increased as the increase of applied voltage (**Figure 16(c)**). The maximum displacement was approximately 130 nm at 10 V. These results verify that the low-temperature-processed PZT film can be applied for actuator applications.

**Figure 16.** (a) Cross-sectional structure and (b) SEM image of the fabricated PZT actuator. (c) Comparison of the

displacement between experiment and simulation results [15].

**4. Applications of low-temperature-processed PZT films**

on the mold.

PZT films.

**4.1. Piezoelectric actuator**

**Figure 15.** PZT films nanoembossing process and results of embossed profiles. (a) A schematic diagram illustrating a one-step embossing process to form a stagger shape in a PZT film with two different thicknesses and (b) AFM images of embossed stagger like profiles of PZT film [67].

a template with an antistick layer. The template with PZT was then placed onto the substrate and applied a pressure. After that, the substrate was separated from template directly. The transferred sample was then crystallized by thermal annealing in air at 650°C for 15 min. The patterned lines appeared to be reasonably straight and well ordered with few defects. The height of the gratings was in the range between 125 and 250 nm depending on the template mold.

Shen et al applied a nanoembossing technique to form a stagger structure in PZT film [67]. **Figure 15(a)** illustrates the nanoembossing process of PZT film. **Figure 15(b)** displays the embossed PZT film profiles measured by AFM; showing an embossed depth of about 160 on a 450-nm thick PZT film with well-quadrate-patterned profiles. After crystallization, the PZT films exhibited a tetragonal structure with (111) preferred orientation. In addition, the morphology of the embossed region remained stable. Chen group demonstrated a low-press, low-temperature direct NIL method for patterning PZT films [68, 69]. In general, conventional direct NIL utilizes ultrahigh pressure or temperature to form patterns on the film. The use of a sharp-tip mold combined with an underlying soft gel film may overcome issues of the conventional NIL. This approach is more favorable because of its simplicity and reduced temperature as well as imprinting pressure. Besides, imprinting on the gel film instead of on the bulk film makes the NIL much easily to be carried out. It was also successfully demonstrated that the NIL on the metal/ferroelectric bilayer structure would help to overcome the flattened problem of pattern in a gel film. Moreover, the metal film prevents the mold from directly contacting with the PZT gel. Therefore, no residual or contaminant will be observed on the mold.
