**3. Micro/nanoscale patterning of PZT films**

#### **3.1. Etching process**

#### *3.1.1. Physical dry etching*

Dry-etching process with a high etch rate, high selectivity to electrode material, and vertical etch profile is preferable for patterning PZT films. Recently, many researchers have studied the etching of PZT films using halogen gases with various etching systems such as reactive ion etching (RIE) [46, 47] and inductively coupled plasma (ICP) [48, 49]. The main problem in the dry etching of PZT films is that the vapor pressures of the etch by-product (mainly metal halogen compounds) are low. Furthermore, the metal halogen compounds have different vapor pressures, which cause compositional variation in the multicomponent PZT films [50]. The etching of PZT films has been studied more widely in chlorinated plasma than in fluorinated plasma because of high vapor pressure of metal chlorides compared with fluoride counterparts. Lee et al. [49] studied the dry-etching mechanism of PZT films in high-density CF<sup>4</sup> and Cl2 /CF<sup>4</sup> ICP. The etching of PZT films in CF<sup>4</sup> -based plasma is chemically assisted sputter etching, and the dominant step of the overall etching process is either the formation or the removal of the etch by-products, depending on the etching conditions. The etching of PZT films in Cl<sup>2</sup> /CF<sup>4</sup> mixed plasma is mainly dominated by the formation of metal chlorides, which depends on the concentration of the atomic Cl and the bombarding ion energy. The PZT film showed a maximum etch rate in 90% Cl<sup>2</sup> /(Cl2 /CF<sup>4</sup> ) plasma where the concentration of atomic Cl was maximum (**Figure 11(a)**). The amount of sidewall residue was greatly reduced in Cl2 /CF<sup>4</sup> mixed plasma compared with in CF<sup>4</sup> plasma. A more vertical etch profile of PZT films was obtained by lowering the process pressure and increasing the substrate bias voltage (**Figure 11(b)**). However, the plasma etch process degrades the structural and electrical properties because of physical damage and chemical residue contamination. The physical damage caused by the bombardment of energetic charged ions to the film surface, which alters the near-surface region and changes its electrical properties. The surface contamination by the etch by-products and penetration of mobile ions into the bulk may also degrade film's quality. Later, Kim et al. reported both a reduction in the etching damage to PZT films during etching in a Cl2 /CF<sup>4</sup> plasma with Ar or O2 added and the recovery of etching damage by using O2 annealing [51].

Lead Zirconium Titanate Films and Devices Made by a Low-Temperature Solution-Based Process http://dx.doi.org/10.5772/intechopen.79378 101

**Figure 11.** (a) The etching rate of PZT films as a function of Cl<sup>2</sup> /(Cl2 + CF<sup>4</sup> ) concentration. (b) Cross-sectional SEM image of PZT pattern [49].

#### *3.1.2. Chemical wet etching*

PZT grains were observed on various kinds of electrodes (Pt, Ru, RuO2

scale) precludes applications with large feature sizes.

and *Ec*

ICP. The etching of PZT films in CF<sup>4</sup>

mixed plasma compared with in CF<sup>4</sup>

**3. Micro/nanoscale patterning of PZT films**

was 170°C. The obtained *Pr*

100 Ferroelectrics and Their Applications

**3.1. Etching process**

CF<sup>4</sup>

and Cl2

PZT films in Cl<sup>2</sup>

reduced in Cl2

etching in a Cl2

annealing [51].

O2

/CF<sup>4</sup>

/CF<sup>4</sup>

/CF<sup>4</sup>

/CF<sup>4</sup>

PZT film showed a maximum etch rate in 90% Cl<sup>2</sup>

plasma with Ar or O2

*3.1.1. Physical dry etching*

crystal growth begins from the film surfaces. However, the small local sintering area (μm to mm

Recently, Borkholder et al. demonstrated a new method of printing and sintering microscale PZT films with low substrate temperature increase [45]. PZT ink was aerosol-jet printed on either stainless steel or PET substrates. After drying at 200°C for 2 h in vacuum, the printed PZT gel was photonically sintered using repetitive sub-ms pulses of high-intensity broad spectrum light in an atmospheric environment. The highest measured substrate temperature

Dry-etching process with a high etch rate, high selectivity to electrode material, and vertical etch profile is preferable for patterning PZT films. Recently, many researchers have studied the etching of PZT films using halogen gases with various etching systems such as reactive ion etching (RIE) [46, 47] and inductively coupled plasma (ICP) [48, 49]. The main problem in the dry etching of PZT films is that the vapor pressures of the etch by-product (mainly metal halogen compounds) are low. Furthermore, the metal halogen compounds have different vapor pressures, which cause compositional variation in the multicomponent PZT films [50]. The etching of PZT films has been studied more widely in chlorinated plasma than in fluorinated plasma because of high vapor pressure of metal chlorides compared with fluoride counterparts. Lee et al. [49] studied the dry-etching mechanism of PZT films in high-density

sputter etching, and the dominant step of the overall etching process is either the formation or the removal of the etch by-products, depending on the etching conditions. The etching of

which depends on the concentration of the atomic Cl and the bombarding ion energy. The

tion of atomic Cl was maximum (**Figure 11(a)**). The amount of sidewall residue was greatly

of PZT films was obtained by lowering the process pressure and increasing the substrate bias voltage (**Figure 11(b)**). However, the plasma etch process degrades the structural and electrical properties because of physical damage and chemical residue contamination. The physical damage caused by the bombardment of energetic charged ions to the film surface, which alters the near-surface region and changes its electrical properties. The surface contamination by the etch by-products and penetration of mobile ions into the bulk may also degrade film's quality. Later, Kim et al. reported both a reduction in the etching damage to PZT films during

mixed plasma is mainly dominated by the formation of metal chlorides,

/(Cl2 /CF<sup>4</sup>

were 32.4 μC/cm2

), which indicates that

and 6.7 kV/cm, respectively (**Figure 10**).


) plasma where the concentra-

plasma. A more vertical etch profile

added and the recovery of etching damage by using

Wet etching is an effectively alternative technique for PZT film's patterning due to its high etching rate, low cost, and high selectivity. Since, PZT can be regarded as a compound of PbO, ZrO2 and TiO2 , etchants containing several compositions are demanded for PZT thin film etching. In recent years, many studies have been performed on wet etching of PZT films using mixtures of various acids, following single-or two-step processes [39, 52–56]. However, problems such as fast etch rate (>400 nm/min), severe undercut, and formation of higher etch residue were encountered. Wang et al. introduced a two-step wet-etching process, using buffered HF acid (BHF) in the first step, and 2HCl:H<sup>2</sup> O at 45°C in the second step, to etch PZT films [54]. However, significant undercutting and brim damage were observed in the achieved pattern. Later, a novel wet-etching process was proposed using 1BHF:2HCl:4NH<sup>4</sup> Cl:4H<sup>2</sup> O solution as the etchant, where NH4 was used as an additive to decrease the undercutting of the obtained PZT pattern. Using this technique, PZT patterns with acceptable undercutting (1.5:1) can be obtained.

Ezhilvalavan et al. proposed a wet-etch recipe using 25% [BOE:HCl:CH<sup>3</sup> COOH:HNO<sup>3</sup> :NH<sup>4</sup> C l:EDTA ethylenediamine tetra acetate trihydrate]:75% H<sup>2</sup> O to pattern PZT films [39]. The etch recipe provided a high etch rate (200 nm/min) and high selectivity with respect to photoresist, limited undercutting (1.5:1, lateral:thickness), and effectively removed the residues on the etched surfaces. Using this recipe, a high-quality patterned PZT film was obtained with a large *Pr* of 30 μC/cm2 , a *Ec* of 150 kV/cm (**Figure 12**), fatigue-free characteristics, and a low leakage current density of 10−<sup>6</sup> A/cm2 at 200 × 10<sup>5</sup> kV/cm. Although various wet-etching procedures have been attempted, the details of the etching mechanism and residue stripping are not properly explained, and more importantly, the ferroelectric/piezoelectric characteristics of the etched PZT structures and its electrical reliability tests have not been studied in details.

#### **3.2. Lift-off process**

Compared to the etching technique, lift-off process is preferable since it has not suffered from the physical and chemical damages caused by etch plasma. The pattern ability of PZT films

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

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

**3.3. Direct UV-patterning**

**Figure 14.** Scheme for patterning of PZT thin film by ultraviolet (UV) light.

sintered for crystallization.

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

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

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http://dx.doi.org/10.5772/intechopen.79378

103

Many studies have reported the use of UV light for direct patterning PZT thin films using photo-

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

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

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

solutions,

sensitive PZT sol solutions [60–64]. Calzada et al. synthesized photo-sensitive PbTiO3

feature sizes of PZT patterns are relatively large (in the order of tens of micrometers).

materials can be textured by NIL, and functional devices are obtained accordingly.

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

**Figure 14.** Scheme for patterning of PZT thin film by ultraviolet (UV) light.
