*2.1.1. Seeded diphasic sol-gel (SDSG) precursors*

It is evident from literatures that there have been few studies of the phase evolution of PZT films at temperature normally considered suitable for pyrolysis (350–450°C) rather than crystallization (600°C). The reaction pathway from nucleation to full growth of perovskite PZT phase plays an important role in optimizing and lowering process-temperature of sol-gel derived films.

To understand the mechanism of transformation from the nucleation to the growth of perovskite PZT, microstructural development, crystallinity and electrical properties of low-temperature pyrolyzed PZT films (<400°C) were systematically investigated. The films were prepared on Pt-coated Si substrates by a sol-gel route, in which different concentrations of nanometric PZT powders were dispersed in the sol (seeded precursor) [10, 16, 17]. It was found that the formation of perovskite phase was facilitated by the seeds as a result of the reduced activation

Lead Zirconium Titanate Films and Devices Made by a Low-Temperature Solution-Based Process http://dx.doi.org/10.5772/intechopen.79378 91

**Figure 1.** Temperature-time-texture diagram for the metastable PbPt3 phase and perovskite PZT for the three-layer films dried at 200°C [19].

energy [17]. The seeded PZT films showed a lesser (111)-preferential orientation, greater nucleation density, and a better ferroelectricity. The formation of the metastable intermetallic Ptx Pb interlayer, between the film and the Pt electrode layer, was also observed. However, the local random perovskite nucleation might result in the decreased (111) orientation of the seeded films [18]. The obtained dielectric permittivity (ε), remnant polarization (*Pr* ), and coercive field (*Ec* ) of the 430°C-pyrolyzed seed-PZT film were 500, 6.71 μC/cm2 , and 80 kV/cm, respectively.

#### *2.1.2. Formation of an early stage seeded PbPtx layer*

It has been reported that an intermetallic PbPt3 phase is formed in the early stages of pyrolysis for PZT thin films deposited on a platinized substrate, which greatly influences on crystallization temperature, microstructure, and electrical properties of resulting films [18, 19]. This metastable phase forms at around 330°C and disappears as elevated heating (**Figure 1**). The pyrolysis and annealing conditions as well as the film thickness determine the formation of this intermetallic phase. These conditions impact on the reduction of Pb2+ into Pb, which drives the formation of the PbPt3 phase. The perovskite nucleation was found on top of the intermetallic phase rather than directly on Pt. This explains why the formation of PZT(111) phase is facilitated by the intermediate ones (**Figure 2**) [20]. Due to very small lattice mismatch (0.4%) between the PbPt<sup>3</sup> and PZT phases, the nucleation activation energy might be reduced. As a result, well (111)-oriented perovskite PZT was able to be fabricated at 440–480°C. The PZT film exhibited a good quality with a pyroelectric coefficient of 1.8 × 10−<sup>4</sup> Cm−<sup>2</sup> K−<sup>1</sup> and a *Pr* of 24 μC/cm2 [20].

#### *2.1.3. Solvothermal synthesis*

relatively low processing temperature (~600°C) compared to organic, lead-free, and the other

Many efforts have been done for lowering the process temperature of device-quality PZT films to below 450°C such as the chemical vapor deposition [5], pulse laser deposition [6], and sputtering [7]. However, most of these technologies are costly and complicated, which are not suitable for practical applications. On the other hand, the chemical solution deposition (CSD) technique offers many advantages such as simplicity, low-cost, large area deposition, and feasibility of material compositional control. Many low-temperature CSD methods, including tailoring precursor solution [8, 9], seeding the film [10], hydrothermal annealing [11], and better lattice matching [12], have been investigated, but all provide insufficient film quality and compromised properties. Hitherto, the relatively successful approaches have been microwave annealing [13], localized heating by pulse laser [14], and ultraviolet-assisted annealing [15]. Nevertheless, microwave heating results in damage of CMOS circuits, while the costly pulse

This chapter presents a critical review on the low-temperature solution-processed PZT films and devices since last 15 years, and addresses challenges for fundamental understanding and practical integration of multifunctional PZT films in devices. Database collection was performed using major searching engines such as ISI Web of Science (Thomson Reuters) and Google Scholar. In the first part, recent advances in fabrication of CSD-derived PZT films at a low temperature (≤450°C) using chemical and physical approaches are thoroughly reviewed. The second part discusses various techniques such as wet/dry-etching, lift-off, and imprinting for patterning PZT into micro-nano-sized patterns. Lastly, some potential applications of the low-temperature CSDderived PZT films and devices for sensor/actuator and energy harvesting are demonstrated.

**2. Recent progress of low-temperature PZT films fabricated by a** 

It is evident from literatures that there have been few studies of the phase evolution of PZT films at temperature normally considered suitable for pyrolysis (350–450°C) rather than crystallization (600°C). The reaction pathway from nucleation to full growth of perovskite PZT phase plays an important role in optimizing and lowering process-temperature of sol-gel derived films.

To understand the mechanism of transformation from the nucleation to the growth of perovskite PZT, microstructural development, crystallinity and electrical properties of low-temperature pyrolyzed PZT films (<400°C) were systematically investigated. The films were prepared on Pt-coated Si substrates by a sol-gel route, in which different concentrations of nanometric PZT powders were dispersed in the sol (seeded precursor) [10, 16, 17]. It was found that the formation of perovskite phase was facilitated by the seeds as a result of the reduced activation

inorganic materials [3, 4].

90 Ferroelectrics and Their Applications

laser processing is unfavorable for industrial application.

**chemical solution deposition (CSD) method**

*2.1.1. Seeded diphasic sol-gel (SDSG) precursors*

**2.1. Chemical pathway**

Solvothermal synthesis is a method of crystallizing solution-derived materials under a high pressure and at a temperature higher than boiling temperatures of used solvents. The method

**Figure 2.** Dark-filed cross-section TEM for a film PZT/Pt/Ti/SiO<sup>2</sup> /Si after annealing at 440°C [20].

has been widely used for the synthesis and growth of various materials and thin films such as metal oxides [21, 22].

> pyrochlore to a perovskite structure. A simple method for the crystallization of solutionprocessed PZT films at 400–450°C by intrinsic change in the crystallization path via circumventing the pyrochlore phase formation was reported [29]. The approach does not need any modification of the precursors nor special facilities. Conventionally, the spin-coated films are normally pyrolyzed at over 300°C for a complete removal of solvent and organic ingredients. However, by this pyrolysis step, the pyrochlore phase is subsequently formed. In this work, by lowering the pyrolysis temperature to a well-below pyrochlore temperature (i.e., 210°C), it was able to retain a proper amount of carbon atoms in the gel film as shown in **Figure 4(a)**.

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

subsequent annealing (**Figure 4(b)** and **(c)**). A significantly enhanced intensity of the PbPt<sup>x</sup> peak for the 210°C-pyrolyzed sample compared to the others indicated that larger amount of

at temperatures as low as 200°C. As a result, the lack of Pb2+ prevented the formation of this intermediate phase (**Figure 4(b)** and **(d)**) that accounts for the high-temperature crystallization of the perovskite phase in the conventional processes. The process was successfully dem-

Recently, ultraviolet (UV)-assisted annealing has been applied for fabrication of various functional oxide thin films since the process is capable of facilitating organic decomposition and condensation of oxide network. Consequently, high-quality oxide thin films can be realized

Shimura et al. reported a low-temperature fabrication of PZT films using a thermal UV/O<sup>3</sup> annealing process [15]. A spin-coated PZT gel film was placed on a heated stage (200°C) and

onstrated on several representative electrode materials (Au, stacked Pt/RuO2

was produced (**Figure 4(b)** and **(c)**). In the presence of sufficient organic carbon, Pb2+ was

, which spontaneously reacted with Pt to form the intermediate PbPtx

when heated up to 400°C in

//STO (100) substrate. (b) Hysteresis

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phase

) in

, and RuO2

The remaining carbon acted as a reagent to reduce Pb2+ to Pb0

**Figure 3.** (a) Cross section of SEM image of the deposited PZT thin film on the SrRuO<sup>3</sup>

//STO (100) [26].

Pbo

reduced to Pb0

curves for PZT thin films on SrRuO<sup>3</sup>

addition to Pt.

**2.2. Physical pathway**

*2.2.1. Ultraviolet-assisted annealing*

at a low temperature [30, 31].

The advantage of the solvothermal method is low reaction temperature, generally below 200°C. It is important to note that this temperature is lower than the Curie temperature of PZT (~200–350°C), and more than 400°C below the reaction temperature required by the other methods. When PZT is used as a transducer, the output force is proportional to the applied voltage, which increases as the film thickness increases. In this regard, the hydrothermal method is advantageous of making micrometer-thick film; consequently, it is a promising feature for developing a microactuator driver. In addition, since the material is synthesized in solution, the film is deposited on all surfaces of the substrate making it a three-dimensional (3D) structure. Such a 3D structure is advantageous not only for actuators but also for FeRAM applications.

The hydrothermal growth of polycrystalline PZT films on Ti-substrates in a two-step process (nucleation and growth) showed that alkaline medium such as KOH was important for the formation of the PZT solid solution [23, 24]. By slightly changing the reaction conditions, PZT films could be grown in a single step [25].

Hetero-epitaxial growth of PZT films on (001) SrTiO<sup>3</sup> (STO) was achieved at 90-150°C (**Figure 3(a)**) [26]. The growth proceeded with the formation of (100)-faceted islands and their coalescence. Full coverage was obtained upon hydrothermal treatment at 150°C for 24 h. However, both the (001) and (100) orientations were detected. In addition, ferroelectric properties were not able to be evaluated due to the lack of conductivity of the STO substrate and the peel-off morphology of the film [27]. Later, these issues were resolved by adjusting the position at which the substrate was suspended in the solution, and by the use a highly conductive SrRuO3 film as a bottom electrode [28]. The 2*Pr* and *Ec* for PZT film on SrRuO<sup>3</sup> /STO (001) were 17.1 μC/cm2 and 36 kV/cm, respectively, and those of PZT on SrRuO3 /STO (111) were 32.7 μC/cm2 and 59 kV/cm, respectively (**Figure 3(b)**).

#### *2.1.4. Excluding pyrochlore phase formation*

It is well known that PZT pyrochlore phase is formed at 300–400°C. Once this stable phase is developed, a high annealing temperature (>600°C) is required to transform it from the Lead Zirconium Titanate Films and Devices Made by a Low-Temperature Solution-Based Process http://dx.doi.org/10.5772/intechopen.79378 93

**Figure 3.** (a) Cross section of SEM image of the deposited PZT thin film on the SrRuO<sup>3</sup> //STO (100) substrate. (b) Hysteresis curves for PZT thin films on SrRuO<sup>3</sup> //STO (100) [26].

pyrochlore to a perovskite structure. A simple method for the crystallization of solutionprocessed PZT films at 400–450°C by intrinsic change in the crystallization path via circumventing the pyrochlore phase formation was reported [29]. The approach does not need any modification of the precursors nor special facilities. Conventionally, the spin-coated films are normally pyrolyzed at over 300°C for a complete removal of solvent and organic ingredients. However, by this pyrolysis step, the pyrochlore phase is subsequently formed. In this work, by lowering the pyrolysis temperature to a well-below pyrochlore temperature (i.e., 210°C), it was able to retain a proper amount of carbon atoms in the gel film as shown in **Figure 4(a)**. The remaining carbon acted as a reagent to reduce Pb2+ to Pb0 when heated up to 400°C in subsequent annealing (**Figure 4(b)** and **(c)**). A significantly enhanced intensity of the PbPt<sup>x</sup> peak for the 210°C-pyrolyzed sample compared to the others indicated that larger amount of Pbo was produced (**Figure 4(b)** and **(c)**). In the presence of sufficient organic carbon, Pb2+ was reduced to Pb0 , which spontaneously reacted with Pt to form the intermediate PbPtx phase at temperatures as low as 200°C. As a result, the lack of Pb2+ prevented the formation of this intermediate phase (**Figure 4(b)** and **(d)**) that accounts for the high-temperature crystallization of the perovskite phase in the conventional processes. The process was successfully demonstrated on several representative electrode materials (Au, stacked Pt/RuO2 , and RuO2 ) in addition to Pt.

#### **2.2. Physical pathway**

has been widely used for the synthesis and growth of various materials and thin films such

/Si after annealing at 440°C [20].

The advantage of the solvothermal method is low reaction temperature, generally below 200°C. It is important to note that this temperature is lower than the Curie temperature of PZT (~200–350°C), and more than 400°C below the reaction temperature required by the other methods. When PZT is used as a transducer, the output force is proportional to the applied voltage, which increases as the film thickness increases. In this regard, the hydrothermal method is advantageous of making micrometer-thick film; consequently, it is a promising feature for developing a microactuator driver. In addition, since the material is synthesized in solution, the film is deposited on all surfaces of the substrate making it a three-dimensional (3D) structure. Such a 3D structure is advantageous not only for actuators but also for FeRAM applications. The hydrothermal growth of polycrystalline PZT films on Ti-substrates in a two-step process (nucleation and growth) showed that alkaline medium such as KOH was important for the formation of the PZT solid solution [23, 24]. By slightly changing the reaction conditions, PZT

(**Figure 3(a)**) [26]. The growth proceeded with the formation of (100)-faceted islands and their coalescence. Full coverage was obtained upon hydrothermal treatment at 150°C for 24 h. However, both the (001) and (100) orientations were detected. In addition, ferroelectric properties were not able to be evaluated due to the lack of conductivity of the STO substrate and the peel-off morphology of the film [27]. Later, these issues were resolved by adjusting the position at which the substrate was suspended in the solution, and by the use a highly

It is well known that PZT pyrochlore phase is formed at 300–400°C. Once this stable phase is developed, a high annealing temperature (>600°C) is required to transform it from the

and *Ec*

and 36 kV/cm, respectively, and those of PZT on SrRuO3

film as a bottom electrode [28]. The 2*Pr*

and 59 kV/cm, respectively (**Figure 3(b)**).

(STO) was achieved at 90-150°C

for PZT film on SrRuO<sup>3</sup>

/STO

/STO (111)

as metal oxides [21, 22].

92 Ferroelectrics and Their Applications

conductive SrRuO3

were 32.7 μC/cm2

(001) were 17.1 μC/cm2

films could be grown in a single step [25].

**Figure 2.** Dark-filed cross-section TEM for a film PZT/Pt/Ti/SiO<sup>2</sup>

*2.1.4. Excluding pyrochlore phase formation*

Hetero-epitaxial growth of PZT films on (001) SrTiO<sup>3</sup>

#### *2.2.1. Ultraviolet-assisted annealing*

Recently, ultraviolet (UV)-assisted annealing has been applied for fabrication of various functional oxide thin films since the process is capable of facilitating organic decomposition and condensation of oxide network. Consequently, high-quality oxide thin films can be realized at a low temperature [30, 31].

Shimura et al. reported a low-temperature fabrication of PZT films using a thermal UV/O<sup>3</sup> annealing process [15]. A spin-coated PZT gel film was placed on a heated stage (200°C) and

**Figure 4.** Effects of pyrolysis temperature on carbon content, valence state of Pb, and phase composition before perovskite crystallization. (a) SIMS analysis for carbon of the as-pyrolyzed samples. (b) XRD patterns for samples further heated to 400°C. (c) Percentages of reduced Pb. (d) XRD patterns for the as-pyrolyzed samples [29].

irradiated with UV light (185 and 254 nm) in O3 ambient before crystallization (**Figure 5**). The thermal UV irradiation facilitated the decomposition of organic components. At a proper temperature, the organic residue such as carbon and hydrogen atoms created a reducing environment within the gel film, which prevented the pyrochlore structure development. As a result, the ferroelectric perovskite structure with (111)-preferential orientation was able to be achieved at 450°C (**Figure 6(a)**). The *Pr* , *Ec* , and leakage current of the PZT film were 23.6 μC/ cm2 , 109.6 kV/cm (**Figure 6(b)**), and 10−<sup>6</sup> A/cm2 , respectively [15]. Similarly, ferroelectric PZT films were fabricated on LaNiO<sup>3</sup> electrode at a low temperature of 450–480°C by a method assisted with UV irradiation [32]. The obtained film annealed at 480°C showed a *Pr* of 21 μC/ cm2 and leakage current of 9.71 × 10−<sup>8</sup> A/cm2 at 100 kV/cm, with good retention and high stability of photocurrent.

the number of nucleation sites in the resulting film, which produced a further reduction of crystallization temperature [34]. The mechanism proposed for the low-temperature processing of *PhS*-PZT thin films is described in **Figure 7**. Combination of the enhanced UV-absorbance and internal nanocrystalline seeds led to a significant improvement in the formation of the PZT perovskite structure at a low temperature, which originated from a decrease of the Gibbs free energy barrier. The 350°C-PZT film deposited on a flexible PI

**Figure 6.** (a) XRD patterns and (b) hysteresis loop characteristics of PZT films prepared by conventional 600°C process

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treatment [15].

Laser annealing (LA) is an alternative technique for fast and low-temperature fabrication of PZT films. This technique uses focused high-energy laser beam in continuous or pulse mode

. This value is close to

, both on rigid Si

, and are

substrate with a thickness of 190 nm, showed value of *Pr* ~ 15 μC/cm2

**Figure 5.** An illustration of UV treatment process for PZT films.

substrates.

*2.2.2. Laser-assisted annealing*

and 450°C process with and without the UV/O3

higher than those reported for organic ferroelectric films, *Pr* ~ 10 μC/cm2

those reported for PZT films processed at temperature over 600°C, *Pr* ~ 20 μC/cm2

In order to enhance absorption in the UV-range, UV-absorber additives are normally added to precursor solutions such as "*photoactive sol*" (*Ph*) [33]. The dip-coated thin gel layer was irradiated under the UV-light, followed by rapid thermal annealing in O2 atmosphere. Formation of highly reactive oxygen radical from ozonolysis facilitated the decomposition of organic components via breaking of the alkyl group-O bonds, resulting in a subsequent formation of the metal–O–metal bonds. As a result, ferroelectric perovskite structure can be obtained at 400°C. Furthermore, incorporation of nanoseeds into the *Ph* sol (*PhS*) increased

**Figure 5.** An illustration of UV treatment process for PZT films.

**Figure 6.** (a) XRD patterns and (b) hysteresis loop characteristics of PZT films prepared by conventional 600°C process and 450°C process with and without the UV/O3 treatment [15].

the number of nucleation sites in the resulting film, which produced a further reduction of crystallization temperature [34]. The mechanism proposed for the low-temperature processing of *PhS*-PZT thin films is described in **Figure 7**. Combination of the enhanced UV-absorbance and internal nanocrystalline seeds led to a significant improvement in the formation of the PZT perovskite structure at a low temperature, which originated from a decrease of the Gibbs free energy barrier. The 350°C-PZT film deposited on a flexible PI substrate with a thickness of 190 nm, showed value of *Pr* ~ 15 μC/cm2 . This value is close to those reported for PZT films processed at temperature over 600°C, *Pr* ~ 20 μC/cm2 , and are higher than those reported for organic ferroelectric films, *Pr* ~ 10 μC/cm2 , both on rigid Si substrates.

#### *2.2.2. Laser-assisted annealing*

irradiated with UV light (185 and 254 nm) in O3

, 109.6 kV/cm (**Figure 6(b)**), and 10−<sup>6</sup> A/cm2

and leakage current of 9.71 × 10−<sup>8</sup> A/cm2

achieved at 450°C (**Figure 6(a)**). The *Pr*

films were fabricated on LaNiO<sup>3</sup>

94 Ferroelectrics and Their Applications

cm2

cm2

ity of photocurrent.

thermal UV irradiation facilitated the decomposition of organic components. At a proper temperature, the organic residue such as carbon and hydrogen atoms created a reducing environment within the gel film, which prevented the pyrochlore structure development. As a result, the ferroelectric perovskite structure with (111)-preferential orientation was able to be

**Figure 4.** Effects of pyrolysis temperature on carbon content, valence state of Pb, and phase composition before perovskite crystallization. (a) SIMS analysis for carbon of the as-pyrolyzed samples. (b) XRD patterns for samples further heated to

In order to enhance absorption in the UV-range, UV-absorber additives are normally added to precursor solutions such as "*photoactive sol*" (*Ph*) [33]. The dip-coated thin gel layer was

Formation of highly reactive oxygen radical from ozonolysis facilitated the decomposition of organic components via breaking of the alkyl group-O bonds, resulting in a subsequent formation of the metal–O–metal bonds. As a result, ferroelectric perovskite structure can be obtained at 400°C. Furthermore, incorporation of nanoseeds into the *Ph* sol (*PhS*) increased

, *Ec*

400°C. (c) Percentages of reduced Pb. (d) XRD patterns for the as-pyrolyzed samples [29].

assisted with UV irradiation [32]. The obtained film annealed at 480°C showed a *Pr*

irradiated under the UV-light, followed by rapid thermal annealing in O2

ambient before crystallization (**Figure 5**). The

, and leakage current of the PZT film were 23.6 μC/

at 100 kV/cm, with good retention and high stabil-

electrode at a low temperature of 450–480°C by a method

, respectively [15]. Similarly, ferroelectric PZT

of 21 μC/

atmosphere.

Laser annealing (LA) is an alternative technique for fast and low-temperature fabrication of PZT films. This technique uses focused high-energy laser beam in continuous or pulse mode

**Figure 7.** Mechanisms for the low-temperature processing of inorganic ferroelectric thin films using the activated *PhS* solutions [34].

and (001)PbTiO3

thin films [14].

with large *Pr*

ficients are ~11 and 9 C/m<sup>2</sup>

/Pt/ Ti/SiO2

(31 and 24 μC/cm2

case, particular sputtered amorphous PZT films were needed.

LA-crystallized PZT film by this method is relatively high (~500°C).

/Si substrates. It was shown that by minimizing nucleation energy

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for (001) and (111) PZT, respectively) and small *Ec*

for (001) and (111) PZT thin films, respectively. However, in this

(86 and

or

with suitable buffer layers, PZT films can be grown in preferred orientation at relatively low temperatures (350–375°C) with good functional properties for thin-film MEMS applications (**Figure 8**). Both (001) and (111) oriented PZT films exhibited relatively good ferroelectricity

**Figure 8.** Polarization hysteresis of laser annealed (a) (001) and (b) (111) PZT thin films. (c) Dielectric permittivity and loss tangents and (d) piezoelectric *e31,f* results as a function of poling field for laser annealed (001) and (111) textured PZT

64 kV/cm for (001) and (111) PZT, respectively). The maximum *e31,f* piezoelectric charge coef-

Considering productivity, a semiconductor diode laser is preferred because of its low cost, small size, and low energy consumption, compared with conventional solid state, CO2

excimer lasers. Chen et al. recently reported a method that can be used to crystallize PZT films derived from sol-gel solution on either Pt or Li-Nb-O-coated Si substrate by LA treatment using a 980 nm continuous wave semiconductor laser [35, 36]. From dielectric constant measurement, it is found that one LA process generates 45-nm-thick crystallized PZT layer. The dielectric constant of the PZT film is about 1200, which is comparable to that of PZT films prepared by conventional RTA technique. However, the substrate temperature required for

to scan the desired film's areas to fuse and bond the powders into a layer of solid mass, and has been used in manufacture of solar cells and power devices. The advantages of LA include the flexibility in manufacturing composites with different geometries with assistance of computer, controllable sintering thickness depending on the laser energy and scanning speed, low influence on the substrate to create the possibility of processing PZT on low melting point substrates. However, LA technique is not suitable for a large area PZT sample due to the limitation of laser spot size (generally ~50 μm in diameter). Many researchers have attempted to apply this technique to PZT crystallization since three decades ago. Bharadwaja et al. [14] reported highly textured (001) and (111) Pb(Zr0.52Ti0.48)O3 thin films (300–350 nm thick) fabricated via excimer laser annealing (248 nm KrF pulsed excimer laser) on (111)Pt/Ti/SiO<sup>2</sup> /Si

Lead Zirconium Titanate Films and Devices Made by a Low-Temperature Solution-Based Process http://dx.doi.org/10.5772/intechopen.79378 97

**Figure 8.** Polarization hysteresis of laser annealed (a) (001) and (b) (111) PZT thin films. (c) Dielectric permittivity and loss tangents and (d) piezoelectric *e31,f* results as a function of poling field for laser annealed (001) and (111) textured PZT thin films [14].

and (001)PbTiO3 /Pt/ Ti/SiO2 /Si substrates. It was shown that by minimizing nucleation energy with suitable buffer layers, PZT films can be grown in preferred orientation at relatively low temperatures (350–375°C) with good functional properties for thin-film MEMS applications (**Figure 8**). Both (001) and (111) oriented PZT films exhibited relatively good ferroelectricity with large *Pr* (31 and 24 μC/cm2 for (001) and (111) PZT, respectively) and small *Ec* (86 and 64 kV/cm for (001) and (111) PZT, respectively). The maximum *e31,f* piezoelectric charge coefficients are ~11 and 9 C/m<sup>2</sup> for (001) and (111) PZT thin films, respectively. However, in this case, particular sputtered amorphous PZT films were needed.

**Figure 7.** Mechanisms for the low-temperature processing of inorganic ferroelectric thin films using the activated *PhS*

to scan the desired film's areas to fuse and bond the powders into a layer of solid mass, and has been used in manufacture of solar cells and power devices. The advantages of LA include the flexibility in manufacturing composites with different geometries with assistance of computer, controllable sintering thickness depending on the laser energy and scanning speed, low influence on the substrate to create the possibility of processing PZT on low melting point substrates. However, LA technique is not suitable for a large area PZT sample due to the limitation of laser spot size (generally ~50 μm in diameter). Many researchers have attempted to apply this technique to PZT crystallization since three decades ago. Bharadwaja et al. [14]

ricated via excimer laser annealing (248 nm KrF pulsed excimer laser) on (111)Pt/Ti/SiO<sup>2</sup>

thin films (300–350 nm thick) fab-

/Si

reported highly textured (001) and (111) Pb(Zr0.52Ti0.48)O3

solutions [34].

96 Ferroelectrics and Their Applications

Considering productivity, a semiconductor diode laser is preferred because of its low cost, small size, and low energy consumption, compared with conventional solid state, CO2 or excimer lasers. Chen et al. recently reported a method that can be used to crystallize PZT films derived from sol-gel solution on either Pt or Li-Nb-O-coated Si substrate by LA treatment using a 980 nm continuous wave semiconductor laser [35, 36]. From dielectric constant measurement, it is found that one LA process generates 45-nm-thick crystallized PZT layer. The dielectric constant of the PZT film is about 1200, which is comparable to that of PZT films prepared by conventional RTA technique. However, the substrate temperature required for LA-crystallized PZT film by this method is relatively high (~500°C).

#### *2.2.3. Microwave-assisted annealing (MV)*

Microwave is an electromagnetic wave with wavelength ranging between 1 and 1 mm and frequency ranging from 1 to 300 GHz [37]. The difference between conventional furnace thermal annealing and MV annealing is the mechanism of these two methods. The thermal approach sinters samples by transferring heat through objects via thermal conduction. Thus, the heating source of this technique is the furnace. However, the MV annealing is different. The materials absorb the electromagnetic energy and transform it into heat to increase the temperature. Therefore, the heating source is materials themselves.

volume of material is determining the heat generation. For a nonuniform film, the heat would generate nonuniformly and result in nonuniform property, and even cracks in the films.

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The photonic sintering is a technique that uses a broadband (UV to IR), short (<ms) and highintensity pulse generated from a xenon gas-filled flash-lamp to heat the films. The thermal budget transferred to the film and substrate can be controlled by the pulse duration allowing PZT sintering while minimizing substrate heating. This unique annealing technique enables a direct formation of perovskite PZT film on low melting point substrates. Amorphous PZT films were successfully transformed to perovskite phase by a flash lamp annealing technique (energy of

**Figure 10.** (a) Cross-sectional SEM image of photonically sintered PZT film on stainless steel substrate. (b) Low-frequency hysteresis loop shows that photonically sintered (green) PZT film has superior remanent polarization (32.4 μC/cm2

) [45].

the thermally sintered (red) film (17.1 μC/cm2

) than

) with a crystallization time of 1.2 ms at a substrate temperature of 350°C [44]. Granular

*2.2.4. Flash-lamp annealing*

27 J/cm2

Recently, the MV processing has been gaining great attention for various types of materials including ceramics and metal-oxide thin films. Compared to the conventional thermal annealing technique, the MV heating offers more thermal uniformity, lower annealing temperature, shorter processing time with extremely high rate, and reduced grain growth [38]. MW-annealing techniques were applied for crystallization of PZT films at relatively low temperatures (<500°C) [13, 39–42]. For instance, Wang et al. reported a crystallization study of sol-gel Pb(Zr0.45Ti0.55)O3 films on platinized Si substrates, pyrolyzed at 400°C, and heated at 430–450°C for 30 min, using a single-mode 2.45 GHz microwave irradiation system in a magnetic field. Good ferroelectric response was obtained upon heating the films at/above 450°C [13] (**Figure 9**). It was found that the MV-annealed PZT films first crystallized into an intermediate pseudo-perovskite phase at 430°C, and then mostly crystallized into the perovskite phase at 450°C. This phenomenon was not observed in PZT films prepared by conventional thermal processing. The crystallization of amorphous PZT films by MV annealing is due to the heat originated from the substrate together with direct MV irradiation onto the films.

Although, the densification of the MV-annealed films is much higher than the conventional thermal process with the same temperature and duration, some fundamental issues limit wide usage of the MV-annealed PZT films. That is because the PZT can only absorb waves with a specific range of frequency, which limits the tools to high frequency (>25 GHz). However, most commercialized MV tools are at approximately 2.4 GHz, which lead to low MV absorption of PZT. Therefore, either preheating is used or absorption aids are added to increase the efficiency of sintering. For example, Sharma et al. added carbon powder in the PZT film to enhance the absorption [43]. Additionally, because the heat is generated internally, the

**Figure 9.** (a) SEM image of PZT film crystallized by MV at 450°C. (b) Hysteresis loops of PZT films crystallized by MV at different temperatures [13].

volume of material is determining the heat generation. For a nonuniform film, the heat would generate nonuniformly and result in nonuniform property, and even cracks in the films.

### *2.2.4. Flash-lamp annealing*

**Figure 9.** (a) SEM image of PZT film crystallized by MV at 450°C. (b) Hysteresis loops of PZT films crystallized by MV

Microwave is an electromagnetic wave with wavelength ranging between 1 and 1 mm and frequency ranging from 1 to 300 GHz [37]. The difference between conventional furnace thermal annealing and MV annealing is the mechanism of these two methods. The thermal approach sinters samples by transferring heat through objects via thermal conduction. Thus, the heating source of this technique is the furnace. However, the MV annealing is different. The materials absorb the electromagnetic energy and transform it into heat to increase the

Recently, the MV processing has been gaining great attention for various types of materials including ceramics and metal-oxide thin films. Compared to the conventional thermal annealing technique, the MV heating offers more thermal uniformity, lower annealing temperature, shorter processing time with extremely high rate, and reduced grain growth [38]. MW-annealing techniques were applied for crystallization of PZT films at relatively low temperatures (<500°C) [13, 39–42]. For instance, Wang et al. reported a crystallization study

430–450°C for 30 min, using a single-mode 2.45 GHz microwave irradiation system in a magnetic field. Good ferroelectric response was obtained upon heating the films at/above 450°C [13] (**Figure 9**). It was found that the MV-annealed PZT films first crystallized into an intermediate pseudo-perovskite phase at 430°C, and then mostly crystallized into the perovskite phase at 450°C. This phenomenon was not observed in PZT films prepared by conventional thermal processing. The crystallization of amorphous PZT films by MV annealing is due to the heat originated from the substrate together with direct MV irradiation onto the films.

Although, the densification of the MV-annealed films is much higher than the conventional thermal process with the same temperature and duration, some fundamental issues limit wide usage of the MV-annealed PZT films. That is because the PZT can only absorb waves with a specific range of frequency, which limits the tools to high frequency (>25 GHz). However, most commercialized MV tools are at approximately 2.4 GHz, which lead to low MV absorption of PZT. Therefore, either preheating is used or absorption aids are added to increase the efficiency of sintering. For example, Sharma et al. added carbon powder in the PZT film to enhance the absorption [43]. Additionally, because the heat is generated internally, the

films on platinized Si substrates, pyrolyzed at 400°C, and heated at

temperature. Therefore, the heating source is materials themselves.

at different temperatures [13].

*2.2.3. Microwave-assisted annealing (MV)*

98 Ferroelectrics and Their Applications

of sol-gel Pb(Zr0.45Ti0.55)O3

The photonic sintering is a technique that uses a broadband (UV to IR), short (<ms) and highintensity pulse generated from a xenon gas-filled flash-lamp to heat the films. The thermal budget transferred to the film and substrate can be controlled by the pulse duration allowing PZT sintering while minimizing substrate heating. This unique annealing technique enables a direct formation of perovskite PZT film on low melting point substrates. Amorphous PZT films were successfully transformed to perovskite phase by a flash lamp annealing technique (energy of 27 J/cm2 ) with a crystallization time of 1.2 ms at a substrate temperature of 350°C [44]. Granular

**Figure 10.** (a) Cross-sectional SEM image of photonically sintered PZT film on stainless steel substrate. (b) Low-frequency hysteresis loop shows that photonically sintered (green) PZT film has superior remanent polarization (32.4 μC/cm2 ) than the thermally sintered (red) film (17.1 μC/cm2 ) [45].

PZT grains were observed on various kinds of electrodes (Pt, Ru, RuO2 ), which indicates that crystal growth begins from the film surfaces. However, the small local sintering area (μm to mm scale) precludes applications with large feature sizes.

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 was 170°C. The obtained *Pr* and *Ec* were 32.4 μC/cm2 and 6.7 kV/cm, respectively (**Figure 10**).

*3.1.2. Chemical wet etching*

of PZT pattern [49].

**Figure 11.** (a) The etching rate of PZT films as a function of Cl<sup>2</sup>

(BHF) in the first step, and 2HCl:H<sup>2</sup>

of 30 μC/cm2

**3.2. Lift-off process**

leakage current density of 10−<sup>6</sup> A/cm2

wet-etching process was proposed using 1BHF:2HCl:4NH<sup>4</sup>

l:EDTA ethylenediamine tetra acetate trihydrate]:75% H<sup>2</sup>

, a *Ec*

and TiO2

where NH4

large *Pr*

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

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

significant undercutting and brim damage were observed in the achieved pattern. Later, a novel

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

cedures 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.

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

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>

, etchants containing several compositions are demanded for PZT thin film etching. In

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

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

was used as an additive to decrease the undercutting of the obtained PZT pattern.

O at 45°C in the second step, to etch PZT films [54]. However,

Cl:4H<sup>2</sup>

of 150 kV/cm (**Figure 12**), fatigue-free characteristics, and a low

at 200 × 10<sup>5</sup> kV/cm. Although various wet-etching pro-

O solution as the etchant,

COOH:HNO<sup>3</sup>

O to pattern PZT films [39]. The etch

) concentration. (b) Cross-sectional SEM image

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

101

:NH<sup>4</sup> C
