Ferroelectrics thin film preparation

This chapter reviews the general aspects of the CSD method for ferroelectric thin-film preparation, with attention given to precursors, solution chemistry, and process development. An additional focus of the chapter is on the structural evolution of the solution precursor into the crystalline (typically perovskite) state and the impact of precursor chemistry and film fabrication conditions on the transformation process. Lastly, the chapter reviews the advantages and disadvantages of the CSD method and discusses industrial implementation of the technique. 

A complete review of the reported properties of ferroelectric thin films prepared by CSD is beyond the scope of this chapter. Suffice it to say that fabrication approaches from each of the three CSD categories noted above have been used to prepare high-quality films in a range of thicknesses. The dielectric response and ferroelectric hysteresis behavior have been widely reported and the reader is referred to References 12 and 13 for representative results. Despite space limitations, three aspects of CSD processing and film properties warrant consideration here. These are (i) the ability to prepare oriented films by CSD (ii) typical stress levels within the films and (iii) the general dielectric properties of the thin film materials compared to the corresponding bulk materials. 

Xu Yuhuan, Mackenzie J.D. Ferroelectric thin films prepared by sol-gel processing. Integrated Fer-roelectrics 1992 1 17 2... 

Xu Yuhuan, Lou Yudan, Cheng C.H., Mackenzie JD. Epitaxial ferroelectric thin films prepared by the sol-gel technique. Ferroelectrics 1997 195 283 Xu Yuhuan, Cheng C.H., Mackenzie J.D. Microstructure in heteroepitaxial potassium niobate thin films. J. Korean Phys. Soc. 1998 32 S1724-S1726 Xu Yuhuan, Mackenzie J.D. A theoretical explanation for ferroelecttic-like properties of amorphous Pb(Zr,Tii )03 and BaTiOs. J. Non-Cryst. Solids 1999 246, 136 Xu Yuhuan, Ye H., Cheng C.H., Mackenzie J.D. Electro-optic effect in nano-crystalUne SbSI-doped glass. Ferroelectrics 2001 259 259-268... 

Funakubo H (2004) Recent development in the preparation of ferroelectric thin films by MOCVD. Ferroelectric Random Access Memories Fundamentals and Applications 93,95-103... 

Another general class of solids that has been prepared as thin films using PLD is ferroelectric materials. A potentially useful characteristic of ferroelectric materials is that they can be polarized by an electric field and retain this polarization when the field is removed. In ferroelectric thin films it may be possible to exploit this polarization phenomenon to make sensors, displays, and memory devices. A number of techniques have been used to prepare ferroelectric thin films. However, it has been difficult to control the stoichiometry (and correspondingly the properties) of these materials using thermal and sputtering techniques. In part, the difficulty in maintaining correct stoichiometry is due to the volatility of a component in the material (e.g. Pb in PbTiOs). 

A broad range of electronic ceramic materials have been prepared by CSD, but three material systems have dominated the field of ferroelectric thin films. These include the perovskites PbZr03-PbTi03 (lead zirconate titanate PZT), BaTi03-SrTi03 (barimn strontium titanate BST), and the layered perovskite SrBi2Ta209 (strontium bismuth tantalate SBT). The extensive solid solubility ranges... 

For CSD processing of ferroelectric thin films, a homogeneous solution of the necessary cation species that may later be applied to a substrate must be prepared. Overall, the basic process involves the steps of solution preparation, film deposition, pyrolysis for removal of organic constituents, and heat treatment to induce crystallization, as shown in Figure 27.2. This section provides some of the experimental details associated with the CSD process, while the next section discusses the process from a more fundamental perspective. 

As ferroelectric material we use poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)). This copolymer is soluble in non-toxic reagents, for example 2-butanone. The preparation of organic and ferroelectric thin films via spin coating from solution is possible [13]. The polarisation field of P(VDF-TrFE) is relatively high, about 50 MV/m [14]. Here, a downscaling of the P(VDF-TrFE) film thickness into a range below 100 nm is necessary in order to use small bias voltages for polarisation. 

Hattori, X, Takahashi, Y, lijima, M., Fukada, E., 1996. Piezoelectric and ferroelectric properties of polyurea-5 thin films prepared by vapor deposition polymerization. J. Appl. Phys. 79,1713-1721. 

Wang, Y, Shao, Q.Y, and Liu, J.-M. (2006) Enhanced fatigue endurance of ferroelectric Pbi Sr (Zro,52Tio.4s)03 thin films prepared by sol-gel method. Appl. Phys. Lett., 88, 122902. 

The successful development of these thin films for device applications requires that two goals be met (1) the preparation of materials with device quality characteristics and (2) for certain applications, successful integration of the thin film with underlying silicon circuitry, without degradation of circuitry performance characteristics. A number of analytical characterization techniques have been employed to study film preparation and thin film—device integration issues. Some of these techniques and their applicability in characterizing ferroelectric thin film device preparation will be briefly discussed. 

Ferroelectric thin films which are being developed for nonvolatile memory applications are most often based on the use of solution-deposited lead zirconate titanate (PZT). Solution deposition generally involves the following steps (1) preparation of the precursor solution from metal alkoxide and carboxylate compounds (2) the tailoring of solution characteristics such as solids content, viscosity. 

Usually, when ferroelectric thin film samples formed as metal-electrode/film/metal-electrode sandwich structures, we can observe a symmetric P-E loop. The ferroelectric P-E hysteresis loop of PZT thin film samples fabricated by the sol-gel method were measiued by using a modified Sawyer-Tower bridge. The PZT films were coated on titanium foil-substrates and were heat-treated at 700°C and 400°C, respectively. The samples were prepared as a sandwich structure of metal electrode (sputtered Au)/PZT film/titanimn-electrode (substrate). The observed P-Eloops ofthese samples are shown in Figure22-8(a) and (b). Similarly, a parallel study was conducted for crystalline and amorphous BaTiOs thin films fabricated by the sol-gel method. A P-E hysteresis loop was observed on the amorphous BaTiOs thin film (Xu and Mackenzie, 1994). 

In this chapter we have introduced some general concepts of the ferroelectric and piezoelectric properties, as well as some typical methods for measurement of the ferroelectric materials fabricated by the sol-gel technique, hi general, sol-gel derived ferroelectric materials can be formed to various shapes, such as bar, disk, fiber, and film, in fact, the most successful product is ferroelectric thin film. In principle, the measurement of ferroelectric thin film derived by the sol-gel processing is similar to the measurement of a bulk material. In general, the properties of carefully prepared ferroelectric films are comparable to those of bulk polycrystalline samples. However, there is a little difference between the resulted... 

Xu Yuhuan, Cheng C.H., Mackenzie J.D. Epitaxial KNbOs and Fe-doped KNb03 thin films prepared by the sol-gel technique. Mater. Res. Soc. Symp. Proc. (Ferroelectric Thin Films V) 1996 433 401... 

Figure 27.10 Ferroelectric strontium bismuth the larger grain size of the surface microstruc-tantalate (Sro BBi2.2Ta209) thin films prepared tures of the films prepared from the tantalum from the sols of Figure 27.9 (a) tantalum pen- pentaglycolate sols and the larger values of taethoxide sols and (b) tantalum pentaglyco- the remanent polarization (5Pr) obtained in late sols. SEM images of the top surfaces of these films compared with the films prepared the films are shown, as well as the ferroelectric from the tantalum alkoxide-based sol [47]. hysteresis loops measured in these films. Note...

Table 27.3 Heterostructural characteristics and electrical properties (dielectric, ferroelectric, and pyroelectric) of two (Ca, Pb)Ti03 thin films prepared from the same precursor solutions but crystallized with different thermal annealing rates for the ceramic conversion.

Martin-Arbella, N Bretos, I., Jimenez, R Calzada, M.L., and Sirera, R. (2011) Metal complexes with A/-methyldiethanolamine as new photosensitive precursors for the low-temperature preparation of ferroelectric thin films. J Mater, Chem.,... 

Combining different ferroelectric layers to enhance comprehensive properties of ferroelectric thin films is also of interest. The aim is to utilize the advantages of the individual materials. Bao, et al. [6,42] prepared BLaT/PZT/BLaT tri-layered thin films by PLD. The sandwiched structure exhibited good ferroelectric properties with high fatigue resistance as shown in Figure 3 [6]. 

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