Processing of ferroelectric thin films

Sheppard, L., Advances in Processing of Ferroelectric Thin Films, CeramicBull, 71(l) 85-95 (1992)... 

A.T. Hunt and H.G. Hornis, CCVD Theory and Processing of Ferroelectric Thin Films at Ambient Pressure. Invited presentation at the MOCVD Workshop for Silicon Processing, Kyungju, Korea, 1995. 

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. 

The first report of a wet chemical processing of ferroelectric thin film was by Fukushima et al. in 1975 [18]. They reported the use of a mixed alkoxide and organic salt precursors in the fabrication of BaTiOs film. Application of sol-gel processing for the PZT thin films was started in 1984 by Wu et al. [19] and Fukushima et al. [20] and followed by Budd et al. in 1985 [21]. More recently, continuing efforts in the processing of PZT family thin films by sol-gel and MOD methods can also be found in the literature [22-27]. 

Calzada, MX., Bretos, L, Jimenez, R., Guillon, H., and Pardo, L. (2004) Low-temperature processing of ferroelectric thin films compatible with silicon integrated circuit technology. Adv, Mater, 16 (18), 1620. 

Precursors for Aqueous and Liquid-Based Processing of Ferroelectric Thin Films... 

Ferroelectric Thin-Film Devices. Since 1989, the study of ferroelectric thin films has been an area of increasing growth. The compositions studied most extensively are in the PZT/PLZT family, although BaTiO, KNbO, and relaxor ferroelectric materials, such as PMN and PZN, have also been investigated. Solution deposition is the most frequentiy utilized fabrication process, because of the lower initial capital investment cost, ease of film fabrication, and the excellent dielectric and ferroelectric properties that result. 

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. 

Recently, efforts have been devoted to the fabrication and characterization of PbZri- Ti c03 family thin films for their potential applications in nonvolatile memory devices (See Ref. 17, for example). Partly because of the convenient stoichiometry control during processing, it was found that chemical methods, such as sol-gel and metal organic decomposition (MOD), are superior to physical means in many aspects. To appreciate better the science and technology of ferroelectric thin-film fabrication, it is important to give a brief account of the past efforts and the present status and, it is hoped, shed some light on the future. 

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, Chen C.J., Xu R., Mackenzie J.D. The self-biased heterojunction effect of ferroelectric thin film on silicon substrate. J. Appl. Phys. 1990 67 2985-2991 Xu Yuhuan, Chen C.J., Xu R., Mackenzie J.D. Ferroelectric thin films on silicon and fused silica substrates by sol-gel process. Mat. Res. Soc. Symp. Proc. 1990 200 13-18 Xu Yuhuan, Chen C.J., Xu R., Mackenzie J.D. Ferroelectric Sro.6Bao.4Nb206 thin films by sol-gel process electrical and optical properties. Phys. Rev. B 1991 44 35 1 Xu Yuhuan. Ferroelectric Materials and Their Applications. Amsterdam, London, New York, Tokyo North-Holland 1991... 

The requirements of thin-film ferroelectrics are stoichiometry, phase formation, crystallization, and microstmctural development for the various device appHcations. As of this writing multimagnetron sputtering (MMS) (56), multiion beam-reactive sputter (MIBERS) deposition (57), uv-excimer laser ablation (58), and electron cyclotron resonance (ECR) plasma-assisted growth (59) are the latest ferroelectric thin-film growth processes to satisfy the requirements. 

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. 

Chemical and physical processing techniques for ferroelectric thin films have undergone explosive advancement in the past few years (see Ref. 1, for example). The use of PZT (PbZri- cTi c03) family ferroelectrics in the nonvolatile and dynamic random access memory applications present potentially large markets [2]. Other thin-film devices based on a wide variety of ferroelectrics have also been explored. These include multilayer thin-film capacitors [3], piezoelectric or electroacoustic transducer and piezoelectric actuators [4-6], piezoelectric ultrasonic micromotors [7], high-frequency surface acoustic devices [8,9], pyroelectric intrared (IR) detectors [10-12], ferroelectric/photoconduc-tive displays [13], electrooptic waveguide devices or optical modulators [14], and ferroelectric gate and metal/insulator/semiconductor transistor (MIST) devices [15,16]. 

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