Chemical Solution Deposition of Functional Oxide Thin Films

Along with the metal carboxylates (Chap. 2), metal alkoxides represent the most frequently applied class of chemical educts for coating solution synthesis. In this chapter, some fundamental aspects of metal alkoxides, which are important for the understanding of the different CSD approaches presented in other chapters of this book, are briefly reviewed. This includes structural aspects and their reactivity towards nucleophilic agents, such as water, alcohols etc. The basics of the sol-gel transition are also given, and how the reactivity of metal alkoxides can be modulated in order to stabilize them for coating solutions.

In this chapter a short review about carboxylate based precursor systems for chemical solution deposition of thin films is presented. First, an introduction into the chemical characteristics of carboxylic acids as well as preparation methods for metal carboxylates is given. Subsequently the MOD (metallo-organic decomposition) concept, which is largely dependent on the use of metal carboxylates is introduced. Important aspects of this method such as deposition, solvent evaporation behavior, and thermal treatment will be discussed. Special emphasis will be given to the decomposition behavior of relevant long- and short chain metal carboxylates, i.e. metal soaps, acetates and propionates. The decomposition of these compounds will be characterized by looking at the degradation mechanisms, intermediate forming and the resulting products. Selected established MOD-routes which are based on these educts are presented and the properties of the resulting layers are described. The last part will be on some basics of the MOD concept for producing superconducting thin films from fluorinated carboxylates.

Solution processing of functional oxides often starts with rather complex precursor systems consisting of metal alkoxides, carboxylates and/or nitrates dissolved, mixed and reacted (modified) in different solvents. Precursor systems of main functional perovskite groups are reviewed in this chapter. The breakthrough was achieved in mid-1980s, when the 2-methoxyethanol route was developed for processing of Pb(Zr,Ti)O₃-based films, in which this ether alcohol is used as the solvent, acetates as the A-site and alkoxides as the B-site ion sources. Two other very popular routes are the diol-route, in which 1,3-propanediol is used as the solvent, and the so-called inverse-mixing-order route, in which the B-site cations are mixed first and the A-site source is added afterwards. In the alkali metals-based systems, such as Li(Nb,Ta)O₃, (K,Na)(Nb,Ta)O₃, and Na_0.5Bi_0.5TiO₃, several methods have been developed in which different starting metallo-organic compounds have been used as the A-site metal sources, while mainly alkoxides have been employed as the B-site precursors. A variety of solvents can be used, such as methanol, ethanol, 2-methoxyethanol, 1,3-propanediol, as well as modifiers, for instance acetic acid, acetylacetone, and polyvinylpyrrolidone. Even water has been employed in some systems. The chapter concludes with the review of (Ba,Sr)TiO₃-based precursor systems, mainly involving A-site carboxylates and stabilized Ti-alkoxides mixed in one of the above mentioned solvents.

The chapter provides detailed analysis of reaction mechanisms for hydrolytic processes, leading to formation of metal oxides in organic media, and explains the role of such factors as hydrolysis ratio, complexation ratio, the nature of heteroligands, and the nature of constituents in chemically complex systems. It presents the MTSAL concept, describing the principles of micellar self-assembly for metal oxides in organic solvents and introduces the principles for construction and application of specially designed single source precursors (SSP). Advantages and disadvantages of SSP approach for the preparation of dense and porous films for different applications are described. The principles for the structure design and synthetic approaches to this class of compounds are outlined and illustrated for a large number of heterometallic and heteroleptic alkoxide complexes.

In this chapter, water based CSD of electronic oxide films from carboxylate based precursors is discussed which has, besides the absence of hazardous solvents, the additional asset of simple and inexpensive synthesis- and deposition equipment. However very attractive, this ‘new’ CSD method had to deal with typical scientific and technological obstacles. First of all, the development of water soluble precursors, especially when high valent metal ions are involved, in a chemical environment suitable for gel formation and film deposition forms a true chemical challenge. A second obstacle is the difficulty to deposit homogeneous thin layers of water based precursor solutions, which is related with wetting incompatibilities. Both issues will be elaborately discussed and reviewed. Moreover it is shown that film properties can be drastically optimized by means of controlling all different processing steps. Finally the achievement of highly demanding challenges such as deposition of epitaxial or ultrathin layers is demonstrated. The aqueous CSD method can therefore be considered as a fairly mature technique enabling the deposition of oxide layers for state of the art applications and providing a means for the fast screening of highly advanced materials systems and processes.

In this chapter, polymer-assisted deposition (PAD) to grow metal-oxide films with desired structural and physical properties is described. In the PAD process, the polymer controls the viscosity and binds metal-ions, resulting in a homogeneous distribution of metal precursors in the solution and the formation of uniform metal-organic films. The latter feature makes it possible to grow a wide range of simple and complex metal-oxide films with desired structural and physical properties. Compared to other chemical solution deposition techniques, the development of PAD technology is still in the early stage. Many issues related to materials science and chemistry need to be investigated, and further optimization of processes is necessary for the high quality materials.

Thermal analysis provides information about the thermal behavior of the investigated material. It comprises a group of analytical techniques performed in controlled conditions and under a controlled temperature program. The measured physical properties include mass, temperature difference, enthalpy, etc. Thermogravimetry alone or in combination with differential thermal analysis and/or differential scanning calorimetry have been used in chemical solution deposition of functional oxide thin films as the analytical methods of choice to study the thermal decomposition of the as-synthesized (liquid) and in most cases dried precursors of the thin-film materials, often in combination with other analytical techniques such as evolved gas analysis by means of infra-red spectroscopy, mass spectrometry, measurements of selected electrical, optical, or other, properties, to provide further insight into the processes taking place in the materials undergoing chemical and structural changes upon heating. There are only a limited number of thermal analysis studies of thin films, mainly due to the extremely low film/substrate mass ratio.

Despite numerous investigations of lead zirconium titanate compounds, little information exists about the microscopic structure of the precursors in solution and their changes in course of the sol-gel process. This fact is not astonishing, since common methods of physical chemistry, like molecular weight determination, IR- and NMR-spectroscopy, conductivity measurements etc. do not provide direct information about the precursor structure. X-ray absorption spectroscopy, however, is a powerful method for studying local environment of metal atoms in liquid and amorphous materials. Nevertheless, this method was rarely applied to the study of the sol-gel process until the late nineties [Malic et al. (J Mater Res 12:2602, 1997)]. In this chapter, EXAFS spectroscopical investigations on the sol-gel-based systems, appropriate for thin film manufacturing via CSD, are presented. After a short introduction about the EXAFS method in theory and practice, we give an overview of the classical thin film compounds, namely the binary systems lead titanate (PT) and lead zirconate (PZ), the ternary lead zirconium titanate (PZT) and quaternary lead zirconium titanate, doped with lanthanum (PLZT). All these systems form ferroelectric perovskite-type films. Our overview will close with an outlook on investigations on new materials produced by CSD like the interesting new high-temperature superconductors (HTS).

One of the first and most frequently-used methods for the investigation of thin films obtained by chemical solution deposition is IR spectroscopy. Infrared spectroscopy (or vibration absorption spectroscopy) provides information about the covalent bonds in the chemical groups of the investigated materials. IR spectroscopy allows both the solutions used for film deposition and the films obtained to be investigated by using different deposition methods. The following characteristics are among the most important results obtained: composition of the initial reagents, molecular structure of the solutions obtained by mixing different reagents, reactions in the solutions during storage, influence of the type of solution (alcoholic or aqueous) on the characteristics of the deposited films, and crystallization by thermal treatment of the structure of as-deposited films which are usually amorphous.

In this chapter a wide a range of examples—from the putative simple dip coating process, where a substrate is withdrawn from the coating solution at a constant rate, to the more advanced evaporation induced self-assembly (EISA) process, which leads to well-ordered nanostructured thin-film materials—is covered. In the first part the fundamentals of classical dip coating are presented. Various physical and chemical effects which influence the thickness and microstructure evolution are discussed. The film thickness is set by the competition among viscous force, capillary force, and gravity. Microstructure and properties of the film e.g. depend on the size and structure of the inorganic precursor species, the magnitude of the capillary pressure exerted during drying, and the relative rates of condensation and drying. After the basic aspects modified dip coating techniques which enable the coating of differently shaped substrates such as cylinders, tubes and bottles are briefly presented. Finally different aspects of the EISA process, which is based on the fact that in dip coating film formation occurs through evaporation of solvents concentrating the system in non-volatile species, is shortly reviewed. Thus by means of the silica/surfactant system it is presented how aggregation & gelation can be controlled and functional nanoscopic materials can be generated. It is also briefly shown that EISA can be used to simultaneously organize hydrophilic and hydrophobic precursors into hybrid nanocomposites that are optically or chemically polymerizable, patternable, or adjustable.

Spin coating is one of the simplest methods for depositing solution-derived thin films onto flat substrates of moderate size. This process is used for the deposition of photoresists, antireflection coatings, dielectric layers, and passivating layers (like spin-on-glass) during the fabrication of integrated circuitry. And, it is one of the key methods for depositing CSD ferroelectric coatings. As the trend toward smaller devices continues, the uniformity of these thin films is increasingly important in achieving optimum device performance. While the process is conceptually quite simple, there are cases where the characteristics of the coating solution may be perturbed by the flow and evaporation processes that are inherent to this process, sometimes causing process-related defects in the final coating. Therefore, the first part of this chapter is aimed at presenting the basic operation of the spin coating process and the connection to basic fluid flow physics. Later parts of this chapter are used to examine some of the cases where coating solution properties can lead to defects and/or thickness variations. In most cases we desire completely flat and uniform coatings, however there may be some situations where texturing is required at the micro and nano scale.

Chemical Solution Deposition (CSD) offers a relatively robust fabrication route in the formation of complex metal oxide thin films. This utility stems largely from the straightforward control of film stoichiometry via the concentration of metallic components in the precursor solutions. There are several techniques by which these precursor solutions can be applied to substrates, each with their own merits that enable specific applications. The purpose of this chapter will be to describe the aerosol deposition of precursor solutions and to compare this technique against other established coating methods. The key processes by which aerosol deposition takes place will be examined and various deposition techniques will be explained on the basis of these processes. Finally, the characteristics of aerosol deposited films will be discussed and the applications in which aerosol deposition has been employed will be presented.

Inorganic printed electronics produced via advanced printing techniques, e.g. direct write, inkjet and microcontact printing are attractive alternatives to conventional photolithography and electroless deposition routes, which are often time-consuming, complicated, and expensive. With the emergence of flexible electronics, the ability to integrate dissimilar materials, i.e., ceramics, metals, semiconductors, and polymers, onto a single, temperature-sensitive substrate enables the fabrication of volumetrically-concise, multi-functional packages. In this chapter, methods to deposit a variety of conductors, transparent conductors and superconductors via printing methods are detailed. A method to integrate such printed elements into self-powered, wireless telemetry units is detailed.

This chapter reviews the transformation of solution-derived ferroelectric thin films from the as-deposited to the crystalline state that occurs during heat treatment. Reaction chemistry and pathways associated with the elimination of organic constituents from the film are discussed, as are the thermodynamic and kinetic aspects of the nucleation and growth processes that lead to crystallization. Related topics discussed in this chapter include structural evolution during pyrolysis, thin film densification processes, the role of interfacial layers on film orientation, and control of thin film crystallization behavior and microstructure. A focus of the chapter is the consideration of solution chemistry effects on pyrolysis, densification and crystallization behavior and illustration of the magnitude of such effects on thin film microstructure.

The chemical solution deposition (CSD) method involves the synthesis of films with a mixture of precursor molecules, dissolved in a common solvent, and uniformly spread on a substrate. When applied to single crystal substrates, sequential events occur during heating that can produce a single crystal, epitaxial film. Upon heating, precursor film decomposes (pyrolysis) to form an inorganic, amorphous phase that crystallizes at a higher temperature, forming a single crystal film at higher temperatures. The fundamentals of CSD will be reviewed to include the relation between cracking and film thickness—one limitation of CSD, why a polycrystalline film with nano grain size is formed before being converted into a single crystal, the crystallization of metastable phases, mechanisms that convert the polycrystalline film into a single crystal film, how polycrystalline films can break into isolated islands with a specific crystallographic orientation that can be used to ‘seed’ a single crystalline film, and why dissimilar crystal structures epitaxially grow on one another.

This chapter deals with the integration of CSD functional oxide thin films into semiconductor devices. Thereby, special emphasis is put on the control of the films’ orientation and microstructure by means of a defined precursor selection, tailored heating procedures, and the choice of a certain substrate material or the use of crystallization seed layers. The basics which are given in the previous Chaps.​ 15 and 16 are applied to two technical important functional oxides, the ferroelectric material Pb(Zr,Ti)O₃ and the high-k material (Ba,Sr)TiO₃. These case studies will exemplarily demonstrate the broad spectrum of control parameters within the CSD processing of complex oxides, even if the choice of the substrate material is limited by the type of application.

Low-temperature processing incorporates a variety of strategies implemented in chemical solution deposition (CSD) of functional oxide thin films with the aim of decreasing the final heating temperature and thus overcoming the problems related to interaction with the substrate and/or integration into microelectronic elements/devices, or deterioration of thin-film materials. The crystallization temperatures of thin films with different material compositions have been decreased by modification of the precursor solution and the design of the heating profile to ensure a high level of chemical homogeneity already in the liquid state and throughout the processing. In Pb(Zr,Ti)O₃ thin films nucleation layers have been successfully implemented to decrease the temperature of perovskite crystallization. Some combined approaches to achieve lower processing temperatures are presented, including hydrothermal reaction, photochemical activation to facilitate the organics thermal decomposition, and laser activation of the as-deposited films. Oxide semiconductors on the other hand can be used as amorphous inorganic films, therefore the focus is on the design of low-temperature decomposable precursors.

Under the broad umbrella of chemical solution depositions (CSD), synthesis of thick films (>1 μm) using a combination of sol and particles (consisting of particles >100 nm size) has first been reviewed. Here the sol is used to both enhance the sintering and performance of conventional powder films as well as being integral to the formation of true powder-sol composite films where the sol forms in integral part of the deposited ink. Advantages and limitation of these composite sol–gel processing techniques are considered and deposition routes explored. The subsequent sections are devoted to outline the novel concept of composite thin film synthesis using molecular precursors. Based on the authors’ own experience and existing literature, the perspective, potential and possibilities of the electro-ceramic thin films synthesized using sub 100 nm particulate precursor sols has been outlined.

UV- and E-beam direct patterning processes using photosensitive precursor films are reviewed in this chapter. For the UV patterning process, discussed in the first part, reaction processes of precursors, such as metal alkoxides modified with β-diketones and metal complexes of carboxylic acids, with UV-irradiation are reported. The discussion of multi-component oxide film, as well as simple oxide film preparation, some applications of direct UV-patterning, and limitation of this process completes this section. The second part covers the progress made in the direct sub-10 nm electron beam patterning of metal oxides over the last 30 years, moving from physical deposition to chemical solution deposition of resists. Patterning of inorganic resists began with thermally evaporated metal halides. They were soon taken over by sputtered metal oxide films due to their excellent environmental stability. However, these inorganic materials, both halides and oxides, suffered from very steep electron dose requirement, thus rendering them useless for practical applications. This gave way to highly electron beam-sensitive chemical solution deposited stabilized metal alkoxides and metal naphthenates, with sensitively close to conventional electron beam resists such as poly(methylmethacrylate), PMMA, and calixarene. Furthermore, they show excellent line edge roughness characteristics at sub-10 nm scale, which is currently unmatched by any other electron beam resist. Recent applications of these resists such as etch mask and their suitability as gate oxides closes the chapter.

By means of ferroelectric nanograins various template based approaches to a registered assembly of functional materials is reviewed. At first the principles of seeds and seedlayers for orientation selection are discussed. Then the generation of artificial seed pattern by a number of top-down and bottom-up methods as well as combination of them are discussed. As such (1) e-beam lithography, (2) e-beam lithography combined with lift off processing, (3) soft template infiltration, (4) a self-assembly approach based on diblock-copolymer micelles and gold hard masks, and (5) FIB to generate defined nucleation sites on platinized silicon within an amorphous TiO₂ layer are detailed. Functional registered ferroelectrics, mainly obtained by subsequent CSD processing, were characterized by means of surface probe microscopy (SPM) methods such as atomic force microscopy (AFM) and piezoelectric force microscopy (PFM). Embedding concepts for the template grown ferroelectric nanograins, which enable to maintain ferroelectric properties, complement the template controlled growth methods with regard to possible integration.

The application of ferroelectric and dielectric materials for capacitors is reviewed in this chapter with a focus on multilayer stacks. As the trend to miniaturization of high-tech electronic devices in recent years requires electric parts mounted in them to be smaller and smaller, the technology of multilayer ceramic capacitors (MLCC) also continues to be developed for downsizing and increasing capacitance. Thus firstly the aspects of the mature technology for fabrication of MLCC are briefly reviewed, followed by introduction of the new concepts for further miniaturization by using thin film deposition technologies. Then the design of thin film multilayer capacitors (TMC) in terms of electrode materials, dielectric film processing issues, patterning aspects, and electrical properties is discussed.

Integrating electroceramic thin films with inexpensive base metal flexible substrates opens new possibilities for realistic mass production of functional oxide thin film technology. Not only do such substrates significantly reduce raw material costs, their flexible nature enables rapid, low-cost roll-to-roll processing. However, integrating an oxide with a base metal is a true scientific challenge. How can an oxide thin film be crystallized without oxidizing the base metal? One possibility is to imitate multilayer capacitor technology and crystallize the film using temperature and oxygen partial pressure (pO₂) conditions that provide thermodynamic equilibrium between the oxide and base metal. Unfortunately, this is not possible for all desired oxide constituents (e.g. PbO, Bi₂O₃). In these cases, the low processing temperatures of a chemical solution deposition method provide another avenue to integration: kinetic avoidance of substrate oxidation. To maintain high quality oxide films at low processing temperatures requires an understanding of chelation chemistry and gel-to-ceramic conversion. This chapter addresses both the thermodynamic equilibrium and kinetic method for integrating chemical solution processed functional oxide films on base metal electrodes and provides the reader with insights into how this state-of-the-art technology is revolutionizing the commercializability of thin film electroceramics.

Polar oxides have outstanding piezoelectric, pyroelectric and optical properties. In this chapter we deal mainly with piezoelectric properties. These play a crucial role in a large number of devices and applications modern society would not like to miss. Mobile communication and ultrasonic imaging are just the most prominent ones. Since two decades, miniaturization of mechanical devices in silicon technology is a major research direction in engineering known under name of MEMS, which stands for micro-electro-mechanical systems. Piezoelectricity fits very well into this concept and was expected right from the beginning to play its role in MEMS. The breakthrough was made with RF filters in mobile phones working on the principle of standing thickness waves in AlN films. What counts here is acoustic quality and stability. The force champion among piezoelectric thin film materials, Pb(Zr, Ti)O₃ gave more problems in processing, and requires more patience to meet requirements and needs for mass applications. It seems, however, that the breakthrough is imminent. This article gives an overview of the field, highlighting major steps in its development, characterized by taking hurdles in processing and integration.

Oxides that are electronic and/or ionic conductors play an important role in many applications. Here we review the use of thin films of oxides for use as base electrodes for ferroelectrics, for use in magnetoresistive devices and for use in solid oxide fuel cells, with focus on chemical solution deposition methods for obtaining the films and on how the processing affects the properties. For base electrodes, LaNiO₃ is being studied as a replacement for Pt. For magnetoresistive devices, Ca- and Sr-doped LaMnO₃ exhibit colossal magnetoresistance and are studied to understand the relation between structure and properties. For solid oxide fuel cells, thinner electrolytes are highly interesting to lower the resistance and thereby enable lower operating temperatures. Chemical solution deposition methods give good stoichiometry control and can lower the processing temperature compared to traditional powder methods, enabling nanostructuring of grain sizes, grain boundaries and the porosity, both for electrolytes and cathodes. New developments in micro-solid oxide fuel cells for integration into portable devices are reviewed in the end.

An introduction to the wet-chemical processing of indium tin oxide (ITO), aluminium-doped zinc oxide (AZO) and antimony-doped tin oxide (ATO) thin films is given, their performance is compared to materials derived from physical and chemical vapour deposition. Strategies to reduce inherent limitations of sol–gel coatings due to residual porosity are emphasized. Printability as a specific advantage of solution-based techniques is highlighted. It also is shown how compositional flexibility and the facile introduction of dopants are used to explore the potential of novel p-type conductive delafossites.

About 25 years after the discovery of high temperature superconductors (HTS) the promises regarding energy and materials efficiency in power technology start becoming reality. Energy densities exceeding 10,000 A/mm² in HTS layers in conjunction with no electric losses may revolutionize the use of electricity. One of the major challenges towards commercialization is the large scale production of HTS wires. In the last years layer architectures processed by chemical solution deposition (CSD) were developed exhibiting competitive performance to physical deposition technologies, while non-vacuum CSD processes offer economic advantages towards large scale production. In this chapter the actual status of CSD processed HTS wires with special focus on epitaxial growth of the complex layer architecture will be presented. In addition applications for HTS technology such as cables and rotating machines are discussed.

Sol-gel techniques are particularly suitable for the preparation of antireflective (AR) coatings and optical filters: Chemical compositions and thus optical properties can be adjusted over a wide range; different films are easily combined to multilayer arrangements. The selective implementation of porosity allows the preparation if films with unique optical characteristics. Additionally micro- and nanopatterns can be fabricated by embossing techniques that are difficult or impossible to be produced otherwise. This chapter shortly reviews the general strategies for AR coatings. Methods for the sol-gel processing of single- and multilayer interference-type films are described; references to respective commercial products are given. Elaborate stack arrangements are used as optical filters beyond the scope of AR-coatings. Index-gradient films are finally introduced including “moth-eye” films prepared by the embossing and subsequent thermal curing of specific coatings.

Inorganic luminescent materials (phosphors) do not absorb visible light and hence they are optically transparent if we could avoid light scattering. Thin films of luminescent materials that are coated on any kinds of substrates are really transparent and have received considerable attention these days. One should know, however, phosphor thin films are usually less bright than powder screens due to the occurrence of multiple internal reflections and light trapping. It is therefore a challenging undertaking to design, synthesize, and utilize luminescent thin films having excellent emission properties. This chapter reviews preparation of luminescent thin films by various kinds of chemical solution deposition methods and their applications. Emphasis is placed on characteristic optical properties of thin films such as transmission, surface Fresnel reflection, total internal reflection, and interference. Applications of luminescent thin films to active waveguides, flat panel displays, X-ray imaging devices, and solar cells are also presented.