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Chemical solution deposition of thin films

Objective

A.BACKGROUND

The introduction of new concepts and potential applications for advanced material thin films has sparked off a new focus on novel synthetic techniques and processing advances for their deposition. As a consequence, the preparation of innovative materials such as complex metal oxides and ferroelectrics using novel synthetic routes is a growing technological area. Chemical solution deposition (CSD) technologies for the fabrication of thin films (including sol-gel, liquid source chemical vapour deposition (LS-CVD), and spray pyrolysis) have gained considerable interest.

These novel process routes make the following possible:

- the synthesis of a broad spectrum of materials,

- the ability to co-synthesise two or more materials simultaneously,

- coating of one or more materials on to other materials (metal or ceramic particulates, and three-dimensional objects),

- production of extremely homogeneous alloys and composites,

- synthesis of ultra-high purity materials,

- very accurate tailoring of the composition even in the early stages of the process, because the synthesis is actually performed on an atomic level,

- precise control of the microstructure of the final products and,

- precise control of the physical, mechanical, and chemical properties of the final products.

The properties and applications of advanced material thin films depends to a large extent on the stoichiometry and microstructure of the thin film. The most prominent advantage of CSD over conventional CVD and sputter deposition methods is the excellent control over film stoichiometry they offer. Because film composition can readily be tuned by adjusting the composition of the precursor solution, a large number of tailored thin films with specific properties can be prepared leading to a diverse family of thin films for numerous applications. The versatility of this technique has already lead to the achievement of high quality thin films with various properties: ferroelectrics (SrBi2Ta2O9 SBT) and Pb(Zr, Ti)O3 (PZT)), piezoelectrics (Pb, Sr)(Zr, Ti)O3, pyroelectrics ((Pb, Ca)(Zr, TiO3)) and electrostrictive materials and mixed composition dielectrics (Ta-Ti oxides).

A1: State of the Art

To date, there are a number of issues facing CSD technologies which must be addressed. Despite the versatility and obvious benefits of CSD techniques, the integration of such processes (and hence such novel materials) into microelectronic, optoelectronic and microsystems devices and components has come up against many difficulties. The problems of large area deposition and compatibility with microelectronic processes, maximum crystallisation temperatures and thermal budget, and thick film deposition have hampered the widespread application of these materials in these devices. Control over grain size is often essential to maintain good optical and electrical quality. In addition, the optimisation of the precursors and crystallisation scheme must be addressed in order to attain the texture and microstructure, which satisfy the requirements of the application. Thin film quality and reliability issues must be investigated in order to apply thin films deposited from chemical solutions in real products and production processes.

To guarantee sufficient device yields, the reduction of the defect density is also an important concern. Defects can take various forms: pores, pinholes, cracks, impurities, poor stoichiometry, excess charge in the film, poor transparency). Chemical purity and particle cleanliness during precursor preparation as well as a rational design of the CSD deposition system are essential for attaining low defect counts. While chemical solution deposition has proven to be most successful for thin films (30 300 nm) for many applications higher thickness (several microns) are often required. While high quality thick films can be achieved by multiple coating using conventional methods, a single step deposition route would enhance manufacturability and quality of thick films drastically. All of the application areas in this COST action could benefit from the development of a single step deposition route for thick films (> 1 (m) by reducing costs and increasing yields.

This Action aims to advance academic research beyond the present state-of-the-art frontiers. At the same time, it aims to push such frontiers in a direction that is useful to industry. To achieve such a dual goal it will be necessary to target the various medium- to long-term needs of European industries. The application projects will aim to provide European companies with an advantageous position in their industrial sector. These applications will demonstrate that the technology developed under the umbrella of this Action can be applied effectively to a very broad range of CSD thin film manufacturing and production processes. This long-term research effort will be determined with a pragmatic balance of specific projects with near term and futuristic goals. In this manner the research will establish new European technical leading edge technologies while providing the European industries with applications for current operating requirements.
A2: Synergy of new COST Action with existing European RTD programs.

The proposed COST Action is an ideal framework for implementation of European cooperation in the domain of solution chemistry thin film technologies since it requires a long-term multi-disciplinary and coherent effort whereby numerous processing parameters can be documented and information distributed. In the long term, development of improved CSD technologies will provide European companies with a competitive advantage in the markets such as ASICs, microsystems, and chips for next generation smart cards, where Europe has traditionally been a strong competitor for the US and Japan. The COST framework provides the best means for harmonising national research activities in this field in many countries including central and Eastern Europe, where significant basic research expertise already exists. By focusing on a five-year COST activity, timely completion of the work is expected. A group of interested parties has defined the core activities of the proposed COST Action as detailed in Section C.

The objective of COST is to foster links between partners of varying capabilities to improve the status of the field at a European level. Therefore this Action will act as a nursery for other groups and as a crucible for generating new ideas to CSD approaches and applications. This research is at a pre-competitive stage. Furthermore, the production equipment for the deposition of such advance material thin films is yet to be realised. The open environment of this COST Action will provide a forum where delegates may interact freely, cross-fertilise disciplines without "intellectual property" worries. This network will foster creative ideas which may develop into genuine proposals with defined aims and work programs specifically tailored for key applications relevant to EU industrial or social needs. The great advantage of the COST approach lies in a relatively rapid exchange of information between scientists and technical experts. The generic horizontal themes employed in this Action (outlined in Section C) will enhance the fundamental understanding of the thin films being studied and also their deposition processes. From this understanding optimum conditions for yields, equipment requirements, utility consumption, and so forth can be balanced for the best overall performance including economic factors.
1.Complementarity of this new COST action and the 5th Framework programme

This COST action can be considered complementary to the specific programme "competitive and sustainable growth" under the 5th Framework Programme that gives support to generic technologies across the application of materials and their production and transformation. Specific objectives of this programme are the development and processing of improved and new functional materials such as electronic materials and devices, sensors and actuators, opto-electronic materials or materials for displays. In this way this proposed COST will focus on new synthesis solution routes for the fabrication of smart and multifunctional thin films that can work integrated in micro-devices with the applications indicated above (micro-sensors, micro-actuators, memories etc.).

2.Complementarity of this proposed COST Action and COST525

The main objective of COST 525 (Advanced Electroceramics: Grain Boundary Engineering) is to develop and to apply numerical optimisation methodologies for automatic materials process design, based on quantified product quality, relating to process targets and constraints, including economic aspects. The proposed CSD Action, although complementary to COST 525 as it is also focused on thin films, covers a much broader range of advanced material thin films, the properties of the final devices and their application in the industrial sector. This COST action will promote cooperation between academia and industry, taking into account the needs of the micro-electronics, optoelectronics and micro-systems industries, and by strengthening the integration of European infrastructures in this domain and related sciences.

B.OBJECTIVES AND BENEFITS

B1:Scientific Objectives and Benefits.

This proposed COST action aims to improve the knowledge of tailored chemical solution deposited thin film, to improve their quality for specific application sectors (microelectronics, optoelectronics and microsystems), to improve compatibility of their processing with industrial production, and to improve environmental compatibility of these chemicals.

The main objective of the proposed Action is to improve the physical and electronic properties of thin films (< 20 ?m) made by chemical solution deposition techniques focusing on the sol-gel, liquid source CVD and spray pyrolysis methods to the quality required for the microelectronics, optoelectronics, and microsystems industries. To that end, knowledge of the precursor chemistry and processing, microstructure and nanostructure, and physics of the resulting thin films will need to be increased, and tailored to the requirements of these industries, especially for integration in larger process flows.

To achieve this goal it will be necessary to coordinate the basic and applied research efforts of European scientists and industries working in this field.

Secondary objectives and benefits are to:

- increase the tailorability of these thin films for industrial applications

- develop improved precursors for chemical solution deposition

- reduce defects (pores, pinholes, cracks, impurities, poor stoichiometry, excess charge in the film, poor transparency) in thin films deposited by chemical solution deposition techniques

- develop chemical solution deposition processes for large-area substrates

- reduce the thermal budget required for specific thin film chemical solution deposition processes

- produce electronic grade dielectrics by chemical solution deposition methods

- produce low loss waveguides for optoelectronic applications

- develop photo-assisted chemical solution deposition processes

- improve the environmental compatibility of chemical solution deposition processes to meet environment, safety, and health requirements

B2:Industrial & Social Objectives and Benefits.

The ultimate goal of this COST Action is to bridge the gap between the fundamental research of these thin films and their final technological application in microelectronics, optoelectronics and microsystems. The Action will enable the innovation of new generation products in these industrial sectors. The involvement of industrial participants is essential to pinpoint innovative applications of these thin films.

By bringing together the specialists in the chemical synthesis, processing and characterisation of advanced material thin films it will be possible to gain a significantly better understanding of these thin films and their manufacture hence reducing the cost of these CSD technologies and that of the final device. This COST Action will develop new miniaturised products and devices prepared by CSD processes that can be applied in the industrial sectors of environment, health, transportation and communication. Fabrication of thin films by CSD technologies will reduce the production cost of the final device as compared with sputter deposition methods or CVD, and improve its performance due to the excellent control of stoichiometry of the films obtained by CSD.

Furthermore, the development of equipment and processes for the production of advanced CSD thin films is at a relatively early stage of development. It is imperative that equipment manufacturers and research groups throughout wider Europe maintain close contact in order to develop this industry to its full and mutually beneficial extent. The interest shown by the Provisional List of COST participants shows the enthusiasm for research cooperation and coordination in this domain. The number of countries willing to participate in this proposed COST Action is at present 18. It is recognised by the technical experts in these countries, that an integrated approach towards CSD technologies is required for multisectoral applications in order to improve the industrial exploitation of these advanced material thin films in Europe. This Action aims to achieve this goal through tailoring of CSD films to the application, improving the material quality and dissemination of the scientific results to industry by integrating research groups across "wider" Europe.

This Action will provide a platform for:

- contribution to the coordination of scientific policies,

- working in a European team,

- development of synergy in advanced material thin film research activities,

- avoidance of duplication of work,

- more rapid dissemination of information through the establishment of a network,

- contribution to the elaboration of a bottom-up strategy for European Action in the area of fundamental research in CSD technologies,

- optimisation of intra European mobility schemes.

Mobility of researchers within Europe encourages successful inter- and multi-disciplinary collaborations, enhances training and facilitates the development of core expertise within the sciences. It helps bridge the frontiers of national practices and strengthens the cohesion of Europe. With the knowledge of the micro- and nano-structural thin film properties and the constraints around those properties, the production process can be fine tuned for enhanced product quality and consistence as well as reduction in quality variation to meet European Industrial expectations and requirements.

C.SCIENTIFIC PROGRAMME

This proposed COST Action will bring together European technical specialists and industrialists working on the common theme of advanced chemical solution deposition of thin films. It is designed to encourage technological exchanges and present real cooperation opportunities. The scientific programme can be structured in the following six working groups:

Note: The groups indicated in the arrowheads refer to the "Provisonal List of COST Participants".

The three horizontal working groups will address the generic themes of precursor chemistry and processing, micro- and nano-structure and finally the physics of the materials, characterisation of their properties and modelling. Interactions are expected between the working groups since specialists will be able to offer support with particular techniques to groups working with other materials and processes. Three vertical working groups will address emerging applications areas for these materials within 3 key industries.

These working groups will form the scientific framework of the Action. The structure of this Action is flexible enough to accommodate projects with either a material or an applications focus. Projects must address the objectives of the Action as outlined in section B. These may be expressed in the following priority themes:

- Research to improve the compatibility of chemical solution deposition with industrial production, addressing issues as large area substrates, and larger process flows. The aim will be to produce high quality films reducing defects (pores, pinholes, cracks, impurities, poor stoichiometry and poor transparency) and render them suitable for specific applications in microelectronics, optoelectronics, microsystems and nanotechnology. The thermal budget needed to produce these films is a key problem that will be investigated.

- The chemistry of the solutions will be studied in order to develop improved precursors, tailor the films and their microstructure. The environmental compatibility of chemicals and processes will be examined.

- The physics and engineering of the devices must be taken into consideration e.g. in the production of low loss waveguides and electronic grade dielectrics.

The horizontal themes are suitably chosen as there are obvious and strong relations between (a) precursor chemistry and processing; (b) micro and nano-structure; and (c) physics/metrology and modelling. To a large extent, processing is responsible for the structures of the films, and these micro and nano-structures have strong effects on their physical properties and characteristics and on the performance of devices in which they are incorporated.

Precursor chemistry and processing:

The focus of this theme is on fundamental development of future generation precursors for advanced solution deposition processing i.e. precursor design for improved material processing. It targets the discovery and exploitation of basic research to enhance expansion into technology-driven areas. Fabrication of complex structures from simple molecular species is the cornerstone of much modern materials synthesis. Applications in high and medium technology rely on synthetic chemistry during one or more steps in the manufacturing process. Molecular precursors can be broadly defined as molecules whose reaction or decomposition, in isolation or in the presence of other compounds, leads to the formation of a solid state material. The use of metal alkoxide precursors for the thin film growth of semiconductor (and other) materials by chemical solution deposition and related techniques requires precursors which must meet a series of chemical and material processing requirements in addition to economic considerations. Synthetic chemistry has made significant advances in all of these areas, for example, improved precursor purity and lower toxicity precursors. However, the ability to control structure, composition, and spatial arrangement are important advantages to materials growth attained by the application of precursor chemistry and will be investigated within the course of this Action.

The main role of the precursor preparation in CSD methods is to ensure a statistical distribution of the different elements over the precursor backbone. Due to the intimate mixing of the constituent oxides after decomposition of the precursor molecules, the CSD technique can yield high quality crystalline films even at relatively low process temperatures. The chemical structure of the precursor solution and its thermal decomposition behaviour can have a large influence on the microstructure of the resulting film and hence it physical properties. Changes in precursor chemistry (molecular structure, cross-linking, ligands) can have a drastic influence on film structure and modifications to the precursor preparation method are often key to improving film properties.
Therefore groups operating within this theme during the Action will synthesise as follows:

(a)precursors (alkoxides) for the deposition of thin films formed by wide variety of metallic and half-metallic oxides,

(b)the organic groups of the precursor molecules strongly influences reactivity as well as gel structure, film structure and physical properties. These organic groups can be modified by synthetic methods. The solution properties relevant for film deposition will be modified (chemical stability, kinematic properties, possible additives and modifiers,

(c)large area deposition (100-150 cm2) will be performed as well,

(d)thermal processing will be optimised,

(e)in addition to films formed by one phase, composite and multilayer films will be prepared. Such films are expected to display interesting physical properties with possible novel applications.

Microstructure and Nanostructure:

As for most materials of technological interest, understanding the microstructure and nanostructure of these thin films is key to understanding their physical and chemical properties, predicting performance, and developing methods to engineer thin films with desired and controllable properties. In order for transfer of the developed technology into industrial applications with a view to the development of industrial scale synthesis, characterisation of the microstructure/nanostructure of the thin films is essential. The objective of this research will be to develop a more comprehensive understanding of the influence of microstructure and nanostructure on the physical, chemical and mechanical properties of these thin films. Understanding a material begins with comprehending its nanostructure (0.1 - 1.0 nm scale) and this activity will be an important factor in this theme. The nanoscale structure will be probed using mass atomic spectroscopy (MAS), NMR, Ca X-ray absorption (XAS), micro-Raman, and infrared (IR) spectroscopies. In addition, small angle scattering techniques and electron microscopy studies will complement spectroscopic data. The properties of electronic ceramics, as of most materials, are to a large degree determined by their defect content. Some examples are: crystal defects such as oxygen vacancies, which are thought to be responsible for poor endurance of PZT capacitors, grain boundaries which lead to increase scattering in waveguide material and second phases lead to resistance degradation and early breakdown. Because in thin film growth the structure and defect content is often dependent on the properties of the growth surface, one approach to influence microstructure development is by introducing seed layers which promote nucleation.

Advances in understanding thin film microstructures and microstructure-property relationships require this integrated research program involving the use of a wide range of modern analytical tools capable of probing thin films. Methods will include but will not be limited to high resolution and analytical TEM, STM/AFM, EPMA, XRD, SEM and other spatially resolved imaging and elemental mapping techniques.

Physics/Metrology and Modelling:

Micro- and nano-structures strongly influence physical properties. More precisely, stresses and strains can give rise to dislocations, defects and inhomogeneities which affect, to a large extent, dielectric properties (for capacitors in microelectronic applications for instance), optical transmission (for optoelectronic applications), and mechanical specifications (for Microsystems applications). It is important to understand the origin of these stresses and strains (i.e. in which step of the process they appear), and to derive some ideas about how to monitor them. To this purpose, it is necessary to measure them (by X-rays or spectroscopies such as infrared, Raman, Brillouin, EPR, etc.), and to build models which could predict their impact on physical properties. Spectroscopic techniques give detailed information on excitations and local fields in the materials studied. These techniques are very sensitive to all of the aforementioned imperfections. Comparison with bulk materials and epitaxial thin films (formed by e.g. laser ablation) will allow us to sensitively position the film quality between these two structural limits.

Microelectronics:

New areas of research in the topic of chemical solution deposition have been opened up by recent studies of thin films. For example, BST is being considered as the dielectric in high density DRAMs and a new method liquid source chemical vapour deposition (LSCVD) has been developed to allow large area growth on Si wafers. However, the integration of novel materials into microelectronic devices has come up against many difficulties which must be addressed with a multi-disciplinary approach. The problems of large area deposition and compatibility with microelectronic processes, maximum crystallisation temperatures and thermal budget and thick film deposition will require imaginative solutions from chemists, equipment manufacturers and material scientists.

Except for the use of BST in DRAM, integration of complex oxide thin films is still in a research or early development stage. Indeed, integration of these films with silicon circuitry requires not only growth of a film with the desired characteristics, but also film patterning, as well as ensuring that the film retains the target properties during the remainder of the process sequence. Because many of the materials under consideration are known to act as contaminants for silicon devices, diffusion barriers and special cleaning steps must be developed to prevent diffusion of heavy metals to regions where they can interfere with device performance. In addition, the development of CVD-type deposition techniques is essential in advanced applications which require conformal deposition over large areas.

Optoelectronics:

Silicon-based optoelectronics is a diversified technology that has grown steadily over the past decade with thin film integrated optics also becoming more and more important in optical-communications technology. At present, optoelectronics suffers from limited capabilities in on-chip integration, whilst the high cost of hybrid packaging limits the scope of applications. Consequently, finding cost effective hybrid opto-solutions remains a formidable challenge. Recently, new demonstrations on sol-gel glass lasers, the densification of sol-gel glass by laser irradiation and the ability to incorporate optically active material has opened up new applications for CSD technology in optics, non-linear optics and optoelectronics. Chemical solution deposition methods are also promising candidates for ferroelectric thin film deposition method for electronic and elecro-optic applications. Ferroelectric films have strong potential for application in optical waveguide devices, with large electro-optic effects observed in many transparent ferroelectric materials. The key challenge is to grow high quality films of the required thickness with low densities of grain boundaries, which lead to scattering losses. In addition, integration of non-standard materials with microelectronic circuits will necessitate special precautions to prevent cross-contamination as discussed above.

One interesting example of an optoelectronics/ferroelectric cross cutting project would be the fabrication of sol-gel processed ferroelectric PLZT (lanthanum doped lead zirconate titanate [(Pb, La)(Zr, Ti)O3]) for optical waveguides. Much more work is required in this area to increase film thickness, to maintain crystal structure, and to develop an active electro-optic device. This COST Action would be an ideal framework in which to implement European cooperation in the cross-cutting domain of optoelectronics with silica, ferroelectric and metal oxide based systems.

Microsystems:

Microsystems are intelligent miniaturised systems comprising sensing, processing and/or actuating functions. Miniaturised, integrated sensors and actuators are a rapidly growing field with great future potential. Nowadays, most integrated microsystems make use of relatively standard microelectronic materials, such as poly SiGe for bolometers and piezoresistive polysilicon for accelerometers. Replacement by CSD thin films with piezo- and pyroelectric properties could lead to enhanced sensitivity and functionality because of their higher figures of merit and the possibility for actuation in MEMS devices. For integration with CMOS, sensor and actuator elements are commonly placed on top of the metallisation, which relaxes concerns of cross contamination in back-end processing, but which puts severe constraints on the thermal budget for growth of the thin film. Novel chemical solution deposition (CSD) and chemical vapor deposition (CVD) routes are expected to play a key role in the development of the required low temperature deposition routes. Magneto-optical and electro-optical effects in materials provide for a large variety of device applications. Realization of the potential of these materials has thus far been impeded by inadequate control of crystalline and chemical perfection during their production. Chemical solution deposition technologies provide methods of compositional and microstructural control for optimising strain energy density for applications in active structures.

Within these themes, researchers from a wide range of scientific disciplines will work with different industrial partners. The innovative application of chemical solution deposition techniques will be promoted by means of an annual research-industry group workshop. It is envisaged that this workshop will modify the priority themes for research for the following year.

D.ORGANISATION MANAGEMENT AND RESPONSIBILITIES

This Action will operate for five years. The Management Committee (MC) of this Action will be organised and operated according to COST/400/94 "Rules and Procedures".

The work in the present proposal will, in principle, be divided among 6 Working Groups. The flow of information between working groups will be stimulated by the interactive nature of the membership of the horizontal and vertical working groups. It is expected that each Working Group will elect a Chairperson to coordinate the work within the group and represent it within the MC.

The MC will invite leading academic and industrial colleagues in several subjects to give plenary talks during the workshop meetings. At the same time attendance of these workshops will enable a participant to exchange ideas with other industrial representatives and to sense the technological prospective of the COST Action members in the areas of interest. This is an effective and economical approach in maintaining a current understanding of the status of industrial and academic efforts in this field. A mid term review is planned at year 2.5. At this review there will be a formal evaluation of the project and assessment of the most appropriate direction for the COST Action.

Delegates representing Signatories will:

- attend and contribute to meetings of the MC (two meetings per year are expected),

- be involved in an active programme in line with the objectives and timescale of the project,

- take responsibility for specific items of the Project,

- set up national working groups for specific items and be responsible for liasing between the MC and national research groups in the participating countries.

Each partner will encourage a wide participation of their respective national groups and of different industrial activities

Dissemination of results.

- The main route to the dissemination of research will be via articles published in peer reviewed journals and oral presentations at key related scientific conferences which this Action will encourage. All publications arising from research carried out under this COST Action will credit COST support, and the MC will encourage and promote co-authored papers.

- Another important route to dissemination will be the research-industry group workshops aimed at discussing progress to date and to bring together the key research technologists and interested industrial parties. All working scientists will be invited to present their findings at these workshops and also in the annual report of the action.



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