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6.1. Thin Film Science

6.1.1. Introduction

In recent years, thin film science has grown world-wide intoa major research area. The importance of coatings and thesynthesis of new materials for industry have resulted in atremendous increase of innovative thin film processing tech-nologies. Currently, this development goes hand-in-handwith the explosion of scientific and technological break-throughs in microelectronics, optics and nanotechnology[1]. A second major field comprises process technologies forfilms with thicknesses ranging from one to several microns.These films are essential for a multitude of production are-as, such as thermal barrier coatings and wear protections,enhancing service life of tools and to protect materialsagainst thermal and atmospheric influences [2, 3]. Pres-ently, rapidly changing needs for thin film materials anddevices are creating new opportunities for the developmentof new processes, materials and technologies (Fig. 6.1).

Therefore, basic research activities will be necessary inthe future, to increase knowledge, understanding, and todevelop predictive capabilities for relating fundamentalphysical and chemical properties to the microstructure

and performance of thin films in various applications. Inbasic research, special model systems are needed for quan-titative investigations of the relevant and fundamental pro-cesses in thin film materials science. In particular, thesemodel systems enable the investigation of i.e. nucleationand growth processes, solid state reactions, the thermaland mechanical stability of thin film systems and phaseboundaries. Results of combined experimental and theo-retical investigations are a prerequisite for the develop-ment of new thin film systems and the tailoring of theirmicrostructure and performance.

6.1.2. State of the Art

The major exploitation of thin film science is still in thefield of microelectronics. However, there are growing ap-plications in other areas like thin films for optical and mag-netic devices, electrochemistry, protective and decorativecoatings and catalysis. Most features of these thin film activities are represented by a relatively new research area,called surface engineering [2]. Surface engineering has beenone of the most expanding scientific areas in the last 10years and includes the design and processing of surfacelayers and coatings, internal interfaces and their charac-terization. Surface engineering is directed by the demandsof thin film and surface characteristics of materials.

(a) Thin film processing techniquesThere exists a huge variety of thin film deposition process-es and technologies which originate from purely physicalor purely chemical processes. The more important thinfilm processes are based on liquid phase chemical tech-niques, gas phase chemical processes, glow discharge pro-cesses and evaporation methods [4]. Recently, a consider-able number of novel processes that utilize a combinationof different processes have been developed. This combi-nation allows a more defined control and tailoring of themicrostructure and properties of thin films. Typical pro-cesses are e.g. ion beam assisted deposition (IBAD) andplasma enhanced CVD (PECVD). Examples for novel thinfilm processing techniques, which are still under develop-ment, are pulsed laser ablation (PLD) and chemical solu-tion deposition (CSD). Both techniques enable the syn-thesis of complex thin film materials (complex oxides, car-bides, and nitrides).

T. Wagner | Max-Planck-Institut für Metallforschung, 70174 Stuttgart, Germany

Fig. 6.1. New processing techniques related to thin film materials and devices.

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Presently, experimental efforts are increasingly supportedby computational approaches that address complex growthprocesses, saving time and money. These approaches enable e.g. the description of the evolution of thin film microstructures as a function of processing parameters.

(b) In situ characterizationThe thin film process equipment can be categorized intoproduction equipment for device manufacturing, equip-ment for research and development, and prototype appa-ratus for fundamental investigations of new or establisheddeposition processes. One reason for the world-wide rapidgrowth of deposition technology is that equipment manu-facturers have successfully met the demands for more sophisticated deposition systems including in situ charac-terization (e.g. reflection high-energy electron diffraction(RHEED), scanning probe microscopy (SPM)) and pro-cess monitoring techniques for measuring process param-eters and film properties (e. g. ellipsometry, plasma analy-sis techniques). Novel experimental tools have enableddiscoveries of a variety of new phenomena at the nano-scale which have in turn opened unexpected opportunitiesfor the development of thin film systems, and tremendousprogress regarding a fundamental understanding of the respective technological processes has been made.

(c) New materialsThin film systems necessitate direct control of materialson the molecular and atomic scale, including surface mod-ifications, deposition and structuring. Many of these tech-niques were improved during the last decade, resulting inremarkable advances in the fundamental understandingof the physics and chemistry of thin films, their micro-structural evolution and their properties. This progress hasled to the development of new materials, expanded appli-cations and new designs of devices and functional thin filmsystems. One of the most outstanding examples is the suc-cessful development of semiconductor devices with novelmaterials like oxides and nitrides (e.g. GaN). Other typicalexamples are advances in the synthesis of hard coatingsbased on borides, carbides and nitrides [3].

6.1.3. Expectations 2000 –2010

The gap between solving fundamental materials problemsand developing new thin film devices for microelectronicand nanotechnological applications is quickly increasing.For example, in many applications the development of thinfilm systems is accompanied by a variety of materials andprocessing problems, which require extensive future efforts to be solved. Prominent examples are the adhesion

and the thermal and environmental stability of thin filmsystems. Future developments are critical to overcomingobstacles to miniaturization as feature sizes in devicesreach the nanoscale. Basic research in this field will referto developments of experimental tools necessary to in situcharacterize and measure thin film structures (e.g. opticaland magnetic characterization), and developments of novel techniques for synthesis and design. These tech-niques may be more reliable, less expensive, or capable ofproducing films with new or improved properties. Typicalexamples are chemical solution deposition (CSD), includ-ing hydrothermal approaches, biomimetic pathways for assembling inorganic thin films, or device applications ofliquid crystalline polymer films [5].

Experiments alone will be insufficient. Theory and model-ling are essential for a complete understanding of the fun-damental growth and deposition processes. Multiscalemodelling of thin film and nanostructuring processes willbe an absolute necessity in the next decade in order to utilize the tremendous potential of thin film science andtechnology. It is expected that time consuming and expen-sive experiments will be replaced by theory and modelling.

Especially, it is still necessary to develop a fundamentalunderstanding of the decisive growth and deposition pro-cesses. It will be important for research institutes to focuson the development of fundamental and novel processesand devices. This will only be realized if a more definedconnection of the activities between single researchgroups and industry can be achieved, based on a fluentexchange of information. Research institutes and compa-nies, which cannot achieve this, will have difficulty com-peting in future. In this field, Europe must compete di-rectly with the U.S. and Japan. In comparison to Europe,there appears to exist an advantageous research environ-ment in the U.S. and Japan, which supports a more fluentconversion of results from basic research into applica-tions. This could be balanced in Europe by improved networking between industry and research laboratories inthe field of basic research.

6.1.4. Expected Needs 2000 –2010

(a) InitiativesEmphasis should be placed on future developments of filmdeposition processes for application in advanced micro-electronic device and nanotechnology applications thatrequire the most demanding approaches. Surface engi-neering is a second important field where similar demandshave to be fulfilled. Effective development of thin film

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systems can only be realized if the fundamental processsteps are well understood. For the future advancement ofthin film research and technology, the following endeavorsare prerequisite:

• Development of improved and novel thin film process technologies and design methodologies

• Development of new materials • More fundamental understanding of the relationships

between microstructure and properties, and how these can be tailored

• Improvement and automation of in situ characterization tools with high spatial resolution and chemical sensitivity

• Strong networking between research laboratories and industry in Europe, and materials research centres in Europe, U.S. and Japan

• Establishment of special competence centres

In the following, some examples are given where new pro-cesses and technologies will challenge established proce-dures:

(b) Thin film model systemsDeposition processes for applications in advanced micro-electronics and surface engineering processes will requirethe most demanding approaches in the near future. Newconcepts and design methodologies are needed to createand synthesize new thin film devices and to integrate theminto architectures for various operations. Examples are thecontrol of surface processes, the development of comput-

er memory chips, and the production of two- and three-di-mensional nanostructures. There will be a rapid increasein the significance of basic research projects due to the needof new materials and devices in these fields. Therefore,purposeful future developments and an understanding ofthe versatility of the basic deposition processes is needed,including microstructural evolution of thin films, e.g. sub-strate and surface preparation, nucleation and growth. Forthe realization of thin film systems, it is of future interestto understand the critical role of surfaces and interfaces.We need to know the structure and bonding at heterophaseinterfaces and grain boundaries, and we have to under-stand how to produce special interfaces experimentallyand how to tailor them for specific properties. Additional-ly, the thermal, chemical and structural stability of the thinfilm systems and devices need to be investigated thorough-ly to allow for the adaptation of fundamental processes. Toinitiate all these processes, special model systems have tobe synthesized and studied experimentally and theoreti-cally (Fig. 6.2).

Typical examples are:

• metal and ceramic multilayers• functionally graded thin films• growth of metastable phase layers • super hard thin films• nanocrystalline layers• superlattice thin films• composite coatings

Fig. 6.2. Research areas for thin film systems.

Thin Film

Atomically abrupt interfaces

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yesno Chemical reactions• material properties• growth parameters

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Significant improvement in both, theory and modellingare necessary to help direct the developments in thin filmscience and technology. This is one of the most promisingattempts for accelerating future developments.

(c) The development of new materials: a combinatorial approachAn accommodation of the equipment to the research anddevelopment of new device structures and materials withnew properties must be performed continuously. For ex-ample, research and development equipment has to offera high degree of flexibility in the accommodation of a mul-titude of substrates, in deposition parameters, and in realtime monitoring capability. Until recently, these facilitiesdid not facilitate a high product throughput, which madethe development of new thin film materials rather tedious.By using a combinatorial approach, parallel thin film pro-cessing can be realized, and by significantly reducing sam-ple size, time and money can be saved (Fig. 6.3).

This requires the development of new and complex depo-sition systems. For example, this can be realized by usingmasks with different patterns in front of the substrate.Then, single or/and multi-beam deposition will result inparallel thin film processing. As a result, the substrate willconsist of thin film patches with varying chemical compo-sition. However, miniaturization complicates the process-ing procedures, and thus an improvement in the resolu-tion of typical thin film characterization tools will be ofgreat importance. Full automation of the deposition andcharacterization routines is required here (e.g. automaticshutter and temperature control, scanning beam RHEEDsystems). A further advantage of the combinatorial ap-proach is the fast characterization of the deposition pro-

cess and the system itself. Worldwide, the developmentsof these processes are still in the initial stages. Presently inEurope, projects of combinatorial chemistry are support-ed in the field of chemical technologies. These researchprogrammes should be extended to all fields of new tech-nologies, which depend on thin film science, and technol-ogy, and it is important to realize these projects in Europethrough networks which consist of research centres andindustry.

(d) Thin film systems via chemical solution deposition(CSD) Conventional CVD and PVD processes are usedroutinely to synthesize thin film systems. Such processtechnologies are rather complex and expensive [4]. De-pending on the applications, these films have to fulfil special demands and very often, low defect densities arenecessary. For most applications, one would prefer filmswith which have a special texture, low grain boundary den-sity, and smooth surfaces. Epitaxial films fulfil these re-quirements. Very promising approaches to synthesis inor-ganic single crystalline thin films are chemical solutiondeposition method (CSD) (Fig. 6.4) and related processes.

CSD enables the growth of thin films fom solutions, eitheraqueous or organic [6]. These solutions contain precursormolecules for a variety of elements in the thin film of inter-est. CSD is inexpensive and enables the synthesis of thinfilm materials with complex chemical compositions. Themain advantage of CSD is the high degree of composition-al control, inherent with other solution synthesis routesfor multi-element, inorganic materials. Recently, singlecrystalline carbide and nitride thin films were synthesizedvia CSD. CSD will become one of the key technologies tosynthesize epitaxial oxide, nitride and carbide films for a

Fig. 6.3. A combinatorial approach for parallel thin film processing.

Fig. 6.4. The chemical solution deposition method for the production ofepitaxial inorganic thin films.

GrowthChamber

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Spin Coating

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Thin Film Deposition: A Combinatorial Approach

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Thin Films1. deposition step 2. deposition step 3. deposition step n. deposition step

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variety of different applications, e.g. opto-electronic de-vice applications, hard coatings and dielectric thin films.A further advantage of CSD will be the possibility of direct patterning of thin films via stampingtechniques. However, for these purposes, appropriate precursor solutions have to be synthesized in future. Onebig challenge is the synthesis of precursor materials withwell-defined doping levels for thin film applications inelectronic devices.

(e) Manpower, interdisciplinary researchContinued progress in thin film research seems to be in-creasingly dependent upon collaborative efforts amongseveral different disciplines, as well as closer coordinationamong funding agencies and effective partnerships includ-ing research laboratories, universities, and industry. Re-cently, a variety of different interdisciplinary programmesfor nanotechnology and microelectronics were initiated inthe U.S., Japan and Europe. However, the topic of thinfilm science and technology is automatically included insuch programmes, although it would be of great benefit toestablish individual programmes, which focus on majorthin film activities in Europe. Interdisciplinary programmesof combinatorial materials science and technology of thinfilms and surface engineering are necessary, to promote theformation of fundamental associations for the improve-ment and development of new techniques, and of novelmaterials and their properties.

The university education of researchers working in the thinfilm area would benefit from an interdisciplinary approach.The materials problems in thin films research are very com-plex and require interdisciplinary training, that is, a com-bination of studies including physics, chemistry, engineer-ing, materials science and biology. In comparison to Eu-rope and Japan, the U.S. seems to occupy a leading positionin this field. In the future, efforts should be taken to com-pensate for this deficit in Europe by modifying the educa-tion system in such a way that new training directions areincluded, helping to create a new generation of research-ers who work across traditional disciplines. This task maybe supported by new centres of excellence in the field ofthin film science and technology.

6.1.5. Expenditures

Thin film process techniques and research are strongly related to the basic research activities in nanotechnology.Thus, government expenditures on nanotechnology research are a reliable indicator of the budget for thin filmresearch. The government expenditures for nanotechnol-

ogy research are presently at similar levels in the U.S., Japan and Western Europe, suggesting that the respectivequantities of research activity are comparable. Accordingto a WTEC study in 1997 [1], large companies in Japanand the U.S. contribute to research to a greater extend thando their counterparts in Europe. While large multination-al companies are developing nanotechnology research activities worldwide, the transfer of new processes to themarket is strongest in the U.S. In Western Europe, a diverse combination of university research, networks andnational laboratories with a special emphasis on coatingsis active. The largest funding opportunities for nanotech-nology are provided by the NSF in the U.S., by MITI in Japan and by BMBF in Germany. Categories such as phys-ical and chemical technologies, materials research, micro-system technologies, electronics and nanotechnologiesare supported by BMBF. Common features of these disci-plines are projects which are located in the area of thinfilm research. In the case of nanotechnology, the BMBFhas established several centres of competence with em-phasis on such topics as opto-electronics, nano-analytics,and ultra-thin films. Ultra-thin films are among the mainelements of thin film science that have varied applicationsin optics, microelectronics and wear protection.

WTEC Panel Report on R & D Status and Trends in Nanoparticles, Nano-structured Materials, and Nanodevices, R. W. Siegel, E. H. Hu, M. C. Roco,Workshop 1997 (http://itri.loyola.edu/nano/us_r_n_d/toc.htm)Surface Engineering, Science and Technology I, Editors : A. Kumar, Y.-W.Chung, J. J. Moore, J. E. Smugeresky, The Minerals, Metals &MaterialsSociety, Warrendale, 1999.Hard Coatings, Edited by A. Kumar, Y.-W. Chung, R. W. J. Chia, The Min-erals, Metals & Materials Society, Warrendale, 1998.Handbook of Thin Film Process Technology, Editors: D. A. Glocker and S.I. Shah, Institute of Physics Publishing, Bristol and Philadelphia, 1998.Thin Films, Science 273 (1996).F. F. Lange, Science 273 (1996) 903.

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References