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Plasma polymers and related materials

Objective


A.BACKGROUND

1.Introduction

It has been known for many years that surfaces exposed to an electrical discharge operated in a gaseous atmosphere containing organic vapours or gases are covered with insulating deposits. The first reports of these deposits were published in the second half of the last century. K(nig and co-workers in 1952 prepared so called carbonaceous films in a glow discharge in benzene vapours using a parallel plate electrode arrangement powered by singly rectified 50 Hz voltage. According to the IR absorption spectra they fabricated not only a hydrocarbon plasma polymer, but also a hard polymer (C:H) and a material approaching an amorphous hydrogenated carbon (a-C:H). Shortly after this the first C:H films were prepared by Schmellenmeier.

Plasma polymerised styrene was announced in real application as dielectric separating film in 1960. During the next thirty years plasma polymerisation has been recognised as an established and rapidly developing field with the basics laid down by M. Schen, A.T. Bell, H. Yasuda, R. d'Agostino and others. A number of applications have been proposed.

Plasma deposited coatings for microelectronics technology, especially silicon nitride and silicon oxide films or mixtures of these film types will also be treated in this context although they are sometimes considered as inorganic films. Nevertheless, organic precursors such as HMDSN, HMDSO and other siloxanes and silazanes are frequently used for their fabrication. Their structures are not fully clarified yet and they certainly always contain polymeric components with a ratio depending on the deposition conditions. In analogy to the hydro- and halo- carbon polymers and hard hydrogenated carbon films or compounds of hard hydrogenated carbon films, very similar questions have to be answered covering the range from silicon polymers to amorphous hydrogenated silicon and silicon compounds. Furthermore, their characteristics, structure and formation kinetics are investigated by common means.

The next topic related to these films is a formation of the so called "blocking" films which are obviously of polymeric nature and play a key role in anisotropic plasma etching in semiconductor processing and are worth investigating within this framework.

The last mentioned two topics from the microelectronics industry underline once more, besides the scientific contents, the important economical aspects of the proposed action.

2.Plasma polymerisation processes

Plasma polymerisation process is usually carried out in one of the two types of reactors: Bell-jar type reactor with internal parallel plate metal electrodes and (b) Tubular-type reactor with external ring electrodes or an external coil for the r.f. discharge excitation. In the case of bell jar type reactor an a.c. voltage (up to 100 kHz) is usually used. Also r.f. (0.5 MHz - 30 MHz) voltage for the discharge excitation is applied. In this case, one of the electrodes is usually grounded as well as the metal components of the reactor (asymmetrical arrangement). Microwave discharges became very popular with various ways of microwave energy applications - usually multimode cavity ("microwave oven") mode is used. Magnetic field has also been employed to assist plasma polymerisation process - ECR discharges and planar or cylindrical magnetrons.

The deposition rate of a plasma polymer is determined by the following parameters: the geometry of the system, the reactivity of the starting monomer, its flow rate, the working gas pressure, the power, and frequency of the excitation, signal, and finally, by the temperature of the substrate.

Structure of plasma polymers has been examined by IR spectroscopy, NMR, ESCA and AES. Several investigators postulated a possible network of plasma polymers e.g. plasma polymerised ethylene using IR absorption spectra and computer analysis. The other proposed networks were for fluorocarbon and silicon containing plasma polymers. It seems to be clear that a network of a plasma polymer has no repeating units but contains a number of various groups. The chains are short and branched and randomly terminated with frequent crosslinks. Such structures may contain a large concentration of trapped free radicals. This often leads to the ageing process when the plasma polymer is exposed to the open atmosphere. If during plasma polymer deposition an energetic (a few eV) ion bombardment takes place the plasma polymer network becomes even more disordered, crosslinked but it is more rigid. Such plasma polymers e.g. hydrocarbon plasma polymers are sometimes called C:H and fluorocarbon ones C:F. When average energy of ions bombarding the growing plasma polymer film increases to several 10th eV or even up to 100 eV so called amorphous hydrogenated carbon films a-C:H and their analogues such as a-C:F or a-Si-C:H and etc. are created. The ageing processes are decreased for these films, however, high internal stresses and sometimes low adhesion to certain substrates is a nuisance. The ageing processes were changed by various e.g. post deposition treatments. However, a more sophisticated and up-dated solution is needed in order to meet demands of present and future technologies based on plasma polymerisation processes. Various types of simultaneous or fast post deposition irradiations seem to be candidates for solving especially adhesion problems and even ageing effects. Especially energetic ion and electron beams and X-ray or UV light come into consideration.

Better understanding of deposition process in relation to the plasma polymer film structure, its properties and ageing effects and its possible elimination will become the main objectives of this Action.

3.Recent development

By changing the energy flux into the growing plasma polymer film, the properties of the film can be substantially varied. The increase of energy causes the film to become more disordered and crosslinked with the excess of carbon. It is also usually harder. On the contrary, if the energy flux is low, a plasma polymer is obtained that retains more molecular structure of the monomer. Therefore plasma polymers resembling conventional polymers may be obtained. Many researchers have tried to extend in this way properties of plasma polymers and also their applications.

The two main directions: (a) new materials including hard coatings and composite films, and (b) plasma polymers resembling conventional polymers were followed. The application of the former is mainly in hard and protective coatings while the latter is in membranes and especially in biomedical field. The use of plasma polymers in sensors for measurements and control of number of effects and especially monitoring of environment is of chief importance.

Additional group of plasma polymers as new materials are metal doped plasma polymers (composite metal/plasma polymers) or even more complex composites such as metal oxide/plasma polymer or ceramics/plasma polymer. These composites present a new class of composite and nanocomposite films with an enormously expanded range of electrical, optical and mechanical properties.

Recently, some attention has also been paid to a new class of thin films of plasma polymers deposited from organic compounds of the carbon family. They reveal especially interesting electronic properties characterised by a very rapid change, from those typical for insulators to those attributed to semiconductors (e.g. the electrical conductivity increases by over a dozen orders of magnitude), when the deposition conditions are changed only very slightly. This effect is called the amorphous insulator - amorphous semiconductor transition. Apart from the understanding of its nature that is a subject of present intensive investigations, this effect opens new possibilities in thin film technology consisting in preparation of novel class of layer structures formed from twin films of amorphous semiconductor and insulator.

In spite of a lot of work done in plasma polymer film deposition during last decades, often serious drawbacks like limited (thermal, mechanical and in some cases optical and electrical) stability or (chemical or physical) non-uniformity prevent a broader application.

New procedures for the improvement of film stability and uniformity are needed. Several candidates that may also remarkably improve the adhesion are simultaneous or short time post-deposition irradiation by energetic ion or electron beams or UV light or X-rays.

Scale-up procedure of promising technology process for large area and especially large volume 3D objects is sometimes a problem because of deposition of plasma polymers on reactor chamber walls and other components. This effect requires their cleaning and means that a way of environmentally safe disposal of the waste must be found. It should be stressed here that the amount of waste is several order of magnitude lower and less dangerous than conventional chemical wet processes used for the same purpose.

It is therefore worth putting efforts into investigations for improvements of plasma polymerisation processes that are sure candidates for environment-friendly technologies of the next century. Using latest developments in physics and chemistry (diagnostics, plasma polymer characterisation, and etc.) and bringing together the knowledge of the participants of the Action from different European countries means that new progress will result in both the fundamental and applied area of plasma polymers.

However, better understanding of the plasma polymerisation process especially how deposition (mainly plasma) parameters influence the resulting properties of plasma polymers and their time and temperature stability (ageing) is needed. For the studies of the plasma, internal parameters, Langmuir probes, integral and/or tomographic optical emission spectroscopy, ion energy analysis, direct ion extraction analysis, mass spectroscopy, down stream mass spectroscopy, laser induced fluorescence (LIF) and in-situ dynamic ellipsometry are employed or applicable. This variety of analytical possibilities combined with the numerous interdependence of the external deposition variables as mentioned before underline once again that better cooperation, rapid exchange of ideas among the European scientists working in this area would be especially beneficial.

B.OBJECTIVES AND BENEFITS

The main objective of the Action is to improve the knowledge of the plasma polymerisation process in relation to the desired physical and chemical properties of resulting plasma polymers with special regard to the understanding and following suppression of ageing processes of plasma polymers at ambient and extreme conditions.

As a consequence, a dramatic expansion of the applications is expected when a solid especially a conventional polymer (plastic) with a given bulk properties is coated by a thin and durable plasma polymer film of tailored properties that are desirable for a specific function of the whole product as a system comprising of a solid and surface plasma polymer coating. In addition to modification or protective function of a plasma polymer coating the new plasma polymer films are top candidates for unusual sensors or even active elements in microelectronics. Plasma polymers are expected to be more widely used as optical coatings and other applications.

Technologies based on plasma polymer process are much more environment-friendly than currently used technologies usually based on wet chemistry for delivery of the same function product.

The following concrete objectives will be followed:

1)better understanding of plasmo-chemical reactions , based on determined deposition and especially plasma parameters, leading to plasma polymers of specified properties - for example:

-plasma diagnostics in relation to resulting film properties in order to create a system of complex in-situ deposition diagnostics for plasma polymerisation processes,

-coupling of reactive plasma generation/transport equations of precursors to desorption/adsorption and chemical reactions at the solid surface (substrate),

-predictive modelling of plasma processes that will improve reactor design and overall modelling of the plasma polymerisation process,

-thorough investigation of mechanical properties of plasma polymers including adhesion and internal stresses related to the structure and the influence of the nature and properties of the substrate,

-investigation of ion beams and other irradiation for changing the structure and overall physical and chemical properties,

-investigation of ion beams and other irradiations for the improvement of adhesion and long term stability of the systems plasma polymer/solid substrate (namely conventional polymer);

2)modification of surface properties of solids, especially conventional polymers, by depositing a plasma polymer film for the improvement of the following functions:

-biocompatibility,

-enzyme anchoring for biochemical reactions,

-membranes and sensors,

-modification of polymer foils, fabrics and glasses in order to change wettability,

-hard and non-abrasive coatings on various solids,

-hard, non abrasive coatings with low oxygen and water vapour permeability on polymer foils for packaging,

-plasma polymer/metal or semiconductor composites for new sensors in microelectronics,

-optical coatings,

-new generation of plasma polymers with unusual electrical properties;

3)A search for plasma polymerisation processes will be carried out which will result in:

-very high deposition rate (1 micron/min and more) while desired properties of plasma polymer are pertained,

-uniform plasma polymer deposition as regard the thickness and property on 3-D substrates (special reactor designs),

-environment-friendly technologies with extremely low waste (especially
solid plasma polymer and gases) that will replace currently used wet-chemistry-rich-waste-technologies. It should be noted here that the waste from plasma polymer process is several orders of magnitude lower than the waste from the conventional chemistry based process used for the same function product,

-low unwanted deposition on walls and components of the reactor that decrease the cleaning cycles,

-formulation of recommendations for reactor cleaning and disposal of vast plasma polymers from reactor walls and components.

Ageing processes when plasma polymers are exposed to ambient atmosphere after deposition take place more or less in dependence on the particular process. European cooperation in all the above items and in the investigation of ageing will be very beneficial for even wider use of plasma polymers especially in microelectronics and optics.

C.SCIENTIFIC PROGRAMME

The proposed COST Action will be implemented through the work of four Working Groups.

These will determine guidelines for the research to be conducted in order to fulfil objectives stated.

Working Group A:Basic issues of plasma polymerisation

Working Group B:Plasma polymers for modification of surfaces of solids

Working Group C:Composites including nanocomposites using plasma polymer matrix and their applications.

Working Group D:Irradiation and temperature post and through deposition treatment of plasma polymer/solid interface for the improvement of adhesion and time and thermal stability

Individual interested groups will propose subprojects through the draft preparation.

D.ORGANISATION AND TIMETABLE

The proposed COST Action will start on 1 January 2000 and will last for five years i.e. until 31 December 2004.

The Management (Scientific) Committee will have two or three meetings a year. Usually one or two individual meetings and one meeting during the COST Action workshop. This workshop will be organised in order to present and discuss the progress reached in the individual subprojects. The main emphasis will be on the discussion among various groups. Therefore the workshop will consist of a few oral contributions, majority of posters and several panel discussions.

A "Research information document" will be regularly delivered through the e-mail to the participants of the Action. This will contain areas and issues needing urgent clarification and related discussion. The bibliographic database will also be provided.

D1.PHASE 1: Introductory Phase (1 year)

Working group coordinators will be appointed by the Management Committee and will begin to organise the work. The "Research information document" will start to be delivered. The main emphasis will be on better understanding of plasma polymerisation process and on determination of various aspects of the ageing process of plasma polymers through all the projects. Irradiation post and through deposition of plasma polymer experiments will be started. Modelling for better reactor design for large area and 3D objects deposition will be started. Corresponding plasma polymerisation processes will be studied that will require decreased reactor cleaning with extremely low production of waste.

D2.PHASE 2: Active research phase (3 years)

The ageing processes will be widely studied in the ambient as well as extreme conditions. Results of all proposed stabilisation processes of the prepared plasma polymers will be discussed. Plasma polymers for modification of solids and plasma polymer composites for sensoring (microelectronics) and optics and on plasma polymers with unusual electrical properties will be studied with the main focus on new generation of materials using plasma polymer matrix. Guidelines for the design and study of the properties for such new materials will be defined.

D3.PHASE 3: Finalisation Phase (1 year)

Conclusions of all the studies related to the ageing processes will be made with the outcome for preparation of stabile plasma polymers that may be deposited at high rate on large area and 3D objects. A summary of the work of the workgroups with special regard to the new generation plasma polymers will be given.

E.ECONOMIC DIMENSIONS

The following COST countries have actively participated in the preparation of the Action or otherwise indicated their interest:

Czech Republic, France, Austria, Poland, Germany, Italy, Turkey (Slovakia, Greece, the Netherlands and England)

Other European non-COST countries (Lithuania) expressed their interest.

E1.Personnel costs:

On the basis of (preliminary) estimates, provided by the representatives from these countries, the total amount of research personnel is:

Phase 1: 60 persons

Phase 2: 70 persons
scientific personnel: 55
technical personnel: 15

Phase 3: 70 persons
scientific personnel: 55
technical personnel: 15

The overall cost of the activities to be carried out under the Action may be estimated at EUR 10 million.

E2.Technical costs:

These costs, which will consist of the use (maintenance and improvements) and acquisition of experimental apparatus and computing, are estimated at 100 kEUR/year.

E3.Coordination costs:

The funding required for the coordination of the research activities, and which will be carried by the COST Action, can be estimated as 30 kEUR/year.

Additionally, a sum of about 45 kEUR/year should be allocated for Short Term Scientific Missions

The global economic dimension of the COST Action may be estimated as EUR 11 million.

F.EUROPEAN ADDED-VALUE

During the work on the projects and through the exchange of the information in the meetings and over the e-mail the new ideas and novel solutions are expected to emerge. This will create a new quality: novel results and expanded knowledge, which will not be possible if researchers work on their own. The participation of the industrial partners will focus attention on particular topics of applications needed especially in Europe.

G.DISSEMINATION OF THE SCIENTIFIC RESULTS

Oral and poster reports during the working groups' meetings and especially during the workshops will present results and progress of particular COST Action projects. Proceedings with the extended abstracts will be prepared for participants. Regular information bulletins over the internet will be set up and serve for fast information of the COST Action participants. Results of research carried out by the working groups under this COST Action will be submitted to international scientific journals and reviews. All publications arising from research carried out under this COST Action will credit COST support, and the Management Committee will encourage and promote all co-authored papers.

Joint meetings among different working groups in this COST Action and with relevant working groups from other COST Actions will be organised in such a way as best to promote interdisciplinary communication.

The Management Committee of this COST Action will, in conjunction with the different working groups of the Action, meet every year with the main aim of presenting results to the Management Committee as a whole and, where possible, invite potential users and interested parties to this meeting.

The Management Committee will, during the first year of the Action, also set up a workplan for interdisciplinary events for the dissemination of the results of the Action.

The Action will be completed by comprehensive publication comprising all results including reprints of publications or unpublished results and scientifically or generally interesting features of the solution. The publication will be broadly available to all interested researchers or the public.

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Czechia

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