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Process and material research for ultra-stable antireflective coatings on glass

Final Report Summary - NOSCRATCH (Process and material research for ultra-stable antireflective coatings on glass)

Executive Summary:
Antireflective (AR) coatings play a key role in many industrial applications. AR coatings are industrially used for the eleminination of the reflectance of computer or mobile phone displays (including iphones, for example) or show screens. In the case of optical instruments with several lenses losses and gost images are a tremendeous problem is no AR coatings would be used. In the case of solar applications the light troughput can be increased significantly, leading to direct improvements of the cell efficiency. However, there is growing demand on ultra-stable AR coatings for the use with glass and sapphire. For glass applications, the coatings must increase the scratch resistance of the glass, too, which is needed for example for harsh environments (for example solar cells in desert).
The main objective of NoScratch is to develop innovative thin film materials as well as an innovative thin film technology for the production of ultra-stable anti-reflective coatings on glass or sapphire substrates. The basic idea of the project has been conceived by the Switzerland coating producer Blösch to realize extremely hard optical anti-reflective coatings with enhanced mechanical resistance against harsh media. The three critical components for the success of this development are newtailored composite mixed nanocrystalline materials (to be developed by Fraunhofer IST), improved magnetron sputter sources for high-rate deposition of defect free and ultrahard films to be realized by the SME Robeko GmbH, new plasma deposition processes based on highly ionized plasma (to be developed by Linköping University and the SME Ionautics). The SME company Blösch AG, as an end user active in the production of optical coatings, will assess and validate the developed technology in precision optical application, especially in the consumer market. Also Ionautics will provide a technology transfer by the end of the project by the fabrication of a pilot coating process for glass based on the new technology. Switzerland, Sweden and Germany will be represented in the consortium, providing a higher impact of the breakthrough in Europe.
The new technology is rated to become a breakthrough and a reference for all end users working in the optical industry. The technological advances achieved by the end of this project will keep European countries at the head of optical industry, with a qualitative jump for at least 3 years.

Project Context and Objectives:
The specific objectives of the NoScratch project are:
- Development of new nanocrystalline materials for optical applications with low refractive index and superior hardness
- Realization of ultra-stable anti-reflective coatings to be applied on glass and sapphire
- Development of a new thin film deposition processes suitable for economic production of optical coatings based on highly ionized plasma leading to superior film properties
- Development of sputter sources to be implemented in the thin-film deposition process and which allow doubled deposition rates and defect-free optical coatings
- Realization of a prototype coating process for the ultra-stable AR coatings on glass

Objectives from the work packages:
Detailed definition of the requirements of the coatings to be tested by means of different application oriented test in the different applications and materials.
- Definition of the real working conditions in terms of productivity, cost effectiveness, quality and accuracy from the end user point of view.
- Definition of the expected improvements to be achieved with the new coatings.
- Definition of the main features and characteristics of the sputter components according to the requirements of the end users and the target materials.
- Detailed definition of the development and testing procedure to be applied in the different phases of the project, from the preliminary single film properties analysis to the final testing of the complete AR-system.
- Definition of the environmental and safety aspects to be introduced into the pilot system.
The final validation will be performed on coated glasses as well as on a industrial coater where the process and materials developments will be applied. In this final system, application tests will be performed to validate the behaviour if the whole system, in terms of robustness of the coatings, productivity of the process, quality requirements and cost effectiveness.

The thin film properties are not only influenced by the material itself, but also by the process parameters which influence the ion energy distribution function and the ion flux density. The latter parameters determine the nucleation and growth processes and therefore are responsible for the mechanical and optical properties of the films. Therefore, the overall goal of the work package is to develop and optimize a thin film deposition process based on pulsed co-sputtering (Fraunhofer IST) with special focus on high power impulse magnetron sputtering (HiPIMS) in order to synthesize nanocrystalline Al-Si-O-F films with high hardness and low refractive index. The work on HiPIMS will be mainly carried out by LiU and Ionautics, since they are experts on the novel thin film deposition technology called high power impulse magnetron sputtering (HiPIMS). Also Fraunhofer IST will work in the process development and will setup the deposition process for the thin film material screening based on cosputtering. Based on the results of process development including the magnetron optimization made by Robeko, Ionautics will develop a suitable HiPIMS process for the growth of the scratch resistant anti-reflective (AR) coatings, which can be industrially scaled up in collaboration with the other project members (WP5).

The goal of WP 3 is to find an optical thin film material with a high hardness (i.e. 20 GPa) and a low refactive index (<= 1.6 at 550 nm). Also of importance is the stress in the film, mostly in compressive form. Layers with a low stress are required, otherwise adhesion to the substrate or to the underlaying layers often is critical. The knowledge of the film density, chemical composition, residual stresses, as well as information about the film microstructure are prerequisites for the synthesis of scratch resistant anti-reflective coatings since the above mentioned physical properties are of key importance for the optical and the mechanical performance of the films.

Once the suitable material for the top layer is found, the complete layer system has to be re-optimized. At first, the optical design will be redesigned. A decision has to be made what designs (i.e. more layers or higher residual reflectivity) will be used for further development. It is known that also the mechanical properties of the underlaying layers influence the stability of the multilayer. Therefore it has to be proven whether for the other layers, standard parameters can be used or whether a further optimization step is required. Also the process technology has to be transferred to the pilot coater (Ionautics) and to production compadible coaters at Blösch.

The objectives are to analyze the film properties regarding the optical, morphological and mechanical properties.These are:
Film density: The energetic bombardment during HiPIMS facilitates densification of the growing films.
Microscopy techniques, such as scanning electron microscopy (SEM) and transmission electron microscopy (TEM), as well as x-ray based techniques (x-ray reflectometry-XRR) will be employed to determine the density of the films deposited at the various conditions.
Film composition: Energetic bombardment during growth of multi-element (compound) films is known to haveimplications for the films’ chemical composition since elements of different masses are affected differently by the energy and momentum transferred by the impinging species (preferential sputtering) [R. Behrisch (ed), Sputtering by Particle Bombardment I, Springer 1982]. In addition, in HiPIMS significant fraction of the film forming species is ions. These ions exhibit spatial distributions which are determined by the pulse characteristics and also differ substantially than those of the neutral species [D. Lundin, et al. Plasma Sources Sci. Technol. 17 (2008) 035021.]. The effect of the process parameters with respect to the pulse characteristics on the chemical composition of the Al-Si-O-F films will be investigated using energy dispersive x-ray spectroscopy (EDX) and elastic detection recoil analysis (ERDA).
Film microstructure: The energetic bombardment during HiPIMS has been shown to enable control over the microstructure of the growing films. One characteristic example is the study by Alami et al. [J Alami et. Al., J. Phys. D: Appl. Phys. 42 (2009) 015304] which demonstrated that CrN films with a microstructure ranging from columnar to a nanocrystalline one can be deposited depending on the HiPIMS pulse characteristics which in turn determine the peak target current during the pulse (see Fig. 2). X-ray techniques (x-ray diffractometry), as well as SEM and TEM will be employed to investigate the microstructure of the Al-Si-O-F films grown at the various deposition conditions. In addition, using these characterization techniques the feasibility of HiPIMS to synthesize nano-composite Al-Si-O-F films will be explored.
Residual stresses: Energetic bombardment is known to generate residual stresses in the growing film [G. Abadias, Surf. Coat. Technol., 202 (2008) 2223]. Depending on their magnitude, the residual stresses can be beneficial for the mechanical performance of the films due to hardening, but also deteriorate the film/substrate adhesion and cause delamination of the film. It is therefore evident that knowledge and control of the magnitude of residual stresses are crucial for synthesizing stable and functional coating/substrate systems. Residual stresses in this work package will be determined during the film growth by measuring in-situ the curvature of the film/substrate system and by employing the Stoney’s formula [G.G. Stoney, Proc. Roy. Soc. London A, 82 (1909) 172.]. At the same time residual stresses will be also determined using ex-situ x-ray based techniques (sin2y method) [A. J. Perry, J. Vac. Sci. Technol. A, 8 (1990) 3186-3193.]. The information by the in-situ and ex-situ techniques will be combined with the film microstructure data in order to unravel the origin of the stresses and optimized the process with respect to the residual stress values.
Film hardness: The hardness of the Al-Si-O-F films will be determined by nano-indentation (NI). The hardness data, as well as the optical properties data determined by one of the project partners (IST) will be interpreted in light of the density, composition, microstructure, and residual stresses data.
This will enable to optimize the process parameters in order to synthesize scratch

The objective of this WP is to deal with the outcome of the project, both externally by participating in congresses, conferences and fairs and internally by the definition of an individual exploitation plan for each of the industrial partners.
RTDs will lead the dissemination activities, while the exploitation plan will be worked together with each of the companies, so that the output is realistic and achievable in 5 years time.

The objective of this WP is the co-ordination of the global project. The co-ordinator will act as the administrative interface to the Commission and as highest organisational authority to the consortium.

Project Results:
The publishable information of S&T results is mainly given in the peer-reviewed publications:
Stefan Bruns, Daniel Rademacher, Michael Vergöhl, Peter Weiss, “Properties of reactive sputtered alumina-silica mixtures”, Applied Optics, Vol. 53, Issue 4 (2014), pp. A334-A338.
Stefan Bruns, Sebastian Montzka, Wilfried Reimann, Michael Vergöhl, „Comparison of abrasive tests for transparent optical coatings“, Thin Solid Films 532 (2013), 73-78.
The papers are attached to this report.

Additional information is given in the final characterisation Deliverable :
D5.1 Characterization report
and also in the previous deliverables D2.1 Task report about modifications, D2.2 Report on IEDF of differen magnetron concepts, D3.4 Summary of materials development.

Additional to the given documents in the periodic report please find this in the attached documents:

Potential Impact:
Please find this in the four final Deliverables of the Dissemination and Exploitation Workpackage:

D6.5 Final Plan for the Use and Dissemination of the Knowledge
D6.6 Compilation of articles, panels… etc. created by partners for project results diffusion
D6.7 Studies, papers and process of patenting/protecting
D6.8 Studies of new markets and new researching paths

List of Websites:

Coordination Office:
Fraunhofer IST
Dr. Michael Vergöhl
Bienroder Weg 54e
38108 Braunschweig - Germany Germany
Tel.: +49 531 2155-628
Fax: +49 531 2155-900

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