NOSCRATCH

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

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 Obiettivo del progetto (Objective)

'Broad band anti-reflective coatings need a low-index material at the top position. Normally SiO2 is used. This layer is in direct contact to the environment. While a number of high- and medium-index layers with a high hardness (up to 30 GPa) have been developed in the past, the hardness of the low-index layer on the top is limited to about 10 GPa. Under hard conditions this layer is then the weak point of the total layer stack. This excludes a number of possible applications.

The goal of the project is to develope a new, ultra-stable AR-coating and the relevant deposition processes. Therefore a new optical material based on a Al-Six-Oy-Fz composite will be developed. The hardness of this material should exceed 20 GPa and must have a refractive index of lower than 1.60 (at 550 nm). The deposition process shall be based on magnetron sputtering with a special focus on highly ionized sputtering. This ensures that the process can be upscaled and can be used for high deposition rates.

The result is a broad band antireflective coating which shows a significant higher scratch resistance (from about 10 to 1000 cycles Taber-abraser test) and a doubled hardness (> 20 GPa). In addition, the coating must be stable against hydrothermal corrosion (1000 cycles autoklave).

The project focusses not only on precision optical products but potentially on all flat glas products such as glasses for instruments, photovoltaic modules, automotive, sensors, displays.'

Introduzione (Teaser)

Anti-reflective glass is a component of products used in devices ranging from mobile phones to solar cells. Extensive materials and process development by EU-funded scientists should help meet the increasing demand for harder coatings that withstand harsh environments.

Descrizione progetto (Article)

Anti-reflective coating layers are made from low-refractive index materials, those that allow the most light to penetrate without being reflected. This layer is in direct contact with the environment and its strength affects the ultimate strength of the component.

Silicon dioxide is the most commonly used low-index coating material, but its strength is limited. The growing demand for anti-reflective coatings with increased scratch resistance for harsh environments provided the focus of the EU-funded project 'Process and material research for ultra-stable antireflective coatings on glass' (http://noscratch.eu/en/about/ (NOSCRATCH)).

Research centred on thin-film materials and processes to lead to highly stable anti-reflective coatings on glass or sapphire substrates. The team developed optical materials based on nanocrystalline composites of aluminium, silicon, oxygen and iron to be used for the anti-reflective coating. Scientists optimised a thin-film deposition process and high-power impulse magnetron sputtering to deliver dense coatings with good durability.

Researchers then analysed the optical, morphological and mechanical properties of the film. They evaluated film density, composition and microstructure, as well as residual stresses and film hardness. A variety of advanced techniques were employed, including scanning electron microscopy, transmission electron microscopy and X-ray reflectometry.

The extensive materials testing and process development conducted within the scope of NOSCRATCH resulted in numerous publications in peer-reviewed scientific journals. Outcomes are expected to aid designers in optimising process parameters to synthesise scratch-resistant anti-reflective coatings for harsh environments such as those to which solar cells are subjected.