UWS optical filter research was led by Prof Frank Placido who has researched advanced optical filters for the last twenty years, leading the development of both advanced materials and process techniques. He pioneered the production of so-called rugate filters for laser notch filters from 1993 to 2002 using a simple reactive sputtering technique, but with very careful control of the reactive gases, oxygen and nitrogen. This allowed the production of narrow band reflection filters based on aluminium oxynitride films, having a periodic variation of refractive index with thickness. This work led to the award of the John Logie Baird prize for Innovation in 1999 and was the catalyst for the subsequent development of the Thin Film Centre at UWS. An award of £1.2 million from the Scottish Funding Council allowed the purchase and development of an advanced sputtering system, an electron-beam system and characterisation tools. This allowed continuing development of expertise in materials and plasma-assist processes to improve thin film properties for advanced optical filters as required by the multi-billion pound filter industry.
Electron-beam evaporation was, and still is, very widely used in industry, but optical filters made by this method suffer from a number of problems because of the relatively low energy of the adatoms arriving at the substrate. In particular, evaporated films have a rather porous nano-structure that makes them susceptible to the ingress of water vapour. This leads to refractive index changes and general deterioration in performance of such filters with time, necessitating expensive encapsulation. These optical filters also tend to have unacceptable levels of absorption due to the difficulty in ensuring stoichiometry of the metal oxides used as high and low index materials. Traditionally, high-temperature deposition and/or subsequent annealing of the films were used to improve the density of evaporated films, but with limited success and considerable extra time and cost. Our research led naturally to the investigation of plasma-assisted techniques where the growing film is bombarded with argon ions and oxygen during deposition to improve densification and stoichiometry. In fact, many research groups and companies have tried to develop ion guns and plasma sources for this type of application, but the production of a stable, wide-area source that can operate reliably at e-beam deposition pressures for the many hours of a deposition run has proved a difficult nut to crack, particularly in a form that can readily be retro-fitted to existing e-beam chambers.
Our research initially developed a compact design of plasma source using an inductively-heated LaB6 cathode and novel ion extraction, producing high ion densities at the optimum energies for thin film enhancement. This has progressed by several innovative steps, such as implementing a hollow cathode effect in the electron emitter, removing the need for inductive heating during operation, thereby leading to considerable savings in operational costs. The latest models use modified magnetics with the aim of allowing operation at higher chamber pressures.