Alfons Zoeller

Plasma ion-assisted deposition: a novel technique for the production of optical coatings

Ion assisted deposition (IAD) is a well known technique to improve the properties of thermally evaporated thin films. A wide range of materials and completed layer systems have already been investigated. Because of the low total ion current and the small beam size of the commercial available ion sources, the useful substrate area is strongly limited. With a newly developed advanced plasma source (APS) we have overcome these problems. A total ion current of up to 5 A with excellent uniformity over a large area substrate holder (approximately equals 1 m2) has been achieved. The plasma source is installed in conventional box coating system. Besides plasma-IAD the APS is also useful for plasma-CVD processes like plasma polymerization. The principle of operation of the plasma assisted processes with the APS is described. Results of dielectric materials and completed layer systems like shift free edge filters and AR-coatings are presented. In particular, in case of organic substrate materials, the advantages of the APS are outstanding. Scratch resistant layers in combination with AR- coatings and hydrophobic surface layers onto organic substrates are successfully introduced in production.

Plasma-ion-assisted deposition: a powerful technology for the production of optical coatings

Plasma-IAD with the APS (advanced plasma source) has been introduced into the market in 1992. Up to now this technique is used worldwide in almost 100 coating systems. A large number of different layer systems has been investigated in R&D and applied in production. For ophthalmic applications plasma-IAD with the APS is used for antireflection systems as well as for wear resistant coatings onto organic substrates. New processes which combine the AR coating and the hardcoating on ophthalmic lenses have been successfully introduced into mass production. Plasma-IAD is also used for laser protection coatings onto plastic substrates. The ability for the production of shift free multilayer coatings is utilized in manufacture for many applications such as steep edge filters for color separation, rugate filters for laser protection, narrow-bandpass filters in the NIR region for wavelength multiplexing in the field of fiber optic communication or for radiometers in the UV-B region. A review of the development of some important APS based coating processes and applications shows the flexibility of plasma-IAD with the APS. Actual evaluations of the optical constants of Ta2O5 layers deposited with APS assistance show a low extinction coefficient and a stable refractive index. New results of the performance, temperature behavior and long term stability of some interference filters confirm high packing density and low absorption of the films produced with plasma-IAD. The result of a reproducibility experiment demonstrates high process stability and high monitoring accuracy.

Nonlinear absorption of thin Al 2 O 3 films at 193 nm

Absorption of thin Al2O3 films was measured at 193 nm with an ArF-laser calorimeter. In addition to the expected high linear absorption coefficient, we found, for the first time to our knowledge, that two-photon absorption and transient color-center formation are nonnegligible loss channels in thin films at 193 nm. The nonlinear absorption coefficient is of the order of several times 10(-4) cm/W.

Plasma-ion-assisted deposition: investigation of film stress

In many applications in the field of optical coatings low dimensions and low weight of the substrates are of increasing importance. Organic substrate materials or thin glass substrates are used in many cases. Due to the low mechanical stability, these substrates are very sensitive against bending caused by the stress of the coatings. The stress of optical coatings deposited with plasma-IAD with the APS (advanced plasma source) was investigated. Based on the bending radius of coated thin glass substrates stress values were evaluated. It was found, that film stress can be influenced significantly with the deposition parameters and the use of the coating starting materials. The results of single layer coatings of various dielectric materials and completed multilayer systems are given.

UV coatings produced with plasma-ion-assisted deposition

Plasma-IAD with the APS has been applied for a large number of different layer systems in production an ind R and D. The ability for the production of shift free multilayer coatings for the visible and NIR spectral range is utilized in manufacture for many applications such as steep edge filters for color separation, rugate filters for laser protection and narrow-bandpass filters for wavelength division multiplexing. An overview was given. Shift free narrowband filters for the UV-B region were published in 1996. The paper reports the result of UV coatings using plasma ion assisted deposition. Tantala/silica and hafnia/silica combinations have been used for multilayer coatings in the UV-A and UV-B spectral range. Single layers of silica and alumina and multilayer systems with both materials were investigated in the UV-B and UV-C region. The coatings were characterized by obtained transmittance and reflectance curves as well as absorption and scattering measurements. The temperature stability results are compared with coatings in the visible and NIR spectral range published.

Accuracy and error compensation with direct monochromatic monitoring

The cut-off and layer thickness accuracy of single layers is discussed. Error compensation effects of complex multilayer systems are investigated with computer simulated process runs and confirmed by practical coating experiments.

Direct Optical Monitoring Enables High Performance Applications in Mass Production

Single wavelength optical monitoring in intermittent mode was investigated. The achieved coincidence between theory and experiment is outstanding. This monitoring technique enables rapid prototyping with tight specifications and high yield in large box coaters.

Large-area IAD with a new plasma source

Ion assisted deposition (lAD) is a well known technique to iniproye the properties of thin films. A wide range of materials and conipleted layer systems have already been investigated by many laboratories. One disadvantage of this technology is the small useful substrate area compared to conventional thermal evaporation. This is due to the limited ion beam size of the available ion sources. Therefore we have developed a new plasma source which is able to irradiate a substrate holder of 800 mm diameter with high plasma current density. The principle of operation and some details of the plasma source are described. The experiments were done in a conventional coating system. The plasma source has been operated at up to 90 V discharge voltage and up to 80 A discharge current. Up to now we have deposited some single layers with dielectric materials. For TiO2 we have achieved an refractive index of 2.55 at 550 nm.