Norbert Kaiser

Review of the fundamentals of thin-film growth

The properties of a thin film of a given material depend on the films real structure. The real structure is defined as the link between a thin films deposition parameters and its properties. To facilitate engineering the properties of a thin film by manipulating its real structure, thin-film formation is reviewed as a process starting with nucleation followed by coalescence and subsequent thickness growth, all stages of which can be influenced by deposition parameters. The focus in this review is on dielectric and metallic films and their optical properties. In contrast to optoelectronics all these film growth possibilities for the engineering of novel optical films with extraordinary properties are just beginning to be used.

Surface modification of PMMA by DC glow discharge and microwave plasma treatment for the improvement of coating adhesion

PMMA shows poor adhesion for evaporated inorganic coatings. The DC and microwave plasma treatments can considerably increase free surface energy but an improvement of coating adhesion is, according to our results, only possible by the DC process. ATR spectroscopy has been applied to investigate the changed chemical structure of DC plasma-treated PMMA. Samples treated by microwave plasma did not show ATR alteration while XPS analysis indicated surface oxygen enrichment.

Antireflection of transparent polymers by advanced plasma etching procedures.

Self-organized nanostructures that provide antireflection properties grow on PMMA caused by plasma ion etching. A new procedure uses a thin initial layer prior to the etching step. Different types of antireflective structures can now be produced in a shorter time and with fewer limitations on the type of polymer that can be used. The durability of the structured surfaces can be improved by the deposition of additional thin films.

Antireflection coating design for plastic optics

The coating of plastics for optical applications is intended to improve the mechanical durability of soft polymers and to serve an antireflection function. Usually a classic four-layer antireflection system is added on top of a single-layer hard coating. With needle optimization, an alternative coating design has been developed. The design is characterized by thin high-refractive-index layers that are almost evenly distributed over the whole stack. Plasma ion-assisted deposition was used to deposit coatings upon poly(methyl methacrylate), polycarbonate, and cyclo-olefin copolymer. Uniform antireflection and high scratch resistance have been achieved.

Mechanical stress and thermal-elastic properties of oxide coatings for use in the deep-ultraviolet spectral region

Mechanical stress and the structures of SiO2, Al2O3, and HfO2 single oxide layers and of high-reflection multilayer coatings deposited by reactive evaporation, plasma ion-assisted deposition, and ion-beam sputtering have been studied. The stress was related to the microstructure and to the incorporation of water by means of infrared spectroscopy. From the slopes of measured stress-temperature curves of these coatings deposited onto two substrate materials (silicon and fused silica), the biaxial moduli and the thermal expansion coefficients of the films were estimated.

Symmetrical periods in antireflective coatings for plastic optics

Plastic optical parts require antireflective as well as hard coatings. A novel design concept for coating plastics combines both functions. Symmetrical three-layer periods with a phase thickness of 3/2pi are arranged in a multilayer to achieve a step-down refractive-index profile. It is shown mathematically that the equivalent index of symmetrical periods can be lower than the lowest refractive index of a material used in the design, if the phase thickness of the symmetrical period is set equal to 3/2pi instead of the usual pi/2. The straightforward application of the concept to the design of antireflection coatings in general is demonstrated by example.

Transparent thermoplastic polymers in plasma-assisted coating processes

Abstract The performance of polymethylacrylate, polycarbonate, Zeonex ® and Topas ® as substrate materials in plasma-assisted physical vapour deposition processes has been studied. Different effects of UV radiation, plasma and ion bombardment on the polymers’ optical features, surface energy and adhesion properties for oxide layers are shown. As a consequence, conditions for the deposition of optical coatings on these substrates are discussed.

Study of a novel ALD process for depositing MgF2 thin films

Magnesium fluoride is an ultraviolet (UV) transparent material which is widely used in optical applications over a wide wavelength range. We have developed a novel atomic layer deposition (ALD) process for depositing magnesium fluoride thin films for the first time. MgF2 films were grown at 250–400 °C using Mg(thd)2 and TiF4 as precursors. The crystallinity, morphology, composition, thicknesses and refractive indices of the films were analyzed by X-ray diffraction/reflection (XRD/XRR), transmission electron microscopy (TEM), atomic force microscopy (AFM), field emission scanning electron microscopy (FESEM), time-of-flight elastic recoil detection analysis (TOF-ERDA), and UV-vis spectrophotometry. Electrical properties were also measured. The growth rate was temperature dependent decreasing from 1.6 A cycle−1 at 250 °C to 0.7 A cycle−1 at 400 °C. The films were polycrystalline at 250–400 °C. The refractive indices were between 1.34–1.42 and the permittivity 4.9. The impurity levels were below 0.6 at.% in the films deposited at 350–400 °C.

Formation and direct writing of color centers in LiF using a laser-induced extreme ultraviolet plasma in combination with a Schwarzschild objective

In order to generate high-energy densities of 13.5nm radiation, an extreme ultraviolet (EUV) Schwarzschild mirror objective with a numerical aperture of 0.44 and a demagnification of 10 was developed and adapted to a compact laser-based EUV source. The annular spherical mirror substrates were coated with Mo∕Si multilayer systems. With a single mirror reflectance of more than 65% the total transmittance of the Schwarzschild objective exceeds 40% at 13.5nm. From the properties of the EUV source (pulse energy 3mJ at 13.5nm and plasma diameter approximately 300μm), energy densities of 73mJ∕cm2 at a pulse length of 6ns can be estimated in the image plane of the objective. As a first application, the formation of color centers in lithium fluoride crystals by EUV radiation was investigated. F2, F3, and F3+ color centers could be identified by absorption spectroscopy. The formation dynamics was studied as a function of the EUV dose. By imaging of a pinhole positioned behind the plasma, an EUV spot of 5μm diameter...

Investigation of the absorption induced damage in ultraviolet dielectric thin films

The interaction of UV laser radiation with optical coatings is investigated by a pulsed two-probe-beam photothermal technique. UV laser damage resistance studies on LaF3/MgF2, Al2O3/SiO2 , HfO2/SiO2 multilayer stacks are performed at ? = 248 nm and t520 ns. By investigating the relationship of the number of high-low (HL) pairs and the substrate material, optical and thermal coating properties are shown to be responsible for UV single-shot laser damage. The damage threshold of selected samples is influenced by the deposition technique. The influence of the bandgap energy of typical UV thin film oxide materials on the damage is investigated. Furthermore, multishot damage measurements on LaF3/MgF2 high-reflection multilayer coatings reveal the accumulation of laser energy in the predamage range and lead to an increase in absorption.