Michael Vergöhl

Effect of deposition parameters on optical and mechanical properties of MF- and DC-sputtered Nb2O5 films

Abstract Niobium-pentoxide films with thicknesses of 100 nm to 1 μm were deposited by both DC and mid-frequency (MF) magnetron sputtering using a dual magnetron (TwinMag®) configuration. Stress measurements were performed by determining the substrate curvature before and after deposition for various process parameters. By varying the argon or oxygen partial pressure, it could be shown that a minimization of the undesired compressive stress, caused by the particle bombardment, is attainable. It was found that the limits for achieving this strongly depend on the applied process (MF or DC) as well as on the target state (oxide or transition mode). The films were characterized using SEM, AFM, vibrating reed, EPMA, and XRD yielding information concerning morphology, stoichiometry, phase, and surface roughness. Furthermore, ellipsometric measurements allowed the determination of the film thicknesses and the refractive indices ( n 550 nm =2.4) which turned out to be highly correlated to the stress values.

Ex situ and in situ spectroscopic ellipsometry of MF and DC-sputtered TiO2 and SiO2 films for process control

Abstract Titanium oxide and silicon oxide films were deposited on floatglass substrates by magnetron sputtering employing both the DC and the MF (mid-frequency) technique. The films were grown at different working points between transition and oxide mode, target power densities up to 7.5 W/cm2. Ex-situ ellipsometry at different angles of incidence was applied to study the optical properties and the morphology of the films. For modeling the spectra, the Lorentz model with one single oscillator was used within the spectral range between 380 and 850 nm. For SiO2, the ellipsometric data could be fitted using a model with one single homogeneous film. The ellipsometric investigation of TiO2 films with thicknesses below 150 nm show that, in general inhomogeneities of the refractive index in the growth direction have to be taken into account. It is concluded that both plasma heating as well as ion bombardment are responsible for these inhomogeneities. Refractive indices of 2.43 for DC sputtered films up to 2.58 for MF sputtered films are observed. A four-layer SiO2–TiO2 antireflective coating on glass was fabricated using both plasma and ellipsometric control. The reflectivity, calculated from the in situ ellipsometric analysis, is in a good agreement with the measured reflectivity.

Mixed oxide coatings for optics

Material mixtures offer new possibilities for synthesizing coating materials with tailored optical and mechanical properties. We present experimental results on mixtures of HfO2, ZrO2, and Al2O3, pursuing applications in UV coating technology, while the mixtures are prepared by magnetron sputtering, ion beam sputtering, plasma ion-assisted deposition (PIAD), and electron beam evaporation without assistance. The properties investigated include the refractive index, optical gap, thermal shift, and mechanical stress. The first high reflectors for UV applications have been deposited by PIAD.

Deposition and spectral performance of an inhomogeneous broadband wide-angular antireflective coating

Gradient index coatings and optical filters are a challenge for fabrication. In a round-robin experiment, basically the same hybrid antireflection coating for the visible spectral region, combining homogeneous refractive index layers of pure materials and linear gradient refractive index layers of material mixtures, has been deposited. The experiment involved three different deposition techniques: electron-beam evaporation, ion-beam sputtering, and radio frequency magnetron sputtering. The material combinations used by these techniques were Nb(2)O(5)/SiO(2), TiO(2)/SiO(2), and Ta(2)O(5)/SiO(2), respectively. The spectral performances of samples coated on one side and on both sides have been compared to the corresponding theoretical spectra of the designed profile. Also, the reproducibility of results for each process is verified. Finally, it is shown that ion-beam sputtering gave the best results in terms of deviation from the theoretical performance and reproducibility.

In situ monitoring of optical coatings on architectural glass and comparison of the accuracy of the layer thickness attainable with ellipsometry and photometry

Abstract Spectroscopic ellipsometry and spectral photometry have been applied for the deposition of optical coatings on architectural glass. For the purpose of achieving a high competitiveness of manufacturing plants, high deposition rates as well as low scrap quantities are important. The first can be obtained by using the mid-frequency (MF) sputtering technique developed at the beginning of the 1990s for the deposition of highly isolating materials. The latter can be improved by using an effective plasma control and a monitor of the optical film properties. In order to obtain constant layer properties, not only the MF but also the direct-current (DC) sputter process has to be stabilized within a relatively narrow process window, which necessitates a suitable plasma control as well as optical monitoring. Spectroscopic ellipsometry as well as spectral photometry are used for monitoring the optical properties of SnO 2 -based low-e-coatings on architectural glass. It is shown that spectroscopic ellipsometry is distinctly more sensitive for monitoring the thickness of low-e coatings than spectral photometry.

New sputtering concept for optical precision coatings

The deposition of optical precision coatings on glass by magnetron sputtering is still a challenging problem regarding particle density and long term stability of coating plants due to target material erosion. A novel approach to increase process stability and reduce drifts is the usage of cylindrical cathodes. These cathodes allow a particle free deposition process as they have virtually no redeposition zones that can lead to destruction of coatings by arcing caused by surface charges. In the present paper optical single layers as well as multilayer coatings were sputtered by means of reactive magnetron sputtering using a double cylindrical cathode setup. The particle density is determined and compared to particles produced with planar magnetrons. A new sputter coater concept will be presented wherein the magnetrons are attached to a rotating disc coater in a sputter-up configuration. The process was stabilized by means of oxygen partial pressure control. Preliminary optical properties as well as deposition rates of different oxide films will be presented.

Real time control of reactive magnetron-sputter deposited optical filters by in situ spectroscopic ellipsometry

Abstract In situ spectroscopic ellipsometry was applied for real-time control of the reactive magnetron sputter process of optical coatings on glass. Single transparent films of SiO 2 , Si 3 N 4 and SiO x N y with varying composition as well as multilayer optical filters on transparent thick floatglass were fabricated. The process control system is based on an optical monitor for the deposition rate and the film composition in combination with a plasma control setup. The latter was developed for the short-term stabilization of the mid-frequency magnetron sputter process. The quality of the filters deposited using the control system is demonstrated by comparing the calculated reflectivity and transmission with the experimental ex situ data. The investigations show that for silicon oxynitride films, the combination of plasma control with ellipsometric control can be used for controlling the stoichiometry of the growing film.

Design and manufacture of spectrally selective reflecting coatings for the use with laser display projection screens

To provide screens for laser projection that improve contrast, a spectrally selective reflecting filter was designed by using genetic algorithms to overcome the problem of unknown starting values. Colormetrics rather than fixed targets were used for evaluation. Various selective filters were deposited upon glass as well as upon solid and flexible plastic substrates by reactive mid-frequency magnetron sputtering. For process control, in situ spectroscopic ellipsometry was applied.

Origin of particles during reactive sputtering of oxides using planar and cylindrical magnetrons

Particles generated during reactive magnetron sputtering cause defects in optical thin films, which may lead to losses in optical performance, pinholes, loss of adhesion, decreased laser-induced damage thresholds and many more negative effects. Therefore, it is important to reduce the particle contamination during the manufacturing process. In the present paper, the origin of particles during the deposition of various oxide films by midfrequency pulsed reactive magnetron sputtering was investigated. Several steps have been undertaken to decrease the particle contamination during the complete substrate handling procedure. It was found that conditioning of the vacuum chamber can help to decrease the defect level significantly. This level remains low for several hours of sputtering and increases after 100 hours of process time. Particle densities of SiO(2) films deposited with cylindrical and planar dual magnetrons at different process parameters as well as different positions underneath the target were compared. It was observed that the process power influences the particle density significantly in case of planar targets while cylindrical targets have no such strong dependence. In addition, the particle contamination caused by different cylindrical target materials was analyzed. No major differences in particle contamination of different cylindrical target types and materials were found.

Optical properties of silicon titanium oxide mixtures prepared by metallic mode reactive sputtering

In this paper different SiO(2)-TiO(2) mixtures are prepared by metallic mode reactive sputtering. The samples were sputtered from cylindrical targets in a sputter-up configuration using an additional plasma source for oxidization. The different ratios of SiO(2) and TiO(2) in the mixtures are prepared by a target sputtering power variation. Optical film properties of the mixtures such as refractive index, which is determined by ellipsometric measurements, and optical bandgap, which is measured by photometric (transmission) measurements, are investigated. The thin-film structure is investigated by x-ray diffraction analysis and the stress of the films is presented. It is shown that the metallic mode reactive sputtering in the present configuration is applicable to continuously tune optical and mechanical properties. Finally the sputtered mixed materials are compared with other optical standard materials such as Nb(2)O(5), Ta(2)O(5), HfO(2), and Al(2)O(3).