Optical thickness monitoring is implemented in almost all coating machines for high precision optical interference filters. Standard broadband transmittance monitoring comes to the limit of thickness resolution when e.g. nanolaminates are deposited. Ellipsometry is more sensitive for material dispersion and interfaces and gives more detailed information on the layers at the beginning of the stack. On the other hand, transmittance measurements can be used for designs with higher layer count with standard materials. In this contribution we show the integration of a broadband ellipsometer for the determination of thickness and material properties during the growing layers in a magnetron sputtering system with a turntable configuration. The ellipsometric angles Psi and Delta were measured at an angle of incidence of 70° and the deposition process was investigated for Ta2O5 and SiO2. The control substrate passes the measurement position every 240 ms. The triggering was optimized to match the exact position on the moving control substrate. In addition, results for nanolaminates are presented from the combination of amorphous silicon and silicon dioxide. The non-reactive magnetron sputtering process with separate oxidation by plasma source gives smooth surfaces even for sub-nm layers as revealed by TEM measurements. The thicknesses are reproducible and in good agreement with ellipsometry.
In broadband dielectric coatings, the wavefront of the reflected wave can change dramatically in a resonance-like manner as a function of wavelength. These wavefront errors can be a significant issue in high precision instruments. In the last years, effort has been undertaken to design and produce coatings to reduce these resonances. However, today there is still limited capability to characterize by measurement the spectral dependence of the wavefront error with high spectral resolution and accuracy. The goal of this paper is to present and analyze a design for a setup to measure the reflected wavefront from a coated flat component with high accuracy as a function of the wavelength. The proposed design is based on a passive system using high-precision off-axis parabolic mirrors. For sensing the wavefront error a Shack-Hartmann sensor is proposed, whose microlens array design is to be modified. According to error analysis and tolerance studies, the setup is capable of measuring wavefront distortion with sub-2 nm RMS accuracy within 510 nm to 950 nm. The angle of incidence and the polarization can also be varied without a loss of accuracy. In order to determine the point spread function (PSF) with high accuracy in addition to the wavefront measurement, the wavefront error of the setup itself needs to be below 50 nm RMS. The tolerancing performed in this study included the light source, shape errors of the mirrors, beam splitter, polarizers, and the sensors. Shape irregularities of the single elements were simulated by Zernike polynomials, and the residual wavefront error of the setup is estimated by Monte Carlo simulations, including uncertainties of the mechanical positioning. From these simulations, specifications for the mirrors have been worked out based on the goal of a system wavefront error lower than 50 nm RMS. The intended broad spectral range makes it challenging to identify a suitable Shack-Hartmann wavefront sensor. Different sensor configurations are evaluated experimentally, and a reproducible wavefront measurement can be achieved by adjusting the focal length of the microlens array. Thereby, the repeatability in wavefront measurements could be reduced from 3 nm to less than 1 nm RMS by modifying the microlens array parameters. Tilting the polarizer and beam splitter by 2° prevents ghost images and multiple reflections in the setup. Finally, considerations about the realization of a suitable reference measurement with an optical flat of sufficient surface quality are presented.
Market drivers for the increasing demand for optical interference coatings on planar substrates and lenses include data communication systems and consumer electronics. Magnetron sputtering can deliver environmentally stable coatings with high throughput. Low defect densities combined with high stability of the process and coating system enable complex optical layer designs with very accurate reproducibility. Cylindrical targets are well known for coatings on large area such as window glass, but are rarely used for coating stacks with high layer counts. With the OPTA X, we are introducing a rotary target-based platform that takes advantage of the long-term stable deposition rate and high material utilization of these sputtering sources. Cylindrical targets have almost no re-deposition zone and no deep racetracks due to rotation. This strongly reduces the potential of particle generating arcs that are caused by surface charges in oxidized target areas. We also use substoichiometric target material sputtered in a pure argon atmosphere without the need for further reactive gas process control. Oxidation is performed by a plasma source in a pressure-separated plasma oxidation zone. The platform can be equipped with sputter sources in a sputter-up or sputter-down configuration or in a combination of both. Interesting applications for double-sided filter coatings include beam splitters for aerospace applications or filter deposition on thin glass substrates to compensate for layer stresses and associated geometric deformations. The elimination of substrate flipping for double-sided coatings also enables shorter production times and offers potential for cost savings.
In this paper, we are presenting preliminary optical properties as well as deposition rates of SiO2 and Nb2O5 oxide films. Furthermore, we are showing results of optical single layers as well as examples of filter coatings prepared by broadband transmittance monitoring and process control.
The present paper addresses uniformity effects in demanding dielectric optical coatings. The origins of spectral resonant wavefront errors (WFE) induced by non-uniformities in complex dielectric filters are investigated in detail. The coating is a broad-band beamsplitter with a high reflectance between 400 and 900 nm and a high transmittance between 920 and 2300nm. The WFE can significantly be reduced with an optimized design. A new setup based on Hartmann-Shack sensors measures the spectrally dependent WFE in the visual spectral range. The paper presents a method for referencing the measured data. The experimental WFE maps obtained by spectral Hartmann-Shack measurements agree well with the expected spectra taken from spectral photometric measurements and the coating design. The paper also addresses coatings on curved surfaces. A band pass filter centered at 670 nm on the convex side of a lens was developed. Using a combination of a sub-rotation and special uniformity masks, a very low spectral shift of the passband position overall the lens surface could be demonstrated. The deposition concept and mask design, respectively, are developed via simulation studies based on a simulation approach shown in [10]. Extension of the model framework by plasma simulation and a concept for computing deposition profiles on moving 3D substrates was required for solving the problem.
In this work the influence of non-uniformity effects on the spectral transmission properties of broad-band dielectric optical coatings was examined. Recently, it was observed that in modern complex dielectric coatings significant spectral, resonantlike errors of the reflected wavefront can occur at specific wavelengths, which are induced by lateral coating nonuniformities [1]. For a detailed investigation of this effect, a setup was developed for monitoring the spectrally dependent wavefront error, utilizing a broad-band monochromatized plasma lamp (spectral range from 400 – 900nm) as light source and a high sensitivity Hartmann-Shack wavefront sensor for detection of reflected or transmitted wavefronts. In addition, a method for absolute and relative calibration of the measured wavefront error is presented. Two broadband dielectric beam splitters (#1 and #2) deposited by magnetron sputtering (high reflectance 400 – 900 nm, high transmittance 920 – 2300 nm) with different coating specifications were analyzed. It could be shown, that for an optimized design the spectral wavefront error can be significantly reduced compared to a standard beam splitter design [2].
A broadband beam splitter coating with the reflecting range from 400 to 900 nm and the transmitting range from 920 to 2300 nm was developed. To avoid absorptive losses at low wavelengths, Ta2O5 was chosen as the high index material. A new setup based on a Hartmann-Shack sensor was built to measure the wavefront error and its spectral dependence. An area of 120 mm diameter was measured. While in a standard coating, large resonant-like wavefront errors occur, an optimized coating with significantly reduced spectral wavefront error was designed and produced.
Particle contamination is a limiting factor for the quality of multilayer thin films, and to evaluate this effect, coated optics are typically subjected to an ex situ inspection. However, to identify the generation mechanisms of particles during the deposition process, it is necessary to record data in situ. In this work we report on a camera based detection method for time resolved particle measurements during plasma deposition processes. We analyze silicon substrates in the vacuum coating chamber by means of dark field illumination to reach high sensitivity to small defects on the substrate surface. We show camera images of the substrate, which document the evolution of particle contamination during the deposition process, and compare the results to microscopy. By providing a cost-efficient monitoring strategy, we take the steps towards an identification of particle sources in the vacuum chamber.
Cylindrical targets give the opportunity to improve the process stability of magnetron sputtering processes although reactive deposition might be a challenge. Sputtering from metal doped oxide targets in connection with a plasma source unlocks the full potential: the process can be driven in well-known mid-frequency mode and the plasma source ensures fully stoichiometric films with low loss.
During the last years different developments for oxide cylindrical targets were done. The suitable composition has to be found regarding e.g. the density and an arc-free process as familiar for planar targets. In the tube geometry new manufacturing methods are required that ensure these properties.
In the present paper we show some examples of the high refractive index materials tantalum oxide: single film characterization as well as realized complex precision optical filters. The results are accompanied by performance measurements in terms of uniformity over 200 mm glass wavers as well as carrier to carrier and batch to batch. These were measured by the position of a quarter-wave stack’s edge.
The generation of third harmonic radiation (THG) is required for many pulsed solid-state laser applications in industry
and science. In this contribution, the coatings for two necessary optical components, dichroic mirrors and nonlinear
optical (NLO) crystals are in the focus of investigation. Because of the high bulk damage threshold lithium triborate
(LBO) crystals are applied for this study. HfO2/SiO2 mixtures are employed as high refractive index material to improve
the power handling capability of the multilayers. All coatings are produced by ion beam sputtering (IBS) using a zone
target assembly for the deposition of material mixtures. The atomic composition and the oxidation ratio of different
HfO2/SiO2 mixtures are analyzed by X-ray photoelectron spectroscopy (XPS). The influence of different deposition
temperatures and post annealing on the optical properties and the amorphous micro structure of the films is investigated
by UV/Vis/NIR spectroscopy and X-ray diffraction (XRD). The laser induced damage thresholds at 355 nm wavelength
for nanosecond pulse durations are measured in a 10,000on1 experiment according with the standard ISO21254.
Furthermore, the optical components are tested under real application conditions.
The optimization of the uniformity of high precision optical filters is often a critical and time consuming procedure. The
goal of the present paper is to evaluate critical factors that influence the thickness distribution on substrates during a
magnetron sputter process. A new developed sputter coater “EOSS” was used to deposit SiO2 and Nb2O5 single films and
optical filters. It is based on dynamic deposition using a rotating turntable. Two sets of cylindrical double magnetrons are
used for the low and the high index layers, respectively. In contrast to common planar magnetrons, the use of cylindrical
magnetrons should yield a more stable distribution during the lifetime of the target. The thickness distribution on the
substrates was measured by optical methods. Homogenization is carried out by shaping apertures. The distribution of the
particle flow from the cylindrical magnetron was simulated using particle-in-cell Monte Carlo plasma simulation
developed at Fraunhofer IST. Thickness profiles of the low index and the high index layers are calculated by numerical
simulation and will be compared with the experimental data. Experimental factors such as wobbling of the magnetron
during rotation, geometrical changes of critical components of the coater such as uniformity shapers as well as gas flow
variations will be evaluated and discussed.
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.
The present study deals with the characterization of hafnia, alumina, and zirconia coatings as well as mixtures
thereof with respect to applications in the UV. Emphasis is placed on optical properties, particularly on the
relation between UV refractive index and absorption edge. The shift of the coatings is investigated as well as the
mechanical stress. Finally, we present the results of stress measurements performed for quarterwave stacks
deposited on different substrates in a broad range of deposition temperatures. In this study, no systematic
dependence of the result of the stress measurement on the substrate material and geometry could be identified.
Ceramic materials such as SiO2 or Ta2O5 are widely used for optical interference coatings. These materials have a high
hardness and mostly offer excellent optical properties. However, there is a growing demand not only for good optical
properties and a high stability, but also for coatings with a high elasticity. Especially coatings on polymer substrates need
layers with improved elasticity since cracks in the layers occur easily when the coated substrates were mechanically
deformed. For such applications flexible layer materials using organics or even polymers are very promising. These may
be used as pure organic layers of with organic-inorganic composites. Unfortunately the chemical reactions to form
polymers layers are more complex than the reactions to form oxides. Thus the deposition techniques for polymer layers
are much more varying. Other important issues are the deposition rate stability and the optical properties of the polymer
layers like haze, refractive and absorption index. In this paper we compare different ways for the deposition of organic
and polymer layers in the gas phase at low pressures. The methods used were: evaporation, sputtering, PECVD and
thermal CVD techniques. The optical parameters (refractive index, absorption and haze) and some mechanical
parameters (adhesion, crack onset strain) of the different polymer layers were characterized. It will be shown that
excellent organic film properties can be obtained by the use of a suitable organic material and deposition process. Also
shown will be results on composite materials to modify the optical properties.
Reactive magnetron sputtering was used to deposit optical thin films. In order to obtain high deposition rates, metallic
targets were used. For the creation of mixed oxides bipolar pulsed sputtering was applied to two different targets. A new
process control setup was developed to monitor the oxidation state of both targets individually. Two different elemental
targets are co-sputtered in oxygen-argon atmosphere within the lambda-probe stabilized transition mode. The
composition is controlled by optical emission spectroscopy. Thus different mixtures are accessible without changing
target material.
Varied mixtures in the system hafnia-silica have been prepared. The optical properties (refractive indices, absorption,
surface roughness, density) as well as mechanical behavior (film stress, hardness) of the mixtures are compared to pure
oxide materials. By mixing the oxides thin film quality can be improved beyond the properties of the single materials.
Reactive magnetron sputtering processes have gained considerable interest for the production of precision optical
coatings. Pulsed sputtering techniques allow the deposition of high quality optical materials at high deposition rates.
However, under high throughput conditions and with tight spectral tolerances, process stabilization has to be included in
the fabrication process. Normally, pulsed magnetron sputter techniques are typically characterized by a low ionization.
Very recently, ionized magnetron sputtering techniques are under investigation, where either the reactive gas or the
metallic sputtered particles are highly ionized. Plasma assisted reactive magnetron sputtering ("PARMS") using an
additional plasma source or the high pulse power magnetron sputtering ("HiPIMS" or "HPPMS") can be applied
therefore. The present paper gives results of film properties of optical materials obtained with these ionized magnetron
sputtering techniques and discusses potentials of the techniques for the use with precision optics.
The optical properties of TaZrOx mixed oxide thin films are investigated. The films were deposited by reactive pulsed magnetron sputtering in a double magnetron set up using two Ta and Zr metallic targets. Depending on the mixture of the materials, crystallization at different temperature occurs. For a specific mixture, temperature stability of more than 950°C could be demonstrated. This is more than another value of TiAlOx thin films reported very recently.
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