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Our employees in the Optical Coating Division are recognised experts in their field and regularly publish scientific work in the form of papers, presentations and lectures.
In addition, we support students who are interested in writing their study, bachelor or master thesis in the field of optical coating.
Below you can find selected publications.
In thin film deposition processes, the lower limit of the deposition temperature is determined by the used coating technology and the duration of the coating process and is usually higher than room temperature. Hence, the processing of thermally sensitive materials and the adjustability of thin film morphology are limited. In consequence, for factual low-temperature deposition processes, an active cooling of the substrate is required. The effect of low substrate temperature on thin film properties during ion beam sputtering was investigated. The SiO2SiO2 and Ta2O5Ta2O5 films grown at 0°C show a trend of lower optical losses and higher laser induced damage threshold (LIDT) compared to 100°C.
1RhySearch, Werdenbergstr. 4, 9471 Buchs, Switzerland
2ThermodynamX, Straubstrasse 11, 7323 Vilters-Wangs, Switzerland
3OST—University of Applied Sciences Buchs, Werdenbergstr. 4, 9471 Buchs, Switzerland
4Leibniz-IPHT, Albert-Einstein-Str. 9, 07745 Jena, Germany
5Fraunhofer IOF, Albert-Einstein-Str. 7, 07745 Jena, Germany
6Empa, Üeberlandstrasse 129, 8600 Dübendorf, Switzerland
*Corresponding author: thomas.gischkat(at)iof.fraunhofer.de
Quantized nanolaminates are a type of optical metamaterials, which were discovered only recently. Their feasibility was demonstrated by atomic layer deposition and ion beam sputtering so far. In this paper, we will report on the successful magnetron sputter deposition of quantized nanolaminates based on Ta2O5-SiO2. We will describe the deposition process, show results and material characterization of films deposited in a very wide parameter range. Furthermore, we will show how quantized nanolaminates deposited by magnetron sputtering were used in optical interference coatings such as antireflection and mirror coatings.
1Evatec AG, Hauptstrasse 1a, 9477 Trübbach, Switzerland
2RhySearch, Werdenbergstrasse 4, 9471 Buchs, Switzerland
3Laser Zentrum Hannover e.V., Optical Components Department, Holleritallee 8, 30419 Hannover, Germany
4Cluster of Excellence PhoenixD (Photonics, Optics, and Engineering innovation across Disciplines), Leibniz University Hannover, Welfengarten 1A, 30167 Hannover, Germany
In many laser applications, there is a higher and higher demand for more efficient coatings with reduced losses, in terms of absorption and scattering as those are contributing factors to diverse laser damage regimes. Ion Beam Sputtering (IBS) is a known technique to provide such high optical quality thin films. Indeed, it allows to achieve high density layers with low absorption and scattering. In this work, various coatings were developed using Bühler IBS technology. Then, total losses were measured using Cavity Ring Down, absorption using Laser Induced Deflection or Laser thermography, and Total Integrated Scatter using dedicated scatterometers. A correlation between the effect of the chosen deposition method and parameters and the measurement performances were made with the aim of a better understanding of the level and the origin of losses in the coatings. Finally, highly reflecting mirror coatings for 1064 nm wavelength were fabricated with different designs and deposition parameters. The results of the different measurements of absorption, scattering and total losses using different equipment are presented and discussed.
1Bühler Alzenau GmbH (Germany)
2Institut Fresnel (France)
4WZW-Optic AG (Switzerland)
5Fraunhofer-Institut für Angewandte Optik und Feinmechanik IOF (Germany)
Tuning ion energies in plasma-enhanced atomic layer deposition (PEALD) processes enables fine control over the material properties of functional coatings. The growth, structural, mechanical, and optical properties of HfO2 thin films are presented in detail toward photonic applications. The influence of the film thickness and bias value on the properties of HfO2 thin films deposited at 100 °C using tetrakis(dimethylamino)hafnium (TDMAH) and oxygen plasma using substrate biasing is systematically analyzed. The HfO2 films deposited without a substrate bias show an amorphous microstructure with a low density, low refractive index, high incorporation of residual hydroxyl (OH) content, and high residual tensile stress. The material properties of HfO2 films significantly improved at a low bias voltage due to the interaction with oxygen ions accelerated to the film. Such HfO2 films have a higher density, higher refractive index, and lower residual OH incorporation than films without bias. The mechanical stress becomes compressive depending on the bias values. Further increasing the ion energies by applying a larger substrate bias results in a decrease of the film density, refractive index, and a higher residual OH incorporation as well as crystalline inclusions. The comparable material properties of the HfO2 films have been reported using tris(dimethylamino)cyclopentadienyl hafnium (TDMACpH) in a different apparatus, indicating that this approach can be transferred to various systems and is highly versatile. Finally, the substrate biasing technique has been introduced to deposit stress-compensated, crack- and delamination-free high-reflective (HR) mirrors at 355 and 532 nm wavelengths using HfO2 and SiO2 as high and low refractive index materials, respectively. Such mirrors could not be obtained without the substrate biasing during the deposition because of the high tensile stress of HfO2, leading to cracks in thick multilayer systems. An HR mirror for 532 nm wavelength shows a high reflectance of 99.93%, a residual transmittance of ∼530 ppm, and a low absorption of ∼11 ppm, as well as low scattering losses of ∼4 ppm, high laser-induced damage threshold, low mechanical stress, and high environmental stability.
1RhySearch, 9471 Buchs, Switzerland
2Institute of Applied Physics, Friedrich Schiller University Jena & Fraunhofer Institute for Applied Optics and Precision Engineering, 07745 Jena, Germany
Influence of Ar-impurities on the wettability of IBS-deposited Y2O3 thin films
Thomas Gischkat1, Max Döbeli2, Andreas Bächli1, Roelene Botha1, Zoltán Balogh-Michels1
The wettability and the control of the contact angle of surfaces are important for various applications. Materials with a water contact angle less than or greater than 90 deg are hydrophilic or hydrophobic, respectively. Most of the binary oxides are hydrophilic, but oxides of rare earth metals and metal atoms with low electronegativity have hydrophobic surfaces. Yttria is one of the materials predicted to have hydrophobic properties. In this work, we investigated the wettability of ion-beam-sputtered Yttria thin films. The measured water contact angles were between 67° and 96°. The concentration of the embedded argon atoms was found to influence the contact angle.
1RhySearch, The Rhine Valley Research and Innovation Centre, Werdenbergstr 4, CH-9471 Buchs, Switzerland
2Ion Beam Physics, ETH Zurich, CH-8093 Zurich, Switzerland
Ion beam sputtered hafnia is a preferred high index coating material for laser applications. It exhibits a mostly amorphous structure and an adequate laser-induced damage (LIDT) threshold. In this work, we investigated the influence of an assisting ion source on the film stress as well as the LIDT of the sputtered hafnia layers. The stress increases with an increasing ion energy of the assisting ion beam. We identified a maximum compressive stress of 3–3.5 GPa before the film cracks, blisters, and delaminates. Different states of stress lead to different laser-induced damage thresholds and damage morphologies
1RhySearch, The Rhine Valley Research and Innovation Center, Werdenbergstrasse 4, 9471 Buchs, Switzerland
2Laboratoire Temps-Fréquence, Institut de Physique, Université de Neuchâtel, 2000 Neuchâtel, Switzerland
Substrate cleaning prior to coating has a strong influence on the performance of the optical component. Exemplary, none or inadequate cleaning reduces the resistance against laser irradiation drastically. Especially in laser components coated with anti-reflective layers, the interface between substrate and coating is one of the most limiting factors. This study investigates different precision cleaning processes and their influence on the laser resistance of ion-beam sputtered anti-reflective coatings. Therefore, a SiO2/Ta2O5 multilayer anti-reflective coating for a wavelength of 1064 nm and a normal angle of incidence was deposited onto high-quality fused silica substrates. Prior to deposition, the substrates were cleaned with various cleaning processes using different solutions and ultrasonic frequencies. To characterize the cleaned surface quality, the surfaces were analyzed with respect to root-mean-square (RMS) roughness and particle density. Laser damage was measured using a 1064 nm ns-pulsed laser test bench. It was found that an alcoholic pre-clean is recommendable to prevent laser damage caused by organic films remaining from the polishing process. The applied ultrasonic frequencies strongly influenced the particle density down to the sub-micrometer range and in consequence, the laser-induced damage threshold (LIDT). Ultrasonic cleaning at excessive power levels can reduce laser resistance.
1 WZW Optic AG, Wegenstrasse 18, 9436 Balgach, Switzerland
2 Institute of Micro Technology and Photonics (IMP), OST—Eastern Switzerland University of Applied Sciences, Werdenbergstrasse 4, 9471 Buchs, Switzerland
3 UCM AG, Langenhagstrasse 25, 9424 Rheineck, Switzerland
We present our investigation on the crystallization of IBS HfO2 on (0001) SiO2. The crystallization was studied by in-situ XRD. The activation energy was 2.6±0.5 eV. The growth follows a two-dimensional mode. LIDT measurements (5000-on-1) with 10 ns pulses at 355 nm on 3QWT HfO2 layers shows that the crystallization leads to increase of the laser irradiation resistance. The 0%-LIDT of the as coated sample was 3.1 J/cm2 and increased to 3.7 J/cm2 after 5h @ 500°C.
1Empa, Center for X-ray Analytics, 8600 Dübendorf ZH, Switzerland
Influence of ultra-sonic frequency during substrate cleaning on the laser resistance of antireflection coatings
Thomas Gischkat,1 Daniel Schachtler,1 Zoltan Balogh-Michels,1 Roelene Botha,1,2 André Mocker,2 Bernd Eiermann,3 Sven Günther3,4
Published: 20. November 2019, Proc. SPIE 11173, Laser-induced Damage in Optical Materials 2019, 1117317.
Cleaning of substrates prior to optical coating is an important step in the manufacturing of high performance optical components. It is well known that the ultra-sonic frequency used during substrate cleaning has a strong influence on the quality of the cleaning process and the number of remaining particles on the surface. Therefore, we have investigated the influence of ultra-sonic frequency during substrate cleaning on the laser resistance of antireflection coatings. For this purpose, a SiO2 / Ta2O5 AR-coating for a normal angle of incidence at 1064 nm was deposited onto fused silica substrates. Prior to deposition, the substrates were cleaned with cleaning processes. The applied ultra-sonic frequencies were 40, 80, 120 and 500 kHz. After deposition the LIDT was measured using a 1064 nm ns-pulsed laser test bench. It turned out that the different ultra-sonic-cleaning processes have a strong influence on the number of remaining particles on the surface of the cleaned samples. The counted number of particles with sizes greater < 83 nm were between 1320 and 12 particles for the different applied ultra-sonic frequencies. In consequence the different cleaned and AR-coated samples show different laser damage behavior. Nevertheless the measured particle density does not totally explain the differences in laser resistance.
2NTB Interstate Univ. of Applied Sciences (Switzerland)
3WZW-Optic AG (Switzerland)
4Ultrasonic Cleaning Machines AG (Ultrasonic Cleaning Machines AG)
Investigating the Long-Term Stability of LiB3O5 (LBO) Frequency Conversion Crystals at 355nm using Photothermal Deflection and LIDT measurements
R. Botha, H. Cattaneo, M. Stahel, Th. Gischkat, I. Stevanovic, Z. Balogh-Michels, C. Ziolek
Published: in Laser Congress 2019 (ASSL, LAC, LS&C), OSA Technical Digest (Optical Society of America, 2019), paper CM2C.4
In order to gain an understanding of the fluctuating long-term stability of LBO crystals, photothermal deflection measurements are used to identify absorbing impurities and defects, followed by multiple pulse LIDT testing to identify possible correlations.
Single layer antireflection coatings (SLAR) consisting of nanoporous silica (NP SiO2) films are developed by selective chemical etching of atomic layer deposited (ALD) Al2O3:SiO2 composite films. The reflective index of the final NP SiO2 film is finely adjusted from 1.132 to 1.400 at 600 nm wavelength by applying an appropriate ratio in the composite. To meet the requirements of the SLAR coatings from the deep UV (DUV) to the near IR (NIR) spectral range, the film thickness is controlled with nanometer precision by the ALD process. The SLAR are simultaneously applied on both sides of flat or highly curved substrates. Transmittance values above 99.4% are achieved even at a wavelength of 193 nm on fused silica substrates. Various characterization methods demonstrate the advantages of these SLAR with regard to impurities, optical losses, laser induced damage threshold (LIDT) properties, and surface super‐hydrophilicity. The absorption losses at 193 nm wavelength as determined by laser induced deflection measurements amount to approximately 200 ppm, and to approximately 2 ppm at a wavelength of 1064 nm, while the scattering losses are around 30 ppm at 532 nm wavelength for quarter‐wave layers. The LIDT values at 1064 nm are in the range of 93 J cm−2 being close to the values measured on the uncoated substrate.
The increasing variety of optical components and materials, combined with stricter surface tolerance requirements, necessitate refining existing polishing processes and developing innovative new polishing solutions and metrology technologies. A fast, reproducible laser polishing process would offer considerable economic benefits over conventional mechanical polishing processes and interest a broad variety of optics manufacturers. In this work, a holistic approach is taken to address the various aspects of glass polishing and form correction via a novel laser polishing system design, the use of a measurement strategy that can be integrated inline and simulation results that are correlated with process parameter studies for different materials.
Atomic layer deposition (ALD) enables coating complex shaped substrates with excellent uniformity along the surface of the optic. Recently developed nanoporous SiO2 layers have been applied as single layer antireflection coatings on fused silica substrates at both 1064 nm and 532 nm wavelengths. The LIDT in the nanosecond regime at both 1064 nm and 532 nm of these nanoporous SiO2 coatings as well as the bare substrates were investigated. The stability of the coatings with respect to LIDT has been evaluated under normal atmospheric conditions, dry air with relative humidity < 10% and nitrogen atmosphere. The multiple pulse damage characteristic for 5000 shots showed in all cases no significant pulse dependence. At 532 nm wavelength, the 0%-LIDT value is between 60 J/cm2 and 70 J/cm2, which is comparable to the values measured on uncoated substrates (80 J/cm2). In case of 1064 nm the 0%-LIDT is only between 40 J/cm2 and 50 J/cm2 (uncoated substrate: 100 J/cm2) which is attributed to generated defects during the fabrication process
Several studies have reported on the detrimental effects of inadequate cleaning on the performance of optical components exposed to laser radiation. The remaining particulates, contaminants or residue located in the coating or on the substrate surface can absorb laser energy and consequently induce damage. To minimize these contaminants, investigations of various cleaning processes have been performed in both the research and industry communities. Transparent published results and comparisons of the different cleaning processes considered, however, are limited due to proprietary considerations. In addition to this, the manufacturing environment, deposition processes, substrate- and deposition materials all have an influence on the effectiveness of a cleaning process. The purpose of this study was to investigate different cleaning procedures and their influence on the laser resistance of ion-beam sputtered antireflective coatings. For this purpose, a SiO2 / Ta2O5 multilayer antireflective coating for a normal angle of incidence at 1064 nm was deposited onto fused silica substrates. Prior to deposition, the substrates were cleaned with a variety of cleaning solutions and procedures and their roughness and surface quality inspected. All samples were characterized in terms of their laser damage threshold using a 1064 nm ns-pulsed test bench and subsequently visually inspected in order to understand the cause of the damage. In this work, the details of the cleaning steps and the corresponding laser damage performance for the different cleaning processes are presented and compared.
In this investigation the influence of the local environment on the laser damage threshold of anti-reflective coatings is reported. For this purpose, HfO2 / SiO2 anti-reflective coatings were deposited on fused silica substrates using an ionbeam sputter system. Laser damage threshold measurements were performed using two test procedures, S-on-1 and Ron- 1, at 355 nm for temperatures ranging from room temperature up to 250 °C and in different atmospheres. The two test procedures had comparable LIDT results with a possible pre-conditioning effect evidenced by a broadening of the transition range of the R-on-1 measured samples. It was found that samples measured in normal atmospheric air showed superior laser resistance compared to samples measured under nitrogen purge or in dry air. Samples measured in normal atmospheric air also showed a temperature dependence with an improved laser resistance at 25 °C. No temperature dependence was observed for samples measured under nitrogen purge or in dry air. In this paper, literature showing similar effects is reviewed and the influence of a water epilayer on the coating as a possible cause for the observed results is discussed.
Optical coatings used in ultraviolet applications are often exposed to harsh environments operating at elevated temperatures. In order to study the impact of the ageing effects optical coatings experience at various operating temperatures, an ultraviolet laser-induced degradation test system has been developed. It allows for flexible use in both a long-term stability test bench as well as in an LIDT measurement system. This work contains the preliminary results of optical degradation tests at 355 nm performed on anti-reflective coatings. As a subsequent step, the LIDT of the samples were measured using a Q-Switched Nd:YAG laser operating at 1064nm.
Comparative study of the laser damage threshold and optical characteristics of Ta2O5-SiO2 multilayers deposited using various methods
R. Botha, S. Schwyn Thöny, M. Grössl, S. Mourad,C. Maissen, J.I. Venter, Th. Südmeyer, M. Hoffmann, P.V. Bulkin, S. Linz-Dittrich, D. Bischof, M. Michler, S.J. Rinner, A. Ettemeyer
Published: 23. November 2015
Manufacturing processes from the private and academic sectors were used to deposit anti-reflective and high-reflective coatings composed of Ta2O5 - SiO2 multilayers. Used deposition techniques included three Ion Assisted Deposition (IAD) systems and an Ion Beam Sputtering (IBS) system. Coatings were performed on fused silica (Corning 7980) substrates polished by two different suppliers. LIDT Measurements were performed using a Q-Switched Nd:YAG laser operating at 1064nm. The paper presents a comparison of the coatings in terms of laser damage threshold values, optical properties and surface quality.
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