Reactive Low Voltage Ion Plating Research Articles

Laser-induced damage of optical thin films submitted to 343, 515, and 1030 nm multiple subpicosecond pulses

Optical materials submitted to multiple subpicosecond irradiations are known to exhibit a decrease of the laser-induced damage threshold (LIDT) with the applied number of pulses, an effect referred to as “fatigue” or “incubation.” In this work, we experimentally investigate this effect for the case of optical thin films submitted to multiple exposures with 500 fs pulses at different wavelengths: 1030, 515, and 343 nm. Niobia, hafnia, and silica films made with dense coating techniques (magnetron sputtering, ion-assisted deposition, and reactive low voltage ion plating) are studied, as well as the surface of a fused silica substrate. These samples have been exposed to different pulse numbers (from 1 to 100,000) at a low-frequency repetition rate (less than 1 kHz) and the LIDT has been measured. The results reveal the differences between materials and for the various wavelengths such as the decrease rate of the LIDT or the stabilization level that is reached after multiple exposures. All the results evidence the role of native- and laser-induced defects that we discuss on the basis of published works on the subject.

ITO-coatings by reactive low-voltage ion plating: film properties and plasma analysis

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Investigation of nanodefect properties in optical coatings by coupling measured and simulated laser damage statistics

We propose a model to link laser damage initiator properties (such as nature, size distribution, and density) to measured laser damage probabilities in optical materials. The model is based on the calculation of light absorption in nanoabsorbers and subsequent heating, coupled to laser damage statistics, and allows to obtain the laser damage probability as a function of laser fluence. Applications to the case of optical coatings irradiated in the nanosecond regime are presented. Laser damage probability curves are measured in hafnia single layer coatings made under different conditions: electron beam deposition and reactive low voltage ion plating. By studying the influence of the laser irradiation parameters (wavelength and beam size) and coating properties on the simulations, we show with our methodology that initiating defects (hafnium inclusions) can be identified. The implications of this approach for physical understanding and metrology applications are discussed.

Laser damage resistance of hafnia thin films deposited by electron beam deposition, reactive low voltage ion plating, and dual ion beam sputtering

A comparative study is made of the laser damage resistance of hafnia coatings deposited on fused silica substrates with different technologies: electron beam deposition (from Hf or HfO(2) starting material), reactive low voltage ion plating, and dual ion beam sputtering. The laser damage thresholds of these coatings are determined at 1064 and 355 nm using a nanosecond pulsed YAG laser and a one-on-one test procedure. The results are associated with a complete characterization of the samples: refractive index n measured by spectrophotometry, extinction coefficient k measured by photothermal deflection, and roughness measured by atomic force microscopy.

Overview about the optical properties and mechanical stress of different dielectric thin films produced by reactive-low-voltage-ion-plating

Thin films of Ta 2O 5, Nb 2O 5, and HfO 2 were deposited by reactive-low-voltage-ion-plating (RLVIP) on unheated glass and silicon substrates. The film thickness was about 200 nm. Optical properties as well as mechanical film stress of these layers were investigated in dependence of various deposition parameters, i.e. arc current and oxygen partial pressure. For an arc current in the range between 40 and 50 A and an oxygen partial pressure of at least 11 · 10 − 4 mbar good results were obtained. The refractive index and film thickness were calculated from spectrophotometric transmission data using the Swanepoel theory. For example at 550 nm wavelength the refractive index for thin RLVIP–Nb 2O 5-films was found to be n 550 = 2.40. The optical absorption was obtained by photo-thermal deflection spectrometry. For the investigated materials absorption coefficients in the range of k = 5 · 10 − 4 at 515 nm wavelength were measured. The mechanical film stress was determined by measuring the difference in bending of silicon substrates before and after the deposition process. For dense films, i.e. no water vapour sorption on atmosphere, the mechanical film stress was always compressive with values of some hundred MPa. In case of films deposited with higher arc currents ( I arc > 60A) and lower oxygen pressure (< 15 · 10 − 4 mbar) the influence of a post deposition heat treatment at 350 °C for 4 h on air was also investigated. For these films the properties could clearly be improved by such treatment. However, by using lower arc currents and higher oxygen partial pressure during the ion plating process, immediately dense and environmental stable films with good optical as well as mechanical properties could be achieved without post deposition heat treatment. All the results obtained will be presented in graphs and diagrams.

Optical and Structural Characteristics of Ion-Plated Nb2O5 and HfO2 Films

Dielectric thin films (Nb2O5, HfO2) were deposited on unheated silicon and glass substrates using reactive low voltage ion plating (RLVIP). Film refractive indices of n550 = 2.39 for Nb2O5 and n550 = 2.13 for HfO2 were determined between an arc current range of 40 and 60 A. Optical absorption coefficients in the low 10−4-range could be obtained for relatively high oxygen partial pressures. The film density was measured by XRR and yielded values higher than the bulk material. Topography and roughness investigations indicated nearly atomical flat film surfaces. XRD and TEM analyses of the film structure showed the presence of fine grained polycrystalline HfO2 films while the deposited Nb2O5 films remained amorphous.

Laser damage resistance of silica thin films deposited by Electron Beam Deposition, Ion Assisted Deposition, Reactive Low Voltage Ion Plating and Dual Ion Beam Sputtering

The laser damage resistance of optical coatings is a key point for a large number of applications. The aim of this work is to test and analyze the laser damage resistance of a thin film material commonly used for high power applications (SiO 2) and deposited with different techniques: Electron Beam Deposition, Ion Assisted Deposition, Low Voltage Reactive Ion Plating and Dual Ion Beam Sputtering. The laser damage thresholds of these coatings were determined at 1064 nm and 355 nm using nanosecond pulsed YAG lasers, with a 1-on-1 test procedure. The results are then compared and discussed: we found different behaviours that we link to laser damage initiators of different nature and densities. The dense thin films (obtained with ion assistance) were found to be highly resistant to laser damage.

Deposited indium-tin-oxide (ITO) transparent conductive films by reactive low-voltage ion plating (RLVIP) technique

Indian-tin-oxide ITO transparent conductive films were deposited by the Reactive low-voltage ion plating (RLVIP) technique. CO 2 and O 2 gases were used as reactive gases coming into the vacuum. The effect of the CO 2 gas flow rate on the optical and electrical properties of the ITO films was studied in detail. The results showed that the CO 2 gas had similar oxidizing action to O 2 except for the magnitude of the flow rate. But, the conductivity of ITO films were not sensitive to the CO 2 gas flow rate over a wide region in contrast to the results to O 2. The optical transmission in the visible spectral region as well as the infrared reflectivity of ITO films varied as the CO 2 flow rate changed. Thus, a better quality ITO film was obtained by the RLVIP technique with reactive gas of 30 sccm CO 2, having of more than 90% transmittance, above 80% reflectance in long-wave IR and 28.7 Ω/□ sheet resistance.

Plasma diagnostic of ion and plasma PVD processes

The aim of this work was to compare different plasma PVD processes in respect to their plasma properties in dependence on the process parameters. The processes under investigation were reactive d.c. pulsed magnetron sputtering magnetically assisted, arc source ion plating, reactive multi plasma supported d.c. magnetron sputtering and reactive low voltage ion plating. The experiments were carried out in specially designed plants under variable vacuum and plasma conditions. The plasma properties, like ion mass, ion energy distribution, ion and electron densities of the different processes were investigated by mass spectroscopy with additional energy distribution analysis (PPM421-Inficon) and a Langmuir probe system (Smart Probe-Scientific Systems). The magnetically forced and medium frequency pulsed biased d.c. magnetron sputter deposition variants, showed a relatively large amount of single and double charged positive ions with kinetic energies up to 55 and 95 eV, as consequence of the applied modifications. The arc source process, even when performed at high pressures of approximately 10 −1 mbar, showed a remarkable amount of ions with energies up to 75 eV.

Gas pressures influence on the optical and mechanical properties of Ta2O5 films produced by reactive low voltage ion plating (RLVIP)

Ta2O5 films were deposited onto unheated glass substrates by reactive low voltage ion plating (RLVIP). From the amorphous films with thicknesses between 200 and 300 nm the optical properties (refractive index n, absorption coefficient k) and the mechanical properties (relative density ρ and intrinsic stress σ) were investigated in dependence on the relation between working gas pressure (Ar) and the reactive gas pressure (O2). The films had k-values lower then 4×10−4 at 550 nm at a gas pressure composition of 0.8×10−3 mbar Ar and 2.0×10−3 mbar O2. With low total pressure high refractive indices, high compressive film stress and a high relative film density were found all decreasing with increasing gas pressure. A post deposition heat treatment at 350°C on air during 4 h resulted in a remarkable decrease in the refractive index and the film stress.

Optical coatings deposited by ion and plasma PVD processes

Abstract Evaporation and sputter deposition and a considerable number of technologically innovative variants of these basic processes are today available for thin film deposition by PVD. Particularly their reactive modes for stoichiometric compound film formation are important. In industrial film manufacture, however, only a limited but clearly growing number of them is in use. The reasons are mainly problems with economy and reliability. Ion and plasma processes with controlled kinetic particle energy of some tens of electron volts are often preferred in reactive evaporative deposition for densification of the layers and improvement in adherence, hardness, abrasion resistance and stoichiometry. Ion beam assisted deposition with various broad-beam ion sources, ion plating, particularly the reactive low-voltage ion plating process, and more recently plasma assisted evaporative coating techniques, as the advanced plasma source process are used mainly for the production of various environmental stable dielectric multilayer products in instrument optics and ophthalmics. For extremely low-loss film products as for instance durable, all-dielectric mirrors, ion beam sputtering is today the most advanced technology. In industrial film production standard bell jar and box coating plants are more and more replaced by modern fully automatic production systems which are designed exactly for the applied process including layer type and layer sequence, and the shape, size and number of the parts to be coated. High rate sputter technologies replace to an increasing degree the traditional evaporative deposition for metal mirror fabrication. New technological solutions as for instance the medium frequency powered twin-magnetron technique are available to overcome troubles in reactive mode DC-sputtering, so that reliable sputtering of dielectric films can be performed in production with improved manufacturing efficiency. Large area coating onto rigid substrates, e.g. for the production of transparent conductive films for liquid crystal display applications, and for the production of various architectural window coatings, is a domain for sputtering and advances assure that it will continue into the future.

Mechanical stress in thin SiO 2 and Ta 2O 5 films produced by reactive-low-voltage-ion-plating (RLVIP)

SiO 2 and Ta 2O 5 films on unheated glass substrates deposited by reactive low voltage ion plating (RLVIP) have high compressive stresses of more than 1.2 GPa for film thicknesses < 100 nm. With increasing film thickness to 700 nm the stresses decrease exponentially to 0.7 GPa for SiO 2 and 0.55 GPa for Ta 2O 5. Increasing the ion current density to 0.45 mA cm −2 during film deposition while maintaining (gas pressure and gas composition) increases the number (ion—molecule-reactions in the plasma) of energetic particles, increase the film density but also the compressive film stress. However, while maintaining stoichiometry parameter sets could be found for dense and therefore stable films with acceptable low compressive stress.

Mechanical Properties of Optical Dielectric Thin Films Deposited by the Ion Plating Technique

Stress in thin films deposited by Reactive Low-Voltage Ion Plating (RLVIP) is studied in air and at room temperature. A multilayer stack, composed of tantalum pentoxide and silicon dioxide layers, is considered and the interactions layer to layer turn out to have no effect as regards to the final bending. The evolutions of plate bending after annealing show the possibility to reduce the stress as well as the absorption for tantalum pentoxide thin films. Finally, ion implantation, with ions such He+ or Xe+ accelerated at energy in the MeV range, proves to be also a way to vary and reduce the stress in thin films.

Reactive low voltage ion plating of aluminium nitride films and their characteristics

By means of reactive low voltage ion plating, AlN films were deposited onto unheated BK-7 glass, fused silica, and Si wafers. The obtained films were investigated mainly with respect to possible optical applications. Refractive index and absorption values were determined from spectrophotometric measurements. Hexagonal film structure, high density and smooth film surface and bulk film morphology were found by electron optical investigations. Mechanical characterization showed high hardness, good adherence to glass and compressive intrinsic film stress.

Comparative photothermal study of reactive low-voltage ion-plated and electron-beam-evaporated TiO_2 thin films

The thickness-dependent optical and thermal properties and the corresponding damage thresholds have been investigated by means of various photothermal techniques on titanium dioxide thin films prepared by the conventional techniques of reactive electron-beam evaporation and reactive low-voltage ion plating (RLVIP). Compared with the reactive-electron-beam-evaporated samples, the RLVIP films exhibit a higher absorption, lower damage threshold, better thermal conductivity, lower defect density, and an almost perfect stability under Ar(+)-laser irradiation. These results are correlated with data from a multimethod approach, and a mechanism is proposed to explain the low damage threshold for the RLVIP TiO(2) films.

Influence of Aging Processes on Adhesion Properties of Metallic and Oxidic Thin Films

AbstractAdhesion of oxidic and metallic films with thicknesses between 40 nm and 350 nm has been investigated by means of a scratch-test method based on a vibrating diamond micro-indenter. SiO2 and TiO2 films are precipitated on fused silica substrates by sol-gel techniques (SG), reactive evaporation (RE), reactive low-voltage ion plating (IP), and plasma impulse chemical vapour deposition (PICVD), and Cr-layers are produced by rf magnetron sputtering (SP). The influence of aging effects on film adhesion is investigated in respect of several conditions, like storage under ambient surroundings with differences in relative humidity or temperature treatment. A method is presented allowing control of the long-term stability of the scratch-test conditions. Temperature treatment up to 600 °C is found to have the most significant impact on adhesion properties. Adhesion increases for some SiO2 films, whereas for TiO2 layers an opposite behavior is observed. Here, the adhesion of the originally amorphous titania films is reduced due to the phase transition to polycrystalline anatase, which correlates with a significant increase in film stress.

Optical waveguide characterization of dielectric films deposited by reactive low-voltage ion plating

We determine the quality of single films of various oxides, which are deposited on thermally oxidized silicon wafers by reactive low-voltage ion plating (RLVIP), by measuring their optical waveguide losses. We use a prism coupler for inserting the radiation of a wavelength-selectable He-Ne laser into the waveguide and a CCD camera for imaging the light scattered from the surface of the films. The waveguide losses of the RLVIP films are typically of the order of 1 to 10 dB/cm. Some data obtained for TiO(2) layers on thermally grown SiO(2) and RLVIP SiO(2) seem to confirm the presence of an absorbing boundary layer between RLVIP SiO(2) and TiO(2) that has been found in SiO(2)-TiO(2) multilayers. The waveguide measurements also reveal unusual index gradients in thick (~ 10 µm) single layers of Al(2)O(3) derived from multimode effective index calculations.

Comparative study of titanium dioxide thin films produced by electron-beam evaporation and by reactive low-voltage ion plating

Titanium dioxide (TiO(2)) is often used as a high refractive-index material for multilayer optical coatings. However, the optical properties of TiO(2) films depend strongly on the deposition process and its parameters. A comparative study of TiO(2) films fabricated by conventional electron-beam evaporation and by reactive low-voltage ion plating that uses different phases of Ti-O as starting materials is reported. Results on the variability of TiO(2) thin films are analyzed in relation to process parameters. The potential of fabricating high and low refractive-index multilayer stacks with TiO(2) only, by employing two different deposition processes, is presented with a practical example.

Emission spectroscopy of reactive low-voltage ion plating for metal-oxide thin films

Excited species in the plasma present during reactive low-voltage ion plating (RLVIP) of refractory oxide films are studied by using emission spectroscopy. We believe we have found a higher ratio of atomic to molecular oxygen then reported for earlier analyses that employed a quadrupole mass spectrometer. The application of emission spectroscopy to the RLVIP process and self-actinometry are discussed with examples of stable and unstable processes.

Critical field patterns in optical Fibonacci multilayers

Optical coatings with 13, 34, 55, and 89 quarter-wave layers of high- and low-index dielectric materials arranged in Fibonacci sequences F6, F8, F9, and F10, respectively, were deposited with use of a reactive low-voltage ion plating process. The calculated average transmission (Tave) for ϕ = (1/2 ± 1/4)π follows a power law Tave ≈ Nm−δ, where Nm is the number of optical quarter-wave layers in a Fibonacci sequence Fm. The critical exponent δ for these multilayers is compared with a renormalization calculation of the exponent governing the total energy width for an electronic system of Fibonacci layers. Spectral-transmittance measurements of the experimental Fibonacci multilayers show some deviation from the calculated spectral transmittance, which is due primarily to absorption in the short-wavelength range, but these measurements do still indicate that the observed optical field patterns are critical.