Ion-assisted Deposition Process Research Articles

Investigation on nano-absorbing precursors in the picosecond regime via the comparative study of HfO2/SiO2 and Ta2O5/SiO2 high-reflectivity coating damage

Laser-induced damage to the final reflective and diffractive optics limits the total output energy of petawatt laser systems with pulse durations ranging from a few hundred femtoseconds (fs) up to a few tens of picoseconds (ps). In this study, the laser damage to HfO2/SiO2 and Ta2O5/SiO2 multilayer dielectric high-reflectivity (HR) coatings induced by a 1053 nm laser with a pulse width of 8.6 ps was studied to investigate the nano-absorbing precursors in ps regimes. The HfO2/SiO2 HR coating exhibited stronger laser resistance than the Ta2O5/SiO2 HR coating. Flat-bottom pits, pinpoints, and funnel pits were the three typical damage morphologies for the experimental HR coatings. The damage to the HfO2/SiO2 HR coating was primarily dominated by flat-bottom pits, whereas dense pinpoints were the most significant damage for the Ta2O5/SiO2 HR coating. The nano-absorbing precursors introduced by the ion-assisted deposition process were proved to be the damage precursors that trigger pinpoints under a strong electric field intensity (EFI). The nano-absorbing precursors located in the second EFI peak of the SiO2 top layer induced the funnel pits. The funnel pits were expected to be the previous stage of the flat-bottom pits. After they grew along the upward-sloping crack and separated from the interface, the flat-bottom pits were formed. In addition, poor-binding interfaces promoted the formation of flat-bottom pits.

Solar Coatings Based on Ag Infrared Reflector with High Stability at Medium and High Temperature

The manufacturing of thermally stable solar coatings with high photo-thermal performance represents a key factor for the further deployment of the CSP technology. Since 2005, ENEA has been developing solar coatings suitable for medium and high temperature applications based on the technology of double nitride cermet, by employing silver and tungsten as infrared reflectors, respectively. Thanks to the high infrared reflectance of silver, the corresponding coatings have better optical performance than those with tungsten; however, the high diffusivity of silver compromises its use at high temperature. In order to improve the structural and chemical stability at medium and high temperature of coatings based on silver, this infrared reflector was placed between compact and uniform layers of metal and cermet manufactured by using high-energy and fast deposition processes. In particular, an Unbalanced Magnetron cathode was adopted to promote an ion-assisted deposition process that improved uniformity and compactness of the metal and cermet films. The new coating shows no photo-thermal parameters degradation after 25 years of service at the operating temperature of 400 °C, while its photo-thermal conversion efficiency decreases by only 1.5% after 25 years of service at an operating temperature of 514 °C.

Concepts for in situ characterization and control of plasma ion assisted deposition processes

In this paper recent results on monitoring and control of plasma ion assisted deposition (PIAD) processes are presented. An industrial PIAD system equipped with diagnostics for deposition rate and optical thickness was augmented by active plasma resonance spectroscopy to access electron density near the substrate. A series of experiments implementing different schemes to control the level of plasma assistance, while depositing a model 5-layer quarterwave stack SiO2/TiO2, were conducted. Data on in situ spectral transmission of the coatings was used to reconstruct optical properties and the thickness of each layer. Conventional control of anode voltage of the assist source Advanced Plasma Source (APS) was found to exhibit remarkable variations of electron density as drifts during deposition and shifts between separate runs. We demonstrate that measuring the electron density and using that signal in the feedback loop control of the APS greatly improves the layer-to-layer repeatability and run-to-run reproducibility of coating performance. Best results were obtained combining electron density control with optical monitoring for endpoint detection.

An inverted gapped-target sputter magnetron for the deposition of thin ferromagnetic films

An inverted gapped-target magnetron sputtering device has been developed for deposition of ferromagnetic thin films under energetic conditions. The main feature of this technique is that the discharge is driven by a positive voltage relative to a grounded target situated away from the magnetic field. As a result, it is possible to efficiently sputter ferromagnetic materials and have a source of energetic plasma ions available in the ion-assisted deposition process at the substrate. Electrical probe measurements have shown that the density and the most likely energy of the ions arriving to the grounded substrate is approximately 1016 m−3 and 200 eV, respectively. In the sputtering of nickel films, no cracking of the film surfaces was observed, without substrate heating or biasing. The energetic ions available with this inverted configuration are thought to be the main contributor to the observed enhancement in overall film qualities.

Effect of boundary continuity on nanosecond laser damage of nodular defects in high-reflection coatings.

Two types of engineered nodules in Ta2O5/SiO2 high-reflection coatings were prepared using electron beam evaporation and an ion-assisted deposition processes to facilitate poor and good boundary continuity, respectively. The influence of nodular boundary continuity on their nanosecond laser damage characteristics was investigated through experimental studies, combined with 3D finite-difference time domain simulations and photo-thermal micro-characterizations. Better boundary continuity led to improved mechanical stability and higher ejection fluence of nodules, in accordance with the thermomechanical damage model. In contrast, the ejected nodules that initially had better boundary continuity exhibited higher localized absorption and lower damage growth fluence, which is attributed to the creation of mechanically induced electronic defects or stronger electric field intensity enhancement at the ejected nodules.

Mechanically Robust Plasma-Activated Interfaces Optimized for Vascular Stent Applications.

The long-term performance of many medical implants is limited by the use of inherently incompatible and bioinert materials. Metallic alloys, ceramics, and polymers commonly used in cardiovascular devices encourage clot formation and fail to promote the appropriate molecular signaling required for complete implant integration. Surface coating strategies have been proposed for these materials, but coronary stents are particularly problematic as the large surface deformations they experience in deployment require a mechanically robust coating interface. Here, we demonstrate a single-step ion-assisted plasma deposition process to tailor plasma-activated interfaces to meet current clinical demands for vascular implants. Using a process control-feedback strategy which predicts crucial coating growth mechanisms by adopting a suitable macroscopic plasma description in combination with noninvasive plasma diagnostics, we describe the optimal conditions to generate highly reproducible, industry-scalable stent coatings. These interfaces are mechanically robust, resisting delamination even upon plastic deformation of the underlying material, and were developed in consideration of the need for hemocompatibility and the capacity for biomolecule immobilization. Our optimized coating conditions combine the best mechanical properties with strong covalent attachment capacity and excellent blood compatibility in initial testing with plasma and whole blood, demonstrating the potential for improved vascular stent coatings.

Compositionally-graded silicon–copper helical arrays as anodes for lithium-ion batteries

Restrictions in silicon based anodes have been the subject of many researches for years. As an innovative approach, we have adopted ion assisted deposition technique to glancing angle deposition method and have used compositionally-graded structuring. A unique helical shaped gradient film has been produced in which the Cu/Si atomic ratio decreases from the bottom to the top of the coating. With such a unique film (high surface area) more spaces have been created promoting mechanical integrity and reaction between active materials (silicon) with lithium ions. The highly adherent film is formed as a result of ion assisted deposition process and the gradual change in Cu/Si atomic ratio diverts stress through the helices. To compare the performance of the SiCu electrode, a pure Si film is deposited in the same experimental condition. Galvanostatic test results show that although the film with pure Si helices fails after 30th cycles, the compositionally graded anode exhibits a capacity of 1228 mAh g−1 at the 100th cycles with 99.5% coulombic efficiencies when cycled at 100 mA g−1, and delivers 815 mAh g−1 when cycled with a rate of 400 mA g−1.

An Effective Thermal Splicing Method to Join Fluoride and Silica Fibers for a High Power Regime**Supported by the National Natural Science Foundation of China under Grant No 61275144, the Innovative Research and Development Project of Nanshan District under Grant No KC2013JSCX0013A, the Shenzhen Science and Technology Innovation Projects under Grant No JCYJ20150324140036862, and the Funding from Shenzhen University under Grant

A repeatable and simple thermal splicing method for low loss splice between fluoride and silica fibers is presented. The minimum splicing loss of 0.58 dB is achieved experimentally with this approach. Meanwhile, the power capacity of this splicing joint is also tested with a high power fiber laser. The maximum input power is up to 15 W, only limited by the available power of the laser source. To the best of our knowledge, this is the first report on thermal splicing between fluoride and silica fibers operating in a high power regime without any complicated ion-assisted deposition process.

Reactive ion-assisted deposition of e-beam evaporated titanium for high refractive index TiO2 layers and laser damage resistant, broad bandwidth, high-reflection coatings

High-reflection coatings with broad bandwidth can be achieved by pairing a low refractive index material, such as SiO2, with a high refractive index material, such as TiO2. To achieve high refractive index, low absorption TiO2 films, we optimized the reactive, ion-assisted deposition process (O2 levels, deposition rate, and ion beam settings) using e-beam evaporated Ti. TiO2 high-index layers were then paired with SiO2 low-index layers in a quarter-wave-type coating to achieve a broader high-reflection bandwidth compared to the same coating composed of HfO2/SiO2 layer pairs. However, the improved bandwidth exhibited by the TiO2/SiO2 coating is associated with lower laser damage threshold. To improve the laser damage resistance of the TiO2/SiO2 coating, we also created four coatings where HfO2 replaced some of the outer TiO2 layers. We present the laser damage results of these coatings to understand the trade-offs between good laser damage resistance and high-reflection bandwidth using TiO2 and HfO2.

Process diagnostics and monitoring using the multipole resonance probe in an inhomogeneous plasma for ion-assisted deposition of optical coatings

The application of a multipole resonance probe (MRP) for diagnostic and monitoring purposes in a plasma ion-assisted deposition (PIAD) process is reported. Recently, the MRP was proposed as an economical and industry compatible plasma diagnostic device (Lapke et al 2011 Plasma Sources Sci. Technol. 20 042001). The major advantages of the MRP are its robustness against dielectric coating and its high sensitivity to measure the electron density. The PIAD process investigated is driven by the advanced plasma source (APS), which generates an ion beam in the deposition chamber for the production of high performance optical coatings. With a background neutral pressure of p0 ∼ 20 mPa the plasma expands from the source region into the recipient, leading to an inhomogeneous spatial distribution. Electron density and electron temperature vary over the distance from substrate (ne ∼ 109 cm−3 and Te,eff ∼ 2 eV) to the APS (ne ≳ 1012 cm−3 and Te,eff ∼ 20 eV) (Harhausen et al 2012 Plasma Sources Sci. Technol. 21 035012). This huge variation of the plasma parameters represents a big challenge for plasma diagnostics to operate precisely for all plasma conditions. The results obtained by the MRP are compared to those from a Langmuir probe chosen as reference diagnostics. It is demonstrated that the MRP is suited for the characterization of the PIAD plasma as well as for electron density monitoring. The latter aspect offers the possibility to develop new control schemes for complex industrial plasma environments.

Numerical study of the characteristics of the ion and fast atom beams in an end-Hall ion source

An end-Hall ion source is a cylindrical magnetized device of few centimeters in length able to generate an ion beam with a current of typically 1 A and ion energies in the range of 100 eV. This ion source does not use acceleration grids, has a relatively large ion beam divergence, and is well suited for ion assisted deposition processes. In this paper, a self-consistent two-dimensional quasi-neutral model of an end-Hall ion source is used to understand the parameters controlling the characteristics of the extracted. The model results underline the role of charge exchange collisions on beam properties. The calculated energy distribution functions reveal the existence of groups of slow ions and fast neutrals. Ion mean energy corresponds to roughly 60% of the discharge voltage, while the root mean square deviation from the mean energy corresponds to about 33% of the discharge voltage, as in experiments. The influence of the position of the electron emitting source on the ion angular distribution is also shown.

Influence of ion assistance on LaF3 films deposited by molybdenum boat evaporation

LaF3 thin films at 193 nm were deposited by the molybdenum boat evaporation with ion-assisted deposition (IAD). Various optical characteristics, stress, and microstructures that formed under different ion-beam voltages of IAD deposition were investigated. The relation between these properties is also discussed. LaF3 films deposited with IAD exhibited small rough surfaces and large optical loss at 193 nm. The largest value of optical loss for films at 193 nm, which were prepared at an ion-beam voltage of 400 V, was 1.55% and the extinction coefficient was smaller than 0.0015. Microstructures and crystalline structures of films were influenced and changed by the ion-assisted deposition process. Tensile stress value of films increased as the ion-beam voltage rose. Refractive index, related to the packing density and microstructures, also increased as the ion-beam voltage rose.

Surface characterization and properties of silicon nitride films prepared by ion-assisted deposition

Silicon nitride (SiN) films had been prepared at low substrate temperature (100 °C) using the ion-assisted deposition (IAD) process. The films had been analyzed by the measurement of X-ray diffraction, atomic force microscopy, Fourier transform infrared spectrometry, nano indenter, and ellipsometry. The effects of N-ion current density on the surface morphology, compositional, mechanical, and infrared optical properties of SiN thin films were investigated. The results showed that the stoichiometric Si3N4 thin film with desirable properties, such as continuous and smooth surface morphology, extremely low hydrogen content, mechanical strong, and low extinction coefficient, could be obtained by using the IAD technique.

Optical and structural properties of silicon nitride thin films prepared by ion-assisted deposition

Silicon nitride (SiN x ) films were prepared by ion-assisted deposition process. The films were analyzed by measurement of scanning electron microscopy (SEM), Fourier transform infrared spectrometry (FTIR), X-ray photoelectron spectrometry, spectrophotometer, and ellipsometer measurements. The effects of N-ion current density ( J) on the structural and optical properties of SiN x thin films were investigated. The SEM studies revealed that films deposited at a higher value of J had highly condensed structure. FTIR analysis indicated an extremely low hydrogen content in the IAD films. It was found that the N/Si ratio of films increased when J increased . The optical constants and the optical band gap of films were determined. The stoichiometric Si 3N 4 thin film was obtained with a high refractive index of 2.073 (at a wavelength of 400 nm), an extinction coefficient less than 6 × 10 −4, and an optical band gap of 4.57 eV.

Optical and structural properties of SiO x films from ion-assisted deposition

This study investigates the optical and structural properties of SiO x ( x ∼ 1) films prepared by an ion-assisted deposition (IAD) process. The films were prepared by evaporating silicon monoxide, with and without simultaneous Ar + bombardment. The stoichiometry of each film was determined as measured by the infrared spectrometry and X-ray photoelectron spectrometry. The variation in the stoichiometry revealed that the oxygen content of the SiO x thin films varied slightly under the different conditions of the Ar + bombardment. The results of the X-ray diffraction and transmission electron microscopy (TEM) measurements illustrated that all of the films had amorphous structures. However, a different interfacial appearance between the film and the substrate was observed from the TEM image. The optical constants of the SiO x thin films were determined by a spectroscopic ellipsometry. The extinction coefficient of all of the films approached zero in the infrared wavelength range from 2 to 7 μm, but the refractive index was varied by the IAD process. The variation of these refractive indices is mainly related to the packing density of the films.

AlF 3 film deposited by IAD with end-Hall ion source using SF 6 as working gas

A novel and effective process to fabricate high quality fluoride thin films was presented. Aluminum fluoride films deposited by a conventional thermal evaporation with an ion-assisted deposition (IAD) using SF 6 as a working gas at around room temperature were investigated. In this study, the optimal voltage and current, 50 V and 0.25 A, were found according to the optical properties of the films: high refractive index (1.489 at 193 nm), low optical absorption and extinction coefficient (<10 −4 at 193 nm) in the UV range. The physical properties of the film are high packing density and amorphous without columnar structure. It was proved that using SF 6 working gas in IAD process is a good choice and significantly improves the quality of AlF 3 films.

Influence and determinative factors of ion-to-atom arrival ratio in unbalanced magnetron sputtering systems

Low pressure sputtering with a controlled ratio of ion flux to deposited atom flux at the condensing surface is one of the main directions of development of magnetron sputtering methods. Unbalanced magnetron sputtering, by producing dense secondary plasma around the substrate, provides a high ion current density. The closed-field unbalanced magnetron sputtering system (CFUBMS) has been established as a versatile technique for high-rate deposition high-quality metal, alloy, and ceramic thin films. The key factor in the CFUBMS system is the ability to transport high ion currents to the substrate, which can enhance the formation of full dense coatings at relatively low value homologous temperature. The investigation shows that the energy of ions incidenced at the substrate and the ratio of the flux of these ions to the flux of condensing atoms are the fundamental parameters in determining the structure and properties of films produced by ion-assisted deposition processes. Increasing ion bombardment during deposition combined with increasing mobility of the condensing atoms favors the formation of a dense microstructure and a smooth surface.

Optical and structural properties of Ge films from ion-assisted deposition

The optical properties and microstructure of germanium (Ge) films, prepared by ion-assisted deposition (IAD) process, were investigated. The Ge films were deposited on sapphire and silicon substrates, with and without simultaneous Ar + bombardment. Higher index films, with a refractive index 7.7% larger than that of the single crystalline Ge wafer, were obtained with the IAD process. The density of the IAD film could be 1.5% greater than that of the e-beam film. The results of the heat treatment indicated that the optical and structural properties of the IAD films were more stable. Ge nano-crystallites could be observed under high ion power density, which induced a crystalline structure in the Ge thin films. The average size of the nano-crystallites, as determined from both the X-ray diffraction data and the transmission electron microscopy images, showed that no systematic change had occurred. The results presented in this work suggest that the optical and structural properties of Ge thin films can be effectively modified by the application of the IAD process.

Antireflection coatings on plastics deposited by plasma polymerization process

Antireflection coatings (ARCs) are deposited on the surfaces of optical elements like spectacle lenses to increase light transmission and improve their performance. In the ophthalmic industry, plastic lenses are rapidly displacing glass lenses due to several advantageous features. However, the deposition of ARCs on plastic lenses is a challenging task, because the plastic surface needs treatment for adhesion improvement and surface hardening before depositing the ARC. This surface treatment is usually done in a multi-stage process—exposure to energetic radiations, followed by deposition of a carbonyl hard coating by spin or dip coating processes, UV curing, etc. However, this treatment can also be done by plasma processes. Moreover, the plasma polymerization process allows deposition of optical films at room temperature, essential for plastics. The energetic ions in plasma processes provide similar effects as in ion assisted physical deposition processes to produce hard coatings, without requiring sophisticated ion sources. The plasma polymerization process is more economical than ion-assisted physical vapour deposition processes as regards equipment and source materials and is more cost-effective, enabling the surface treatment and deposition of the ARC in the same deposition system in a single run by varying the system parameters at each step. Since published results of the plasma polymerization processes developed abroad are rather sketchy and the techniques are mostly veiled in commercial secrecy, innovative and indigenous plasma-based techniques have been developed in this work for depositing the complete ARCs on plastic substrates.

Measurement of energy transfer at an isolated substrate in a pulsed dc magnetron discharge

The power density delivered by particles to an electrically isolated substrate in an asymmetric bipolar pulsed dc unbalanced magnetron has been quantified. The plasma source was operated in argon with a titanium target, and measurements were made using both a calorimeter probe and time-resolved Langmuir probe incorporated into a specially made substrate holder. The main results from the calorimeter probe show clearly that with increased pulse frequency (from dc to 350kHz) and reduced duty cycle (90%–50%), the particle power density (from ions, electrons, sputtered Ti, and backscattered Ar) at the substrate increases significantly. For instance, at 350kHz and 60% duty cycle, the total power density is 83mW∕cm2, about 60% higher than in dc mode for the same time-average discharge power. However, from an inventory of the individual particle contributions to the total power density derived from time-resolved Langmuir measurements and a simple model of the substrate sheath and plasma internal processes, we predict values of power density much lower than those measured. The measured and calculated values are in close agreement for the results obtained in dc mode but diverge at high frequencies. It is believed that this is due to the Langmuir probe measurements being unable to observe the presence of high-energy ions, created during the transient peaks in the electron temperature at the transitions from on off and off on [J. W. Bradley et al., Plasma Sources Sci. Technol. 11, 165 (2002)] which subsequently bombard the substrate. This paper shows conclusively the benefit of pulsing the magnetron over and above dc operation for enhancing the ion power per depositing neutral in the ion assisted deposition process.