Laser damage thresholds of optical coatings

Since the very beginning of laser technology, Laser Induced Damage Thresholds (LIDT) of optical components were always an obstacle for the application of laser systems operating at high power levels. Also, further progresses in the development of new high power laser concepts are often directly limited by the availability of advanced optical components with high quality and LIDT-values. Nowadays, in the course of the development of optical materials with excellent quality and power handling capability, the problem of laser induced damage has shifted from the bulk to the surface of the optical component. The optical surface is objected to various production steps and environmental influences, which modify its structure and composition. Especially, the thin film coating, which is deposited on the optical surface to adapt its reflectance and transmittance to the application, contributes predominantly to the reduction of the LIDT-values. As a consequence, the measurement and optimization of the power handling capability of thin films is considered as one of the primary research areas in modern optics technology and is supported by an extensive scientific community.In the present paper, a brief review will be given on selected fundamental damage mechanisms in thin films considering different operation conditions of modern laser systems. Also, the current standards for the measurement of LIDT will be described, and examples illustrating some practical aspects of high power optical coatings will be presented. Finally, recent trends in laser technology will be discussed in respect to research in laser induced damage.     

Vacuum properties of palladium thin film coatings

A recent development, carried out at CERN for particle accelerator applications, showed that a vacuum chamber coated with a thin getter film and then exposed to ambient air may be transformed into a pump by “in situ” heating at temperatures as low as 180°C.Heating activates the diffusion into the film of the oxygen present in the surface passivation layer. Repeated air exposure-activation cycles progressively enrich the film with oxygen, reducing its performance and shortening its operating life. To overcome this inconvenience, noble metal coatings were considered. At distinction with getters, noble metals may release all the pumped gases by heating, resulting in a practically unlimited life.Thin film coatings of palladium were studied by surface analysis, electron stimulated desorption and pumping speed measurements. These coatings were found to pump H₂ and CO, even without activation by heating, but not N₂ or CO₂. Thin Pd and Pd-Ag films were also used as overlayers for protecting a getter film from oxidation while not impairing its H₂ pumping.The result of these studies are presented and discussed.     

Tantalum-based thin film coatings for wear resistant arthroprostheses

Cobalt-chromium-molybdenum alloys with high carbon content (HC-CoCrMo) are widely used as materials for arthroprosthesis, in particular in metal-on-metal (MoM) hip joints. In spite of their good wear and corrosion resistance, production of metallic wear particles and metal ion release will occur on a large time-scale. An enhancement of the metal ion level in the patient's blood and urine is often reported in clinical data. Hypersensitivity, inflammatory response and cell necrosis can occur as consequence. So implants on young patients and women on childbearing age are not so widespread. The aim of this research is the realization of a thin film coating in order to improve the biocompatibility of Co-based alloys and to reduce debris production, ion release and citotoxicity. The innovative process consists of a thermal treatment in molten salts, in order to obtain a tantalum enriched thin film coating. Tantalum is chosen because it is considered a biocompatible metal with high corrosion resistance and low ion release. Three HC-CoCrMo alloys, produced by different manufacturing processes, are tested as substrates. The coating is a thin film of TaC or it can be composed by a multilayer of two tantalum carbides and metallic tantalum, depending on the temperature of the treatment and on the carbon content of the substrate. The thin films as well the substrates are characterized from the structural, chemical and morphological point of view. Moreover mechanical behaviour of treated and untreated materials is analyzed by means of nanohardness, scratch and ball-on-disc wear tests. The coating increases the mechanical and tribological properties of HC-CoCrMo.     

沉积硬质涂层技术的新发展

本文介绍了新型硬质涂层和硬质涂层沉积技术中电弧离子镀和磁控溅射镀技术的新进展。     

Laser damage of CdS and ZnS thin films

A method which has been used previously for three-dimensional space groups was applied to the determination of zero-slope points for all possible special wavevectors in the Brillouin zone for each of the plane groups. The results are tabulated for both non-magnetic and magnetic crystals and are relevant to electronic energy bands and to quasiparticle dispersion relations for surface states or thin film states. They enable one to identify the sources of peaks in density-of-states curves, and they also provide an aid to drawing energy bands or quasiparticle dispersion relations in the vicinity of wavevectors of special symmetry.     

Three dimensional imaging of deformation modes in TiN-based thin film coatings

A TiN thin film coating, approximately 4 μm in thickness, deposited on a ductile steel substrate, was subject to surface deformation via nanoindentation using a spherical indenter, 5 μm in radius, with loads up to 500 mN. Pop-ins were observed during loading, which are characteristic of the onset of cracking and the formation of shear steps at the coating-substrate interface. Focused ion beam microscopy was used to prepare cross-sections through the indentation that revealed the presence of both intercolumnar and inclined cracks. Three-dimensional reconstructions of the deformation zone beneath the indentation were performed using a dual-beam focus ion beam instrument. These constructions provided more detailed images of the morphology of cracks, which were observed to be consistent with theoretical models of plastic deformation of such brittle coatings.     

Dielectric reference coatings for the evaluation of thin film characterization techniques

Thin film technology has an increasing demand for industrial reliable characterization techniques. A precise absolute determination of layer thickness, interface width and the quantification of depth profiles in dependence on resolution limits of the measurement are required. Certified reference materials, certified reference coatings (CRCs) and non-destructive evaluation techniques can meet these requirements. Dielectric reference coatings (SiO₂, Si₃N₄) were used for metallographic preparation (e.g. bevelled cross-sections), optical characterization techniques (e.g. spectroscopic ellipsometry (SE)), and films of SiO₂, Si₃N₄ and Al₂O₃ were applied to reference measurements in depth profiling of layer stacks (e.g. radio frequency glow discharge optical emission spectroscopy). Thickness and refractive index of these dielectric single-and multilayer coatings on different substrate materials are accurately determined in advance by means of SE. These values are subsequently used for precise angle determination of bevelled cross-sections, for reference and re-calibration purposes in thin film characterization (system reproducibility) and in surface analysis (determination of sputter and erosion rates, depth profiles). Examples are discussed for different applications and the calculated data are compared with experimental results. It is shown that reproducible commercial coatings are also of importance for use as CRCs.     

Instrument for close-to-process light scatter measurements of thin film coatings and substrates

Scatter analysis is an effective method for the characterization of thin film components. The new highly sensitive table top system ALBATROSS-TT (3D-Arrangement for Laser Based Transmittance, Reflectance and Optical Scatter Measurement-Table Top) has been developed at the Fraunhofer Institute in Jena to meet the specific requirements for close-to-process applications. Extremely high sensitivity with a noise equivalent angle resolved scatter level of 2×10(-8)?sr(-1), full three-dimensional spherical measurement capability, and an instrument size as small as 0.8?m×0.8?m×0.8?m have been achieved. Details of specifications, optical components, and software are presented, including a comparison to our laboratory system. Anisotropy analysis of diamond-turned aluminum substrates as well as substrate and coating characterization are demonstrated as examples of application.     

Tensile and high cycle fatigue test of copper thin film

This paper presents the results of tensile and high cycle fatigue tests for copper thin film. Copper films coated by Sn are often used in various electro devices. Especially, when the film is used in tape carrier package (TCP), the film is repeatedly exposed to mechanical or(and) thermal stresses which results often in the failure of the component. Therefore, to guarantee the reliability of the electrical devices using a film, tensile and fatigue characteristics of the film are important. In this study, to obtain the tensile and fatigue characteristics of the film, the specimen was fabricated by an etching process to make a smooth specimen of 2000 μm width, 8000 μm length and 15.26 μm thickness. The tensile and high cycle fatigue tests were performed with the specimen using the test machine developed by the authors. These specimens had measured values of Young's modulus (72 GPa) and a 0.2 % offset yield and an ultimate strength of 358 MPa and 462 MPa, respectively. A closed‐loop feedback control of the magnetic‐electric actuator allowed load‐controlled fatigue tests with 20 Hz frequency, in ambient environment and at two levels of mean stresses. The fatigue strength coefficient and exponent at 0.5 times of the ultimate tensile strength were 431 MPa and ‐0.0843, respectively. The fatigue strength coefficient and exponent at 0.6 times of the ultimate tensile strength were 371 MPa and ‐0.0923, respectively. The Goodman method is recommended when the fatigue life of thin film with mean stress will be estimated. The fatigue strength coefficient and exponent of copper thin film modified using Goodman method were 910 MPa and ‐0.0896, respectively.     

用中频溅射沉积的大面积SiO2膜层损伤机制的探讨

大面积SiO2厚膜沉积可创造很高的附加值，但在规模生产中会出现严重的膜层损伤。本文针对这一现象进行机理探讨。     

Laser transfer processing for the integration of thin and thick film ferroelectrics

Laser transfer processing (LTP) offers the potential to overcome the problems of integrating ferroelectric thin and thick film materials with polymers and other technologically useful substrate materials that cannot sustain the high process temperatures, 600–1,000 °C, required for normal film deposition. The LTP technique involves the fabrication of a ceramic film on a high-temperature substrate material such as sapphire, and subsequent release by application of pulsed ultra-violet laser radiation. Here, the LTP technique is reviewed in the context of ferroelectric thin and thick films, and current developments are presented. Micro- and nanostructural features of the films before and after transfer to a second substrate are revealed using scanning and transmission electron microscopy. The consequences of laser-generated structural changes on ferroelectric properties are illustrated, and measures to mitigate the effects of an amorphous damage-layer are discussed.     

A thin film phantom for blood flow simulation and Doppler test

The thin film phantom is a new type of ultrasound resolution test object. It consists of a thin planar substrate that is acoustically matched to the surrounding media. Precisely located scatterers on the surface of the substrate generate echo signals. The patterning of scatterers on the substrate allows echogenicity to be controlled as a function of position, which enables the production of a test object with highly reproducible and controllable scattering characteristics. We show that by vibrating the substrate in a suitable manner, an echo signal may be generated that simulates bidirectional flow. We demonstrate that a vibration of low amplitude at frequency f₀ produces a Doppler spectral signal at f₀ and -f₀, within the limits of aliasing. Furthermore, by driving the film with a bandlimited noise signal, we illustrate how a velocity distribution may be simulated. A time-varying flow velocity may be simulated by varying the noise bandwidth with time. Finally, using this technique, we demonstrate a system that simulates an arterial flow pattern, including its characteristic velocity distribution in forward and reverse directions simultaneously     

Mechanical properties of polymer thin film measured by the bulge test

The bulge test was used to measure the mechanical properties of polymer thin films with thickness in the range of 77 nm to 352 nm. The mechanical properties of polymeric thin films were extracted by comparing differences between curves of load vs. bulge height obtained from composite film configurations with and without the polymer layer. Both square and long rectangular windows were used to obtain the Poisson ratio and Young's Modulus. Composite film with 230 nm silicon nitride layer and 30 nm Al layer has a composite Poisson ratio of 0.29 and a Young's Modulus of 234 ± 0.8 GPa. The Poisson ratio extracted for a 352 nm Poly(methyl methacrylate)-based thermoplastic polymeric thin film was 0.39. The Young's Modulus extracted for the 77 nm thick polymeric film is 4.9 ± 0.8 GPa and for the 352 nm thick film is 5.8 ± 0.2 GPa. In the thickness range investigated, no clear thickness dependence of the Young's modulus was observed using the Bulge test.     

Efficient charge carrier control on single walled carbon nanotube thin film transistors using water soluble polymer coatings

SWCNT-based thin-film transistors (TFTs) typically display unipolar p-type electrical characteristics in ambient condition due to the O₂/H₂O redox couple. However, complementary circuits that combine both p and n channels are preferred due to lower power requirements. Typical approaches with small molecule or polymeric dopants often yield ambipolar devices, or unstable n-type devices while concomitantly suppressing the on-current and mobility. Herein, we demonstrate a charge carrier control strategy using aqueous-based polymeric coatings that enable n-type devices with comparable performance to p-type devices. Specifically, we used a polyvinyl alcohol (PVA) coating layer containing a minority fraction of polyethyleneimine (PEI) (0.06–1.1?% w/w) to effectively switch the transfer characteristics from p-type to n-type, while maintaining decent electrical characteristics. Moreover, we demonstrate the ability to fine-tune the n branch threshold voltage via the annealing temperature. A similar strategy provides a balanced p branch on-current by incorporating PVA as a minor component (0.1-6?% w/w) into a polyacrylic acid (PAA) matrix. Through effective n-type conversion and p-type balancing, we demonstrate a simple SWCNT-based inverter. Considering the low-cost, environmentally friendly compositions and aqueous processability, this approach is attractive for large scale complementary printable circuits.

     

Grain boundary and vacancy diffusion model for electromigration-induced damage in thin film conductors

A theoretical model is proposed to explain the formation of both voids and hillocks in electromigration-induced damage. In this model, grain boundary mass transport is assumed to be responsible for the depletion and accumulation of diffusing atoms.The previously observed high compressive stresses generated in regions of atom accumulation can be explained using this model. In regions of atom depletion, vacancies are created but the expected tensile stresses are not observed because of efficient stress relaxation by the rapid diffusion of the vacancies into the neighboring lattice. The annihilation of these diffusing vacancies at film surfaces causes local film thinning which leads to the formation of electromigration-induced through-voids at grain boundaries.     

Laser recrystallized,faceted, cellular and dendritic morphologies of thin film silicon-on-insulator

Configurations of faceted, cellular and dendritic growth morphologies, as observed on laser beam recrystallized thin film silicon on amorphous, insulating substrates, are reported. These configurations are viewed as first-order growth stability breakdown, from which the sub-boundaries develop. The cause of breakdown is discussed in terms of constitutional supercooling and absolute (bath) supercooling.     

无机薄膜电致发光研究进展

薄膜电致发光显示是平板显示的主要技术之一。文章在介绍了无机薄膜电致发光器件结构和工作原理的基础上，介绍了薄膜电致发光研究的一般方法，描述了无机薄膜电致发光的研究现状，分析了薄膜电致发光所面临的问题，重点探讨了解决蓝光问题的几种方案。蓝光亮度已能满足全色显示的要求，薄膜电致发光具有广阔的发展前景。     

Contact damage in TiN coatings on steel

Damage mechanisms beneath Vickers indentations are examined on 5 μm TiN film deposited on stainless steel substrate as a function of load. Prominent mode of cracking includes surface edge cracks and subsurface inclined cracks. No interfacial delaminations were noted at the TiN/steel interface. The tangential traction, radial stress and shear stress distribution around an axisymmetric indentation field are used to assess the driving force for crack propagation.