L. Martinu

Plasma deposition of optical films and coatings: A review

Plasma enhanced chemical vapor deposition(PECVD) is being increasingly used for the fabrication of transparent dielectric optical films and coatings. This involves single-layer, multilayer, graded index, and nanocomposite optical thin filmsystems for applications such as optical filters, antireflective coatings, optical waveguides, and others. Beside their basic optical properties (refractive index, extinction coefficient, optical loss), these systems very frequently offer other desirable “functional” characteristics. These include hardness, scratch, abrasion, and erosion resistance, improved adhesion to various technologically important substrate materials such as polymers, hydrophobicity or hydrophilicity, long-term chemical, thermal, and environmental stability, gas and vapor impermeability, and others. In the present article, we critically review the advances in the development of plasma processes and plasmasystems for the synthesis of thin film high and low index optical materials, and in the control of plasma–surface interactions leading to desired film microstructures. We particularly underline those specificities of PECVD, which distinguish it from other conventional techniques for producing optical films (mainly physical vapor deposition), such as fabrication of graded index (inhomogeneous) layers, control of interfaces, high deposition rate at low temperature, enhanced mechanical and other functional characteristics, and industrial scaleup. Advances in this field are illustrated by selected examples of PECVD of antireflective coatings, rugate filters, integrated optical devices, and others.

Mechanical and optical properties of hard SiCN coatings prepared by PECVD

Novel amorphous SiCN coatings are becoming increasingly attractive because of their mechanical, optical and electronic properties. In the present work, SiCN films were fabricated by PECVD from SiH4/CH4/N2/Ar gas mixtures at a temperature of 400 °C. Mechanical properties such as hardness, Youngs modulus, friction coefficient and stress were evaluated, respectively, by depth-sensing indentation, pin-on-disk, micro-scratch and curvature methods. Films deposited under optimized conditions exhibited a hardness >30 GPa, Youngs modulus >190 GPa, elastic rebound of 85% and a compressive stress of approximately 1 GPa. A friction coefficient against Al2O3, ranging from 0.75 to 0.25 and a low surface roughness of approximately 1 nm were found to be accompanied by a refractive index ranging from 1.85 to 2.10 (at 550 nm) and an extinction coefficient between 1.0×10−4 and 4.5×10−2. The film behavior is correlated with the microstructure and composition determined by SEM, XPS, AFM and broad-range UV–VIS–NIR–IR spectroscopic ellipsometry.

Critical ion energy and ion flux in the growth of films by plasma‐enhanced chemical‐vapor deposition

Dual‐mode microwave/radio frequency plasma‐enhanced chemical‐vapor deposition allows one to decouple ion bombardment effects from processes in the discharge volume. This approach has been used to deposit three types of hydrogenated amorphous films at low substrate temperature and high deposition rate (∼10–20 A/s): SiNx, SiO2, and a‐C:H. For each of these materials, we have determined critical values of the negative bias potential, VB,C, of the average ion energy, Ei,c, and of the ion/condensing‐atom flux ratio (φi/φn)c, which characterize the transition from a porous to a densely packed microstructure. The evaluations are based on measurements of the films’ resistivity, dielectric loss tangent, microhardness, density, and stress. The Ei,c, (φi/φn)c values found are: 170 eV, 0.60 for SiNx; 70 eV; 0.26 for SiO2; and 80 eV, 0.28 for a‐C:H. Ion bombardment at energies above Ei,c has been found to account for a large portion of hydrogen in the films which is not chemically bonded. The results are interprete...

OpenFilters: open-source software for the design, optimization, and synthesis of optical filters.

The design of optical filters relies on powerful computer-assisted methods. Many of these methods are provided by commercial programs, but, in order to adapt and improve them, or to develop new methods, one needs to create his own software. To help people interested in such a process, we decided to release our in-house software, called OpenFilters, under the GNU General Public License, an open-source license. It is programmed in Python and C++, and the graphical user interface is implemented with wxPython. It allows creation of multilayer and graded-index filters and calculation of reflection, transmission, absorption, phase, group delay, group delay dispersion, color, ellipsometric variables, admittance diagram, circle diagram, electric field distribution, and generation of reflection, transmission, and ellipsometric monitoring curves. It also provides the refinement, needle, step, and Fourier transform methods.

Spectroellipsometric characterization of plasma-deposited Au/SiO2 nanocomposite films

Nanocomposite films consisting of metal nanoparticles embedded in a dielectric matrix were fabricated by simultaneous sputtering of a gold target and plasma-enhanced chemical vapor deposition of hydrogenated SiO2. The optical constants of the films were determined from spectroscopic ellipsometry measurements and were modeled using the Maxwell–Garnett effective medium theory. The particle size dependence of the free electron absorption was included according to the limited electron mean free path effect using a broadening parameter A=0.16 determined from the comparison of the measured spectra with transmission electron microscopy micrographs. Using bulk interband optical constants for gold, very good agreement was obtained between the model and the measured spectra but only in the narrow particle size range ∼10–20 nm, the latter of which marks the onset of phase retardation effects. For smaller particles, the energy of the surface plasmon resonance was progressively blueshifted with respect to the predicte...

Dual‐frequency N2 and NH3 plasma modification of polyethylene and polyimide

Surfaces of polyethylene and polyimide films were treated in NH3 and N2 plasma using ‘‘dual‐frequency’’ excitation: The samples were exposed to a microwave (2.45 GHz) glow discharge, while variable radio frequency (13.56 MHz)—induced negative direct current bias voltage values were simultaneously applied. The surface chemical structure was determined by high‐resolution x‐ray photoelectron spectroscopy. Up to 40 at. % of nitrogen was incorporated onto the sample surface by exposure to a microwave discharge in N2, while systematically lower N uptake was found in NH3 plasma. Nitrogen was found to be bonded predominantly in amine (C–N) groups by NH3 ‐type plasma, and in imine groups (C=N) by N2 plasma. Surface ‘‘damage’’ of polyimide, including opening of benzene rings and breaking of carbonyl groups, accompanied by the formation of increasing concentrations of chemically reactive receptor sites, has been demonstrated with increasing the energy of bombarding species.

Micro-scratch analysis and mechanical properties of plasma-deposited silicon-based coatings on polymer substrates

Abstract Advanced optical applications require multifunctional coatings with specific mechanical properties, such as resistance to damage and good adhesion to different types of substrates, including polymers. In the present study we deposited amorphous hydrogenated silicon nitride (SiN 1.3 ) and oxide (SiO 2 ) films on polycarbonate and on silicon substrates by plasma enhanced chemical vapor deposition (PECVD), using a dual-mode microwave/radio frequency plasma system. The film adhesion was determined by the micro-scratch test. Depth-sensing indentation and substrate curvature measurements were used to evaluate the microhardness. Youngs modulus and residual stresses of the films. The adhesion strength, represented by the critical load, L c , when the film starts to delaminate, was determined as a function of the substrate material and the energy of bombarding ions. A direct correlation between the L c values and the mechanical properties of the films was found. The formation of different crack patterns in the coatings during the scratch procedure is explained in terms of stress release mechanism depending on the mechanical properties of the film, the substrate and the interface region. In addition, different models applicable to the evaluation of the work of adhesion in the case of hard coatings on soft substrates are critically reviewed.

Relationship between the mechanical properties and the microstructure of nanocomposite TiN/SiN1.3 coatings prepared by low temperature plasma enhanced chemical vapor deposition

Abstract Nanocomposite hard coatings were fabricated by PECVD from TiCl4/SiH4/N2/H2/Ar gas mixtures at substrate temperatures of 300 and 500 °C. The mechanical characteristics such as micro- and nanohardness, Youngs modulus, toughness and stress were evaluated, respectively, by depth-sensing and classical indentations and by curvature method. The mechanical and tribological properties are systematically correlated with the film microstructure and composition determined by XRD, SEM, ERD-TOF, XPS and AFM. For optimized nanocomposite films consisting of approximately 8 nm size TiN grains incorporated in an amorphous SiN1.3 matrix, we found Youngs modulus >270 and >350 GPa, hardness >25 and >40 GPa and compressive stresses ∼1.0 GPa and ∼2.5 GPa for low and high deposition temperatures, respectively. The effect of microstructure on the mechanical characteristics is discussed and the methodology of hardness measurements, in particular, the correlation between the depth-sensing indentation and the indentation size effect are addressed in detail.

Tribological study of CNx films prepared by reactive d.c. magnetron sputtering

Abstract The effects of both substrate bias voltage during fabrication and countersurface materials during sliding, on the tribology of CN, films are reported. Amorphous CN, films (12–24 at.% N), about 1 μm thick, were deposited onto Si(100) substrates at 600°C by d.c. magnetron sputtering of graphite in a nitrogen plasma. Tribological behaviour of the CN x films was evaluated in continuous, unidirectional sliding against different materials (Al 2 O 3 , 52100 steel, and diamond film-coated Si 3 N 4 balls) in pin-on-disk tests. Low friction coefficient values (μ = 0.1–0.25) were found for soft, but rougher, films prepared at lower bias voltage (−300 V). Such films exhibited a relatively high wear rate (≥ 1.3 × 10 −13 m 3 N −1 m −1 ), partial transfer to the alumina ball, but the wear debris acted as a lubricant during sliding. Harder CN x films prepared at higher bias voltage (−700 V) resulted in lower wear but higher friction levels (μ = 0.25–0.5) when sliding against alumina. The hardest film (23 GPa) prepared at a pressure of 0.5 Pa also showed lower wear, but relatively high and variable friction, behaviour when sliding against 52100 steel (μ = 0.4–0.5). Sliding against the rough, polycrystalline diamond coated countersurface eventually led to film delamination. Friction coefficients calculated from scratch test measurements are also compared with previous results obtained for diamond-like carbon films.

Optical properties and microstructure of plasma deposited Ta2O5 and Nb2O5 films

Advanced optical filter applications require an appropriate control of the optical constants, as well as of other suitable film properties such as mechanical performance, thermal and environmental stability, absence of refractive index inhomogeneities, and others. In the present work we studied the characteristics of two high index optical materials, namely amorphous tantalum pentoxide (Ta2O5) and niobium pentoxide (Nb2O5) prepared by plasma enhanced chemical vapor deposition, using penta-ethoxy tantalum Ta(OC2H5)5 and penta-ethoxy niobium, Nb(OC2H5)5, precursors. We particularly investigated the effect of energetic conditions on the film growth by using different modes of plasma excitation, namely rf, microwave, and dual-mode microwave/radio frequency discharges. Under sufficient ion bombardment, controlled by the rf-induced negative substrate bias, the dense Ta2O5 and Nb2O5 films exhibited a refractive index of 2.16 and 2.26 (at 550nm), respectively, while the extinction coefficient was below 10−5, as d...