Daniel Poitras

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.

Effect of interface on the characteristics of functional films deposited on polycarbonate in dual-frequency plasma

Functional coatings are used in increasingly demanding applications that require specific optical characteristics, resistance to damage and good adhesion to different types of substrate materials, including polymers. In the present work we investigate thin films fabricated by low pressure plasma-enhanced chemical vapor deposition, using a dual-mode microwave/radio-frequency (MW/RF) plasma approach. The substrates are exposed to the principal dense microwave (MW, 2.45 GHz) plasma, while applying a RF-induced negative bias voltage. This technique provides independent control of the energy and flux of bombarding ions, and it allows one to deposit dense films at ambient substrate temperature at a high rate over large areas. We optimized the deposition process for amorphous hydrogenated silicon nitride (SiN1.3) obtained from SiH4/NH3 mixture. Using an average ion energy of about 150 eV, we obtained low-stress (<100 MPa) films with a refractive index of 1.89, at deposition rates around 30 A/s. MW plasma pretreatments with different gases have been investigated in order to further enhance adhesion of the SiN1.3 coatings on polycarbonate (PC) substrates—the highest adhesion, determined by the microscratch- and the adhesive-tape peel tests, was found when PC was pretreated in N2 plasma. The adhesion is related to the thickness and chemical structure of the interface. In fact, spectrophotometry and x-ray photoelectron spectroscopy analysis suggest the presence of a graded interface region, up to 40 nm thick, part of which contains Si–C, Si–O–C, and Si–N–C chemical links.Functional coatings are used in increasingly demanding applications that require specific optical characteristics, resistance to damage and good adhesion to different types of substrate materials, including polymers. In the present work we investigate thin films fabricated by low pressure plasma-enhanced chemical vapor deposition, using a dual-mode microwave/radio-frequency (MW/RF) plasma approach. The substrates are exposed to the principal dense microwave (MW, 2.45 GHz) plasma, while applying a RF-induced negative bias voltage. This technique provides independent control of the energy and flux of bombarding ions, and it allows one to deposit dense films at ambient substrate temperature at a high rate over large areas. We optimized the deposition process for amorphous hydrogenated silicon nitride (SiN1.3) obtained from SiH4/NH3 mixture. Using an average ion energy of about 150 eV, we obtained low-stress (<100 MPa) films with a refractive index of 1.89, at deposition rates around 30 A/s. MW plasma pretrea...

Design and plasma deposition of dispersion-corrected multiband rugate filters

Inverse Fourier transform method has been commonly used for designing complex inhomogeneous optical coatings. Since it assumes dispersion-free optical constants, introducing real optical materials induces shifts in the position of reflectance bands in multiband inhomogeneous minus (rugate) filters. We propose a simple method for considering optical dispersion in the synthesis of multiband rugate filter designs. Model filters designed with this method were fabricated on glass and polycarbonate substrates by plasma-enhanced chemical vapor deposition of silicon oxynitrides and SiO2/TiO2 mixtures with precisely controlled composition gradients.

Properties and stability of diamond-like carbon films related to bonded and unbonded hydrogen

Abstract Hydrogenated amorphous carbon (a-C:H) films with diamond-like characteristics have been deposited in dual-mode microwave-radio-frequency discharges in methane or methane-argon mixtures. Comparative measurements by elastic recoil detection and Fourier transform IR spectroscopy have been used to evaluate the amount of chemically bonded (IR-active) and unbonded (IR-inactive) hydrogen in the films. Three categories of structures have been distinguished: (a) hard hydrocarbon with hydrogen predominantly bonded, (b) hard hydrocarbon with hydrogen predominantly unbonded and (c) a mixture of hard hydrocarbon and polymer-like phases. Film properties such as refractive index (1.8– 2.5), resistivity (1012–1017 Ω cm) and thermal stability (between 25 and 550°C) are correlated with the amount of bonded hydrogen.

Mechanical properties of plasma-deposited silicon-based inhomogeneous optical coatings

Abstract Amorphous hydrogenated silicon nitride, oxide and oxynitride films are deposited by plasma-enhanced chemical vapour deposition (PECVD) using a dual-mode microwave/radio-frequency (MW/RF) plasma system. Optical filters are prepared by varying the film composition either abruptly (discrete, homogeneous multilayer structure) or continuously (graded-index, inhomogeneous structure). The coatings are characterised both optically, by spectrophotometry and spectroscopic ellipsometry, and mechanically, with depth-sensing indentation and low-load microscratch testing. A comparison is made between the properties of the homogeneous multilayer and the inhomogeneous multilayer structure with the corresponding optical performance. This multiple technique approach for characterisation was proved to be efficient for analysis of the optical and mechanical behaviour of coatings, and it provides a possibility for optimising the deposition process. It is demonstrated that the graded system exhibits a higher mechanical strength and a better toughness than the discrete structure.

Characterization of homogeneous and inhomogeneous Si‐based optical coatings deposited in dual‐frequency plasma

Silicon-compound optical coatings (silicon nitride, dioxide, oxynitrides) were prepared by plasma-enhanced chemical vapor deposition (PECVD) at near-ambient substrate temperature from SiH4/N2O/NH3 mixtures, using a dual-mode microwave/radiofrequency plasma system. The refractive index profile of the films was adjusted with depth in two ways: (1) between 1.45 (SiO2) and 1.90 (SiN1.3) by varying the N2O/NH3 feed gas ratio, and (2) between 1.65 and 1.90 for SiN1.3 films, by increasing the energy of bombarding ions from 5 eV to 400 eV. The film composition was changed either abruptly (multilayers) or continuously (graded-index, inhomogeneous layers). The resulting optical properties are correlated with the depth profile analysis of the chemical composition, and with mechanical characteristics such as adhesion, stress, and scratch resistance.

Metallized viscoelastic control layers for light-valve projection displays

Abstract In the first part of the three-paper series on light-valve projection with metallized viscoelastic control layers, the technologies for producing suitable layer systems as well as the observed performance of such light modulators are described. Optimum conditions for elastomer preparation and gel-layer formation by means of spin coating are given. Before metallization in a multistep vacuum evaporation process, the elastomer layers were coated with a thin pellicle whose mechanical properties and thermal expansion lie between those of the silicone gel and the silver mirror. For pellicle formation, a novel ‘soft’ plasma process was developed. The deformation behaviour of the completed control-layer systems was tested by means of an interferometry set-up and found to be sufficient for light-valve applications. Furthermore, the long-term and elevated-temperature performance of the metallized viscoelastic layers was investigated. Continuous operation over more than 1 year did not yield any appreciable deterioration, while the maximum deformation amplitude increased very remarkably if the operating temperature was elevated by a few tens of degrees Celsius. Finally, possible improvements of the present control-layer technology are proposed.

Interphase in plasma-deposited films on plastics: effect on the spectral properties of optical filters

The plasma-enhanced chemical vapor deposition of optical coatings on plastic substrates leads to the formation of a physically thick (approximately 50-100-nm) interfacial region (or interphase). We propose, based on our earlier spectroellipsometric (in situ and ex situ) and spectrophotometric (ex situ) studies, an optical model for the description of the interphase refractive-index profile n(z). We study in detail the effect of such an interphase on the spectral performance of various optical filters (antireflective V-coat, W-coat, achromatic W-coat, and minus filter). It is shown that considering the inhomogeneous n(z) profile in the design can improve the optical performance of some filters.

Diamond-like carbon films deposited in a dual microwave-radio-frequency plasma

Abstract Diamond-like carbon (DLC) films were deposited from CH4 gas in microwave-radio-frequency (MW-RF) plasma consisting of a microwave discharge with RF power applied to the substrate. We report the effect of the negative d.c. substrate bias voltage on the deposition rate, ion flux and film structure. Substantially higher fluxes were measured in the MW-RF mode than in the “pure” RF mode owing to a higher rate of fragmentation and ionization of the CH4 molecules in the gas phase, as also indicated by optical emission spectroscopy. It has been found by Fourier transform IR spectroscopy that the DLC films deposited in the MW-RF discharge exhibit more sp3 character than the deposits from a “pure” RF plasma. It is shown that DLC films prepared at lower ion energy but higher ion flux in the dual-frequency mode exhibit superior characteristics.

Simple method for determining slowly varying refractive-index profiles from in situ spectrophotometric measurements

Reliable control of the deposition process of optical films and coatings frequently requires monitoring the refractive-index profile throughout the layer. In the present research a simple in situ approach is proposed that uses a WKBJ matrix representation of the optical transfer function of a single thin film on a substrate. Mathematical expressions are developed that represent the minima and the maxima envelopes of the curves transmittance versus time and reflectance versus time. The refractive index and the extinction coefficient depth profiles of different films are calculated from simulated spectra as well as from experimental data obtained during the PECVD (plasma-enhanced chemical vapor deposition) of silicon-compound films. Variation in the deposition rate with time is also evaluated from the position of the spectra extrema as a function of time. The physical and mathematical limitations of the method are discussed.