Silver growth studied by UV reflectometry
Un article de Surface du verre et interfaces.
Marco De Grazia, Sergey Grachev, Etienne Barthel, Elin Sondergard
Sommaire |
Multilayers on glass
Multilayers on glass are complex dielectrics/metal systems that add functionalities to the whole stack, such as antireflection, light filtering and blocking etc. Silver is commonly used as metal layer in the glass industry. The quality of the Ag/dielectrics interface is of major importance, as it controls the film conductivity leading to the performances in the infrared (IR) light reflection. The thin layers are obtained by magnetron sputtering, their thickness and homogeneity determines the optical properties in the visible range. The study of specific material responses, such as adhesion properties, residual stress and microstructure of the multilayers structure aims for the understanding of the mechanical reliability of the stack (ANR project, MERETHIF). The optimisation of an optical spectroscopy tool should allow for the in-situ investigation of layers of 1Å at the very beginning of the growth (< 1 nm). The main achievement of the forthcoming experiments is the monitoring, in real time, of the Ag deposition by optical means, in order to distinguish the growth mode of the film, as dictated by the deposition and surface conditions.
Ag growth mode
The growth mode depends on the surface conditions and the environment of deposition chamber. Ag is a noble metal and tends to dewet from the substrate and to form islands. The film is formed by islands nucleation and coalescence and becomes conductive when percolation occurs. The transit from a wetting to a non wetting growth reduces the quality of the Ag/dielectrics interface, impacting dramatically the optical performance of the stack.
Optical spectroscopy and techniques
There are two main reasons why optical spectroscopy is well adapted for the investigation of thin layer deposition: the first is the fundamental importance of the electromagnetic field for many processes, e.g. for photoemission. The other reason is the practical usefulness of light as a surface probe.
- Ellipsometry: measuring a change in the polarization of reflected or transmitted light, the technique allows the investigation of the dielectrics properties (refraction index or dielectric function) of thin film.
- Anisotropic reflectivity probes the difference in the normal-incidence reflectivity along two mutually perpendicular orientations of the polarization vector. Usually one or both of these directions coincide with the principal crystallographic directions in the surface.
- The Surface Differential Reflectivity (SDR) measures the variation of the reflectivity when a change in surface conditions occurs (adsorbates deposition, heat, gas exposure, etc.) [1] .
These techniques allow for the investigation of optical transitions and the electronic structure of the surface; they are non-invasive and non-destructive. Furthermore they do not need UHV conditions and can be applied in liquid ambient.
SDR Spectroscopy: experimental and simulation
The SDR set-up consists of a source, whose collimated light enters in the experiment chamber (MISSTIC) and is reflected from the sample surface. Specular symmetric at the chamber exits, a Wollaston prism separates the s- and p-polarized light that are both simultaneously collected and detected. The instrumentation has recently been upgraded with a new Deuterium-Halogen lamp, providing light from 200-1500 nm and a multichannels spectrometer with CCD cameras as detector. Optical and optomechanical components are added for adjustment: collimating and focussing lenses optimize the signal intensity and a spatial filter reduces to background level, the parasite light coming from the magnetron during the deposition.
Intensity and stability of the signal are the most important issue for identifying small changes in variation, in particular at the beginning when the reflectivity of a transparent substrate is low. The differential reflectance spectrum is recorded as a function of the photon energy:
where RS is the reflectivity recorded in real time, R is the reference of the substrate and BG is the background light. The analysis of the high energy plasma resonances, shown in the spectrum below, allows determining the morphology characteristics of the thin layer: the islands size (R) the aspect ratio (Tr) and the surface coverage (L) [2] . The Granfilm software, developed by R. Lazzari and Ingve Simonsen calculates the polarizability of truncated spheres or spheroids deposited on a substrate and all the derived Fresnel quantities for island layers. The islands are approximated to truncated sphere (or spheroids) of the same size and located on a square grid [3] . Granfilm simulates and fits the experimental data to determine the surface morphology, taking into account the deposition and surface conditions.
Collaborations
Remi Lazzari, Institut des NanoScience de Paris INSP, Campus Boucicaut, 75015 Paris
References
- [1] P. Chiaradia and R. Del Sole, Surface Review and Letters., 1999, 6, 517–528.
- [2] A. Liebsch, Phys. Rev. B, 1993, 48(15), 11317–11328.
- [3] R. Lazzari and I. Simonsen, Thin Solid Films, 2002, 419(1-2), 124 – 136.