Showing papers by "Ludvik Martinu published in 2001"

Optical properties of discontinuous gold films: finite-size effects

Abstract: The optical constants of discontinuous gold films consisting of both embedded and unembedded spherical particles in the size range 5–14 nm were determined from spectroellipsometric and spectrophotometric measurements. The optical response was modeled with the Yamaguchi anisotropic dielectric function. The energy and width dependence of the surface-plasmon resonance owing to the finite size of the gold particles was determined with a phenomenologically modified metal dielectric function. The expected blueshift and broadening of the resonance with decreasing particle size was observed. The magnitude of the shift suggests that lattice contraction, including its effect on both the free-electron response and the core response, plays an important role in determining the resonant energy. The width dependence on particle size was found to be well described by broadening parameters A=0.15 and A=0.22 for embedded and unembedded particles, respectively.

Structural properties of silicon dioxide thin films densified by medium-energy particles

Abstract: Classical molecular-dynamics simulations have been carried out to investigate densification mechanisms in silicon dioxide thin films deposited on an amorphous silica surface, according to a simplified ion-beam assisted deposition scenario. We compare the structures resulting from the deposition of near-thermal (1 eV) ${\mathrm{SiO}}_{2}$ particles to those obtained with increasing fraction of 30 eV ${\mathrm{SiO}}_{2}$ particles. Our results show that there is an energy interval---between 12 and 15 eV per condensing ${\mathrm{SiO}}_{2}$ unit, on average---for which the growth leads to a dense, low-stress amorphous structure, in satisfactory agreement with the results of low-energy ion-beam experiments. We also find that the crossover between low- and high-density films is associated with a tensile-to-compressive stress transition, and a simultaneous healing of structural defects of the $a\ensuremath{-}{\mathrm{SiO}}_{2}$ network, namely, threefold and fourfold rings. It is observed, finally, that densification proceeds through significant changes at intermediate length scales $(4--10 \mathrm{\AA{}}\mathrm{}),$ leaving essentially unchanged the ``building blocks'' of the network, viz. the $\mathrm{Si}({\mathrm{O}}_{1/2}{)}_{4}$ tetrahedra. This latter result is in qualitative agreement with the mechanism proposed to explain the irreversible densification of amorphous silica recovered from high pressures $(\ensuremath{\sim}15--20 \mathrm{GPa}).$

The Adhesion of Evaporated Copper to Dow Cyclotene 3022®, Determined by Microscratch Testing

Abstract: The mechanical integrity, stability, and strong interfacial adhesion between Cu, a high conductivity metal, and Dow Cyclotene 3022®, a low permittivity polymer, are important for their application in future high-speed microelectronic devices. In the present study, Cu was deposited by both evaporation and sputtering, and various Cyclotene surface modifications were carried out. These modifications included low pressure N2 plasma and Ar+ treatments and the use of a Ti interlayer. The adhesion was evaluated by use of the microscratch test, and complemented by an adhesive tape peel test and XPS. The N2 plasma treatment was found to lead to a dramatic increase in adhesion, which was influenced to a minor extent by the adhesion promoter that was used at the Cyclotene/Si substrate interface. This significant Cu/Cyclotene adhesion enhancement is interpreted in terms of the chemical groups present at the Cyclotene surface and the bonds formed on Cu deposition.

Design and plasma-deposition of dispersion-corrected multiband rugate filters

Abstract: The inverse Fourier transform method is one of the most efficient synthesis method for the design of inhomogeneous optical coatings. However, this method assumes dispersion-free optical constants, which can induce undesired shifts in the spectral properties of coatings with complex features when “real” optical materials are introduced during fabrication. We describe a simple method for considering the dispersion in the synthesis of multiband rugate filter designs. The filters were then fabricated on glass substrates using plasma-deposition of silicon oxynitrides and silicon oxide-titanium oxide mixtures with precisely controlled composition gradients.

Plasma deposition of anti-reflective coatings on spherical lenses

Abstract: Plasma-enhanced chemical vapor deposition (PECVD) was used to fabricate multilayer anti-reflective coatings (ARCs) on spherical fiber couplers. Two- and four-layer designs were applied for single- and double-band ARCs centered at 1300 nm and 1550 nm, respectively. The systems consist of SiO2 as a low index material, and SiN1.3 or TiO2 as high index materials, obtained from different precursors (SiH4, SiCl4, and TiCl4). The deposition was controlled in-situ by single wavelength (632.8 nm) reflection monitoring. The optical and mechanical performance of the lenses was evaluated and related to the deposition conditions and the film microstructure.

Admittance diagrams of accidental and premeditated inhomogeneous optical coatings

Abstract: Intuitive interpretation of the effect of inhomogeneous layers in optical coatings is often difficult. In this work, we use admittance diagrams to interpret some properties of accidental and premeditated optical inhomogeneities. The effect of accidental homogeneous and inhomogeneous transition layers on the spectral properties of a single film is presented. We also give an original explanation for the suppression of harmonics and sidelobes in the spectra of rugate filters.

Structural and vibrational properties of silicon dioxide thin films densified by medium-energy particles bombardment

Abstract: Using classical molecular-dynamics simulations, we worked out a simple model of Ion Beam Assisted Deposition (IBAD) of silicon dioxide on an amorphous silica substrate, in view to in- vestigate the modifications of the structural and vibrational properties induced by medium-energy bombardment. Atoms are assumed to interact via the two- and three-body potential developed by Nakano et al.[1]. Analysis of the films grown with increasing ratio, R, of medium-(30 eV) to low- (1 eV) kinetic energy SiO2 particles shows that the density rises rapidly, from 1.3 g/cm3 for R =0 to about 2.3 g/cm3 for R = 0.7. This effect can be associated primarily with structural changes occurring at an intermediate length scale (4-10 A), as it manifests itself by changes in the so-called first sharp diffraction peak (FSDP), the finger-print of medium range order (MRO) in a-SiO2 glass [2, 3]. We found also that the densification results in a significant decrease of the number of “soft” vibrational modes, occurring in the 0.5-3.5 THz frequency range.