L. Le Brizoual

Reactive DC magnetron sputtering of aluminum nitride films for surface acoustic wave devices

A piezoelectric film combined with high velocity substrate as sapphire or diamond, seems very promising for surface acoustic wave (SAW) devices operating at high frequency. In this work, we concentrated on the optimization of growth parameters to perform AlN films with required properties for SAW devices: high resistivity, low roughness and high piezoelectricity coupling. AlN films are deposited by reactive DC magnetron sputtering on silicon substrate as a function of N concentration in Ar-N gas 22 mixtures. The duration of the process was modulated to obtain a constant film thickness (2 mm) to permit a better comparison. X-Ray diffraction (XRD) shows that the AlN films deposited in the range of 20-100% N , 400 8C, and 6=10 mbar, exhibit y3 2 a columnar structure textured in (002) orientation corresponding to hexagonal wurtzite structure with a c-axis perpendicular to the surface. In regard to XRD and electrical characterizations, the optimum film properties are obtained in the range of 60-80% N concentration. The higher peak intensity of (002) AlN diffraction and the higher resistivity are obtained for the AlN films 2 synthesized with 75% N. AFM analysis of AlN films demonstrate a low roughness at approximately 3 nm. The SAW device 2 (filter) was formed by development of interdigital transducers of 32 mm wavelength on the AlNySi structure by photolithography. The frequency response shows a center frequency of 158 MHz corresponding to a phase velocity of 5055 mys. This value is almost constant for all samples performed at optimum conditions. However, the insertion loss of device seems varies with N2 percentage and the low attenuation is obtained with 70% of N. This study is extrapolated on a sapphire substrate, and a phase 2 velocity of 5536 mys is recorded showing the effect of substrate to increase the center frequency of SAW devices. � 2002 Elsevier Science B.V. All rights reserved.

GHz frequency ZnO/Si SAW device

The demand for high-frequency low-loss filters generates intensive research on innovative wave guide solutions. In this work, a GHz SAW device based on a ZnO/Si structure was fabricated using classical UV photolithography. The thickness of the piezoelectric thin film was optimized and a specific interdigital transducer structure was used to generate third and fifth harmonic guided waves at 2.5 GHz and 3.5 GHz, respectively, with an aluminum strip larger than 1 micrometer. Different modes have been measured and theoretically identified thanks to an advanced finite-element/boundary-element- based model. Good agreement is found between theory and experiments. The high-frequency modes have been fully characterized, allowing for accurate design of SAW devices exploiting such modes.

Thermal properties of carbon nanowall layers measured by a pulsed photothermal technique

We report the thermal properties of carbon nanowall layers produced by expanding beam radio-frequency plasma. The thermal properties of carbon nanowalls, grown at 600 °C on aluminium nitride thin-film sputtered on fused silica, were measured with a pulsed photo-thermal technique. The apparent thermal conductivity of the carbon at room temperature was found to increase from 20 to 80 Wm−1 K−1 while the thickness varied from 700 to 4300 nm, respectively. The intrinsic thermal conductivity of the carbon nanowalls attained 300 Wm−1 K−1 while the boundary thermal resistance with the aluminium nitride was 3.6 × 10−8 Km2 W−1. These results identify carbon nanowalls as promising material for thermal management applications.

Analysis of Ti-Si-N diffusion barrier films obtained by r.f. magnetron sputtering

Abstract Thin films of Ti–Si–N are deposited by r.f. magnetron sputtering in a Ar/N 2 gas mixture. The magnetron discharge is operated at 10 mTorr with 5 and 10% N 2 in the gas mixture and r.f. powers ranging from 100 to 200 W. The composition and electrical resistivity of the thin films were determined by energy dispersive X-ray spectroscopy and the four-point probe method, respectively. The structure of the films was determined by high-resolution transmission electron microscopy. The Ti–Si–N films were either amorphous or contained cubic TiN nanosized grains in an amorphous phase. The diffusion barrier properties of 10-nm thick film between Cu and Si were studied from 500 to 700°C. The highest failure temperature of 650°C was obtained for Ti 37.5 Si 27 N 35.5 which. contains 4-nm TiN crystallites in an amorphous phase.

Hierarchical carbon nanostructure design: ultra-long carbon nanofibers decorated with carbon nanotubes

Hierarchical carbon nanostructures based on ultra-long carbon nanofibers (CNF) decorated with carbon nanotubes (CNT) have been prepared using plasma processes. The nickel/carbon composite nanofibers, used as a support for the growth of CNT, were deposited on nanopatterned silicon substrate by a hybrid plasma process, combining magnetron sputtering and plasma-enhanced chemical vapor deposition (PECVD). Transmission electron microscopy revealed the presence of spherical nanoparticles randomly dispersed within the carbon nanofibers. The nickel nanoparticles have been used as a catalyst to initiate the growth of CNT by PECVD at 600°C. After the growth of CNT onto the ultra-long CNF, SEM imaging revealed the formation of hierarchical carbon nanostructures which consist of CNF sheathed with CNTs. Furthermore, we demonstrate that reducing the growth temperature of CNT to less than 500°C leads to the formation of carbon nanowalls on the CNF instead of CNT. This simple fabrication method allows an easy preparation of hierarchical carbon nanostructures over a large surface area, as well as a simple manipulation of such material in order to integrate it into nanodevices.

Forouhi-Bloomer and Tauc-Lorentz optical dispersions applied using spectroscopic ellipsometry to plasma-deposited fluorocarbon films

Optical properties of the fluorocarbon (FC) films plasma deposited on Si substrates are evaluated in this work using multiple sample analysis (MSA)-based spectroscopic ellipsometry (SE) with representing the film optical constants by the Forouhi-Bloomer (FB) and Tauc-Lorentz (TL) optical dispersions. This SE analysis supported also with other film investigations results in a two-layer optical model consisting of an interface assimilated to FC species-permeated Si layer beneath a surface smooth, homogeneous, and isotropic FC bulk film. Both dispersions yield a low-dielectric constant quality visible range refractive index of 1.39 and almost identical model layer thicknesses. Deposition-dominated linear film growth is thus asserted. Specifically, the FB dispersion better describes the region near absorption cutoff with taking up a lower optical band gap (OBG) than that of the TL dispersion, indicating thereby that particular FC film absorptions included in the FB dispersion are excluded in the other. Also, ...

Experimental study of Ti–Si–N films obtained by radio frequency magnetron sputtering

Abstract Thin films of Ti–Si–N were deposited by magnetron sputtering in an Ar–N2 gas mixture. The magnetron discharge was operated at 10 mTorr with less than 10% N2 in the gas mixture. The r.f. power varied from 100 to 200 W. The composition and resistivity of the thin films were determined by energy dispersive X-ray spectroscopy and the four-point probe method respectively. At low r.f. power the films were richer in nitrogen. The resistivity of the films varied from 250 to 1150 μΩ cm. Low resistivity films are obtained below 35 at.% nitrogen content. Furthermore, a Monte Carlo simulation previously developed for a W7Ti3 target was adapted to the Ti5Si3 target and used to determine the transport characteristics of the sputtered atoms. The results were compared with optical emission spectroscopy measurements of Ti and Si emission lines, revealing that the ejection cone from the target is wider for Ti atoms than for Si atoms.

a-SiC x N y thin films deposited by a microwave plasma assisted C VD process using a CH 4 /N 2 /Ar/HMDSN mixture: methane rat e effect

Amorphous silicon carbonitride thin films were deposited using a microwave plasma assisted chemical vapour deposition process fed with a mixture of methane, nitrogen, argon and hexamethyldisilazane (Si2C6H19N). Effects of the methane rate on thin films composition, n anostructuration and characteristics are investigated by means of various techniques such as X-ray Photoelectron Spectroscopy, Fourier Transform Infrared Spectroscopy, Transmission Electron Microscopy and UV-Visible absorption. The raise of the methane rate results in less organic, denser films and in an increase of refractive index.

Theoretical and Experimental Identification of Love Wave Frequency Peaks in Layered Structure ZnO/Quartz SAW Device

In this paper, the layered structure ZnO/Quartz (90deg rotated ST-cut) is investigated theoretically and experimentally. Both waves, Rayleigh and Love, are analyzed. Dispersion curves of phase velocities, electromechanical coupling coefficient (K 2) and temperature coefficient of frequency (TCF) were calculated as a function of normalized thickness ZnO film (kh ZnO = 2pih ZnO /lambda) and the optimum value of h ZnO was determined for experimental study. Experimental results combined with simulation lead to clearly identify the generated waves and their higher modes in this structure except the mode 0 that shows comparable velocity for both Rayleigh and Love waves. The identification of the wave type was performed by studying the frequency response of the device with or without a droplet of water in the wave path. We also demonstrate that the highest elastic velocity is obtained for the mode 1 of the Love wave. This Love wave mode exhibits very interesting electrical characteristics, good K 2, high-frequency rejection, low TCF, and very low attenuation in liquid making it very attractive for gas and liquid sensor applications.

Magnetic tunnel junctions with a zinc oxide–cobalt oxide composite tunnel barrier

Composite CoO–ZnO tunnel junctions showing nonlinear and asymmetric current-voltage characteristics with significant magnetoresistance ratios (up to 8% at 77 K) have been prepared by using reactive sputtering from a zinc target. Electron transmission microscopy demonstrates the formation of a zinc oxide–cobalt oxide bilayer. Observed asymmetries, which are directly linked to the difference in zinc oxide and cobalt oxide barrier heights are in good agreement with calculations done within the framework of a parabolic bands model, using thicknesses extracted from transmission electron microscopy (TEM) images and barrier heights found in literature.