K. Jung

Investigations of diamond nucleation on a-C films generated by d.c. bias and microwave plasma

Abstract By the application of a negative d.c. bias to a Si(100) substrate in a CH 4 (5–15%)-H 2 microwave plasma, a SiC interface is formed covered by an amorphous carbon (a-C) layer. The diamond nucleation on these a-C films as a function of the bias voltage and the methane concentration has been studied by Raman spectroscopy, scanning electron microscopy, X-ray diffraction and atomic force microscopy. Measurements of the actual ion energy distribution by a retarding field probe showed that the ion energies for maximum nucleation are about 90 eV and are over a rather wide range independent of the methane concentration. In contrast, C + ions with energies of about 90 eV possess the optimum energy for sp 3 cluster formation in a-C films because of subplantation. Owing to the different etching efficiencies of a-C and sp 3 clusters by atomic hydrogen, the films become granular, with grain sizes of up to 100 nm. The grain number density after 10 min and the nucleation density of diamond (after 30 min diamond deposition conditions) are identical and between 10 9 and 10 10 . From XRD line width measurements we estimate sp 3 cluster diameters of 20 nm.

Experimental characterisation of bias-enhanced nucleation of diamond on Si

Abstract This paper reports an extensive characterisation of the bias process used to increase the nucleation density of diamond on Si. The nucleation density has been measured as a function of bias voltage, methane gas flow ratio and temperature. The nucleation density is found to be increased above 650 °C and reach a maximum at around −250 V. The nucleation density increases rapidly with time, up to a saturation value of about 10 10 cm −2 . The ion energy distribution is measured by a retarding field probe and has a maximum at ≈ 70–90 eV. This is close to the optimum energy for ion subplantation, responsible for sp 3 bonding in diamond-like carbon, which suggests that bias aids nucleation by some form of subplantation process.

Critical properties of nanoporous low dielectric constant films revealed by Brillouin light scattering and surface acoustic wave spectroscopy

Thin porous films with nanometer pore sizes are the subject of intense interest, primarily because of their reduced dielectric constant k. The lack of useful characterization tools and the reduction in film mechanical properties with increasing porosity have severely hindered their development and application. We show that both Brillouin light scattering and surface acoustic wave spectroscopy allow one to measure density, porosity and stiffness properties of nanoporous methylsilsesquioxane films of low-k value. Excellent correlations are observed among independent measurements of density, porosity and the Young’s modulus which show that the results obtained are reliable and reveal properties of the films which are difficult or impossible to obtain using other techniques.

Influence of ion energy and flux composition on the properties of plasma-deposited amorphous carbon and amorphous hydrogenated carbon films

Abstract The impact energies of C + ions strongly influence the density of sp 2 and sp 3 CC bonds in amorphous carbon and hydrogenated amorphous carbon films via collisions causing vacancies and subplantations. The importance of these momentum-transferring collisions has been shown experimentally for three different particle fluxes, namely those from magnetron sputtering, plasma beam and r.f. plasma deposition. The concentrations of neutrals, radicals and ions in the different fluxes and the energies of these particles are discussed and compared with the film properties density, hardness and internal stress (typical properties which strongly depend on sp 3 CC bonds), and band gap and spin density (properties which depend mostly on sp 2 CC bonds). It is found that collisions producing vacancies and interstitials (subplantations) play the predominant role with regard to the macroscopic properties of the films, even for the complex particle fluxes emerging from r.f. discharges.

Elastic properties of thin h-BN films investigated by Brillouin light scattering

Hexagonal BN films have been deposited by rf-magnetron sputtering with simultaneous ion plating. The elastic properties of the films grown on silicon substrates under identical coating conditions have been de-termined by Brillouin light scattering from thermally excited surface phonons. Four of the five independent elastic constants of the deposited material are found to be c11 = 65 GPa, c13 = 7 GPa, c33 = 92 GPa and c44 = 53 GPa exhibiting an elastic anisotropy c11/c33 of 0.7. The Youngs modulus determined with load indenta-tion is distinctly larger than the corresponding value taken from Brillouin light scattering. This discrepancy is attributed to the specific morphology of the material with nanocrystallites embedded in an amorphous matrix.

Plasma beam deposition of highly tetrahedrally bonded amorphous carbon

Abstract A new type of amorphous hydrogenated carbon was produced by an r.f. plasma beam. Using C2H2 as the precursor gas, the film-forming particle flux consists mainly of C2H+2 ions with well-defined energies. Corresponding to the subplantation (Lifshitz et al., Phys. Rev. B, 41 (1990) 10486) model of Robertson (Diamond Relat. Mater., 2 (1993) 984) and Davis (Thin Solid Films, 30 (1993) 276), the sp3 content and density are correlated with the ion energy and current density. The films are analysed by optical spectroscopy and photodeflection spectroscopy. The hardness and elastic properties are studied by indenter measurements. The film properties are strongly determined by the energy per C atom. The current density only governs the mass deposition rate. For an energy of about 90 eV an sp3 content of about 70% ± 10% can be achieved, which corresponds to a density of about 2.9 g cm−3, whereas the hydrogen content is still as high as about 25 at.%. The optical band gap of these 50–200 nm films is in the range 2.3–2.4 eV. The room temperature conductivity of 10−9 Ω−1 cm−1 is shown to be thermally activated with an activation energy of about 0.45 eV.

Elastic properties of indium tin oxide films

The Brillouin light scattering (BLS) technique is used to observe thermally excited acoustic surface phonons in backscattering geometry. A series of DC sputtered ITO films with thicknesses ranging from 31 to 3600 nm were deposited at a substrate temperature of 1508C on hydrogen terminated (100) silicon. From the dispersion of the Rayleigh mode and of the higher order Rayleigh-like modes in the discrete part of the spectrum, three independent stiffness constants, c , c and c , are determined 11 33 55 with reasonable accuracy. These results are obtained using a hexagonal model, which takes the axial symmetry of the film material into account. The elastic anisotropy, c yc f5y4, is attributed to the single crystal anisotropy of the strongly textured ITO. The 11 33 Youngs moduli obtained from BLS and a microindentation analysis are in good agreement. 2001 Elsevier Science B.V. All rights reserved.

Characterization of a CH4-RF-plasma by ion flux, Langmuir probe, and optical emission spectroscopy measurements

Abstract Methane plasmas excited with 13.56 MHz have been used for a-C:H deposition. Plasma parameters are studied by Langmuir probes and optical emission spectroscopy. In order to correlate macroscopic film properties and the plasma parameters, the ion energy distributions and the different ion masses impinging on the substrate have been determined by a 90° energy analyser and a quadrupole mass spectrometer. A very important parameter for the plasma and consequently for the a-C:H properties is the flow rate of CH 4 into the reactor. Various experiments have been performed at constant pressure and varying flow rate in the range from 21 seem to 1 seem by reducing the pumping speed. For an a-C:H film with 1300 kgf/mm 2 Knoop hardness, 1.55 g/cm 3 mass density and 1.3 GPa internal stress details of the ionic flux rate have been determined. The ionic flux rate is rather high with 0.3 mA/cm 2 and contains 65% CH − x and 30% C 2 H − x ions. The ions have a mean energy of 170 eV (4.4 × 10 −2 mbar pressure, 21 seem flow rate, residence time of the gas in the plasma∼1 × 10 −2 s). The ionic flux rate is about 32% of the total particle flux to the surface, which forms the film. In the plasma a rather high portion of the gas is transformed into C 2 H 2 and C 2 H 4 , both in the order of 5–10% of the CH 4 pressure.

Brillouin light scattering study on the elastic properties of thick sputtered c-BN films

The Brillouin light scattering technique is used to observe thermally excited acoustic phonons in backscattering geometry in a transparent, 1.8 μm thick diode sputtered c-BN film. The Rayleigh mode at the film surface and the quasilongitudinal bulk mode were detected under various angles of light incidence. The angular dependent phase velocity of the bulk wave provides evidence for an elastic anisotropy of the film material. Complete descriptions of the elastic properties are presented, assuming either isotropic or hexagonal film symmetry. The reduction of the film stiffness in comparison to the single crystal and the origin of the elastic anisotropy of c11/c33≈4/5, where c11 and c33 are the respective extensional stiffness constants parallel and perpendicular to the film, is discussed. Both effects are predominantly caused by sp2-bonded material and a structured film growth.

Brillouin light scattering from surface phonons in hexagonal and cubic boron nitride films

Abstract Phase velocities of surface acoustic waves in several boron nitride films were investigated by Brillouin light scattering. In the case of films with a predominantly hexagonal crystal structure, grown under conditions close to the nucleation threshold of cubic BN, four independent elastic constants have been determined from the dispersion of the Rayleigh and the first Sezawa mode. The large elastic anisotropy of up to c11/c33=0.1 is attributed to a pronounced texture with the c-axes of the crystallites parallel to the film plane. In the case of cubic BN films, the dispersion of the Rayleigh wave provides evidence for the existence of a more compliant layer at the substrate-film interface. The observed broadening of the Rayleigh mode is identified to be caused by the film morphology.