Electron Cyclotron Wave Resonance Research Articles

Protective diamond-like carbon coatings for future optical storage disks

Diamond-like carbon (DLC) is a promising protective coating for future optical storage technologies. The requirements place conflicting constraints on the nature of the DLC. It must be transparent at 400 nm, hard and wear-resistant, uniform, pinhole-free, have a low stress and be deposited at high rate without a high heat load on a plastic substrate. In order to optimise the films under these constraints, we have studied in detail the band gap, stress, density and Young's modulus of hydrogenated amorphous carbon (a-C:H) films deposited by plasma enhanced chemical vapour deposition using a large area electron cyclotron wave resonance source of 14 in. diameter and a variety of hydrocarbon gas sources. We have been able to produce wear-resistant carbon coatings with a high transparency at 400 nm without damaging the plastic disks. We also show that for these films the refractive index can be used as a rapid empirical means of property correlation.

Chemical structure of silicon-, oxygen- and nitrogen-containing a-C:H films prepared by RF plasma beam CVD

Si-, SiO x - and SiN x -containing a-C:H films (denoted as DLCSi, DLCSiO and DLCSiN) were deposited respectively from tetramethylsilane, hexamethyldisiloxane and hexamethyldisilazane precursors onto silicon wafer and aluminium substrates by electron cyclotron wave resonance (ECWR) RF plasma beam CVD. The chemical composition and the bonding states of the constituent elements were characterised by X-ray photoelectron spectroscopy and X-ray induced Auger electron spectroscopy. Compared to the composition of the precursors, significant loss of C for each layer and some loss of N for the DLCSiN layers were obtained, while the O/Si ratios for the DLCSiO layers remained practically unaltered. The modified Auger parameter of Si (Si α) decreased with increasing oxygen content and increased with increasing N content for the DLCSiO and DLCSiN layers. Si α was proposed to reflect the degree of crosslinking. For the DLCSiO and DLCSiN layers Si α could be altered by the deposition conditions (self-bias). The appearance of Si–O bonds was inferred from the valence band analysis of an Ar + ion bombarded DLCSi layer containing O-contamination.

Nanomechanical properties of silicon-, oxygen- and nitrogen-containing a-C:H films prepared by RF plasma beam CVD

The mechanical properties like hardness and reduced modulus of Si-, SiO x - and SiN x -containing a-C:H films (denoted as DLCSi, DLCSiO and DLCSiN), deposited respectively from tetramethylsilane, hexamethyldisiloxane and hexamethyldisilazane precursors onto silicon wafer by electron cyclotron wave resonance (ECWR) RF plasma beam CVD, have been determined by nanoindentation method. Relationships have been established between the nanomechanical properties and the self-bias developed during deposition, as well as the chemical structural parameters determined previously by X-ray photoelectron spectroscopy and X-ray induced Auger electron spectroscopy. In particular, an increase of the self-bias up to about 200 V during deposition led to a significant increase in the hardness of the layers. The hardness had a tendency to decrease with increasing O-content, and to increase with the increase of the modified Auger parameter for Si. The latter was attributed to the increase in the degree of crosslinking. For the first time, the hardness was found to increase with the increase of the bulk plasmon loss energy of the C 1s peak, which was interpreted in terms of the increase in the mass density.

Deposition of ta-C:N:H as Function of Experimental Parameters

The incorporation of nitrogen into tetrahedral amorphous carbon films (ta-C:H) was investigated as a function of different gas sources, ion energies and substrate temperatures. Samples were deposited using a highly energetic plasma method: electron cyclotron wave resonance. Composition was measured by nuclear techniques or by XPS. The optical gap was determined by reflection and transmission measurements of samples deposited on quartz substrates and the bonding state was investigated by Raman and infrared (ir) spectroscopy. All the results suggest that tetrahedral bonding is lost, as the nitrogen content increases independently of the deposition parameters used. Structural changes indicated a more polymeric structure with the formation of >C=N, -C≡N and >NH groups.

Large area deposition of hydrogenated amorphous carbon films for optical storage disks

Homogenous wear resistant carbon coatings with an optical gap of 3 eV but low stress are needed to increase the storage density in optical storage disks in order to reach 100 GB in 12-cm diameter disks. Here, hydrogenated amorphous carbon (a-C:H) films are deposited at room temperature using a large area electron cyclotron wave resonance plasma beam source of 14.2-inch diameter. Methane is used as the precursor. The ion flux and energy, plasma pressure, distance between the extraction grid and the substrate were varied. The films are characterized in terms of hydrogen content, stress, optical gap, refractive index, wear resistance, surface roughness and homogeneity by a variety of characterization techniques. Resonant Raman spectroscopy is used in order to non-destructively monitor the disk quality. We show how it is possible to produce homogenous, wear resistant a-C:H coatings with an optical gap of 3 eV and low stress (<500 MPa), without damaging the plastic disks.

Gas evolution studies for structural characterization of hydrogenated carbon nitride samples

Gas evolution experiments have been performed on hydrogenated carbon nitride samples in order to study the thermal stability of such samples and to investigate the presence of molecular nitrogen within the samples. The films were deposited by a combination of nitrogen and acetylene as precursor gases using an electron cyclotron wave resonance source. The results show for the release of molecular hydrogen broad effusion transients extending from approximately 300 to 900 °C. In addition to H 2 and hydrocarbons C x H y , usually observed for hydrogenated carbon films, there is a significant release of HCN molecules. This effusion also sets in at temperatures near 300 °C and shows a broad maximum near 700 °C. Above approximately 500 °C, for most of the samples the evolution of nitrogen rises steadily with increasing temperature, i.e. the effusion maximum is at temperatures exceeding the temperature range (100–1050 °C) investigated. However, in the nitrogen content range from 15 at.% to approximately 25 at.%, a peak near 300 °C is also observed. It becomes less pronounced at higher nitrogen content. This peak is probably a consequence of molecular nitrogen that is included in the sample without bonding, in (isolated) voids or bubbles. The implications for film properties, composition and thermal stability are discussed.

Plasma enhanced deposition of silicon carbonitride thin films and property characterization

Silicon carbonitride thin films were elaborated by reactive co-sputtering of graphite and silicon onto Si(111) substrates. The low-pressure electron cyclotron wave resonance (ECWR) plasma was radio-frequency biased both to sputter the target and to bombard the growing film at moderate energies (up to 180 eV). X-Ray photoelectron spectroscopy (XPS), Fourier-transform infrared spectroscopy (FTIR), X-ray diffraction (XRD) and atomic force microscopy (AFM) were applied to characterize the deposits. While films grown with pure nitrogen are exclusively amorphous, nanocrystallites are observed in specimens deposited with N 2+Ar mixture and the grain size can be well controlled. Depending on the processing parameters, the films displayed a variety of surface morphologies. The Vickers hardness for amorphous samples is roughly 17 GPa, slightly larger than the maximum value 13.8 GPa measured in crystalline specimens.

Properties of amorphous carbon–silicon alloys deposited by a high plasma density source

The addition of silicon to hydrogenated amorphous carbon can have the advantageous effect of lowering the compressive stress, improving the thermal stability of its hydrogen, and maintaining a low friction coefficient up to high humidity. Most experiments to date have been on hydrogenated amorphous carbon–silicon alloys (a-C1−xSix:H) deposited by rf plasma enhanced chemical vapor deposition. This method gives alloys with sizeable hydrogen content and only moderate hardness. Here we use a high plasma density source known as the electron cyclotron wave resonance source to prepare films with higher sp3 content and lower hydrogen content. The composition and bonding in the alloys is determined by x-ray photoelectron spectroscopy, Rutherford backscattering, elastic recoil detection analysis, visible and ultraviolet (UV) Raman spectroscopy, infrared spectroscopy, and x-ray reflectivity. We find that it is possible to produce relatively hard, low stress, low friction, almost humidity insensitive a-C1−xSix:H alloys with a good optical transparency and a band gap well over 2.5 eV. The friction behavior and friction mechanism of these alloys are studied and compared with that of a-C:H, ta-C:H, and ta-C. We show how UV Raman spectroscopy allows the direct detection of Si–C, Si–Hx, and C–Hx vibrations, not seen in visible Raman spectra.

Deposition of a-C:H films with an ECWR-reactor at 27 MHz: plasma diagnostics and film properties

For the first time, an electron–cyclotron-wave resonance (ECWR) source was used to deposit thin amorphous hydrocarbon (a-C:H) films. The deposition experiments have been supported by intensive plasma diagnostics with Langmuir probe (LP) measurements and energy mass spectrometry (EMS). The LP-investigations yielded a set of external parameters for homogeneously grown hard films at deposition rates of approximately 1.5 μm/h. By calibrating the EMS-system for particle number densities of stabile hydrocarbons and by using an appropriate fit-formula to evaluate absolute ion flux densities, the growth rates were in good agreement with predictions of the ‘thermally activated re-etching’-model (TR-model).

Comparative determination of the electron temperature in Ar- and N2-plasmas with electrostatic probes, optical emission spectroscopy OES and energy dispersive mass spectrometry EDMS

The electron temperature T e as the often most essential parameter for applications of low pressures plasma has been derived from comparative in situ measurements with a single and a double electrostatic probe, and with optical emission spectroscopy OES and energy dispersive mass spectrometry EDMS as remote techniques. Electrodeless rf discharges maintained by electron cyclotron wave resonance ECWR in pure Ar and N 2 in the pressure regime from 10 −2 to 1 Pa have been used as sample plasmas. The evaluation of the OES- and EDMS-signals is described in detail. The pressure dependence of the T e -results derived therefrom is found to compare well with the data from the probe measurements, and with calculations from a charge carrier balance equation. By matching the OES data to the absolute T e -values from the probe measurements, numerical expressions have been obtained by which T e can be quantitatively calculated from the intensity ratios between selected emission lines from the Ar- and the N 2 -plasma. Furthermore, the EDMS-results are also shown to deliver quantitative information about T e .

Chemical sputtering of ta-C: Implications for the deposition of carbon nitride

The majority of attempts to synthesize the theoretically predicted superhard phase β−C3N4 have been driven towards the use of techniques which maximize both the carbon sp3 levels and the amount of nitrogen incorporated within the film. However, as yet no attempt has been made to understand the mechanism behind the resultant chemical sputter process and its obvious effect upon film growth. In this work, however, the chemical sputtering process has been investigated through the use of an as-deposited tetrahedrally bonded amorphous carbon film with a high density nitrogen plasma produced using an rf-based electron cyclotron wave resonance source. The results obtained suggested the presence of two distinct ion energy dependent regimes. The first, below 100 eV, involves the chemical sputtering of carbon from the surface, whereas the second at ion energies in excess of 100 eV exhibits a drop in sputter rate associated with the subplantation of nitrogen within the carbon matrix. Furthermore, as the sample temperature is increased there is a concomitant decrease in sputter rate suggesting that the rate is controlled by the adsorption and desorption of additional precursor species rather than the thermal desorption of CN. A simple empirical model has been developed in order to elucidate some of the primary reactions involved in the sputter process. Through the incorporation of various previously determined experimental parameters including electron temperature, ion current density, and nitrogen partial pressure the results indicated that molecular nitrogen physisorbed at the ta-C surface was the dominant precursor involved in the chemical sputter process. However, as the physisorption enthalpy of molecular nitrogen is low this suggests that activation of this molecular species takes place only through ion impact at the surface. The obtained results therefore provide important information for the modeling and growth of high density carbon nitride.

Dual ion plasma-beam sources used to maximise sp 3 C–C bonds in carbon nitride

Amorphous carbon nitride films grown by plasma or ion sources never achieve the limit of β-C 3N 4, because the nitrogen fraction saturates below 57% and the carbon tends to become sp 2-bonded at high N content. When a-CN x is grown from a single ion or plasma beam, high nitrogen pressures are needed to promote higher N contents, but this leads to a loss of ionisation. We use a dual ion beam method to grow a-CN x with a filtered cathodic arc (FCVA) to supply carbon and a low pressure, high plasma density electron cyclotron wave resonance (ECWR) source to supply atomic nitrogen ions. The film composition and fraction of unsaturated π* bonding at C and N sites was measured by electron energy loss spectroscopy. We find that the C sp 3 fraction decreases linearly with nitrogen content, rather than showing the sharp fall at N/C=0.08–0.1 found by others. Thus, we achieve the highest CC sp 3 content for a-CN x films with N/C>0.1. We find a rather sharp increase in the fraction of empty N π * states, from 10 to 30% at N/C=0.2. Whereas previous work suggests that N contents above the critical value of 0.1 induce a transition for all C sites to sp 2, in our results only those C sites bonded to N revert to sp 2. The film density was observed to change in a similar way to the density of ta-C films with respect to the C sp 2 fraction. However, strong differences in the optical gap are observed.

Electron cyclotron wave resonance plasma assisted deposition of cubic boron nitride thin films

Cubic boron nitride (cBN) thin films have been deposited by sputtering the hexagonal boron nitride (hBN) target with nitrogen ions from a low-pressure plasma source. The electron-cyclotron-wave resonance (ECWR) plasma served both to sputter the hBN target and to simultaneously bombard the growing film. Deposits of over 90% cBN content were obtained with ion energies between 70 and 135 eV. The stoichiometry of the films was warranted by the extremely dense nitrogen ions arriving at the substrate. The frequency of the TO-mode for cBN on the Fourier-transform infrared (FTIR) spectra shifted up only slightly from the bulk value at 1065 cm−1. A maximum thickness of 350 nm for the cBN layer was achieved. A cross-sectional transmission electron microscopy (TEM) image showed that the film grew in the turbostratic boron nitride (tBN), aligned hBN-arrays and cBN sequence. At the hBN–cBN interface, some nanoarches were identified. The corresponding interplanar distances for the hBN (0002) and the cBN (111) planes are 0.321 and 0.212 nm, approximately ideal for the 2:3 lattice matching. Both the high-resolution TEM image and the arc pattern of the (111) reflection from selected-area electron diffraction revealed the textured structure of the cubic phase.

Plasma oxidation of silicon using an electron cyclotron wave resonance (ECWR) oxygen plasma

A high plasma density system, the electron cyclotron wave resonance (ECWR) has been used for the room temperature plasma oxidation of silicon. The oxidation efficiency, defined in terms of the ratio of initial growth rate to total anodisation current, is very high. The quality of the oxide as measured by refractive index, infra-red spectra and composition is compared to that of thermally grown silicon dioxide.

Stoichiometric cBN films deposited by electron-cyclotron-wave-resonance plasma sputtering of hBN

Electron-cyclotron-wave-resonance (ECWR) plasma of pure nitrogen was employed to deposit cBN thin films onto Si(100) substrates by hBN target sputtering. The ion current density and ion energy can be varied fairly independently. Deposition was achieved with ion energies between 70 and 230 eV. The cBN phase was identified with both Fourier transform-infrared spectroscopy (FT-IR) and high-resolution transmission electron microscopy (TEM). Thicker films with nearly 100% cBN phase in the upper layer appeared bright gray or transparent. A maximum thickness of 350 nm for the cBN layer was measured. The film growth follows the aBN→hBN→cBN sequence, with nanoarches being located at the hBN–cBN interface. Energy-dispersive X-ray (EDX) analysis verified the perfect stoichiometry of the deposits. Depending on the processing parameters the films displayed varied morphology. In particular, the surface of the film deposited at 74 eV exhibited large islands with diffuse boundaries on a compact base plane, suggesting the reduction of compressive stress at low ion energy.

Deposition of carbon nitride films using an electron cyclotron wave resonance plasma source

Abstract Hydrogenated amorphous carbon nitride (a-C:N:H) has been synthesised using a high plasma density electron cyclotron wave resonance (ECWR) technique using N2 and C2H2 as source gases, at different ratios and a fixed ion energy (80 eV). The composition, structure and bonding state of the films were investigated and related to their optical and electrical properties. The nitrogen content in the film rises rapidly until the N2/C2H2 gas ratio reaches 2 and then increases more gradually, while the deposition rate decreases steeply, placing an upper limit for the nitrogen incorporation at 30 at%. For nitrogen contents above 20 at%, the band gap and sp3-bonded carbon fraction decrease from 1.7 to 1.1 eV and ∼65 to 40%, respectively. The transition is due to the formation of polymeric C N, C N and NH groups, not an increase in CH bonds. Films with higher nitrogen content are less dense than the original hydrogenated tetrahedral amorphous carbon (ta-C:H) film but, because they have a relatively high band gap (1.1 eV), high resistivity (109 Ω cm) and moderate sp3-bonded carbon fraction (40%), they should be classed as polymeric in nature.

Nitrogen Incorporation into Tetrahedral Hydrogenated Amorphous Carbon

Structural changes induced by the incorporation of nitrogen into ta-C : H films have been studied by Electron Energy Loss Spectroscopy, X-Ray Photoelectron Spectroscopy, Fourier Transformed Infrared Spectroscopy and Ultraviolet-Visible Spectroscopy. ta-C:H films have been synthesised using a low pressure Electron Cyclotron Wave Resonance (ECWR) source which provides a plasma beam with a high degree of ionisation and dissociation. Nitrogen was incorporated by adding N 2 to the C 2 H 2 plasma used for the deposition of ta-C : H films. The N/C atomic ratio in the films rises rapidly until the N 2 /C 2 H 2 gas ratio reaches three, and then increases more gradually, while the deposition rate decreases steeply. Chemical sputtering of the forming films and the formation of molecular nitrogen within the films limit the maximum nitrogen content to about N/C = 0.6. For low nitrogen content the films retain their diamond-like properties, however as N/C atomic ratio increases, a polymeric-like material is formed, with > C=N- structures and terminating C=N and NH groups that decrease the connectivity of the network.

The Preparation, Characterization and Tribological Properties of TA-C:H Deposited Using an Electron Cyclotron Wave Resonance Plasma Beam Source

A compact electron cyclotron wave resonance (ECWR) source has been developed for the high rate deposition of hydrogenated tetrahedral amorphous carbon (ta-C:H). The ECWR provides growth rates of up to 900 Å/min over a 4″ diameter and an independent control of the deposition rate and ion energy. The ta-C:H was deposited using acetylene as the source gas and was characterized in terms of its sp3 content, mass density, intrinsic stress, hydrogen content, C–H bonding, Raman spectra, optical gap, surface roughness and friction coefficient. The results obtained indicated that the film properties were maximized at an ion energy of approximately 167 eV, corresponding to an energy per daughter carbon ion of 76 eV. The relationship between the incident ion energy and film densification was also explained in terms of the subsurface implantation of carbon ions into the growing film.

ECWR-Plasma CVD as a novel technique for phase controlled deposition of semiconductor films

Plasma excitation by electron cyclotron wave resonance (ECWR) as a rather novel technique for direct and remote PACVD processes is briefly described from its physical background. ECWR-plasmas operated in pure SiH 4 and GeH 4 have been employed for the production of amorphous and nanocrystalline hydrogenated Si and Ge films with favourable properties for photovoltaic applications.

High rate deposition of ta-C:H using an electron cyclotron wave resonance plasma source

A compact electron cyclotron wave resonance (ECWR) source has been developed for the high rate deposition of hydrogenated tetrahedral amorphous carbon (ta-C:H). The ECWR provides growth rates of up to 1.5 nm/s over a 4-inch diameter and an independent control of the deposition rate and ion energy. The ta-C:H was deposited using acetylene as the source gas and was characterized as having an sp 3 content of up to 77%, plasmon energy of 27 eV, refractive index of 2.45, hydrogen content of about 30%, optical gap of up to 2.1 eV and RMS surface roughness of 0.04 nm.