Hard Amorphous Hydrogenated Carbon Films Research Articles

Stages in the interaction of deuterium atoms with amorphous hydrogenated carbon films: Isotope exchange, soft-layer formation, and steady-state erosion

We report studies of the interactions of quantified deuterium (hydrogen) atom beams with hard amorphous hydrogenated carbon films at a substrate temperature of ∼330 K in an ultrahigh-vacuum chamber. The modification/erosion of a-C:H (a-C:D) films was monitored in situ by ellipsometry in real time. By interpreting the ellipsometric information and combining it with measurements of the absolute D areal density changes in the a-C:H (a-C:D) films by ion beam analysis as a function of D (H) atom fluence, we are able to distinguish three sequential stages of D interaction with hard a-C:H films. The first stage is replacement of bonded hydrogen by deuterium up to an areal density of ∼5×1015 D cm−2 to a depth of ∼1.4 nm from the surface. This phase is complete after a deuterium fluence of ≈2×1018 cm−2. The effective cross section for isotopic exchange of H with D atoms for the a-C:H layer is found to be σ=2.0×10−18 cm2, and is close to the cross section for H abstraction from a carbon surface. This may indicate that H abstraction by D from the a-C:H surface is the rate limiting step for isotope exchange in this situation. Hydrogen replacement is followed by creation of additional C–D bonds in the near-surface region and increases the D areal density by about 2.5×1015 D cm−2. By ellipsometry this process can be observed as the formation of a soft a-C:D layer on top of the hard a-C:H bulk film, with the soft layer extending about 1.4 nm from the surface. This stage is complete after a deuterium fluence of about 2×1019 cm−2. Subsequently, steady-state erosion of the a-C:H film takes place. Here, a soft a-C:D layer with roughly constant thickness (∼1.4 nm) remains on the hard a-C:H substrate and is dynamically reformed as the underlying hard a-C:H film becomes thinner. A similar sequence of processes takes place at a substrate temperature of 650 K, albeit at a much faster rate.

Deposition of hard amorphous hydrogenated carbon films by radiofrequency parallel-plate hollow-cathode plasmas

Hard amorphous hydrogenated carbon (a-C:H) films were deposited by plasma decomposition of CH 4 gas in a RF parallel-plate hollow-cathode system. The deposition system was built by placing a metallic plate in parallel to and in electrical contact with an usual RF-PECVD planar cathode. Self-bias versus RF power curves were used to make an initial characterization of plasma discharges in nitrogen gas atmospheres, for pressures between 10 and 100 mTorr. The strongly increased power consumption to obtain the same self-bias in the hollow-cathode system evidenced an increase in plasma density. The a-C:H films were deposited onto Si single crystalline substrates, in the − 50 to − 500 V self-bias range, at 5, 10 and 50 mTorr deposition pressures. The film deposition rate was found to be about four times than that usually observed for single-cathode RF-PECVD-deposited films, under methane atmosphere, at similar pressure and self-bias conditions. Characterization of film structure was carried out by Raman spectroscopy on films deposited at 10 and 50 mTorr pressures. Gaussian deconvolution of the Raman spectra in its D and G bands shows a continuous increase in the I D/ I G integrated band intensity ratio upon self-bias increase, obeying the expected increasing behavior of the sp 2 carbon atom fraction. The peak position of the G band was found to increase up to − 300 V self-bias, showing a nearly constant behavior for higher self-bias absolute values. On the other hand, the G band width showed a nearly constant behavior within the entire self-bias range. Nanohardness measurements have shown that films deposited with self-bias greater than 300 V are as hard as films obtained by the usual PECVD techniques, showing a maximum hardness of about 18 GPa. Films were also found to develop high internal compressive stress. The stress dependence on self-bias showed a strong maximum at about − 200 V self-bias, with a maximum stress value of about 5 GPa.

Fine determination of interatomic distances on surface using extended energy‐loss fine structure (EELFS) data: peculiarities of the technique

AbstractFor determination of interatomic distances on a surface we use spectra below the lines of backscattered electrons and below the Auger lines. In contrast to other authors, who record spectra in dN/dE mode for extended energy‐loss fine structure (EELFS) analysis, the integral N(E) mode is used with further filtering of the structure‐dependent oscillations by original software. The sequence of structure‐dependent oscillation extraction from the energy‐loss spectrum is presented, including the following main stages: synthesis of peak and inelastic background and detection of inelastic oscillations; normalization of the inelastic spectrum; recalculation of the spectrum to K‐space; and Fourier transformation.Good agreement between theoretical and experimentally obtained interatomic distances for various systems (multicomponent grain boundary segregations in steels, hard diamond‐like coatings and films generated on the surface of cutting tools with engineered coatings) is observed. Detailed results of application of EELFS for investigation of the atomic structure of these systems are presented.In particular, the results of EELFS analysis of hard amorphous hydrogenated carbon films show the ability of this method to resolve interatomic bonds of different types (sp, sp2, sp3, aromatic bonds). The ‘carbon–hydrogen’ distances are also determined. Copyright © 2002 John Wiley & Sons, Ltd.

Optical investigations and Raman scattering characterisation of carbon bonding in hard amorphous hydrogenated carbon films

Hydrogenated amorphous carbon films were prepared by plasma enhanced chemical vapour deposition (PECVD) of methane–argon (5%) gas mixtures at low pressure, in a dual electron cyclotron resonance (ECR)-rf glow discharge. Optical transmission spectroscopy, Raman spectroscopy, elastic recoil detection analysis (ERDA) and infra-red (IR) absorption spectra were combined to examine the relationship between the local microstructure (CH and CC bonds) and the optical properties at different negative bias voltage. The amount of bonded H, obtained by IR spectra, is higher in series deposited at low bias voltage, in qualitative agreement with the ERDA results. In all cases, most of the incorporated H is bonded to sp 3C sites, with a predominance of CH 3 methyl groups. These results are also consistent with the Raman spectroscopy measurements. The positions, widths, and relative intensities of the two characteristic features, the D and G peaks are found to vary systematically with deposition conditions and film properties. The series prepared at high bias voltage shows a higher disorder (D) over the graphitic (G) band ratio, which also indicates a structure with a high disorder form of Csp 2 sites.

Formation of Hard Amorphous Hydrogenated Carbon Films with Low Hydrogen Concentration and Their Erosion in Air

Hard amorphous hydrogenated carbon (a-C:H) films are prepared by radio-frequency plasma deposition from a mixture of CH4 and Ar. When these films are heated up to 1220 K under vacuum, they release Ar, a large amount of H2 and a small amount of hydrocarbon, but the thickness of the films does not decrease. In addition, their micro-Vickers hardness decreases, but they remain hard. If these hydrogen-free semihard a-C:H films are heated at 670 K in air for 1 h, they become soft and uptake oxygen and hydrogen. On the other hand, if the original hard a-C:H films are heated under similar conditions, they remain hard without releasing argon or uptaking oxygen. It is concluded that the reduction of hydrogen does not lead to the formation of new cross linking C–C bonds, and the pore size of the films increases after heating allowing the penetration of oxygen and water vapor into the films.

Fluorine Incorporation Into Hard Amorphous Hydrogenated Carbon Films Deposited by PECVD

ABSTRACTThe incorporation of fluorine into amorphous hydrogenated carbon films deposited by the plasma enhanced chemical vapor deposition technique (PECVD) was investigated. Different mixtures of CH4 and CF4 were employed as the plasma atmosphere, with the partial pressure of CF4 ranging from 0 to 100 %. For all depositions, the self-bias was kept at –350 V. In the case of atmospheres richer than ∼80 % of CF4, no film deposition but substrate erosion was observed. The composition of the films was determined by Ion Beam Analysis (IBA), revealing that fluorine is incorporated into the amorphous network by replacing hydrogen. Infrared (IR) transmission measurements confirmed these results. It was also found that the incorporation of fluorine has the effect of reducing both the internal stress and hardness due to atomic density reduction.

Hard Amorphous Hydrogenated Carbon Films Deposited from an Expanding Thermal Plasma

AbstractDiamondlike amorphous hydrogenated carbon is deposited from an expanding thermal argon/acetylene plasma. It is observed that the film quality improves with increasing deposition rate. To obtain the best material quality the admixed acetylene flow has to be of comparable magnitude as the argon ion flow from the plasma source (critical loading). A new method to determine the ion density in an argon/acetylene plasma, by probe measurements, is presented. They reveal that the deposition during critical loading is governed by radicals. It is suggested that acetylene is dissociated once and that the C2H radical is formed dominantly.

Nitrogen implantation into amorphous carbon films: XPS, AES and Raman analyses

Hard amorphous hydrogenated carbon films (350 nm thick) deposited by self-bias glow discharge were implanted at room temperature with 70 keV nitrogen ions at fluences of 0.6, 1.0 and 2.0 × 10 17 N/cm 2. The implanted energy was chosen so that the projected range plus straggling ( R p + ΔR p) was smaller than the film thickness. The implanted samples were analyzed by Auger electron spectroscopy, X-ray photoelectron spectroscopy and micro-Raman spectroscopy. Factor analysis was applied to the crater-edge Auger profile. For micro-Raman depth profiles, a well focused argon laser beam was scanned across the wall of the same crater produced by 2 keV Ar + bombardment for the AES measurements. XPS depth profiles were also obtained. The Auger and Raman results indicated that the structural modifications (increase of the relative amount of carbon sp 2-bonds) are related to the energy deposited by the incident ion and depend on the ion fluence. Nitrogen incorporation into the amorphous carbon network can be inferred from XPS results.

Characterization of hard amorphous carbon films implanted with nitrogen ions

Hard amorphous hydrogenated carbon films (a-C:H) and nitrogen-doped hydrogenated carbon films (a-C:H(N)) deposited by self-bias glow discharge have been implanted with 70 keV nitrogen ions at fluences of 0.6 and 2.0 × 10 17 ions/cm 2. The implanted films were characterized by nuclear techniques (Rutherford backscattering spectrometry, nuclear reaction and elastic recoil detection analysis), Auger electron spectroscopy and Raman scattering. ERDA profiles show that nitrogen implantation produces a hydrogen-depleted layer. A study of the hydrogen loss as a function of deposition parameters (self-bias voltage, partial pressure of nitrogen) and ion fluence was made. A nitrogen depletion was also determined for the a-C:H(N) films, Auger spectroscopy, using factor analysis, and Raman scattering give some insights about the correlation between the energy loss mechanism of nitrogen ions and structural modifications.

7225. Polymer-like and hard amorphous hydrogenated carbon films prepared in an inductively coupled rf glow discharge: W Dworschak et al,Thin Solid Films, 189, 1990, 257–267

7225. Polymer-like and hard amorphous hydrogenated carbon films prepared in an inductively coupled rf glow discharge: W Dworschak et al,Thin Solid Films, 189, 1990, 257–267

Polymer-like and hard amorphous hydrogenated carbon films prepared in an inductively coupled R.F. glow discharge

An inductively coupled r.f. discharge was used to prepare amorphous hydrogenated carbon (a-C:H) films in a wide pressure range, at different ion energies, and at different substrate temperatures. Because of the inductive coupling the substrate bias and the r.f. power can be varied independently. The gas pressure ranged from 3 × 10 −2 to 5 × 10 −4 mbar and rather high deposition rates of 30 Å s −1 at 8 × 10 −4 mbar have been obtained. Depending on the plasma parameters and the substrate temperature, polymer-like or hard a-C:H films were prepared. Their properties were analysed by IR and visible spectrometry, and measuring the electrical conductivity, the hydrogen content, the Knoop hardness, the adhesion to different substrates, and the internal stress. Strong correlations exist between the optical gap, the relative IR absorption coefficients for the CC vibrational mode, for the integrated CH x ( x = 1,2,3) stretching modes, and the conductivity of the polymer-like films. Measurements of the temperature dependence of the conductivity indicate a superposition of activated conductivity and hopping conductivity at the Fermi level. The hard a-C:H films were prepared with average ion energies between 80 and 330 eV. The films exhibited significant enhancement of the Knoop hardness from 700 kgf mm −2 (7 GPa) to 2000 kgf mm −2 (20 GPa) and density up to 2.0 g cm −3 with increasing ion energy from 80 to 160 eV. A further increase in the ion energy up to about 300 eV does not influence the film properties. Films deposited on silicon wafers had a stress of 1–6 GPa, and the critical load in scratch tests was between 20 and 30 N, being equivalent to an adhesion of about 300 MPa. The influence of the r.f. power (for constant ion energy) on the film properties was also examined.