Stress Of Diamond-like Carbon Films Research Articles

Alleviating internal stress in diamond-like carbon films for enhanced tribological performance under high contact pressure via interface texturing

Diamond-like carbon (DLC) films have found widespread applications across numerous fields due to their exceptional properties. Nevertheless, their significant internal stress has restricted their further utility, particularly in scenarios involving high-contact pressure conditions. In response to this challenge, we fabricated diamond-like carbon films with nano-depth interface texture. This nano-depth interface texturing technique helps alleviate the internal stress within the carbon film, thereby enhancing the tribological performance of diamond-like carbon films with textured interfaces, especially under high contact pressure. We successfully fabricated textured carbon films with a thickness of 1 μm, which exhibited excellent tribological performance even under pressures exceeding 1 GPa. Through atomistic molecular dynamics simulations, internal stress testing, and cross-sectional transmission electron microscopy imaging, we observed that the incorporation of nano-depth interface texturing resulted in an approximate 47.62 % increase in the thickness of the transition layer between the silicon substrate and the carbon film. Furthermore, there was a notable reduction of about 30 % in the internal stress exhibited by the carbon films. These findings are expected to expand the application of carbon films under higher contact pressure and facilitate the production of thicker films.

Impact of temperature on residual stress and bonding in diamond-like carbon film: A molecular dynamics study under various deposition conditions

Diamond-like carbon (DLC) films exhibit excellent frictional properties; however, the high residual stress within the film, which causes detachment issues, limits their application in mechanical sliding surfaces. We have previously clarified via molecular dynamics (MD) simulations that the growth of sp3 clusters after the transition from sp2 to sp3 bonds during film formation causes the in-plane compressive stress in DLC films. In this study, we investigated whether this stress generation mechanism holds under various deposition conditions in the MD simulations, wherein incident energy, atomic flux, and substrate temperature were systematically changed. The results of the MD simulations consistently revealed that the compressive stress generation mechanism remains unchanged across different deposition conditions. With increasing incident energy and atomic flux and decreasing substrate temperature, in-plane compressive stress in the film increased owing to increased sp3 content. Conversely, when the incident energy was further increased, both stress and sp3 content decreased. Based on a molecular statistical calculation, it was shown that the stable bonding state changed from sp3 to sp2 at high temperatures. Because the temperature monotonically increased with increasing incident energy, it was concluded that the change in the stable bonding state caused the non-monotonic change in the stress and sp3 content at high temperatures.

Evaluation of Stoney equation for determining the internal stress of DLC thin films using an optical profiler

Excessive internal stresses in Diamond-like Carbon (DLC) films induced by the energetic ion bombardment during the deposition cause poor adhesion even delamination of the thin films from the substrate surfaces. As a result, determination of internal stresses in DLC films is essential for their applications. In this study, DLC thin films were prepared on thin silicon wafers with different shapes and sizes by end hall ion beam deposition. Optical profiler was used to measure the curvature of the samples before and after DLC deposition and then the Stoney equation was used to calculate the residual stress of DLC thin films in order to evaluate the feasibility of this method for measurement of DLC stress. For this purpose, the curvature radius in different directions of all the samples was measured before and after DLC deposition by the optical profiler. The calculated stress values using Stoney equation for all the samples are the same within reasonable errors. These results show that this simple method is appropriate and reliable for stress measurement of DLC films.

First principles investigation of interaction between impurity atom (Si, Ge, Sn) and carbon atom in diamond-like carbon system

The interaction between impurity atom (Si, Ge, and Sn) and carbon atom in diamond-like carbon (DLC) system was investigated by the first principles simulation method based on the density functional theory. The tetrahedral configuration was selected as the calculation model for simplicity. When the bond angle varied in a range of 90°–130° from the equivalent state of 109.471°, the distortion energy and the electronic structures including charge density of the highest occupied molecular orbital (HOMO) and partial density of state (PDOS) in the different systems were calculated. The results showed that the addition of Si, Ge and Sn atom into amorphous carbon matrix significantly decreased the distortion energy of the system as the bond angles deviated from the equilibrium one. Further studies of the HOMO and PDOS indicated that the weak covalent bond between Si(Ge, Sn) and C atoms was formed with the decreased strength and directionality, which were influenced by the electronegative difference. These results implied that the electron transfer behavior at the junction of carbon nano-devices could be tailored by the impurity element, and the compressive stress in DLC films could be reduced by the incorporation of Si, Ge and Sn because of the formation of weaker covalent bonds.

Micro‐Raman spectroscopy as a tool to analyze mechanical stress in nano‐structured diamond‐like carbon films

AbstractIn this work, we deposited nano‐structured diamond‐like carbon (DLC) films by high density plasma chemical vapour deposition system (HDPCVD) and the study of mechanical stress of these films has been made using micro‐Raman spectroscopy. Recently developed high‐density plasma chemical vapour deposition techniques have made it possible to produce nano‐structured DLC films with low surface roughness and high sp3/sp2 hybridization ratio. Thus, these parameters have been studied in this work. Nano‐structured DLC films were obtained using four methods for the surface modification of the silicon wafers: diamond powder, surface roughness generated by plasma etching, surface roughness generated by wet chemical etching solution and reference sample (silicon wafer only submitted to a chemical clean). The samples were analysed by AFM and SEM, too. We related the mechanical stress with the position of G and D bands, creating a new method to analyze these films (analyzing the displacement of central position G band). Thus, the results obtained in this work, related the Raman spectroscopy bands with the mechanical stress in DLC films obtained with different surface topographies (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Buffered internal stresses in diamond-like carbon films reinforced with single-walled carbon nanotubes

Diamond-like carbon (DLC) films reinforced with single-walled carbon nanotubes (SWCNTs) were fabricated by sputter-deposition of DLC onto a few monolayers of spray-coated SWCNTs on glass substrates. The thickness-averaged internal stress was reduced by 1.5 GPa by incorporation of SWCNTs into 10-nm-thick DLC films. Stress analysis indicates that the internal stress is reduced by 1.8 GPa at the SWCNT-DLC nanocomposite layer and decreases exponentially as a function of film thickness. Microscopy reveals significant cracking and delamination in 150-nm-thick DLC films, while the SWCNT-reinforced films remain essentially intact. The results demonstrate that SWCNTs in DLC films influence the early stage of DLC film growth and act as an effective stress-buffering layer near the boundary between the film and substrate.

Sp<sup>2</sup> Hybridization Effects on Friction of Diamond-Like Carbon Film (110) Surfaces Studied by First Principles Molecular Dynamics

The influence of sp2hybridization on friction and structure of diamond-like carbon (DLC) films was studied by first-principles molecular dynamics. First-principles calculations have been performed using the pseudo-potential plane wave method. Our results show that: 1) The buffer role of sp2-hybridized state carbon between grain boundaries greatly affected the residual stress of DLC film, and hence the friction decreased steadily with the increased in sp2hybridization content. 2) For thinner films, there was very greatly differences in stress between diamond-like carbon film (110) surfaces in <001> and<1-10> directions. The difference in friction force between soft and hard directions was nonexistent, when the sp2-hybridized state carbon atoms increased to 80%, perhaps because the originally hard C-C and C-H bonds changed, and steadily reject interaction was drive break.

Residual stress in diamond-like carbon films: Role of growth conditions and ion irradiation

The dependence of the internal residual stress in thin diamond-like carbon films grown by the plasma-enhanced chemical vapor deposition technique on the most important growth parameters, namely, the radio-frequency (RF) power and direct-current (DC) bias voltage, was studied. It was shown that the compressive stress reaches its highest value of 2.7 GPa at the low RF power and DC bias values. Both increase and decrease of these parameters result in stress decrease. The stress increases with increase in the growth temperature from 250 to 350°C. Thermal annealing of up to 350°C does not affect the stress value. Subsequent irradiation of the films by In+ and P+ ions with the energy of 350 and 120 keV, respectively, results in the decrease in the compressive stress followed by its inversion to the tensile one. The stress linearly depends on the effective dose of up to 0.5 DPA with the slope of (8.7 ± 1.3) Gpa/DPA.

Influence of ion bombardment on residual stresses in diamond-like carbon films

The dependence of internal residual stresses in thin diamond-like carbon films grown by the PECVD technique on the most important growth parameters such as the power of the exciting RF discharge and the substrate bias potential is considered. The results have shown that the mechanical stresses in films reach the uppermost value of 1.9 GPa at the smallest values of power and potential. The stress decreases with the growth of both parameters and has only a slight dependence on the film thickness in the range 0.1–1 μm. The bombardment of the obtained films by argon ions with energy of 300 keV and phosphorus ions with energy of 200 keV has resulted in the reduction of compressive stress with the ion dose growth down to its inversion. AFM study of the bombarded films has revealed significant changes in their surface morphology.

Comparing internal stress in diamond-like carbon films with different structure

Diamond-like carbon (DLC) films with different structures were deposited on Si (100) and stainless steel substrates in a hybrid deposition system with Ar and CH 4 as the feedstocks. The effects of the bias voltage, Ti-interlayer, Ti functional gradient layer and Ti-doping on the internal stress in DLC films were investigated. The results show that the internal stress in DLC films arises from both the intrinsic stress generated during the film growth and the thermal stress generated due to the mismatching of the thermal expansion coefficient between the DLC films and the substrate materials. The intrinsic stress can be released through doping titanium element at the expense of reducing the sp 3/sp 2 ratio. The thermal stress in DLC films can be decreased through introducing Ti-interlayer or Ti functional gradient layer. Noticeably, DLC films with very low internal stress deposited on stainless steel can be obtained through the combination of Ti-doping and Ti functional gradient layer.

Measurement of residual stress in DLC films prepared by plasma-based ion implantation and deposition

A hybrid process of plasma-based ion implantation and deposition (PBIID) was used to achieve relaxation of compressive residual stress in diamond-like carbon (DLC) films. The residual stress in each film was determined from the curvature of quartz glass plate using Stoney's equation. The compressive residual stress in the DLC film prepared using acetylene gas was about 0.46 GPa without the negative pulsed voltage for ion implantation. The residual stress decreased considerably with the increase of the negative pulsed voltage during deposition and reduced to about 0.16 GPa at the negative pulsed voltage of −20 kV. The residual compressive stress in the DLC films prepared using toluene gas was reduced to about several MPa at the region of negative pulsed voltage of −5 to −7 kV. These reduction of stress might be due to the thermal spike effect by ion implantation. However, at the negative pulsed voltage exceeding about −10 kV, the reduction of residual stress was not observed because of generation of glow discharge by the negative pulsed voltage. In either case, the observed residual stress of the DLC film was almost two orders less than that of conventional DLC films.

In situ measurement of stresses in diamond-like carbon films

In situ determination of stresses in thin films can be used as an important tool to assist process development as well as to understand the thermodynamics of film formation. A simple technique for the measurement of stresses in growing films is described here. The technique consists of measuring the displacement of a laser beam reflected from the film surface. Displacement is induced by changes in the radius of the curvature of the substrate resulting from stresses in the film. The detector sensitivity at the used wavelength (635 nm) is approximately 12 mV μm −1, for which our experimental set-up is equivalent to 4 mV μrad −1. The actual data collected consist of the reflected beam displacement vs. time, and provides at any instant the value of the average stress. By knowing the deposition rate, time is directly correlated with film thickness, and the local stress can be determined. Examples of measurement of stresses in tetragonally bonded amorphous carbon films prepared by filtered cathodic arc are presented, as well as how this technique can be used to design the deposition process to virtually eliminate intrinsic stresses.

Characterization and enhanced properties of plasma immersion ion processed diamond-like carbon films

The formation and properties of diamond-like carbon (DLC) films prepared on silicon substrates at room temperature, using C2H2–Ar plasma immersion ion processing, are investigated with respect to film deposition parameters. Decreases in the reactive gas-flow ratios of C2H2 to Ar(FC2H2/FAr) or the gas pressure were found to decrease the hydrogen content, increase the density and hardness, and improve the surface finish of the DLC films, all of which led to enhanced tribological properties. Decreasing the friction coefficient requires increasing the hardness of the film and smoothing its surface, whereas increasing the wear resistance correlates with reducing both the hydrogen content and residual stress in DLC films. High hardness and optimum tribological properties were reached as the growth of DLC films was subjected to low-energy ion impingement, which was induced by a −150 V pulsed bias from the C2H2–Ar plasma produced at low reactive gas pressures with low FC2H2/FAr ratios.

Possible solution to the problem of high built-up stresses in diamond-like carbon films

The various issues relating to the nature of high built-up stresses in diamond like carbon (DLC) films are presented and analyzed and the utility of pulse plasma technique in growing low residual stress DLC films is emphasized. Subsequently, sufficiently thick (2.2 μm) and hard (2000 kg/mm2) DLC films of significantly low stress (≈0.1 GPa) were deposited by the pulse plasma enhanced chemical vapor deposition (PECVD) technique. Stress values were found to be less than 0.5 GPa even with wide variation in pulse parameters (power density 0.4–2.0 W/cm2, dwell time 10–150 ms and duty cycle 10%–70%). A possible growth mechanism operating during pulse plasma discharge of such low residual stress and hard DLC films appears to involve the three phenomena: (i) relaxation of adions/adatoms, (ii) control of the substrate temperature, and (iii) creation of a hard/soft multilayer structure. To examine the role of substrate heating during the pulse plasma discharge, films were also deposited on deliberately heated substrates, using pulse plasma discharge, by using methane, acetylene, and benzene as hydrocarbon sources. An observation of direct correlation of the residual stresses and the degree of order of the film network has been made. Nitrogen dilution of the feedstock was also investigated, and further stress reduction has been observed, but not to the extent that occurs in continuous wave (cw) discharge grown films. This may be because constituent atoms in the film already approach close to a critical coordination number set by the fully constrained network (FCN) model. Other film properties like optical band gap (Eg), refractive index, and room temperature electrical conductivity (σRT) have also been estimated.

Doping Induced Internal Stress Reduction in Diamondlike Carbon Films Deposited by Pulsed Laser Ablation

ABSTRACTWe have investigated the effect of dopants on the reduction of internal compressive stress in diamond-like carbon (DLC) films prepared by pulsed laser deposition on Si(100) substrates. A novel target configuration was used to incorporate dopants into DLC films by sequential pulsed laser ablation of two targets. These dopants include copper, titanium and silicon. The thickness of the DLC films deposited was measured in the range 400nm - 600nm using a profilometer. Raman spectroscopy was employed to analyze the chemistry of the films. The shifts of the G-peak position in the Raman spectrum, due to different concentrations of dopant, were used to estimate the internal stress changes. All of the films showed a Raman spectrum typical of DLC films containing a high fraction of sp3 species, with the G-peak centered at around 1510–1560cm−1. The shift of the G-peak due to the presence of dopants was observed for all the DLC films as compared to the undoped one. It was found that Ti has the strongest tendency to reduce the compressive stress of DLC films. This effect increases with increasing concentration of dopants. Silicon was also observed to have this effect, but the G-peak position did not appear to shift with different Si concentrations. Buckling occurred in the as-deposited, undoped DLC film because of the relief of the large compressive stress accumulated in the film, while all the doped DLC films showed good adhesion to the substrate. The results are discussed combining the atomic structure of DLC and the structure and properties of the dopants.

A Study on the Residual Stress of Diamond-Like Carbon Films Deposited by Magnetically Enhanced RF PECVD

AbstractHydrogenated DLC films were synthesized by RF plasma deposition with and without magnetic enhancement, and their film stresses were investigated as a function of process parameters. Under investigated process conditions, Vb/P1/2 (where Vb is the self-bias voltage and P is the working pressure) is the appropriate scaling factor representing impinging ion energy. Film stress is influenced by not only ion impinging energy but also by ion to adspecies flux ratios. As ion energy increases, film stresses increase to a maximum value corresponding to the highest number of sp3 carbon bonds. As ion/adspecies flux ratio increases, the maximum stress value decreases and the corresponding ion energy increases. Induction of a magnetic field promotes film stresses as high as 15.2 GPa, which is one of the highest value reported in hydrogenated DLC films. The magnetic-induced increase of stress can be explained by increased ion/adspecies flux ratio, thus, enhanced sp3 formation. Rapid reduction of stresses observed at high ion energies may stem from the formation of graphite (sp2 bond) phases. Inclusion of hydrogen in the films is not directly responsible for the stress generation.

Stresses in diamond-like carbon films

Internal stresses have been measured in diamond-like carbon (DLC) films deposited by d.c. plasma assisted chemical vapor deposition from methane, acetylene, or cyclohexane, and in nitrogen containing DLC films deposited from acetylene, or cyclohexane and nitrogen. The total hydrogen content in the films and the fraction of bound hydrogen have been analyzed by forward recoil elastic scattering and Fourier transform infrared spectroscopy respectively. It was found that in pure DLC films the stresses increase with increasing fraction of unbound hydrogen. The highest compressive stresses were obtained in the films deposited from methane and the lowest stresses in films deposited from cyclohexane. In the nitrogen containing DLC films the stresses decrease with increasing nitrogen content in the films. Stresses as low as 0.22 GPa were obtained in the films deposited from cyclohexane and nitrogen at a ratio of 1 15 in the plasma.

Dependence of Residual Stress of Diamond-Like Carbon Films on Precursor Gases and Process Parameters of RF PACVD

ABSTRACTResidual compressive stress of diamond-like carbon (DLC) films was measured by beam deflection method. DLC films were deposited on thin Si wafers using r.f. plasma decomposition of methane and benzene. Negative bias voltage of the cathode was varied from -100 to -800 V and deposition pressure from 3 to 100 mTorr. When using benzene as precursor gas, the residual stress monotonically increases as increasing . (Here, Vb is the negative bias voltage of cathode and P the deposition pressure.) In case of using methane, however, the residual stress has a maximum value at between 70 and 100 V/mTorr1/2. Because of the difference in molecular size between benzene and methane, the mean free path of ions in benzene discharge is 5 times shorter than that in methane discharge. The contrasting behavior of residual stress is discussed in terms of the difference in ion energies at the specimen surface due to the difference in mean free path. On the other hand, total hydrogen concentration decreases as increasing in both cases. This result thus shows that the total hydrogen concentration cannot be a key to understand the behavior of residual stress.

Intrinsic stress in diamond-like carbon films and its dependence on deposition parameters

The compressive stresses and the hydrogen content inside diamond-like carbon films were measured for various deposition conditions. Four methods of film deposition were tested: primary beam containing CH 4; primary beam containing CH 4 with simultaneous bombardment of second beam of energetic argon ions; primary beam containing C 4H 10, and sputter deposition of graphite with the addition of hydrogen gas. The compressive stresses in the films were attributed mainly to the bombarding energy of the ion beam. An additional contribution to the compressive stresses probably came from the complex species in the discharge. The contribution of the hydrogen to the stresses in the films did not seem to be obvious.

Summary Abstract: Energy dependence of the stress in diamondlike carbon films

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