The Mo-DLC film composed of the transition metal Mo and DLC possesses not only novel mechanical properties but also biological compatibility, and has been widely used on the surface of artificial limbs, intravascular stents and other medical alloys. But its inner stress leading to film cracking as well as flaking off from the substrate is association with the component and microstructure. To address the challenge of accurately characterizing the atomic contents and depth distribution of Mo and C in Mo-DLC films, this paper proposes a Rutherford Backscattering Spectroscopy (RBS) analysis scheme based on a tuned-energy ionic accelerator and multilayer structure fitting by corresponding fitting data software SIMNRA7.02. By utilizing the resonant scattering between protons and C to resolve the fitting error caused by overlapping C/Si signals in traditional RBS, and revealing the actual film structure through multilayer fitting, this study can provide an ion beam detection method for the indirect characterization of internal stress in metal-doped DLC films.
Mo-DLC/Mo composite films are fabricated on single-crystal silicon substrates using an ion-assisted multi-arc ion plating system: a 99.9at.% Mo target is used as the Mo source, and acetylene is ionized by a hollow cathode ion source to provide the C source. To enhance film-substrate adhesion, a Mo transition layer is first deposited under the parameters: Mo target current of 80 A, bias voltage of -100 V, Ar partial pressure of 1.2 Pa and deposition time of 5 min. Subsequently, the Mo-DLC target layer is deposited with a Mo target current of 70 A, hollow cathode current of 50 A, bias voltage of -100 V, C2H2/Ar mixed gas pressure of 1.2 Pa, and deposition time of 60 min. The entire deposition process is conducted at 300 ℃ with a target-substrate distance of 100 mm, and the base vacuum is maintained below 6×10-³ Pa. The substrate is pretreated by Ar plasma bombardment (bias of -1 000 V, duration of 10 min) to remove surface contaminants. Signal optimization in RBS testing is achieved by designing different incident proton energies: measurements are performed on a 2×1.7 MV tandem accelerator with the target chamber vacuum better than 3×10-4 Pa. A proton beam with a current of 5 nA and spot size of 1 mm is incident perpendicularly, with a fixed backscattering angle of 170°. The backscattered ions are collected by a PIPS (passivated implanted planar silicon) detector and analyzed by MCA (multichannel analyzer).
When the proton energy is 2.00 MeV, the non-Rutherford scattering signal of C overlaps with that of the Si substrate, resulting in significant fitting errors. However, when the energy is reduced to 1.75 MeV (close to the 12C(p, p)12C resonance energy of 1.74 MeV), the resonance scattering cross section of C increases to 60 times larger than the Rutherford scattering cross section, forming an isolated peak completely separated from Si, which effectively eliminates signal interference. SIMNRA7.02 details the multilayer fitting results in comparison with single-layer fitting for RBS spectra. Under the incident condition of 1.75 MeV protons, the areal density of the Mo-DLC layer is about 1.47×1019 atoms/cm2, and its thickness is calculated to be approximately 2.90 μm based on a theoretical density of 9.08 g/cm3, which was smaller than 3.57 μm obtained at 2.00 MeV. The both thicknesses of the Mo transition layers are nearly equivalent. Multilayer fitting further reveals that the average atomic ratio of Mo to C is 75:25 (at.%), and the Mo content is significantly enriched near the transition layer with a stoichiometric ratio of 9∶1, which is attributed to interfacial atomic interdiffusion induced by the deposition temperature at 300 ℃. Meanwhile, it weakens the fitting deviation at the low-energy region caused by proton multiple scattering. This study verifies that 1.75 MeV is the optimal proton energy for RBS analysis of the Mo-DLC/Si system, realizing accurate separation of C/Si signals through resonant scattering. Additionally, multilayer fitting is more consistent with the actual film formation structure of arc ion plating, and also clarifies the phenomenon of interfacial atomic interdiffusion in Mo-DLC films. Furthermore, based on the quantitative results of Mo and C contents and depth distribution, it can indirectly correlate with film internal stress, providing a reliable ion beam experimental technology for the internal stress characterization of most metal-doped DLC films.
Key words
vacuum arc ion plating /
Mo-DLC film /
backscattering spectroscopy /
areal density /
fitting and calculation
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Funding
Hainan Provincial Graduate Students' Scientific Research Project (Hys2025-537); Scientific Research Foundation of Hainan Tropical Ocean University (RHDRCZK202505)
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