Micro-structural and optical properties of diamond-like carbon films grown by magnetic field-assisted laser deposition
At a Glance
Section titled âAt a Glanceâ| Metadata | Details |
|---|---|
| Publication Date | 2024-01-01 |
| Journal | Acta Physica Sinica |
| Authors | Yimin Lu, Yujie Wang, Manman Xu, Hai Wang, Lin Xi |
| Citations | 1 |
| Analysis | Full AI Review Included |
Executive Summary
Section titled âExecutive SummaryâThis research investigates the use of an inhomogeneous magnetic field (MF) during Pulsed Laser Deposition (PLD) to control the properties of Diamond-Like Carbon (DLC) films, focusing on enhancing the desirable sp3 bonding content.
- Core Achievement: The magnetic field successfully constrains the carbon plasma (C2+ ions), forcing them into helical trajectories and concentrating them toward the center of the permanent magnet source.
- Microstructural Enhancement: Raman analysis confirms that this plasma confinement significantly increases the local pressure on the substrate, promoting the conversion of sp2 bonds into high-quality sp3 bonds, evidenced by a lower ID/IG ratio (as low as 0.331).
- Optical Quality: The central, high-pressure region exhibits a very low extinction coefficient (k < 0.01), indicating minimal light absorption and high optical quality.
- Major Drawback (Non-Uniformity): The inhomogeneous magnetic field creates severe non-uniformity in the film. Thickness and optical properties rapidly degrade (extinction coefficient increases dramatically) moving away from the center (0 mm to 18 mm).
- Stress and Delamination: The highest magnetic field tested (B3, 840 mT) resulted in excessive localized ion bombardment and internal stress at the center, causing the film to delaminate from the substrate.
- Future Direction: The study suggests optimizing the setup by fixing the magnet (not rotating with the substrate) and using plasma shielding to filter low-energy particles and large clusters, aiming for uniform, high-sp3 DLC films.
Technical Specifications
Section titled âTechnical Specificationsâ| Parameter | Value | Unit | Context |
|---|---|---|---|
| Laser Type | UV Excimer (PLD20) | N/A | Wavelength 248 nm |
| Pulse Duration | 30 | ns | Nanosecond regime |
| Repetition Rate | 20 | Hz | N/A |
| Energy Density | 5.2 | J/cm2 | Ablation of graphite target |
| Base Vacuum Pressure | 1 x 10-4 | Pa | PLD operating condition |
| Substrate Material | Single-crystal Silicon | N/A | 60 mm diameter |
| Max Magnetic Field (B1) | 340 | mT | Near magnet edge (5 mm thick magnet) |
| Max Magnetic Field (B3) | 840 | mT | Near magnet edge (15 mm thick magnet) |
| Film Thickness (B1, center) | ~480 | nm | Maximum thickness at 0 mm position |
| Thickness Non-uniformity (B2) | 52.9 | % | Calculated over 18 mm radius |
| Refractive Index (n) Range | 2.56 - 2.63 | N/A | At 1000 nm wavelength |
| Extinction Coefficient (k) Range | 0.008 - 0.227 | N/A | At 1000 nm wavelength (increases with radius) |
| Lowest ID/IG Ratio (B2, center) | 0.331 | N/A | Indicates highest sp3 content |
| G-peak Position (B1, center) | 1570.8 | cm-1 | Raman analysis (indicates high local stress) |
| Thickness Difference per Fringe | ~120 | nm | Calculated from He-Ne laser interference (632.8 nm) |
Key Methodologies
Section titled âKey MethodologiesâThe experiment utilized a specialized Magnetic Field-Assisted PLD setup combined with computational modeling and advanced material characterization.
- PLD Setup: A 248 nm UV excimer laser (30 ns pulse, 20 Hz) was used to ablate a rotating graphite target (99.99% purity) at 5.2 J/cm2 energy density in a 1 x 10-4 Pa vacuum.
- Magnetic Field Generation: Rectangular permanent magnets (NdFeB) of varying thicknesses (5 mm, 10 mm, 15 mm) were placed directly beneath the silicon substrate to generate three inhomogeneous magnetic fields (B1, B2, B3).
- Synchronous Rotation: The substrate and the magnet were rotated synchronously during the 30,000 pulse deposition process.
- Magnetic Field Modeling: The magnetic field distribution and flux lines were calculated using the Biot-Savart Law, confirming the fieldâs convergence toward the magnet center.
- Ion Trajectory Simulation: The flight paths of carbon ions (C2+) were simulated using an iterative calculation of the Lorentz force (F = qB x v), demonstrating the confinement and helical motion of the plasma toward the substrate center.
- Optical and Thickness Analysis: Film thickness and non-uniformity were measured using surface interference patterns (632.8 nm He-Ne laser) and fitted via Ellipsometry (GenOsc model) to determine the refractive index (n) and extinction coefficient (k).
- Microstructural Analysis: Raman spectroscopy (532 nm laser) was performed at various radial positions (0 mm, 6 mm, 12 mm, 18 mm). The spectra were deconvoluted into D and G peaks to calculate the ID/IG ratio, which correlates directly with the sp3/sp2 bonding ratio and local stress.
Commercial Applications
Section titled âCommercial ApplicationsâThe ability to significantly enhance the sp3 content and local density of DLC films through MF-PLD opens doors for applications requiring superior mechanical and optical performance.
- High-Performance Protective Coatings: Used in micro/nano-electronic devices and MEMS/NEMS components, where the enhanced hardness, density, and wear resistance provided by high sp3 content are critical for device longevity and reliability.
- Advanced Tribological Systems: Applications in high-load, low-friction environments (e.g., aerospace bearings, automotive engine components) benefit from the improved adhesion and reduced friction coefficient associated with dense, high-sp3 DLC films.
- Optical Windows and Lenses: The central region of the deposited film exhibits a very low extinction coefficient (k), making it suitable for protective coatings on optical components operating in the visible and near-infrared spectrum, particularly in harsh environments.
- Biomedical Devices: High-quality, dense DLC films are known for their biocompatibility and chemical inertness, making them ideal candidates for coatings on surgical tools and implants.
- Semiconductor Manufacturing: DLC films are utilized as hard masks in deep etching processes. The increased density and stress control offered by MF-PLD could lead to more robust and precise masking layers.
View Original Abstract
Inhomogeneous magnetic field is introduced into pulsed laser deposition process, in order to find new properties of diamond-like carbon film grown under magnetic field, thereby offering the theoretical and experimental basis for further enhancing sp<sup>3</sup>-bond content in this film. Distribution of the magnetic strength and flux lines induced by a rectangular permanent magnet is calculated. And then, flying trace of the carbon ions in the magnetic field is also simulated by the iterative method, which indicates that the carbon ions cannot expand freely and they are confined and accumulate around the center region of the magnet source. Beside the surface interference, the measurement and the fitted results of ellipsometry parameters show that magnetic field exerts an important influence on layer-thickness distribution and optical constant of the pulsed laser deposition-grown diamond-like carbon film. Meanwhile, it is indicated that the inhomogeneity of the layer-thickness distribution and optical constant increase when the magnetic strength is higher. Micro-structure of diamond-like carbon film is affected seriously by magnetic field, which is indicated by Raman spectra. Magnetic field can enhance the local stress in the carbon matrix net, increasing the sp<sup>3</sup>-bond content. Theoretical research and experimental research both show that a suitable magnetic strength can excite micro-structure of diamond-like carbon film significantly, and the high-quality diamond-like carbon coating with practical application value will be obtained by technological adjustment.