800 kHz Femtosecond Laser Cleaning of Microwave Plasma Chemical Vapor Deposition Diamond Growth Substrate

At a Glance

Metadata	Details
Publication Date	2025-05-28
Journal	Crystals
Authors	Xiwang Wu, Xin Chen
Institutions	Xuchang University
Analysis	Full AI Review Included

Executive Summary

This study validates the use of an 800 kHz high-repetition-rate femtosecond laser for the non-destructive cleaning of Microwave Plasma Chemical Vapor Deposition (MPCVD) diamond growth substrates.

Core Problem Solved: Effective removal of strongly adhered MPCVD residues, primarily polycrystalline diamond and graphite, from molybdenum alloy substrates, which previously degraded subsequent single-crystal diamond growth.
Performance Achievement: The cleaning process significantly reduced the average surface roughness (Sa) of the Foundation Trench Region (FTR) groove from approximately 6.56 µm to 5.23 µm, promoting high-quality regrowth.
Optimal Parameters: Cleaning was achieved using a laser power of 2.38 W, a repetition rate of 800 kHz, a scanning speed of 1800 mm/s, and a 10 µm scanning interval.
Non-Destructive Cleaning: EDS analysis confirmed that the chemical composition of the molybdenum substrate was preserved (Mo content restored to 86.24 wt%), ensuring the base’s reusability and original performance.
Mechanism: Residue removal relies on a combination of vaporization, thermoelastic expansion (spalling), and elastic forces generated by acoustic waves, effectively clearing layered residues without excessive thermal damage.
Structural Cleaning Capability: The method successfully cleaned complex structures, including the inclined Inwall Region (IR) grooves and micro-cracks (3 µm width) on the Mo substrate surface.

Technical Specifications

Parameter	Value	Unit	Context
Laser Repetition Rate	800	kHz	Optimal cleaning parameter
Average Laser Power	2.38	W	Optimal cleaning parameter
Pulse Width	250	fs	Femtosecond laser specification
Laser Wavelength	1035	nm	Femtosecond laser specification
Spot Diameter	15	µm	Focused spot size
Scanning Speed	1800	mm/s	Optimal cleaning parameter
Scanning Interval	10	µm	Optimal cleaning parameter
Initial Surface Roughness (Sa)	6.557	µm	Average roughness of uncleaned FTR groove
Post-Cleaning Roughness (Sa)	5.231	µm	Average roughness of cleaned FTR groove
Mo Content (Post-Clean FTR)	86.24	wt%	Demonstrates chemical restoration of substrate surface
Residue Thickness (IR)	30 to 50	µm	Thick-layered structure on Inwall Region (IR)
Mo Substrate Crack Width Cleaned	3	µm	Width of strip cracks successfully cleaned and smoothed
Diamond Ablation Threshold (Single Pulse)	8.80	J/cm²	Reference threshold for single-crystal diamond

Key Methodologies

The cleaning process utilized a high-repetition-rate femtosecond laser system combined with specialized sample positioning and comprehensive characterization techniques.

Substrate Preparation and Characterization:
- Molybdenum alloy MPCVD growth substrates (35 mm radius) containing non-uniform residues (polycrystalline diamond, graphite) were analyzed.
- Raman spectroscopy confirmed the presence of significant diamond content (1304 cm^-1 peak) within the residues.
- Confocal microscopy measured the initial high surface roughness (Sa ≈ 6.56 µm) caused by residue accumulation.
Laser System Configuration:
- An 800 kHz femtosecond laser (250 fs pulse width) was focused via an F-theta lens to a 15 µm spot size.
- The system employed a two-dimensional galvanometer and a three-dimensional translation stage for precise scanning.
Foundation Trench Region (FTR) Cleaning:
- The substrate was placed flat for FTR cleaning.
- Optimal parameters (2.38 W power, 800 kHz rate, 1800 mm/s speed) were applied across multiple scans to remove aggregated granular residues.
- Removal mechanism involved spalling due to thermoelastic expansion and vaporization.
Inwall Region (IR) Cleaning (Tilted Focusing):
- A special processing platform was used to incline the substrate (45°) to allow laser access to the IR groove walls.
- Due to the short Rayleigh length (0.7 mm) relative to the IR height (±1 mm), the laser focus was positioned at the midpoint of the IR to ensure comparable energy density across the inclined surface.
- Multiple scans were performed to clear thick, layered residues (30-50 µm thick).
Post-Cleaning Analysis:
- Scanning Electron Microscopy (SEM) confirmed the smooth, residue-free surface morphology and the successful cleaning of strip cracks.
- Energy Dispersive Spectroscopy (EDS) verified the restoration of the Mo substrate’s chemical composition, confirming non-destructive cleaning.
- Confocal microscopy confirmed the significant reduction in surface roughness (Sa ≈ 5.23 µm).

Commercial Applications

The successful, non-destructive cleaning and surface smoothing of MPCVD growth substrates using high-repetition-rate femtosecond lasers have direct implications for industries requiring high-quality, reusable diamond growth platforms.

Advanced Semiconductor Manufacturing: Essential for producing high-quality single-crystal diamond wafers used in high-power electronic devices, thermal management solutions (heat spreaders for GaN/SiC), and high-frequency RF components.
Substrate Reclamation and Cost Reduction: Enables the efficient and environmentally friendly reuse of expensive molybdenum alloy growth bases, significantly reducing operational costs in diamond synthesis facilities.
Precision Tool and Mold Cleaning: The technique is broadly applicable to cleaning complex, high-precision molds (e.g., in automotive or aerospace industries) where traditional methods cause thermal damage or chemical waste.
Optical and Quantum Materials: Supports the growth of high-purity diamond required for quantum sensing applications (NV centers) and high-power optical components, where surface defects must be minimized.
Industrial Laser Processing: Provides a proven methodology for using high-repetition-rate femtosecond lasers to achieve surface smoothing concurrently with contaminant removal, a critical requirement for high-throughput industrial processes.

View Original Abstract

Microwave Plasma Chemical Vapor Deposition (MPCVD) plays a crucial role in the growth of high-quality diamonds. However, during the MPCVD process, residues such as polycrystalline diamond, and graphite often adhere to the high-temperature growth substrate surfaces, potentially degrading diamond growth quality. To effectively remove these contaminants and improve the quality of diamond growth, this study employed an 800 kHz femtosecond laser to clean growth substrates with residual deposits. We assessed the effects of multiple cleaning cycles on residue removal from the Foundation Trench Region (FTR) and Inwall Region (IR) and on substrate quality. The results indicate that multiple scans at a laser power of 2.38 W, a repetition rate of 800 kHz, a scanning speed of 1800 mm/s, and a scan spacing of 10 μm significantly removed residues, reduced substrate surface roughness, and restored substrate cleanliness. This approach enhances the quality and efficiency of diamond growth via MPCVD. The application of high-repetition-rate femtosecond laser cleaning techniques for growth substrates significantly improves the quality of regenerated diamond films, providing crucial support for the preparation of high-quality diamond materials.

Tech Support

Original Source

DOI: https://doi.org/10.3390/cryst15060517

References

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