A New Slurry for Photocatalysis-Assisted Chemical Mechanical Polishing of Monocrystal Diamond

At a Glance

Metadata	Details
Publication Date	2023-06-20
Journal	Machines
Authors	Junyong Shao, Yanjun Zhao, Jianhui Zhu, Zewei Yuan, Haiyang Du
Institutions	Northeastern University, Shenyang University of Technology
Citations	9
Analysis	Full AI Review Included

Executive Summary

This research introduces a novel Photocatalysis-Assisted Chemical Mechanical Polishing (PCMP) method utilizing a specialized slurry for achieving ultra-smooth surfaces on monocrystal diamond.

Core Mechanism: The PCMP slurry uses titanium dioxide (TiO₂) photocatalysts activated by UV light to generate highly oxidative hydroxyl radicals (·OH), which chemically react with the diamond surface (C*).
Slurry Optimization: The slurry formulation requires a photocatalyst (P25 TiO₂), an abrasive (Al₂O₃), an electron capture agent (H₂O₂), and a pH regulator (H₃PO₄) to maximize the oxidation-reduction potential (ORP).
Optimal Catalyst: P25 TiO₂ (a mixed crystal anatase/rutile form) demonstrated the most stable and effective photocatalytic activity compared to single-crystal TiO₂ variants.
Surface Quality Achievement: The process successfully reduced the surface roughness (Ra) of CVD diamond workpieces from an initial Ra 33.6 nm to an ultra-smooth Ra 2.6 nm after 8 hours of polishing using the P25 TiO₂ slurry.
Scratch Removal: PCMP effectively removed residual mechanical scratches left by prior lapping steps, yielding a smooth, atomic-level surface finish.
Safety and Environment: The developed slurry is non-toxic and environmentally safer than traditional CMP slurries that rely on highly corrosive or toxic strong oxidants (e.g., CrO₃, KMnO₄).

Technical Specifications

Parameter	Value	Unit	Context
Initial Surface Roughness (Ra)	33.6	nm	Before PCMP
Final Surface Roughness (Ra)	2.6	nm	After 8 h polishing with P25 TiO₂ slurry
Best AFM Roughness (Ra)	1.6	nm	Measured after 8 h polishing with 5 nm TiO₂ slurry
Polishing Time	8	h	Duration to achieve Ra 2.6 nm
Polishing Pressure	1.09	MPa	Applied pressure during PCMP
Rotational Speed	60	r/min	Polishing head speed
Optimal Photocatalyst	P25 TiO₂	N/A	Mixed crystal (80:20 anatase:rutile)
P25 Particle Size	~25	nm	Typical size of P25 TiO₂ particles
TiO₂ Bandgap (E_g)	3.2	eV	Required for photocatalysis
Required UV Wavelength	Less than 387.5	nm	Wavelength needed to activate TiO₂
Slurry TiO₂ Concentration	0.5	g / 100 mL	P25 TiO₂ in water
Slurry Abrasive Concentration	6	g / 100 mL	Aluminum oxide (Al₂O₃) in water
Electron Capture Agent Dose	3	mL / 100 mL	H₂O₂ concentration
Maximum ORP Value	~501	mV	Achieved with 5 nm TiO₂ + H₂O₂ + H₃PO₄ under UV

Key Methodologies

The Photocatalysis-Assisted Chemical Mechanical Polishing (PCMP) process involved specific preparation and operational steps:

Workpiece Preparation:
- CVD diamond workpieces were mounted onto the polishing head base using paraffin.
- Prior to PCMP, rough asperities were removed via stepwise mechanical lapping using diamond abrasives ranging from 3-6 µm down to 0-0.5 µm.
Slurry Formulation (PCMP Slurry Recipe):
- Photocatalyst: 0.5 g of P25 TiO₂ particles (or 5 nm TiO₂ for comparison) per 100 mL water.
- Abrasive: 6 g of aluminum oxide (Al₂O₃) per 100 mL water. Al₂O₃ was chosen as it cannot be oxidized by the slurry oxidant, isolating the chemical effect.
- Electron Capture Agent: 3 mL of H₂O₂ per 100 mL water. H₂O₂ was preferred over K₂FeO₄ due to its superior chemical stability.
- pH Regulator: Phosphoric acid (H₃PO₄) was used to maintain an acidic environment, which significantly boosts the slurry’s ORP and oxidizability.
PCMP Operation:
- Equipment: UNIPOL-1202 automatic lapping and polishing machine.
- Polishing Plate: Aluminum oxide plate.
- Mechanical Parameters: Polishing rotational speed was set to 60 r/min, and polishing pressure was maintained at 1.09 MPa.
- Chemical Activation: A Merc-1000 W mercury lamp (UV light source) was used to irradiate the slurry and the diamond surface, activating the TiO₂ photocatalyst.
Slurry Characterization:
- Oxidizability Test: Oxidation-Reduction Potential (ORP) and conductivity were measured using an AZ86505 potentiometer.
- Photocatalytic Efficiency Test: Methyl orange degradation test was performed. The time required for the orange dye to fade (due to oxidation by ·OH radicals) indicated the slurry’s oxidizing power (60 min for complete degradation).

Commercial Applications

The ability to achieve ultra-smooth, damage-free surfaces on large-area monocrystal diamond is critical for several high-technology sectors:

Electronic Semiconductors: Diamond is a next-generation semiconductor material. Ultra-smooth polishing is essential for fabricating high-performance electronic devices, especially those requiring large-area wafers.
Optical Windows and Components: Diamond’s wide-range transparency and high hardness make it ideal for optical windows (e.g., high-energy lasers, harsh environments). Surface roughness must be minimized to prevent scattering and maintain optical clarity.
High-Fidelity Loudspeakers: Diamond diaphragms require extremely precise surface finishes to ensure optimal acoustic performance and fidelity.
High-Energy Accelerators: Diamond components used in accelerators require high structural integrity and smooth surfaces to meet stringent operational criteria.
Advanced Manufacturing (CMP/PCMP Slurries): The development of non-toxic, high-efficiency, and stable PCMP slurries provides a safer and more effective alternative to traditional chemical mechanical polishing formulations.

View Original Abstract

Diamond needs to have a perfectly smooth surface due to the growing requirements in the fields of electronic semiconductors, optical windows and high-fidelity loudspeakers. However, the polishing of diamonds is highly challenging due to their exceptional hardness and chemical stability. In this study, a new polishing slurry is prepared for the proposed photocatalysis-assisted chemical mechanical polishing (PCMP) approach to obtain an ultra-smooth surface for large-area diamond. The analyses and experimental findings revealed the significance of the photocatalyst, abrasive, electron capture agent and pH regulator as essential components of the PCMP slurry. TiO2 with a 5 nm pore size and P25 TiO2 possess improved photocatalysis efficiency. Moreover, diamond removal is smooth under the acidic environment of H3PO4 due to the high oxidation-reduction potential (ORP) of the slurry, and, during the methyl orange test, P25 TiO2 exhibits reasonable photocatalytic effects. Moreover, in 8 h, a smooth surface free of mechanical scratches can be obtained by reducing the surface roughness from Ra 33.6 nm to Ra 2.6 nm.

Tech Support

Original Source

DOI: https://doi.org/10.3390/machines11060664

References

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