A Systematic Study of the Factors Affecting the Surface Quality of Chemically Vapor-Deposited Diamond during Chemical and Mechanical Polishing

At a Glance

Metadata	Details
Publication Date	2024-03-28
Journal	Micromachines
Authors	Zewei Yuan, Zhihui Cheng, Yusen Feng
Institutions	Shenyang University of Technology
Citations	5
Analysis	Full AI Review Included

Technical Documentation & Analysis: Ultra-Smooth CVD Diamond Polishing

Executive Summary

This research validates Chemical Mechanical Polishing (CMP) as the superior method for achieving ultra-smooth surfaces on CVD diamond, a critical requirement for advanced semiconductor, optical, and thermal management applications.

CMP Superiority: CMP effectively utilizes the crystal anisotropy of CVD diamond to efficiently erase uneven steps and grain boundaries, achieving surface roughness (Ra) significantly lower than traditional mechanical polishing (MP).
Roughness Achievement: The study successfully demonstrated surface roughness below Ra 3 nm using optimized CMP techniques, confirming the feasibility of producing device-ready diamond wafers.
Material Dependence: Single Crystal Diamond (SCD) is inherently easier to polish, achieving Ra values below 1 nm, as its surface quality is independent of grain boundaries and porosity.
PCD Limitations: For Polycrystalline Diamond (PCD), grain boundaries and pores become the primary limiting factors for surface quality once roughness drops below Ra 20 nm, necessitating precise control of abrasive size and polishing time.
Abrasive and Time Control: Achieving optimal surface quality requires a carefully planned, multi-stage polishing process, transitioning from large abrasives (W10, W5) to ultra-fine abrasives (W0.1, 0.2 µm CMP slurry) over extended processing times (up to 150 hours).
6CCVD Value Proposition: 6CCVD specializes in delivering SCD and large-area PCD wafers pre-polished to industry-leading specifications (Ra < 1 nm for SCD, Ra < 5 nm for PCD), eliminating the need for customers to undertake complex, time-consuming, and defect-prone in-house polishing development.

Technical Specifications

The following hard data points were extracted from the analysis of CVD diamond polishing parameters and results:

Parameter	Value	Unit	Context
Target Surface Roughness (CMP)	< 3	nm	Achieved roughness for device applications.
Achieved SCD Roughness	< 1	nm	Achieved via polishing, independent of anisotropy.
6CCVD Guaranteed SCD Roughness	< 1	nm	Standard specification for optical grade SCD.
6CCVD Guaranteed PCD Roughness	< 5	nm	Standard specification for inch-size PCD.
Critical PCD Roughness Threshold	20	nm	Below this, grain boundaries/pores limit MP effectiveness.
Initial PCD Roughness (Growth Surface)	13-18	µm	Roughness before initial lapping/grinding.
Required Removal Height (PCD)	180	µm	Minimum layer removal needed to achieve smooth surface.
CMP Abrasive Particle Size	0.2	µm	Boron carbide used in the optimized CMP slurry.
MP Fine Abrasive Limit	0.1	µm	W0.1 diamond powder used in final MP stage (Ra 8 nm).
Polishing Time (W0.1 Abrasive)	150	hours	Time required to reach stable Ra 8 nm state.
Diamond Hardness Range	80 to 120	GPa	Varies significantly based on crystal plane orientation.

Key Methodologies

The study focused on optimizing the Chemical Mechanical Polishing (CMP) process, contrasting it with traditional Mechanical Polishing (MP) and grinding.

Sample Preparation:
- Single Crystal Diamond (SCD) samples were 0.8 mm thick, cut into 5 mm x 5 mm pieces.
- Polycrystalline Diamond (PCD) samples were 2-inch diameter, 1 mm thick, produced by DC Arc Plasma Jet method.
- Initial rough surfaces (Ra up to 15 µm) required removal of at least 180 µm of material.
Mechanical Polishing (MP):
- Utilized diamond lapping and polishing equipment (rotational speed 750 r/min).
- Abrasive progression: W10, W5, W2, W0.5, and W0.1 diamond powders.
- Grinding stages used ceramic-based diamond grinding wheels (2000# and 10,000#) at 5000 r/min.
Chemical Mechanical Polishing (CMP) Slurry Recipe:
- Oxidant: 30 g of K₂FeO₄ and 10 mL of H₂O₂ in 500 mL of deionized water.
- Abrasive: Boron carbide powder (0.2 µm particle size).
- Accelerants: Phosphoric acid and ferrous sulfate solutions were added to accelerate the chemical reaction (generating Fenton’s reagent).
CMP Process Control:
- Sample rotation was critical to minimize unidirectional scratches and achieve optimal surface quality.
- Chemical action strength must be controlled to prevent the formation of chemical etching pits, especially at grain boundaries and pores.
Characterization:
- Surface morphology analyzed using OLUMPUS laser scanning confocal microscope.
- Surface roughness (Ra, Rz) measured using Mitutoyo SJ-411 and Zygo New View 5022 profilers.

6CCVD Solutions & Capabilities

This research confirms that achieving the surface quality required for high-performance applications (Ra < 3 nm) is highly complex, demanding specialized CMP expertise, extended processing times, and precise control over material structure. 6CCVD eliminates this development burden by providing application-ready diamond materials.

Applicable Materials for Replication and Extension

To replicate or extend the high-quality surface results demonstrated in this study, 6CCVD recommends the following materials, pre-polished to specification:

Application Requirement (Paper)	6CCVD Material Solution	Key 6CCVD Specification
Semiconductor Substrates (Ra < 1 nm)	Optical Grade Single Crystal Diamond (SCD)	Guaranteed Ra < 1 nm polishing.
Large-Area Optical Windows (Ra 1-3 nm)	High-Purity Polycrystalline Diamond (PCD)	Wafers up to 125 mm diameter, Ra < 5 nm standard.
High Thermal Conductivity Applications	SCD or High-Purity PCD Substrates	Thicknesses available from 0.1 µm up to 10 mm.
Electrodes/Chemical Fields	Boron-Doped Diamond (BDD)	Custom doping levels available for electrochemical stability.

Customization Potential

The study highlights the need for specific dimensions (5 mm x 5 mm SCD, 2-inch PCD) and the importance of material removal (180 µm layer removal). 6CCVD’s in-house capabilities directly address these requirements:

Custom Dimensions: While the paper used 2-inch PCD, 6CCVD offers PCD wafers up to 125 mm (5 inches) in diameter, suitable for next-generation large-area SAW devices and optical windows.
Thickness Control: 6CCVD provides precise thickness control for both SCD and PCD films (0.1 µm to 500 µm) and substrates (up to 10 mm), ensuring the material is delivered post-polishing, meeting the required final thickness without the need for extensive material removal by the customer.
Advanced Polishing: 6CCVD guarantees ultra-smooth surfaces, exceeding the paper’s Ra 3 nm achievement. Our proprietary CMP techniques ensure Ra < 1 nm for SCD and Ra < 5 nm for inch-size PCD, minimizing defects like etching pits and scratches.
Metalization Services: For device integration (e.g., semiconductor contacts, X-ray detectors), 6CCVD offers internal metalization capabilities, including Au, Pt, Pd, Ti, W, and Cu deposition, providing a complete, ready-to-use substrate solution.

Engineering Support

The paper emphasizes that polishing success hinges on managing crystal anisotropy, grain boundaries, and optimizing the chemical action of the slurry—challenges that require deep material science expertise.

6CCVD’s in-house PhD engineering team specializes in diamond growth and post-processing optimization. We offer consultation services to assist researchers and engineers with:

Material Selection: Guiding the choice between SCD and PCD based on required surface roughness and application size (e.g., selecting PCD for large-area optical windows or SCD for high-fidelity semiconductor substrates).
Process Integration: Advising on the optimal material specifications (crystal orientation, thickness, doping) for similar high-power infrared laser, X-ray detector, or SAW device projects.
Custom Polishing Recipes: Developing specific polishing protocols to mitigate defects inherent to the customer’s application, ensuring the delivered material meets stringent quality criteria (e.g., minimizing sub-surface damage or eliminating chemical etching pits).

For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly.

View Original Abstract

Diamond surfaces must be of high quality for potential use in semiconductors, optical windows, and heat conductivity applications. However, due to the material’s exceptional hardness and chemical stability, it can be difficult to obtain a smooth surface on diamond. This study examines the parameters that can potentially influence the surface quality of chemically vapor-deposited (CVD) diamonds during the chemical and mechanical polishing (CMP) process. Analysis and experimental findings show that the surface quality of polished CVD diamonds is significantly influenced by the crystal structure and the growth quality of the diamond. In particular, when the surface roughness is below Ra 20 nm, the pores and grain boundaries on CVD diamond obstruct surface roughness reduction during mechanical polishing. To obtain a smooth polished surface, careful consideration of the size of diamond abrasives and polishing methods is also a prerequisite. Chemical mechanical polishing is a novel method to achieve a surface quality with roughness below Ra 3 nm, as in this method, the anisotropy of the CVD diamond allows the uneven steps to be efficiently erased. However, the chemical actions of polishing slurry should be controlled to prevent the formation of chemical etching pits.

Tech Support

Original Source

DOI: https://doi.org/10.3390/mi15040459

References

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