Surface finish and subsurface damage distribution during diamond turning of silicon

At a Glance

Metadata	Details
Publication Date	2018-07-11
Journal	Advanced Materials Proceedings
Authors	Rohit Sharma, Neha Khatri, Vinod Mishra, Harry Garg, Vinod Karar .
Citations	2
Analysis	Full AI Review Included

Diamond Materials for Ultra-Precision Machining: Subsurface Damage Analysis in Silicon Turning

Executive Summary

This documentation analyzes a study on achieving sub-nanometer surface roughness on optical-grade Silicon wafers using Single Point Diamond Turning (SPDT) and correlates the findings to the necessity and specifications of high-performance CVD diamond products offered by 6CCVD.

Precision Target Achieved: The study successfully utilized diamond turning to achieve an optimized surface roughness (Ra) of 0.243 nm on Si substrates, a critical benchmark for high-performance optical and infrared components.
High-Pressure Phase Change: Diamond turning applied sufficient pressure (estimated >10 GPa) to induce a structural phase change, transforming the crystalline Si surface layer into amorphous silicon (a-Silicon), indicating the formation of Subsurface Damage (SSD).
Critical Machining Recipe: Optimal ductile-regime machining was achieved using controlled parameters: 1000 rpm Spindle Speed, 2.5 µm/rev Tool Feed Rate, and a shallow 0.5 µm Depth of Cut (DOC).
SSD Detection Method: Laser µ-Raman Spectroscopy was confirmed as a primary, non-destructive method for characterizing SSD, identified by a sharp Raman shift at 409 cm^-1 corresponding to the amorphous phase.
Tool Requirements: The experiment required a robust, precisely shaped diamond tool blank featuring a 1.50 mm Tool Nose Radius (TNR) and a highly negative rake angle (-20°), necessitating high-purity Single Crystal Diamond (SCD) material.
6CCVD Value Proposition: Replicating or extending this ultra-precision work requires SCD blanks with stringent structural integrity and precise orientation, matching 6CCVD’s core manufacturing capability for optical and mechanical grades.

Technical Specifications

Data extracted from the analysis of diamond turning parameters and resulting material characteristics in the Silicon substrate.

Parameter	Value	Unit	Context
Achieved Surface Roughness (Ra)	0.243	nm	Best combination result for Si substrate
Spindle Speed (SS)	1000	rpm	Optimized machining speed
Tool Feed Rate (TFR)	2.5	µm/rev	Optimized machining parameter
Depth of Cut (DOC)	0.5	µm	Used for achieving minimum surface roughness
Tool Nose Radius (TNR)	1.50	mm	Specification of the diamond tool
Tool Rake Angle	-20	°	Highly negative angle used
Raman Amorphous Shift	409	cm^-1	Characteristic shift indicating a-Silicon SSD layer
Estimated Machining Pressure	>10	GPa	Required threshold to cause phase change in Si
Workpiece Material	Optical grade Si (100)	N/A	Substrate machined
Machine Feedback Accuracy	0.09	nm	Capability of the Nanoform-250 equipment

Key Methodologies

The following ordered list outlines the critical steps and material specifications used to achieve ductile-regime machining and characterize the resulting subsurface damage (SSD).

Workpiece Preparation: A circular disc of optical grade Silicon with (100) structural orientation (50 mm diameter, 15 mm thickness) was prepared for testing.
Equipment Setup: A 2-axis Diamond Turning Machine (Nanoform-250) was used, mounted on an air floating table to minimize environmental vibrations and ensure mechanical feedback accuracy of 0.09 nm.
Tool Specification: A single-point diamond tool with a fixed overhang was utilized, specified with a 1.50 mm Tool Nose Radius (TNR) and a -20° negative rake angle.
Machining Recipe (Optimum Run): Machining was performed in the ductile regime using the following parametric combination:
- Spindle Speed (SS): 1000 rpm
- Tool Feed Rate (TFR): 2.5 µm/rev
- Depth of Cut (DOC): 0.5 µm
Surface Characterization: Surface finish was measured using a Coherence Correlation Interferometer (CCI-OPTICS).
Subsurface Damage (SSD) Characterization: Laser µ-Raman Spectroscopy (using a 633 nm laser) was employed for non-destructive detection of the amorphous layer (a-Silicon) formed beneath the machined surface, observed via the 409 cm^-1 shift.

6CCVD Solutions & Capabilities

This study highlights the extreme demands placed on cutting tool materials during ultra-precision machining. 6CCVD is positioned as the primary supplier for the diamond materials necessary to achieve or surpass these demanding specifications.

Applicable Materials

The achievement of nanometer-scale roughness and the maintenance of a sharp cutting edge under high stress (>10 GPa) explicitly requires the highest grade of material.

Optical Grade Single Crystal Diamond (SCD): This is the ideal material for high-precision SPDT tools. 6CCVD supplies SCD blanks optimized for tool fabrication, offering:
- Superior Hardness and Wear Resistance: Essential for maintaining the critical 1.50 mm TNR and -20° rake angle required for ductile-regime machining of brittle materials like Si.
- Low Defect Density: High-purity MPCVD SCD ensures maximum thermal and mechanical stability, critical for enduring the high localized pressures that cause amorphous layer formation.
Polycrystalline Diamond (PCD): While SCD is preferred for monolithic SPDT tools, 6CCVD’s large-area PCD plates (up to 125 mm) are ideal for large-scale tooling inserts or wear parts in related micro-mechanical systems where Ra < 5 nm finish is acceptable.

Customization Potential

The experimental setup required specific geometries and mounting processes, areas where 6CCVD provides direct, in-house technical support.

Paper Requirement	6CCVD Solution & Service	Benefit to Engineer
Complex Tool Geometry (1.50 mm TNR, -20° Rake)	Custom Dimensions & Orientations	Supply SCD blocks pre-cut to engineer specifications, ensuring correct crystallographic alignment for optimal edge strength.
Tool Mounting and Bonding	Custom Metalization Services	Internal capability for standard and custom metal stacks (Au, Pt, Pd, Ti, W, Cu) essential for creating strong ohmic or structural contacts for tool integration and thermal dissipation.
Ultra-low Roughness requirement	Precision Polishing	Guaranteed SCD polishing to Ra < 1 nm, enabling researchers to achieve the desired surface finish immediately upon receipt of the material blank.
Substrate Size (50 mm)	Large Format Capability	6CCVD offers SCD and PCD plates/wafers up to 125 mm, allowing researchers to scale up testing or develop larger optical components.

Engineering Support

6CCVD’s in-house PhD team provides expert consultation essential for advancing high-stress and micro-machining research. We assist clients with material selection, orientation optimization, and thickness specifications for projects involving brittle materials, high-pressure cells, or high-wear surfaces (e.g., machining Si, Ge, and various compound semiconductors).

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

View Original Abstract

Subsurface Damage (SSD), which is introduced to optical materials by diamond turning processes, affects the performance in optical, laser and infrared applications. For optical applications, SSD can be the source of component instability (e.g., surface stress) and flaw. The objective of the present study is to investigate the subsurface damage in silicon. Interferometry and Raman Spectroscopy are used to detect the surface finish and SSD. The surface roughness of 0.243 nm is achieved at best combination. A sharp Raman shift at 409 cm-1 is obtained, which reveals that a thin layer of Silicon has transformed to amorphous state resulting in subsurface damages. Copyright © 2017 VBRI Press.

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Original Source

DOI: https://doi.org/10.5185/amp.2017/706