High aspect ratio diamond nanosecond laser machining

At a Glance

Metadata	Details
Publication Date	2023-06-15
Journal	Applied Physics A
Authors	Natalie C. Golota, David Preiss, Zachary P. Fredin, Prashant Patil, Daniel Banks
Institutions	Massachusetts Institute of Technology, MIT-Harvard Center for Ultracold Atoms
Citations	16
Analysis	Full AI Review Included

Technical Documentation & Analysis: High Aspect Ratio Diamond Nanosecond Laser Machining

This document analyzes the research on achieving high aspect ratio (AR) and low taper structures in diamond using nanosecond laser machining, focusing on material requirements and process optimization. The findings directly inform the selection and customization of Chemical Vapor Deposition (CVD) diamond materials supplied by 6CCVD.

Executive Summary

High Aspect Ratio Achievement: Demonstrated successful nanosecond laser machining of high aspect ratio structures in diamond, achieving a maximum AR of 66.4:1 (average ~40:1) using rotary assisted drilling and high pulse accumulation (> 1.6 M equivalent pulses).
Low Taper Control: Developed a ramped pulse energy machining profile to minimize internal taper angle to an unprecedented 0.11°-0.17° over 3 mm depth in 10:1 AR tubes, crucial for microfluidic and NMR rotor applications.
Ablation Regimes: Identified distinct gentle and strong ablation regimes in HPHT diamond, characterized by an incubation coefficient of 0.919 ± 0.008, indicating that ablation threshold decreases with accumulated laser-induced damage.
Laser-Induced Damage Quantification: Confocal Raman spectroscopy confirmed the formation of surface graphite (sp² carbon) and quantified laser-induced tensile strain, which increased by up to 36% following high-energy irradiation.
Strain Mitigation: Successfully mitigated laser-induced internal tensile strain by approximately 50% using a post-machining oxidative heat treatment at 600 °C for 24 hours, resulting in considerable strain homogenization.
Material Requirement: The demanding nature of high AR drilling necessitates high-quality, low-defect diamond material, making high-purity Single Crystal Diamond (SCD) the optimal choice for industrial replication.

Technical Specifications

The following hard data points were extracted from the research detailing the laser parameters and achieved material characteristics:

Parameter	Value	Unit	Context
Laser Wavelength	532	nm	Nanosecond Nd:YAG system
Pulse Duration	~20	ns	Fixed parameter
Repetition Rate	5	kHz	Fixed parameter
Maximum Single Pulse Energy	594	µJ	Used for percussion drilling
Maximum Aspect Ratio (Rotary)	66.4	:1	Achieved at the exit hole
Average Aspect Ratio (Rotary)	~40	:1	Achieved over 1-3 machining cycles
Minimum Internal Taper Angle	0.11 - 0.17	°	Taper excluding chamfer, 10:1 AR tubes
Ablation Threshold Fluence (10,000 pulses)	12.4 ± 1.4	J/cm²	Gentle ablation regime
Single Pulse Threshold Fluence	29.5 ± 1.3	J/cm²	Strong ablation regime
Laser-Induced Strain Increase	Up to 36	%	Following high-energy irradiation
Post-Machining Heat Treatment	600	°C	Applied for 24 hours in air
Strain Reduction Post-Treatment	~50	%	Reduction in laser-induced tensile strain
Surface Graphite Layer Thickness	1 - 2	µm	Confirmed by Raman spectroscopy

Key Methodologies

The experiment relied on precise control over nanosecond laser parameters and advanced rotary mechanics to overcome the challenges of plasma attenuation and debris expulsion inherent in high aspect ratio diamond machining.

Material Selection: Type Ib HPHT diamond was used, characterized by its low absorption coefficient and high thermal stability, contributing to large ablation thresholds.
Laser System Configuration: A Q-switched 532 nm diode-pumped Nd:YAG laser was used, featuring a pulse duration of ~20 ns and a repetition rate of 5 kHz. Laser power was controlled via a motorized optical attenuator.
Percussion Hole Drilling: Holes were drilled using single pulse energies up to 594 µJ and pulse counts up to 10,000 to characterize gentle and strong ablation regimes.
Rotary Assisted Drilling (High AR): An in-house precision rotary drilling apparatus (up to 166 RPM) was employed. High AR was achieved using negative focus increments into the material and extremely high pulse accumulation (up to 1.68 M equivalent pulses per cycle).
Rotary Assisted Drilling (Low Taper Tubes): Taper minimization (0.11°-0.17°) was achieved by modulating the irradiation profile, specifically using ramped pulse energy during rough and finishing cuts (e.g., average pulse energies of 422 µJ or 507 µJ).
Characterization: MicroCT scanning (2.1 µm voxel size) and SEM were used for dimensional analysis (depth, diameter, taper).
Damage Analysis and Mitigation: Confocal Raman spectroscopy was used to map internal tensile strain. Laser-induced damage was mitigated by subjecting machined samples to oxidative heat treatment at 600 °C for 24 hours.

6CCVD Solutions & Capabilities

This research validates nanosecond laser machining as a viable, accessible method for fabricating complex diamond microstructures, provided the material quality and post-processing are rigorously controlled. 6CCVD offers the high-specification CVD diamond required to replicate and advance these demanding processes for next-generation devices.

Applicable Materials

The paper utilized Type Ib HPHT diamond. For high-precision applications like quantum devices, microelectronics, and advanced sensing, 6CCVD recommends materials with superior purity and consistency:

Optical Grade Single Crystal Diamond (SCD): Ideal for replicating this research, especially for applications requiring minimal defects (e.g., NV centers for quantum sensing) and maximum thermal stability. 6CCVD SCD ensures the lowest possible initial strain, optimizing the subsequent laser machining and thermal mitigation steps.
High Purity Polycrystalline Diamond (PCD): For larger area microfluidic or thermal management devices where the highest aspect ratios are needed over large areas, 6CCVD PCD wafers (up to 125 mm diameter) provide excellent thermal and mechanical properties.

Customization Potential

The fabrication of high aspect ratio tubes (e.g., for NMR rotors) requires precise material dimensions and specific surface treatments. 6CCVD’s capabilities directly address these needs:

Research Requirement	6CCVD Capability	Benefit to Customer
Thick Substrates (up to 4 mm)	SCD/PCD Substrates up to 10 mm thick.	Enables deeper drilling and higher aspect ratio structures than demonstrated.
High Aspect Ratio Drilling	SCD/PCD Thicknesses from 0.1 µm up to 500 µm (SCD/PCD wafers).	Provides the necessary material depth and quality for achieving 66:1 AR and beyond.
Low Taper/High Precision	SCD Polishing to Ra < 1 nm; PCD Polishing to Ra < 5 nm (Inch-size).	Minimizes surface defects that can initiate damage or interfere with laser ablation dynamics.
Custom Metalization	Internal capability for Au, Pt, Pd, Ti, W, Cu metalization.	Essential for integrating diamond structures into microelectronic or MEMS devices (e.g., bonding, electrical contacts).
Custom Shaping/Cutting	Advanced laser cutting and shaping services.	Allows for precise pre-shaping of diamond rods or wafers required for rotary assisted drilling setups.

Engineering Support

The successful mitigation of laser-induced tensile strain via 600 °C heat treatment highlights the importance of controlled post-processing. 6CCVD’s in-house PhD team can assist researchers and engineers with:

Material Selection Optimization: Guiding the choice between SCD and PCD based on specific application requirements (e.g., thermal management vs. quantum coherence).
Process Integration: Consulting on optimal laser parameters (fluence, pulse accumulation, and ramping profiles) necessary to achieve target aspect ratios and taper angles while minimizing damage.
Post-Processing Protocols: Developing and implementing customized thermal annealing or surface treatment protocols to mitigate laser-induced damage and strain for projects involving Diamond Rotors for Nuclear Magnetic Resonance (NMR) or high-power microelectronics.

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

Tech Support

Original Source

DOI: https://doi.org/10.1007/s00339-023-06755-2