Microstructures Manufactured in Diamond by Use of Laser Micromachining

At a Glance

Metadata	Details
Publication Date	2020-03-06
Journal	Materials
Authors	Mariusz Dudek, Adam Rosowski, Marcin Kozanecki, Malwina Jaszczak, W. Szymański
Institutions	Lodz University of Technology, Oxford Lasers (United Kingdom)
Citations	8
Analysis	Full AI Review Included

Executive Summary

This research details the successful application of UV nanosecond laser micromachining for creating high-precision microstructures in polycrystalline diamond synthesized via Microwave Plasma-Enhanced Chemical Vapor Deposition (MW PECVD).

High-Precision Geometry: Achieved deep, narrow grooves with exceptional geometry, including depths up to 270 µm, widths as narrow as 10 µm, and near-vertical wall slopes (> 88.9°).
Surface Quality: Optimal processing parameters resulted in ultra-smooth surfaces, achieving a roughness (Ra) as low as 0.135 µm.
Material Integrity Control: Raman spectroscopy confirmed that the degree of diamond modification (conversion of sp³ diamond to sp² graphite/amorphous carbon) is critically dependent on scan speed.
Optimal Recipe: The best results for geometry and material preservation were obtained using 9.5 W average power, 100 mm/s scan speed, and 5 µm hatching distance.
Mechanism Insight: Low scan speeds facilitate better heat transfer away from the modified zone, preventing the rapid temperature increase that drives the phase transition to graphitic carbon.
Core Value Proposition: The demonstrated technique enables the manufacturing of complex, high-aspect-ratio features necessary for advanced diamond-based microfluidic and MEMS devices.

Technical Specifications

Parameter	Value	Unit	Context
Diamond Synthesis	MW PECVD	N/A	Polycrystalline plate
Diamond Thickness	530	µm	Starting material
Microwave Power	3.6	kW	PECVD condition
Substrate Temperature	820	°C	PECVD condition
Methane Content	2	%	CH₄/H₂ gas mixture
Diamond Growth Rate	1	µm/h	Deposition speed
Hardness	85.1 ± 10.2	GPa	Material property
Young’s Modulus	1114.5 ± 183.8	GPa	Material property
Thermal Conductivity	1040 / 1280	W/mK	Material property
Laser System	DPSS (Coherent AVIA)	N/A	Diode Pumped Solid State
Laser Wavelength	355	nm	UV range (high absorption)
Pulse Repetition Rate	50	kHz	Fixed frequency
Pulse Duration	25 / 35	ns	Nanosecond regime
Focused Spot Size	~21	µm	Using 163 mm F-Theta lens
Machining Results (Optimal)
Average Power Range Tested	5 to 12	W	Variable parameter
Scan Speed Range Tested	50 to 1000	mm/s	Variable parameter
Hatching Distance Range	5 to 20	µm	Variable parameter
Optimal Groove Depth	270	µm	Achieved maximum
Optimal Groove Width	10	µm	Achieved minimum
Optimal Wall Slope	> 88.9	°	High perpendicularity
Best Roughness (Ra)	0.135	µm	Achieved with optimal parameters
Undamaged Diamond Peak	1332	cm^-1	Raman shift (sp³ carbon)

Key Methodologies

Diamond Synthesis (MW PECVD)

Substrate Preparation: Silicon substrates were seeded using a suspension of detonation nanodiamond in ethanol via ultrasonic treatment.
Deposition Parameters: Diamond was grown using a CH₄/H₂ gas mixture (2% methane content) at a total flow rate of 800 sccm.
PECVD Conditions: The process was maintained at 87 Torr pressure, 820 °C substrate temperature, and 3.6 kW microwave power, yielding a 1 µm/h growth rate.

Laser Micromachining Process

Laser Setup: A 355 nm nanosecond DPSS laser (50 kHz) was directed via a galvanometer scanner head and focused by a 163 mm F-Theta lens to a spot size of approximately 21 µm.
Ablation Technique: A hatching technique was used, filling defined shapes with bidirectional lines (0° and 90° double-pattern) to achieve deep engraving.
Parameter Variation: Experiments varied three main parameters:
- Average Power: 5 W to 12 W.
- Scanning Speed: 50 mm/s to 1000 mm/s (affecting pulse overlap from 0% to 90%).
- Hatching Distance: 5 µm to 20 µm.
Atmosphere Control: No purge gas was used during machining, which resulted in the formation of debris (hillocks) on groove edges, particularly at high power and low speed.
Optimal Processing: The best geometric results (deep, perpendicular grooves) were achieved at 9.5 W power, 100 mm/s scan speed, and 5 µm hatching distance.

Characterization

Geometry and Roughness: Measured using Confocal Laser Scanning Microscopy (CLSM, Nikon MA200) and confirmed by Scanning Electron Microscopy (SEM, S-3000N Hitachi).
Material Modification: Analyzed using Confocal Raman Microspectrometry (Jobin-Yvon T-64000) with a 514.5 nm Ar line excitation source.
Raman Findings: High scan speed (400 mm/s) or high power (11 W) led to the complete disappearance of the sp³ diamond peak (1332 cm^-1) and the appearance of D and G peaks (sp² carbon), indicating graphitization due to excessive local heating.

Commercial Applications

The ability to precisely shape bulk polycrystalline diamond with minimal structural damage opens doors for several high-value engineering applications:

Microfluidic Devices: Manufacturing complex, high-aspect-ratio channels and reservoirs for diamond-based microfluidic chips, leveraging diamond’s chemical inertness and high thermal conductivity.
MEMS (Microelectromechanical Systems): Creation of three-dimensional diamond structures for robust sensors, actuators, and mechanical components operating in harsh environments.
Biomedical Technology: Utilizing diamond’s superior biocompatibility for advanced biomedical devices, including electrophoretic microchips and implants.
Thermal Management: Fabrication of micro-channels within diamond substrates for highly efficient micro-channel heat sinks, capitalizing on diamond’s exceptional thermal conductivity.
Optoelectronics: Patterning diamond surfaces for applications such as antireflection structures or patterned “black diamond” for future optoelectronic devices.

View Original Abstract

Different microstructures were created on the surface of a polycrystalline diamond plate (obtained by microwave plasma-enhanced chemical vapor deposition—MW PECVD process) by use of a nanosecond pulsed DPSS (diode pumped solid state) laser with a 355 nm wavelength and a galvanometer scanning system. Different average powers (5 to 11 W), scanning speeds (50 to 400 mm/s) and scan line spacings (“hatch spacing”) (5 to 20 µm) were applied. The microstructures were then examined using scanning electron microscopy, confocal microscopy and Raman spectroscopy techniques. Microstructures exhibiting excellent geometry were obtained. The precise geometries of the microstructures, exhibiting good perpendicularity, deep channels and smooth surfaces show that the laser microprocessing can be applied in manufacturing diamond microfluidic devices. Raman spectra show small differences depending on the process parameters used. In some cases, the diamond band (at 1332 cm−1) after laser modification of material is only slightly wider and shifted, but with no additional peaks, indicating that the diamond is almost not changed after laser interaction. Some parameters did show that the modification of material had occurred and additional peaks in Raman spectra (typical for low-quality chemical vapor deposition CVD diamond) appeared, indicating the growing disorder of material or manufacturing of the new carbon phase.

Tech Support

Original Source

DOI: https://doi.org/10.3390/ma13051199

References

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