Fabrication of Diamond Membranes by Femtosecond Laser Ablation for MEMS Sensor Applications

At a Glance

Metadata	Details
Publication Date	2020-12-10
Authors	Johann Zehetner, Alexander Kromka, Tibor Izsák, G. Vanko, Lenka Gajdošová
Institutions	Czech Academy of Sciences, Institute of Physics, Institute of Electrical Engineering of the Slovak Academy of Sciences
Analysis	Full AI Review Included

Executive Summary

This research demonstrates a robust fabrication pathway for creating free-standing diamond membranes suitable for Micro-Electro-Mechanical Systems (MEMS) and specialized sensor applications using femtosecond (fs) laser ablation.

Core Achievement: Successful selective removal of the entire 525 µm thick Si substrate layer to release a high-quality, 22 µm thick Chemical Vapor Deposition (CVD) diamond membrane.
Material Stack: Diamond films were grown on 525 µm (100) Si substrates buffered by a 1.3 µm thick SiO₂ layer, leveraging the high ablation threshold difference between diamond and Si.
Process Optimization: Critical issues like corner cavity formation and pinholes were mitigated by rotating the laser polarization direction and employing a three-step reduction of the bore diameter (2 mm down to 1 mm).
Surface Functionalization: The fs laser process inherently generates Laser-Induced Periodic Surface Structures (LIPSS), which can be utilized to tailor the surface chemistry for enhanced sensor performance (e.g., gas sensing).
Methodology: The membrane was fabricated entirely by fs laser ablation, requiring at least 700 consecutive scans to remove the bulk Si material, demonstrating the precision and control of the technique.

Technical Specifications

Parameter	Value	Unit	Context
Substrate Material	(100) Si	-	Thickness 525 µm
Buffer Layer	SiO₂	-	Thickness 1.3 µm
Diamond Thickness (Achieved)	21.7	µm	Grown over 32 hours
CVD Reactor Type	Ellipsoidal Cavity	-	Microwave Plasma Reactor
CVD Power	4.2	kW	Microwave (MW) power
CVD Pressure	90	mbar	Deposition pressure
CVD Temperature	960	°C	Deposition temperature
Gas Mixture (CH₄)	5	%	Methane concentration
Gas Mixture (CO₂)	1.5	%	Carbon Dioxide concentration
Laser Type	Femtosecond (fs)	-	SPIRIT (Spectra Physics)
Laser Wavelength	520	nm	Ablation setting
Effective Pulse Frequency	100	kHz	Every tenth pulse used from 1 MHz base
Scan Speed	500	mm/s	Ablation rate
Line Distance (Scans)	5	µm	Spacing between consecutive scan lines
Focus Diameter	~18	µm	Using 170 mm scanner lens
Si Removal Depth	~525	µm	Required for membrane release
Final Membrane Notch Diameter	~1	µm	Shallow notches due to Si pinholes

Key Methodologies

The fabrication process combines standard diamond growth techniques with highly controlled femtosecond laser ablation for precise micromachining.

Substrate Preparation and Seeding:
- 525 µm thick (100) Si substrates covered with a 1.3 µm SiO₂ layer were used.
- Substrates were ultrasonically seeded using nanodiamond powder suspended in DI water prior to growth.
Diamond Chemical Vapor Deposition (CVD):
- Deposition was performed in an ellipsoidal cavity microwave plasma reactor.
- Key parameters included 4.2 kW MW power, 90 mbar pressure, 960 °C temperature, and a gas mixture of 5% CH₄ and 1.5% CO₂ in H₂.
- Deposition times of 16 and 32 hours yielded diamond films of 7.2 µm and 21.7 µm, respectively.
Femtosecond Laser Ablation Setup:
- A 520 nm fs laser was used, operating at an effective pulse frequency of 100 kHz (derived from a 1 MHz source).
- The ablation pattern used a 5 µm line distance and a scan speed of 500 mm/s.
Ablation Optimization for Deep Etching:
- To prevent excessive ablation and cavity formation at the corners of the bore, the laser polarization direction was rotated during the procedure.
- The bore diameter was reduced in three steps (2 mm, 1.5 mm, 1 mm) to attenuate corner effects.
Membrane Release:
- Selective laser ablation was performed on the backside of the Si/SiO₂/Diamond stack.
- At least 700 consecutive ablation scans were required to remove the approximately 525 µm of Si substrate, resulting in the release of the free-standing diamond membrane.

Commercial Applications

This technology provides a scalable and precise method for manufacturing robust components from wide-bandgap materials, targeting extreme environment applications.

Application Area	Specific Product/Function	Technical Advantage of Diamond
MEMS & Sensors	Pressure sensors, accelerometers, micro-actuators.	High mechanical hardness and chemical resistivity allow operation in harsh environments (high temperature, corrosive media).
Gas Sensing	Chemical sensor functionalization.	Laser-Induced Periodic Surface Structures (LIPSS) tailor surface chemistry, enhancing selectivity and sensitivity.
Electronics & Batteries	Electronic devices, high-power components.	Diamond’s wide bandgap and thermal properties are ideal for high-power or high-frequency electronics.
Photocatalysis	Solar cells, water splitting devices.	Diamond membranes can be functionalized (e.g., by LIPSS) to enhance surface area and catalytic activity.
Extreme Environment Devices	Downhole drilling sensors, aerospace components.	Ability to withstand high pressure and temperature where conventional Si-based MEMS fail.

View Original Abstract

We present the feasibility in fabricating membranes and cantilevers made of diamond grown on Si/SiO2 substrates by femtosecond laser ablation. In the ablation process, we generated nano- and microstructures on the membrane surface. Such laser-induced periodic surface structures (LIPSS) are useful in tailoring the surface chemistry. In combination with wet or reactive ion etching, smooth membranes were generated.

Tech Support

Original Source

DOI: https://doi.org/10.3390/proceedings2020056013

References

2013 - Bulk micromachining of SiC substrate for MEMS sensor applications [Crossref]
2016 - Manufacturing of membranes by laser ablation in SiC, sapphire, glass and ceramic for GaN/ferroelectric thin film MEMS & pressure sensors [Crossref]
2017 - Femtosecond Laser Processing of Membranes for Sensor Devices on different Bulk Materials