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, Slovak Academy of Sciences
Analysis	Full AI Review Included

Technical Documentation & Analysis: Diamond Membranes for MEMS Sensors

Executive Summary

This research successfully demonstrates the feasibility of fabricating high-precision diamond membranes and cantilevers for MEMS sensor applications using MPCVD diamond films and femtosecond (fs) laser ablation.

Core Achievement: Successful fabrication of thin, robust diamond membranes (up to 22 µm thick) by selectively removing the underlying 525 µm Si substrate using fs laser ablation.
Material Basis: Diamond films were grown via Microwave Plasma Chemical Vapor Deposition (MPCVD) on standard Si/SiO₂ substrates, confirming compatibility with existing semiconductor platforms.
Precision Processing: Femtosecond laser ablation (520 nm, 100 kHz) provides a non-contact, precise micromachining method, overcoming the chemical resistivity and hardness challenges of diamond.
Surface Functionalization: The process generated Laser-Induced Periodic Surface Structures (LIPSS), which are crucial for tailoring surface chemistry in applications like gas sensors, photocatalysis, and electronic devices.
Future Potential: The material properties suggest the ability to produce even thinner membranes (< 7 µm) using this selective laser ablation technique, pushing the limits of high-sensitivity MEMS.
6CCVD Relevance: This work validates the need for high-quality, precisely controlled, thin-film PCD material, a core offering of 6CCVD, for advanced sensor development.

Technical Specifications

The following hard data points were extracted from the research paper detailing the deposition and ablation parameters:

Parameter	Value	Unit	Context
Substrate Material	(100) Si	N/A	Standard semiconductor wafer
Substrate Thickness	525	µm	Thickness removed by ablation
Buffer Layer Thickness	1.3	µm	SiO₂ layer on Si
Final Membrane Thickness (Achieved)	22	µm	Laser-generated diamond membrane
Target Diamond Thickness Range	7.2 and 21.7	µm	Corresponding to 16h and 32h deposition times
CVD MW Power	4.2	kW	Microwave Plasma Reactor Setting
CVD Pressure	90	mbar	Deposition condition
CVD Temperature	960	°C	Deposition condition
Gas Mixture (CH₄)	5	%	In H₂ carrier gas
Gas Mixture (CO₂)	1.5	%	In H₂ carrier gas
Laser Wavelength	520	nm	Femtosecond (fs) laser ablation source
Ablation Pulse Frequency	100	kHz	Effective frequency used for ablation
Focus Diameter	18	µm	Scanner lens focus size
Scan Speed	500	mm/s	Ablation process parameter
Bore Diameter Reduction	2 to 1	mm	Optimization to reduce corner cavity formation

Key Methodologies

The experiment relied on a multi-step process combining standard semiconductor preparation, high-power MPCVD, and optimized femtosecond laser processing.

Substrate Preparation: 525 µm thick (100) Si substrates, pre-coated with a 1.3 µm SiO₂ layer, were used as the growth platform.
Seeding: Substrates were ultrasonically seeded using nanodiamond powder suspended in DI water to promote nucleation density for subsequent CVD growth.
MPCVD Growth: Diamond films were deposited using an ellipsoidal cavity microwave plasma reactor.
- Recipe: 5% CH₄ and 1.5% CO₂ mixed with H₂.
- Conditions: 4.2 kW MW power, 90 mbar pressure, and 960 °C deposition temperature.
Femtosecond Laser Ablation: A 520 nm fs laser (SPIRIT) was applied at a 100 kHz effective pulse frequency to selectively remove the 525 µm Si substrate from the backside.
Process Optimization: To ensure smooth membrane edges and minimize defects (pinholes/cavities):
- The direction of laser polarization was rotated during ablation.
- The bore diameter was reduced in three steps (2 mm to 1 mm) to attenuate corner cavity effects.
Surface Structuring: Laser-Induced Periodic Surface Structures (LIPSS) were generated on the diamond surface to enhance functionalization for chemical sensing applications.

6CCVD Solutions & Capabilities

6CCVD is uniquely positioned to supply the high-quality MPCVD diamond materials and precision processing required to replicate and advance this MEMS membrane fabrication technique.

Applicable Materials

The research requires high-quality, thin-film diamond grown on standard Si substrates. 6CCVD offers two primary solutions depending on the final sensor application:

Optical Grade Polycrystalline Diamond (PCD):
- Application Match: Ideal for mechanical MEMS, pressure sensors, and applications requiring high thermal conductivity and mechanical hardness.
- Thickness Control: We supply PCD films with precise thickness control from 0.1 µm up to 500 µm, easily meeting the 7.2 µm to 22 µm range demonstrated in the paper.
- Substrate Compatibility: Standard growth on Si/SiO₂ wafers up to 125 mm diameter.
Heavy Boron-Doped Diamond (BDD):
- Application Match: If the MEMS device is intended for electrochemical sensing (e.g., pH, heavy metal detection), BDD provides the necessary conductivity and chemical stability.
- Doping Levels: Custom doping levels available to optimize conductivity for specific electronic or sensing requirements.

Customization Potential for MEMS Fabrication

The paper highlights the need for precise material removal and surface structuring. 6CCVD provides integrated services that streamline the transition from raw material to functional MEMS component:

Research Requirement	6CCVD Capability	Technical Advantage
Thin Film Precision	SCD/PCD Thickness Control (0.1 µm - 500 µm)	Guarantees the exact thickness required for specific mechanical resonance or pressure sensitivity.
Smooth Membrane Surfaces	Advanced Polishing Services	Achieves surface roughness (Ra) < 5 nm for inch-size PCD, minimizing stress concentration and maximizing membrane lifetime.
Complex Geometry Definition	Custom Laser Cutting & Etching	We offer high-precision laser cutting and Reactive Ion Etching (RIE) to define cantilevers, membranes, and bore holes, providing an alternative or complement to fs laser ablation.
Electrical Contact Integration	In-House Metalization	We apply custom metal stacks (e.g., Ti/Pt/Au, W, Cu) directly to the diamond surface, essential for creating electrical contacts for sensor readout or heating elements (as needed for gas sensors).
Large Area Processing	PCD Wafers up to 125 mm	Supports high-volume MEMS production and integration with standard semiconductor fabrication lines.

Engineering Support

6CCVD’s in-house PhD team specializes in diamond material science and device integration. We can assist engineers and scientists with material selection, optimizing film quality (e.g., grain size, stress control) for similar MEMS sensor and high-frequency cantilever projects. We offer global shipping (DDU default, DDP available) for rapid delivery of custom materials worldwide.

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

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