Diamond Coated LW-SAW Sensors-Study of Diamond Thickness Effect

At a Glance

Metadata	Details
Publication Date	2017-08-08
Authors	L. Drbohlavová, Jean-Claude Gerbedoen, Andrew Taylor, Abdelkrim Talbi, Ladislav Fekete
Institutions	Université de Lille, Czech Academy of Sciences
Analysis	Full AI Review Included

Technical Documentation and Analysis: Diamond Coated LW-SAW Sensors

This analysis focuses on the successful implementation of Nano-Crystalline Diamond (NCD) coatings for Love Wave Surface Acoustic Wave (LW-SAW) sensors, demonstrating high sensitivity crucial for advanced biosensing applications. The thin, chemically inert NCD layer provides superior stability, addressing a critical need for long-term real-time monitoring sensors.

Executive Summary

Application Focus: Fabrication and characterization of Love wave acoustic sensors leveraging thin Nano-Crystalline Diamond (NCD) coatings for enhanced sensitivity and stability in liquid biosensing environments.
Material Achievement: Demonstrated high surface stiffness and chemical stability using thin NCD films deposited via MW-LA-PECVD, suitable for subsequent biomolecule functionalization.
Thickness Optimization: Experimentally verified that both phase velocity and center frequency increase proportionally with increasing NCD layer thickness, consistent with theoretical modeling.
Key Sensitivity Metric: Achieved a remarkable sensor sensitivity of 1170 cm²/g for a 100 nm thick NCD layer on a normalized silica guiding layer thickness (h_SiO2/λ = 0.1).
Methodology: Utilized a low-temperature (<500 °C) Plasma Enhanced Chemical Vapor Deposition (PECVD) process to preserve the piezoelectric properties of the underlying ST-cut quartz substrate.
Liquid Testing Insight: Identified the substrate’s low dielectric constant (ST-cut quartz) as a limiting factor in high dielectric constant liquids (like water/PBS), resulting in high insertion losses. Future work suggests utilizing substrates like LiTaO₃ or LiNbO₃.

Technical Specifications

The following hard data points were extracted from the investigation into diamond-coated LW-SAW sensors:

Parameter	Value	Unit	Context
Substrate	ST-cut Quartz	Crystal	Piezoelectric material base
IDT Thickness	200	nm	Aluminum interdigital transducers
IDT Spatial Periods (λ)	16 and 32	µm	Used for SAW generation
SiO₂ Guiding Layer Thickness	1.45	µm	PECVD deposited amorphous layer
NCD Thickness Range Tested	30 to 295	nm	Effect on phase velocity investigated
Maximum Sensitivity Achieved	1170	cm²/g	Achieved with 100 nm NCD layer (h_SiO2/λ = 0.1)
Diamond Deposition Temperature	<500	°C	Low temperature required to preserve quartz properties
Characteristic Diamond Raman Peak	1332	cm^-1	Clear diamond signature observed
Trans-Polyacetylene Raman Peak	1490	cm^-1	Attributed to sp² carbon content (NCD morphology)
Water Dielectric Constant (20 °C)	≈ 80.1	-	High value contributing to insertion loss
Water Viscosity (20 °C)	1.002	mPa·s	Baseline fluid property

Key Methodologies

The LW-SAW sensors were fabricated and tested using precise, multilayer deposition and characterization techniques:

Substrate Preparation: ST-cut quartz crystals were used as the piezoelectric base material.
IDT Fabrication: 200 nm thick Aluminum Interdigital Transducers (IDTs) were patterned using photolithography and lift-off, utilizing 16 µm and 32 µm spatial periods.
Guiding Layer Deposition: An amorphous SiO₂ layer (1.45 µm) was deposited via Plasma Enhanced Chemical Vapor Deposition (PECVD) to act as the LW wave guiding layer.
Selective Seeding: Clean lab tape was applied to protect the IDT contact pads before selective diamond seeding.
NCD Deposition: Thin Nano-Crystalline Diamond (NCD) layers were deposited using a Microwave Linear Antenna Plasma Enhanced Chemical Vapor Deposition (MW-LA-PECVD) system.
Process Control: Deposition temperature was strictly controlled at <500 °C to prevent degradation of the quartz piezoelectric properties. Consecutive deposition steps were used to vary and investigate the effect of diamond thickness.
Sensitivity Testing: Sensor response was measured as a function of deposited LOR (lift-off resist) polymer thickness, which simulated mass loading.
Liquid Characterization: Sensors were integrated into a home-made microfluidics set-up and tested with water, methanol (MeOH), isopropyl alcohol (IPA), and Phosphate Buffer Saline (PBS).

6CCVD Solutions & Capabilities

6CCVD’s expertise in customized MPCVD diamond synthesis is perfectly suited to meet the demanding material specifications of high-performance LW-SAW sensors, enabling researchers to replicate, optimize, and scale this promising biosensing technology.

Applicable Materials

The study utilizes Nano-Crystalline Diamond (NCD), a specific morphology of Polycrystalline Diamond (PCD). 6CCVD offers solutions tailored to this research:

Research Requirement	6CCVD Material Solution	Technical Advantage
Thin NCD Coating (30 nm to 295 nm)	Ultra-Thin Polycrystalline Diamond (PCD/NCD)	6CCVD guarantees thickness control from 0.1 µm (100 nm) to 500 µm, allowing precise optimization of the mass loading and stiffness layers far beyond the scope of this initial study.
Substrate Compatibility	Custom Thin Film Deposition Service	We can deposit thin diamond layers (<1 µm) onto customer-supplied exotic substrates (e.g., LiTaO₃, LiNbO₃, ST-cut Quartz) crucial for resolving the observed dielectric mismatch issues.
Enhanced Stability/Bio-Receptor Attachment	Optical Grade SCD or PCD	Diamond’s inertness provides superior long-term stability over gold or silicon, and our MPCVD material ensures high purity and surface quality (Ra < 5 nm for inch-size PCD).

Customization Potential for LW-SAW Devices

6CCVD’s in-house capabilities directly address the complexity of acoustic sensor fabrication:

Precision Thickness Control: We can tailor the diamond layer thickness in increments as small as 10 nm across the critical range (0.1 µm - 1 µm) to achieve optimal phase velocity and sensitivity parameters (e.g., maximizing the 1170 cm²/g sensitivity achieved in this paper).
Advanced Metalization Services: While this study used Al IDTs, long-term stability and high-frequency SAW devices often require robust contact pads. 6CCVD offers integrated custom metalization (Ti/Pt/Au, Au, Pt, Pd, W, Cu) for contact pads and transducer structures, ensuring stable electrical contacts compatible with microfluidics.
Custom Dimensions and Wafer Scale-Up: We provide diamond plates/wafers up to 125 mm (5 inches) in diameter, allowing researchers to move from small-scale academic testing to commercial batch production of LW-SAW sensors.
Surface Preparation: Achieving a smooth surface is vital for reliable microfluidic integration and sensor function. Our high-precision polishing achieves roughness levels down to Ra < 5 nm on large-area PCD substrates.

Engineering Support

6CCVD’s in-house PhD team provides consultative support, ensuring material selection and synthesis parameters are optimized for high-frequency acoustic devices:

We can assist with selecting alternative guiding layer materials or optimizing the diamond deposition recipe (e.g., refining NCD grain structure) to achieve lower acoustic losses and maximize sensitivity for similar Love Wave Acoustic Sensor projects.
We specialize in low-stress, low-temperature diamond growth techniques necessary to maintain the integrity and piezoelectric performance of temperature-sensitive substrates (like quartz or LiNbO₃).

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

View Original Abstract

This study focuses on the fabrication and characterization of Love wave surface acoustic wave (LW-SAW) sensors with a thin nano-crystalline diamond (NCD) coating with an integrated microfluidics system. The effect of diamond layer thickness on the acoustic wave phase velocity and the sensor’s sensitivity have been investigated experimentally and compared with theoretical simulations. The fabricated sensors have been tested with a several liquids using a home-made microfluidics system.

Tech Support

Original Source

DOI: https://doi.org/10.3390/proceedings1040540

References

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