Flexible, diamond-based microelectrodes fabricated using the diamond growth side for neural sensing

At a Glance

Metadata	Details
Publication Date	2020-07-12
Journal	Microsystems & Nanoengineering
Authors	Bin Fan, Cory A. Rusinek, Cort H. Thompson, Monica B. Setien, Yue Guo
Institutions	Michigan State University, Fraunhofer USA
Citations	69
Analysis	Full AI Review Included

Technical Documentation & Analysis: Flexible BDD Microelectrodes for Neural Sensing

Executive Summary

This documentation analyzes the fabrication and performance of flexible, boron-doped polycrystalline diamond (BDD) microelectrodes optimized for simultaneous neurochemical and neurophysiology sensing. The core innovation lies in utilizing the BDD growth side as the active sensing surface, which significantly enhances device performance.

Material Superiority: The BDD growth side exhibited superior electrochemical properties, including higher sp³ content, larger grain size (0.5 µm), and lower non-diamond sp² impurities compared to the nucleation side.
Enhanced Sensitivity: The optimized BDD growth side electrodes achieved high sensitivity for Dopamine (DA) detection, demonstrating a low Limit of Detection (LOD) of 830 nM, even in the presence of high concentrations of Ascorbic Acid (AA).
Improved Kinetics & Stability: The growth side showed faster electron transfer kinetics (∆E_p = 80 mV for Ru(NH₃)₆²⁺/³⁺) and significantly reduced chemical fouling/absorption compared to the nucleation side.
Low Impedance for Recording: The rougher growth surface morphology reduced the 1 kHz electrochemical impedance to ~200-250 kΩ, minimizing noise for high-quality extracellular neural recording.
Flexible Platform: The BDD electrodes were successfully integrated onto flexible Parylene C substrates, validating their capability for both in vitro (cortical neurons) and in vivo (rat V1 cortex) neural recording.
Wafer-Scale Fabrication: The methodology employed a robust, wafer-scale fabrication and transfer process, demonstrating scalability for high-channel-count array production.

Technical Specifications

Parameter	Value	Unit	Context
BDD Film Thickness (Target)	5.5	µm	Optimized for low sp² impurities (Ref. 43)
BDD Bulk Resistivity	5 x 10⁻³	Ω·cm	Highly conductive, heavily boron-doped
BDD Growth Temperature	850	°C	MW-PACVD standard synthesis condition
BDD Growth Pressure	65	Torr	Chamber pressure during deposition
Boron/Carbon (B/C) Ratio	37,500	ppm	Required for high conductivity
Working Electrode (WE) Area	0.0079	mm²	Circular electrode geometry
BDD Growth Side Grain Size (Avg.)	0.5	µm	Microcrystalline morphology
BDD Growth Side Impedance (1 kHz)	203.4 to 254.2	kΩ	Measured in 0.1 M PBS solution
BDD Growth Side Double-Layer Capacitance (C_dl)	~10	µF/cm²	Low background noise for sensing
Dopamine (DA) Limit of Detection (LOD)	830	nM	Measured via Square-Wave Voltammetry (SWV)
Electron Transfer Kinetics (Growth Side, ∆E_p)	80	mV	Quasireversible reaction (Ru(NH₃)₆²⁺/³⁺ redox couple)

Key Methodologies

The fabrication relies on Microwave Plasma-Assisted Chemical Vapor Deposition (MW-PACVD) for BDD synthesis, followed by a complex transfer process to expose the desired growth surface on a flexible polymer substrate.

Substrate Preparation: 4-inch silicon wafers coated with 1 µm SiO₂ were scratch seeded to promote microcrystalline diamond nucleation.
BDD Growth: Films were grown using MW-PACVD at 850 °C and 65 Torr, utilizing a gas chemistry of 1% CH₄ in H₂. Heavy boron doping was achieved using Diborane (B₂H₆) at a B/C ratio of 37,500 ppm.
Patterning and Etching: Aluminum was thermally evaporated and patterned via UV photolithography to serve as a hard mask. The BDD film was subsequently plasma etched using Electron Cyclotron Resonance Reactive Ion Etching (RIE) with SF₆/Ar/O₂ chemistry.
Anchor Formation: The underlying SiO₂ layer was slightly overetched using Buffered Oxide Etchant (BOE) to create undercut structures, forming anchors for the subsequent Parylene C layer.
Parylene C Deposition (Backside): A ~15 µm Parylene C layer was deposited via CVD onto the patterned BDD film (nucleation side).
Silicon Release: The BDD-Parylene film was released from the rigid silicon substrate by etching the silicon in 35% KOH solution at 70 °C.
Growth Side Exposure: A second 10 µm Parylene C layer was deposited to encapsulate the nucleation side. Custom metalization (Aluminum) was patterned and etched to expose the BDD growth side, which serves as the final sensing surface.

6CCVD Solutions & Capabilities

This research demonstrates the critical role of precise material engineering—specifically, controlling the BDD surface morphology and sp³/sp² ratio—for high-performance neural interfaces. 6CCVD is uniquely positioned to supply and optimize the materials required to replicate and advance this technology.

Applicable Materials

To replicate the high-performance electrodes described, researchers require heavily boron-doped diamond films with controlled thickness and morphology.

Heavy Boron-Doped Polycrystalline Diamond (PCD/BDD): 6CCVD offers custom BDD films optimized for electrochemical applications. We can precisely control the B/C ratio (up to 40,000 ppm) during MW-PACVD growth to achieve the required bulk resistivity (as low as 5 x 10⁻³ Ω·cm) necessary for highly conductive electrodes.
Morphology Control: The paper highlights the superior performance of the rougher, larger-grain growth side. 6CCVD provides detailed surface characterization (SEM, Raman, AFM) and can tune growth parameters (temperature, pressure, gas mixture) to ensure the desired microcrystalline morphology (average grain size 0.5 µm) and high sp³ content are achieved consistently across large wafers.

Customization Potential

The fabrication process detailed in the paper requires precise control over film dimensions, thickness, and multi-layer metalization, all of which are core 6CCVD capabilities.

Research Requirement	6CCVD Capability	Value Proposition
Large-Scale Substrates	Custom PCD/BDD wafers up to 125 mm diameter.	Enables wafer-scale fabrication and high-throughput production of large electrode arrays.
Precise Thickness Control	SCD/PCD thickness range from 0.1 µm to 500 µm.	Allows researchers to optimize BDD thickness (e.g., 5.5 µm used here) to minimize sp² impurities and maximize electrochemical performance.
Custom Metalization	Internal capability for Au, Pt, Pd, Ti, W, Cu deposition and patterning.	We can supply BDD films pre-metalized with the required Ti/Cu/Al layers for subsequent transfer and encapsulation processes, streamlining device fabrication.
Surface Finishing	Polishing services available (Ra < 5 nm for inch-size PCD).	While this research favored the rougher growth side, 6CCVD can provide ultra-smooth SCD/PCD for applications requiring low friction or specific optical properties.
Global Logistics	Global shipping (DDU default, DDP available).	Ensures reliable and rapid delivery of sensitive diamond materials worldwide, supporting international research collaborations.

Engineering Support

The successful replication of this work hinges on optimizing the BDD material recipe to maximize sp³ content and control surface roughness for low impedance.

Electrochemical Optimization: 6CCVD’s in-house PhD team specializes in BDD material selection for advanced electrochemical applications, including neurotransmitter sensing and neural interfaces. We can assist researchers in tuning the B/C ratio and film thickness to achieve the lowest possible double-layer capacitance (C_dl < 10 µF/cm²) and fastest electron transfer kinetics (low ∆E_p).
Integration Support: We provide consultation on integrating BDD films with flexible polymer substrates (like Parylene C) and optimizing the RIE etching process to maintain material integrity and pattern fidelity.

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

Tech Support

Original Source

DOI: https://doi.org/10.1038/s41378-020-0155-1