Fluorine-Terminated Polycrystalline Diamond Solution-Gate Field-Effect Transistor Sensor with Smaller Amount of Unexpectedly Generated Fluorocarbon Film Fabricated by Fluorine Gas Treatment

At a Glance

Metadata	Details
Publication Date	2022-04-19
Journal	Materials
Authors	Yukihiro Shintani, Hiroshi Kawarada
Institutions	Chiba Institute of Technology, Waseda University
Citations	3
Analysis	Full AI Review Included

Technical Documentation & Analysis: Fluorine-Terminated Diamond SGFETs

This document analyzes the research detailing the fabrication of high-performance fluorine-terminated polycrystalline diamond (C-F PCD) solution-gate field-effect transistor (SGFET) sensors using a novel $F_{2}$ gas treatment method. This method significantly reduces detrimental fluorohydrocarbon (CxFy) film formation compared to conventional ICP techniques, leading to superior device characteristics.

Executive Summary

The following points summarize the core technical achievements and commercial value derived from the research:

Novel Surface Functionalization: Introduction of a direct fluorine gas ($F_{2}$ gas) treatment method for creating C-F terminated diamond surfaces, specifically targeting high-performance SGFET sensors.
Damage Mitigation: The $F_{2}$ gas method successfully minimizes or eliminates the formation of unexpected fluorohydrocarbon (CxFy) films, which typically degrade the semiconductor interface quality during conventional ICP fluorination (e.g., using C${3}$F${8}$ gas).
Material Basis: The study utilized highly (110)-oriented Polycrystalline Diamond (PCD) substrates, subsequently boron-doped (BDD) to form the active SGFET channel.
Analytical Confirmation: X-ray Photoelectron Spectroscopy (XPS) and Time-of-Flight Secondary Ion Mass Spectrometry (TOF-SIMS) confirmed that the $F_{2}$-treated surface exhibited significantly fewer C-F${3}$ and C-F${2}$ fragments associated with CxFy film contamination.
Ideal Device Performance: The resulting C-F BDD SGFET demonstrated nearly ideal drain-source current (Ids) vs. drain-source voltage (Vds) characteristics, closely matching standard metal-oxide-silicon semiconductor field-effect transistor (MOSFET) theory.
Electrochemical Advantage: The C-F termination provides unique properties, including hydrophobicity and reduced pH sensitivity, making it highly attractive for advanced chemical and biochemical sensing applications.

Technical Specifications

Parameter	Value	Unit	Context
Material Substrate	Polycrystalline Diamond (PCD)	N/A	Highly (110)-oriented
Active Channel Material	Boron-Doped Diamond (BDD)	N/A	Used for SGFET fabrication
Novel Fluorination Method	Direct $F_{2}$ Gas Treatment	N/A	Used KF$_{2}$HF electrolysis source
$F_{2}$ Gas Reaction Pressure	ca. 101	kPa	Ambient pressure reaction
$F_{2}$ Gas Reaction Time	30 min to 24	h	Range tested for optimization
Conventional ICP Gas Source	Perfluoropropane (C${3}$F${8}$)	N/A	Comparison method
Conventional ICP Pressure	3	Pa	Low pressure plasma treatment
Conventional ICP Power	100-500	W	Power range applied
C-H Ids (Vds=-1.0V, Vgs=0V)	-22	µA/mm	Reference C-H BDD SGFET
C-F Ids (Vds=-1.0V, Vgs=0V)	-11	µA/mm	$F_{2}$-treated C-F BDD SGFET (50% decrease)
Threshold Voltage (Vth) Shift	0.8	V	Negative shift due to C-F termination
XPS C-F$_{3}$ Binding Energy	292.8	eV	Indicator of CxFy film presence
*C-F${3}$ Coverage ($F{2}$ Treated)*	2.2	%	Significantly lower than ICP method
C-F$_{3}$ Coverage (ICP Treated)	10.8	%	High CxFy film presence

Key Methodologies

The fabrication and characterization process involved precise material preparation and comparative surface treatment techniques:

CVD Growth and Cleaning: Highly (110)-oriented Polycrystalline Diamond (PCD) was synthesized via Chemical Vapor Deposition (CVD). Substrates were cleaned using standard solvents (ultrapure water, ethanol, acetone, isopropyl alcohol).
Hydrogen Termination (C-H Precursor): A nearly full C-H termination was achieved using an ASTeX-type Microwave Plasma CVD system, preparing the surface for subsequent fluorination.
Novel $F_{2}$ Gas Treatment:
- C-H substrates were placed in a lab-made Nickel (Ni) reactor.
- The reactor was purged with Argon (Ar) gas, followed by decompression.
- $F_{2}$ gas, generated by the electrolysis of potassium diacid fluoride (KF$_{2}$HF) melted at ca. 100 °C, was introduced at approximately 101 kPa.
- Reaction times ranged from 30 minutes to 24 hours.
Conventional ICP Treatment (Comparison):
- Perfluoropropane (C${3}$F${8}$) gas was used as the fluorocarbon source.
- Treatment was conducted under low pressure (3 Pa) with a gas flow of 20 standard cubic centimeters per minute (sccm) and 100-500 W power for up to 30 seconds.
Surface Analysis: X-ray Photoelectron Spectroscopy (XPS) and Time-of-Flight Secondary Ion Mass Spectrometry (TOF-SIMS) were used to quantify the bonding states (C-F, C-F${2}$, C-F${3}$) and the depth profile of fluorocarbon fragments.
SGFET Characterization: Boron-doped diamond (BDD) SGFETs were fabricated and tested in a 1 mM Phosphate-Buffered Saline (PBS; pH 7.4) solution using a common-source configuration with an Ag/AgCl reference electrode.

6CCVD Solutions & Capabilities

The successful replication and extension of this advanced diamond sensor technology require highly controlled material synthesis, doping, and surface engineering—all core competencies of 6CCVD.

Applicable Materials

To replicate or extend this research into scalable, high-performance sensor arrays, 6CCVD recommends the following materials:

6CCVD Material	Specification Relevance	Customization Potential
Boron-Doped Polycrystalline Diamond (BDD-PCD)	Essential for the semiconductor channel (SGFET). We offer precise, uniform boron doping profiles necessary for stable FET operation.	Wafers available up to 125mm in diameter, with thickness control from 0.1 µm to 500 µm.
High-Quality PCD Substrates	Required for the highly (110)-oriented growth used in the study. 6CCVD ensures high crystal quality and low defect density precursors.	Custom substrate thicknesses up to 10mm for robust device integration.
C-H Termination Precursors	The $F_{2}$ gas treatment relies on a high-quality C-H terminated surface. 6CCVD provides in-house, controlled C-H termination services prior to functionalization.	Guaranteed surface termination quality for optimal subsequent chemical reactions.

Customization Potential

6CCVD’s advanced manufacturing capabilities directly address the complex requirements of diamond FET fabrication:

Custom Dimensions and Scaling: While the paper used laboratory-scale samples, 6CCVD can provide inch-size PCD wafers up to 125mm for industrial scaling of SGFET sensor arrays.
Precision Polishing: Achieving ideal FET characteristics requires extremely smooth surfaces. 6CCVD offers ultra-low roughness polishing on PCD (Ra < 5nm) to minimize scattering and maximize interface quality, crucial for maintaining the “nearly ideal” I-V characteristics observed.
Integrated Metalization Services: Although the paper focuses on surface chemistry, robust FET devices require reliable source/drain contacts. 6CCVD offers custom metalization stacks (including Ti, Pt, Au, W, Cu) applied directly to the diamond surface, ensuring low-resistance contacts compatible with electrochemical environments.
Advanced Surface Engineering: 6CCVD’s in-house expertise extends beyond standard CVD growth to include precise surface modification, enabling clients to transition from the C-H precursor to the desired C-F termination with high repeatability.

Engineering Support

6CCVD’s in-house PhD team specializes in the physics and chemistry of CVD diamond interfaces. We offer comprehensive engineering support for projects involving advanced surface functionalization, such as:

Material Selection: Assisting researchers in selecting the optimal diamond type (SCD vs. PCD) and doping level for specific electrochemical or biosensing applications.
Process Integration: Consulting on the integration of post-processing steps, including the implementation of novel gas treatments (like the $F_{2}$ method) and subsequent metal contact deposition.
Application Extension: Providing expertise to extend the C-F BDD SGFET technology to long-term stability studies, ion sensitivity analysis, and interference mitigation, as suggested for future work in the paper.

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

View Original Abstract

In this study, a partially fluorine-terminated solution-gate field-effect transistor sensor with a smaller amount of unexpectedly generated fluorohydrocarbon film on a polycrystalline diamond channel is described. A conventional method utilizing inductively coupled plasma with fluorocarbon gas leads the hydrogen-terminated diamond to transfer to a partially fluorine-terminated diamond (C-F diamond); an unexpected fluorohydrocarbon film is formed on the surface of the diamond. To overcome this issue, we newly applied fluorine gas for the fluoridation of the diamond. Analytical results of X-ray photoelectron spectroscopy and time-of-flight secondary ion mass spectrometry suggest that the fluorocarbon film does not exist or only a smaller amount of fluorocarbon film exists on the diamond surface. Conversely, the C-F diamond fabricated by the conventional method of inductively coupled plasma with a perfluoropropane gas (C3F8 gas) source possesses a certain amount of fluorocarbon film on its surface. The C-F diamond with a smaller amount of unexpectedly generated fluorohydrocarbon film possesses nearly ideal drain-source-voltage vs. gate-source-current characteristics, corresponding to metal-oxide-silicon semiconductor field-effect transistor theory. The results indicate that the fluorine gas (F2 gas) treatment proposed in this study effectively fabricates a C-F diamond sensor without unexpected semiconductor damage.

Tech Support

Original Source

DOI: https://doi.org/10.3390/ma15092966

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