An All-Solid-State pH Sensor Employing Fluorine-Terminated Polycrystalline Boron-Doped Diamond as a pH-Insensitive Solution-Gate Field-Effect Transistor

At a Glance

Metadata	Details
Publication Date	2017-05-05
Journal	Sensors
Authors	Yukihiro Shintani, Mikinori Kobayashi, Hiroshi Kawarada
Institutions	Yokogawa Electric (Japan), Waseda University
Citations	8
Analysis	Full AI Review Included

Technical Documentation and Analysis: All-Solid-State Diamond pH Sensor

Executive Summary

This documentation analyzes the successful fabrication and characterization of an all-solid-state pH sensor utilizing the advanced surface properties of MPCVD Boron-Doped Diamond (BDD). The key findings and value proposition for 6CCVD clients are summarized below:

Novel Device Architecture: Achieved a highly stable, all-solid-state pH sensor using differential FET sensing, eliminating the need for bulky liquid-junction reference electrodes (e.g., Ag/AgCl reference electrode).
Material Innovation: Demonstrated the crucial role of surface termination: a pH-sensitive channel (Oxygen-Terminated BDD, C-O BDD) was paired with a highly pH-insensitive Reference FET (Fluorine-Terminated BDD, C-F BDD).
High Sensitivity and Stability: The differential FET system achieved a sensitivity of 27 mV/pH across a wide physiological range (pH 2-10), suitable for practical applications.
Reference Stability: The C-F BDD SGFET successfully served as a solid-state REFET, exhibiting minimal pH response (sensitivity < 3 mV/pH), demonstrating superior stability compared to conventional polymeric REFETs.
Manufacturing Relevance: The device was fabricated on Polycrystalline Diamond (PCD) films deposited via Microwave Chemical Vapor Deposition (MPCVD), a core capability of 6CCVD.
Process Control: Validation of precision surface modification (ICP plasma fluorination and UV oxidation) to engineer the charge transfer characteristics essential for differential sensing.

Technical Specifications

Parameter	Value	Unit	Context
Differential Sensor Sensitivity	27	mV/pH	Across pH range 2-10
pH-Insensitive (C-F BDD) Sensitivity	< 3	mV/pH	Measured at fixed I_ds and V_ds
pH-Sensitive (C-O BDD) Sensitivity	37	mV/pH	Baseline performance (10x higher than C-F BDD)
Linearity (R<sup>2</sup>)	0.99	-	Differential output voltage vs. pH (pH 2-10)
Sensor Stability (24h)	0.096	delta-pH	Meets JIS standard (below 0.1)
BDD Sheet Carrier Density	3.7 x 10<sup>13</sup>	/cm<sup>2</sup>	Before oxygen treatment
BDD Hall Mobility	8.6	cm<sup>2</sup>/Vs	Before oxygen treatment
BDD Sheet Resistance	24	kΩ/cm<sup>2</sup>	Before oxygen treatment
Drain-Source Voltage (V<sub>ds</sub>)	-0.5	V	Used for I<sub>ds</sub>-V<sub>gs</sub> Characterization
Threshold Voltage (V<sub>TH</sub>) C-F BDD	-0.09	V	Estimated in normally-on mode

Key Methodologies

The core devices employed Polycrystalline Boron-Doped Diamond (PCD BDD) channels fabricated via MPCVD, followed by precise surface engineering and metallization steps:

Material Deposition: Boron-doped diamond layers were grown on 10-mm square PCD substrates using a quartz-type Microwave Chemical Vapor Deposition (CVD) reactor.
Metallization: Source and drain electrodes were formed by depositing Titanium (Ti, 20 nm) / Gold (Au, 100 nm) contact pads using lift-off photolithography techniques.
Device Geometry: Channel length was defined between 5-10 mm, and channel width between 0.3-0.6 mm. Electrodes were subsequently encapsulated in nonconductive epoxy resin.
pH-Insensitive Surface Treatment (C-F BDD): Partial fluorination was achieved using Inductively Coupled Plasma (ICP).
- Gas Source: Octafluoropropane (C<sub>3</sub>F<sub>8</sub>).
- Pressure: 2-5 Pa.
- Power/Time: 300-600 W for 10-120 seconds.
pH-Sensitive Surface Treatment (C-O BDD): Partial oxidation of the BDD surface was achieved via Ultraviolet (UV) irradiation in an oxygen atmosphere.
Sensing Setup: Differential FET measurements utilized a lab-made ISFET pH-mV meter in conjunction with a platinum (Pt) Quasi-Reference Electrode (QRE).

6CCVD Solutions & Capabilities

The successful implementation of this all-solid-state sensor hinges on controlled MPCVD growth of Boron-Doped Diamond (BDD) and precision surface modification—both areas where 6CCVD offers world-class expertise and materials ready for immediate deployment and scale-up.

Research Requirement	6CCVD Material Specification	6CCVD Capability Advantage
Base Material	Boron-Doped Polycrystalline Diamond (BDD)	Custom BDD films grown via high-quality MPCVD. We can match or exceed the reported sheet resistance (24 kΩ/cm<sup>2</sup>) or customize doping levels from light to heavy (>10<sup>20</sup> atoms/cm<sup>3</sup>).
Substrate Size & Scale	Custom PCD Plates/Wafers	The paper used 10 mm squares. 6CCVD offers PCD substrates up to 125 mm in diameter, enabling mass production scale-up of all-solid-state diamond sensors.
Thickness Control	PCD/BDD Layer Thickness	We offer active layer thicknesses from 0.1 µm up to 500 µm, allowing fine-tuning of electrical and mechanical properties for optimal SGFET performance and durability.
Metallization	Custom Ti/Au Contacts	The required Ti (20 nm)/Au (100 nm) contacts are standard. 6CCVD provides in-house metal deposition services (Au, Pt, Pd, Ti, W, Cu) for immediate integration into FET architectures.
Surface Engineering	Engineering Support	While fluorination (C-F) and oxidation (C-O) were performed in-house by the researchers, 6CCVD supplies pristine Hydrogen-Terminated (C-H) BDD ready for customer-specific functionalization recipes (e.g., ICP, UV, electrochemical modification).
Post-Processing	Precision Cutting and Polishing	For high-density sensor arrays or specific channel geometries, 6CCVD offers precision laser cutting and advanced polishing (Ra < 5 nm for inch-size PCD) to define complex microfluidic or sensor channels.

6CCVD’s in-house PhD team can assist with material selection and specification for developing robust all-solid-state pH sensors, ensuring the correct BDD doping concentration and substrate dimensions are selected for optimal differential FET performance.

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

View Original Abstract

A fluorine-terminated polycrystalline boron-doped diamond surface is successfully employed as a pH-insensitive SGFET (solution-gate field-effect transistor) for an all-solid-state pH sensor. The fluorinated polycrystalline boron-doped diamond (BDD) channel possesses a pH-insensitivity of less than 3mV/pH compared with a pH-sensitive oxygenated channel. With differential FET (field-effect transistor) sensing, a sensitivity of 27 mv/pH was obtained in the pH range of 2-10; therefore, it demonstrated excellent performance for an all-solid-state pH sensor with a pH-sensitive oxygen-terminated polycrystalline BDD SGFET and a platinum quasi-reference electrode, respectively.

Tech Support

Original Source

DOI: https://doi.org/10.3390/s17051040

References

1970 - Development of an ion-sensitive solid-state device for neurophysiological measurements [Crossref]
2001 - Study of the pH-ISFET and EnFET for Biosensor Applications
1997 - Diamond-like carbon-gate pH-ISFET [Crossref]
2001 - Electrolyte-solution- gate FETs using diamond surface for biocompatible ion sensors [Crossref]
2012 - Boron δ-doped (111) diamond solution gate field-effect transistors [Crossref]
2012 - Growth and electrical characterisation of δ-doped boron layers on (111) diamond surfaces [Crossref]
2010 - Low drift and small hysteresis characteristics of diamond electrolyte-solution-gate FET [Crossref]
2016 - Polycrystalline boron-doped diamond with an oxygen-terminated surface channel as an electrolyte-solution-gate field-effect transistor for pH sensing [Crossref]