Wirelessly Powered High-Temperature Strain Measuring Probe Based on Piezoresistive Nanocrystalline Diamond Layers

At a Glance

Metadata	Details
Publication Date	2016-07-18
Journal	Metrology and Measurement Systems
Authors	A. Bouřa, Pavel Kulha, M. Hušák
Institutions	Czech Technical University in Prague
Citations	2
Analysis	Full AI Review Included

6CCVD Technical Documentation: High-Temperature Nano-Crystalline Diamond Piezoresistors

This document analyzes the research on wirelessly powered, high-temperature strain measuring probes utilizing Nano-Crystalline Diamond (NCD) piezoresistors. It extracts key performance metrics and outlines 6CCVD’s capabilities to supply the necessary Boron-Doped Diamond (BDD) materials, custom dimensions, and metalization required for replication, scale-up, and advancement of this technology in harsh-environment sensing applications (e.g., turbomachinery).

Executive Summary

The reported research successfully demonstrates a wireless, high-temperature strain sensor utilizing Boron-Doped Nano-Crystalline Diamond (BDD NCD) films. This system is optimized for harsh, energy-demanding environments where conventional sensors fail, such as internal turbine blade monitoring.

Harsh Environment Suitability: NCD piezoresistors demonstrated stable operation up to 250°C, significantly exceeding standard silicon component limits.
High Sensitivity: Achieved Gauge Factors (GF) up to 12.8, demonstrating high deformation sensitivity, which decreases predictably with increasing temperature.
Optimal Doping Control: TCR (Temperature Dependences of Resistance) values were successfully controlled from 270 to 2400 ppm/K by adjusting the Boron-to-Carbon (B:C) ratio (500 ppm to 6000 ppm).
Wireless Power Transfer: The system achieved remote powering, transferring 100 mW at 4 cm distance with 14% efficiency, optimized via resonant frequency calculations (up to 1.954 MHz).
Material Foundation: The sensors relied on high-quality, ultra-thin (250-300 nm) BDD NCD films, essential for reliable high-temperature piezo-resistive performance.
Core Application: Validation performed on a real sample intended for stress monitoring of steam turbine blades, confirming suitability for industrial and aerospace control systems.

Technical Specifications

The following performance and material metrics were achieved utilizing the MPCVD BDD NCD strain sensors:

Parameter	Value	Unit	Context
Max Operating Temperature	250	°C	Theoretical/Simulation Limit
Tested Temperature Range	25 - 200	°C	Measured Performance
Max Gauge Factor (GF)	12.8	(-)	500 ppm B:C @ 25°C
Minimum GF (Tested)	2.1	(-)	500 ppm B:C @ 200°C
Temperature Coefficient of Resistance (TCR)	270	ppm/K	High Doping (6000 ppm B:C)
TCR Range	270 - 2400	ppm/K	Dependence on B:C ratio (6000 ppm to 500 ppm)
Diamond Film Thickness	250 - 300	nm	Nano-crystalline structure
Diamond Crystal Size	100 - 200	nm	Nano-Crystalline Diamond (NCD)
Max Power Transfer	100	mW	At 4 cm axial distance
Power Transfer Efficiency	14	%	At 4 cm axial distance
Optimal Resonance Frequency	1.954	MHz	Determined by coil geometry and load (C₂=127 pF)
Metal Contact Thickness	90	nm	Ti/Au contact pads (100 nm Ti / 100 nm Au films used)
Load Resistance (R_L)	500	Ω	Minimal required for HT1104 amplifiers (5 V @ 10 mA)

Key Methodologies

The Boron-Doped Nano-Crystalline Diamond (BDD NCD) films were fabricated via MPCVD, followed by standard lithographic processing.

Substrate Preparation: SiO₂/Si₃N₄/Si substrates (8 x 25 mm²) were cleaned, seeded, and prepared for growth.
MPCVD Growth Parameters (ASTeX 6500 System):
- Gas Pressure: 50 torr
- Microwave Power: 4000 W
- Gas Mixture: 2% Methane (CH₄) diluted in Hydrogen (H₂)
- Growth Duration: 30 minutes
Boron Doping: Trimethylboron (TMB) was introduced to the gas phase to achieve B:C ratios of 500 ppm, 3000 ppm, or 6000 ppm.
Patterning: Standard lithography was used to define piezoresistive structures (full-bridges, meanders).
Etching: Reactive Ion Etching (RIE) was performed using an O₂/CF₄ gas mixture to form the final diamond structures.
Metalization: Thin films of Titanium (Ti) and Gold (Au) (100 nm/100 nm) were evaporated onto the piezoresistive structures to form low-resistance ohmic contacts.
Packaging: Ti/Au contact pads were connected to a gold-plated terminal board using ball bonding.

6CCVD Solutions & Capabilities

The development of high-performance, high-temperature piezoresistive sensors relies entirely on tight control over diamond film structure, doping uniformity, and integrated metalization. 6CCVD is an expert MPCVD supplier uniquely positioned to support the replication and industrial scaling of this research.

Applicable Materials

The paper utilized a thin, Boron-Doped Nano-Crystalline Diamond (BDD NCD) film. 6CCVD provides custom materials engineered specifically for this application:

Heavy Boron Doped PCD (Polycrystalline Diamond): Ideal for achieving the high Gauge Factors observed (GF > 10) and ensuring low resistivity essential for sensor operation and input impedance matching. Our ability to precisely control the TMB flow ensures repeatable B:C ratios up to and exceeding the 6000 ppm required.
Ultra-Thin Films: The required film thickness of 250-300 nm falls perfectly within our SCD/PCD thin film capability range (0.1µm - 500µm), allowing for precise adjustment of sensor resistance and strain characteristics.

Customization Potential

The success of this probe depends on integrated fabrication. 6CCVD offers end-to-end services that eliminate supply chain complexity:

Paper Requirement	6CCVD Capability & Advantage
Substrate Size (8 x 25 mm²)	Large Area Supply: We provide custom PCD wafers up to 125 mm diameter, enabling the fabrication of high-density sensor arrays or large-scale full-bridge structures far exceeding the research dimensions.
Ti/Au Metal Contacts	In-House Metalization: 6CCVD offers the exact Ti/Au metal stacks (as well as Pt, Pd, W, Cu) required for high-temperature ohmic contacts, optimizing adhesion and reducing contact resistance drift up to 250°C.
High Precision Polishing	Surface Finish Optimization: For applications requiring precise lithography or low friction (e.g., adjacent to rotating turbine blades), our industry-leading polishing achieves Ra < 5 nm for inch-size PCD, ensuring minimized stress concentration points and improved film homogeneity.
Laser Cutting/Dicing	Complex Geometry Support: We provide custom laser cutting and shaping services to produce the exact dimensions and meander geometries required for optimal cantilever or blade mounting structures.

Engineering Support

This research demonstrates the need for precise material characterization, particularly the relationship between B:C ratio, TCR, and high-temperature GF degradation.

6CCVD’s in-house PhD team provides consultative support on material selection, particularly optimizing the B:C doping concentration to minimize the temperature dependency of the Gauge Factor (as seen when increasing doping from 500 ppm to 6000 ppm).
We specialize in supplying diamond optimized for high-temperature piezo-resistive strain measurement and other harsh-environment sensor projects (e.g., pressure, temperature, chemical sensing) up to and exceeding 250°C.
Global Shipping is standard (DDU default, DDP available) to ensure fast delivery for critical aerospace and industrial development programs worldwide.

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

View Original Abstract

Abstract A high-temperature piezo-resistive nano-crystalline diamond strain sensor and wireless powering are presented in this paper. High-temperature sensors and electronic devices are required in harsh environments where the use of conventional electronic circuits is impractical or impossible. Piezo-resistive sensors based on nano-crystalline diamond layers were successfully designed, fabricated and tested. The fabricated sensors are able to operate at temperatures of up to 250°C with a reasonable sensitivity. The basic principles and applicability of wireless powering using the near magnetic field are also presented. The system is intended mainly for circuits demanding energy consumption, such as resistive sensors or devices that consist of discrete components. The paper is focused on the practical aspect and implementation of the wireless powering. The presented equations enable to fit the frequency to the optimal range and to maximize the energy and voltage transfer with respect to the coils’ properties, expected load and given geometry. The developed system uses both high-temperature active devices based on CMOS-SOI technology and strain sensors which can be wirelessly powered from a distance of up to several centimetres with the power consumption reaching hundreds of milliwatts at 200°C. The theoretical calculations are based on the general circuit theory and were performed in the software package Maple. The results were simulated in the Spice software and verified on a real sample of the measuring probe.

Tech Support

Original Source

DOI: https://doi.org/10.1515/mms-2016-0036

References

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