Raman Microscopic Analysis of Internal Stress in Boron-Doped Diamond

At a Glance

Metadata	Details
Publication Date	2015-05-22
Journal	Materials
Authors	Kevin E. Bennet, Kendall Lee, Jonathan R. Tomshine, Emma Sundin, James Kruchowski
Institutions	The University of Texas at El Paso, WinnMed
Citations	9
Analysis	Full AI Review Included

Technical Analysis and Commercial Solutions for Stress Management in Boron-Doped Diamond Films

Executive Summary

This study details the critical role of internal stress management in Boron-Doped Diamond (BDD) thin films intended for high-reliability neurosurgical biosensing applications (DBS/FSCV). Key technical and commercial takeaways are:

Stress Mitigation via Doping: Boron incorporation is scientifically proven to reduce intrinsic compressive stress within the diamond film, primarily by increasing the diamond’s thermal expansion coefficient, leading to improved reliability and stability.
High Compressive Stress Identified: Undoped diamond films and regions of pure diamond crystallites showed significantly higher compressive stress (1.5 to 6.7 GPa), correlating directly with lattice mismatch at the substrate interface (Tungsten rods).
Impurity Correlation: sp² carbon impurities were found to accumulate at crystallite boundaries and interfaces, contributing to enhanced surface stress, validating the need for ultra-high purity MPCVD growth environments.
Critical Application Reliability: The long-term stability of BDD electrodes for neurosurgical applications depends directly on minimizing delaminations and dislocations caused by intrinsic and extrinsic residual stresses.
6CCVD Advantage: Utilizing advanced Microwave Plasma CVD (MPCVD), 6CCVD can precisely control boron concentration and manage impurity phases (sp² carbon) to engineer films with optimal stress profiles, lattice constants, and high electrical conductivity.

Technical Specifications

Parameter	Value	Unit	Context
CVD Type Used	Hot Filament CVD (HFCVD)	N/A	Reactor utilized substrate rotation for uniform deposition on cylindrical rods.
Chamber Pressure	20	Torr	Constant pressure maintained during diamond growth.
Filament Temperature	2300	°C	Required temperature for filament operation.
Substrate Temperature	~800	°C	Substrate temperature during deposition.
Nominal Gas Mixture	99% H₂, 1% CH₄	Volume %	Baseline precursor gas composition.
TMB Concentration (Light Dope)	10 (2 sccm TMB/H₂)	ppm	Chamber concentration for lightly doped BDD films.
TMB Concentration (Heavy Dope)	100 (20 sccm TMB/H₂)	ppm	Chamber concentration for heavily doped BDD films.
Stress/Raman Shift Conversion	3	cm^-1 / 1 GPa	Standard conversion factor for diamond Raman peak shift.
Observed Compressive Stress (General)	1.5 to 4	GPa	Typical stress values in doped films/surface regions.
Observed Compressive Stress (Interface)	Up to 6.7 (20 ± 1 cm^-1 shift)	GPa	Peak stress found near the diamond/tungsten interface.
Raman Excitation Source	532	nm	Frequency-doubled Nd:YAG laser for confocal Raman mapping.
Characteristic Diamond Raman Peak	1332 ± 2	cm^-1	Used for stress mapping analysis based on peak shifts.
sp² Carbon Impurity Peak	1500	cm^-1	Used to identify non-diamond carbon content.
Boron Pair Signature	~500	cm^-1	Used to identify aggregation of boron interstitial atoms/pairs.

Key Methodologies

The BDD films were fabricated using a specialized Hot Filament Chemical Vapor Deposition (HFCVD) process designed to manage stress and improve deposition uniformity on complex geometries.

Substrate Preparation:
- Substrates (Tungsten rods) were electrochemically etched in 1 M NaOH.
- Abrasion via sonication for 30 minutes in a slurry of 100 nm diamond powder and isopropyl alcohol.
- Final rinse in deionized water.
Reactor Design & Control:
- Custom-built HFCVD reactor employed substrate rotation to achieve uniform film deposition on the cylindrical tungsten rods.
- Filament (2300 °C) and Substrate (~800 °C) temperatures were maintained using a PID software-based control loop (ramping power up to 450 W).
Deposition Recipe (Undoped/Baseline):
- Total Chamber Pressure: 20 Torr.
- Gas Flow: 2 sccm Methane (CH₄) and 198 sccm Hydrogen (H₂).
Deposition Recipe (Boron Doped):
- Trimethylborane (TMB, 1000 ppm in H₂) was introduced as the doping source.
- Lightly Doped: 2 sccm TMB/H₂, 2 sccm CH₄, 196 sccm H₂ (10 ppm TMB chamber concentration).
- Heavily Doped: 20 sccm TMB/H₂, 2 sccm CH₄, 178 sccm H₂ (100 ppm TMB chamber concentration).
Characterization (Raman Mapping):
- Confocal Raman measurements acquired using a WITec system with 532 nm excitation (Nd:YAG laser).
- Side-wall mapping was performed on cross-sectionally cleaved samples using high magnification objectives (100X/NA 0.90, 50X/NA 0.75).
- Stress analysis utilized the Advanced Fitting Tool (WITec Project Plus software) to fit the 1332 cm^-1 peak, determining stress based on vibrational shift.

6CCVD Solutions & Capabilities

This research confirms that controlled boron doping is a vital engineering tool for mitigating internal stress and optimizing device reliability in advanced electrochemical applications like biosensing. 6CCVD’s specialized MPCVD capabilities exceed the scope of the HFCVD methods used in this paper, enabling superior material quality and stress engineering.

Applicable Materials

To replicate or extend this highly specialized neurosensing research, 6CCVD offers the following optimized materials:

Material Grade	Recommended Specification	Application Alignment
Heavy Boron Doped PCD (PCD-B)	Doping concentration > 1 x 10²⁰ atoms/cm³ (Metallic-like conductivity).	Direct replication/enhancement for electrode fabrication requiring high conductivity and maximum stress release.
Low Stress BDD Wafers	Thickness range 0.1 µm - 500 µm, grown on non-diamond substrates (Si, W, etc.) or free-standing.	Provides optimized lattice matching and thermal expansion coefficients for long-term stability and prevention of film delamination.
Optical/High Purity SCD	Low nitrogen concentration (< 1 ppb) for NV center applications; essential for high-purity baselines.	Provides the foundation for highly controlled, single-crystal BDD films, eliminating grain boundary sp² accumulation.

Customization Potential

The experimental use of custom-shaped (cylindrical) tungsten rods highlights the necessity for flexible material supply. 6CCVD offers comprehensive engineering services tailored to complex device architectures:

Custom Dimensions and Shapes: While the paper used rods, 6CCVD provides PCD and SCD wafers/plates up to 125 mm in diameter. We offer precision laser cutting and shaping services to match custom geometries, including micro-electrode arrays or non-standard substrate profiles.
Interface and Adhesion Engineering: The paper found maximum stress at the W/Diamond interface (up to 6.7 GPa). 6CCVD utilizes MPCVD techniques for optimized nucleation layers and precise temperature ramping, which are superior to HFCVD for managing extrinsic thermal stress and lattice mismatch on diverse substrates.
Custom Metalization: For integration into neurosurgical probes or contact electrodes, 6CCVD offers in-house deposition of standard and custom metal stacks, including Au, Pt, Pd, Ti, W, and Cu, ensuring robust electrical contact and biocompatibility.
Ultra-Smooth Surface Finish: High-performance biosensors require minimal surface roughness. 6CCVD provides industry-leading polishing services: Ra < 1 nm for SCD and Ra < 5 nm for inch-size PCD, drastically improving electrochemical performance and repeatability compared to typical as-grown CVD films.

Engineering Support

6CCVD’s in-house PhD team provides expert consultation on leveraging boron doping for advanced Electrochemical Sensing and High-Reliability Implants. We specialize in:

Stress Profile Modeling: Assisting clients in selecting specific material thicknesses and doping levels required to achieve target residual stress values (e.g., maximizing the beneficial compressive stress component while preventing delamination).
Material Selection for Biosensing: Guiding material choices based on desired conductivity (metallic vs. semiconducting BDD) and surface quality needed for fast-scan cyclic voltammetry (FSCV) and Deep Brain Stimulation (DBS) projects.
Scaling and Manufacturing Transition: Supporting the transition of lab-scale experimental designs (like those described in this paper) to high-volume, uniform production using our large-area MPCVD capabilities.

Call to Action: For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly. We enable the next generation of high-reliability diamond biosensors.

View Original Abstract

Analysis of the induced stress on undoped and boron-doped diamond (BDD) thin films by confocal Raman microscopy is performed in this study to investigate its correlation with sample chemical composition and the substrate used during fabrication. Knowledge of this nature is very important to the issue of long-term stability of BDD coated neurosurgical electrodes that will be used in fast-scan cyclic voltammetry, as potential occurrence of film delaminations and dislocations during their surgical implantation can have unwanted consequences for the reliability of BDD-based biosensing electrodes. To achieve a more uniform deposition of the films on cylindrically-shaped tungsten rods, substrate rotation was employed in a custom-built chemical vapor deposition reactor. In addition to visibly preferential boron incorporation into the diamond lattice and columnar growth, the results also reveal a direct correlation between regions of pure diamond and enhanced stress. Definite stress release throughout entire film thicknesses was found in the current Raman mapping images for higher amounts of boron addition. There is also a possible contribution to the high values of compressive stress from sp2 type carbon impurities, besides that of the expected lattice mismatch between film and substrate.

Tech Support

Original Source

DOI: https://doi.org/10.3390/ma8052782

References

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