Fabrication and characterization of boron-doped nanocrystalline diamond-coated MEMS probes

At a Glance

Metadata	Details
Publication Date	2016-03-07
Journal	Applied Physics A
Authors	Robert Bogdanowicz, Michał Sobaszek, Mateusz Ficek, Daniel Kopiec, Magdalena Moczała
Institutions	Institute of Fluid Flow-Machinery, Gdańsk University of Technology
Citations	23
Analysis	Full AI Review Included

Technical Documentation & Analysis: Boron-Doped Nanocrystalline Diamond for High-Performance MEMS Probes

Executive Summary

This document analyzes the fabrication and characterization of highly conductive, thin boron-doped nanocrystalline diamond (B-NCD) films for Micro-Electromechanical Systems (MEMS) probes. The results demonstrate the transformative potential of B-NCD coatings supplied by 6CCVD for advanced AFM/KPFM and biosensing applications.

Core Achievement: Successful deposition of fully encapsulated, thin B-NCD films (~60 nm) onto silicon cantilevers via Microwave Plasma Assisted Chemical Vapour Deposition (MW PA CVD).
Enhanced Conductivity: Boron doping (5000 ppm [B]/[C]) achieved high surface conductivity, resulting in a low surface resistivity of 30 mΩ cm, crucial for electrical surface analysis (KPFM).
Material Quality: B-NCD layers exhibited superior morphology, drastically reducing RMS roughness to 17 nm (compared to 63 nm for undoped NCD) and yielding smaller, uniform crystallite sizes (153 nm).
Mechanical Modification: The B-NCD coating significantly stiffened the cantilevers, increasing the spring constant from 1.1 N/m to 3.2 N/m and shifting the resonance frequency from 21.57 kHz to 35.87 kHz.
Electrical Utility: KPFM measurements confirmed B-doping increased carrier density and conductivity, calculating a work function of 5.15 eV for the doped film, ideal for low-noise electrical probing.
6CCVD Value: These findings validate the critical role of custom, high-quality Boron-Doped Diamond (BDD) materials—a core competency of 6CCVD—in enabling next-generation nanoscale metrology.

Technical Specifications

Parameter	Value	Unit	Context
Film Thickness (B-NCD)	Approx. 60	nm	Fully encapsulated film thickness
RMS Roughness (B-NCD)	17	nm	Measured via AFM (compared to 63 nm for undoped NCD)
Surface Resistivity (B-NCD)	30	mΩ cm	Achieved high conductivity for electrical probing
Crystallite Grain Size (B-NCD)	153	nm	Determined by AFM/SEM analysis
Gas Phase Boron Concentration [B]/[C]	5000	ppm	Used Diborane (B₂H₆) precursor
B-NCD Work Function	5.15	eV	Calculated via KPFM (Undoped: 4.65 eV)
Cantilever Spring Constant (Shift)	1.1 → 3.2	N/m	Increase due to B-NCD coating/stiffening
Cantilever Resonance Frequency (Shift)	21.57 → 35.87	kHz	Frequency shift post-coating
CVD Deposition Temperature	500	°C	Molybdenum stage controlled temperature
Microwave Power / Frequency	1000 @ 2.45	W / GHz	Optimized for diamond synthesis

Key Methodologies

The highly conductive B-NCD films were synthesized using a carefully tuned MW PE CVD process focusing on plasma distribution and in-situ doping control.

Substrate Preparation (Seeding):
- p-type (100) monocrystalline silicon cantilevers were pre-treated by dip-coating.
- Suspension used: Detonation Nanodiamond (DND) in Dimethyl Sulphoxide (DMSO) with 0.5% Polyvinyl Alcohol (PVA).
- Final nanodiamond concentration: 0.25% w/w.
CVD System Configuration:
- System: MW PE CVD (SEKI Technotron AX5400S).
- Substrate Holder: Truncated cone-shaped molybdenum stage, designed to strongly focus microwave plasma energy and enable growth of thin films (< 100 nm).
Deposition Parameters (B-NCD Sample):
- Base Pressure: 10^-5 Torr.
- Working Pressure: 50 Torr.
- Deposition Temperature: 500 °C (Controlled by induction heater).
- Gas Flow Rate (Total): 156 sccm.
- Gas Mixture: H₂ (135 sccm), CH₄ (6 sccm), B₂H₆ (15 sccm).
- Methane Molar Ratio: 4%.
- Dopant Precursor: Diborane (B₂H₆) diluted in H₂.
- Boron Level: 5000 ppm [B]/[C] in gas phase.
- Deposition Time: 60 min.
Characterization Techniques:
- Morphology/Roughness: SEM (Helios NanoLab^TM 600i) and Tapping Mode AFM.
- Chemical/Structure: Micro-Raman spectroscopy (1148 cm^-1 B-NCD peak confirmed).
- Electrical Properties: Kelvin Probe Force Microscopy (KPFM) using PtIr-coated probes for Contact Potential Difference (CPD) and work function calculation.
- Mechanical Parameters: Laser Vibrometer (SP-S 120, SIOS GmbH) for resonance frequency and spring constant changes.

6CCVD Solutions & Capabilities

This research highlights the demand for highly tailored diamond films that offer superior electrical and mechanical properties for micro- and nanoscale devices. 6CCVD is uniquely positioned to supply the materials and processing required to replicate and advance this technology for researchers and engineers globally.

Applicable Materials

The requirements of this project necessitate materials with precise doping control and tailored morphology.

Material Requirement	6CCVD Solution	Rationale / Benefit
High Conductivity Diamond	Heavy Boron-Doped Polycrystalline Diamond (BDD PCD)	Standard production BDD material, capable of achieving semi-metallic conductivity required for KPFM and electrochemical sensing.
Nanocrystalline Structure (NCD)	Polycrystalline Diamond (PCD) with Controlled Grain Size	We offer PCD deposition with grain size control, easily fulfilling the NCD morphology (153 nm crystallite size) for ultra-smooth surfaces (Ra < 5 nm achievable for inch-size wafers).
Thin Film Deposition	PCD or SCD Films (0.1µm - 500µm)	Our minimum thickness (0.1 µm) is comparable to the research’s 60 nm film, ensuring the tight dimensional control needed for MEMS applications. We can target sub-100 nm films upon request.
High Sp³ Content / Purity	High-Quality MPCVD Diamond	Our standard MPCVD growth maximizes sp³ content, ensuring superior hardness, chemical inertness, and stability demonstrated in the paper.

Customization Potential

The success of this research depended on custom material size, thickness, and specialized surface treatment. 6CCVD capabilities directly address these needs:

Substrate Deposition: While the paper used silicon cantilevers, 6CCVD offers custom deposition onto customer-supplied substrates (e.g., specific MEMS wafers, SiC, or Mo) for seamless integration into existing fabrication flows.
Custom Dimensions: We supply plates and wafers up to 125 mm in diameter, allowing for high-throughput coating of MEMS and NEMS arrays, far exceeding the scale implied by single cantilever experiments.
Precision Thickness Control: We can precisely control the film thickness from 0.1 µm up to 500 µm, ensuring that the critical mechanical properties (e.g., resonance frequency and spring constant) of the final device are perfectly tuned.
Advanced Metalization: The researchers relied on external PtIr probes for KPFM. For integrated electrical measurements, 6CCVD offers in-house custom metalization (Au, Pt, Pd, Ti, W, Cu) allowing researchers to define specific contact pads or internal electrodes on the diamond film.
Ultra-Low Roughness Polishing: For demanding AFM/KPFM tip applications requiring minimal artifact generation, 6CCVD offers precision polishing down to Ra < 1 nm for SCD and Ra < 5 nm for large-area PCD, providing smoother surfaces than the 17 nm RMS achieved in the paper.

Engineering Support

This study demonstrates the complexity of tailoring diamond material properties (structural, electrical, mechanical) simultaneously.

6CCVD’s in-house PhD engineering team specializes in diamond material science and is available to assist customers with:

Recipe Optimization: Selecting the precise BDD doping concentration (ppm [B]/[C]) to hit target resistivity (e.g., 30 mΩ cm) for specific electrochemical or KPFM projects.
Structural Tuning: Adjusting CVD parameters to control crystallite size (NCD vs. PCD) and surface roughness for optimal device performance in AFM/MEMS Probing and Biosensor Applications.
Mechanical Modeling: Advising on required film thickness and Young’s Modulus for predetermined cantilever spring constant shifts.

Call to Action:

For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly. We ship high-performance CVD diamond solutions globally (DDU default, DDP available).

View Original Abstract

Fabrication processes of thin boron-doped nanocrystalline diamond (B-NCD) films on silicon-based micro- and nano-electromechanical structures have been investigated. B-NCD films were deposited using microwave plasma assisted chemical vapour deposition method. The variation in B-NCD morphology, structure and optical parameters was particularly investigated. The use of truncated cone-shaped substrate holder enabled to grow thin fully encapsulated nanocrystalline diamond film with a thickness of approx. 60 nm and RMS roughness of 17 nm. Raman spectra present the typical boron-doped nanocrystalline diamond line recorded at 1148 cm−1. Moreover, the change in mechanical parameters of silicon cantilevers over-coated with boron-doped diamond films was investigated with laser vibrometer. The increase of resonance to frequency of over-coated cantilever is attributed to the change in spring constant caused by B-NCD coating. Topography and electrical parameters of boron-doped diamond films were investigated by tapping mode AFM and electrical mode of AFM-Kelvin probe force microscopy (KPFM). The crystallite-grain size was recorded at 153 and 238 nm for boron-doped film and undoped, respectively. Based on the contact potential difference data from the KPFM measurements, the work function of diamond layers was estimated. For the undoped diamond films, average CPD of 650 mV and for boron-doped layer 155 mV were achieved. Based on CPD values, the values of work functions were calculated as 4.65 and 5.15 eV for doped and undoped diamond film, respectively. Boron doping increases the carrier density and the conductivity of the material and, consequently, the Fermi level.

Tech Support

Original Source

DOI: https://doi.org/10.1007/s00339-016-9829-9

References

2002 - MEMS and NEMS: Systems, Devices, and Structures
2012 - Atomic Force Microscopy: Understanding Basic Modes and Advanced Applications [Crossref]