Microcrystalline diamond film evaluation by spectroscopic optical coherence tomography

At a Glance

Metadata	Details
Publication Date	2022-09-30
Journal	Photonics Letters of Poland
Authors	Paulina Strąkowska, Marcin R. Strąkowski
Institutions	Gdańsk University of Technology
Citations	2
Analysis	Full AI Review Included

Executive Summary

This study focuses on the non-invasive evaluation of microcrystalline diamond (MCD) thin films deposited on Ti6Al4V substrates, primarily intended for implant coatings.

Core Achievement: Successful application of Spectroscopic Optical Coherence Tomography (Sc-OCT) to measure thickness variations in a highly heterogeneous MCD layer.
Thickness Range: The average film thickness was approximately 1.5 µm, which is below the standard axial resolution limit (12 µm) of the OCT system used.
Resolution Enhancement: Sc-OCT, utilizing time-frequency analysis of backscattered light spectra, confirmed its ability to track thickness changes in the submicrometer range (observed variation of approximately 180 nm).
Film Structure: The deposited layer was identified as a heterogeneous MCD/TiC structure, featuring 1-2 µm diamond crystallites covering about 5% of the surface.
Methodological Advantage: Sc-OCT overcomes the limitations of standard OCT and other methods (like reflectometry) by analyzing spectral shifts caused by interference within the thin film, making it suitable for complex, multi-layered structures.
Deposition Method: Films were grown using Microwave Plasma Assisted Chemical Vapor Deposition (MWPA-CVD) at 700 °C.

Technical Specifications

MCD Film Deposition Parameters (MWPA-CVD)

Parameter	Value	Unit	Context
Substrate Material	Ti6Al4V (ASTM 136)	Alloy	Cylindrical specimens (16 mm diameter)
Deposition Method	MWPA-CVD	Technique	Astex AX6500 system
Methane (CH₄) / Hydrogen (H₂) Ratio	1:99	Percentage	Process gas mixture
Process Pressure	50	Torr	CVD chamber environment
Total Gas Flow Rate	300	sccm	Standard cubic centimeters per minute
Microwave Power (PMW)	1.3	kW	Plasma excitation power
Substrate Temperature (T_S)	700	°C	Deposition temperature
Deposition Time	180	min	Total growth duration
Average Film Thickness	~1.5	µm	Estimated MCD layer thickness
Thickness Variation Observed	~180	nm	Difference between high/low points (Sc-OCT result)

Spectroscopic OCT (Sc-OCT) System Features

Parameter	Value	Unit	Context
Light Source Type	Swept-source	20 kHz	Scanning speed
Central Wavelength	1290	nm	Center of the light spectrum
Wavelength Range	140	nm	Total spectral width
Average Output Power	10	mW	Power delivered to the sample
Axial Resolution	12	µm	In air (Standard OCT limit)
Lateral Resolution	15	µm	Standard OCT limit
Scan Area (C-scan)	5 x 5	mm²	Area evaluated for thickness mapping

Film Characterization Results

Parameter	Value	Unit	Context
Diamond Crystallite Size	1-2	µm	Observed via SEM
Surface Coverage (Diamond)	~5	%	Estimated coverage of the TiC layer
Raman Band 1	391	cm^-1	Presence of competitive Titanium Carbide (TiC) phase
Raman Band 2	612	cm^-1	Presence of competitive Titanium Carbide (TiC) phase
AFM Scan Area	20 x 20	µm²	Area used for roughness measurement

Key Methodologies

The evaluation involved a three-stage process: substrate preparation and deposition, structural characterization, and non-invasive thickness mapping using Sc-OCT.

1. Substrate Preparation and Nucleation

Mechanical Preparation: Ti6Al4V cylindrical specimens were ground and polished, finishing with a 1-3 µm diamond suspension.
Cleaning: Samples were sonicated for 20 minutes each in 1% Triton X-100/deionized water, acetone, and ethanol.
Nucleation: Samples were immersed or sonicated for 1 hour in a nanodiamond suspension (0.5% mass concentration in DMSO) to promote high-density nucleation sites.
Drying: Samples were dried using a stream of nitrogen before placement in the CVD chamber.

2. MWPA-CVD Deposition

MCD layers were deposited using the Astex AX6500 system.
A gas mixture of CH₄:H₂ (1:99 ratio) was introduced at a total flow of 300 sccm.
Plasma was excited using 1.3 kW microwave power at a process pressure of 50 Torr.
Deposition was maintained for 180 minutes, achieving a substrate temperature of 700 °C.

3. Structural and Topographical Characterization

Morphology: Scanning Electron Microscopy (SEM) was used to confirm the presence of 1-2 µm diamond crystallites and the heterogeneous structure.
Composition: Raman Spectroscopy confirmed the presence of Titanium Carbide (TiC) and amorphous carbon phases (bands at 391 cm^-1 and 612 cm^-1).
Roughness: Atomic Force Microscopy (AFM) was used to measure average roughness (Ra) and root-mean-square roughness (Rq) over a 20 x 20 µm² area.

4. Spectroscopic OCT (Sc-OCT) Analysis

The standard OCT system was enhanced with time-frequency signal processing (Sc-OCT).
A C-scan (en-face plane) covering 5 mm x 5 mm was performed on the MCD layer.
For each point, the spectral characteristic of the backscattered light was obtained.
Thickness changes were estimated by monitoring the shift of local minima in the backscattered light spectra, which is directly related to the interference pattern within the thin film.
The resulting data was presented as a wrapped function map showing thickness variations (e.g., 180 nm elevation difference).

Commercial Applications

The development and precise characterization of thin, functional diamond coatings on titanium alloys are critical for high-performance engineering and biomedical fields.

Biomedical Implants: The primary application cited is the use of MCD films as biocompatible coatings for medical components and implants (e.g., orthopedic joints, dental fixtures) based on the inertness and mechanical strength of diamond.
Wear Resistance: Diamond films provide superior hardness and low friction, making them ideal for mechanical engineering components requiring high wear resistance in demanding environments.
Semiconductor Industry: Thin diamond films are utilized in semiconductor manufacturing for heat dissipation (due to diamond’s high thermal conductivity) and as protective layers.
Corrosion Protection: Applying continuous, uniform diamond coatings to reactive metals like Ti6Al4V enhances corrosion resistance, crucial for long-term implant stability.
Advanced Metrology: The Sc-OCT technique itself is valuable for non-invasive quality control and thickness mapping of complex, multi-layered optical or electronic devices where submicrometer resolution is required without physical contact.

View Original Abstract

This study has focused on the microcrystalline diamond film (MCD) thickness evaluation. For this purpose, optical coherence tomography (OCT) enhanced by spectroscopic analysis has been used as a method of choice. The average thickness of the tested layer was about 1.5 µm, which is below the standard 2-point OCT resolution. In this case, the usefulness of the spectroscopic analysis was confirmed for the evaluation of the thickness changes in the submicrometer range. Full Text: PDF ReferencesM.D. Drory, J.W. Hutchinson, “Diamond Coating of Titanium Alloys”, Science, 263 (1994). CrossRef J. Wang, J. Zhou, H.Y. Long, Y.N. Xie, X.W. Zhang, H. Luo, Z.J. Deng, Q. Wei, Z.M. Yu, J. Zhang, Z.G. Tang, “Tribological, anti-corrosive properties and biocompatibility of the micro- and nano-crystalline diamond coated Ti6Al4V”, Surf. Coat. Technol., 258 (2014). CrossRef P.A. Nistor, P.W. May, F. Tamagnini, A.D. Randall, M.A. Caldwell, “Long-term culture of pluripotent stem-cell-derived human neurons on diamond - A substrate for neurodegeneration research and therapy”, Biomaterials, 61 (2015). CrossRef C.A. Love, R.B. Cook, T.J. Harvey, P.A. Dearnley, R.J.K. Wood, “Diamond like carbon coatings for potential application in biological implants—a review”, Tribol. Int., 63 (2013). CrossRef P. Strąkowska, R. Beutner, M. Gnyba, A. Zielinski, D. Scharnweber, “Electrochemically assisted deposition of hydroxyapatite on Ti6Al4V substrates covered by CVD diamond films — Coating characterization and first cell biological results”, Materials Science and Engineering: C, 59 (2016). CrossRef T.S. Ho, P. Yeh, C.C. Tsai, K.Y. Hsu, S.L. Huang., “Spectroscopic measurement of absorptive thin films by Spectral-Domain Optical Coherence Tomography”, Opt. Express 22, 5 (2014). CrossRef N. Bosschaart, T.G. van Leeuwen, M.C.G. Aalders, D.J. Faber, “Quantitative comparison of analysis methods for spectroscopic optical coherence tomography”, Biomedical Opt. Express 4, 11 (2013). CrossRef F.E Robles, C. Wilson, G. Grant, A. Wax, “Molecular imaging true-colour spectroscopic optical coherence tomography”, Nat. Photonics 5, 12 (2011). CrossRef A.F. Fercher, W. Drexler, C.K. Hitzenberger, T. Lasser, “Optical coherence tomography - principles and applications”, Rep. Prog. Phys. 66, 239 (2003). CrossRef A.M. Kamińska, M.R. Strąkowski, J. Pluciński, “Spectroscopic Optical Coherence Tomography for Thin Layer and Foil Measurements”, Sensors 20, 19, (2020). CrossRef M. Kraszewski, M. Strąkowski, J. Pluciński, B.B. Kosmowski, “Spectral measurement of birefringence using particle swarm optimization analysis”, Appl. Opt. 54, 1 (2015). CrossRef

Tech Support

Original Source

DOI: https://doi.org/10.4302/plp.v14i3.1156