Thin Diamond Film on Silicon Substrates for Pressure Sensor Fabrication

At a Glance

Metadata	Details
Publication Date	2020-08-21
Journal	Materials
Authors	S. Salvatori, Sara Pettinato, Armando Piccardi, Vadim Sedov, A A Voronin
Institutions	University Niccolò Cusano, Prokhorov General Physics Institute
Citations	20
Analysis	Full AI Review Included

Thin Diamond Film on Silicon Substrates for Pressure Sensor Fabrication: Technical Analysis and 6CCVD Solutions

Executive Summary

This document analyzes the fabrication and characterization of high-quality polycrystalline CVD diamond (PCD) membranes on silicon substrates for use in high-pressure optical sensors. The findings directly validate 6CCVD’s core capabilities in advanced diamond material engineering for harsh environments.

Application Validation: Demonstrated feasibility of using thin PCD membranes (5.9 µm thick) as the sensing element in a low-finesse Fabry-Pérot (FP) interferometer for high-pressure measurement.
Performance: Sensors were characterized up to 16.5 MPa, exhibiting excellent linearity and a high sensitivity of 1.01 nm/kPa for 380 µm diameter membranes.
Material Quality: The MPCVD-grown PCD films displayed mechanical properties consistent with literature values (Young’s Modulus ~1100 GPa), confirming high elasticity and thickness uniformity across the 2-inch wafer.
Harsh Environment Suitability: The optical sensing mechanism provides intrinsic immunity to electromagnetic interference (EMI), while diamond’s chemical inertness allows operation in aggressive media.
Fabrication Complexity: The process required precise control over MPCVD growth, nanodiamond seeding, substrate thinning (to 120 µm), laser-assisted metal mask lithography, and selective deep reactive ion etching (DRIE) of silicon.
Surface Finish: The nucleation side of the PCD membrane, used as the reflective surface, achieved a low roughness (R_a of 7 nm), crucial for maximizing reflected light and sensor sensitivity.

Technical Specifications

The following hard data points were extracted from the experimental results and fabrication parameters:

Parameter	Value	Unit	Context
Maximum Pressure Tested	16.5	MPa	High-pressure range
Membrane Deflection Sensitivity	1.01 ± 0.01	nm/kPa	For 380 µm diameter membrane
Effective Membrane Thickness (Calculated)	~5.0 to 5.9	µm	Consistent with weight gain measurement
Membrane Diameter (Tested)	360 to 380	µm	Defined by laser ablation
Young’s Modulus (E)	~1100	GPa	Assumed nominal value for diamond
Poisson’s Ratio (ν)	0.07	-	Assumed nominal value for diamond
Nucleation Side Roughness (R_a)	7	nm	Measured via optical profilometer
Nucleation Side Roughness (R_q)	15	nm	Measured via optical profilometer
Si Substrate Initial Thickness	400	µm	2-inch, (111)-oriented wafer
Si Substrate Final Thickness	~120	µm	After ICP thinning
CVD Growth Time	5	h	Total deposition time
Early Stage Growth Rate	~1.2	µm/h	First 100 min
Metal Mask Layers	100 nm Ti / 2 µm Al	-	Used for selective etching

Key Methodologies

The fabrication of the diamond-on-silicon membranes involved a six-step procedure utilizing advanced MPCVD and microfabrication techniques:

Substrate Seeding: A 2-inch, 400 µm thick, (111)-oriented Si wafer was pre-treated by seeding with 5 nm Nanodiamond (ND) particles in an ultrasonic bath to ensure high nucleation density (> 10⁹ cm^-2) and uniform PCD film growth.
MPCVD Deposition: Polycrystalline diamond (PCD) films were grown using a Microwave Plasma-Assisted CVD (MPCVD) system under the following recipe parameters:
- Gas Mixture: CH₄/H₂ (6% Methane content).
- Total Gas Flow: 500 sccm.
- Pressure: 55 Torr.
- Microwave Power: 5.0 kW.
- Substrate Temperature: Maintained at ~800 °C.
Substrate Thinning: The backside of the Si substrate was homogeneously thinned down to ~120 µm using Inductively Coupled Plasma (ICP) etching (etch rate ~5 µm/min).
Metal Mask Formation: A bi-layer metal mask (100 nm Ti adhesion layer / 2 µm Al top layer) was deposited via electron beam physical vapor deposition.
Patterning: Circular windows (150-400 µm diameter) were opened in the Ti/Al mask using laser ablation (KrF excimer laser, 248 nm) via an optical projection scheme.
Membrane Release: Selective ICP etching was performed through the patterned windows to remove the exposed silicon, automatically stopping at the diamond/Si interface, followed by chemical wet etching to remove the residual metal mask, resulting in free-standing PCD membranes.

6CCVD Solutions & Capabilities

The research demonstrates a critical need for high-quality, thin PCD films with precise thickness control and advanced post-processing capabilities—all core competencies of 6CCVD. We are uniquely positioned to supply the materials and services required to replicate, scale, and advance this high-pressure optical sensing technology.

Applicable Materials

To replicate or extend this research, 6CCVD recommends the following materials:

Optical Grade Polycrystalline Diamond (PCD): Required for the Fabry-Pérot cavity reflector. 6CCVD provides high-uniformity PCD wafers up to 125 mm in diameter, ensuring the mechanical consistency and low surface roughness (R_a < 5 nm for inch-size PCD) necessary for optimal optical reflection and minimal scattering loss.
Custom Thickness PCD: The paper utilized a 5.9 µm film. 6CCVD specializes in growing PCD films with thickness control from 0.1 µm up to 500 µm, allowing researchers to precisely tune the flexural rigidity ($D$) and sensitivity of the diaphragm for different pressure ranges, as defined by the relationship $\Delta d \propto t^{-3}$.
Boron-Doped Diamond (BDD) (Optional Extension): While the paper focused on optical sensing, BDD films could be used to integrate piezoresistive sensing elements directly onto the membrane, creating a hybrid sensor. 6CCVD offers BDD films with controlled doping levels.

Customization Potential

6CCVD’s in-house engineering and fabrication services directly address the complex post-processing steps detailed in the paper:

Research Requirement	6CCVD Capability	Value Proposition
Substrate Size & Thinning	Plates/wafers up to 125 mm (PCD)	Scale-up capability far exceeding the 2-inch wafer used in the study, enabling high-volume sensor array production.
Precise Membrane Thickness	SCD/PCD thickness control from 0.1 µm to 500 µm	Guaranteed thickness uniformity across large areas, critical for predictable diaphragm deflection and sensor calibration.
Metal Mask Deposition	Custom Metalization (Au, Pt, Pd, Ti, W, Cu)	We offer the Ti adhesion layer used in the paper, along with high-reflectivity metals (e.g., Au, Pt) for enhanced FP cavity performance, deposited via internal PVD/E-beam capabilities.
Membrane Definition	Custom Laser Cutting & Etching	Precise laser cutting and advanced etching services to define circular, square, or complex membrane geometries, eliminating the need for external laser ablation steps.
Surface Finish	Polishing down to R_a < 1 nm (SCD) or R_a < 5 nm (PCD)	We can provide membranes with superior surface finish compared to the 7 nm R_a achieved in the study, further reducing optical scattering and improving signal-to-noise ratio.

Engineering Support

6CCVD’s in-house PhD team provides expert consultation on material selection and optimization for high-performance devices. We can assist engineers and scientists in:

Modeling and Simulation: Optimizing PCD film thickness and membrane diameter to achieve specific sensitivity and pressure range targets for harsh environment pressure sensing.
Interface Engineering: Developing robust metalization schemes for improved adhesion and long-term stability in aggressive media.
Optical Integration: Advising on material specifications (e.g., refractive index, surface roughness) to maximize the finesse and signal quality of the Fabry-Pérot cavity.

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

View Original Abstract

Thin polycrystalline diamond films chemically vapor deposited on thinned silicon substrates were used as membranes for pressure sensor fabrication by means of selective chemical etching of silicon. The sensing element is based on a simple low-finesse Fabry-Pérot (FP) interferometer. The FP cavity is defined by the end-face of a single mode fiber and the diamond diaphragm surface. Hence, pressure is evaluated by measuring the cavity length by an optoelectronic system coupled to the single mode fiber. Exploiting the excellent properties of Chemical Vapor Deposition (CVD) diamond, in terms of high hardness, low thermal expansion, and ultra-high thermal conductivity, the realized sensors have been characterized up to 16.5 MPa at room temperature. Preliminary characterizations demonstrate the feasibility of such diamond-on-Si membrane structure for pressure transduction. The proposed sensing system represents a valid alternative to conventional solutions, overcoming the drawback related to electromagnetic interference on the acquired weak signals generated by standard piezoelectric sensors.

Tech Support

Original Source

DOI: https://doi.org/10.3390/ma13173697

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