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	Institute for Microelectronics and Microsystems, Prokhorov General Physics Institute
Citations	20
Analysis	Full AI Review Included

Executive Summary

This research details the design, fabrication, and preliminary characterization of a high-pressure fiber-optic sensor utilizing a thin polycrystalline Chemical Vapor Deposition (CVD) diamond membrane.

Core Technology: The sensor operates as a low-finesse extrinsic Fabry-Pérot (FP) interferometer, where the cavity is defined by the end-face of a single mode fiber (SMF) and the reflective diamond diaphragm surface.
Material Advantage: Exploits the superior properties of CVD diamond—high hardness (Young’s modulus ~1100 GPa), chemical inertness, and ultra-high thermal conductivity (2200 W m^-1 K^-1)—making the sensor suitable for harsh and aggressive environments.
Fabrication Method: A six-step process was used, combining Microwave Plasma-Assisted CVD (MPCVD) growth of 5.9 µm thick polycrystalline diamond on a thinned silicon substrate (120 µm), followed by laser-assisted metal mask lithography and selective Inductively Coupled Plasma (ICP) etching of the silicon.
Sensing Principle: Pressure difference (ΔP) causes deflection (Δd) of the diamond diaphragm, changing the FP cavity length, which is measured optoelectronically via interference signal modulation.
Performance: Feasibility was demonstrated up to 16.5 MPa at room temperature. A sensitivity of 1.01 nm/kPa was achieved for a 380 µm diameter membrane.
Value Proposition: The optical sensing system provides a robust alternative to conventional piezoelectric sensors, offering intrinsic immunity to electromagnetic interference (EMI) in noisy environments.

Technical Specifications

Parameter	Value	Unit	Context
Diamond Film Thickness (Measured)	5.9	µm	Estimated via weight gain during growth
Diamond Film Thickness (Calculated)	~5 to ~6	µm	Calculated from FP fringe period (confirms measured value)
Si Substrate Thickness (Thinned)	~120	µm	After ICP etching
Si Etch Rate (ICP)	~5	µm/min	Inductively Coupled Plasma etching process
Maximum Pressure Tested	16.5	MPa	High-pressure characterization range
Sensitivity (380 µm membrane)	1.01 ± 0.01	nm/kPa	Evaluated in 0-16.5 MPa range
Young’s Modulus (Assumed)	~1100	GPa	Literature value for diamond
Poisson’s Ratio (Assumed)	0.07	-	Literature value for diamond
Thermal Conductivity (Diamond)	2200	W m^-1 K^-1	Key material property
Raman Peak (1st Order)	1332.9	cm^-1	Diamond quality confirmation
FWHM of Raman Peak	3.8	cm^-1	Diamond quality confirmation
Growth Side Roughness (R_rms)	160	nm	Coarse-grain surface
Nucleation Side Roughness (R_rms)	15	nm	Smooth surface (used as reflective layer)
Interferometry Wavelength (λ)	1550	nm	IR fiber-coupled laser source
Fringe Period (360° Phase Shift)	775	nm	Corresponds to 1/2 λ displacement
Membrane Diameter (Tested)	360 ± 20 or 380 ± 20	µm	Defined by laser ablation mask

Key Methodologies

The diamond membranes were fabricated using a six-step procedure on a 2-inch, 400 µm thick, (111)-oriented single crystal silicon wafer.

Substrate Seeding: The Si wafer was seeded with nanodiamond (ND) particles (5 nm average size) using an ultrasonic bath to achieve a high nucleation density (>10⁹ cm^-2).
Diamond Deposition (MPCVD): Polycrystalline diamond (PCD) film was grown using Microwave Plasma-Assisted CVD (MPCVD).
- Process Parameters: 6% CH₄ in H₂, 500 sccm total flow, 55 Torr pressure, 5.0 kW microwave power.
- Temperature: Substrate maintained at ~800 °C.
- Result: A uniform 5.9 µm thick PCD film was achieved with an average grain size of ~4 µm.
Substrate Thinning: The Si substrate was thinned from the back side to a thickness of ~120 µm using Inductively Coupled Plasma (ICP) etching (~5 µm/min etch rate).
Masking and Patterning: A bi-layer metal mask (100 nm Ti / 2 µm Al) was deposited on the thinned Si back side. Circular windows (150-400 µm diameter) were opened in the mask using KrF excimer laser ablation (λ = 248 nm).
Membrane Formation (Selective Etching): ICP etching was used to selectively remove the exposed Si through the mask windows. The etching automatically stopped upon reaching the diamond/Si interface, creating free-standing diamond membranes.
Mask Removal: The residual Al-Ti metal layer was removed via chemical wet etching.
Sensor Assembly and Testing:
- The diamond-on-Si die was mounted on a brass holder and aligned to a Single Mode Fiber (SMF) using a micrometric x-y stage.
- The smooth diamond nucleation side was used as the reflective surface for the FP cavity.
- Pressure was applied using a hydraulic test pump, and membrane deflection was measured by monitoring the interference signal (photocurrent/photovoltage) generated by a 1550 nm laser source coupled via an optical circulator.

Commercial Applications

The unique combination of diamond’s mechanical and chemical properties with fiber-optic sensing provides significant commercial opportunities, particularly in demanding environments.

High-Temperature and Harsh Environment Sensing:
- Monitoring pressure in chemical reactors, furnaces, and industrial boilers where temperatures exceed the limits of conventional sensors.
- Aerospace and Automotive applications (e.g., engine diagnostics, combustion monitoring) requiring high thermal stability and chemical resistance.
Electromagnetic Interference (EMI) Immunity:
- Pressure monitoring in high-voltage power grids, particle accelerators, or military/defense systems where electrical noise is prohibitive for piezoresistive sensors.
- Remote sensing applications where long-distance signal transmission via fiber is advantageous.
High-Pressure Hydraulics:
- Measuring static and dynamic pressures up to 16.5 MPa (and potentially higher) in hydraulic machinery and deep-sea exploration equipment.
Biomedical Devices:
- Diamond’s biocompatibility and inertness make it suitable for implantable sensors requiring long-term stability in the human body.
Advanced MEMS/NEMS Fabrication:
- The established fabrication process for thin, high-quality diamond membranes can be leveraged for other diamond-based microelectromechanical systems, such as high-frequency resonators or capacitive ultrasonic transducers.

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

References

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