Development of Diamond and Silicon MEMS Sensor Arrays with Integrated Readout for Vapor Detection

At a Glance

Metadata	Details
Publication Date	2017-05-24
Journal	Sensors
Authors	Maira Possas-Abreu, Farbod Ghassemi, Lionel Rousseau, Emmanuel Scorsone, Emilie Descours
Institutions	CEA LIST, Laboratoire d’électronique, systèmes de communication et microsystèmes
Citations	26
Analysis	Full AI Review Included

Technical Analysis: MPCVD Diamond Microcantilevers for Integrated VOC Detection

Source Paper: Development of Diamond and Silicon MEMS Sensor Arrays with Integrated Readout for Vapor Detection (Sensors 2017, 17, 1163)

Executive Summary

This paper validates the superior performance of MPCVD-grown Polycrystalline Diamond (PCD) microcantilevers as highly sensitive MEMS components for advanced gas sensing (Electronic Nose applications).

Core Material Validation: Synthetic PCD is confirmed as a highly promising material for resonant microcantilevers due to its exceptional mechanical properties, high elastic modulus (~10³ GPa), and robust nature.
High Sensitivity & Resolution: Sensors operate in dynamic (resonant) mode, demonstrating mass sensitivity in the range of hundreds of Hz/ng and achieving mass resolution down to the nanogram (ng) level.
Target Application: Successful application demonstrated in the detection and discrimination of 13 different Volatile Organic Compounds (VOCs) using pattern recognition (PCA) of the sensor array responses.
Integrated Readout: The system utilizes polysilicon piezoresistive strain gauges integrated directly into the cantilever anchorage, connected in a Wheatstone half-bridge configuration for low-noise, differential electrical readout.
Fabrication Process: PCD films are fabricated via Microwave Plasma Enhanced Chemical Vapor Deposition (MPECVD) on 4-inch silicon substrates using a precise nano-seeding technique (diamond nanoparticles in PVA).
Performance Metrics: Cantilevers consistently exhibited a high Quality Factor (Q > 600). The lowest Limit of Detection (LOD) achieved was 7.5 ppm for Ppy-Phenyl Acetate.
Modular Design: A patented, autonomous gas cell accommodates up to eight sensors with removable electrical connections, facilitating easy cleaning, replacement, and functionalization adaptation.

Technical Specifications

Parameter	Value	Unit	Context
Material Base	Polycrystalline Diamond (PCD)	N/A	Fabricated via MPECVD.
PCD Elastic Modulus	~10³	GPa	High modulus contributes to higher sensitivity.
Substrate Size	4-inch	Wafer	Used for integrated fabrication process.
Cantilever Lengths (L)	360, 660	µm	Examples of various geometries studied.
Resonance Frequency Range	20 to 150	kHz	Across five different geometries tested.
Minimum Quality Factor (Q)	> 600	N/A	Criterion for selected cantilevers.
Average Diamond Grain Size	1	µm	Measured via SEM.
Mass Sensitivity	Hundreds of Hz/ng	Hz/ng	Demonstrated performance range.
Estimated Noise Level	4	Hz	Fluctuation under reference gas flow.
Lowest LOD (7.5 ppm)	7.5	ppm	Achieved with Ppy-Phenyl Acetate coating.
Internal Gas Cell Volume	1	cm³	Optimized for homogeneous flow and small sample size.
Polymer Film Thickness (Max)	1.6	µm	Controlled thickness applied via spray coating.

Key Methodologies

The experimental approach combined advanced MEMS fabrication of diamond films with specialized electronic integration and chemical functionalization for dynamic mode operation.

Diamond Synthesis (MPECVD): Polycrystalline diamond films were grown onto 4-inch Silicon-on-Insulator (SOI) wafers using Microwave Plasma Enhanced Chemical Vapor Deposition (MPECVD) from methane gas, following a nano-seeding step utilizing diamond nanoparticles suspended in Poly-vinyl alcohol (PVA).
Integrated Piezoresistive Gauges: Polysilicon strain gauges were sputter-deposited and patterned onto the substrate prior to diamond growth to serve as transducer elements for electrical readout.
Etch Stop Layer: A layer of Tungsten (W) was deposited and patterned to act as an etch stop during subsequent etching steps required for diamond structuration and gauge access.
Microcantilever Structuring: Diamond and silicon layers were patterned and released using Deep Reactive Ion Etching (DRIE) techniques (both front-side and back-side silicon etching) to define the final cantilever geometries.
Surface Functionalization: Cantilever surfaces were coated with various polymeric thin films (e.g., poly(epichlorohydrin) (PECH), Polydimethylsiloxane (PDMS)) via optimized spray coating techniques to confer chemical selectivity toward specific VOCs.
Dynamic Mode Actuation & Readout: Cantilevers were excited at their resonance frequency (first transverse mode) using a piezoelectric cell. Resonance frequency shifts ($\Delta$f) caused by mass adsorption were measured via the low-noise Wheatstone half-bridge electronic circuit.
Data Analysis: Sensor response patterns were analyzed using Principal Component Analysis (PCA) to successfully discriminate between 13 tested Volatile Organic Compounds (VOCs).

6CCVD Solutions & Capabilities

This research highlights a compelling application for high-quality, customized MPCVD PCD material, requiring tight specifications in geometry, thickness, and integration interfaces—all core competencies of 6CCVD.

Applicable Materials

To replicate or extend this research, 6CCVD recommends leveraging its specialized Polycrystalline Diamond (PCD) substrates grown via MPCVD.

Application Requirement	6CCVD Solution	Rationale / Advantage
Core Sensor Material	Optical Grade PCD Wafers	Essential for high-Q resonance, chemical inertness, and superior mechanical strength (> 10³ GPa).
High Sensitivity/Q-factor	SCD or Highly Polished PCD	Achieving Q > 600 requires minimal internal stress and exceptional surface finish. 6CCVD polishing (Ra < 5 nm for PCD) ensures optimal surface preparation for polymer coating and maximizes Q.
Bio/Chemical Grafting	Functionalization-Ready PCD	Diamond’s carbon nature facilitates stable covalent C-C binding for advanced bio-receptor or polymer integration, a key future direction identified in the paper (Section 2.2).

Customization Potential

The success of MEMS devices depends critically on dimensional precision, thin-film control, and electrical integration. 6CCVD offers full customization services to meet these stringent requirements:

Custom Dimensions and Etching: The cantilevers required specific µm-scale geometries cut from 4-inch wafers. 6CCVD offers laser cutting and shaping services for PCD plates up to 125 mm, allowing for rapid prototyping and precise definition of microstructures.
Tight Thickness Control: The paper notes that diamond thickness variation was observed across the wafer. 6CCVD ensures ultra-precise thickness control for PCD layers, available from 0.1 µm up to 500 µm, essential for matching target resonance frequencies ($f_{0}$).
Integrated Metalization: The device fabrication required Tungsten (W) for etch stops and Chromium/Gold (Cr/Au) for electrical contacts. 6CCVD provides in-house multi-layer thin-film metalization capabilities, including Au, Pt, Pd, Ti, W, and Cu, directly onto SCD or PCD wafers to facilitate complex piezoresistive integration.

Engineering Support

6CCVD’s in-house PhD team specializes in optimizing CVD diamond materials for advanced electromechanical and sensing applications. We can assist researchers and engineers in VOC discrimination and Electronic Nose projects by consulting on:

PCD Film Optimization: Tailoring diamond growth recipes to minimize stress and defects, thereby maximizing the Quality Factor (Q) of the resonant structure.
Surface Engineering: Advising on the optimal surface roughness (polishing specification) to ensure repeatable and effective polymer coating adhesion and functionalization stability.
Substrate Compatibility: Selecting the ideal PCD or SCD thickness and integration strategy for integration with existing CMOS/SOI platforms, minimizing processing steps and costs.

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

View Original Abstract

This paper reports on the development of an autonomous instrument based on an array of eight resonant microcantilevers for vapor detection. The fabricated sensors are label-free devices, allowing chemical and biological functionalization. In this work, sensors based on an array of silicon and synthetic diamond microcantilevers are sensitized with polymeric films for the detection of analytes. The main advantage of the proposed system is that sensors can be easily changed for another application or for cleaning since the developed gas cell presents removable electrical connections. We report the successful application of our electronic nose approach to detect 12 volatile organic compounds. Moreover, the response pattern of the cantilever arrays is interpreted via principal component analysis (PCA) techniques in order to identify samples.

Tech Support

Original Source

DOI: https://doi.org/10.3390/s17061163

References

2014 - Humans can discriminate more than 1 trillion olfactory stimuli [Crossref]
1998 - ‘Electronic noses’ and their application to food [Crossref]
2001 - An electronic nose for the discrimination of forane 134a and carbon dioxide in a humidity controlled atmosphere [Crossref]
2003 - Peer reviewed: detection of explosives by electronic noses [Crossref]
2007 - An electronic nose in the discrimination of patients with asthma and controls [Crossref]
2009 - An optoelectronic nose for the detection of toxic gases [Crossref]
2010 - Diagnostic performance of an electronic nose, fractional exhaled nitric oxide, and lung function testing in asthma [Crossref]
2011 - Advances in electronic-nose technologies developed for biomedical applications [Crossref]
1982 - Analysis of discrimination mechanisms in the mammalian olfactory system using a model nose [Crossref]