SiOCH Hits the Gas
Authors: Jousseaume Vincent , El Sabahy Julien , Ricoul Florence 
Published: May 2020

There is an increasing need for on-site and real-time analyses of complex gas mixtures in many application fields such as industry, air quality, security and medicine. One challenge consists in developing complete and portable multi-gas analysis systems allowing in situ and real-time quantitative analysis of complex gas mixtures. A promising approach is based on the integration of resonating devices on silicon chips by using “standard” microelectronic technologies. The fabrication of these gravimetric gas sensors based on Nano Electro Mechanical System (NEMS) requires the use of a chemical sensitive layer to ensure a better collection and concentration of the target molecules. This enhances the sensitivity in conjunction with the limit of detection. The integration of a chemical sensitive layer on to NEMS fabricated on Si wafers leads to specific material requirements.

The use of solvent is prohibited in order to avoid the degradation of the Si nanocantilevers due to capillarity forces,

Very thin film thickness (less than few hundred of nanometers) should be deposited on the Si beam in order to avoid filling the gap between the resonant cantilever and the electrodes,

The deposition technique should provide good uniformity over a large surface (typically on 200 mm Si wafers) for a collective sensor fabrication.

And the material should be optimized to absorb the highest level of targeted gas for a given thickness, reversibly and quickly.

In this work, the objective was to develop materials deposited using microelectronic compatible techniques in order to detect light alkanes and aromatic volatile organic compounds such as BTEX (for Benzene, Toluene Ethylbenzene and Xylene). Organosilicate materials are potentially good candidates for this application especially because they are nonpolar or weakly polar materials. In order to better understand the impact of the material chemistry on the detection of hydrocarbon gas and optimize the sensitivity of the chemical layer, a large panel of organosilicate thin films were deposited by different chemical vapor deposition (CVD) techniques. First, SiOCH were deposited by Plasma Enhanced Chemical Vapor Deposition (PECVD), this technique being the most used in the microelectronic industry for dielectric thin-film deposition. Several precursors (diethoxymethylsilane, trimethylsilane or octamethyltrisiloxane) and deposition conditions were used in order to vary their chemical composition and their physical properties. In these SiOCH, the carbon content is mainly under the form of methyl groups bonded to silicon. At the same time, to allow a better flexibility in term of chemical functions, filament assisted chemical vapor deposition (FACVD) was used for the deposition of organosilicate thin films. Indeed, FACVD allow a better control over the precursor fragmentation pathway in comparison to plasma-based techniques. Then, it is possible to add polar groups (such as ethoxy to in case of FACVD of methyltriethoxysilane) or to introduce siloxane rings in the materials (in case of initiated CVD of cyclosiloxane). Finally, the introduction of additional porosity was investigated as another way to potentially improve the sensitivity of the chemical sensitive layers. By using a porogen approach, it is possible to introduce up to 40 % porosity in a PECVD SiOCH. Using a foaming approach, we can go even further (> 50 %). Film properties after deposition and after potential annealing were investigated using multiple characterization techniques (ellipsometry, FTIR and Tof-SIMS). Porosity was characterized by ellipsometric porosimetry and grazing incidence small angle X-ray Scattering. In these material, pores (if any) are nanometrics and porosity is mainly open and interconnected. Thin film response under toluene or pentane exposures was studied using Quartz Crystal Microbalance functionalized with the different SiOCH.
Through the synthesis and characterization of these various SiOCH thin films, the role of hydrophobicity, carbon content and specific chemical bonding can be highlighted. It is shown that the hydrophobic nature of SiOCH materials composed of a Si-O-Si backbone with methyl groups bonded to silicon, combined with the presence nanoporosity, lead to very high sensibility. The presence of isolated polar bonds such as ethoxy groups seems beneficial especially for BTEX detection. High partition coefficients toward toluene and pentane can be obtained, at least ten time higher than those measured on more classical polymers such as PDMS. Both deposition techniques (PECVD or FACVD) are able to produce good chemical sensitive layers for the detection of light alkanes and aromatics VOCs and these organosilicates constitute promising solution for the functionalization of NEMS-based gas sensors.
Acknowledgment: Developments on FACVD were performed in the frame of a collaboration with TEL, US-Technology Development Center, America.