Multi-layered mesoporous silicon membrane for flow-through optical sensing applications
(2019) SCOPe 2019 — Location: Thiais, France (19.June.2019)
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- Abstract
- In this work, a transverse porous silicon (PSi) membrane was fabricated for flow-through optical sensing applications. An open-ended porous silicon membrane helps to overcome the infiltration limitations of close-ended PSi layers. Indeed, in a close-ended PSi matrix, the analyte is transported over the film and its permeation into the depth of the pores is slow and limited, mainly governed by diffusion. In an open-ended PSi membrane, the sample is guided through the pores, enabling the fast and increased penetration of the analyte inside the porous matrix. A reduction in response time is therefore possible, enabling real-time optical sensing applications. The response amplitude is also improved, as more analyte is captured. The manufacturing process of the porous membrane starts with a heavily p-doped silicon wafer (0.8-0.9 mOhmcm, Siltronix) onto which a 400 nm-thick film of silicon nitride (Si3N4) is deposited, densified and then patterned. Over 300 μm of silicon is removed at the backside of these patterns using a time-stopped deep-reactive ion etching (DRIE) technique and a thick resist mask, leaving a few tens of microns-thick membrane. Electrochemical anodization is then performed in a double-bath etch cell, using different current densities in order to obtain multiple porous layers. The first layer, which is the layer of interest for the optical characterisation, is porosified at 200 mA/cm2, resulting in a 45 nm-average pore diameter and a more than 75% porosity. The second layer presents much smaller pores in order to create a good contrast with the first layer and thus to improve the optical response, as well as to maintain the mechanical integrity of the membrane. The third layer, with a slightly lager pore size, serves only for the toughness of the membrane. The optical characterisation consists in measuring the reflectance spectrum of the porous silicon membrane. The spectrum, which indicates only response of the top layer, displays Fabry-Pérot fringes. These can be analysed using the Reflective Interferometric Fourier Transform Spectroscopy (RIFTS) technique, which enables the determination of the effective optical thickness (EOT) of the first porous layer. This EOT, being proportional to the refractive index of the porous matrix, is affected by changes of the environment inside the pores. These variations can be exploited for sensing applications in which a flow-through setup is required. When the sensing is carried out in aqueous media, the chemical stability of the membrane must however be ensured. This is achieved by growing a thin Al2O3 passivation layer inside the pores using atomic layer deposition (ALD). This oxide layer reduces the pore size and porosity of the layers and changes the refractive index of the porous membrane, thereby affecting its optical response. A possible implementation of this passivated PSi membrane is the detection of bacteria in biological media using selective lytic enzymes. The flow-through setup shows improved results compared to the flow-over set up, promising a decrease of the limit of detection.
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Citations
Vercauteren, R., & Francis, L. (2019). Multi-layered mesoporous silicon membrane for flow-through optical sensing applications. SCOPe 2019, Thiais, France. https://hdl.handle.net/2078.5/227829