Abstract
An optical sensor based on a self-standing porous silicon (PS) membrane is presented. The sensor was created by electrochemically etching a heavily doped p-type silicon wafer with an organic electrolyte that contained dimethylformamide. After fabrication, a high-current density close to electropolishing was applied in order to allow the detachment from the substrate using a lift-off method. The PS membrane was integrated in a microfluidic cell for sensing purposes, and reflectance spectra were continuously obtained while the target substance was flowed. A comparison of the bulk sensitivity is achieved when flowing through and over the pores is reported. During the experiments, a maximum sensitivity of 770 nm/RIU measured at 1700 nm was achieved. Experimental sensitivity values are in good agreement with the theoretical calculations performed when flowing through the PS membrane, it means that the highest possible sensitivity of that sensor was achieved. In contrast, a drop in the sensitivity of around 25% was observed when flowing over the PS membrane.
Highlights
P OROUS silicon (PS) [1] is a good material for fabricating high sensitivity optical sensors
A shift of the reflectance spectrum towards higher wavelengths was observed due to the refractive index increase when the substance filling the pores was changed from Deionized water (DIW) to EtOH and/or IPA
All recorded data of each experiment were processed in order to obtain the position of a local maximum as the different substances were flowed and it was plotted versus the refractive index of the medium, as shown in Fig. 5 (b)
Summary
P OROUS silicon (PS) [1] is a good material for fabricating high sensitivity optical sensors. As the reactions occur almost exclusively at the pore tip, changes in the anodization conditions during the etching will not affect the PS already formed, which allows the formation of layers with different porosities and thicknesses throughout the structure. Both reactions are strongly dependent on the chemical and electrical conditions of the etching process. The sensitivity obtained when flowing through the PS membrane perfectly matches the one obtained in theoretical calculations, confirming that a complete filling of the sensor is accomplished, unlike when the flow is driven over the PS sensing membrane
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