POROUS SILICON BASED OPTICAL DEVICES FOR BIOCHEMICAL SENSING

Abstract

This thesis summarizes a three years scientific research investigation on the design and fabrication of porous silicon based optical devices for applications in the field of biochemical sensing. Porous silicon is an ideal transducer material due to its sponge-like morphology, characterized by a specific surface area up to 500 m2 cm-3, which assures an effective interaction with gas and liquid substances. Moreover, porous silicon is a low cost material, completely compatible with standard microelectronic processes. In this work, different porous silicon structures such as Fabry-Perot interferometer, Bragg mirror, optical microcavity, Thue-Morse sequences and optical waveguide have been realized and characterized as optical transducers for the monitoring of chemical and biological interactions. The selectivity, reversibility and sensitivity of these devices as optical sensors have been discussed. The porous silicon surface has been modified in order to gain chemical stability, proper wettability, and specific features such as biomolecules immobilization. Standard chemical functionalizations, but also an innovative pure biological passivation method based on selfassembled biofilms of the Hydrophobins proteins, have been successfully experimented. Some standard micromachining techniques, such as HF wet etching and anodic bonding, have been optimized to integrate the porous silicon sensing element into a Lab-on-Chip prototype. The integrated devices have been characterized as fast sensors of chemical compounds and response times shorter than 100 ms have been demonstrated. The Direct-Laser-Writing of the porous silicon surface, as alternative process to the photolithographic patterning in the device miniaturization has been also exploited. Finally, a bottom-up approach in microoptics has been developed by using the silica shells of some marine Diatoms, microalgae which show impressive morphological and physical analogies with porous silicon.