Development of opto-mechanical sensors based on silicon nanopillars for biological applications

Abstract

[eng] The scope of this thesis is the advancement on the theoretical simulation and nanofabrication of photonic crystals based on a periodic array of silicon pillars. This research is intended to be part of the preliminary research within the framework of an ambitious project for the development of mechanical biosensors for in vivo monitoring of tissues growth and regeneration based on photonic crystal structures. This application introduces limitations and requirements on the photonic structures. Among other requirements, the system should work within the near infrared (NIR) wavelength range of the electromagnetic spectrum, and the materials used for the fabrication should be biocompatible. The main two goals of this thesis have been the evaluation of the feasibility of such structures to behave as mechanical biosensor and the development and optimisation of a fabrication route and guidelines for the successful fabrication of sensor protoypes. This thesis compiles the work done and results obtained for the fulfilment of these two goals. The theoretical evaluation of the structures has been carried out using well known finite element modelling and finite-difference time domain programs for the simulation of the band diagrams and transmission spectra of the analysed devices, respectively. The outcome of these simulations has been used to set the main photonic crystal parameters suitable for this project, such as the radius of the pillars, the pitch between them and their length. These simulations have been performed considering air and water backgrounds. The simulations have shown, as well, that the introduction of defects into the main photonic crystal structure allow tuning the optical response of the system. Two types of defects have been considered: line and point defects (or cavities). The first ones, consisting in the variation of the size of the pillars in one row, introduces extra transmission bands inside the photonic band gap of the crystal. On the other hand, adding a point defect into this system a transmission peak appears within the photonic bandgap. An important result from the simulation is that this peak is sensitive to small perturbations of the cavity shape and size. Therefore, the proper design of the photonic crystal with defects can bring a system that can be sensitive to perturbations of its geometry. The fabrication process has been built based on the results of the simulations. The nanofabrication of the structures has a crucial role for the success of the project. For this reason, the second part of the thesis has been devoted to provide a very well detailed and optimised process flow for the fabrication of the arrays. In addition, this process has proved to be robust and versatile among different sensor designs. The fabrication process is based on an electron beam lithography exposure, followed by a liftoff for creating a hard mask to be used for the dry etching of the pillars. This last step is the one that has been more broadly studied and optimised due to its high impact on the resulting structures. The shape and roughness of the pillars and the consistency of the etching over different samples and structure configurations have been analysed to determine the optimal conditions of the dry etching process. A thorough analysis of the surface elements and compounds left after the fabrication process has been performed as well to ensure the biocompatibility of the structures. Finally, a short attempt to fabricate more exotic structures to enhance the photonic performance of the devices has been carried out. The results have shown that it is possible to further nanostructure the bottom of the photonic crystal pillars to improve the vertical light confinement within the photonic crystal structure.

[cat] El text d’aquesta tesi compila els resultats obtinguts de la simulació teòrica i la nanofabricació de cristalls fotònics basats en pilars de silici. L’objectiu d’aquesta recerca ha estat el d’establir les bases per a desenvolupar un projecte interdiciplinari i ambiciós pel desenvolupament de biosensors mecànics basats en cristalls fotònics per a la mesura del creixement i la regeneració de teixits in vivo . En el marc d’aquesta recerca, els objectius principals d’aquesta tesi es poden agrupar en dos grans grups. Per una banda, a valuació teòrica de la viabilitat de cristalls fotònics basats en pillars com a sensors mecànics . De l’altra , el desenvolupament i l’optimització de la nanofabricació de prototips del sensor esmentat. En la memòria s’hi compilen els resultats per l’assoliment d’aquests dos grans objectius. L’anàlisi teòric de la resposta fotònica d’aquestes estructures s’ha dut a terme considerant una estructura de cristall fotònic, a més de la introducció de defectes de línia i defectes puntuals dins de l’estructura. També s’ha dut a terme un estudi de la l’alçada òptima dels pilars. Els resultats d’aquest estudi han demostrat que, amb el disseny adequat aquestes estructures produeix en una resposta òptica única que, a la vegada, és sensible a alteracions geomètriques dels seus pilars. Aquest fet fa que aquestes estructures siguin viables com a sensors mecànics. Els resultats de les simulacions han estat utilitzats, després, per a l optimització del procés de fabricació per tal que aquesta última s’adeqüi als requeriments del projecte. El procés de fabricació s’ha basat en l’ús d’una litografia per feixos d’electrons, combinada amb la creació d’una màscara metàl·lica generada per lift off, seguida d’un procés de gravat sec. S’ha fet especialment èmfasi en l’impacte d’aquest últim procés degut al seu paper crucial en la forma i les dimensions dels pilars. El procés de fabricació resultant d’aquesta tesi ha estat pensat per proporcionar als nous investigadors d’aquest projecte una guia versàtil sobre com adaptar i optimitzar la fabricació de les estructures.