UV-induzierte Brechzahländerung zur Herstellung von Wellenleitern und Integration von Siliziumphotodioden

Abstract

This thesis describes the fabrication process for polymer waveguides by UV-induced refractive index modification on silicon substrate. This process was used for the integration of waveguides with silicon photodiodes. The fabrication process is based on the local modification of the refractive index of polymers by deep UV-exposure. The local structuring of waveguides was performed by conventional photolithographic technique using a quartz/chromium mask. In order to investigate the influence of the material selection on the photochemical process, different optical polymers were used. Only modified acrylate (alicyclic methacrylate copolymer, OPTOREZ ® ) showed enough refractive index change for the waveguide fabrication by UV-exposure. This new material owns a higher refractive index, higher glass transition temperature and lower water absorption than conventional polymethyl methacrylate (PMMA). The photochemical reaction of modified acrylate was shown to be the same as that of PMMA by spectroscopic analysis. The first investigations for the immobilization of living cells (L929 fibroblast) on the surface of modified acrylate have been successfully carried out. This modified acrylate is suitable for the fabrication of waveguides and the immobilization of living cells. OZ-1100 was selected from OPTOREZ ® series for the waveguide fabrication and characterized. The exposure time and exposure wavelength were optimised with respect to the optical properties of the waveguides. By using a maskaligner EVG 620 with shorter exposure time, single-mode waveguides with smaller propagation loss (0.8 dB/cm at 1550 nm and 0.6 dB/cm at 808 nm) were achieved. Furthermore the integration of waveguides with silicon photodiodes was demonstrated. PMMA waveguides was fabricated on the SiO 2 coated silicon wafer and characterized. 10 μm wide waveguides were multimode at 635 nm. The propagation loss was 0.16-0.25 dB/cm at 650 nm. The fiber to chip coupling loss was 1.2-2.7 dB/facet, depending on the quality of the diced edged. This value is much better than the already reported value (1-1.2 dB/cm and 7 dB/facet) for SU-8 waveguide on silicon wafer. PMMA waveguide was integrated with silicon photodiode on a substrate. The photocurrent was measured and coupling efficiency was determined. Due to the uncertainness of the total losses in the waveguide, the rough estimation of the coupling efficiency was between 30% and 90%. The approach presented here is a first step to the integrated biophotonic sensor which contains optical waveguides, detector and light sources.