Integrated evanescent waveguide detector for optical sensing

This paper presents the analysis and development of an evanescent waveguide sensor system, based on a hydrogenated amorphous silicon (a-Si:H) photodiode and a double ion-exchanged waveguide diffused in a borosilicate BK7 glass substrate. The a-Si:H sensor is a p-doped/intrinsic/n-doped diode, with a metal top electrode and an indium tin oxide (ITO) transparent bottom contact. Simulations on the confinement of a monochromatic light in a channel waveguide and its coupling into a thin-film photodetector were performed using COMSOL Multiphysics. The system design considers a deep investigation on refractive index and thickness of the ITO layer, which are key parameters for the optical coupling optimization between the waveguide and the photodetector. The first prototype was made using standard microelectronics techniques: physical and chemical vapor deposition of the thin films of the structure, wet and dry etching of the different materials, and photolithography for the thin-film pattern definition. The electrooptical characterization of the fabricated device shows the effectiveness of the optical coupling and suggests its use for on-chip detection in lab-on-chip applications.