Plasmonic filters for ambient and near infrared sensing on CMOS

Abstract

The light sensors market is growing, driven largely by increased use of proximity detection and ambient light sensing (ALS) in consumer electronics. There is high demand for reduced cost and physical size of light sensors, however the spectral filter technology used on complementary metal-oxide semiconductor (CMOS) chips has not advanced significantly. Plasmonic filters have been proposed as a superior alternative offering reduced cost and thickness, among other advantages. In this work plasmonic filters are investigated in the near infrared (NIR) range for proximity sensing applications, and the visible range for ALS applications using CMOS compatible materials and fabrication processes. The plasmonic filters are thin metallic films nanostructured with an array of subwavelength holes that facilitate coupling with surface plasmon polaritons (SPP) and localised surface plasmons (LSP). They exhibit extraordinary optical transmission with peak transmission wavelengths controlled by the geometry and size of the hole array. Filters were designed on glass substrate by electromagnetic simulations using a finite-difference time-domain (FDTD) method, created using micro and nano-fabrication techniques, and then measured by microspectrophotometry to evaluate their spectral response. Following characterisation, the NIR filter was fabricated directly onto a CMOS chip and the spectral response was assessed by chip measurement for a proof-of-concept demonstration of an integrated device. The NIR plasmonic filter exhibited poor suitability on CMOS due to high order plasmonic resonances in the visible range that were enhanced by Fabry-Pérot resonances supported by the CMOS stack. The most common plasmonic filter, a circular-shaped hole nanostructure, is sensitive to angle of incidence (AOI) making it unsuitable for ALS applications. Preliminary designs for plasmonic ALS filters with low sensitivity to AOI were demonstrated, by characterisation on glass, using a cross-shaped hole nanostructure. Design dimensions that produced this quality were decreased array period and decreased ratio of the cross arm-length to arm-width, due to increased separation between the SPP and LSP resonances generated by the plasmonic hole array filter