P11 - Polarization and Spectral Filter Arrays Based on Sub-Wavelength Structures in CMOS

SENSOR+TEST Conferences 2011
2011-06-07 - 2011-06-09
Proceedings OPTO 2011
OP - Poster Session
S. Junger, W. Tschekalinskij, N. Verwaal, N. Weber - Fraunhofer Institute for Integrated Circuits -IIS-, Erlangen (Germany)
161 - 165


In order to achieve polarization or spectral selectivity, conventional detectors and detector arrays require additional optical elements. Polarization filters, prisms, or beam splitters are used for polarization sensing, color filters or dispersive elements like diffraction gratings for color and multispectral sensors. Especially a pixelwise alternating filter array for image sensors is difficult to fabricate and align and only standard filter arrangements like the red/green/blue filter mosaic of the well known Bayer pattern for color image sensors are established.

We demonstrate a different approach, where metallic nanostructures – combined with underlying photodiodes – are used for filtering the light. Sub-wavelength gratings act as wire grid polarizers and sub-wavelength hole arrays are used as spectral filters. The optical properties of these nanostructures are well understood, but fabrication is usually done using technologies like electron beam lithography or focused ion beam milling. As opposed to that, we used deep submicron CMOS (complementary metal-oxide-semiconductor) manufacturing processes, allowing for high-volume fabrication of photodiodes including optical filters ("More than Moore").

The simulation of the nanostructures was performed using the finite-difference time-domain (FDTD) method and the layout was generated during the IC design flow. Several test chips were fabricated using the CMOS processes of different semiconductor foundries and the successful patterning of the nanostructures was verified by scanning electron microscopes. Each test chip comprises of photodiodes covered by different nanostructures; photodiodes without nanostructures are used as a reference. The spectral transmission of the nanostructures is obtained by calculating the ratio of the absolute spectral response of test photodiodes and reference photodiodes.

We show the measured performance of a grating with a period of 400 nm, and the expected polarizing effect is evident for wavelengths beyond 800 nm, where TEpolarized light is suppressed efficiently and the transmission of TM-polarized light is approximately 30 %. Spectral filtering is achieved by nanohole arrays, and an example for a band-pass filter with a peak wavelength of 520 nm is given. The peak wavelength can be tuned by modifying the lateral geometry of the nanostructures at constant layer thickness. The demonstrated on-chip polarization and spectral filters can be applied to polarimetry, polarization imaging, spectral monitoring of LEDs, color detection, and multispectral sensing.