Optical Performances of Lensless Sub- 2micron Pixel for Application in Image Sensors

In this paper we will report on the optical performances of submicron planar lensless pixels arranged in the 2 × 2 Bayer cell configuration, the basic element of CMOS colour image sensors. The 2D microlens array placed in front of each pixel in commercial devices has been replaced by a 2D array of submicron holes realised on a thin metal film. Each pixel has been designed to present a lightpipe inside its structure acting as an optical waveguide that confines the light up to photodiode surface. This pixel design is fully compatible with large scale industry production since its fabrication involves only standard lithographic and etching procedures. Simulations of the light propagation inside the lensless pixel has been performed by using full 3D electromagnetic analysis. In this way it was possible to determine the optical performances of the Bayer cell in terms of the normalized optical efficiency and crosstalk effects between adjacent pixels that result to be up to 30% and a factor 10, respectively, better than those ones obtained for the microlens counterpart. The significant increase of the achievable values of the normalized optical efficiency and crosstalk can foresee the possibility to reduce the pixel size down to 1μm, i.e., beyond the limit imposed by the diffraction effects arising in microlens equipped pixel.