A technique for phase-detection auto focus under near-infrared-ray incidence in a back-side illuminated CMOS image sensor pixel with selectively grown germanium on silicon

Abstract

A novel phase-detection auto focus (PDAF) technique for incident 850 nm plane wave is demonstrated using Ge-on-Si layer and deep trench isolation (DTI), which are locally arranged on light receiving surface (LRS) of crystalline silicon (c-Si). No metal light shielding film (LSF) for pupil division is formed. The key concept of the present work for PDAF is to perform the pupil division by the locally arranged Geon-Si layer in a pixel according to incident angle. The present pixel is based on a back-side illuminated CMOS image sensor pixel; the pixel pitch is 1.85 μm and the thickness of c-Si is around 3 μm. The simulation, based on three-dimensional finite difference time domain (3D-FDTD) method, shows that the external quantum efficiency (EQE) of the present pixel exhibits above 44.3 % with the maximum of 76.0 % for incident angles of - 30° to + 30°, owing to the selectively arranged Ge-on-Si layer; it exhibits 3.6 times improvement in the EQE at normal incidence compared to that of current state-of-the-art pixel with half metal-shielded aperture; the EQE is 49.2 % and 13.8 %, respectively. The present technique can enhance the accuracy of AF under low-illuminated condition.