Cross-Point Alignment for All AF Pixels in CMOS Image Sensors

The phase detection autofocus (PDAF) method is vital for mobile cameras to capture rapidly occurring moments. In particular, for off-axis pixels, which are located in the excepting center of the image sensor, sensitivity difference occurs in each pixel having the same color due to oblique incident light. The sensitivity difference causes a misaligned autofocus (AF) cross point that determines the incorrect focus. To eliminate the sensitivity difference, the optical stacks are shifted toward the incident light. The use of in-pixel deep-trench isolation (DTI), which physically distributes the incident light, is also suggested to find the in-focus position regardless of the location of the image sensor. However, the chromatic aberration of the microlens and the refractive index difference of the optical stacks depending on wavelength still cause misalignment in the AF cross point. In this article, a pixel structure is suggested to find the in-focus position by eliminating sensitivity differences and aligning the AF cross point. The suggested pixel changed the width of the in-pixel DTI and changed the refractive index of the passivation layer (PL). To investigate its effectiveness, an optical simulation was used for 1.0-
$\mu \text{m}$
pixels. As a result, a sensitivity difference of up to 23.5% was completely eliminated. Moreover, the AF cross points that were misaligned up to 1.4° from the in-focus position determined the correct focus. For all pixels in the image sensor, the AF cross points were confirmed to align exactly regardless of the pixel positions and transmitted wavelength from the color filter (CF).