KEYWORDS: Quantum efficiency, Silicon, Optical filters, Signal to noise ratio, Sensors, CMOS sensors, Electrons, Photons, Signal processing, Back end of line
Photo collection efficiency (proportional to the sensitivity of the photo sensor) and color crosstalk, both optical and electrical, are extremely important CMOS image sensor (CIS) pixel parameters. In measured QE data, photo collection efficiency and crosstalk information are mixed and it is difficult to disentangle the contributions of each to the raw QE spectrum. In our pixel optimization work, it is desirable to extract each component and to further separate the contribution of the color filter array (CFA) from the fundamental processes in and above the Silicon. In this paper, a new approach is introduced to extract the QE data related to the Si processing and decompose it into two components, the Mono QE and crosstalk spectrum, respectively. Using this approach one may gauge the impact of pixel structure differences, realize the sensor design goals to achieve the targeted system performance.
The demand of a large resolution CMOS image sensor (CIS) in a small package drives the pixel pitch size down
to the neighborhood of 2 μm. Double-micro-lens (ML) structure is a promising technology to obtain the high
focusing capability required by such a small pixel. In this work, an optical model of a double-ML is derived from
the well-known lens maker equation. This model predicts the critical back focal length (BFL) and the effective
focal length (EFL) of the double-ML embedded in the Back-End-Of-The-Line (BEOL) stack. Explained by this
model, a design guideline is provided to optimize the amount of light collected by the photo diode area for a
good quantum efficiency (QE), which is crucial to the sensitivity of the sensor.
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