CMOS Image Sensor Macropixel Area Allocation
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Solution Overview
Problem
Miniaturization of CMOS image sensors has led to a decrease in pixel fill factor due to increased consumption of area by active circuit elements and metal interconnects, resulting in reduced signal-to-noise ratio (SNR) and image quality, particularly in applications requiring high resolution and low power consumption like medical endoscopes.
Innovation Solution
The implementation of a macropixel block structure in CMOS image sensors, where the light-collection area is allocated differently among color pixels, with a larger allocation to green pixels to improve SNR, and the use of macropixel shapes and primary color sets like RGB or magenta-yellow-cyan to optimize image quality.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If pixel pitch is reduced to increase resolution, then image resolution is improved, but pixel fill factor decreases due to increased area consumption by circuit elements and interconnects
Solution Approach 1:
The patent transitions from planar 2D pixel layout to a 3D stacked architecture where photodiodes are positioned vertically above transistors and interconnects. This vertical stacking allows light-sensitive areas to be separated from circuit elements in the third dimension, effectively increasing fill factor without reducing pixel pitch.
Solution Approach 2:
The pixel structure is segmented into distinct functional layers: photodetector layer, transistor layer, and interconnect layer. This segmentation allows each component to be optimized independently and positioned in optimal spatial locations, maximizing light collection area while minimizing circuit area consumption.
2Reliability
If photodiode area is increased to improve signal-to-noise ratio, then image quality is improved, but pixel area consumption increases
Solution Approach 1:
By stacking photodiodes vertically above other pixel components rather than placing them side-by-side in 2D, the patent enables larger photodiode area without increasing the planar pixel footprint. The vertical dimension provides additional space for enlarged photodetectors while maintaining compact pixel pitch.
Solution Approach 2:
The patent implements a nested arrangement where photodiodes are positioned over transistors and interconnect structures. This nesting allows photodetectors to occupy the vertical space above existing components, effectively utilizing the pixel area that would otherwise be wasted on circuit elements.
3Measurement precision
If more area is allocated to color filters and circuit elements, then color accuracy is improved, but light-sensitive area decreases
Solution Approach 1:
The patent positions color filters and circuit elements in vertical layers below or beside the photodiodes rather than competing for the same planar space. This layered approach allows full-color filtering capability while maintaining maximum photodetector area for light collection.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach enhances the signal-to-noise ratio and maintains high image resolution while reducing pixel size, particularly benefiting applications where color accuracy is a lower priority, such as automotive, security, and machine vision systems.
Implementation Method 1
use a microscale lens (microlens) directly above each pixel to focus the light directly towards the photosensitive portion of the area within the pixel
Implementation Method 2
the photosensitive element within the substrate facing a backside of an image sensor die. For BSI image sensors, the majority of photon absorption occurs near the backside silicon surface.
Implementation Method 3
photon absorption occurs near the backside silicon surface
Data Source
AI summary
Embodiments of an apparatus comprising a pixel array comprising a plurality of macropixels. Each macropixel includes a pair of first pixels each including a color filter for a first color, the first color being one to which pixels are most sensitive, a second pixel including a color filter for a second color, the second color being one to which the pixels are least sensitive and a third pixel including a color filter for a third color, the third color being one to which pixels have a sensitivity between the least sensitive and the most sensitive, wherein the first pixels each occupy a greater proportion of the light-collection area of the macropixel than either the second pixel or the third pixel. Corresponding process and system embodiments are disclosed and claimed.