Capacitor Over Red Pixel for CMOS Imager Fill Factor
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Solution Overview
Problem
In CMOS active pixel imagers, the inclusion of capacitors to increase charge storage capacity reduces the fill factor, as they occupy space that could be used for photo-conversion devices, leading to a tradeoff between charge storage and light sensitivity.
Innovation Solution
Capacitors are placed over red pixels, where red light is absorbed deeper in the silicon, and shared with neighboring non-red pixels, allowing for increased size of photosensitive regions without significantly affecting the fill factor.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Quantity of substance
If capacitors are included in the pixel array to increase charge storage capacity, then charge storage capacity is improved, but fill factor deteriorates due to space occupation
Solution Approach 1:
The capacitor is moved from the lateral plane to the vertical dimension by placing it over the red pixel photosensitive region. This three-dimensional arrangement allows the capacitor to share space with the red pixel, enabling increased charge storage capacity without reducing the fill factor of other pixels.
Solution Approach 2:
The capacitor over the red pixel serves multiple functions: it provides charge storage for the red pixel and simultaneously serves as a shared resource for neighboring non-red pixels. This multi-functionality maximizes the utility of the capacitor structure and optimizes space utilization across the pixel array.
2Quantity of substance
If capacitor size is increased to enhance charge storage, then charge storage capacity is improved, but space for photo-conversion devices is reduced
Solution Approach 1:
By positioning the capacitor in the vertical dimension over the red pixel, the patent enables the capacitor to occupy space that would otherwise be unused or underutilized. This vertical placement allows for larger capacitor structures without encroaching on the photosensitive regions of blue or green pixels, thereby maintaining their photo-conversion capabilities.
Solution Approach 2:
The capacitor is specifically placed over the red pixel region where red light is absorbed deeper in the silicon. This localized placement exploits the specific optical properties of red pixels, allowing the capacitor to be positioned in a region where it minimally interferes with photo-conversion while maximizing charge storage capacity.
3Area of stationary object
If capacitors are placed over red pixels, then fill factor is improved for blue and green pixels, but red pixel photosensitive region may be affected
Solution Approach 1:
The patent exploits the local optical property that red light is absorbed deeper in the silicon compared to blue and green light. By placing the capacitor over the red pixel, the structure primarily affects the upper silicon layers where blue and green light are absorbed, while red light penetrates deeper and is less affected. This local quality difference allows the capacitor to be positioned without significantly compromising red pixel photosensitivity.
Solution Approach 2:
The capacitor structure over the red pixel serves as a shared resource that is functionally copied for use by multiple pixels (the red pixel and neighboring non-red pixels). This sharing arrangement allows the capacitor to benefit multiple pixels simultaneously, improving overall array efficiency without requiring separate capacitors for each pixel.
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 both the fill factor and charge storage capacity of the pixel array by optimizing the use of space, particularly for red pixels, while minimizing cross-talk with blue or green pixels.
Implementation Method 1
a photo-conversion device 23 for generating and collecting charge generated by light incident on the pixel 10
Implementation Method 2
a capacitor 20 is also included... increasing the charge storing capacity of the floating diffusion node 5
Implementation Method 3
red light is absorbed deeper in the silicon
Data Source
AI summary
A red pixel having a capacitor formed over the photo-conversion region of the pixel. The capacitor can be used by other pixels as a common capacitor. The capacitor is coupled to floating diffusion region shared by a plurality of pixels. The plurality of pixels also share readout circuitry.