CMOS Imaging Device Storage Capacitance Lines Dynamic Range
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
CMOS imaging devices face challenges in achieving a large dynamic range while maintaining sensitivity on the low-illuminance side without increasing the pixel area or costs, as traditional storage capacitance elements restrict the area of photodiodes and incur high-k material and stacked capacitor complexities.
Innovation Solution
The imaging device employs a configuration with storage capacitance lines that are not formed on the semiconductor substrate, using the wiring layer to increase the area of the photodiode, allowing for efficient storage of overflowing charges without increasing the capacitance value of the floating diffusion capacitor, thereby enhancing dynamic range and suppressing clipped whites.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a storage capacitance element is formed on the semiconductor substrate to suppress clipped whites on the high illuminance side, then the dynamic range is enlarged, but the pixel area increases and it becomes difficult to realize a large number of pixels
Solution Approach 1:
The invention extracts the storage capacitance function from the semiconductor substrate and implements it using wiring layers (metal interconnect layers) instead. By taking out the capacitance formation location from the substrate plane, the photodiode area is no longer constrained by substrate real estate, enabling larger photodiodes without increasing pixel area.
Solution Approach 2:
The invention moves the storage capacitance implementation from the two-dimensional substrate plane to the vertical dimension by utilizing wiring layers stacked above the substrate. This dimensional transition allows the storage capacitance to coexist with the photodiode without competing for planar space, thus enlarging the photodiode area while maintaining compact pixel structure.
2Measurement precision
If the capacity of the floating diffusion is set small to improve sensitivity on the low illuminance side, then sensitivity is enhanced, but clipped whites appear on the high illuminance side
Solution Approach 1:
The invention segments the charge storage function into two distinct components: the floating diffusion (maintained small for sensitivity) and the storage capacitance element (enlarged for high-illuminance handling). This segmentation allows each component to be optimized independently - the floating diffusion for low-illuminance sensitivity and the storage capacitance for preventing clipped whites.
Solution Approach 2:
The invention introduces a transfer transistor as an intermediary mechanism between the photodiode and the storage capacitance element. This intermediary enables controlled charge redistribution, allowing charges to be transferred to the storage capacitance when the floating diffusion becomes saturated, thus preventing clipped whites while maintaining the small floating diffusion capacity needed for sensitivity.
3Reliability
If a MOS capacitor is formed in a semiconductor substrate or over a semiconductor substrate as a storage capacitance element, then charge storage is achieved, but it becomes difficult to secure the area of a photodiode and high-k material and stacked capacitor push up the cost
Solution Approach 1:
The invention makes the wiring layers (metal interconnect layers) serve a dual function: their primary function for electrical interconnection and an additional function as storage capacitance elements. This multi-functionality eliminates the need for separate capacitance structures, reducing manufacturing complexity and cost while maintaining charge storage capability.
Solution Approach 2:
The invention utilizes existing wiring layer structures that are already part of the standard CMOS fabrication process, rather than introducing expensive specialized materials like high-k dielectrics or complex stacked capacitor structures. This approach leverages readily available, cost-effective materials and processes to achieve the storage capacitance function.
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 configuration allows for a larger photodiode area, maintaining sensitivity on the low-illuminance side while preventing clipped whites on the high-illuminance side, thus realizing a high dynamic range imaging device without increasing costs or complexity.
Implementation Method 1
a photodiode which generates and stores a charge when subjected to light
Data Source
AI summary
An imaging device is provided which can secure the dynamic range of a COMS imaging sensor, by storing a charge overflowing from a floating diffusion in a storage capacitance element and suppressing the increase of a pixel area which occurs if the storage capacitance element is formed by a MOS capacitor. The imaging device includes plural pixel circuits arranged in the row direction and the column direction, and plural storage capacitance lines arranged in the row direction and extending in the column direction. Each of the storage capacitance lines is coupled to the pixel circuits arranged in the same column. The pixel circuit includes a first photoelectric conversion element which stores a charge generated by being subjected to light, a floating diffusion to which the charge stored in the first photoelectric conversion element is transferred, and a first switching transistor coupling the floating diffusion and the storage capacitance line.