CMOS Image Sensor Rear-Side ADC Placement for Aperture Ratio
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Solution Overview
Problem
Conventional CMOS solid-state image pickup elements face a decrease in aperture ratio due to the integration of analog-to-digital conversion circuits, leading to reduced sensitivity and difficulties in increasing pixel density and frame rate.
Innovation Solution
The analog-to-digital conversion circuits are placed on a surface opposite to the image pickup surface, allowing for XY address control and digital signal output, thereby maintaining the aperture ratio and enabling high-density pixel arrangements without occupying valuable light-receiving surface area.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If analog-to-digital conversion circuits are provided for each pixel on the image pickup surface, then peripheral circuit configuration is simplified, but aperture ratio decreases and sensitivity is lowered
Solution Approach 1:
The patent moves the analog-to-digital conversion circuits from the image pickup surface (2D plane) to the rear surface of the semiconductor chip, utilizing the third dimension (depth/thickness) of the substrate. This spatial relocation allows the conversion circuits to be positioned away from the light-receiving area, preserving the aperture ratio while maintaining functional integration.
Solution Approach 2:
The patent divides the semiconductor chip into distinct functional layers: the front surface contains only the pixel array for light reception, while the rear surface contains the analog-to-digital conversion circuits. This segmentation separates the optical function from the electrical conversion function, allowing each to be optimized independently without compromising the other.
2Area of moving object
If pixel area is increased to maintain aperture ratio, then sensitivity is improved, but area of pixel cell increases and number of pixels cannot be increased
Solution Approach 1:
By relocating the analog-to-digital conversion circuits to the rear surface, the front surface is cleared of non-optical components. This allows pixels to occupy maximum area on the light-receiving surface without interference from conversion circuits, thereby maintaining high pixel density while preserving aperture ratio and sensitivity.
3Quantity of substance
If analog-to-digital conversion circuits are created at high density to maintain aperture ratio, then pixel density is improved, but manufacturing difficulty increases
Solution Approach 1:
The patent segments the chip into two functional surfaces: the front surface dedicated to pixel array fabrication and the rear surface dedicated to analog-to-digital conversion circuit fabrication. This segmentation allows each surface to be optimized for its specific function, simplifying the manufacturing process compared to integrating both functions on the same surface at high density.
4Device complexity
If pixels are scanned in row units to output image pickup results, then processing is simplified, but sampling rate cannot be increased when number of pixels is increased
Solution Approach 1:
By moving the analog-to-digital conversion circuits to the rear surface, the patent enables parallel processing architecture where multiple pixels can be processed simultaneously without row-by-row scanning constraints. This spatial separation allows independent operation of conversion circuits for different pixel rows, thereby increasing the sampling rate while maintaining manageable processing complexity.
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 effectively prevents a decrease in aperture ratio, allowing for improved sensitivity, increased pixel density, and higher frame rates while simplifying the peripheral circuitry and enabling stable digital signal processing.
Implementation Method 1
a pixel for photoelectric conversion; and a plurality of pixels arranged in a matrix form
Data Source
AI summary
The present invention is applied to an image pickup device with a CMOS solid-state image pickup element, in which an analog-to-digital conversion circuit is disposed in a surface on an opposite side from an image pickup surface in a semiconductor chip 2.