Stacked Image Sensor Pixel Reflective Structure
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Solution Overview
Problem
Miniaturization of CMOS image sensors has led to insufficient light absorption, particularly for long-wavelength light, due to reduced pixel size, which affects the performance of backside illumination technology, necessitating new approaches to enhance light collection efficiency and reduce crosstalk.
Innovation Solution
Incorporating a vertical channel transfer transistor and a reflective structure between the photodiode and readout circuit in a stacked image sensor pixel cell, where the reflective structure reflects unabsorbed light back towards the photodiode for improved absorption, reducing crosstalk and enhancing signal-to-noise ratio.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If pixel size is reduced to increase resolution, then higher resolution is achieved, but light absorption efficiency deteriorates
Solution Approach 1:
The patent introduces a reflective structure positioned beneath the photodiode to reflect unabsorbed light back through the pixel. This adds a second optical path dimension, allowing light to pass through the thin substrate twice (downward and reflected upward), thereby compensating for the insufficient absorption depth caused by miniaturized pixel sizes in BSI technology.
Solution Approach 2:
The reflective structure acts as an intermediary element that intercepts light passing through the photodiode and redirects it back toward the light-sensitive region. This mediator enables additional light absorption opportunities without requiring increased substrate thickness, thus maintaining high resolution while improving light absorption efficiency.
2Loss of energy
If substrate thickness is increased to improve long-wavelength light absorption, then light absorption efficiency improves, but device complexity and manufacturing difficulty increase
Solution Approach 1:
Instead of increasing substrate thickness in the vertical dimension, the patent employs a reflective structure to create an additional optical path. This approach maintains the thin substrate geometry while effectively doubling the light interaction length through reflection, thereby improving long-wavelength absorption without complicating the substrate structure.
Solution Approach 2:
The patent changes the optical path length parameter by introducing reflection rather than physically thickening the substrate. This parameter modification achieves enhanced light absorption for long wavelengths while preserving the original thin-substrate design, avoiding increased device complexity and manufacturing difficulty.
3Loss of energy
If reflective structure is added to improve light absorption, then light absorption efficiency improves, but device complexity increases
Solution Approach 1:
The reflective structure serves as a simple intermediary component positioned between the substrate and the underlying layer. It performs the single function of reflecting unabsorbed light back through the photodiode, improving light absorption efficiency while adding minimal structural complexity to the pixel design.
Solution Approach 2:
The reflective structure performs multiple beneficial functions simultaneously: it reflects unabsorbed light back through the photodiode for additional absorption opportunities, reduces pixel crosstalk by directing light more precisely, and enhances the signal-to-noise ratio. This multi-functionality justifies the added structural element while maximizing its utility.
4Device complexity
If circuitry is integrated on the same chip as photodiodes, then device complexity is reduced, but manufacturing flexibility and optimization opportunities are lost
Solution Approach 1:
The patent divides the image sensor into two separate chips: a first chip containing the photodiode array and a second chip containing the circuitry. This segmentation allows each chip to be independently optimized for its specific function using appropriate manufacturing processes and materials, while still achieving a compact integrated final product through stacking.
Solution Approach 2:
Instead of integrating circuitry laterally alongside photodiodes on the same chip plane, the patent moves circuitry to a separate chip and stacks it vertically above the photodiode array. This vertical stacking in the third dimension achieves both manufacturing flexibility (separate chip fabrication) and compact integration (reduced overall footprint).
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 enhances the collection efficiency of longer wavelength light, reduces pixel array size, and lowers manufacturing costs by optimizing the placement of circuit elements, thereby improving the overall performance of the image sensor.
Implementation Method 1
the reflective structure reflects unabsorbed light back through the pixel
Implementation Method 2
the image sensor converts the light into electrical signals
Data Source
AI summary
A pixel cell has a photodiode, a readout circuit, a vertical transfer transistor and a reflective structure. The photodiode is disposed within a first substrate of a first semiconductor chip for accumulating an image charge in response to light incident upon the photodiode. The readout circuit is disposed within a second substrate of a second semiconductor chip. The vertical transfer transistor is coupled between the photodiode and the readout circuitry to transfer the image charge from the photodiode to the readout circuitry. The reflective structure is positioned between the readout circuit and the photodiode to reflect incident light, that passes through the photodiode without being absorbed, back towards the photodiode for a second chance at being absorbed.