Raised Photosensitive Elements in Image Sensors
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Solution Overview
Problem
Conventional frontside illuminated image sensors face challenges in achieving reduced stack height, smaller pixel sizes, and higher fill factor without compromising quantum efficiency or image quality, due to the height of the image sensor stack and difficulty in precisely controlling photodiode depletion regions.
Innovation Solution
The process involves forming periphery circuitry separately from the pixel array circuitry, using a selective epitaxial growth process to create an epitaxial layer within the pixel array area, which allows for raised photodiode formation, reducing stack height and pixel size while increasing fill factor, and controlling depletion region characteristics for improved quantum efficiency.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Length of moving object
If conventional frontside illuminated image sensor formation techniques are used, then the image sensor can be manufactured with standard processes, but the stack height remains large, pixel size cannot be reduced, and fill factor is limited
Solution Approach 1:
The patent segments the image sensor formation process into distinct stages: first forming periphery circuitry on the substrate, then selectively forming photosensitive elements in raised regions within the pixel array area. This segmentation allows the photosensitive elements to be positioned at different heights, reducing the overall stack height while maintaining manufacturing feasibility through separate processing steps for periphery and pixel array regions.
Solution Approach 2:
The patent introduces vertical dimensionality by forming photosensitive elements in raised regions, creating a three-dimensional structure where photosensitive elements are positioned at different heights relative to the substrate. This vertical arrangement reduces the horizontal footprint and allows for smaller pixel sizes and higher fill factors without increasing the lateral dimensions of the sensor.
2Area of stationary object
If periphery circuitry is formed together with pixel array circuitry, then the manufacturing process is simplified, but the position of photosensitive elements cannot be raised, limiting fill factor improvement
Solution Approach 1:
The patent divides the circuitry formation into two separate processes: periphery circuitry is formed first on the substrate, followed by selective formation of photosensitive elements in the pixel array area. This segmentation enables the photosensitive elements to be positioned in raised regions that would be inaccessible if periphery and pixel array circuitry were formed simultaneously, thereby increasing the fill factor.
Solution Approach 2:
The periphery circuitry is formed in advance before the photosensitive elements are created. This preliminary action establishes the foundational circuitry and defines the regions where photosensitive elements will subsequently be formed, allowing for optimized positioning and raised structure formation that would not be possible with simultaneous formation.
3Reliability
If photodiode depletion regions are not precisely controlled, then the manufacturing process is simpler, but quantum efficiency and image quality are degraded
Solution Approach 1:
The patent applies different properties to different regions of the substrate: periphery regions contain standard circuitry, while the pixel array area contains selectively formed photosensitive elements with controlled depletion regions. This local differentiation allows precise control of depletion region characteristics in the pixel array area through selective epitaxial growth, ensuring high quantum efficiency while maintaining simpler processes in periphery regions.
Solution Approach 2:
The patent controls the depletion region characteristics by adjusting parameters of the selective epitaxial growth process, including temperature, pressure, and gas flow rates. These parameter changes enable precise control over the depth and shape of depletion regions in the photosensitive elements, optimizing quantum efficiency while maintaining manufacturing feasibility.
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 enables reduced stack height, smaller pixel sizes, and higher fill factor, resulting in improved quantum efficiency and image quality, comparable to backside illuminated sensors, with fewer process steps and using a single semiconductor wafer.
Implementation Method 1
forming an epitaxial layer in the opening... The epitaxial layer may be formed by performing a selective epitaxial growth process
Implementation Method 2
ensure sufficient charge carriers... Failure to configure the photodiode depletion regions to provide sufficient charge carriers can degrade quantum efficiency
Data Source
AI summary
An image sensor having a pixel array comprises periphery elements formed over a substrate, an oxide layer formed over the periphery elements, an epitaxial layer formed in an opening in the oxide layer in a pixel array area, and a plurality of photosensitive elements of the pixel array formed in the epitaxial layer. Formation of an initial metallization layer occurs after the formation of the photosensitive elements in the epitaxial layer. The photosensitive elements can thus be formed in the epitaxial layer at a higher level within an image sensor stack than that of the initial metallization layer. This advantageously allows stack height and pixel size to be reduced, and fill factor to be increased. The image sensor may be implemented in a digital camera or other type of digital imaging device.