Elevated Photosensor Trenches for CMOS Image Sensors
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Solution Overview
Problem
CMOS image sensors face challenges such as electron/carrier recombination, low fill factor, charge leakage, and reduced quantum efficiency due to surface defects and potential barriers, leading to incomplete charge transfer and loss of incident light.
Innovation Solution
The implementation of a raised photosensitive region with a u-shaped or v-shaped trench configuration that redirects reflected light for increased absorption, combined with a hydrogenated amorphous silicon layer and epitaxial structure to enhance fill factor and quantum efficiency.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If the n-type silicon layer of the photosensor is located close to the surface, then the device structure is simplified and manufacturing is easier, but electron/carrier recombination occurs due to surface defects
Solution Approach 1:
The patent raises the photosensor above the substrate surface to form an elevated structure. This dimensional change moves the photosensitive region away from the defective surface plane while maintaining electrical connection to the substrate, thereby reducing surface defect impact on charge transfer efficiency
Solution Approach 2:
The patent introduces an insulating layer between the elevated photosensor and the substrate surface. This intermediary structure provides electrical isolation and creates a controlled interface that reduces charge recombination at the surface while maintaining the simplified n-type silicon layer structure
2Reliability
If a transfer transistor gate is used to transfer charge, then charge transfer is enabled, but potential barriers at the gate hinder complete charge transference
Solution Approach 1:
The patent removes the transfer transistor gate structure from the charge transfer path. By eliminating this component, the potential barriers it creates are removed, allowing complete charge transference from the photosensor to the floating diffusion region without charge loss at gate interfaces
Solution Approach 2:
The patent creates a direct electrical connection between the elevated photosensor and the floating diffusion region through doped regions, establishing an equipotential path that eliminates potential barriers and enables complete charge transfer without energy loss
3Device complexity
If a conventional planar photosensor is used, then the device structure is simple, but the fill factor is only approximately fifty percent
Solution Approach 1:
The patent elevates the photosensor above the substrate surface to form a three-dimensional structure. This allows the photosensitive area to extend vertically while maintaining a compact planar footprint, effectively doubling the photosensor area without proportionally increasing the device footprint, thus achieving high fill factor with relatively simple structure
Solution Approach 2:
The patent nests the elevated photosensor structure within the pixel cell architecture, with the photosensor rising above the substrate plane while remaining integrated with the underlying circuit elements. This nested configuration maximizes the photosensitive area within the available pixel cell space
4Productivity
If incident light strikes the surface of a photosensor, then photon to charge conversion occurs, but electron/carrier recombination reduces quantum efficiency
Solution Approach 1:
The patent raises the photosensor above the substrate surface, creating an elevated photosensitive region that is physically separated from the defective surface plane. This dimensional separation reduces electron/carrier recombination losses while maintaining efficient photon absorption and charge generation
Solution Approach 2:
The patent introduces an insulating layer as an intermediary between the elevated photosensor and the substrate. This layer reduces charge recombination at the interface while allowing photons to effectively reach the photosensitive region, thereby improving quantum efficiency without compromising conversion rate
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 significantly increases the fill factor and quantum efficiency of the pixel cell, reducing electron recombination and charge leakage, while effectively capturing reflected light, thereby improving the overall sensitivity and responsivity of the image sensor.
Implementation Method 1
a hydrogenated amorphous silicon layer and epitaxial structure to enhance fill factor and quantum efficiency
Implementation Method 2
when incident light strikes the surface of a photosensor, electron/hole pairs are generated in a p-n junction of the photosensor
Data Source
AI summary
An elevated photosensor for image sensors and methods of forming the photosensor. The photosensor may have light sensors having indentation features including, but not limited to, v-shaped, u-shaped, or other shaped features. Light sensors having such an indentation feature can redirect incident light that is not absorbed by one portion of the photosensor to another portion of the photosensor for additional absorption. In addition, the elevated photosensors reduce the size of the pixel cells while reducing leakage, image lag, and barrier problems.