Back Side Illuminated Image Sensor Charge Storage Design
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Solution Overview
Problem
Conventional CMOS image sensors with small pixel sizes face challenges in maintaining image performance due to kTC-reset noise and limited photodiode charge storage capacity, often requiring complex pixel circuits and stacking of substrates, which increases manufacturing costs and complexity.
Innovation Solution
A back-side illuminated image sensor pixel design where charge is stored in a potential well at the back side of the substrate and transferred to a floating diffusion node at the front side for readout, minimizing the area occupied by transfer gates and other circuitry, allowing for improved charge storage and reduced noise through correlated double sampling.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If charge is stored in a potential well at the back side of the substrate and transferred to a floating diffusion node at the front side, then pixel dynamic range and noise performance are improved, but the device complexity increases due to the need for charge transfer mechanisms
Solution Approach 1:
The pixel substrate is divided into two distinct surfaces with specialized functions: the back side contains the charge storage potential well for photon detection, while the front side contains the floating diffusion node for charge readout. This segmentation allows each surface to be optimized independently, improving noise performance while managing complexity through functional separation.
Solution Approach 2:
The charge storage function is moved from the traditional front-side planar configuration to the back side of the substrate, utilizing the third dimension (depth) of the substrate. This vertical separation of charge storage and readout functions enables improved dynamic range and noise performance without proportionally increasing lateral device complexity.
2Ease of manufacture
If small pixel sizes are used, then manufacturing cost is reduced, but image sensor performance is sacrificed due to limited charge storage capacity
Solution Approach 1:
By forming the potential well at the back side of the substrate and utilizing vertical depth for charge storage, the patent achieves sufficient charge storage capacity in small pixels without requiring larger lateral dimensions. This enables reduced pixel size and lower manufacturing cost while maintaining adequate charge storage capacity through the third dimension.
Solution Approach 2:
The separation of charge storage (back side potential well) and charge readout (front side floating diffusion) allows for optimized charge storage capacity within small pixel footprints. The back side potential well can be sized independently to provide sufficient charge storage without increasing the lateral pixel dimensions, thus reducing manufacturing cost while maintaining performance.
3Device complexity
If conventional front-side illuminated pixels are used, then device structure is simpler, but kTC-reset noise is generated during charge-to-voltage conversion
Solution Approach 1:
The charge storage function is extracted from the front side where the floating diffusion node is located, and placed in a separate back side potential well. This extraction separates the charge accumulation function from the charge readout function, allowing the floating diffusion node to be reset without disturbing a large charge storage region, thereby eliminating kTC-reset noise while maintaining relatively simple device structure.
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 design enhances pixel dynamic range and noise performance while eliminating kTC-reset noise and reducing manufacturing complexity by eliminating the need for substrate stacking, thus achieving improved image sensor performance at lower costs.
Implementation Method 1
Photons 90 that enter p-type doped layer 115 generate carriers that are collected in the potential well of the photodiode formed in region 108
Implementation Method 2
charge from the back side charge storage region can be transferred to the front side charge readout node
Data Source
AI summary
A back side illuminated image sensor may be provided with an array of image sensor pixels. Each image sensor pixel may include a substrate having a front surface and a back surface. The image sensor pixels may have a charge storage region formed at the back surface and a charge readout node formed at the front surface of the substrate. The image sensor pixels may receive image light at the back surface of the substrate. Photo-generated charge may be accumulated at the charge storage region during a charge integration cycle. Upon completion of the charge integration cycle, a transfer gate formed at the front surface may be pulsed high to move the charge from the charge storage region to the charge readout node. The charge may be converted to a voltage at the charge readout node and may be read out using a rolling shutter readout mode.