CMOS Image Sensor Doping Profile for Sensitivity and Charge Capacity
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Solution Overview
Problem
CMOS image sensors face a trade-off between sensitivity at low illuminance and dynamic range due to the expansion of the depletion region, which improves photoelectric conversion efficiency but reduces capacitance and charge capacity.
Innovation Solution
A CMOS image sensor design with a second conductive bottom region of higher impurity concentration and a first conductive high concentration region, allowing for a wide depletion region while maintaining high capacitance, is implemented. This includes forming a field region and active region in a semiconductor substrate, with specific depth profiles for conductive regions to enhance sensitivity and dynamic range.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If the depletion region is expanded to improve photoelectric conversion efficiency and sensitivity at low illuminance, then sensitivity is improved, but capacitance and charge capacity are reduced
Solution Approach 1:
The patent applies local quality by creating different doping concentration zones within the photodiode structure. Specifically, a first doped region with higher doping concentration is formed adjacent to a second doped region with lower doping concentration. This spatial variation in doping quality allows different regions to serve different functions: the high-concentration region maintains capacitance while the low-concentration region enables a wide depletion region for improved sensitivity.
Solution Approach 2:
The patent segments the doped region into multiple distinct zones with different doping concentrations. The photodiode is divided into a first doped region (higher concentration) and a second doped region (lower concentration), allowing each segment to contribute differently to the overall performance - one segment maintains charge capacity while the other enhances sensitivity through extended depletion region.
2Measurement precision
If the depletion region is expanded to improve photoelectric conversion efficiency, then sensitivity at low illuminance is improved, but dynamic range is reduced due to lower capacitance
Solution Approach 1:
The patent uses local quality by implementing spatially varying doping concentrations within the photodiode. The first doped region with higher concentration maintains capacitance values necessary for dynamic range, while the second doped region with lower concentration creates the wide depletion region needed for sensitivity at low illuminance, thus resolving the trade-off between these two performance metrics.
Solution Approach 2:
The patent segments the photodiode structure into multiple doped regions with different electrical characteristics. This segmentation allows the device to simultaneously achieve wide depletion region (for sensitivity) and adequate capacitance (for dynamic range) by having different segments contribute different properties to the overall device performance.
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
The design improves sensitivity at low illuminance and increases charge capacity, thereby enhancing the dynamic range of the pixel device without significant loss in capacitance.
Implementation Method 1
The photodiode is a light receiving element. The CMOS device provides an electric signal using electric charge accumulated in the photodiode.
Implementation Method 2
If electrons and holes generated by incident light are generated in the depletion region, the electrons and holes are not recombined and are isolated in respective different directions by an electric field formed within the depletion region.
Data Source
AI summary
A CMOS image sensor and a method for manufacturing the same. In one example embodiment, a CMOS image sensor includes a field region and an active region, a second conductive bottom region, a first conductive well region, a second conductive top region, and a first conductive high concentration region. The field region and the active region are formed in a first conductive semiconductor substrate. The second conductive bottom region has a first depth in part of the active region. The first conductive well region is formed in the active region. The second conductive top region has a depth that is less than the first depth. The first conductive high concentration region has a depth that is less than the depth of the second conductive top region.