CMOS Image Sensor Optical Black Layer Dark Current Reduction
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Solution Overview
Problem
Conventional CMOS image sensor manufacturing processes face issues with high dark current due to dangling bonds on photodiode surfaces, which are not efficiently passivated by hydrogen annealing when a metal light shielding layer is present, leading to impaired light shielding and increased dark current in the optical black region.
Innovation Solution
The method involves forming an optical black layer without a metal light shielding layer, allowing for efficient dangling bond passivation at lower temperatures, using materials like titanium or titanium nitride, and forming the optical black layer after passivation, thereby reducing dark current and maintaining good light shielding properties.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If a metal light shielding layer is formed over the photodiodes, then light shielding properties are improved, but dark current increases due to impeded dangling bond passivation
Solution Approach 1:
The light shielding function is segmented into two separate layers: a metal light shielding layer for primary light blocking, and an optical black layer for secondary light absorption and dark current suppression. This segmentation allows each layer to perform its specific function optimally without interfering with the passivation process.
Solution Approach 2:
The optical black layer is formed after the metal light shielding layer and passivation processes are completed. This preliminary action sequence ensures that the metal layer does not impede the passivation of dangling bonds, while the subsequently formed optical black layer provides additional light shielding without causing the same problem.
2Object-generated harmful factors
If hydrogen annealing is performed to passivate dangling bonds, then dark current is reduced, but the metal layer reacts with hydrogen and impedes passivation
Solution Approach 1:
The metal light shielding layer is formed before the hydrogen annealing process, allowing the passivation to occur without the metal layer interfering. The optical black layer is then formed after passivation is complete, ensuring that the metal-hydrogen reaction does not impede the passivation effectiveness.
Solution Approach 2:
The optical black layer acts as an intermediary between the metal light shielding layer and the photodiode surface. It provides the necessary light shielding function while allowing the hydrogen annealing process to effectively passivate dangling bonds on the photodiode surface without being blocked by the metal layer.
3Ease of manufacture
If conventional manufacturing processes are used, then production cost is reduced, but dark current problem persists
Solution Approach 1:
The invention merges the light shielding function with the dark current suppression function by adding an optical black layer that performs both roles. This combination approach maintains the cost-effectiveness of conventional CMOS manufacturing while solving the dark current problem through a relatively simple additional process step.
Solution Approach 2:
The invention changes the process parameters by forming the optical black layer at a specific temperature and timing (after metal layer formation and passivation) to optimize both light shielding and dark current suppression performance while maintaining compatibility with existing manufacturing processes.
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 results in a CMOS image sensor with significantly reduced dark current and improved light shielding, as demonstrated by measurements showing lower dark current levels in the optical black region compared to conventional techniques.
Implementation Method 1
an optical black layer is formed over the second planarized layer in the optical black region
Implementation Method 2
A conventional technique using a hydrogen annealing process is performed to solve the problem
Implementation Method 3
after the passivation layer 130 is formed, the photodiodes often have plenty of dangling bonds on the surface, leading to a current leakage (that is, dark current) problem
Data Source
AI summary
An image sensor, in which, a planarized layer is formed on a semiconductor substrate including a pixel array region, an optical black region, and a logic region to cover a photo sensing unit array in the pixel array region, a patterned metal layer is formed on the planarized layer corresponding to the pixel array region and the logic region, but not the optical black region. An optical black layer is formed in the optical black region after a passivation layer is formed and before a color filter array is formed at a temperature less than about 400° C., and preferably contains metal material.