Image Sensor Device STI and LOCOS Isolation Dark Current Reduction
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Solution Overview
Problem
Conventional CMOS image sensors face challenges in minimizing dark current due to surface defects and plasma damage during manufacturing, which affects the dynamic range and integration density, with existing solutions like LOCOS and STI having limitations in space consumption and process complexity.
Innovation Solution
The method involves forming shallow trench isolations (STIs) and local oxidation of silicon (LOCOS) layers separately, with the STI for electrical isolation and the LOCOS layer as a protection layer for the photodiode, along with a gate insulator of varying thickness to reduce dark current.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a LOCOS layer is used to protect the photodiode surface, then dark current is reduced, but manufacturing complexity increases due to the LOCOS process
Solution Approach 1:
The patent divides the isolation structure into two separate components: STI for electrical isolation between devices and LOCOS layer for surface protection of the photodiode. This segmentation allows each component to be optimized for its specific function, with STI providing electrical isolation through trench structures and LOCOS providing surface protection through oxidized regions, thereby reducing dark current while maintaining manufacturing feasibility
Solution Approach 2:
The LOCOS layer serves multiple functions: it protects the photodiode surface from plasma damage during manufacturing, provides electrical isolation, and reduces surface defects that generate dark current. By combining multiple functions into a single structure, the patent reduces the need for additional separate protection layers, thereby managing manufacturing complexity while achieving reliable dark current reduction
2Productivity
If STI is used for electrical isolation, then integration density improves, but process complexity increases due to trench etching and planarization
Solution Approach 1:
The patent segments the isolation function into STI for electrical isolation and LOCOS for surface protection. The STI structure uses shallow trenches filled with dielectric material to provide electrical isolation between active areas, enabling higher integration density. By separating the electrical isolation function from surface protection, the patent avoids the need for deeper trenches and complex planarization processes that would be required if a single structure had to perform both functions
Solution Approach 2:
The STI trenches are positioned only where electrical isolation is needed between active areas, while the LOCOS layer provides surface protection specifically over the photodiode regions. This localized approach allows each structure to be optimized for its specific location and function, reducing overall process complexity compared to applying uniform isolation structures across the entire wafer
3Reliability
If LOCOS layer covers the entire photodiode, then surface protection is improved, but valuable wafer space is occupied reducing integration
Solution Approach 1:
The LOCOS layer is applied selectively only over the photodiode regions where surface protection is needed to prevent plasma damage and reduce dark current, rather than covering the entire wafer. The STI structures provide electrical isolation between active areas without occupying additional space beyond the device footprints. This localized application strategy maintains surface protection while maximizing wafer space utilization for higher integration density
Solution Approach 2:
The patent segments the protection function into localized LOCOS layers over photodiodes and STI structures for electrical isolation. This segmentation allows each protection element to be precisely positioned only where needed, avoiding unnecessary occupation of valuable wafer space and enabling more devices to be integrated on each wafer while maintaining adequate surface protection
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 effectively reduces dark current occurrence without compromising integration density, as the STI isolates active areas and the LOCOS layer protects the photodiode surface, while the varying gate insulator thickness ensures the gate is turned off below the threshold voltage, minimizing noise.
Implementation Method 1
performing a local oxidation of silicon (LOCOS) process to form a LOCOS layer on the photo sensing region
Data Source
AI summary
A semiconductor substrate is provided on which a plurality of shallow trench isolations (STI) defining a plurality of active areas are formed. The active areas comprise a photo sensing region, and a plurality of photodiodes are formed in each photo sensing region. Then a local oxidation of silicon isolation (LOCOS) layer is formed by performing a LOCOS process. Thereafter a plurality of gates are respectively formed in each active area, where the gates partially overlap the LOCOS layer. Finally doped regions are formed in the semiconductor substrate where the gate does not cover the LOCOS layer.