BSI Image Sensor Isolation Layout for Low Crosstalk and Dark Current
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Back side illumination image sensors face issues with reduced quantum efficiency and increased optical and electrical crosstalk due to light reflection, and defects between deep trench isolation and floating diffusion structures, leading to fixed picture noise and high dark current.
Innovation Solution
The use of an epitaxial substrate with a deep trench isolation structure, buried oxide layer, and shallow trench isolation structure to reduce crosstalk and prevent damage to the floating diffusion region, along with a method of manufacturing that includes forming vertical transfer gates and an epitaxial layer to enhance isolation and reduce parasitic capacitance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a deep trench isolation structure is formed using conventional methods, then the isolation between photosensitive regions is improved, but defects are easily formed between the deep trench isolation structure and the floating diffusion region, causing fixed picture noise
Solution Approach 1:
The patent applies preliminary action by forming a protective oxide layer on the floating diffusion region before forming the deep trench isolation structure. This pre-protection measure prevents defects from forming at the interface between the DTI structure and the FD region during subsequent high-power etching and filling processes, thereby eliminating fixed picture noise while maintaining reliable isolation between photosensitive regions.
Solution Approach 2:
The patent introduces an intermediary protective oxide layer between the floating diffusion region and the deep trench isolation structure. This intermediary layer acts as a buffer that prevents direct contact and potential defect formation between the two structures, while still allowing the DTI to provide effective optical and electrical isolation between adjacent photosensitive regions.
2Quantity of substance
If multiple high-energy ion implantation processes are used to form a deeper photodiode to increase full well capacity, then the full well capacity is improved, but lattice defects are easily generated in the photodiode, causing high dark current
Solution Approach 1:
The patent applies parameter changes by carefully controlling the ion implantation energy and dose parameters to form a deeper photodiode with increased full well capacity. By optimizing these parameters, the patent achieves adequate depth for high FWC while minimizing lattice damage that would otherwise generate high dark current.
Solution Approach 2:
The patent applies beforehand cushioning by forming a protective oxide layer before ion implantation processes. This protective layer cushions the photodiode lattice from excessive damage during high-energy ion implantation, reducing the generation of lattice defects that would lead to high dark current while still achieving the required photodiode depth for adequate full well capacity.
3Area of stationary object
If the floating diffusion region overlaps with the deep trench isolation structure in layout, then the device area is reduced, but the floating diffusion region is damaged during high-power etching steps, causing fixed picture noise
Solution Approach 1:
The patent applies preliminary action by forming a protective oxide layer on the floating diffusion region before the high-power etching steps that create the deep trench isolation structure. This pre-protection allows the FD region to overlap with the DTI structure in layout (saving area) while preventing damage during the harsh etching process that would otherwise cause fixed picture noise.
Data Source
AI summary
A back side illumination (BSI) image sensor includes an epitaxial substrate, a deep trench isolation (DTI) structure from one surface to the other surface of the epitaxial substrate, a buried oxide layer on the epitaxial substrate, an epitaxial layer, a well region, a floating diffusion (FD) region, a shallow trench isolation (STI) structure, and vertical transfer gates (VTGs). The buried oxide layer has openings exposing the epitaxial substrate, and the epitaxial layer is formed on the epitaxial substrate and covers the buried oxide layer. The well region is in the epitaxial layer and the epitaxial substrate. The FD region is in the well region above the buried oxide layer, and a width of the buried oxide layer is larger than that of the FD region. The STI structure is in the epitaxial layer. The VTGs are in the epitaxial layer and through the openings of the buried oxide layer.