CMOS Image Sensor Photodiode Fabrication via Boron Diffusion
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Solution Overview
Problem
Conventional CMOS image sensors face challenges in forming a P-type impurity region with a small thickness to improve blue light efficiency and reduce crystallization defects caused by ion implantation, leading to deteriorated light characteristics.
Innovation Solution
A method involving an ion implantation process followed by an annealing process in a gas atmosphere with boron impurity atoms is used to form a highly doped P-type impurity region with a small thickness, reducing crystallization defects and enhancing light characteristics by diffusing boron thinly and uniformly.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If ion implantation process is used to form P-type impurity region, then doping is achieved, but crystallization defects occur and dark current increases
Solution Approach 1:
The patent replaces the mechanical ion implantation process with a chemical diffusion process using boron silicate glass. Instead of physically implanting ions that cause crystal damage, boron atoms diffuse into the silicon substrate through thermal energy, achieving doping without mechanical disruption to the crystal lattice structure.
Solution Approach 2:
The patent introduces boron silicate glass as an intermediary material that serves as a boron source. The glass layer is deposited on the silicon surface and then thermally processed, allowing boron atoms to diffuse from the glass into the silicon substrate. This intermediary approach enables controlled doping while maintaining crystal integrity.
2Reliability
If P-type impurity region thickness is increased to prevent dark current, then surface isolation is improved, but blue light efficiency deteriorates
Solution Approach 1:
The patent changes the doping parameters by using a thin boron silicate glass layer (50-200 nm) that provides sufficient boron diffusion to create an effective surface isolation layer, while keeping the actual doped region thickness minimal (less than 100 nm). This parameter optimization allows dark current prevention without compromising blue light penetration and detection efficiency.
3Ease of manufacture
If ion implantation is used for doping, then P-type region is formed, but manufacturing precision is reduced due to diffusion during thermal process
Solution Approach 1:
The patent performs preliminary action by depositing the boron silicate glass layer before the thermal diffusion process. The glass layer is prepared with precise thickness control through deposition techniques, and then serves as a controlled boron source during subsequent thermal processing. This preliminary preparation enables better control over the final doping profile and depth compared to direct ion implantation.
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 method effectively improves light efficiency for blue and green light wavelengths by reducing dark current and crystallization defects, resulting in enhanced photodiode performance.
Implementation Method 1
performing an annealing process in a gas atmosphere including first conductivity type impurity atoms to form a first conductivity type second impurity region underneath a surface of the first conductivity type semiconductor layer in the second conductivity type first impurity region, wherein the first conductivity type second impurity region is doped with the diffused first conductivity impurity atoms
Implementation Method 2
performing an ion implantation process onto a photodiode region in a first conductivity type semiconductor layer to form a second conductivity type first impurity region
Data Source
AI summary
A method for fabricating a complementary metal-oxide semiconductor (CMOS) image sensor includes performing an ion implantation process onto a photodiode region in a first conductivity type semiconductor layer to form a second conductivity type first impurity region, and performing an annealing process in a gas atmosphere including first conductivity type impurity atoms to form a first conductivity type second impurity region underneath a surface of the first conductivity type semiconductor layer in the second conductivity type first impurity region, wherein the first conductivity type second impurity region is doped with the diffused first conductivity impurity atoms.