Vertical Transfer Gate Boron Passivation for Low-Dark-Current Pixels
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Solution Overview
Problem
The formation of silicon dangling bonds in vertical transfer gates of CMOS image sensors reduces the sensitivity and optical responsivity of pixel sensors due to electron and photon diffusion, leading to increased dark current levels.
Innovation Solution
A boron passivation layer is formed epitaxially in a recess where the vertical transfer gate is to be created, forming a boron-silicon interface that reduces silicon dangling bonds and enhances the diode junction, thereby reducing dark current and increasing optical responsivity and sensitivity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If a vertical transfer gate is formed in a CMOS image sensor, then the electron collection efficiency is improved, but silicon dangling bonds are generated that increase dark current and reduce optical responsivity
Solution Approach 1:
A boron passivation layer is introduced as an intermediary between the vertical transfer gate and the surrounding silicon substrate. This passivation layer mediates the interaction by bonding to silicon dangling bonds at the gate interface, preventing them from acting as recombination centers that generate dark current, while allowing the vertical gate structure to maintain its electron collection efficiency
Solution Approach 2:
The silicon dangling bonds, which are harmful as they generate dark current, are converted into a beneficial state by introducing boron atoms that preferentially bond to these dangling bonds. This transforms the harmful dangling bonds into stable boron-silicon bonds, reducing dark current generation while maintaining the vertical gate's electron collection capability
2Productivity
If a vertical transfer gate is formed in a CMOS image sensor, then the electron collection efficiency is improved, but optical responsivity is reduced due to silicon dangling bonds
Solution Approach 1:
The boron passivation layer serves as an intermediary that protects the vertical transfer gate interface from generating spurious electrical signals. By passivating silicon dangling bonds, it prevents noise generation that would interfere with the precise measurement of optical signals, thereby maintaining high optical responsivity while preserving electron collection efficiency
3Device complexity
If silicon dangling bonds are present at the vertical transfer gate interface, then the device structure is simplified, but electron diffusion into the gate increases
Solution Approach 1:
The boron passivation layer acts as an intermediary barrier at the vertical transfer gate interface. It selectively passivates silicon dangling bonds that would otherwise cause electron diffusion into the gate, while maintaining the simple vertical gate structure. This ensures accurate electron transfer by preventing unwanted electron diffusion into the gate region
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 boron-silicon interface effectively minimizes electron and photon penetration into the vertical transfer gate, resulting in reduced dark current levels and increased optical responsivity and sensitivity of the pixel sensor.
Implementation Method 1
A boron passivation layer is formed in a recess of the vertical transfer gate using epitaxial growth
Implementation Method 2
The passivation layer may minimize diffusion of photons and electrons into the vertical transfer gate
Data Source
AI summary
A boron (B) layer may be formed as a passivation layer in a recess in which a vertical transfer gate is to be formed. The recess may then be filled with a gate electrode of the vertical transfer gate over the passivation layer (and/or one or more intervening layers) to form the vertical transfer gate. The passivation layer may be formed in the recess by epitaxial growth. The use of epitaxy to grow the passivation layer enables precise control over the profile, uniformity, and boron concentration in the passivation layer. Moreover, the use of epitaxy to grow the passivation layer may reduce the diffusion length of the passivation layer into the substrate of the pixel sensor, which provides increased area in the pixel sensor for the photodiode.