Image Sensor Passivation Film for White Pixel and Dark Current Control
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Solution Overview
Problem
CMOS image sensors suffer from 'white pixel' defects due to electrical and optical cross-talk between pixels, which are exacerbated by defects in the recessed regions created during the etching process, leading to increased dark current and degraded image quality.
Innovation Solution
A multilayer film with high-κ material layers is applied to the recessed regions of the semiconductor substrate, featuring pairs of layers with specific fixed-charge densities that form dipoles, reducing the electric potential barrier and enhancing passivation, thereby minimizing dark current and image artifacts.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If deep trench isolation (DTI) structure is formed by etching the back surface to isolate pixels, then optical and electrical isolation between adjacent pixels is improved, but defects are created on the recessed region which increases dark current and causes white pixel defects
Solution Approach 1:
The patent changes the physical and chemical parameters of the recessed surface by forming a multilayer film structure with varying κ values and fixed charge densities. The inner layer has higher fixed charge density than the outer layer, creating an electric field that repels minority carriers (holes) from the defect-prone recessed region, thereby reducing dark current while maintaining the benefits of deep trench isolation.
Solution Approach 2:
The patent applies a composite multilayer film structure consisting of dielectric materials with different κ values and fixed charge densities. The inner layer (higher κ, higher fixed charge density) and outer layer (lower κ, lower fixed charge density) work together to provide both strong electric field passivation at the interface and reduced defect impact, achieving superior dark current reduction compared to single-layer structures.
2Reliability
If measures are taken to reduce electrical and optical cross-talk between pixels, then image quality is improved, but other image artifacts are introduced
Solution Approach 1:
The patent modifies the electrical parameters at the recessed surface interface by introducing a controlled electric field through the multilayer film's fixed charge densities. This electric field parameter change creates a potential barrier that prevents carrier leakage and cross-talk, thereby improving image quality without introducing additional artifacts that would result from alternative isolation methods.
3Ease of manufacture
If plasma etching is used to create the recessed region, then the trench isolation structure is formed, but the semiconductor lattice is damaged creating dangling bonds and charge trap sites
Solution Approach 1:
The patent converts the harmful effect of plasma etching damage into a beneficial structure by applying a multilayer film that specifically addresses the created defects. The inner layer with high fixed charge density is positioned exactly where plasma damage occurs (at the recessed surface interface), creating an electric field that passivates the damaged lattice, reduces dangling bonds, and mitigates the harmful effects of the etching process.
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 multilayer film effectively reduces dark current and 'white pixel' defects, improving image quality by diffusing holes and increasing electron tunneling, which enhances the passivation around recessed regions and isolates adjacent pixels optically and electrically.
Implementation Method 1
relative charge densities of adjacent layers result in dipoles at the interface of the adjacent layers. The electric dipole moment of each dipole points away from the recessed surface.
Implementation Method 2
The reduced barrier height induces electron tunneling from the semiconductor substrate toward a dielectric filling material on the recess region
Implementation Method 3
The negative end of the dipole attracts holes in the semiconductor substrate to the recessed region. This diffusion of holes reduces the height of the electric-potential barrier at the interface
Implementation Method 4
High-κ dielectric films on the recessed region and back surface passivate these surfaces; which reduces white pixel defects.
Data Source
AI summary
An image sensor includes a semiconductor substrate and a multilayer film. The semiconductor substrate includes a photodiode and a back surface having a recessed region that surrounds the photodiode. The multilayer film is on, and conformal to, the recessed region, and includes N layer-groups of adjacent high-κ material layers. Each pair of adjacent high-κ material layers of a same layer-group of the N layer-groups includes (i) an outer-layer having an outer fixed-charge density and (ii) an inner-layer, located between the outer-layer and the recessed region, that has an inner fixed-charge density. Each of the outer and inner fixed-charge density is negative. The inner fixed-charge density is more negative than the outer fixed-charge density.