Conductive Isolation Regions for CMOS Image Sensors
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Solution Overview
Problem
Conventional methods for forming isolation regions in CMOS image sensors, such as ion implantation and deep trench isolation, face challenges in precisely controlling ion diffusion and lattice mismatch, leading to increased dark current and noise, which compromises image quality by reducing full well capacity and increasing cross-talk between pixels.
Innovation Solution
The formation of isolation regions with a conductive gate electrically connected to a bias voltage, which creates a charge inversion to prevent dark current and enhance isolation, while optionally using a thin liner oxide or omitting it to reduce lattice mismatch defects, allowing for increased full well capacity and reduced cross-talk.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If ion implantation is used to form isolation regions, then isolation between photodiodes is improved, but manufacturing precision deteriorates due to difficult control of ion diffusion and inability to produce abrupt junctions
Solution Approach 1:
The patent changes the physical-chemical parameters of the isolation region by using thermal oxidation to grow silicon dioxide, transforming the isolation mechanism from ion-based to oxide-based. This allows precise control of isolation depth and width through oxidation time and temperature parameters, achieving both good isolation and abrupt junctions.
Solution Approach 2:
The patent replaces the mechanical ion implantation process with a thermal field-based oxidation process. Instead of physically implanting ions, the invention uses thermal energy to grow oxide layers, substituting a mechanical/chemical process with a thermal process that offers better controllability and precision.
2Reliability
If deep trench isolation with liner oxide is used, then isolation between photodiodes is improved, but object-generated harmful factors increase due to lattice mismatch defects causing higher dark current
Solution Approach 1:
The patent changes the material composition parameter of the isolation region by using pure silicon dioxide grown through thermal oxidation, eliminating the need for liner oxides with different lattice structures. This parameter change removes the source of lattice mismatch defects that generate dark current, while maintaining effective isolation through the oxide layer itself.
3Productivity
If pixel density is increased, then productivity is improved, but object-affected harmful factors increase due to greater cross-talk between closer pixels
Solution Approach 1:
The patent applies local quality by creating isolation regions with precisely controlled spatial characteristics. The thermal oxidation process allows the isolation width and depth to be locally optimized for each pixel region, providing sufficient electrical isolation even when pixels are densely packed, thereby preventing cross-talk while maintaining high pixel density.
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 and cross-talk, enabling higher pixel density with improved image quality by increasing full well capacity and quantum efficiency, while minimizing noise and hot pixels.
Implementation Method 1
forming a conductive layer in each isolation region... Each conductive layer may be electrically connected to a bias voltage supply line
Implementation Method 2
this method introduces defects due to lattice mismatch, thus resulting in higher dark current
Data Source
AI summary
An image sensor may include isolation regions that are formed in between photodiodes. These isolation regions may prevent cross-talk and improve the performance of the image sensor. The isolation regions may include a conductive layer that is electrically connected to a bias voltage supply line. Biasing the conductive layer may result in a charge inversion in the substrate adjacent to the conductive layer. The charge inversion may prevent the generation of dark current. The conductive layer may be formed on a liner oxide layer in trenches formed in epitaxial silicon. A connecting layer may be used to electrically connect each conductive layer. The connecting layer may be formed integrally with the conductive layer or formed from a separate material.