Via-Level Conductive Light Shield for CMOS Image Sensor Noise Reduction
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
CMOS image sensor pixels face challenges with charge leakage and photogeneration noise at the floating diffusion node, leading to image degradation, especially in high-speed image capture applications, where existing light shields restrict metal wiring and affect fill factor or add unnecessary capacitance.
Innovation Solution
A conductive light shield is formed within the via level of the metal interconnect structure to effectively block light over the floating drain, reducing noise and maintaining metal wiring flexibility without increasing wiring channels.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If a light shield is formed at the interconnect level to block light over the floating diffusion, then photogeneration noise is reduced, but metal wiring flexibility is restricted and fill factor is affected
Solution Approach 1:
The light shield is moved from the interconnect level (horizontal plane) to the via level (vertical dimension), allowing metal wiring to proceed freely at the interconnect level while the via-level light shield blocks light vertically over the floating diffusion node
Solution Approach 2:
A via structure serves as an intermediary element that combines light-blocking functionality with electrical connectivity, allowing the light shield to be formed through the dielectric layer without obstructing metal wiring paths
2Object-affected harmful factors
If a light shield is formed at the interconnect level to block light, then photogeneration noise is reduced, but unnecessary capacitance is added to the floating diffusion
Solution Approach 1:
The light shielding function is extracted from the interconnect metal layers and placed in the via structure, eliminating the need for light-blocking metal interconnects that would add capacitance to the floating diffusion node
3Productivity
If the hold time between exposure and readout is increased for high-speed image capture, then more frames can be captured, but charge leakage at the floating diffusion increases causing image degradation
Solution Approach 1:
The via-level light shield is positioned to preemptively block light from reaching the floating diffusion node during the hold time period, preventing photogeneration of charge that would otherwise leak and degrade image quality in high-speed capture modes
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 conductive light shield significantly reduces noise and enhances signal fidelity by suppressing photogeneration, thereby improving the quality of images captured by CMOS image sensor pixels without compromising metal interconnect design.
Implementation Method 1
A conductive light shield is formed within the via level of the metal interconnect structure to effectively block light over the floating drain, reducing noise and maintaining metal wiring flexibility
Data Source
AI summary
A conductive light shield is formed over a first dielectric layer of a via level in a metal interconnect structure. The conductive light shield is covers a floating drain of an image sensor pixel cell. A second dielectric layer is formed over the conductive light shield and at least one via extending from a top surface of the second dielectric layer to a bottom surface of the first dielectric layer is formed in the metal interconnect structure. The conductive light shield may be formed within a contact level between a top surface of a semiconductor substrate and a first metal line level, or may be formed in any metal interconnect via level between two metal line levels. The inventive image sensor pixel cell is less prone to noise due to the blockage of light over the floating drain by the conductive light shield.