Via-Level Conductive Light Shield for CMOS Noise Reduction
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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 capacitance.
Innovation Solution
A conductive light shield is integrated within the via level of the metal interconnect structure, positioned between dielectric layers, to effectively block incident light and reduce noise without increasing wiring channels or adding capacitance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If a light shield is added to block incident light at the floating diffusion node, then photogeneration noise is reduced, but metal wiring is restricted and fill factor is affected
Solution Approach 1:
The light shield is moved from the interconnect level to the via level, changing its vertical position in the three-dimensional structure. This dimensional relocation allows the shield to block light at the floating diffusion node while leaving the interconnect metal wiring paths unrestricted, thus resolving the contradiction between noise reduction and wiring flexibility
Solution Approach 2:
A via-level conductive light shield is introduced as an intermediary element between the incident light and the floating diffusion node. This mediator blocks photogeneration noise while being positioned such that it does not interfere with the metal wiring structure, thus protecting the signal without compromising wiring adaptability
2Object-affected harmful factors
If a light shield is added to block incident light at the floating diffusion node, then photogeneration noise is reduced, but capacitance is increased
Solution Approach 1:
The light shield is implemented as a thin conductive film at the via level rather than a bulky interconnect-level structure. This thin-film approach provides effective light blocking to reduce photogeneration noise while minimizing the added capacitance to the floating diffusion node, thus resolving the contradiction between noise reduction and capacitance increase
3Productivity
If the hold time between exposure and readout is increased for high-speed capture, then image capture speed is improved, but charge leakage increases
Solution Approach 1:
The via-level light shield is positioned to preemptively block incident light from reaching the floating diffusion node before charge leakage can occur during the hold time. This preliminary protective action maintains charge integrity throughout the extended hold period required for high-speed capture, thus resolving the contradiction between capture speed and charge leakage
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 in the floating drain signal, enhancing image fidelity and maintaining the integrity of captured images during high-speed capture without compromising the metal wiring structure.
Implementation Method 1
A conductive light shield is integrated within the via level of the metal interconnect structure, positioned between dielectric layers, to effectively block incident light and reduce noise
Data Source
AI summary
A CMOS image sensor pixel includes a conductive light shield, which is located between a first dielectric layer and a second dielectric layer. At least one via extends 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 CMOS image sensor pixel enables reduction of noise in the signal stored in the floating drain.