Tunnel Contact for Pixel Cell Parasitic Capacitance Reduction
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Solution Overview
Problem
Conventional CMOS image sensors face challenges in achieving high conversion gain and low noise due to parasitic capacitance introduced by metal interconnections, which affects their dynamic range and low-light sensitivity.
Innovation Solution
The introduction of a tunnel contact that couples the floating diffusion directly to the source follower gate and reset transistor, replacing traditional metal lines, reduces parasitic capacitance and increases conversion gain.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If traditional metal lines in metallic interconnection layers are used to connect floating diffusion to source follower gate, then electrical connection is established, but parasitic capacitance increases and conversion gain decreases
Solution Approach 1:
The patent extracts and removes the metal interconnection layer and dielectric material between the floating diffusion and source follower gate, directly connecting the two components. This eliminates the parasitic capacitance introduced by the traditional metal line while maintaining the necessary electrical connection, thereby resolving the contradiction between reliable electrical connection and minimizing parasitic capacitance.
Solution Approach 2:
The patent transitions from a planar metal line connection to a vertical tunnel contact structure that penetrates through the dielectric layer. By changing the dimensional approach from lateral metal interconnection to vertical semiconductor tunneling, the parasitic capacitance is significantly reduced while maintaining electrical connectivity, thus resolving the contradiction.
2Reliability
If multiple contacts coupled via metal line through dielectric material are used, then floating diffusion is connected to source follower gate, but effective capacitance of floating diffusion increases and conversion gain lowers
Solution Approach 1:
The patent removes the metal line and surrounding dielectric material that form the traditional interconnection structure. By extracting these components, the parasitic capacitance is eliminated, which directly improves the conversion gain of the floating diffusion while maintaining the essential electrical connection to the source follower gate.
Solution Approach 2:
The patent replaces the mechanical metal line interconnection system with a semiconductor-based tunnel contact mechanism. This substitution eliminates the parasitic capacitance associated with metal-dielectric structures while maintaining electrical connectivity, thereby improving conversion gain without sacrificing connection reliability.
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 solution enhances the conversion gain, reduces noise, and improves low-light detectivity while maintaining compatibility with existing manufacturing methods, thereby increasing the dynamic range of image sensors.
Implementation Method 1
The electric coupling between the floating diffusion and the source follower gate through the inter-layer dielectric layer can introduce parasitic capacitance, which increases the effective capacitance of the floating diffusion and lowers the conversion gain.
Data Source
AI summary
A pixel cell includes an electrically conductive tunnel contact formed across a surface of a source follower gate, the tunnel contact having a first end, a second end, and an intermediate portion between the first and second ends. The first end is coupled to a floating diffusion FD, the second end is coupled to the first doped region of a reset transistor RST. The tunnel contact is formed in physical and in electrical contact with the surface of the source follower gate for a length of the intermediate portion substantially equal to a width of the source follower gate. Methods of forming the pixel cell are also described.