Stacked Image Sensor FD Wiring for Higher Conversion Gain
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Solution Overview
Problem
As pixels in image sensors become smaller, the increase in floating diffusion capacitance leads to a decrease in conversion gain, necessitating an improvement in conversion gain to maintain effective image capture.
Innovation Solution
The image sensor is designed with a layered structure comprising a top layer, middle layer, and bottom layer, incorporating a floating diffusion wiring structure and shield structure to enhance capacitance and improve conversion gain.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If pixels are made smaller to increase resolution, then the pixel density and image quality improve, but the floating diffusion capacitance increases which decreases conversion gain
Solution Approach 1:
The patent introduces a vertical stacking architecture where photodetectors, floating diffusion nodes, and readout circuits are arranged in multiple layers along the vertical dimension. This allows the pixel footprint to remain small for high resolution while providing sufficient physical space in the vertical direction to implement capacitance enhancement structures without increasing the horizontal pixel area.
Solution Approach 2:
The patent introduces a shield structure as an intermediary element positioned between the floating diffusion wiring structure and other circuit elements. This shield structure acts as a mediator to control electrical field distribution and reduce parasitic capacitance from neighboring structures, thereby improving conversion gain in small pixels.
2Reliability
If floating diffusion capacitance is increased to improve conversion gain, then the signal amplification improves, but the pixel area increases which reduces pixel density
Solution Approach 1:
The patent moves the capacitance enhancement function from the horizontal plane to the vertical dimension by stacking multiple functional layers. The floating diffusion node and associated capacitance structures are positioned in vertical layers above or below the photodetector, allowing capacitance increase without expanding the horizontal pixel footprint.
Solution Approach 2:
The patent implements a nested structure where the floating diffusion node is positioned within or adjacent to the photodetector structure in the vertical dimension. The capacitance enhancement features are integrated within the same vertical column as the photodetector, creating a compact nested arrangement that maximizes capacitance within minimal pixel area.
3Reliability
If shield structures are added to reduce noise and improve conversion gain, then the signal-to-noise ratio improves, but the device complexity increases
Solution Approach 1:
The patent combines the shield structure with existing circuit elements by positioning the shield to also serve as a connection element or structural support. The shield is integrated into the vertical stacking architecture and shares space with other components, reducing the need for separate dedicated shield elements and minimizing overall device complexity.
Solution Approach 2:
The shield structure is designed to perform multiple functions: it provides noise shielding, serves as an electrical connection path, and acts as a structural element in the vertical stacking architecture. This multi-functionality reduces the total number of separate components needed, thereby limiting the increase in device complexity.
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 layered structure effectively increases conversion gain by forming capacitance between the floating diffusion wiring structure and shield structure, enhancing the image sensor's performance.
Implementation Method 1
The layered structure effectively increases conversion gain by forming capacitance between the floating diffusion wiring structure and shield structure
Data Source
AI summary
An image sensor is provided and includes a top layer including a first floating diffusion (FD) wiring structure connected to a FD node, and a first shield structure next to the first FD wiring structure. The image sensor further includes a middle layer bonded to and below the top layer, wherein the middle layer includes a source follower gate, a source follower source region, a second FD wiring structure connected to the first FD wiring structure and the source follower gate, a second shield structure connected to the first shield structure and the source follower source region, and a source follower landing pad between the second shield structure and the source follower source region and spaced apart from the source follower gate.