Vertical Transfer Gate Structure for Low-Noise Pixel Sensors
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Solution Overview
Problem
Pixel sensors in image sensors face challenges such as parasitic capacitance between the transfer gate electrode and the conductive contact, and leakage between the transfer gate electrode and the floating diffusion node, leading to fixed-pattern noise (FPN) in images.
Innovation Solution
The design includes a vertical transfer transistor structure where the top conductive body of the transfer gate electrode is laterally separated by a large distance from the floating diffusion node, and the corner of the transfer gate electrode adjacent to the floating diffusion node is rounded, reducing parasitic capacitance and leakage.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Area of stationary object
If the transfer gate electrode is positioned close to the floating diffusion node, then the device area is reduced, but parasitic capacitance increases leading to fixed-pattern noise
Solution Approach 1:
The transfer gate electrode is designed with rounded corners instead of sharp angles. This curvature reduces the concentration of electric field lines at the corners, thereby reducing parasitic capacitance between the transfer gate electrode and the floating diffusion node while maintaining compact device area.
Solution Approach 2:
The transfer gate electrode has non-uniform geometry with different corner radii. Specifically, the corner adjacent to the floating diffusion node has a larger radius of curvature than other corners. This local modification targets the specific location where parasitic capacitance is most problematic, reducing noise without unnecessarily increasing overall device area.
2Area of stationary object
If the transfer gate electrode is positioned close to the floating diffusion node, then the device area is reduced, but leakage current increases
Solution Approach 1:
The rounded corners of the transfer gate electrode, particularly the corner adjacent to the floating diffusion node, reduce the electric field concentration that drives leakage current. This geometric modification effectively suppresses leakage while allowing the electrode to remain positioned close to the floating diffusion node for compactness.
Solution Approach 2:
The transfer gate electrode employs selective corner rounding where only the corner adjacent to the floating diffusion node has a large radius of curvature, while other corners may have smaller radii. This localized approach addresses the leakage problem at the critical interface without unnecessarily increasing device area.
3Ease of manufacture
If the transfer gate electrode has sharp corners, then the manufacturing process is simpler, but fixed-pattern noise increases due to electric field concentration
Solution Approach 1:
The transfer gate electrode is designed with rounded corners that can be formed using standard semiconductor manufacturing techniques such as isotropic etching or controlled deposition processes. The rounding radius is within the capability of typical fabrication processes, balancing manufacturing simplicity with noise reduction performance.
Solution Approach 2:
The corner radius of the transfer gate electrode is optimized to a specific range that effectively reduces electric field concentration and fixed-pattern noise while remaining compatible with standard manufacturing process capabilities. This parameter optimization achieves noise reduction without significantly complicating fabrication.
Data Source
AI summary
Various embodiments of the present disclosure are directed towards a method for forming a pixel sensor. The method comprises forming a photodetector in a substrate. The substrate is patterned to define an opening above the photodetector. A gate electrode is formed within the opening, where the gate electrode has a top conductive body overlying a bottom conductive body. A first segment of a sidewall of the top conductive body contacts the bottom conductive body. A floating diffusion node is formed in the substrate laterally adjacent to the gate electrode. A second segment of the sidewall of the top conductive body overlies the floating diffusion node.