CMOS Imaging Pixel Shielding Electrode for Low Noise
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Solution Overview
Problem
Current CMOS imaging sensors suffer from high readout noise, particularly at low light levels, limiting the lowest light levels that can be utilized to form an image, as the noise is of the order of 2-10 electrons RMS, which is not ideal for capturing images with signal generated by one electron.
Innovation Solution
The introduction of a shielding electrode between the transfer gate and the floating diffusion node to reduce capacitive coupling noise, combined with replacing the source follower with a capacitive transimpedance amplifier to provide additional gain and reduce overall noise levels.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a source follower is used to drive the bit line capacitance, then the pixel can be read out, but the voltage gain is insufficient and readout noise increases to 2-10 electrons RMS
Solution Approach 1:
A shielding electrode is introduced as an intermediary element between the transfer gate and the floating diffusion node. This electrode acts as a mediator that blocks the capacitive coupling path, preventing noise from the transfer gate from directly coupling to the sensitive floating diffusion node where charge is stored, thereby improving signal-to-noise ratio without complicating the amplifier configuration.
2Measurement precision
If the floating diffusion node is directly coupled to the transfer gate, then the structure is simple, but capacitive coupling causes noise that limits low light performance
Solution Approach 1:
The shielding electrode serves as a physical intermediary that interrupts the direct capacitive coupling between the transfer gate and floating diffusion node. By placing this electrode in between, the noise pathway is blocked while maintaining the essential functional connections, achieving lower readout noise with minimal structural modification.
Solution Approach 2:
The harmful capacitive coupling effect is extracted or removed from the system by introducing the shielding electrode. This electrode effectively takes out the noise pathway from the signal path, separating the harmful capacitive interaction from the necessary charge transfer function, thereby reducing readout noise to below one electron RMS.
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 significantly reduces noise interference and enhances the ability to capture images at low light levels by minimizing capacitive coupling noise and distributing gain across multiple amplifiers, resulting in improved signal quality and reduced overall noise levels.
Implementation Method 1
Each pixel cell includes a photodiode that converts photons received during an exposure to a charge having a magnitude that is proportional to the light received
Implementation Method 2
A shielding electrode shields the floating diffusion node from the first gate signal. The shielding electrode is held at a potential that prevents charge from accumulating under the shielding electrode
Implementation Method 3
the output stage includes a capacitive transimpedance amplifier having a gain greater than one
Data Source
Figure 1
Figure 2
Figure 3~4B
AI summary
A pixel cell and imaging arrays using the same are disclosed. The pixel cell, includes a photodiode that is connected to a floating diffusion node by a transfer gate that couples the phoiodiode to the floating diffusion node in response to a first gate signal. A shielding electrode shields the floating diffusion node from the first gate signal. An output stage generates a signal related to a charge on the floating diffusion node. In one aspect of the invention, the photodiode is connected to the floating diffusion node by a buried channel, and the shielding electrode includes an electrode overlying the channel and positioned between the transfer gate and the floating diffusion node. The shielding electrode is held at a potential that prevents charge from accumulating under the shielding electrode when the floating diffusion is at the second potential.