Image Sensor Floating Diffusion Interconnect Capacitor
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Solution Overview
Problem
Active pixel CMOS image sensors face issues with dark current flowing to the floating diffusion, leading to increased fixed pattern noise and temporal noise, as well as geometric distortion due to light from the next frame reaching the floating diffusion before being read out in global shutter mode.
Innovation Solution
The image sensor design incorporates a local interconnect layout that forms a dual capacitor, reducing the size of the floating diffusion area while maintaining sufficient charge storage capacity, and includes a global shutter transistor to allow for simultaneous resetting of all pixels, minimizing the time between integration periods and reducing dark current noise.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-generated harmful factors
If the floating diffusion area is reduced to minimize dark current, then dark current noise is reduced, but charge storage capacity is insufficient
Solution Approach 1:
The patent divides the charge storage function into two separate components: the floating diffusion region and the interconnect capacitor. The floating diffusion is minimized in area to reduce dark current, while the interconnect capacitor provides the additional charge storage capacity needed to hold the full well charge. This segmentation allows each component to be optimized for its specific function without compromise.
Solution Approach 2:
The interconnect capacitor acts as an intermediary charge storage element between the photodiode and the readout circuitry. It receives charge from the photodiode through the transfer gate and can hold this charge independently of the floating diffusion size, thereby mediating the conflict between small floating diffusion area and sufficient charge storage capacity.
2Measurement precision
If global shutter mode is implemented to capture entire frame simultaneously, then image quality is improved, but dark current accumulation during readout time increases
Solution Approach 1:
The patent implements preliminary action by having the transfer gate transfer charge from the photodiode to the floating diffusion/interconnect capacitor system before the readout process begins. This allows the charge to be stored in a minimized floating diffusion area that accumulates less dark current during the extended global shutter readout period, while still maintaining the ability to read out the entire frame simultaneously.
3Object-affected harmful factors
If floating diffusion area is minimized, then light from next frame is prevented from reaching it, but charge storage capacity becomes insufficient
Solution Approach 1:
The patent segments the charge storage function between the minimized floating diffusion and the interconnect capacitor. The small floating diffusion area prevents light from the next frame from reaching it, while the interconnect capacitor provides sufficient charge storage capacity to hold the full well charge transferred from the photodiode.
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 approach effectively reduces dark current noise and prevents light from the next frame from interfering with the current frame, enhancing image quality by minimizing fixed pattern noise and allowing for higher frame rates in global shutter mode.
Implementation Method 1
By overlapping the interconnect with the transfer gate line and/or the amplifier gate, a dual capacitor is formed. The dual capacitor allows the floating diffusion area to be reduced while still providing sufficient storage capacity for the full well charge.
Implementation Method 2
Digital image capturing devices use image sensors to convert incident light energy into electrical signals. Each pixel in the array produces an electrical signal corresponding to an amount of light incident upon the pixel during an integration period.
Data Source
AI summary
An image sensor includes a pixel circuit that includes a light sensing element and a charge storage node formed in a substrate, an output element having a control electrode formed in an electrode layer above the substrate, the output element generating an output signal corresponding to charge generated by the light sensing element and held by the charge storage node, and a local metal interconnect that electrically connects the charge storage node to the control electrode. Control wirings that control operations of the pixel circuit are formed in wiring layers that are located above the electrode layer. The metal interconnect is formed in a layer that is located above the electrode layer and below a lowest one of the wiring layers such that a given one of the control wirings overlaps the metal interconnect so as to form a parallel plate capacitor in a region of overlap.