3D Separation Pixel Image Sensor Charge Transmission
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Solution Overview
Problem
The existing image sensor pixel design limits the area available for the photodiode due to the presence of multiple transistors, reducing the efficiency of charge transmission from the photodiode to the floating diffusion area.
Innovation Solution
A separation type unit pixel with a 3D structure is implemented, where the photodiode on a first wafer generates charge, and the transistors on a second wafer convert and reset this charge, with the photodiode's positive region receiving a lower N_ground voltage than the ground voltage used on the second wafer, enhancing charge transmission efficiency.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If multiple transistors are formed on the same wafer as the photodiode, then the unit pixel can be fully integrated, but the area allocated to the photodiode is reduced due to space occupied by transistors
Solution Approach 1:
The unit pixel components are segmented into two separate wafers: the first wafer contains the photodiode and transmission transistor, while the second wafer contains the source follower transistor, select transistor, and reset transistor. This segmentation allows the photodiode to occupy maximum area on the first wafer without being constrained by transistor space requirements, while still achieving full integration through wafer bonding.
Solution Approach 2:
The patent transitions from a planar 2D integration approach to a 3D stacked architecture by bonding two wafers together. This dimensional change allows components to be distributed across different layers (wafer 1 and wafer 2), increasing the effective integration density without reducing the photodiode area on either wafer.
2Productivity
If the photodiode area is increased to improve charge generation, then the area for transistors is reduced, limiting circuit functionality
Solution Approach 1:
By segmenting the circuit into two functional groups on separate wafers, the photodiode can be maximized for charge generation on wafer 1, while all necessary transistors are fully integrated on wafer 2, eliminating the area trade-off between photodiode and transistor regions.
3Productivity
If a higher voltage is applied to the positive region of the photodiode to improve charge transmission, then charge transmission efficiency increases, but power consumption increases
Solution Approach 1:
The patent changes the voltage parameter by applying a negative voltage (N_ground voltage) to the positive region of the photodiode instead of a positive voltage. This parameter change creates a voltage difference that enhances charge transmission efficiency from the photodiode to the floating diffusion area while avoiding the increased power consumption associated with higher positive voltages.
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 configuration effectively transmits charge to the floating diffusion area and allows for efficient resetting, improving the overall charge transmission efficiency and pixel density.
Implementation Method 1
a photodiode configured to generate a charge corresponding to incident light
Data Source
AI summary
The present invention introduces a separation type unit pixel of an image sensor having a three-dimensional (3D) structure, which is capable of maximizing transmission efficient of a charge generated through a photodiode to a floating diffusion area. The separation type unit pixel may include a first wafer on which a photodiode and a transmission transistor are formed and a second wafer on which a reset transistor and a source follower transistor are formed. In particular, the photodiode has a positive region to which an N_ground voltage is applied, the N_ground voltage having a lower voltage level than a ground voltage used in the second wafer.