Transparent Transfer Gate CMOS Image Sensor Sensitivity
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Solution Overview
Problem
The reduction in size of photodiodes in CMOS image sensors leads to reduced sensitivity due to decreased optical charge generation, necessitating an improvement in the amount of generated optical charge despite the smaller pixel size.
Innovation Solution
Incorporating a transparent transfer gate formed from materials like nitrides, oxides, or compounds (e.g., GaN, ZnO, AlGaN, ITO) that allow light to permeate and generate additional optical charges in the channel area, increasing the area capable of generating optical charges and thus improving sensitivity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Area of moving object
If the size of photodiode is reduced to decrease unit pixel size, then the device complexity is reduced and integration density is improved, but the sensitivity is deteriorated due to decreased optical charge generation
Solution Approach 1:
The transfer gate is changed from opaque polysilicon to transparent material, fundamentally altering its optical properties. This allows light to pass through the transfer gate and generate optical charges in the channel area, thereby improving sensitivity while maintaining the reduced pixel size
Solution Approach 2:
The transfer gate is formed using transparent materials such as transparent oxide semiconductors (e.g., IGZO - indium gallium zinc oxide) or transparent nitrides, creating a composite structure that combines electrical functionality with optical transparency. This enables the transfer gate to serve dual purposes: charge transfer and additional photocharge generation
2Area of moving object
If the size of photodiode is reduced to decrease unit pixel size, then the manufacturing precision requirements are reduced, but the amount of generated optical charge is deteriorated
Solution Approach 1:
The light-receiving function is extended from the photodiode area to the channel area under the transfer gate. By making the transfer gate transparent, the system utilizes the previously non-functional channel region for charge generation, effectively adding a new dimension to photocharge production
Solution Approach 2:
The transparent transfer gate acts as an intermediary that simultaneously performs charge transfer function and serves as an additional light-receiving element. The transparent material allows incident light to pass through and generate optical charges in the channel area, mediating between the photodiode and the readout circuit while contributing to charge generation
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 use of transparent transfer gates enhances the generation of optical charges, thereby improving sensitivity in CMOS image sensors even with reduced pixel size, as demonstrated by increased sensitivity measurements with materials like ZnO and AlGaN compared to polysilicon.
Implementation Method 1
The transfer gate may be formed, at least in part, of transparent material... light to permeate and generate additional optical charges in the channel area
Data Source
AI summary
A complementary metal-oxide-semiconductor image sensor may include: a semiconductor substrate; a photodiode formed on a first portion of the semiconductor substrate; a transfer gate formed on the semiconductor substrate, near the photodiode, to transfer optical charges accumulated in the photodiode; a floating diffusion area formed on a second portion of the semiconductor substrate, on an opposite side of the transfer gate from the photodiode, to accommodate the optical charges; and/or a channel area formed under the transfer gate and contacting a side of the photodiode to transfer the optical charges. The transfer gate may be formed, at least in part, of transparent material. A method of manufacturing a complimentary metal-oxide-semiconductor image sensor may include: forming the photodiode; forming the floating diffusion area, separate from the photodiode; and/or forming the transfer gate, near the photodiode, to transfer optical charges accumulated in the photodiode.