Back-Side Illumination Pixel Structure Using Stacked OS FETs
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Solution Overview
Problem
BSI image sensors face challenges in reducing size, improving photo-electric conversion efficiency, sensitivity, and low noise while maintaining low power consumption and integration efficiency.
Innovation Solution
The BSI image sensor pixel structure incorporates two oxide semiconductor field effect transistor (OS FET) devices and a capacitor stacked on the front side of the substrate, overlapping the sensing device, which serve as charge and discharge control devices, reducing power consumption without occupying valuable substrate area.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Use of energy by stationary object
If conventional FET devices are used in BSI image sensors, then integration is achieved, but power consumption is high due to short channel effects
Solution Approach 1:
The patent changes the material parameter of the FET channel from conventional semiconductor to oxide semiconductor, which fundamentally alters the electrical characteristics to achieve ultra-low off-state current and reduced power consumption while maintaining device reliability
2Adaptability or versatility
If more devices are placed on the substrate to improve functionality, then integration efficiency decreases, but area consumption increases
Solution Approach 1:
The patent transitions from planar device layout to three-dimensional stacked architecture, placing the OS FET device, capacitor, and color filter in vertical layers above the sensing device, thereby improving integration efficiency without increasing substrate footprint
3Length of moving object
If device size is reduced to decrease sensor size, then manufacturing precision becomes more difficult
Solution Approach 1:
The patent implements a nested structure where the OS FET device, capacitor, and color filter are stacked in layers, with each component containing or supporting the next, allowing compact integration while maintaining manufacturable dimensions through standardized layer thicknesses
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 reduces power consumption and maintains high integration efficiency by suppressing short channel effects and storing charges efficiently, even when the charge control OS FET device is turned off.
Implementation Method 1
Since the OS FET devices suppress short channel effect, it has the advantage of extremely low off-state current. Therefore, power consumption of the image sensor pixel structure and/or BSI image sensor pixel structure is efficaciously and extraordinarily reduced
Implementation Method 2
when the charge control OS FET device is turned on, electrical charges accessed from the sensing device are stored in the capacitor
Implementation Method 3
a sensing device formed in the substrate to receive an incident light through the back side of the substrate
Data Source
AI summary
A back side illumination image sensor pixel structure includes a substrate having a front side and a back side opposite to the front side, a sensing device formed in the substrate to receive an incident light through the back side of the substrate, two oxide-semiconductor field effect transistor (OS FET) devices formed on the front side of the substrate, and a capacitor formed on the front side of the substrate. The two OS FET devices are directly stacked on the sensing device and the capacitor is directly stacked on the OS FET devices. The two OS FET devices overlap the sensing device, and the capacitor overlaps both of the OS FET devices and the sensing device.