Backside Illuminated Image Sensor Stacked Structure
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Solution Overview
Problem
Front-side illuminated CMOS image sensors face reduced quantum efficiency due to light obstruction from additional layers, whereas back-side illuminated sensors have direct light paths, but existing methods for fabricating back-side illuminated sensors are complex and costly, limiting their widespread adoption.
Innovation Solution
A method for fabricating back-side illuminated image sensors involving a stacked semiconductor structure with vertically integrated photodiodes and logic circuits, where the first wafer with photoactive regions is thinned and bonded to a second wafer with logic circuits, allowing direct light exposure and reducing form factor and power consumption.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If front-side illuminated structure is used with additional layers, then circuit integration is improved, but quantum efficiency deteriorates due to light obstruction
Solution Approach 1:
The patent inverts the traditional front-side illuminated structure by making the backside the light-receiving surface. This allows light to directly reach the photodiode without passing through opaque interconnect layers, thereby maintaining high quantum efficiency while still integrating circuits on the front side of the substrate.
Solution Approach 2:
The patent separates the light-receiving function to the backside dimension while keeping circuit integration on the front side dimension. This spatial separation in different dimensions allows both high quantum efficiency and circuit integration to coexist without interfering with each other.
2Reliability
If back-side illuminated structure is used, then quantum efficiency is improved, but fabrication complexity increases
Solution Approach 1:
The patent divides the substrate into distinct front and back sides with specialized functions. The front side contains circuitry while the back side is optimized for light reception. This segmentation allows each side to be optimized independently, simplifying the overall fabrication process compared to trying to optimize a single side for both functions.
Solution Approach 2:
The substrate serves multiple functions: the front side handles circuit integration while the back side handles light reception. This multi-functionality approach allows a single substrate structure to achieve both high quantum efficiency and circuit integration without requiring separate components, thereby reducing overall fabrication complexity.
3Adaptability or versatility
If additional dielectric and interconnect layers are added, then circuit functionality is improved, but light absorption is reduced
Solution Approach 1:
By inverting the light-receiving surface to the backside, the patent eliminates the problem of interconnect layers blocking light. The additional dielectric and interconnect layers remain on the front side for circuit functionality, while light enters from the backside where no such obstructing layers exist, maintaining full light absorption efficiency.
Solution Approach 2:
The patent places circuit functionality layers on the front side dimension and light reception function on the backside dimension. This dimensional separation ensures that additional circuit layers do not interfere with light absorption, as light travels through a different spatial path that avoids these layers entirely.
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 enhances quantum efficiency by eliminating light obstructions, reduces power consumption, and improves circuit density while simplifying the fabrication process, making back-side illuminated sensors more viable.
Implementation Method 1
A CMOS image sensor utilizes light-sensitive CMOS circuitry to convert photons into electrons. The light-sensitive CMOS circuitry typically comprises a photo-diode formed in a silicon substrate. As the photo-diode is exposed to light, an electrical charge is induced in the photo-diode.
Data Source
AI summary
A backside illuminated image sensor comprises a photodiode and a first transistor located in a first substrate, wherein the first transistor is electrically coupled to the photodiode. The backside illuminated image sensor further comprises a plurality of logic circuits formed in a second substrate, wherein the second substrate is stacked on the first substrate and the logic circuit are coupled to the first transistor through a plurality of bonding pads.