CMOS Image Sensor Inner Microlens for Crosstalk Reduction
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Solution Overview
Problem
CMOS image sensors face reduced photosensitivity due to small pixel size and optical crosstalk caused by microlenses with long focal lengths, leading to light being diverted to neighboring pixels instead of the photodiode.
Innovation Solution
The integration of inner microlenses between the semiconductor substrate and the photodiodes, along with a silicon layer etched to form an inverse triangle profile, reduces focal length and enhances light condensation by forming concave recesses and nitride layers to improve light absorption.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Illumination intensity
If a microlens with long focal length is used to condense light, then light condensing effect is improved, but optical crosstalk between neighboring pixels increases
Solution Approach 1:
The invention divides the microlens system into two separate components: an inner microlens formed within the semiconductor substrate and an outer microlens formed on the top layer. This segmentation allows each microlens to have optimized focal length characteristics, with the inner microlens having a shorter focal length to reduce optical crosstalk while the outer microlens provides additional light condensing capability.
Solution Approach 2:
The inner microlens is nested within the semiconductor substrate, positioned between the photodiode and the top layer where the outer microlens is formed. This nested configuration enables the inner microlens to pre-condense light before it reaches the outer microlens, creating a cascaded optical system that achieves both strong light condensing and reduced optical crosstalk.
2Productivity
If pixel size is reduced to increase integration degree, then device integration is improved, but photosensitivity deteriorates
Solution Approach 1:
The invention addresses the photosensitivity limitation in small pixels by adding a vertical dimension to light condensing. Instead of relying solely on the horizontal pixel area, the dual microlens system (inner and outer) creates vertical light concentration through the substrate thickness, effectively increasing the light collection capability without increasing pixel area.
Solution Approach 2:
The inner microlens performs preliminary light condensing action within the substrate before light reaches the photodiode active area. This preliminary condensation ensures that even in reduced-size pixels, sufficient light intensity is achieved at the photodiode, maintaining photosensitivity despite smaller pixel dimensions.
3Reliability
If fill factor is increased to improve photosensitivity, then light absorption is improved, but logic circuit area is reduced
Solution Approach 1:
The invention replaces the mechanical approach of increasing fill factor (enlarging photodiode area within the pixel) with an optical approach using dual microlenses. The optical system concentrates light from the entire pixel area onto the photodiode, achieving high photosensitivity without requiring a large photodiode area, thus preserving space for logic circuits.
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 increases the light intensity on photodiodes, reduces optical crosstalk, and enhances image quality by ensuring more light is absorbed by the photodiodes, thereby improving the overall sensing capability.
Implementation Method 1
A nitride layer 41 having a large refractive index is stacked on the semiconductor substrate 30 including the concave recesses
Data Source
AI summary
A fabricating method of a CMOS image sensor is disclosed, by which a light condensing effect is enhanced by providing an inner microlens to a semiconductor substrate. The CMOS image sensor includes a plurality of photodiodes on a semiconductor substrate, a plurality of inner microlenses on a plurality of the photodiodes, an insulating interlayer on a plurality of the inner microlenses, a plurality of metal lines within the insulating interlayer, a device protecting layer on the insulating interlayer, and a plurality of microlenses on the device protecting layer.