Back-Illuminated Image Sensor Dishing Depression Fabrication
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
The existing methods for fabricating back-illuminated image sensors face challenges in achieving accurate cross-wafer and wafer-to-wafer thickness uniformity during the thinning process, which can damage sensing elements and affect the performance of optical members, leading to reduced image sensor quality.
Innovation Solution
A fabricating method that employs trench isolations as a polishing stop layer in a chemical mechanical polishing (CMP) process to thin the silicon wafer, allowing for high polishing rate selectivity and forming dishing depressions on the wafer surface, upon which a microlens with a smoothly curved surface is formed to improve light focusing.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If a thinning process is performed on the back side of the silicon wafer to reduce thickness from several hundred micrometers to several micrometers, then light beams can be introduced from the back surface without being blocked by metal wires, but it is very difficult to accurately control the thinning process such that the thinned wafer has good cross-wafer uniformity and wafer-to-wafer uniformity
Solution Approach 1:
Trench isolations are formed on the front surface of the silicon wafer before the thinning process. These trench isolations serve as a polishing stop layer that prevents over-thinning and ensures uniform thickness control during the CMP process, while still allowing the back surface to be thinned to the required several micrometers for back-illuminated operation
Solution Approach 2:
The trench isolations act as an intermediary structure that mediates between the thinning process and the silicon wafer substrate. By using the trench isolations as a polishing stop layer, the system achieves precise thickness control without requiring complex real-time monitoring during the CMP process
2Ease of manufacture
If the cross-wafer is over-thinned or unevenly thinned, then the light beams can pass through the back surface, but the sensors on the front surface would be damaged or the performance of the optical members on the back surface would be reduced
Solution Approach 1:
Trench isolations are formed on the front surface before the thinning process to establish a polishing stop layer. This preliminary structure prevents over-thinning that would damage front surface sensors or reduce optical member performance, while still achieving the necessary thinness for back-illuminated operation
Solution Approach 2:
The trench isolations provide a cushioning effect by acting as a mechanical stop during the CMP thinning process. This beforehand protection ensures that even if the CMP process continues, the trench isolations prevent further thinning that would compromise sensor integrity or optical performance
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 ensures precise control over the thinning process, achieving good cross-wafer and wafer-to-wafer thickness uniformity, enhancing the performance and quality of back-illuminated image sensors by accurately focusing incident light onto the image sensing members.
Implementation Method 1
a first chemical mechanical polishing (CMP) process is performed on the second surface to thin the silicon wafer
Data Source
AI summary
A fabricating method of a back-illuminated image sensor includes the following steps. First, a silicon wafer having a first surface and a second surface is provided, wherein a number of trench isolations are formed in the first surface, and at least one image sensing member is formed between the trench isolations. Then, a first chemical mechanical polishing (CMP) process is performed to the second surface using the trench isolations as a polishing stop layer to thin the silicon wafer. Because the polishing rate of the silicon material in the silicon wafer is different with that of the isolation material of the trench isolations in the first CMP process, at least one dishing depression is formed in the second surface of the silicon wafer. Finally, a microlens is formed above the dishing depression, and a surface of the microlens facing the dishing depression is a curved surface.