Backside Illuminated Image Sensor Surface Roughness Control
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Solution Overview
Problem
As device scaling continues, existing backside illuminated (BSI) image sensor devices face challenges in achieving optimal image quality due to surface roughness issues arising from annealing processes, which can degrade image quality and result in defects like striped patterns.
Innovation Solution
A method is proposed that involves forming a doped layer at the back surface of the substrate, followed by a polishing process to remove surface roughness, optimizing the annealing process parameters, and reducing the substrate thickness to enhance quantum efficiency and reduce dark current and white pixel defects.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If annealing process is performed to activate dopants at the back surface, then dark current is reduced, but surface roughness increases causing image quality degradation
Solution Approach 1:
A protective layer is formed over the back surface before the annealing process to prevent surface roughness from developing during dopant activation. This preliminary protective action allows the annealing to proceed effectively for dark current reduction while the protective layer prevents the harmful surface roughness formation.
Solution Approach 2:
A protective layer acts as an intermediary between the annealing process and the back surface. This intermediary layer allows thermal energy to reach the dopants for activation while preventing direct contact between the annealing environment and the surface, thereby avoiding surface roughness formation.
2Reliability
If substrate thickness is reduced to enhance quantum efficiency, then light sensitivity improves, but mechanical strength and handling difficulty worsen
Solution Approach 1:
The substrate is constructed as a composite structure combining a thin active region for high quantum efficiency with supporting layers that provide mechanical strength. This composite approach allows the substrate to be thin enough for enhanced light sensitivity while maintaining sufficient mechanical strength for handling through the supportive composite layers.
3Productivity
If device geometry is scaled down to increase integration density, then fabrication cost and device density improve, but image quality control becomes more difficult
Solution Approach 1:
Process parameters such as annealing temperature, duration, and protective layer properties are optimized specifically for scaled-down geometries. By adjusting these parameters, the patent maintains image quality control despite the reduced feature sizes that increase integration density, ensuring that smaller devices do not suffer from degraded manufacturing precision.
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 polishing process effectively reduces surface roughness, improving image quality by eliminating defects and maintaining the benefits of the doped layer, such as reduced dark current, in a cost-effective and time-effective manner.
Implementation Method 1
a silicon polishing process is performed on the implanted back surface of the silicon substrate
Implementation Method 2
performing a green laser annealing process to the implanted back surface of the silicon substrate
Implementation Method 3
performing a green laser annealing process to the implanted back surface of the silicon substrate
Implementation Method 4
performing an ion implantation process to the back surface of the silicon substrate
Data Source
AI summary
A method of forming an image sensor device includes forming a light sensing region at a front surface of a silicon substrate and a patterned metal layer there over. Thereafter, the method also includes performing an ion implantation process to the back surface of the silicon substrate and performing a green laser annealing process to the implanted back surface of the silicon substrate. The green laser annealing process uses an annealing temperature greater than or equal to about 1100° C. for a duration of about 100 to about 400 nsec. After performing the green laser annealing process, a silicon polishing process is performed on the back surface of the silicon substrate.