Backside Image Sensor Epitaxial Structure for Dark Current Control
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Solution Overview
Problem
The existing process for forming back-illuminated image sensors results in incomplete removal of the substrate, leading to lattice defects and increased dark current due to the expansion of the depletion region, affecting the performance of the image sensor.
Innovation Solution
A method involving the formation of multiple epitaxial layers on a substrate with varying ion concentrations, allowing controlled thinning to preserve the high-doped epitaxial layers and prevent depletion region expansion, utilizing etching and planarization techniques to maintain uniformity and prevent ion diffusion.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If the substrate is completely removed using the epitaxial layer as a stop layer, then the substrate removal process is simplified, but lattice defects are generated and dark current increases due to depletion region expansion
Solution Approach 1:
The patent applies local quality by creating different doping concentrations in different regions of the epitaxial layer. Specifically, a first doped region with higher doping concentration is formed near the substrate interface, while a second doped region with lower doping concentration is formed above it. This local variation in doping concentration prevents depletion region expansion into the low-doped area, thereby reducing dark current while maintaining the substrate removal process.
Solution Approach 2:
The patent implements beforehand cushioning by pre-forming a highly-doped region (first doped region) at the interface between the epitaxial layer and substrate before complete substrate removal. This high-doped region acts as a cushion or barrier that prevents the depletion region from expanding into the low-doped area during sensor operation, thereby preventing dark current generation before it can occur.
2Ease of manufacture
If only a portion of the low-doped epitaxial layer is preserved, then the substrate removal is facilitated, but the performance of the final image sensor is affected
Solution Approach 1:
The patent uses local quality by creating spatially varying doping concentrations within the epitaxial layer. The first doped region has higher doping concentration than the second doped region, and this local differentiation allows the structure to maintain both manufacturability (through controlled substrate removal) and performance (by preventing depletion region expansion in the low-doped area).
Solution Approach 2:
The patent applies parameter changes by varying the doping concentration parameter throughout the epitaxial layer. By changing the doping concentration from high in the first doped region to low in the second doped region, the patent optimizes both the substrate removal process and the final sensor performance, preventing dark current while maintaining manufacturing feasibility.
3Reliability
If multiple epitaxial layers with varying ion concentrations are formed, then dark current generation is reduced, but the device complexity increases
Solution Approach 1:
The patent applies segmentation by dividing the epitaxial layer into distinct doped regions. The first doped region and second doped region are segmented with different doping concentrations, allowing each region to serve a specific function: the first region prevents depletion expansion while the second region maintains low dark current characteristics. This segmentation achieves dark current reduction while keeping the structure manageable.
Solution Approach 2:
The patent uses composite materials by creating a composite epitaxial structure with multiple doped regions having different doping concentrations. This composite structure combines the benefits of high-doped regions (depletion region containment) and low-doped regions (low dark current) into a single integrated layer, achieving performance improvement without excessive complexity.
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 the performance of the semiconductor structure by reducing dark current generation and improving thickness uniformity, ensuring better radiation reception and sensor performance.
Implementation Method 1
because of the ion concentration difference between the first epitaxial layer and the first substrate, the thinning process may first stop at the interface of the first epitaxial layer and the first substrate
Implementation Method 2
the etching is a wet etching process, and processing parameters of the wet etching include an etching solution including an HNA solution
Implementation Method 3
the planarization treatment is a chemical mechanical polishing; and processing parameters of the chemical mechanical polishing include: a polishing liquid including SiO2
Data Source
AI summary
Method for forming a semiconductor structure includes: providing a first substrate including a first surface and second surface opposite to each other, where the first substrate includes first ions with a first concentration; forming a first epitaxial layer on the first surface of the first substrate, where the first epitaxial layer includes second ions with a second concentration smaller than the first concentration; forming a second epitaxial layer on the first epitaxial layer and a third epitaxial layer located on the second epitaxial layer, where the second epitaxial layer includes third ions with a third concentration and the third epitaxial layer includes fourth ions with a fourth concentration smaller than the third concentration; and thinning the first substrate from the second surface of the first substrate until the surface of the second epitaxial layer is exposed.