CMOS Image Sensor Buffer Layer for Dark Current Reduction
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Solution Overview
Problem
Conventional CMOS image sensors face challenges with high power consumption, complex manufacturing processes, and difficulty in integrating with other devices, leading to limitations in miniaturization and performance issues such as dark currents and reduced light sensitivity due to impurity accumulation during high-temperature heat-treatment.
Innovation Solution
A CMOS image sensor manufacturing method involving a buffer layer to absorb impurities and reduce dark currents, which includes forming an isolation layer, defining active regions, creating gate electrodes and diffusion regions, and using a buffer layer to prevent substrate damage and impurity accumulation, along with selective removal and thermal treatment processes to form a metal silicide layer and interlevel dielectric layer.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If high-temperature heat-treatment is applied to activate dopant ions and repair lattice damage, then device activation and repair are improved, but impurity accumulation occurs leading to increased dark currents
Solution Approach 1:
A buffer layer comprising a first insulating layer and a second insulating layer is introduced as an intermediary between the substrate and the processing environment. This buffer layer absorbs impurities generated during high-temperature heat-treatment, preventing them from accumulating in the active device regions. The buffer layer is selectively removed in the photo diode region to maintain light sensitivity while retaining impurity absorption capability in transistor regions, thus resolving the contradiction between device activation and dark current reduction.
2Reliability
If multiple photolithography processes are used to form transistors and diffusion regions, then device functionality is improved, but manufacturing complexity increases
Solution Approach 1:
The manufacturing process is segmented into distinct stages with clearly defined buffer layer formation, selective removal, and dopant implantation steps. The buffer layer itself is segmented into multiple insulating layers with different etching selectivities, allowing selective removal in different regions. This segmentation enables systematic control of the manufacturing process, reducing overall complexity while maintaining device functionality.
Solution Approach 2:
The buffer layer is formed preliminarily before dopant ion implantation and high-temperature heat-treatment processes. This preliminary action prepares the structure to absorb impurities during subsequent processing steps, preventing dark current issues before they occur. The selective removal of the buffer layer is also performed preliminarily in photo diode regions to preserve light sensitivity before final device assembly.
3Reliability
If the buffer layer is completely removed to improve light sensitivity in photo diode regions, then light sensitivity is improved, but impurity absorption capability is lost
Solution Approach 1:
The buffer layer is selectively removed only in the photo diode region where light sensitivity is critical, while being retained in transistor regions where impurity absorption is more important. This local differentiation of buffer layer presence optimizes both light sensitivity in photo diodes and impurity absorption in transistors, resolving the contradiction between these two requirements.
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 method effectively reduces dark currents, improves reproducibility, and enhances the dynamic range and light sensitivity of CMOS image sensors by managing impurities and substrate damage during high-temperature processing.
Implementation Method 1
forming a buffer layer over an entire surface of the substrate, the buffer layer selectively removed to cover the photo diode region
Implementation Method 2
dopant ions are implanted in the active region 10 where each transistor is formed
Implementation Method 3
forming a metal silicide layer on a surface of the high-concentration diffusion region
Data Source
AI summary
Disclosed are a CMOS image sensor and a manufacturing method thereof. The method includes the steps of: forming an isolation layer on a semiconductor substrate, defining an active region that includes a photo diode region and a transistor region; forming a gate in the transistor region, the gate including a gate electrode and a gate insulating layer; forming a first low-concentration diffusion region in the photo diode region; forming a second low-concentration diffusion region in the transistor region; forming a buffer layer over the substrate, the buffer layer covering the photo diode region; forming first and second insulating layers over the entire surface of the substrate, the first and second insulating layer having a different etching selectivity from each other; forming an insulating sidewall on sides of the gate electrode by selective removal of the second insulating layer; removing the first insulating layer from the transistor region; forming a high-concentration diffusion region in the exposed transistor region, partially overlapping the second low-concentration diffusion region; and forming a metal silicide layer on the high-concentration diffusion region.