Back-illuminated CMOS Image Sensor Pixel Separation
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Solution Overview
Problem
Conventional CMOS image sensors face challenges in achieving high aperture ratios and preventing electrical and optical color mixing due to wiring layers and transistors on the light incident side, leading to poor light use efficiency and increased noise from trench separators.
Innovation Solution
A back-illuminated solid-state imaging device design with tapered pixel separating regions and a low refractive index material film on the light-illuminated side, combined with a manufacturing method using etching solutions to control impurity density and trench formation, reduces optical crosstalk and dark currents.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If high-density impurity ions are implanted to strengthen pixel separation by potentials, then electrical color mixing is prevented, but pixel separating regions become wider and sensitivity in short-wavelength regions decreases
Solution Approach 1:
The pixel separating region is divided into two functional parts: an upper region with high-density impurity ions for electrical separation and a lower region with low-density impurity ions for maintaining sensitivity. This segmentation allows each region to perform its specific function optimally without compromising the other.
Solution Approach 2:
Different impurity density regions are created within the pixel separating region to provide different local properties. The upper part has high impurity density for strong potential separation, while the lower part has low impurity density to maintain optical transparency and sensitivity in short-wavelength regions.
2Reliability
If trench separators are used for pixel separation, then electrical color mixing is prevented, but dark current from interface states increases noise components
Solution Approach 1:
Instead of completely removing the oxide film from trench separators, the patent utilizes the oxide film's presence but modifies the trench geometry to reduce its harmful effects. The inverted tapered shape minimizes the oxide film's exposure to incident light while maintaining its electrical separation function.
Solution Approach 2:
The trench separator's geometric parameters are changed from a conventional tapered shape to an inverted tapered shape, with the aperture being smaller at the light incident face and larger at the wiring face. This parameter change reduces the interface area exposed to light while maintaining electrical separation effectiveness.
3Reliability
If trench separators with wider aperture on wiring side are used, then electrical separation is achieved, but optical color mixing increases due to narrower aperture on light incident face
Solution Approach 1:
The conventional tapered shape of trench separators is inverted. Instead of having a wider aperture at the wiring side and narrower at the light incident side, the patent creates a structure with a narrower aperture at the light incident face and wider aperture at the wiring face, reversing the traditional geometry to simultaneously achieve both electrical separation and optical isolation.
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 solution enhances light use efficiency by minimizing optical crosstalk and dark currents, maintaining sensitivity across various wavelengths while preventing electrical color mixing.
Implementation Method 1
pixel separating regions separating the pixels from one another, the pixel separating region being formed between the pixels, each of the pixel separating regions including a second semiconductor layer of the first conductivity type covering faces in contact with the photoelectric conversion elements, and an insulating film with a lower refractive index than a refractive index of the second semiconductor layer to cover the second semiconductor layer
Implementation Method 2
each of the pixels including a photoelectric conversion element that converts light entering through a second face of the first semiconductor layer on the opposite side from the first face into a signal charge
Data Source
AI summary
A solid-state imaging device according to an embodiment includes: a plurality of pixels arranged on a first face of a first semiconductor layer, each of the pixels including a photoelectric conversion element converting light entering through a second face of the first semiconductor layer on the opposite side from the first face into a signal charge, the photoelectric conversion element having a pn junction formed with a first semiconductor region formed on the first face and a second semiconductor region formed on a surface of the first semiconductor region; pixel separating regions separating the pixels from one another and formed between the pixels, each of the pixel separating regions including a second semiconductor layer covering faces in contact with the photoelectric conversion elements, and an insulating film with a lower refractive index than a refractive index of the second semiconductor layer to cover the second semiconductor layer.