Back-Illuminated Image Sensor Dark Current Reduction
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Solution Overview
Problem
Image sensors suffer from property degradation due to dark current noise, which is generated at the surface of the substrate where the photoelectric conversion device is formed, affecting the quality of the pixel signal.
Innovation Solution
A back side illumination image sensor is fabricated with a photoelectric conversion region, an interlayer insulation layer, a carbon-containing layer doped with impurities on the back side of the substrate, and a color filter and micro-lens on top, along with an anti-reflective layer to minimize light loss and control electric charges, thereby preventing dark current generation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a conventional image sensor structure is used with photoelectric conversion devices formed on the substrate surface, then the device can be fabricated using standard processes, but dark current is generated at the substrate surface which degrades image sensor properties
Solution Approach 1:
The patent inverts the conventional structure by forming the photoelectric conversion devices on the back surface of the substrate rather than the front surface. This inversion allows the front surface to be used for other purposes while the back surface handles light detection, thereby eliminating dark current generation at the front substrate surface and improving image sensor properties.
Solution Approach 2:
The patent transitions from a single-sided (front surface) photoelectric conversion structure to a back-illuminated structure utilizing the third dimension (substrate thickness). By moving the photoelectric conversion region to the back surface and introducing an interlayer insulation layer on the front surface, the design exploits vertical spatial arrangement to separate functions and eliminate dark current interference.
2Reliability
If the substrate surface is used for photoelectric conversion, then the structure is simple, but light loss occurs and sensitivity is reduced
Solution Approach 1:
The patent inverts the illumination direction by forming photoelectric conversion devices on the back surface of the substrate. This back-illuminated structure allows light to enter through the back surface directly reaching the photoelectric conversion region without passing through the substrate thickness, thereby reducing light absorption loss and improving sensitivity.
3Reliability
If a back side illumination structure is implemented, then light loss is minimized and sensitivity is enhanced, but additional layers and fabrication steps are required
Solution Approach 1:
The patent segments the substrate into distinct functional regions: the front surface with interlayer insulation layer for electrical isolation and the back surface with photoelectric conversion devices for light detection. This segmentation allows each surface to be optimized independently, achieving high sensitivity while managing fabrication complexity through clear functional separation.
Solution Approach 2:
The substrate serves multiple functions: it provides mechanical support, electrical isolation (through the interlayer insulation layer on the front surface), and the medium for back-illuminated photoelectric conversion. This multi-functionality reduces the need for additional separate components, thereby managing device complexity while achieving enhanced sensitivity.
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 effectively reduces dark current noise, enhances the sensitivity of the image sensor by minimizing light loss, and improves the reliability of the device by controlling defects during fabrication, leading to improved image sensor performance.
Implementation Method 1
a carbon-containing layer doped with impurities and formed over a back side of the substrate
Implementation Method 2
an anti reflective layer formed between the back side of the substrate and the carbon-containing layer
Implementation Method 3
Each pixel accumulates photocharges corresponding to the incident light through a photoelectric conversion device, i.e., a photo diode
Data Source
AI summary
An image sensor includes a photoelectric conversion region formed in a substrate, an interlayer insulation layer formed over a front side of the substrate, a carbon-containing layer doped with impurities and formed over a back side of the substrate, and a color filter and a micro-lens formed over the carbon-containing layer.