CMOS Image Sensor Surface Patterning for Near-Infrared Absorption
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Solution Overview
Problem
Existing solid state imaging elements face challenges in achieving high sensitivity to long wavelength light without increasing the thickness of the silicon layer, leading to issues like increased defects, dark current, and low light absorption efficiency, particularly when using concave-convex structures on the pixel transistor surface.
Innovation Solution
A solid state imaging element with a periodic concave-convex pattern on both the light receiving surface and the opposite surface of the silicon light absorbing layer, where the pattern's period can be infinitely small and varies with the sensed wavelength, combined with an element isolation structure and a reflecting mirror structure to enhance light absorption efficiency.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the thickness of the Si layer is increased to improve light absorption efficiency at long wavelengths, then light absorption efficiency is improved, but manufacturing difficulty increases and material cost increases
Solution Approach 1:
The patent applies curvature to the Si layer surface by forming a convex-concave pattern (microlens array) on the light-receiving surface. This curved structure increases the optical path length and light absorption efficiency without increasing the physical thickness of the Si layer, thereby avoiding manufacturing difficulties associated with thick layers while maintaining high light absorption efficiency at long wavelengths
Solution Approach 2:
Instead of increasing light absorption efficiency by increasing the thickness dimension, the patent transitions to modifying the surface topology dimension by adding convex-concave structures. This dimensional shift allows the light to travel a longer path through the Si layer without requiring the layer to be physically thicker, thus avoiding the manufacturing challenges of thick Si layers
2Reliability
If the thickness of the Si layer is increased to improve light absorption efficiency at long wavelengths, then light absorption efficiency is improved, but image quality degrades due to increased color mixing
Solution Approach 1:
The convex-concave pattern creates curved light paths that confine light more effectively within each pixel region. This curvature helps prevent light from laterally spreading into adjacent pixels, thereby reducing color mixing and maintaining image quality while still achieving high light absorption efficiency through the extended optical path
Solution Approach 2:
The light-receiving surface is segmented into multiple microlenses corresponding to individual pixels. Each microlens focuses and confines light to its respective pixel region, preventing inter-pixel color mixing while maintaining high absorption efficiency. This segmentation approach allows thin Si layers to achieve high efficiency without the color mixing problems of thick layers
3Loss of energy
If a concave-convex structure is formed on the pixel transistor surface to suppress etalon phenomenon, then light loss is reduced, but defects in crystal planes increase and dark current increases
Solution Approach 1:
The patent forms convex-concave structures (microlenses) on the light-receiving surface rather than on the pixel transistor surface. This positional distinction is critical: the convex-concave structures are formed on the illuminated surface where they suppress etalon effects without interfering with the underlying transistor structure, thereby avoiding crystal defects and dark current increases while still reducing light loss
Solution Approach 2:
The patent uses a mold to replicate the convex-concave pattern across the entire light-receiving surface in a single process step. This copying approach creates uniform microlenses that effectively suppress etalon phenomena without requiring complex direct patterning that could damage the crystal structure or increase dark current
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 configuration effectively suppresses reflection in the visible to near-infrared range, improving light absorption sensitivity on the long wavelength side without increasing the silicon layer thickness, thereby reducing defects and enhancing image quality.
Implementation Method 1
a structure in which the loss of light caused by an etalon phenomenon based on the interference of light is suppressed by forming a fine, random concave-convex structure on the surface on the opposite side to the light receiving surface
Implementation Method 2
single-crystal silicon (Si) is generally used for the light absorbing layer of the light detecting element of them that performs photoelectric conversion
Data Source
AI summary
The present disclosure relates to a solid state imaging element and an electronic device that make it possible to improve sensitivity to light on a long wavelength side. A solid state imaging element according to a first aspect of the present disclosure has a solid state imaging element in which a large number of pixels are arranged vertically and horizontally, the solid state imaging element includes a periodic concave-convex pattern on a light receiving surface and an opposite surface to the light receiving surface of a light absorbing layer as a light detecting element. The present disclosure can be applied to, for example, a CMOS and the like installed in a sensor that needs a high sensitivity to light belonging to a region on the long wavelength side, such as light in the infrared region.