Back Side Illumination Photodiode Recess for Quantum Efficiency
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Solution Overview
Problem
BSI-type photodiodes with small lateral dimensions face low quantum efficiency due to limited photon absorption, which is exacerbated by crosstalk issues from thick planarization layers and manufacturing complexities in forming pad structures on the rear surface.
Innovation Solution
A recess with a material of lower optical index, such as silicon oxide, is created on the light-receiving surface of BSI photodiodes, with ring-shaped dimensions optimized to enhance photon absorption, specifically with internal lateral dimensions between 200 and 500 nm and external dimensions between 450 and 650 nm, to improve quantum efficiency.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Area of stationary object
If the lateral dimensions of photodiodes are reduced to increase pixel density, then the area per photodiode decreases, but quantum efficiency deteriorates due to limited photon absorption
Solution Approach 1:
The patent introduces surface relief structures (recesses or protrusions) on the light-receiving surface of photodiodes. These three-dimensional modulations create additional optical path lengths and multiple internal reflections, effectively increasing the light absorption probability without increasing the lateral footprint of the photodiode. This dimensional transition from 2D planar surface to 3D structured surface resolves the contradiction between small area and high quantum efficiency.
2Reliability
If pad structures are added on the rear surface to improve quantum efficiency, then photon absorption increases, but device complexity and manufacturing difficulty increase
Solution Approach 1:
The patent integrates the optical enhancement function directly into the photodiode's light-receiving surface structure itself, rather than adding separate pad structures. The surface relief features (recesses or protrusions) are formed as part of the photodiode fabrication process, merging the optical enhancement function with the existing device structure. This eliminates the need for additional discrete pad components and their associated manufacturing steps, thereby reducing device complexity while maintaining quantum efficiency improvement.
3Ease of manufacture
If planarization layers are thickened to accommodate pad structures, then manufacturing is facilitated, but crosstalk between adjacent photodiodes increases
Solution Approach 1:
The patent removes the need for thick planarization layers by eliminating the pad structures that necessitated them in the first place. By integrating the optical enhancement features directly into the photodiode surface and avoiding separate pad components, the patent extracts the problematic planarization layer requirement from the device architecture. This eliminates the source of crosstalk while maintaining manufacturing feasibility through standard fabrication processes.
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 increases photon absorption by up to 4% compared to conventional structures, reducing crosstalk and manufacturing challenges while maintaining transparency to the operating wavelength.
Implementation Method 1
at least one area of the light-receiving surface of the photodiode comprises a recess filled with a material having an optical index lower than that of the semiconductor material of the photodiode
Implementation Method 2
said material of lower optical index being transparent to the operating wavelength of the photodiode
Data Source
AI summary
A back side illumination photodiode includes a light-receiving back side surface of a semiconductor material substrate. An area of the light-receiving back side surface includes a recess. The recess is filled with a material having an optical index that is lower than an optical index of the semiconductor material substrate. Both the substrate and the filling material are transparent to an operating wavelength of the photodiode. The recess may be formed to have a ring shape.