Back-Side Illuminated Image Sensor Blue Light Absorption
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Solution Overview
Problem
Downscaled CMOS image sensors face challenges in maintaining low-light sensitivity and reducing image noise due to reduced photodiode area, which limits blue light absorption as blue light is absorbed at the surface and blocked by typical light filters, and increasing semiconductor thickness does not improve blue pixel quantum efficiency.
Innovation Solution
The use of back side illuminated image sensors with doped regions at varying depths and unconventional light filters that allow blue light to be absorbed by specific doped regions, preventing red and green light absorption, thereby enhancing blue light absorption without increasing semiconductor thickness.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If semiconductor thickness is increased to improve quantum efficiency, then light absorption improves for longer wavelengths, but blue light absorption does not improve because blue light is absorbed at the surface
Solution Approach 1:
The patent inverts the conventional light incident direction by using back-side illumination. Light enters through the back surface of the semiconductor substrate, allowing blue light to reach doped regions at varying depths before being absorbed, rather than being absorbed at the front surface. This inversion enables blue light absorption improvement without increasing semiconductor thickness.
Solution Approach 2:
The patent introduces doped regions at varying depths within the semiconductor substrate, creating local variations in light absorption properties. Different doped regions at different depths absorb different wavelengths, with shallower regions absorbing blue light and deeper regions absorbing red and green light, enabling wavelength-selective absorption throughout the substrate thickness.
2Productivity
If photodiode area is reduced to achieve higher resolution and miniaturization, then device integration is improved, but low-light sensitivity and image noise performance deteriorate
Solution Approach 1:
The patent transitions from two-dimensional surface absorption to three-dimensional volumetric absorption by utilizing doped regions at varying depths within the semiconductor substrate. This dimensional extension allows the photodiode to collect photons throughout the substrate volume rather than only at the surface, effectively increasing the light collection area without increasing the device footprint, thus improving low-light sensitivity while maintaining high resolution.
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 improves blue light absorption across the entire surface area of the image sensor, maintaining sensitivity to red and green light while enhancing blue light collection, thus improving the signal-to-noise ratio and addressing low-light sensitivity issues.
Implementation Method 1
blue light is absorbed at the very surface of the semiconductor... improve blue pixel quantum efficiency... blue light absorption
Implementation Method 2
typical light filter color choices (e.g. red, green, blue) block much of the blue light incident on the surface
Data Source
AI summary
A back side illuminated image sensor includes a semiconductor material having a front side and a back side. The semiconductor material is disposed between image sensor circuitry and a light filter array. The image sensor circuitry is disposed on the front side, and the light filter array is disposed proximate to the back side. The image sensor includes a first pixel with a first doped region that extends from the image sensor circuitry into the semiconductor material a first depth. The first pixel also includes a second doped region that is disposed between the back side of the semiconductor material and the first doped region. The second doped region is electrically isolated from the first doped region. A second pixel with a third doped region is also included in the image sensor. The third doped region extends from the image sensor circuitry into the semiconductor material a second depth.