Backside Electrode Pixel Structure for Low-Noise SPAD Imaging
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Solution Overview
Problem
In SPAD technology, miniaturization leads to challenges in insulating electrodes, increasing noise and reducing photoelectric conversion efficiency, particularly affecting short-wavelength sensitivity due to inter-pixel interference and variations.
Innovation Solution
An imaging device with a semiconductor substrate featuring a first and second semiconductor layer of opposite conductivity types, a pixel separation unit, and electrodes connected to each layer, allowing for efficient signal processing and improved sensitivity while minimizing noise and inter-pixel interference.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If a pair of electrodes are provided side by side on the front surface of a substrate to apply high bias voltage for electron multiplication, then light incidence can be extracted as a large signal, but it becomes difficult to reliably insulate the pair of electrodes as miniaturization advances, leading to increased noise and reduced photoelectric conversion efficiency
Solution Approach 1:
The patent moves the second electrode from the front surface to the back surface of the substrate, changing the spatial arrangement from a two-dimensional side-by-side configuration to a three-dimensional configuration spanning both surfaces. This dimensional change allows the electrodes to be separated by the substrate thickness, providing reliable insulation while maintaining the ability to apply high bias voltage for signal extraction.
2Ease of manufacture
If a transparent electrode is provided on the light irradiation surface to enable electrode connection, then electrical connection can be established, but noise occurs at the contact portion with the substrate
Solution Approach 1:
The patent extracts the second electrode from the light irradiation surface and relocates it to the back surface of the substrate. This extraction eliminates the problematic contact between the electrode and the light irradiation surface, thereby removing the noise source while still maintaining electrical connection capability through the substrate.
3Reliability
If an electrode is formed by an impurity layer with high-concentration impurities to lower resistance, then electrical resistance can be reduced, but a depletion layer cannot be formed in the impurity layer region and the thickness must be increased, causing sensitivity of short-wavelength light to decrease
Solution Approach 1:
The patent relocates the second electrode to the back surface of the substrate, allowing the use of high-concentration impurity layers for low resistance without compromising short-wavelength light sensitivity. The back surface positioning ensures that the impurity layer does not interfere with light absorption in the pixel region, as light enters through the front surface and the depletion layer can be properly formed in the pixel region while the electrode resides on the opposite surface.
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 enables low noise, high quantum efficiency, and enhanced short-wavelength sensitivity while suppressing inter-pixel interference and variations, achieving miniaturization of pixels.
Implementation Method 1
it is possible to extract light incidence as a large signal according to electron multiplication by applying a high bias voltage
Implementation Method 2
the second semiconductor layer is completely converted into a depletion layer by applying a bias voltage thereto
Data Source
AI summary
To realize miniaturization of a pixel, reduction in noise, and high quantum efficiency, and to improve short-wavelength sensitivity while suppressing inter-pixel interference and variations for each pixel. According to the present disclosure, there is provided an imaging device including: a first semiconductor layer formed in a semiconductor substrate; a second semiconductor layer of a conductivity type opposite to a conductivity type of the first semiconductor layer formed on the first semiconductor layer; a pixel separation unit which defines a pixel region including the first semiconductor layer and the second semiconductor layer; a first electrode which is connected to the first semiconductor layer from one surface side of the semiconductor substrate; and a second electrode which is connected to the second semiconductor layer from a light irradiation surface side that is the other surface of the semiconductor substrate, and is formed to correspond to a position of the pixel separation unit.