Silicided Polysilicon Reflective Gate for Backside Illuminated Sensor Crosstalk
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Solution Overview
Problem
Backside illuminated imaging sensors face challenges with limited fill factor and high optical crosstalk due to thin silicon wafers, leading to reduced signal quality and increased noise from light scattering and reflection between pixels.
Innovation Solution
Incorporating a silicided polysilicon light reflecting layer on the frontside of the photodiode region, which reflects unabsorbed light back into the photodiode, reducing optical crosstalk and enhancing light absorption, while also serving as a protective layer to prevent etching damage and current leakage.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Illumination intensity
If the silicon wafer is thinned to improve sensitivity to light, then light sensitivity is improved, but optical crosstalk increases
Solution Approach 1:
The patent applies this principle by converting the harmful reflected light that causes optical crosstalk into a beneficial effect. The silicided polysilicon layer is specifically designed to reflect unabsorbed light back into the photodiode region, giving the reflected light a useful function rather than letting it cause interference. This transforms the harmful reflection into a mechanism that enhances light absorption and improves quantum efficiency.
Solution Approach 2:
The silicided polysilicon layer serves multiple functions simultaneously: it acts as a light-reflecting layer to improve light absorption, a protective layer to prevent etching damage during fabrication, and a gate structure for the transistor. This multi-functionality resolves the contradiction by adding a single structure that addresses both sensitivity enhancement and crosstalk reduction.
2Illumination intensity
If the silicon wafer is thinned to improve sensitivity, then sensitivity to shorter wavelengths is improved, but light scattering and reflection between pixels increases
Solution Approach 1:
The patent captures scattered and reflected light that would otherwise be lost or cause crosstalk, and redirects it back into the photodiode region through the silicided polysilicon reflective layer. This converts harmful light scattering into beneficial additional light absorption opportunities, improving quantum efficiency without requiring increased wafer thickness.
3Object-generated harmful factors
If a silicided polysilicon light reflecting layer is added to reduce optical crosstalk, then optical crosstalk is reduced, but device complexity increases
Solution Approach 1:
The silicided polysilicon layer is integrated into the existing transistor gate structure, serving simultaneously as the transistor gate and the light-reflecting layer. This multi-functional design adds the light management capability without requiring a completely separate structural layer, thereby minimizing the increase in device complexity while achieving crosstalk reduction.
Solution Approach 2:
The patent merges the transistor gate function with the light-reflecting function into a single silicided polysilicon structure. By combining these two functions that were previously separate, the design reduces overall structure complexity while achieving both electrical control and optical management in one integrated component.
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 improves light absorption and reduces noise by minimizing optical crosstalk and protecting the photodiode region from processing damage, thereby enhancing the sensitivity and quality of the imaging sensor without compromising the wafer thickness.
Implementation Method 1
a silicided polysilicon light reflecting layer formed on the frontside of the thin insulator layer... reflects unabsorbed light back into the photodiode
Data Source
AI summary
A backside illuminated imaging sensor includes a semiconductor having an imaging pixel that can include a photodiode region, an insulation layer, and a reflective layer. The photodiode is typically formed in the frontside of the semiconductor substrate. A surface shield layer can be formed on the frontside of the photodiode region. A light reflecting layer can be formed using silicided polysilicon on the frontside of the sensor. The photodiode region receives light from the back surface of the semiconductor substrate. When a portion of the received light propagates through the photodiode region to the light reflecting layer, the light reflecting layer reflects the portion of light received from the photodiode region towards the photodiode region. The silicided polysilicon light reflecting layer also forms a gate of a transistor for establishing a conductive channel between the photodiode region and a floating drain.