Polysilicon Gate-Integrated Optical Filter for IR Imager Noise Reduction
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Solution Overview
Problem
Avalanche photodiodes used in imagers for infrared detection suffer from low quantum efficiency and poor signal-to-noise ratio due to high detection of visible radiation, which existing filters do not adequately address.
Innovation Solution
An optical filter is integrated into the integrated circuit with an electrically conductive filter layer having a high product of thickness and imaginary refractive index, positioned above or below the photosensitive zone, connected to the interconnection part to block visible radiation and improve electrical behavior.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If existing filters are used to block visible radiation, then visible radiation detection is reduced, but the filters are not efficient enough and require additional manufacturing steps
Solution Approach 1:
The patent merges the optical filter function with the transistor gate structure by using the same polycrystalline silicon layer for both purposes. The filter layer is integrated into the existing gate formation process, eliminating the need for separate filter manufacturing steps while achieving effective visible radiation blocking.
Solution Approach 2:
The polycrystalline silicon layer serves multiple functions simultaneously: it acts as both the transistor gate electrode and the optical filter. This multi-functional design blocks visible radiation while maintaining electrical control of the photodiode, resolving the contradiction between filtering efficiency and manufacturing simplicity.
2Reliability
If an optical filter layer is added to block visible radiation, then signal-to-noise ratio is improved, but manufacturing complexity increases
Solution Approach 1:
The filter layer is combined with the gate structure, using the same polycrystalline silicon material and formation process. This integration improves signal-to-noise ratio by blocking visible radiation while avoiding additional device complexity through process consolidation.
3Object-affected harmful factors
If the filter layer thickness is increased to improve filtering efficiency, then visible radiation blocking is enhanced, but quantum efficiency for infrared detection may be affected
Solution Approach 1:
The patent optimizes the filter layer thickness parameter to achieve the right balance: a thickness of 180 nm provides sufficient visible radiation blocking while maintaining transparency for infrared wavelengths. The product of thickness and imaginary refractive index is controlled to be greater than 1 nm for effective filtering without compromising infrared detection efficiency.
4Object-affected harmful factors
If polycrystalline silicon is used for the filter layer, then visible radiation is blocked effectively, but the layer cannot be polarized without additional contacts
Solution Approach 1:
The gate contact serves dual purposes: providing electrical control for the photodiode and enabling polarization of the polycrystalline silicon filter layer. This eliminates the need for additional polarization contacts while achieving both electrical functionality and optical filtering.
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 effectively enhances the signal-to-noise ratio by blocking visible radiation, reducing dark count rates and dark current while maintaining quantum efficiency, without requiring additional manufacturing steps.
Implementation Method 1
an optical filter is integrated into the integrated circuit with an electrically conductive filter layer having a high product of thickness and imaginary refractive index, positioned above or below the photosensitive zone, connected to the interconnection part to block visible radiation
Implementation Method 2
said electrically conductive filter layer is connected to said interconnection part by at least one contact suitable for polarizing the filter layer... to improve the electrical behavior of the detection device which comprises the photosensitive zone. It will thus be possible in particular to improve the diffusion of electrons in the substrate
Data Source
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AI summary
A method for implementing an optical filter within an integrated circuit comprising a substrate and an interconnection portion (ITC) above the substrate, and a corresponding integrated circuit. The implementation of the filter includes forming, above a photosensitive area (ZP) located in the substrate, at least one filter layer (CF), the product of the thickness of the filter layer and the imaginary part of the refractive index of the filter layer being greater than 1, said at least one filter layer being located between the substrate and the interconnection portion (ITC).