Photodiode Thin-Film Phase Modulation for NIR Light Absorption
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Solution Overview
Problem
Silicon-based CMOS image sensors have insufficient sensitivity and low quantum efficiency for near-infrared wavelengths due to low light absorption, with most incident power passing through without being absorbed, necessitating improved photon management techniques to reach or exceed 50% quantum efficiency.
Innovation Solution
The integration of phase-modulating thin-film optics, including metasurfaces and nanostructures, which provide a unidirectional π phase shift to enhance light absorption within the photodiode, utilizing materials with a refractive index less than that of the photodiode and reflecting light back towards the photodiode to increase absorption, particularly effective for near-infrared wavelengths.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the thickness of Si is increased to improve quantum efficiency, then light absorption is enhanced, but manufacturing cost increases, jitter time increases, and dark-noise issues worsen
Solution Approach 1:
The patent changes the optical parameters of the system by introducing a thin-film layer with specific refractive index properties (lower than silicon) and controlled thickness (lambda/4 to lambda/2). This modifies the optical path and phase relationships without changing the physical thickness of the silicon absorber, thereby improving quantum efficiency while avoiding the drawbacks of thicker silicon structures
Solution Approach 2:
The thin-film layer acts as an intermediary optical element between the incident light and the silicon photodiode. It provides phase modulation and optical path control, enabling enhanced light absorption in the silicon without requiring increased silicon thickness, thus resolving the contradiction between quantum efficiency and manufacturing complexity
2Reliability
If nanostructures are used to scatter incident NIR light to increase path length, then light absorption is improved, but the scattering angle is limited by the low refractive index and small size of nanostructures
Solution Approach 1:
The patent replaces the mechanical/scattering-based approach (relying on physical nanostructure geometry to scatter light) with an optical phase-modulation approach. The thin-film layer uses optical interference and phase shift mechanisms to redirect and trap light within the silicon, achieving superior light absorption without being constrained by the scattering angle limitations of physical nanostructures
3Reliability
If previous metaphotonics techniques are used to bend incident NIR light at high angles, then light absorption is increased, but the implementation complexity increases
Solution Approach 1:
The patent simplifies the implementation by using a single thin-film layer with specific optical parameters (refractive index lower than silicon, thickness of lambda/4 to lambda/2) instead of complex metaphotic structures. This parameter-based approach achieves high-angle light bending and enhanced absorption through optical interference while maintaining CMOS compatibility and reducing fabrication complexity
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 significantly improves quantum efficiency and light absorption in near-infrared CMOS image sensors, providing a cost-effective and practical solution compatible with CMOS technology, enhancing performance in low-light applications.
Implementation Method 1
The thin-film layer may be on the first side of the photodiode and may provide a unidirectional phase-shift to light passing from the photodiode to the thin-film layer
Implementation Method 2
The reflective layer may be on the second side of the photodiode and may reflect light passing from the photodiode to the reflective layer toward the first side of the photodiode
Implementation Method 3
Silicon-based CIS may have insufficient sensitivity and low Quantum Efficiency (QE) (At Near Infrared (NIR) wavelengths, light absorption by silicon is low
Data Source
Figure 1A
Figure 1B
Figure 2~3
AI summary
A pixel for an image sensor is disclosed that includes a photodiode, a thin-film layer and a reflective layer. The photodiode includes a first side and a second side that is opposite the first side, and receives incident light on the first side. The thin-film layer is formed on the first side of the photodiode and provides a unidirectional phase-shift to light passing from the photodiode to the thin-film layer. The thin-film layer has a refractive index that less than a refractive index of material forming the photodiode. The unidirectional phase-shift may be a unidirectional π phase shift at a target near-infrared light wavelength. The reflective layer is formed on the second side of the photodiode and reflects light passing from the photodiode to the reflective layer toward the first side of the photodiode. The reflective layer may be a thin-film layer, a Distributed Bragg Reflector layer, or a metal.