Diffractive Lenses for Image Sensors to Reduce Optical Cross-Talk
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Solution Overview
Problem
Conventional image sensors with curved microlenses suffer from optical cross-talk due to peripheral light passing through without being focused, leading to suboptimal image quality.
Innovation Solution
The use of diffractive lenses with varying refractive indices and thicknesses over each pixel in an image sensor to redirect incident light effectively, focusing it onto the photodiodes and minimizing cross-talk.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional curved microlenses are used to focus light, then light focusing capability is provided, but peripheral light passes through without being focused causing optical cross-talk
Solution Approach 1:
The patent changes the fundamental optical parameter from refraction to diffraction by using diffractive lenses instead of conventional refractive microlenses. The diffractive lens uses a periodic structure with varying thickness to diffract light, where the diffraction angle is determined by the grating period and wavelength. This parameter change enables the lens to focus light through diffraction rather than refraction, effectively addressing the optical cross-talk issue by redirecting peripheral light that would otherwise pass through unfocused.
Solution Approach 2:
The patent replaces the mechanical/refractive focusing mechanism with an optical diffraction mechanism. Instead of relying on the curved surface geometry to refract and focus light, the diffractive lens uses a periodic modulation of the optical path length to create constructive interference at the focal point. This substitution of the focusing mechanism fundamentally changes how light is directed, eliminating the limitation of peripheral light passing through without focusing.
2Manufacturing precision
If diffractive lenses with varying refractive indices and thicknesses are used, then light focusing accuracy is improved, but device complexity increases
Solution Approach 1:
The diffractive lens is segmented into multiple zones or rings with different thicknesses, where each zone contributes to focusing light from specific angular ranges. The lens structure is divided into concentric annular regions with varying optical path lengths, allowing independent optimization of each zone's diffraction characteristics. This segmentation enables precise control over light focusing while maintaining a relatively simple overall lens geometry that can be fabricated using standard semiconductor processing techniques.
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 enhances image quality by ensuring that incident light is focused accurately onto the photodiodes, reducing optical cross-talk and improving the overall performance of image sensors.
Implementation Method 1
The use of diffractive lenses with varying refractive indices and thicknesses over each pixel in an image sensor to redirect incident light effectively, focusing it onto the photodiodes
Implementation Method 2
Each image pixel in the array includes a photodiode that is coupled to a floating diffusion region via a transfer gate. Each pixel receives incident photons (light) and converts the photons into electrical signals.
Implementation Method 3
The microlenses of conventional image sensors typically have curved surfaces and use refraction to focus light on an underlying photodiode.
Data Source
AI summary
An image sensor may include an array of imaging pixels. Each imaging pixel may have a photosensitive area that is covered by a respective diffractive lens to focus light onto the photosensitive area. The diffractive lenses may have a higher index of refraction than the surrounding materials. The diffractive lenses may be formed on an upper or lower surface of a planarization layer or may be embedded within the planarization layer. In some cases, multiple diffractive lenses may be formed over the imaging pixels. Some of the multiple diffractive lenses may have refractive indexes lower than the planarization layer such that the diffractive lenses defocus light. Focusing and defocusing diffractive lenses may be used to tune the response of the imaging pixels to incident light.