Silicon Pixel Optics for Longer Near-IR Photodiode Light Paths
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Solution Overview
Problem
The quantum efficiency of existing silicon photodiodes in integrated image sensors is low, particularly for near-infrared wavelengths, due to poor light absorption and absorption pathlength in the photodiode.
Innovation Solution
A pixel design incorporating a silicon photodiode with a diffractive structure and layers of materials with lower refractive indices to reflect and diffract light, increasing the light pathlength within the photodiode, comprising a substrate with a photodiode portion extending vertically, covered by layers of dielectric materials and metallic layers, and a diffractive structure on the side of the substrate that diffracts light into two orthogonal directions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a conventional silicon photodiode is used without additional optical structures, then the device complexity is low, but the quantum efficiency for near-infrared wavelengths is low
Solution Approach 1:
The patent introduces intermediary optical structures (diffractive structure and light confinement layers) between the incident light and the photodiode to enhance light absorption. The diffractive structure acts as an intermediary to redirect light into the photodiode, while the light confinement layers serve as intermediaries to trap and extend the light path within the active region, thereby improving quantum efficiency without requiring fundamental changes to the photodiode itself.
Solution Approach 2:
The patent employs a diffractive structure that operates in the spatial domain to redirect light propagation directions. By introducing lateral dimensionality through diffraction gratings or similar structures, light is redirected at oblique angles into the photodiode, increasing the optical path length and absorption probability without increasing the vertical thickness of the photodiode.
2Reliability
If the photodiode thickness is increased to improve light absorption, then the quantum efficiency improves, but the absorption pathlength for near-infrared wavelengths remains insufficient
Solution Approach 1:
The patent uses diffractive structures to create curved or angled light paths within the photodiode instead of straight vertical propagation. The diffracted light travels along oblique paths, effectively increasing the absorption pathlength through the active region without requiring increased photodiode thickness. This curved trajectory approach mimics the effect of increased thickness while maintaining a compact structure.
Solution Approach 2:
By introducing lateral light redirection through diffractive structures, the patent extends the light absorption path in the lateral dimension. Light that would otherwise pass through the photodiode in a straight vertical line is redirected to traverse a longer, angled path through the active region, effectively increasing the absorption pathlength without increasing the vertical dimension.
3Length of stationary object
If light confinement layers with lower refractive index are added to reflect light, then the light pathlength increases, but the device complexity increases
Solution Approach 1:
The patent utilizes changes in refractive index parameters by introducing light confinement layers with lower refractive index than the photodiode material. This parameter change creates optical total internal reflection at the interfaces, effectively trapping light within the photodiode and extending its absorption pathlength. The refractive index contrast serves as a simple yet effective mechanism to achieve light confinement without complex structures.
4Reliability
If a diffractive structure is added to redirect light, then the quantum efficiency improves, but the manufacturing precision requirements increase
Solution Approach 1:
The diffractive structure can be segmented into discrete elements such as diffraction grating lines or zones. This segmentation allows the complex diffractive function to be broken down into simpler, repeatable manufacturing units that can be produced using standard semiconductor fabrication techniques like photolithography and etching, thereby reducing the overall manufacturing precision requirements despite the functional 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
The design significantly enhances quantum efficiency by increasing light pathlength within the photodiode, improving charge collection efficiency by over twice that of similar pixels without the diffractive structure and light confinement layers.
Implementation Method 1
a diffractive structure disposed on a face of the photodiode on the side of the second face of the substrate
Implementation Method 2
each of the first and second materials is configured so that light at an operating wavelength of the pixel reaching an interface between the photodiode and said material with an angle of incidence greater than 30° is fully reflected
Data Source
AI summary
The present disclosure relates to a pixel comprising: a photodiode comprising a portion of a substrate of a semiconductor material, extending vertically from a first face of the substrate to a second face of the substrate configured to receive light; a layer of a first material covering each of the lateral surfaces of the portion; a layer of a second material covering the portion on the side of the first face, first and second material having refractive indexes lower than that of the semiconductor material; and a diffractive structure disposed on a face of the photodiode on the side of the second face.