Grating Photo Detector Lateral Absorption SOI CMOS
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Solution Overview
Problem
The implementation of photo detectors in CMOS integrated circuits is challenging due to narrow depletion regions and thin active silicon layers in silicon on insulator (SOI) processes, resulting in low responsivity and high junction capacitances, which limit frequency response and absorption of photons.
Innovation Solution
A photo detector design featuring a grating of grating material on top of an active semiconductor layer with positively and negatively doped regions, arranged in parallel strips, allowing for increased absorption length and efficient photon capture, independent of the active layer thickness, and enabling integration in CMOS and SOI manufacturing processes.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If high doping levels are used in pn-junctions to achieve proper electrical function, then the photo detector can be manufactured in CMOS process, but the depletion region becomes very narrow (50-100 nm) resulting in small absorption volume and low responsivity
Solution Approach 1:
The patent extends the absorption path from the vertical dimension (through the thin depletion region) to the lateral dimension by using a meandering waveguide structure. The light propagates horizontally through the active layer multiple times, effectively increasing the absorption length without requiring a thicker depletion region. This dimensional transformation allows the thin-film structure to achieve high absorption efficiency while maintaining compatibility with standard CMOS processes.
Solution Approach 2:
The patent introduces a grating structure at the input surface that performs preliminary diffraction of incident light before it enters the waveguide. This pre-diffraction distributes the light into multiple propagation modes within the waveguide, increasing the interaction length between light and the active layer. By preparing the light distribution in advance, the system achieves enhanced absorption without requiring increased doping levels or thicker depletion regions.
2Device complexity
If the active silicon layer thickness is reduced to 70 nm for SOI CMOS process, then integration is improved, but the layer becomes virtually transparent for light at 850 nm wavelength (absorption length is 15 μm) resulting in poor photon absorption
Solution Approach 1:
The patent transforms the absorption mechanism from vertical (through-layer) to lateral (along the waveguide). The meandering waveguide structure allows light to travel an extended horizontal path through the thin active layer, accumulating absorption over distance rather than requiring thickness. This enables the use of ultra-thin 70 nm SOI layers while maintaining high absorption efficiency through the extended optical path length.
Solution Approach 2:
The patent employs a meandering waveguide geometry that dynamically extends the optical path length within the thin active layer. Instead of a straight path, the waveguide follows a serpentine route, allowing light to interact with the active material multiple times across different locations. This dynamic path extension achieves effective absorption in thin layers without increasing physical thickness or complexity of the SOI structure.
3Ease of manufacture
If the depletion region is made narrow for high doping levels, then manufacturing is simplified, but junction capacitance increases severely limiting frequency response
Solution Approach 1:
The patent replaces the traditional vertical pn-junction detection mechanism with a lateral waveguide-based absorption mechanism. Instead of relying on carrier generation in a vertical depletion region, the system uses direct optical absorption in the active layer followed by lateral charge collection. This substitution eliminates the fundamental link between doping level and depletion region width, allowing high doping for manufacturing ease without the penalty of narrow depletion regions and high capacitance.
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 achieves high responsivity and efficient photon absorption, capturing both s- and p-polarizations, and is suitable for high-speed optical chip-to-chip communication with a simple structure and low manufacturing complexity, even with thin active semiconductor layers.
Implementation Method 1
The grating has the function of a diffraction grating that bends incident electromagnetic waves into the active semiconductor layer
Implementation Method 2
Silicon has an absorption length of roughly 15 μm for light with a wavelength of 850 nm. As a result, 1/e of the light is absorbed after 15 μm in silicon
Data Source
AI summary
A photo detector comprising a grating (PC). The grating (PC) is arranged on top of a surface of an active semiconductor layer. The grating (PC) is patterned in uninterrupted first strips (ST1), that are arranged in a first direction (x) in a first predetermined interval (a), and second strips (ST2), that are arranged in a second direction (y) in a second predetermined interval (b). The second strips (ST2) each comprise at least one interruption in a region between each two neighboring first strips (ST1) in form of a predetermined gap (d). Positively doped regions (P) and negatively doped regions (N) each are arranged as strips in parallel with the first strips (ST1) such that in a region between each two neighboring first strips (ST1) alternately either one of the positively doped regions (P) or one of the negatively doped regions (N) is arranged.