Backside Illumination Demodulation Pixel for 3D Time-of-Flight Imaging
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Solution Overview
Problem
Three-dimensional (3D) time-of-flight (TOF) cameras face challenges in achieving high pixel sensitivity and quantum efficiency due to low fill factor and inefficient photon collection, especially with near-infrared light, which penetrates deeply into the substrate, and requires fast electron transfer to storage nodes.
Innovation Solution
The implementation of backside illumination demodulation sensors with a photo-sensitive area, a demodulation area, a switching element, and a charge barrier to prevent electron leakage, along with micro-lenses to direct photons effectively, enhances fill factor and quantum efficiency by separating the sensitive and storage areas and utilizing built-in drift fields for electron transfer.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If photons are received in the photo-sensitive area and transferred through the substrate, then near-infrared photons can be detected, but electrons generated deep in the substrate take too long to reach storage nodes
Solution Approach 1:
The pixel structure is segmented into distinct functional regions: a photo-sensitive area for photon absorption, a demodulation area for electron transfer control, and separate storage nodes. This segmentation allows optimized electron transfer paths from deep substrate regions to storage nodes, improving both quantum efficiency for near-infrared photons and electron transfer speed.
Solution Approach 2:
The patent implements backside illumination, changing the photon entry direction from the front to the back of the substrate. This dimensional change allows photons to enter directly into the substrate where they generate electrons, and enables the use of drift fields applied through the demodulation area to rapidly extract electrons from deep regions before they diffuse slowly to storage nodes.
2Measurement precision
If backside illumination is used to achieve 100% fill factor, then photon collection efficiency improves, but electron leakage from sensitive area to storage areas occurs
Solution Approach 1:
A demodulation area is introduced as an intermediary region between the photo-sensitive area and storage nodes. This intermediate zone contains drift field control electrodes that actively manage electron transport, preventing direct leakage from the sensitive area to storage areas while maintaining the benefits of backside illumination and high fill factor.
3Measurement precision
If micro-lenses are used to direct photons, then photon collection efficiency improves, but the layer stack complexity increases
Solution Approach 1:
Instead of adding complex micro-lens structures on the front surface to focus photons, the patent inverts the approach by using backside illumination where photons enter directly through the back surface. This eliminates the need for complex front-side optical structures while achieving high photon collection efficiency, and the drift field control in the demodulation area provides the necessary directional guidance.
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 increases pixel sensitivity to up to 100% fill factor and improves quantum efficiency for long wavelengths, enabling faster and more efficient capture of photo-generated charges deep in the silicon, particularly effective for near-infrared illumination.
Implementation Method 1
the incoming photons generate charge carriers, meaning electron-hole pairs within the substrate
Implementation Method 2
a charge barrier to prevent said electrons from flowing from said sensitive area to said storage areas
Implementation Method 3
a switching element that directs said electrons from said demodulation area to alternative storage areas
Implementation Method 4
utilizing built-in drift fields for electron transfer
Data Source
AI summary
A high-speed, high-sensitivity demodulation sensor usable for e.g. time-of-flight application uses a back side illuminated (BSI) image sensor chip, in which the photo-generated charges are first transferred to a demodulation area, from which the charges are then sampled and stored on at least one specific storage node. The storage node is electrically isolated from the sensitive area. Such a pixel might find its use specifically in 3D time-of-flight imaging given its improvements in sensitivity because the presented invention allows to design pixel with up to 100% fill factor and enables charge detection even if the charge generation by the photon occurs deep in the silicon substrate.