Backside-Electrode SPAD Fabrication for Uniform Contact Resistance
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Solution Overview
Problem
Existing single-photon avalanche diodes (SPADs) with electrodes on the front and backside of the substrate suffer from non-uniform contact resistance and inconsistent performance due to substrate thickness variations, affecting pixel density and uniformity in advanced detectors.
Innovation Solution
A method involving the removal of the substrate and forming the second electrode on the backside of the epitaxial layer through ion implantation, ensuring uniform thickness and dopant concentration, with trench isolation structures to enhance uniformity and performance consistency.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Length of stationary object
If the substrate is thinned using a regular grinding process, then the substrate thickness is reduced, but the substrate thickness uniformity deteriorates
Solution Approach 1:
The patent removes the substrate entirely from the final device structure. The semiconductor layer is separated from the substrate through selective removal processes (mechanical polishing followed by wet etching), eliminating the substrate thickness uniformity problem since the active device is formed only in the epitaxial layer on the substrate surface, not in the substrate itself.
Solution Approach 2:
The patent divides the semiconductor structure into distinct segments: the substrate serves only as a temporary support during manufacturing, the epitaxial layer contains the active device structure, and the substrate is completely removed after device formation. This segmentation allows the substrate to be optimized for mechanical support while the epitaxial layer is optimized for device performance.
2Productivity
If two electrodes are arranged on the front side and backside of the substrate, then pixel density can be increased, but contact resistance uniformity deteriorates due to substrate thickness variations
Solution Approach 1:
The substrate is extracted/removed from the final device structure. By completely removing the substrate after device formation, the patent eliminates the source of thickness variations that cause non-uniform contact resistance, while still allowing the backside electrode to be formed on the backside of the epitaxial layer for high pixel density.
Solution Approach 2:
The patent changes the reference plane for electrode formation from the substrate to the epitaxial layer. The backside electrode is formed on the backside surface of the epitaxial layer (which has uniform thickness) rather than on the substrate surface (which has non-uniform thickness after thinning), thereby achieving both high pixel density and uniform contact resistance.
3Ease of manufacture
If both electrodes are arranged on the front side of the substrate, then contact region isolation is simplified, but area shrinkage is limited due to spacing requirements
Solution Approach 1:
The patent utilizes the third dimension (front and back sides) of the epitaxial layer for electrode placement. The first electrode is on the front side and the second electrode is on the back side, allowing maximum area shrinkage while maintaining proper electrical isolation through the epitaxial layer structure and depletion region formation.
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 method achieves improved uniformity and consistent performance of single-photon detectors by ensuring uniform photon absorption and travel distance, along with adjustable contact resistance, facilitating higher pixel density and efficient photon detection.
Implementation Method 1
based on a reverse bias voltage higher than a breakdown voltage of a p-n junction therein. The p-n junction in the SPAD is reversely biased at a voltage higher than its breakdown voltage, allowing an avalanche current to develop due to the internal photoelectric effect (the liberation of electrons or another species of current carriers from a material stricken by photons)
Implementation Method 2
allowing an avalanche current to develop due to the internal photoelectric effect
Implementation Method 3
implanting ions of the first doping type to the epitaxial layer from its backside to form a contact region for a second electrode
Data Source
AI summary
A method of manufacturing single-photon detector includes forming a first electrode on a front side of a substrate, removing the substrate and preforming ion implantation on a backside of an epitaxial layer to form a contact region for a second electrode, which extends from the surface of the epitaxial layer to a first predetermined depth within the epitaxial layer. The second electrode is be electrically connected to the contact region for the second electrode. Since the substrate is removed, the epitaxial layer, which is provided as a semiconductor layer, has a uniform thickness. The contact region for the second electrode has a uniform thickness, and its dopant concentration is easy to control and adjust. Thus, the second electrode can be formed so as to have uniform contact resistance across its different regions.