Small-Pitch Micro-LED Bonding With Post-Bonded Electrodes
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Solution Overview
Problem
The challenge in fabricating micro-LED display devices lies in precisely aligning metal bonding pads on small-pitch micro-LED arrays with pixel drive circuits for reliable bonding, which is difficult due to the small dimensions and high sensitivity to misalignment, leading to issues with contact resistance and metal diffusion, and the need for precise annealing and alignment in hybrid bonding processes.
Innovation Solution
A method involving the formation of mesa structures on a micro-LED wafer, alignment error-tolerant bonding to a backplane wafer, and post-bonding processing to form individual electrodes and light extraction structures, using distributed Bragg reflectors instead of metal back reflectors to enhance light extraction efficiency and reduce the need for precise alignment.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If metal bonding pads are used for bonding micro-LED arrays to pixel drive circuits, then electrical connection is established, but precise alignment is required which increases manufacturing difficulty and reduces yield
Solution Approach 1:
The patent extracts the alignment-critical metal bonding pads from the bonding interface and replaces them with alignment-tolerant oxide bonding pads. This allows the metal electrodes to be formed after bonding, eliminating the need for precise pre-alignment of metal pads while maintaining reliable electrical connection through the oxide interface.
Solution Approach 2:
The patent performs preliminary bonding using oxide pads before forming the metal electrodes. This preliminary action establishes the bonded structure with alignment tolerance, and the metal electrodes are subsequently formed through the oxide layer to create the final electrical connection, avoiding the need for precise initial metal pad alignment.
2Strength
If annealing is applied to metal bonding pads to improve bonding, then bonding strength increases, but metal diffusion occurs which degrades device performance
Solution Approach 1:
The patent introduces an oxide layer as an intermediary bonding pad material between the bonding interfaces. This oxide intermediary allows for annealing to improve bonding strength while preventing direct metal-to-metal contact, thereby eliminating metal diffusion issues that would otherwise occur during the annealing process.
3Productivity
If small-pitch micro-LED arrays are fabricated to increase resolution, then packing density increases, but alignment sensitivity increases making bonding more difficult
Solution Approach 1:
The patent extracts the alignment-critical metal bonding pads from the small-pitch micro-LED array structure and replaces them with oxide bonding pads that have larger effective bonding area and higher alignment tolerance. This enables small-pitch arrays to be bonded successfully without requiring proportionally reduced bonding pad sizes, maintaining both high packing density and manufacturability.
4Measurement precision
If precise alignment is performed during bonding, then bonding accuracy improves, but manufacturing complexity and process time increase
Solution Approach 1:
The patent uses oxide bonding pads that can be formed using standard semiconductor processing techniques without requiring complex alignment equipment or procedures. The oxide pads serve as a disposable alignment-tolerant interface that simplifies the bonding process, eliminating the need for expensive and complex precise alignment systems while maintaining adequate bonding accuracy.
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 improves yield and reliability by eliminating the need for precise alignment and annealing, reduces metal diffusion, and significantly increases light extraction efficiency for small micro-LEDs, enabling the production of high-intensity, directionally focused light with improved quantum efficiency.
Implementation Method 1
using distributed Bragg reflectors instead of metal back reflectors to enhance light extraction efficiency
Implementation Method 2
post-bonding processing (e.g., from n-side) to form micro-LED electrodes and other features (e.g., light extraction structures)
Data Source
AI summary
A micro-LED device and a method of fabricating the micro-LED device are disclosed. The method includes processing from a first side (e.g., p-side) of epitaxial layers of a micro-LED wafer to form individual mesa structures and a first solid metal bonding layer on the mesa structures, bonding a second solid metal bonding layer on a backplane wafer to the first solid metal bonding layer of the micro-LED wafer, removing the substrate of the micro-LED wafer and processing from a second side (e.g., n-side) of the epitaxial layers to isolate the solid metal bonding layers and form individual electrodes (e.g., anodes) for individual micro-LEDs, forming a dielectric material layer on surfaces in regions between the mesa structures, and depositing one or more metal materials in the regions between the mesa structures to form mesa sidewall reflectors and a common electrode for the micro-LEDs.