Series-Connected Flip Chip LEDs with Removed Sapphire Substrate
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Solution Overview
Problem
There is a need for an efficient technique to form series-connected light emitting diodes (LEDs) in a compact structure that can reliably undergo substrate removal and etching processes to increase light extraction efficiency.
Innovation Solution
A wafer-scale process is developed to form serially connected LEDs on a single submount, using a semi-insulating GaN layer for mechanical support and electrical biasing during the substrate removal and etching processes, which includes trenching to isolate LEDs, metal interconnections, and optional photo-electrochemical etching to enhance light extraction.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If individual LEDs are mounted and interconnected on a circuit board to achieve series connection, then the LEDs can be connected in series configuration, but the light emitter becomes relatively large
Solution Approach 1:
Multiple LEDs are integrated onto a single semiconductor chip substrate, merging what would traditionally be separate discrete components into one unified device. The chip contains multiple LED structures with shared electrical connections, eliminating the need for separate mounting and interconnection of individual LEDs on a circuit board.
Solution Approach 2:
Multiple LED structures are nested within the confines of a single chip substrate. The patent describes a chip containing multiple LED structures that share common electrical connections, effectively nesting multiple functional units within one compact package.
2Illumination intensity
If the sapphire growth substrate is removed to increase light extraction efficiency, then light extraction is improved, but the LED layers become mechanically vulnerable and may break
Solution Approach 1:
A suspended membrane structure acts as an intermediary between the removed sapphire substrate and the LED layers. This membrane provides mechanical support and prevents breakage of the thin LED layers while allowing light to pass through, effectively mediating between the need for substrate removal and the need for mechanical strength.
Solution Approach 2:
A thin flexible membrane is introduced to provide mechanical support to the LED layers after substrate removal. This thin film maintains the structural integrity of the device while being optically transparent, allowing light extraction to proceed effectively.
3Illumination intensity
If the exposed GaN layer is etched to thin the layer and create a roughened surface to increase light extraction, then light extraction efficiency is improved, but the structural integrity may be compromised
Solution Approach 1:
The etching process is applied partially - only to the extent needed to create sufficient surface roughness for enhanced light extraction. The membrane structure provides the additional mechanical strength needed, allowing the etching to be performed without compromising overall structural integrity.
4Reliability
If a thick semi-insulating GaN layer is used to provide mechanical support during substrate removal, then mechanical support is improved, but the layer resistivity must be precisely controlled to prevent electrical shorting
Solution Approach 1:
The semi-insulating GaN layer serves multiple functions simultaneously: it provides mechanical support during substrate removal, acts as an electrical isolation layer to prevent shorting between LEDs, and serves as a structural foundation for the LED structures. This multi-functionality reduces the need for separate components.
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 method allows for the creation of compact, reliable LED structures that improve light extraction efficiency and mechanical support, enabling the formation of interconnected LEDs that can be efficiently connected in series and parallel configurations.
Implementation Method 1
The thick SI-GaN layer mechanically supports the thin N-GaN, active, and P-GaN layers during the lift-off process to prevent breakage
Implementation Method 2
The laser causes the top layer of GaN to release a gas to push off the sapphire substrate from the GaN layer
Implementation Method 3
A PEC etch involves electrically biasing the material to be etched, immersing the material in a base solution, and exposing the material to UV light
Data Source
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AI summary
LED layers (18-22) are grown over a sapphire substrate. Individual flip chip LEDs are formed by trenching or masked ion implantation. Modules containing a plurality of LEDs are diced and mounted on a submount wafer (44). A submount metal pattern or a metal pattern formed on the LEDs connects the LEDs in a module in series. The growth substrate is then removed, such as by laser lift-off. A semi-insulating layer is formed, prior to or after mounting, that mechanically connects the LEDs together. The semi-insulating layer may be formed by ion implantation of a layer between the substrate and the LED layers. PEC etching of the semi-insulating layer, exposed after substrate removal, may be performed by biasing the semi-insulating layer. The submount is then diced to create LED modules containing series-connected LEDs.