LED Substrate Removal Tolerance via Wafer Etching
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Solution Overview
Problem
Existing techniques for flip-chip LEDs face challenges in accurately affixing optical elements, such as phosphor plates, due to misalignment caused by raised underfill walls after substrate removal, leading to suboptimal light emission properties and potential delamination.
Innovation Solution
An additional etching step is performed on the LED/substrate wafer to create gaps between LEDs, allowing for an 'enlarged' substrate with extended edges, which relaxes the tolerance for placing optical elements and ensures proper alignment and adhesion during the affixing process.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Illumination intensity
If the substrate is removed after flip-chip bonding, then the light extraction efficiency is improved, but the alignment precision for affixing optical elements deteriorates due to raised underfill walls
Solution Approach 1:
The patent performs the etching step to create gaps between LEDs on the wafer before singulation and mounting. This preliminary action ensures that when the substrate is later removed, the underfill does not form raised walls that would interfere with optical element alignment. The gaps are created while the LEDs are still on the wafer, allowing precise control before the final mounting step.
Solution Approach 2:
The patent segments the wafer into individual LED units with gaps between them through etching. This segmentation creates isolated regions for each LED that prevent the underfill from forming continuous raised walls across the substrate surface, thereby improving alignment precision for optical elements after substrate removal.
2Strength
If the underfill is deposited to cover the sides of the substrate, then structural support is improved, but the tolerance for positioning optical elements deteriorates
Solution Approach 1:
The etching step creating gaps between LEDs is performed before underfill deposition and substrate removal. This preliminary action anticipates the problem of raised underfill walls and prevents it by creating physical separations, thereby maintaining both structural support and positioning tolerance.
Solution Approach 2:
The patent applies different properties to different regions: the gaps between LEDs provide local separation zones where underfill cannot form raised walls, while the areas under each LED maintain full underfill coverage for structural support. This local differentiation resolves the contradiction between support and tolerance.
3Device complexity
If the phosphor plate is affixed directly to the LED surface without gap compensation, then the device complexity is reduced, but the light emission uniformity deteriorates due to misalignment
Solution Approach 1:
The gaps are created in advance during wafer processing, before mounting and optical element affixing. This preliminary preparation eliminates the need for complex alignment mechanisms or procedures, maintaining simple fabrication while ensuring uniform light emission through proper spacing.
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 provides increased tolerance and uniform light emission by allowing optical elements to be positioned flush and securely on the LED surface, enhancing the light color consistency and preventing delamination.
Implementation Method 1
The photon energy of the laser (e.g., an excimer laser) is selected to be above the band gap of the LED material and below the absorption edge of the sapphire substrate
Implementation Method 2
Pulses from the laser through the sapphire are converted to thermal energy within the first 100nm of the LED material. The generated temperature is in excess of 1000°C and dissociates the gallium and nitrogen
Implementation Method 3
The underfill 30 may be liquid silicone that is then cured to harden
Data Source
AI summary
An etching step is performed on an LED/substrate wafer to etch through the LED epitaxial layers entirely around each LED on the substrate wafer to form a gap between each LED on the wafer. The substrate is not etched. When the LEDs/substrates are singulated, edges of each substrate extend beyond edges of the LED die. The LEDs are flip-chips and are mounted on a submount with the LED die between the submount and the substrate. An insulating underfill material is injected under the LED die and also covers the sides of the LED die and "enlarged" substrate. The substrate is then removed by laser lift-off. The raised walls of the underfill that were along the edges of the enlarged substrate are laterally spaced from the edges of the LED die so that a phosphor plate can be easily positioned on top to the LED die with a relaxed positioning tolerance.