Gallium Pick and Place for Micro-LED Transfer
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Solution Overview
Problem
Conventional pick and place techniques are unsuitable for transferring very small light emitting diodes (LEDs) due to their small size, requiring innovative methods to attach and detach LEDs from pick-up tools without damaging them or using additional adhesive materials.
Innovation Solution
The method employs a laser lift-off process to detach gallium semiconductor layers from substrates, utilizing the naturally formed gallium layer for attachment and detachment from a pick-up tool, eliminating the need for additional adhesive materials and minimizing risk of damage due to temperature exposure.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If conventional pick and place techniques are used for transferring very small LEDs, then the process is simple and straightforward, but the techniques are unsuitable due to the small size of LEDs requiring innovative attachment methods
Solution Approach 1:
The gallium layer that is naturally present on the LED surface during laser lift-off is utilized as the bonding agent for attachment to the pick-up tool. The LED's own gallium layer serves the dual purpose of both separation from substrate and bonding to transfer tool, eliminating the need for separate adhesive materials and simplifying the overall process while enabling transfer of very small LEDs
Solution Approach 2:
The bonding state of the gallium layer is controlled by changing temperature parameters. Heating above gallium's melting point (29.76°C) creates a liquid state for bonding to the pick-up tool, while cooling below this temperature solidifies the gallium for secure attachment. This parameter change enables reliable pick and place of very small LEDs without complex mechanical gripping systems
2Reliability
If additional adhesive materials are used to attach LEDs to pick-up tools, then attachment is achieved, but additional materials increase process complexity and potential damage risk
Solution Approach 1:
The gallium layer inherently present on the LED surface is used as the bonding agent instead of applying separate adhesive materials. This self-service approach uses the LED's own material properties to achieve reliable attachment, reducing the number of materials required and simplifying the process while maintaining attachment reliability through controlled phase changes of the gallium
Solution Approach 2:
The function of adhesive bonding is extracted from a separate material application step and integrated into the existing gallium layer on the LED surface. By taking out the need for additional adhesives and utilizing the naturally present gallium, the process complexity is reduced while attachment reliability is maintained through thermal control of the gallium phase
3Ease of operation
If high temperatures are used to melt gallium for bonding, then liquid gallium forms for attachment, but temperature exposure may damage LEDs or pick-up tools
Solution Approach 1:
The bonding process utilizes a very low temperature parameter change, heating only to above gallium's melting point of 29.76°C rather than high temperatures. This minimal temperature increase is sufficient to create liquid gallium for bonding while minimizing thermal damage risk to the LED structure and pick-up tool, enabling easy operation with thermal safety
Solution Approach 2:
The potential harmful effect of temperature exposure is converted into a benefit by using the very low melting point of gallium (29.76°C) as an advantage. The low temperature required to melt gallium for bonding actually reduces thermal damage risk compared to high-temperature bonding methods, transforming what could be a harmful thermal exposure into a safe and effective bonding approach
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 enables efficient and precise picking and placing of LEDs without additional materials, improving manufacturing efficiency and reducing the risk of damage to LEDs or pick-up tools, while allowing for subsequent reuse of the gallium layer.
Implementation Method 1
exposing the gallium semiconductor layer to a laser to detach the substrate layer and the gallium semiconductor layer. The laser is absorbed by a portion of the gallium semiconductor layer to form gallium material on each of the LED dies of the array
Implementation Method 2
The gallium material is heated to melt the gallium material and form liquid gallium on the surface of the gallium semiconductor layer
Implementation Method 3
cooling the liquid gallium into solid gallium to attach the PUT with the at least a subset of the LED dies via the solid gallium
Data Source
Figure 1
Figure 2A
Figure 2B
AI summary
An LED die containing a gallium semiconductor layer is placed on a target substrate using a pick-up tool (PUT) attached to the LED die using metallic gallium. As a result of a laser lift-of (LLO) process to separate the gallium semiconductor layer from a substrate layer on which the gallium semiconductor layer is formed, a layer of gallium metal is formed on a surface of the LED die. The gallium layer is melted to form liquid gallium. A head of the PUT is contacted with the liquid gallium, whereupon the LED die is cooled such that the liquid gallium solidifies, attaching the LED die to the PUT. The PUT picks up and places the LED die at a desired location on a target substrate. The LED die can be heated to melt the gallium layer, allowing the PUT to be detached.