What is Laser Lift-Off (LLO)?

Laser Lift-Off (LLO) is a process that uses high-energy laser pulses (typically UV excimer lasers) to decompose the interface between a GaN epilayer and a sapphire substrate, allowing the film to be separated and transferred to a new carrier.

Why is LLO used for MicroLEDs?

MicroLEDs often require flexible or conductive substrates. Since GaN is grown on insulating sapphire, LLO allows the LEDs to be transferred to a new substrate (like silicon or flexible polymer) for better thermal management and electrical contact.

Laser Lift-Off (LLO) Process for GaN on Sapphire

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Research Case Studies

MicroLED Optimization
A research group required 2" Sapphire wafers with epitaxial GaN layers for MicroLED fabrication. The goal was to perform laser lift-off and then CMP (Chemical Mechanical Polishing) on the backside of the GaN.

Our Solution (Ref #265446): We provided custom GaN on Sapphire wafers tailored for LLO experiments.

PhD Research Spec
"I am beginning to learn about MicroLEDs and need a GaN on Sapphire wafer for lift-off purposes."

Diameter	50.8mm (2")
Orientation	<0001> C-Plane
Substrate	Sapphire 430µm
GaN Layer	5µm - 6.5µm
Type	N-type (Si Doped)
Polish	SSP

Related Resources

The Laser Lift-Off (LLO) Process Explained

Laser lift-off (LLO) is a critical technique for separating GaN thin films from sapphire substrates. Because sapphire is an electrical insulator and has poor thermal conductivity compared to metals or silicon, separating the GaN layer allows engineers to transfer devices onto better heat sinks or flexible substrates.

How it Works

The process typically uses a 248 nm KrF excimer laser. The laser pulses are directed through the transparent sapphire substrate. The high energy is absorbed at the GaN/Sapphire interface, decomposing a thin layer of GaN into metallic Gallium (Ga) and Nitrogen gas (N₂). Heating the metallic Ga slightly above 30°C melts it, allowing the sapphire substrate to slide off or be lifted away.

Substrates for LLO

The most common stack for this process is Gallium Nitride (GaN) on Sapphire. However, variations exist depending on the application:

AlGaN on Sapphire: Used for UV LEDs.
GaN on ZnO: Sacrificial layers like Zinc Oxide can be used to assist delamination.

Applications in MicroLEDs & Flexible Displays

LLO is the enabling technology for flexible OLEDs and MicroLED displays. By growing high-quality crystals on sapphire and then transferring them to polyimide (PI) or glass, manufacturers can create:

Flexible displays that bend without breaking.
High-brightness MicroLED arrays on silicon backplanes.
Vertical LED structures for better current distribution.

Role of SU-8 Photoresist

In many research setups, GaN nanoLEDs are embedded in SU-8 photoresist before lift-off. This polymer acts as a mechanical support, keeping the tiny LED pixels aligned during the transfer process from sapphire to the target substrate.

Challenges: Thermal Mismatch & Cracking

One of the main challenges in LLO is the thermal mismatch between the GaN epilayer and the substrate. The laser pulse generates significant localized heat. If not managed, this can cause the released GaN film to crack due to residual stress. Advanced "Line Beam" LLO techniques and sacrificial buffer layers are used to mitigate this issue and enable mass production.

Video: Laser Lift-Off Explained

Watch this visual explanation of how the excimer laser separates the semiconductor layers.

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