Optical Layer for Laser Lift-Off Substrate Reflection
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Solution Overview
Problem
Laser lift-off (LLO) techniques face challenges in achieving high laser energy transmission through non-polished, rough temporary substrates, leading to unacceptable reflection and potential defects in the process layer, especially in heteroepitaxially grown device layers like GaN on sapphire substrates, due to the need for lapping and polishing which can render substrates non-reusable and increase the risk of cracking or delamination.
Innovation Solution
The implementation of an optical layer or optical liquid with a refractive index within ±0.25 of the temporary substrate and an optical extinction coefficient of 0 to 0.0005 at the operational wavelength, applied to the back side of the temporary substrate to reduce reflection and increase laser fluence, allowing for efficient decomposition of the process layer and separation from the substrate without requiring extensive polishing or increasing laser power.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Use of energy by moving object
If lapping and polishing is performed on the back side of the temporary substrate to improve laser energy transmission, then laser transmission increases, but the substrate becomes non-reusable and the process complexity increases
Solution Approach 1:
An optical layer is deposited on the back side of the temporary substrate before the laser lift-off process. This preliminary optical treatment creates a low-reflection interface that improves laser energy transmission without requiring subsequent mechanical polishing or lapping operations, thereby preserving substrate reusability
Solution Approach 2:
An optical layer with specific refractive index properties is introduced as an intermediary between the laser beam and the temporary substrate. This intermediate layer reduces reflection at the air-substrate interface, improving laser energy transmission while avoiding the need for mechanical surface modification that would compromise substrate reusability
2Productivity
If laser power is increased to improve decomposition of the process layer, then decomposition efficiency increases, but defects in the process layer increase
Solution Approach 1:
The refractive index of the optical layer is specifically optimized to match the temporary substrate, reducing reflection and increasing laser energy transmission. This parameter optimization allows effective process layer decomposition at lower laser power levels, avoiding the defects that occur with excessive laser power
3Ease of manufacture
If the back side of the temporary substrate is left unpolished to maintain substrate reusability, then substrate can be reused, but laser energy transmission decreases due to reflection
Solution Approach 1:
An optical layer is deposited on the unpolished back side of the temporary substrate to serve as an intermediary that reduces reflection. This allows the substrate to remain unpolished and reusable while the optical layer compensates for the increased reflection that would otherwise occur with a rough surface
Solution Approach 2:
A composite structure is created by depositing an optical layer with specific optical properties onto the temporary substrate. This composite structure combines the mechanical properties of the reusable substrate with the optical properties of the low-reflection coating, achieving both reusability and high laser energy transmission
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 ensures greater than 90% laser energy transmission through the substrate, minimizing defects and enabling the reuse of temporary substrates by maintaining their original thickness and smoothness, thus enhancing the efficiency and reliability of the LLO process.
Implementation Method 1
The optical layer may be characterized by a front surface on the back side of the temporary substrate, an index of refraction within +/−0.25 of the temporary substrate at an operational wavelength
Implementation Method 2
A laser at the operational wavelength is directed through the optical layer and the temporary substrate to decompose the process layer
Data Source
AI summary
Laser lift-off methods are described in which optical flatness is provided on the back side of a temporary substrate using either an optical layer or optical liquid. A laser is directed through the optical layer or optical liquid and a back side of the temporary substrate to decompose a portion of a process layer supported on a front side of the temporary substrate, followed by separation of the process layer and the temporary substrate.