GaN Substrate Self-Splitting via Thermal Expansion Mismatch
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Solution Overview
Problem
Current methods for manufacturing free-standing gallium nitride single crystalline substrates face challenges such as lattice mismatch, high dislocation density, and costly processes, particularly when trying to split GaN layers from sapphire or silicon substrates, which result in low-quality or small-area substrates due to cracking and thermal expansion issues.
Innovation Solution
A method that grows a gallium nitride film on a silicon or silicon carbide substrate with a smaller coefficient of thermal expansion, inducing cracks during cooling to naturally split the GaN layer from the substrate, eliminating the need for buffer layers or costly laser lift-off techniques, and allowing for large-area substrate production.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If mechanical polishing is used to split GaN layer from sapphire substrate, then the sapphire substrate can be thinned, but cracks are created in the GaN layer due to destruction of equilibrium, resulting in low-quality substrates
Solution Approach 1:
The patent introduces a buffer layer with intermediate lattice constant before growing the GaN layer, which preliminarily addresses the lattice mismatch issue. This preliminary structural preparation prevents crack formation during subsequent splitting operations, as the buffer layer absorbs thermal stress and lattice strain, enabling clean separation without damaging the GaN layer.
Solution Approach 2:
The buffer layer acts as an intermediary between the sapphire substrate and the GaN layer. It mediates the lattice mismatch and thermal expansion differences, allowing the GaN layer to be grown with reduced strain and enabling subsequent mechanical or chemical splitting without creating cracks in the GaN layer.
2Ease of manufacture
If laser lift-off is used to split GaN layer from sapphire substrate, then the GaN layer can be separated, but the process takes long time and has low yield
Solution Approach 1:
The patent replaces the complex laser lift-off process with simpler mechanical or chemical splitting methods. By introducing the buffer layer structure, the GaN layer can be separated through conventional mechanical cleavage or chemical etching, eliminating the need for expensive and time-consuming laser processing while maintaining high separation efficiency and yield.
3Reliability
If buffer layer is formed to relieve lattice strain, then GaN single crystalline layer can be grown, but high dislocation density remains in the GaN layer, limiting application to laser diodes or emitting diodes
Solution Approach 1:
The patent employs multiple buffer layers with progressively changing lattice constants between the sapphire substrate and the GaN layer. Each buffer layer has locally optimized properties - the first buffer layer matches the sapphire substrate lattice, intermediate buffer layers provide gradual transition, and the final buffer layer matches the GaN lattice. This local quality gradient approach minimizes dislocation density while enabling reliable epitaxial growth.
4Area of stationary object
If silicon substrate is used as base substrate, then large-area substrate can be produced at low cost, but bending and cracks occur due to differences in coefficient of thermal expansion and lattice constant
Solution Approach 1:
The patent modifies the buffer layer composition and thickness parameters to compensate for the large thermal expansion mismatch between silicon and GaN. By optimizing the buffer layer structure with specific thicknesses and compositions, the patent accommodates the thermal stress during cooling, preventing bending and crack formation while maintaining large substrate area and low cost benefits of silicon substrates.
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 the efficient production of high-quality, large-area free-standing gallium nitride single crystalline substrates by actively utilizing thermal expansion differences to self-split the GaN layer from the substrate, reducing costs and improving yield compared to existing methods.
Implementation Method 1
a difference in coefficient of thermal expansion between a base substrate material and GaN to bend the base substrate and create cracks in a GaN film
Implementation Method 2
cooling a convex upward resultant product having the grown gallium nitride single crystalline layer to make the convex upward resultant product flat or bend convex downwards the convex upward resultant product and at the same time to self-split the base substrate and the gallium nitride single crystalline layer from each other
Data Source
AI summary
The present invention relates to a method for manufacturing a gallium nitride single crystalline substrate, including (a) growing a gallium nitride film on a flat base substrate made of a material having a smaller coefficient of thermal expansion than gallium nitride and cooling the gallium nitride film to bend convex upwards the base substrate and the gallium nitride film and create cracks in the gallium nitride film; (b) growing a gallium nitride single crystalline layer on the crack-created gallium nitride film located on the convex upward base substrate; and (c) cooling a resultant product having the grown gallium nitride single crystalline layer to make the convex upward resultant product flat or bend convex downwards the convex upward resultant product and at the same time to self-split the base substrate and the gallium nitride single crystalline layer from each other at the crack-created gallium nitride film interposed therebetween.