GaN Semiconductor Stress Relief Layer for Warpage Suppression
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Solution Overview
Problem
Conventional GaN HEMT manufacturing techniques face challenges in suppressing warpage and cracking due to thermal expansion coefficient differences between GaN, AlGaN, and Si substrates, which affect the crystallinity and properties of electron travel and supply layers.
Innovation Solution
A stress relief layer with recesses of 5 nm or more depth and a number density of 2×10^10 cm^-2 is introduced between the substrate and the GaN compound semiconductor multilayer structure, generating compressive stress to counteract tensile stress and reduce warpage and cracking.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If a GaN layer and AlGaN layer are grown on a Si substrate by heteroepitaxial growth, then large-size high-quality Si substrates can be used at low cost, but the Si substrate becomes warped or cracked due to the large difference in thermal expansion coefficient during high-temperature treatment
Solution Approach 1:
A stress relief layer composed of AlN and GaN layers with a superlattice structure is introduced as an intermediary between the Si substrate and the GaN/AlGaN device layers. This intermediate structure gradually transitions the thermal expansion coefficient mismatch, preventing direct stress concentration at the Si/GaN interface while maintaining the cost advantage of using Si substrates.
Solution Approach 2:
The stress relief layer employs a composite superlattice structure alternating AlN and GaN layers. This composite design allows optimization of both mechanical stress management and thermal expansion properties, creating a gradient transition zone that mitigates warpage and cracking while preserving substrate cost benefits.
2Reliability
If buffer layers with a super-lattice structure are provided between GaN/AlGaN layers and Si substrate to solve thermal expansion coefficient differences, then some warpage and cracking are reduced, but it is still difficult to sufficiently suppress cracking and warpage, and it is difficult to obtain good crystallinity in electron travel and supply layers
Solution Approach 1:
The patent optimizes specific parameters of the stress relief layer including the thickness of AlN and GaN layers (each 5-50 nm), the number of periods (5-50 repetitions), and the growth conditions. By precisely controlling these parameters, the layer achieves both effective stress relief and good crystallinity in the subsequent device layers.
Solution Approach 2:
The stress relief layer is designed with localized AlN/GaN alternating structure specifically at the interface region where stress concentration occurs, while the upper device layers maintain high crystallinity. This local structural differentiation addresses stress relief needs at the interface without compromising the overall crystalline quality of the active device regions.
3Temperature
If a layer with thermal expansion coefficient close to GaN is formed on the back surface of Si substrate, then thermal expansion mismatch is reduced, but warpage occurs during layer formation which causes variations in substrate temperature
Solution Approach 1:
Instead of forming a single thick compensating layer on the substrate back surface, the patent segments the stress relief function into multiple thin AlN and GaN layers stacked in alternation. This segmentation distributes the stress relief function throughout the interface region, achieving thermal expansion matching without inducing significant substrate warpage that would affect temperature uniformity.
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
The solution effectively suppresses cracking and warpage, maintaining good crystallinity and desired properties of the semiconductor layers, while simplifying the manufacturing process and reducing costs.
Implementation Method 1
generating compressive stress to counteract tensile stress and reduce warpage and cracking
Implementation Method 2
there are large differences in thermal expansion coefficient between a GaN layer, an AlGaN layer, and a Si substrate
Implementation Method 3
conventional GaN compound semiconductor layers such as GaN layers and AlGaN layers have been formed mainly over a Si substrate, a sapphire substrate, or a SiC substrate by heteroepitaxial growth
Implementation Method 4
strain due to the difference in lattice constant between AlGaN and GaN is caused in the AlGaN layer. Therefore, piezoelectric polarization occurs and a high-concentration two-dimensional electron gas (2DEG) is obtained
Data Source
AI summary
A compound semiconductor device includes: a substrate; a GaN compound semiconductor multilayer structure disposed over the substrate; and a stress relief layer which is AlN-based and which is disposed between the substrate and the GaN compound semiconductor multilayer structure, wherein a surface of the stress relief layer that is in contact with the GaN compound semiconductor multilayer structure includes recesses that have a depth of 5 nm or more and that are formed at a number density of 2×1010 cm−2 or more.