Buffer Region Design for Semiconductor Wafer Warpage Control
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Solution Overview
Problem
Semiconductor wafers with nitride semiconductors grown on silicon substrates face significant warpage issues due to mismatched thermal expansion coefficients and lattice constants, leading to cracking and dislocations, especially as wafers increase in size and thickness for higher antivoltage strength.
Innovation Solution
A new buffer region configuration with a first and second multilayered subregion, where the first subregion consists of alternating multi-sublayered and non-sublayered buffer layers with specific lattice constants, and the second subregion is designed to provide tensile stress to counteract compressive stress in the main semiconductor region, allowing for thicker wafer growth while minimizing warpage.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a multilayered buffer is used to mitigate stress and prevent cracking, then the nitride semiconductor region is protected against dislocations, but wafer warpage increases as the wafer size and thickness increase
Solution Approach 1:
The buffer region is divided into multiple subregions (first buffer subregion, second buffer subregion, third buffer subregion) with progressively different lattice constants. Each subregion is further segmented into alternating layers of different materials (AlN, AlGaN, GaN) with specific thickness ratios. This segmentation allows gradual stress transition and prevents sudden warpage while maintaining crack resistance.
Solution Approach 2:
Different portions of the buffer region are assigned different lattice constants and material compositions tailored to local stress requirements. The first buffer subregion has lattice constants closer to silicon, the second has intermediate values, and the third has values closer to GaN. This local quality variation enables precise stress control at each interface, reducing overall warpage while preventing cracking.
2Strength
If the nitride semiconductor region is made thicker to increase antivoltage strength, then device voltage withstand capability improves, but wafer warpage and susceptibility to warpage increases
Solution Approach 1:
The buffer region is designed with alternating layers of materials having different lattice constants (AlN with smaller lattice constant, GaN with larger lattice constant) to create preliminary counteracting stresses. The thickness ratios are specifically controlled (AlN:AlGaN:GaN = 1:2:3 or 1:3:6) to generate tensile stresses that pre-compensate for the compressive stresses that will develop when thick nitride semiconductor layers are added, thereby preventing warpage even as antivoltage strength increases.
Solution Approach 2:
The lattice constants of the buffer layers are systematically varied across different subregions and within alternating layers. By changing the aluminum content in AlGaN layers and selecting specific thickness ratios, the patent creates a gradient of lattice constants that transitions from silicon-matched values at the substrate interface to GaN-matched values at the nitride semiconductor interface. This parameter variation enables thick nitride growth without warpage.
3Reliability
If a superlattice buffer with alternating AlGaN and GaN layers is used to improve crystallinity, then cracking is prevented, but wafer warpage is not effectively eliminated
Solution Approach 1:
The buffer structure introduces intermediate layers with specific lattice constants between the silicon substrate and the GaN-based nitride semiconductor. The first buffer subregion uses AlN and AlGaN layers with lattice constants closer to silicon, the second buffer subregion uses intermediate compositions, and the third buffer subregion uses layers closer to GaN lattice constants. These intermediary layers gradually bridge the lattice mismatch, improving crystallinity while the specific thickness ratios control stress to minimize warpage.
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 configuration significantly reduces wafer warpage, allows for thicker nitride semiconductor regions for enhanced antivoltage strength, and stabilizes the crystallinity of the main semiconductor region, making it suitable for mass production with reduced dislocations and fluctuations.
Implementation Method 1
a problem has been encountered in use of a silicon substrate in this type of wafer by reason of an inconveniently great difference in coefficient of linear thermal expansion between the silicon substrate and the nitride semiconductors grown thereon. The linear expansion coefficient of silicon is approximately 4.70×10−6/K whereas that of gallium nitride, for example, is 5.59×10−6/K. Other semiconducting nitrides are more or less equally higher in linear expansion coefficient than silicon. What is worse, silicon and semiconducting nitrides also differ in lattice constant.
Implementation Method 2
a problem has been encountered in use of a silicon substrate in this type of wafer by reason of an inconveniently great difference in coefficient of linear thermal expansion between the silicon substrate and the nitride semiconductors grown thereon
Data Source
AI summary
A main semiconductor region of semiconducting nitrides is formed on a silicon substrate via a buffer region of semiconducting nitrides to provide devices such as HEMTs, MESFETs and LEDs. In order to render the wafer proof against warping, the buffer region is divided into a first and a second multilayered buffer subregion. The first buffer subregion comprises multiple alterations of a multi-sublayered first buffer layer and a non-sublayered second buffer layer. Each multi-sublayered first buffer layer of the first buffer subregion comprises multiple alternations of a first and a second buffer sublayer. The second buffer sublayers of each multi-sublayered first buffer layer either do not contain aluminum or do contain it in a higher proportion than do the first buffer sublayers. The second multilayered buffer subregion comprises multiple alternations of a first and a second buffer layer. The first buffer layers of the second multilayered buffer subregion are less in aluminum proportion than the fourth buffer layers of the second multilayered buffer subregion.