Heteroepitaxial Wafer Buffer Structure for Low-Warp GaN-on-Silicon
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Solution Overview
Problem
Large wafer warp during epitaxial deposition on silicon substrates of 150 mm or 200 mm diameter leads to temperature non-uniformity and material alloy fluctuations, increasing the risk of cracks and reducing the effectiveness of AlGaN/GaN high-electron-mobility transistor (HEMT) devices.
Innovation Solution
A heteroepitaxial wafer structure comprising a silicon substrate with alternating strain building and strain preserving layers, including AlGaN and AlN layers, deposited using MOCVD, to manage curvature and maintain a flat substrate, thereby reducing warping and enhancing breakdown performance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the thickness of the AlGaN buffer is increased to improve breakdown characteristics, then breakdown performance is improved, but wafer warp increases causing temperature non-uniformity and layer thickness variation
Solution Approach 1:
The AlGaN buffer is segmented into multiple thin layers with different Al contents (first strain building layer, first strain preserving block, second strain building layer, second strain preserving block) rather than using a single thick layer. This segmentation allows each sub-layer to contribute to strain management while maintaining overall buffer thickness for breakdown performance.
Solution Approach 2:
Different regions of the buffer have different Al contents optimized for their specific functions: higher Al content in strain preserving blocks (y>z, w>x) for strain management, and lower Al content in strain building layers for gradual strain introduction. This local optimization resolves the contradiction between thickness and uniformity.
2Manufacturing precision
If alternating strain building and strain preserving blocks are implemented to manage curvature, then wafer warp is reduced and temperature uniformity is improved, but device complexity increases
Solution Approach 1:
The buffer uses composite AlGaN structures with varying Al contents arranged in alternating strain building and strain preserving blocks. This composite approach manages wafer curvature through controlled strain distribution while maintaining a systematic structure that is manageable in fabrication.
Solution Approach 2:
The buffer structure utilizes controlled changes in Al content parameters (z, y, x, w) across different layers to manage strain and curvature. By systematically varying these compositional parameters, the patent achieves wafer flatness control without requiring overly complex structural arrangements.
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 approach results in a crack-free epilayer with reduced warp and improved breakdown field, enabling the production of high-quality HEMT devices with reduced production costs and increased reliability.
Implementation Method 1
a heteroepitaxial wafer comprising... an AlN nucleation layer, an AlGaN buffer and a GaN layer
Implementation Method 2
deposited using MOCVD
Implementation Method 3
Lattice mismatch between the substrate and the layers deposited thereon and differences with respect to their thermal coefficient of expansion will lead to warping of the multi-layer structure
Implementation Method 4
Lattice mismatch between the substrate and the layers deposited thereon and differences with respect to their thermal coefficient of expansion
Data Source
AI summary
A heteroepitaxial wafer comprises, in the following order: a silicon substrate having a diameter and a thickness;an AlN nucleation layer;a first strain building layer which is an AlzGal-zN layer having a first average Al content z, wherein 0<z;a first strain preserving block comprising ≥5 and ≤50 units of a first sequence of layers, the first sequence comprising an AlN layer and at least two AlGaN layers, and having a second average Al content y, wherein ya second strain building layer which is an AlxGal-xN layer having a third average Al content x, wherein 0≤x<y;a second strain preserving block comprising ≥5 and ≤50 units of a second sequence of layers, the sequence comprising an AlN layer and at least one AlGaN layer, and having a fourth average Al content w, wherein x<w<y, anda GaN layer, wherein the layers between the AlN nucleation layer and the GaN layer form an AlGaN buffer.