Engineered RF Substrate Structure for GaN Thermal Mismatch
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Solution Overview
Problem
High frequency, high performance RF devices face issues due to lattice mismatch and thermal expansion coefficient disparities between GaN and carrier substrates, leading to defects, dislocations, and increased manufacturing costs.
Innovation Solution
The use of an engineered substrate with a polycrystalline ceramic core and interlayer structure, including silicon oxide and silicon nitride layers, coupled with a single crystalline silicon layer, to match thermal expansion coefficients and reduce thermal resistance, inductance, and impedance, enabling high-frequency operations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If GaN is deposited on a carrier substrate with a different lattice structure, then RF devices can be fabricated, but defects and dislocations are created due to lattice mismatch
Solution Approach 1:
A multi-layer buffer structure is introduced between the GaN layer and the carrier substrate. This buffer structure includes layers with gradually changing lattice constants that serve as an intermediary, reducing the abrupt lattice mismatch and minimizing defects and dislocations in the GaN layer.
Solution Approach 2:
The lattice constant parameter is gradually changed through the buffer layers, transitioning from the carrier substrate's lattice constant to the GaN layer's lattice constant. This gradual parameter change reduces stress and defect formation compared to direct deposition on mismatched substrates.
2Productivity
If thermal processing is performed on GaN layers with carrier substrates, then epitaxial growth can be achieved, but cracking and delamination occur due to different coefficients of thermal expansion
Solution Approach 1:
The buffer structure is specifically designed with materials and thicknesses that account for thermal expansion differences. The multi-layer structure distributes thermal stress across multiple interfaces, preventing catastrophic cracking and delamination during epitaxial growth and subsequent thermal processing.
Solution Approach 2:
A composite buffer structure comprising multiple materials with different thermal expansion coefficients is used. This composite structure gradually transitions the thermal expansion properties from the carrier substrate to the GaN layer, reducing thermal stress and improving reliability during thermal processing.
3Ease of manufacture
If carrier substrates are used for GaN fabrication, then RF devices can be produced, but substrate bowing and breaking occur during thermal processing
Solution Approach 1:
The buffer structure compensates for thermal expansion mismatches that cause substrate bowing. By distributing thermal stress across multiple layers with graded thermal expansion properties, the structure prevents excessive bowing and subsequent substrate breaking during thermal processing.
4Ease of manufacture
If different coefficients of thermal expansion between GaN and carrier substrate are accommodated, then thermal stress is reduced, but substrate wafer size is restricted and manufacturing cost increases
Solution Approach 1:
The buffer structure is segmented into multiple thin layers rather than a single thick layer. This segmentation allows better stress distribution and thermal management while maintaining a relatively simple overall structure that can be integrated into standard fabrication processes, avoiding excessive complexity and cost.
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 results in high-frequency RF devices with reduced defects, lower manufacturing costs, and improved thermal performance, allowing for larger wafer sizes and more complex circuitry.
Implementation Method 1
The GaN layers and carrier substrate may have different coefficients of thermal expansion (CTEs). Thermal processing (e.g., GaN epitaxial growth) can crack or delaminate the GaN, or bow and, in some cases, break the carrier substrate.
Implementation Method 2
epitaxial layers, such as gallium nitride (GaN), may be formed by a heteroepitaxial (epi) growth process that involves depositing GaN on a semiconductor carrier substrate
Data Source
Figure 1
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Figure 3A
AI summary
A substrate for RF devices includes a polycrystalline ceramic core and an interlayer structure. The interlayer structure includes a first silicon oxide layer coupled to the polycrystalline ceramic core, a polysilicon layer coupled to the first silicon oxide layer, a second silicon oxide layer coupled to the polysilicon layer, a barrier layer coupled to the second silicon oxide layer, a third silicon oxide layer coupled to the barrier layer, and a substantially single crystalline silicon layer coupled to the third silicon oxide layer.