SOS Substrate Stress Reduction via Ion Implantation Bonding
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Solution Overview
Problem
The silicon-on-sapphire (SOS) substrate faces challenges due to high defect density and excessive stress in the silicon film, limiting its use in high-frequency devices, primarily because of lattice mismatch and thermal expansion differences between silicon and sapphire, which degrade electron mobility and introduce compressive stress.
Innovation Solution
A method involving ion implantation, surface activation, bonding at controlled temperatures, and mechanical impact to transfer the silicon film onto the sapphire substrate, reducing stress by managing the bonding temperature and embrittling the interface, thereby alleviating warpage and stress variations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If heteroepitaxial growth is used to form silicon on sapphire, then an SOS substrate can be obtained, but high defect density occurs due to lattice mismatch
Solution Approach 1:
The process is segmented into distinct stages: first forming silicon on a silicon substrate, then bonding to sapphire, and finally separating the silicon film. This avoids continuous heteroepitaxial growth and reduces defect accumulation.
Solution Approach 2:
A silicon substrate acts as an intermediary during the bonding process, allowing the silicon film to be transferred to sapphire without direct heteroepitaxial growth, thereby reducing lattice mismatch defects.
2Stability of the object's composition
If silicon film is formed at high temperature (900°C to 1000°C), then proper crystal structure is achieved, but excessive compressive stress occurs due to thermal expansion difference
Solution Approach 1:
The silicon film is formed on a silicon substrate at high temperature first, allowing proper crystal structure development. The bonding to sapphire and subsequent cooling occurs after film formation, preventing stress accumulation during growth.
Solution Approach 2:
Instead of forming silicon directly on sapphire at high temperature (which causes stress), the process inverts the sequence: form silicon on silicon substrate, bond to sapphire, then separate. This reverses the traditional approach and eliminates thermal stress.
3Ease of manufacture
If conventional bonding temperature is used, then substrates can be bonded, but warpage occurs due to thermal expansion mismatch
Solution Approach 1:
The bonding temperature is optimized to a specific range that balances bonding effectiveness with minimal thermal expansion difference between silicon and sapphire, reducing warpage while maintaining bond strength.
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 method achieves a significant reduction in silicon film stress to 2.5 × 10^8 Pa or less across the in-plane area, improving the electrical characteristics and handling ease of the SOS substrate by controlling film thickness and stress variations.
Implementation Method 1
implanting ions into a silicon substrate or a silicon substrate having an oxide film thereon to form an ion-implanted layer therein
Implementation Method 2
embrittling an interface of the ion-implanted layer of the bonded body
Implementation Method 3
applying mechanical impact to the interface of the ion-implanted layer to split the bonded body along the interface
Data Source
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Figure 2
AI summary
There is provided an SOS substrate with reduced stress. The SOS substrate is a silicon-on-sapphire (SOS) substrate comprising a sapphire substrate and a monocrystalline silicon film on or above the sapphire substrate. The stress of the silicon film of the SOS substrate as measured by a Raman shift method is 2.5 × 108 Pa or less across an entire in-plane area of the SOS substrate.