Semiconductor Light Emitting Device Stress Relief via Substrate Bonding
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Solution Overview
Problem
The manufacturing of nitride semiconductor light emitting devices is limited by the use of sapphire substrates, which are electrical insulators with poor thermal conductivity, leading to stress-induced luminous efficiency degradation due to differences in lattice constants and coefficients of thermal expansion between the substrate and the light emitting structure layer.
Innovation Solution
A method involving the bonding of a second substrate with a different coefficient of thermal expansion to the light emitting structure layer at a temperature higher than room temperature, followed by removal of the first substrate using laser lift-off, to relieve stress and enhance luminous efficiency, where the second substrate is chosen based on whether compressive or tensile stress is induced in the active layer.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a sapphire substrate is used to form the light emitting structure layer, then the electrical insulation is improved, but the thermal conductivity deteriorates and stress is induced affecting luminous efficiency
Solution Approach 1:
The substrate system is segmented into two distinct substrates: the first substrate (sapphire) provides electrical insulation during growth, while the second substrate (Si, SiC, AlN, GaP, InP, or graphite) provides superior thermal conductivity for heat dissipation. This segmentation allows each substrate to fulfill its optimal function without compromise.
Solution Approach 2:
The light emitting structure layer acts as an intermediary between the first and second substrates. It is grown on the electrically insulating first substrate, then transferred to the thermally conductive second substrate, mediating the transition between the two substrate properties.
2Ease of manufacture
If a sapphire substrate is used to form the light emitting structure layer, then the growth process is simplified, but stress is induced due to differences in lattice constants and coefficients of thermal expansion affecting luminous efficiency
Solution Approach 1:
The coefficient of thermal expansion is changed by switching from a sapphire substrate to a second substrate with a coefficient closer to the light emitting structure layer. This parameter change reduces thermal mismatch stress during cooling, improving luminous efficiency while maintaining manufacturing simplicity.
3Manufacturing precision
If the second substrate is bonded at a high temperature and then cooled to room temperature, then the stress in the active layer is relieved, but the bonding process becomes more complex
Solution Approach 1:
The bonding process utilizes phase transition (melting and solidification) of a eutectic bonding metal layer. The bonding metal is heated to its eutectic temperature to melt and bond the substrates, then cooled to solidify and relieve stress. This phase transition approach simplifies the bonding process despite the temperature variations required.
4Strength
If a bonding metal with eutectic temperature of 200°C or higher is used, then the bonding strength is improved, but the bonding temperature increases
Solution Approach 1:
The eutectic temperature of the bonding metal is changed to 200°C or higher, which increases the bonding strength while managing the bonding temperature. This parameter change allows for stronger bonds that can withstand subsequent processing and provide adequate thermal stress relief during cooling.
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 method effectively relieves residual stress in the active layer, enhancing the internal quantum efficiency of the semiconductor light emitting device by selectively using substrates like Si, SiC, or graphite with varying thermal expansion coefficients.
Implementation Method 1
A coefficient of thermal expansion of the second substrate is different from a coefficient of thermal expansion of the active layer
Implementation Method 2
The second substrate and the light emitting structure are cooled to reach room temperature. Compressive or tensile stress induced in the active layer may be relieved while the light emitting structure layer is cooled to reach room temperature
Implementation Method 3
The first substrate is removed from the light emitting structure layer at a second temperature higher than room temperature. The removing of the first substrate may be performed by laser lift-off (LLO)
Data Source
AI summary
A method of manufacturing a semiconductor light emitting device includes forming a light emitting structure layer including an active layer on a first substrate. A second substrate is bonded to the light emitting structure layer at a first temperature higher than room temperature. The first substrate is removed from the light emitting structure layer at a second temperature higher than room temperature. The second substrate and the light emitting structure are cooled to reach room temperature. A coefficient of thermal expansion of the second substrate is different from a coefficient of thermal expansion of the active layer.