Laser Diode Submount CTE Mismatch Stress Reduction
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
High power semiconductor laser diodes experience degradation due to thermal mismatch between the diodes and the cooling system, leading to stress and reliability issues, particularly with existing mounting technologies that fail to maintain planarity and optical performance under thermal cycling.
Innovation Solution
A submount with tailored tensile strength is designed by selecting a material with an average thermal expansion coefficient between that of the laser diode and the cooler, and pre-stressing the submount to balance forces, ensuring zero force exertion on the laser diode bar, thus maintaining planarity and reducing stress-induced aberrations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a submount with CTE-matched material is used to reduce thermal mismatch stress, then reliability improves, but manufacturing complexity increases due to the need for specialized materials and multi-layer structures
Solution Approach 1:
The patent employs a multi-layer composite submount structure consisting of a first submount layer (e.g., CuW alloy with CTE ≈ 6.5×10^-6/K) and a second submount layer (e.g., Mo with CTE ≈ 5.0×10^-6/K). This composite structure achieves an average CTE that matches the laser diode bar while distributing thermal and mechanical stresses across layers with different properties, thereby improving reliability without requiring a single complex material.
Solution Approach 2:
The submount is segmented into multiple functional layers: a first layer for CTE matching with the laser diode, a second layer for additional stress management, and a third layer for thermal conduction to the heat sink. This segmentation allows each layer to be optimized for its specific function, simplifying the overall design compared to seeking a single material that satisfies all requirements simultaneously.
2Power
If hard solder is used for mounting the laser bar to achieve higher power handling, then power capacity improves, but thermal mismatch stress increases leading to degradation
Solution Approach 1:
The patent changes the CTE parameter of the submount material to match that of the laser diode bar (CTE ≈ 6.5×10^-6/K). By selecting a CuW alloy with this specific CTE value, the submount undergoes similar thermal expansion as the laser diode during hard soldering processes, eliminating differential stress that would otherwise cause degradation at high power levels.
Solution Approach 2:
The CTE-matched submount acts as an intermediary layer between the laser diode bar and the heat sink. It mediates the thermal and mechanical interface, absorbing expansion differences and preventing stress transmission from the rigid heat sink to the laser diode, thereby enabling hard soldering without degradation.
3Temperature
If the laser bar is directly mounted to the cooler to improve thermal conductivity, then heat dissipation improves, but stress and smile increase reducing reliability
Solution Approach 1:
The patent introduces a CTE-matched submount as an intermediary layer between the laser diode bar and the heat sink. This submount maintains excellent thermal conductivity while acting as a stress buffer that compensates for thermal expansion differences, thereby preventing smile and reliability issues associated with direct mounting.
4Object-affected harmful factors
If soft solder is used for mounting to reduce stress, then stress on laser diodes decreases, but stability under thermal cycling deteriorates limiting operating current
Solution Approach 1:
The patent changes the material parameter (CTE) of the submount to match the laser diode bar. This CTE matching eliminates the root cause of thermal mismatch stress, allowing the use of hard solder for stable thermal cycling performance while maintaining low stress levels on the laser diodes during operation.
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 low smile values, high reliability, and precise spectral output, enabling increased rated output power without deformation and allowing for optimized epitaxial structure design.
Implementation Method 1
the mismatch of the thermal properties, especially the thermal expansion coefficient (CTE), between the laser diodes and the submount and/or cooling system or mount
Implementation Method 2
such a laser diode bar is arranged on a submount, mostly junction side down, which submount serves as 'stress buffer' and transfers the heat to a cooling system
Data Source
Figure 1a~1d
Figure 2
Figure 3
AI summary
A high power laser source comprises a bar of laser diodes, a submount onto which said laser bar is affixed, and a cooler onto which said submount is affixed. The laser bar has a first coefficient of thermal expansion (CTEbar), the submount has a second coefficient of thermal expansion (CTEsub), and the cooler has a third coefficient of thermal expansion (CTEcool), the third coefficient (CTEcool) being higher than both said first coefficient (CTEbar) and said second coefficient (CTEsub). According to the invention, the second coefficient (CTEsub) is selected lower than both the first coefficient (CTEbar) and the third coefficient (CTEcool), contrary to the usual approach with the CTEsub matching the CTEbar. A preferred range is CTEsub= k * CTEbar, with 0.4 < k < 0.9. The submount may consist of or comprise two or more layers of different materials having different CTEs, e.g. a Cu layer of about 10-20µm thickness and a Mo layer of about 200-300µm thickness, resulting in a CTEsub which varies across the submount's thickness. Alternatively, the submount may consist of a single, more or less homogeneous material with a CTEsub varying across the submount's thickness. A method for making such a high power laser source includes selecting a submount whose CTEsub lies between the CTEcool of the cooler and the CTEbar of the bar of laser diodes and hard soldering the bar and the cooler to the submount.