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Advanced Metrology of High Thermal Conductivity Materials and Interfaces


OUSD (R&E) CRITICAL TECHNOLOGY AREA(S): Microelectronics OBJECTIVE: Develop a turnkey system to measure the thermal conductivity and thermal boundary resistance of wide bandgap semiconductor films, interfaces, and substrates. DESCRIPTION: Future military platforms will require high-power converters for propulsion, sensors, and directed energy systems. The power densities for these converters necessitate high-voltage, high-efficiency power switches based on the application of wide bandgap (WBG) and ultra-wide bandgap (UWBG) semiconductor thin films, because of their wide bandgaps and high breakdown fields. An added benefit of these materials is their intrinsically large thermal conductivities, which can help to mitigate extreme temperature rises during power cycling and high-power operation. For example, isotopically pure diamond can have thermal conductivities of 3000 W∙m-1∙K-1, low defect AlN films were recently shown to have thermal conductivities over 300 W∙m-1∙K-1, and homoepitaxially grown GaN films can have thermal conductivities near 200 W∙m-1∙K-1. However, these large thermal conductivities are often not observed when WBG materials are integrated into power devices. It is well known that defects arising from material growth, interfaces from heterogeneous integration, and dopant species used to tune electrical properties all scatter phonons leading to reductions in thermal conductivity. The resultant reduced thermal conductivity, itself a temperature-dependent quantity, in integrated materials compromise high power devices and can lead to device failure and/or dictate lower max power thresholds. Given the large thermal resistances that occur at heterogeneous interfaces, especially at interfaces of WBG and UWBG materials, measurements of thermal boundary resistances in the 1-100 m2·K/GW range are similarly crucial to and predictive of reliable device operation. The current state-of-the-art for laboratory measurements are thermoreflectance-based techniques that can measure the thermal conductivity of thin films with accuracy significantly higher than that available in commercial systems. Further, control of laser spot sizes these techniques allows for micron-scale spatial resolution of thermal conductivity on sample surfaces, which can reveal spatial inhomogeneities due to dislocations, defects, grain boundaries, and other growth-related phenomena. A major limiting factor in the use of these thermoreflectance techniques for wide scale materials characterization is their complicated, free space design on open optical tables that is not conducive to turnkey operation, even with a highly-skilled technician operating and aligning these systems full time. An additional limitation is the need for user-friendly and versatile instrument control software and analysis codes that can be widely used to acquire and analyze measurement data. The development of a reliable, repeatable, and fully automated tool that harnesses the sensitivities and resolution of free-space thermoreflectance systems is thus a key to establishing consistent and common measurements of materials for DoD applications. This necessitates a system design that does not require optical maintenance or alignment of laser paths to ensure day-to-day repeatability and accuracy when operated by different users. Such a tool would be of significant use for testing a wide array of materials for both military and civilian applications, including hybrid electric vehicles. Recent advances in fiber-optic-based thermoreflectance systems show promise to meet these requirements; however, any approach that has potential to achieve the desired measurement capabilities will be considered. PHASE I: Establish design of a temperature-dependent (25-225 °C) thermal conductivity measurement system that can produce highly accurate (± 5%) and reproducible (± 1%) measurements of the thermal conductivity in both thin films and bulk substrates of wide bandgap semiconductors, as well as thermal boundary resistance in the 1-100 m2·K/GW range across semiconductor interfaces at the wafer scale. The system should have micrometer area resolution and depth resolution capable of measuring atomically thin interfaces/contacts to thin films and buried substrates with device relevant length scales. The system should be designed for ease and repeatability of measurements among multiple users. Demonstrate the capability of measuring the thermal conductivity of materials ranging from 0.1 W∙m-1∙K-1 to 2000 W∙m-1∙K-1 through experimentation or detailed modeling. Perform an initial estimate of size, weight, and cost of production unit, as well as technical risks to be addressed during potential Phase II. PHASE II: Refine Phase I design and fabricate a fully-functional prototype system having automated data collection and analysis capabilities. The system should be able to measure thermal conductivity of materials as high as 3000 W∙m-1∙K-1, measure thermal boundary resistance independently from thermal conductivity of materials, and resolve these properties and thermal resistances of thin film stacks with dimensions and temperatures appropriate for power electronic devices, as well as perform these measurements on an electronic device or test structure under nominal voltage and current operating conditions. Data reduction should be available in analysis codes with GUIs that can rapidly analyze large data sets. Deliver a fully operational prototype of the measurement system, including appropriate control and analysis software, to the Navy for evaluation. PHASE III DUAL USE APPLICATIONS: Develop final design and manufacturing plans using the knowledge gained during Phases I and II in order to support transition of the technology for Navy use and adoption in the WBG/UWBG device community. A thermal conductivity measurement tool of this design will enable cost- and time-effective material evaluation of high-power devices. REFERENCES: 1. D. G. Cahill, "Analysis of heat flow in layered structures for time-domain thermoreflectance," Review of Scientific Instruments 75, 5119-5122 (2004); 2. A. J.. Schmidt, R. Cheaito and M. Chiesa, "A frequency-domain thermoreflectance method for the characterization of thermal properties," Review of Scientific Instruments 80, 094901 (2009); 3. J. L. Braun, D. H. Olson, J. T. Gaskins and P. E. Hopkins, "A steady-state thermoreflectance method to measure thermal conductivity," Review of Scientific Instruments 90, 024905 (2019); 4. Naval Power and Energy System Technology Development Roadmap.; 5. U. S. Drive Electrical and Electronics Technical Team Roadmap. KEYWORDS: Thermal Conductivity; Thermal Boundary Resistance; Thermoreflectance; Wide Bandgap Semiconductors
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