Advanced Thermal Characterization

Thermal characterization of the micro-to-nanoscale energy transport in semiconductors, phase change heat transfer on nanoengineered surfaces as well as the conduction-radiation interaction is of great importance to enhance the fundamental understanding in heat transfer physics and inspire innovative industrial applications.

At the Device Research Laboratory, we developed state-of-art techniques to measure the complex heat transfer process. We designed and built a free-space Raman spectroscopy system and extended the capability of Raman thermometry in understanding the coupling effects of multiphysics in semiconductor materials, transient temperature response, thermal properties and heat transfer of 2D materials, nanoscale temperature probe, phase change heat transfer on nanoengineered surfaces, and radiation and conduction heat transfer in aerogel.
Micro-Raman Multiphysics Characterization

Semiconductor materials, such as gallium nitride (GaN) high electron mobility transistors (HEMTs) which are widely used for high-power radio frequency amplifier and high voltage power conversion applications offer high power density and efficiency. However, the high-power density presented in the device elevates the channel temperature, which leads to high thermoelastic stresses. The strong electric field in the material can also induce piezoelectric stress as well as electrochemical phenomena. All these temperature, stresses and electric field induced effects contribute to the degraded performance and even failure of devices. In this project, we are investigating this the coupling effect of multiphysics, and measuring the highly localized temperature, stresses and electric field using spectroscopy techniques.

A group theory based theoretical framework was developed to understand the effect of temperature rise, stresses and electric field on the change of phonon frequency. We used Raman spectroscopy to probe different phonon modes directly and converted the changes in phonon frequency to the strength of different fields. This implementation of Raman spectroscopy offers an exciting opportunity to simultaneously probe thermal, mechanical and electric phenomena in semiconductor materials. This multiphysics characterization technique has been successfully applied to measure the complex physics in GaN HEMTs, providing unique insight in describing the behavior of the device.
Micro-Raman Thermometry for Transient Temperature Response

For semiconductor materials applied for power amplifiers, the devices are operated in pulse mode which leads to transient heat transfer problem. It is critical to understand the transient temperature response and corresponding time constant of the material. In this project, we developed the transient micro-Raman thermometry technique and compact electrothermal modelling tool to investigate the transient heat transfer process. The time-resolved micro-Raman thermometry has a temporal resolution of 30 ns, and a broad spectrum of time constants spanning from 130 nm to 3.2 ms were measured. The proposed theoretical and experimental methodology can be applied to characterize a variety of high-power dissipation devices.
Micro-Raman Thermometry for 2D Materials

2D materials such as graphene, black phosphorous as well as metal dichalcogenide (TMD) monolayers are extensively studied in recent researches, which shows unique mechanical, optical, electrical and thermal properties. For example, the indirect band gap in bulk TMD transits to direct electronic band when the material becomes atomic thin monolayer. For this reason, 2D materials show great potential for the next-generation low-power transistor applications. Consequently, understanding the thermal properties of 2D materials is important for both physical level understanding and device level characterization. In this project, we developed a theoretical framework to understand the effects of temperature rise and stresses on the phonon modes of MoS2. We used the micro-Raman thermometry to investigate the temperature dependent Raman spectrum of MoS2. The thermal characterization enables the high-sensitive temperature measurement on 2D-material based transistors. We also measured the thermal properties such as the thermal conductivity and thermal expansion coefficient of monolayer flakes. This developed techniques not only enables direct measurements on atomic thin monolayer materials but shows great possibility to use 2D materials as a thin film sensor for planar temperature measurements.

Plasmon Enhanced Nano-Raman Sensor for Nanoscale Temperature Measurement -- In the study of nanoscale heat transport process, the temperature is usually highly localized and the power dissipation is often confined within a narrow region (10 nm to 100 nm). For this reason, characterization techniques with very high spatial resolution are needed to capture the peak temperature rise. In our previous studies, we demonstrated simultaneous measurement of temperature, stress, and electric field components via micro-Raman with a spatial resolution of 1 um. However, due to the restriction of diffraction limit, improving the spatial resolution below 1 um is challenging. In this project, we proposed a plasmon enhanced nano-Raman sensor that can probe length scales as low as 50 nm. This nano-Raman sensor concentrates the laser excitation at the near-field region and can also emit strong Raman signal that is highly temperature sensitive. This nano-Raman sensor can be either used for in situ temperature measurement at nanoscale, or for mapping the temperature distribution of any Raman-inactive materials.
Micro-Raman Thermometry for Thin Film Evaporation on Structured Surfaces

Micro and nanostructures to enhance liquid-to-vapor phase change heat transfer for cooling high-performance electronics have attracted significant attention owing to their ability to generate capillary flow and thin-film area. Typically, heat transfer measurements are performed remotely (i.e., away from the three-phase contact line) due to limitations of conventional contact-mode temperature sensors such as thermocouples and resistance temperature detectors (RTDs), or averaged over an area of 20-50 um with infrared cameras. However, as evaporation mainly occurs in the thin-film region near the three-phase contact line, fundamental understanding of the enhancement mechanism requires a microscopic measurement technique capable of probing temperature near the contact line with high spatial resolution. In this project, we reported a novel platform using micro-Raman spectroscopy to perform in situ temperature measurement of micropillar structures during thin-film evaporation. We interfaced our Raman spectroscopy system with conventional thin film evaporation setup. We performed thin film evaporation experiment on the silicon micropillar arrays structured surfaces (diameters of 20 um, heights of 50 um and pitches of 40-100 um). We measured temperature on the top of silicon micropillars near the liquid-vapor interface at various locations on the sample and heat flux conditions. We can also probe the liquid-vapor interface directly. Our measurements on structured surface as well as liquid-vapor interface enable fundamental understanding into phase change heat transfer at the interface. The experimental results also provide a guideline for optimizing the wick structures to increase evaporation heat transfer coefficient. The local, in situ temperature measurement platform presented in this study serves as a new tool to aid mechanistic understanding of phase change heat transfer.

GaN Electronics (Past work) -- Gallium Nitride (GaN)-based electronics are one of the most exciting semiconductor technologies for high power, high frequency power amplifiers and high voltage power switching devices. However, the very high power densities enabled by the excellent electrical properties of GaN and its related alloys lead to high device temperatures and degraded performance. Due to recent advances in the design of heat sinks, thermal spreaders, and interface materials, the thermal resistance within the first ~100 µm of the electrical junction may be the dominant thermal resistance in the system. This very challenging thermal management problem requires a paradigm shift from the traditional thermal stack to embedded cooling solutions.

At the Device Research Laboratory, we are investigating thermal issues in GaN devices. We are developing electro-thermal and thermal modeling tools to aid in understanding the relationship between the electrical operation and thermal transport in GaN-based devices. Although often limited to simple 2D configurations, electro-thermal models give valuable information about the heat source and the dependence of the heat source size and shape on the bias condition and device geometry. On the other hand, more flexible thermal models provide the 3D temperature and heat flux distributions for predicting device temperature and optimizing the device design.

Most recent work has included computationally-efficient, analytical thermal models for GaN epitaxial structures and high electron mobility transistors (HEMTs). These models are helpful in parametric studies to understand the key dependencies of temperature rise on device structure and layout. In addition, we are pursuing thermal metrology techniques based on micro-Raman and infrared microscopy and in-situ thermal sensors. The future goal is to develop an innovative, near-junction thermal management strategy based on high thermal conductivity solid materials and liquid-vapor phase change cooling.