Water is often considered as the highest performance working fluid for liquid-vapor phase change due to its high thermal conductivity and large enthalpy of vaporization. However, a wide range of industrial systems require using low surface tension liquids where heat transfer enhancement has proved challenging for boiling and evaporation. Here, we enable a new paradigm of phase change heat transfer, which favors high volatility, low surface tension liquids rather than water. We utilized a nanoporous membrane of ≈600 nm thickness and <140 nm pore diameters supported on efficient liquid supply architectures, decoupling capillary pumping from viscous loss. Proof-of-concept devices were microfabricated and tested in a custom-built environmental chamber. We used R245fa, pentane, methanol, isopropyl alcohol, and water as working fluids with devices of total membrane area varying from 0.017 to 0.424 cm2. We realized a device-level pure evaporation heat flux of 144 ± 6 W/cm2 for water, and the highest evaporation heat flux was obtained with pentane at 550 ± 90 W/cm2. We developed a three-level model to understand vapor dynamics near the interface and thermal conduction within the device, which showed good agreement with experiments. We then compared pore-level heat transfer of different fluids, where R245fa showed approximately 10 times the performance of water under the same working conditions. Finally, we illustrate the usefulness of a figure of merit extracted from the kinetic theory for evaporation. The current work provides fundamental insights into the evaporation of low surface tension liquids, which can impact various applications such as refrigeration and air conditioning, petroleum and solvent distillation, and on-chip electronics cooling.

The simultaneous imaging of magnetic fields and temperature (MT) is important in a range of applications, including studies of carrier transport and semiconductor device characterization. Techniques exist for separately measuring temperature (e.g., infrared (IR) microscopy, micro-Raman spectroscopy, and thermoreflectance microscopy) and magnetic fields (e.g., scanning probe magnetic force microscopy and superconducting quantum interference devices). However, these techniques cannot measure magnetic fields and temperature simultaneously. Here, we use the exceptional temperature and magnetic field sensitivity of nitrogen vacancy (NV) spins in conformally coated nanodiamonds to realize simultaneous wide-field MT imaging at the device level. Our "quantum conformally attached thermo-magnetic" (Q-CAT) imaging enables (i) wide-field, high-frame rate imaging (100-1000 Hz); (ii) high sensitivity; and (iii) compatibility with standard microscopes. We apply this technique to study the industrially important problem of characterizing multifinger gallium nitride high-electron mobility transistors (GaN HEMTs). We spatially and temporally resolve the electric current distribution and resulting temperature rise, elucidating functional device behavior at the microscopic level. The general applicability of Q-CAT imaging serves as an important tool for understanding complex MT phenomena in material science, device physics, and related fields.

We present the development of a self-referencing Yb-fiber laser frequency comb based on octave-spanning supercontinuum generation from dispersion-engineered silicon nitride waveguides, fabricated with the standard, readily available plasma-enhanced chemical vapor deposition (PECVD) process.

Atomically thin two-dimensional (2D) materials have shown great potential for applications in nanoscale electronic and optical devices. A fundamental property of these 2D flakes that needs to be well characterized is the thermal expansion coefficient (TEC), which is instrumental to the dry transfer process and thermal management of 2D material-based devices. Yet, most of current studies of 2D materials' TEC extensively rely on simulations due to the difficulty of performing experimental measurements on an atomically thin, micron-sized, and optically transparent 2D flake. In this work, we present a three-substrate approach to characterize the TEC of monolayer molybdenum disulfide (MoS2) using micro-Raman spectroscopy. The temperature dependence of the Raman peak shift was characterized with three different substrate conditions, from which the in-plane TEC of monolayer MoS2 was extracted based on lattice symmetries. Independently from two different phonon modes of MoS2, we measured the in-plane TECs as (7.6±0.9)×10-6 1/K and (7.4±0.5)×10-6 1/K, respectively, which are in good agreement with previously reported values based on first principle calculations. Our work is not only useful for thermal mismatch reduction during material transfer or device operation, but provides a general experimental method that does not rely on simulations to study key properties of 2D materials.

Corrections have been made to the above named paper.

We present a high-heat-flux cooling device for advanced thermal management of electronics. The device incorporates nanoporous membranes supported on microchannels to enable thin-film evaporation. The underlying concept takes advantage of the capillary pressure generated by small pores in the membrane, and minimizes the viscous loss by reducing the membrane thickness. The heat transfer and fluid flow in the device were modeled to determine the effect of different geometric parameters. With the optimization of various parameters, the device can achieve a heat transfer coefficient in excess of 0.05 kW/cm2-K, while dissipating a heat flux of 1 kW/cm2. When applied to power electronics, such as GaN high-electron-mobility transistors, this membrane-based evaporative cooling device can lower the near-junction temperature by more than 40 K compared with contemporary single-phase microchannel coolers.

With current trends toward increased power in smaller devices and packages, the search continues for high-performance thermal management solutions for MOSFETs, high-electron mobility transistors, and other power electronics. Microjet impingement cooling has been shown to produce high heat transfer capabilities for electronics cooling. In this paper, microjets based on well-studied geometries were embedded within the substrate of a heat-producing device, removing several layers of thermal resistance typically found in advanced packages. In contrast with competing liquid cooling solutions that use expensive materials and processes, industry-standard silicon microfabrication techniques were used to build a low-cost microjet cooler. Numerical analysis of the embedded microjet device showed average heat transfer coefficients greater than 250 kW/m(2) . K and low peak temperature rises in high power-density devices. Using an innovative micro-Raman thermography technique, experimental measurements with a 1-mu m spatial resolution were taken, supporting the numerically predicted heat transfer coefficients on a device with a heat flux of over 5 kW/cm(2) and a temperature rise of 46 degrees C. As devices continue to become miniaturized, both low-cost embedded microjet cooling technology and micro-Raman thermography may play a vital role in the design, operation, and evaluation of advanced high power-density electronics.

High power density electronics are severely limited by current thermal management solutions which are unable to dissipate the necessary heat flux while maintaining safe junction temperatures for reliable operation. We designed, fabricated, and experimentally characterized a microfluidic device for ultra-high heat flux dissipation using evaporation from a nanoporous silicon membrane. With ~100 nm diameter pores, the membrane can generate high capillary pressure even with low surface tension fluids such as pentane and R245fa. The suspended ultra-thin membrane structure facilitates efficient liquid transport with minimal viscous pressure losses. We fabricated the membrane in silicon using interference lithography and reactive ion etching and then bonded it to a high permeability silicon microchannel array to create a biporous wick which achieves high capillary pressure with enhanced permeability. The back side consisted of a thin film platinum heater and resistive temperature sensors to emulate the heat dissipation in transistors and measure the temperature, respectively. We experimentally characterized the devices in pure vapor-ambient conditions in an environmental chamber. Accordingly, we demonstrated heat fluxes of 665 ± 74 W/cm 2 using pentane over an area of 0.172 mm × 10 mm with a temperature rise of 28.5 ± 1.8 K from the heated substrate to ambient vapor. This heat flux, which is normalized by the evaporation area, is the highest reported to date in the pure evaporation regime, that is, without nucleate boiling. The experimental results are in good agreement with a high fidelity model which captures heat conduction in the suspended membrane structure as well as non-equilibrium and sub-continuum effects at the liquid–vapor interface. This work suggests that evaporative membrane-based approaches can be promising towards realizing an efficient, high flux thermal management strategy over large areas for high-performance electronics.

Due to the high dissipated power densities present in GaN high-electron-mobility transistors (HEMTs) in high-power radio frequency applications, thermal analysis and thermal management of these devices are important in achieving their full potential. In this paper, we present a fundamental study of the transient thermal behavior of GaN HEMTs to aid in understanding the complex contributions of multidimensional thermal spreading, multiple epitaxial layers, multiple gate fingers, and thermal boundary resistance to the temperature rise. This complex behavior cannot be accurately described by one or two thermal time constants. Rather, a broad spectrum of time constants from ~1 ns up to several milliseconds are present in the device due to aggressive multidimensional thermal spreading from the narrow region of power dissipation next to each HEMT gate through the substrate, die attach, and package. In order to accurately model the temperature response, at least one time constant per decade over the timescale of interest is required. These findings are crucial in developing an intuitive understanding of the transient thermal behavior of GaN HEMTs and properly accounting for transient temperature rise in electrothermal modeling of high-power GaN-based amplifiers.

Thin film evaporation on microstructured surfaces is a promising strategy for high heat flux thermal management. To enhance fundamental understanding and optimize the overall heat transfer performance across a few microns thick liquid film, however, requires detailed thermal characterizations. Existing characterization techniques using infrared thermometry or contact-mode temperature sensors such as thermocouples and resistance temperature detectors cannot accurately measure the temperature of the thin liquid film near the three-phase contact line due to the restriction of low spatial resolution or temperature sensitivity. In this work, we developed a non-contact, in situ temperature measurement approach using a custom micro-Raman spectroscopy platform which has a spatial resolution of 1.5 μm and temperature sensitivity within 0.5 °C. We utilized this method to characterize thin film evaporation from fabricated silicon micropillar arrays. We showed that we can accurately measure the local thin film temperature and map the overall temperature distribution on the structured surfaces at different heat fluxes. We investigated the effects of micropillar array geometries and showed that the temperature rise of the liquid was reduced with the decreasing micropillar pitch due to the increased fraction of the thin film area. This work offers a promising method with micro-Raman to quantify phase change heat transfer on microstructured surfaces. This characterization technique can significantly aid mechanistic understanding and wick structure optimization for various phase-change based thermal management devices.

Gallium nitride (GaN) high-electron mobility transistors (HEMTs) are a key technology for realizing next generation high-power RF amplifiers and high-efficiency power converters. However, elevated channel temperatures due to self-heating often severely limit their power handling capability. Although the steady-state thermal behavior of GaN HEMTs has been studied extensively, significantly fewer studies have considered their transient thermal response. In this paper, we report a methodology for measuring the transient temperature rise and thermal time constant spectrum of GaN HEMTs via time-resolved micro-Raman thermometry with a temporal resolution of 30 ns. We measured a broad spectrum of time constants from approximate to 130 ns to approximate to 3.2 ms that contribute to the temperature rise of an ungated GaN-on-SiC HEMT due to aggressive, multidimensional heat spreading in the die and die-attach. Our findings confirm previous theoretical analysis showing that one or two thermal time constants cannot adequately describe the transient temperature rise and that the temperature reaches steady-state at approximate to 16L(2)/pi(2)alpha, where L and alpha are the thickness and thermal diffusivity of the substrate. This paper provides a practical methodology for validating transient thermal models of GaN HEMTs and for obtaining experimental values of the thermal resistances and capacitances for compact electrothermal modeling.

As semiconductor devices based on silicon reach their intrinsic material limits, compound semiconductors, such as gallium nitride (GaN), are gaining increasing interest for high performance, solid-state transistor applications. Unfortunately, higher voltage, current, and/or power levels in GaN high electron mobility transistors (HEMTs) often result in elevated device temperatures, degraded performance, and shorter lifetimes. Although micro-Raman spectroscopy has become one of the most popular techniques for measuring localized temperature rise in GaN HEMTs for reliability assessment, decoupling the effects of temperature, mechanical stress, and electric field on the optical phonon frequencies measured by micro-Raman spectroscopy is challenging. In this work, we demonstrate the simultaneous measurement of temperature rise, inverse piezoelectric stress, thermoelastic stress, and vertical electric field via micro-Raman spectroscopy from the shifts of the E2 (high), A1 longitudinal optical (LO), and E2 (low) optical phonon frequencies in wurtzite GaN. We also validate experimentally that the pinched OFF state as the unpowered reference accurately measures the temperature rise by removing the effect of the vertical electric field on the Raman spectrum and that the vertical electric field is approximately the same whether the channel is open or closed. Our experimental results are in good quantitative agreement with a 3D electro-thermo-mechanical model of the HEMT we tested and indicate that the GaN buffer acts as a semi-insulating, p-type material due to the presence of deep acceptors in the lower half of the bandgap. This implementation of micro-Raman spectroscopy offers an exciting opportunity to simultaneously probe thermal, mechanical, and electrical phenomena in semiconductor devices under bias, providing unique insight into the complex physics that describes device behavior and reliability. Although GaN HEMTs have been specifically used in this study to demonstrate its viability, this technique is applicable to any solid-state material with a suitable Raman response and will likely enable new measurement capabilities in a wide variety of scientific and engineering applications.

Massachusetts Institute of Technology. Microsystems Technology Laboratories. GaN Energy Initiative

We investigated the evaporative cooling performance of a nanoporous membrane based thermal management solution designed for ultra-high heat flux dissipation from high performance integrated circuits. The biporous evaporation device utilizes thermally-connected, mechanically-supported, high capillarity membranes that maximize thin film evaporation and high permeability liquid supply channels that minimize viscous pressure losses. The 600 nm thick membrane was created on a silicon on insulator (SOI) wafer, fusion-bonded to a separate wafer with larger liquid channels. Overall device performance arising from non-uniform heating and evaporation of methanol was captured experimentally. Heat fluxes up to 412 W/cm 2 over an area of 0.4×5 mm, at a temperature rise of 24.1 K from the heated substrate to ambient vapor, were obtained. These results are in good agreement with a high-fidelity coupled fluid convection and solid conduction compact model that incorporates non-equilibrium and sub-continuum effects at the liquid-vapor interface. This work provides a proof-of-concept demonstration of our biporous evaporation device. Simulations of the validated model at optimized operating conditions and with improved working fluids, predict heat dissipation in excess of 1 kW/cm 2 with a device temperature rise under 30 K, for this scalable cooling approach.

We demonstrate simultaneous wide-field (>1000 μm2) measurements of magnetic fields and temperature on GaN HEMTs using nitrogen vacancy centers in nanodiamonds.

Due to the high dissipated power densities in gallium nitride (GaN) high electron mobility transistors (HEMTs), temperature measurement techniques with high spatial resolution, such as micro-Raman thermography, are critical for ensuring device reliability. However, accurately determining the temperature rise in the ON state of a transistor from shifts in the Raman peak positions requires careful decoupling of the simultaneous effects of temperature, stress, strain, and electric field on the optical phonon frequencies. Although it is well-known that the vertical electric field in the GaN epilayers can shift the Raman peak positions through the strain and/or stress induced by the inverse piezoelectric (IPE) effect, previous studies have not shown quantitative agreement between the strain and/or stress components derived from micro-Raman measurements and those predicted by electro-mechanical models. We attribute this discrepancy to the fact that previous studies have not considered the impact of the electric field on the optical phonon frequencies of wurtzite GaN apart from the IPE effect, which results from changes in the atomic coordinates within the crystal basis and in the electronic configuration. Using density functional theory, we calculated the zone center E2 (high), A1 (LO), and E2 (low) modes to shift by −1.39 cm−1/(MV/cm), 2.16 cm−1/(MV/cm), and −0.36 cm−1/(MV/cm), respectively, due to an electric field component along the c-axis, which are an order of magnitude larger than the shifts associated with the IPE effect. Then, we measured changes in the E2 (high) and A1 (LO) Raman peak positions with ≈1 μm spatial resolution in GaN HEMTs biased in the pinched OFF state and showed good agreement between the strain, stress, and electric field components derived from the measurements and our 3D electro-mechanical model. This study helps to explain the reason the pinched OFF state is a suitable reference for removing the contributions of the electric field and the IPE-induced stress from the temperature rise in the ON state and suggests that the IPE-induced stress in the GaN buffer is an order of magnitude smaller than previously believed. Our analysis and experimental results support previous theoretical studies discussing the electric field dependence of optical phonon frequencies apart from the IPE effect and suggest that this is a general phenomenon occurring in all wurtzite and zincblende crystals. The total electric field dependence of the optical phonon frequencies in piezoelectric crystals is a critical consideration in accurately characterizing the stress, strain, electric field, and temperature distributions in microelectronic devices via micro-Raman spectroscopy.

The present work describes the method by which the thermal resistance of a flat heat pipe spreader can be more accurately computed. The total effectiveness of the heat spreader is dependent on the one-dimensional (R-1D) thermal resistance and the thermal spreading resistance (R-s). Recently developed more accurate methods from the literature were used to calculate the spreading resistance by taking into account all the multiple layers of the heat pipe. Both R-1D and R-s depend to a large extent on the effective thermal conductivity of the wick. The calculation of effective thermal conductivity of wick is demonstrated using well-defined silicon micropillars. The effect of interfacial heat transfer resistance, which is usually neglected on the wick's thermal resistance is also discussed. Finally, the effective thermal conductivity of a flat heat pipe as a function of vapor chamber size and effective wick thermal conductivity is calculated. As the overall device performance strongly depends on the estimation of wick thermal resistance, the results of this study show that an effective thermal conductivity that is equal to or better than diamond can be attained with proper design. Furthermore, this work provides design tools that can be used to optimize the overall device level thermal performance.

Micro-Raman thermography is one of the most popular techniques for measuring local temperature rise in gallium nitride (GaN) high electron mobility transistors with high spatial and temporal resolution. However, accurate temperature measurements based on changes in the Stokes peak positions of the GaN epitaxial layers require properly accounting for the stress and/or strain induced by the inverse piezoelectric effect. It is common practice to use the pinched OFF state as the unpowered reference for temperature measurements because the vertical electric field in the GaN buffer that induces inverse piezoelectric stress/strain is relatively independent of the gate bias. Although this approach has yielded temperature measurements that agree with those derived from the Stokes/anti-Stokes ratio and thermal models, there has been significant difficulty in quantifying the mechanical state of the GaN buffer in the pinched OFF state from changes in the Raman spectra. In this paper, we review the experimental technique of micro-Raman thermography and derive expressions for the detailed dependence of the Raman peak positions on strain, stress, and electric field components in wurtzite GaN. We also use a combination of semiconductor device modeling and electro-mechanical modeling to predict the stress and strain induced by the inverse piezoelectric effect. Based on the insights gained from our electro-mechanical model and the best values of material properties in the literature, we analyze changes in the E2 high and A1 (LO) Raman peaks and demonstrate that there are major quantitative discrepancies between measured and modeled values of inverse piezoelectric stress and strain. We examine many of the hypotheses offered in the literature for these discrepancies but conclude that none of them satisfactorily resolves these discrepancies. Further research is needed to determine whether the electric field components could be affecting the phonon frequencies apart from the inverse piezoelectric effect in wurtzite GaN, which has been predicted theoretically in zinc blende gallium arsenide (GaAs).

Although GaN high electron mobility transistors (HEMTs) are one of the most promising semiconductor technologies for high power and high frequency applications, high device temperatures often lead to degraded performance and reliability. Most reports on the thermal characterization of GaN HEMTs have focused on the steady state temperature rise in spite of the prevalence of time-dependent power dissipation in aerospace, defense, and communications applications. In this work, we utilize analytical solutions to the transient heat conduction equation to investigate the thermal time constants associated with self-heating in GaN HEMTs. In contrast to previous reports and commonly held notions in the GaN device industry, we demonstrate that one or two thermal time constants does not adequately describe transient self-heating. Due to aggressive heat spreading from the small heat source in GaN HEMTs, a wide range of thermal time constants up to ≈10 ms are important for devices on sapphire substrates. These theoretical arguments are supported with transient temperature measurements obtained by time-resolved micro-Raman spectroscopy for a GaN-on-sapphire ungated HEMT.