Transient Thermoreflectance Measurements Research Articles

Polydimethylsiloxane (PDMS)-assisted non-destructive transient thermoreflectance characterizations.

The conventional pump-probe thermoreflectance (pump-probe TR) techniques are routinely performed on a thin (typically <100nm) metal transducer deposited on samples, ensuring the most fundamental principle of thermoreflectance: the reflectance change (∆R/R) of the sample surface is directly and linearly related to the temperature change (∆T) within a finite temperature range. However, general metal coating methods may damage the sample by forming a mixed layer with the sample. In this article, we present a non-destructive pump-probe transient thermoreflectance (TTR) characterization using the polydimethylsiloxane (PDMS)-assisted metal transfer technique. Our method utilizes PDMS dry transfer to transfer common transducer metals (Au and Al) to the target substrate. This method effectively avoids impurities and damage to the sample. In particular, we have utilized Au as a transition layer to study the application of Al transfer in TTR measurements. The substrate thermal conductivity measurement results show that they can all be used for TTR measurement, and the thermal boundary conductance results show that the transferred metals have low bonding to the substrate. In addition, Al/Au can be transferred over a large area at room temperature compared to Au. After measurement, the samples can be restored to their original state by a simple cleaning method. This method provides a simple and reliable way to characterize samples without metal coating for TTR.

Deep learning-based data processing method for transient thermoreflectance measurements

Pump–probe thermoreflectance has been commonly applied for characterizing the thermal properties of materials. Generally, a reliable and efficient non-linear fitting process is often implemented to extract unknown thermal parameters during the pump–probe thermoreflectance characterizations. However, when it comes to processing large amounts of data acquired from similar structural samples, non-linear fitting process appears to be very time-consuming and labor-intensive to search for the best fitting for every testing curve. Herein, we propose to apply deep learning (DL) approach to nanosecond transient thermoreflectance technique for high-throughput experimental data processing. We first investigated the effect of training set parameters (density and bounds) on the predictive performance of the DL model, providing a guidance to optimize the DL model. Then, the DL model is further verified in the measurement of the bulk sapphire, SiC, diamond samples, and GaN-based multilayer structures, demonstrating its capability of analyzing the results with high accuracy. Compared to the conventional non-linear fitting method (such as Global Optimization), the computation time of the new model is 1000 times lower. Such a data-driven DL model enables the faster inference and stronger fitting capabilities and is particularly efficient and effective in processing data acquired from wafer-level measurements with similar material structures.

Efficient thermal dissipation in wafer-scale heterogeneous integration of single-crystalline β-Ga2O3 thin film on SiC

The semiconductor, β-Ga2O3 is attractive for applications in high power electronic devices with low conduction loss due to its ultra-wide bandgap (∼4.9 eV) and large Baliga's figure of merit. However, the thermal conductivity of β-Ga2O3 is much lower than that of other wide/ultra-wide bandgap semiconductors, such as SiC and GaN, which results in the deterioration of β-Ga2O3-based device performance and reliability due to self-heating. To overcome this problem, a scalable thermal management strategy was proposed by heterogeneously integrating wafer-scale single-crystalline β-Ga2O3 thin films on a highly thermally conductive SiC substrate. Characterization of the transferred β-Ga2O3 thin film indicated a uniform thickness to within ±2.01%, a smooth surface with a roughness of 0.2 nm, and good crystalline quality with an X-ray rocking curves (XRC) full width at half maximum of 80 arcsec. Transient thermoreflectance measurements were employed to investigate the thermal properties. The thermal performance of the fabricated β-Ga2O3/SiC heterostructure was effectively improved in comparison with that of the β-Ga2O3 bulk wafer, and the effective thermal boundary resistance could be further reduced to 7.5 m2K/GW by a post-annealing process. Schottky barrier diodes (SBDs) were fabricated on both a β-Ga2O3/SiC heterostructured material and a β-Ga2O3 bulk wafer. Infrared thermal imaging revealed the temperature increase of the SBDs on β-Ga2O3/SiC to be one quarter that on the β-Ga2O3 bulk wafer with the same applied power, which suggests that the combination of the β-Ga2O3 thin film and SiC substrate with high thermal conductivity promotes heat dissipation in β-Ga2O3-based devices.

Time Resolved Hyperspectral Quantum Rod Thermography of Microelectronic Devices: Temperature Transients in a GaN HEMT

The trend of miniaturization and rapid progress in the cost-competitive microelectronic industry require high resolution, fast, accurate and cost-effective thermal characterization techniques. These techniques aid the assessment of reliability and performance bench-marking of new device designs for the realistic operation conditions. We present a time resolved, surface sensitive, sub-micron resolution wide field thermal imaging technique, exploiting fast radiative recombination rates of quantum rod photoluminescence to probe temperature transients in semiconductor devices. We demonstrate a time resolution of $20~\mu \text{s}$ on a single finger AlGaN/GaN HEMT. This technique provides an image of the surface temperature transients regardless of the device design/material system under test. The results were verified with transient thermo-reflectance measurements.

Measurement and multilayer model of cooling of gold nanoparticles: Transient thermoreflectance experiments and multilayer analytical modeling

We derive an analytical model of diffusive thermal transport in multilayer structures of spherical symmetry and apply it to transient thermoreflectance measurements of gold nanoparticles embedded in a polymer matrix. This multilayer approach significantly improves the quantitative measurement of material thermal properties, in comparison with single-layer methods. The model adapts the typical planar transfer matrix model to a spherical geometry, and we apply it to transient thermoreflectance (TTR) experiments on gold nanoparticles embedded in a polymer matrix, to published TTR data for aqueous platinum nanoparticles, and also to example systems of aqueous gold and platinum nanoparticles. We measure a thermal boundary conductance value of 410MW/m2K at the nanoparticle gold/polymer interface. The sensitivity of the TTR signal to system thermal properties is predicted as a function of the particle/matrix thermal boundary resistance (TBR), and we discuss the differentiation of TBR and capping layer effects on a TTR signal.

Thermal characterization of gallium nitride p-i-n diodes

In this study, various thermal characterization techniques and multi-physics modeling were applied to understand the thermal characteristics of GaN vertical and quasi-vertical power diodes. Optical thermography techniques typically used for lateral GaN device temperature assessment including infrared thermography, thermoreflectance thermal imaging, and Raman thermometry were applied to GaN p-i-n diodes to determine if each technique is capable of providing insight into the thermal characteristics of vertical devices. Of these techniques, thermoreflectance thermal imaging and nanoparticle assisted Raman thermometry proved to yield accurate results and are the preferred methods of thermal characterization of vertical GaN diodes. Along with this, steady state and transient thermoreflectance measurements were performed on vertical and quasi-vertical GaN p-i-n diodes employing GaN and Sapphire substrates, respectively. Electro-thermal modeling was performed to validate measurement results and to demonstrate the effect of current crowding on the thermal response of quasi-vertical diodes. In terms of mitigating the self-heating effect, both the steady state and transient measurements demonstrated the superiority of the tested GaN-on-GaN vertical diode compared to the tested GaN-on-Sapphire quasi-vertical structure.

Transient Thermoreflectance for Gate Temperature Assessment in Pulse Operated GaN-Based HEMTs

An experimental method to measure the gate metal temperature of GaN-based high electron mobility transistors is demonstrated. The technique is based on transient thermoreflectance measurements performed from the backside of the device. The thermoreflectance coefficient of the gate metal was calibrated by correlating the relative change of its optical reflectivity with the temperature change measured in the GaN layer using time-resolved Raman thermography during the device cooling transient. Simulated temperature transients were in good agreement with the experimental data. The main advantage of this new method is that it enables the direct assessment of gate metal temperature under device pulsed operation regardless of the device design.

Adhesion, Modulus and Thermal Conductivity of Porous Epoxy Film on Silicon Wafers

An 8 μm epoxy film deposited on a 350 μm Si (100) Si wafer with a 0.4 μm Au transducer film deposited on top of the polymer film was used to evaluate the thermal conductivity, the modulus of the porous film, and the initiation of spalling upon laser beam irradiation on the back side of the Si wafer. The polymer films were characterized for pore microstructure using scanning electron microscopy and energy dispersive spectrometry. The polymer films were characterized using transient thermo reflectance (TTR) with laser beams illuminating the Au layer. The TTR signal from the polymer film showed only the thermal component and was characteristic of variations associated with thermal conduction into the film. To induce spalling, the back side was illuminated with a Nd-YAG laser beam with a 532 nm wavelength, pulse energy density 1.8 J/cm2, and a repetition rate of 10 Hz for 10 s in conjunction with TTR measurements on the front side. The TTR signal from the polymer film subjected to laser beam incidence from the backside of the Si wafer showed both the thermal and the acoustic components. The acoustic component was used to detect the initial stages of spalling or delamination. The acoustic oscillations were modeled using a modified wave equation to determine the velocity of sound and the modulus of the film. The results were also used to determine the effect of porosity on the modulus of the polymer film. The TTR signal was found to be very sensitive to detection of delamination without complete separation of the film.

Interface Thermal Conductance Between Metal Films and Copper

Transient thermo-reflectance measurements were made on metals films deposited on Cu film. The Cu film was deposited on Si (001) and sapphire (0001) substrates. The metal films that were deposited include Al, Au, Sn, Zn, and In. The results were modeled using one-dimensional heat equation to determine thermal conductance of interfaces between the metal film and Cu (film/Cu) and Cu and Si (Cu/Si), or Cu and sapphire (Cu/sapphire) in each sample. The results were used to determine the importance of microstructural parameters such as surface roughness, lattice mismatch, solid solubility, and surface energy of the metal film on Cu. The experimental values of interface thermal conductance, although smaller in magnitude, were interpreted in terms of the predicted values by diffuse mismatch model after including the effect of the microstructural parameters. In particular, interface roughness, lattice mismatch, solid solubility, and wettability were found to be important parameters as these are responsible for good atomic level contact between the metal film and Cu.

Heat Transfer Across Metal-Dielectric Interfaces During Ultrafast-Laser Heating

Heat transfer across metal-dielectric interfaces involves transport of electrons and phonons accomplished either by coupling between phonons in metal and dielectric or by coupling between electrons in metal and phonons in dielectric. In this work, we investigate heat transfer across metal-dielectric interfaces during ultrafast-laser heating of thin metal films coated on dielectric substrates. By employing ultrafast-laser heating that creates strong thermal nonequilibrium between electrons and phonons in metal, it is possible to isolate the effect of the direct electron–phonon coupling across the interface and thus facilitate its study. Transient thermo-reflectance measurements using femtosecond laser pulses are performed on Au–Si samples while the simulation results based on a two-temperature model are compared with the measured data. A contact resistance between electrons in Au and phonons in Si represents the coupling strength of the direct electron–phonon interactions at the interface. Our results reveal that this contact resistance can be sufficiently small to indicate strong direct coupling between electrons in metal and phonons in dielectric.

Ultrafast-laser Modification of Thermoelectric Sb2Te3 Thin Films

ABSTRACTWe have modified Sb2Te3 thin film thermoelectric materials by scanning a femtosecond laser across the film surface to create track-like nanostructures. These nanotracks have widths of 50-80 nm and a periodicity of ∼ 130 nm. We show that the nanotrack morphology is highly dependent on laser fluence and scan speed. Moreover, we performed transient thermoreflectance measurements on a laser-irradiated film and found a thermal conductivity reduction of 4.5% in the nanostructured regions compared to that of the unmodified regions. These results suggest the potential use of femtosecond pulsed lasers to create nanostructured thermoelectric materials with improved performance.

Substrate influence in electron–phonon coupling measurements in thin Au films

Accurate understanding and measurement of the energy transfer mechanisms during thermal nonequilibrium between electrons and the surrounding material systems is critical for a wide array of applications. With device dimensions decreasing to sizes on the order of the thermal penetration depth, the equilibration of the electrons could be effected by boundary effects in addition to electron–phonon coupling. In this study, the rate of electron equilibration in 20 nm thick Au films is measured with the Transient ThermoReflectance (TTR) technique. At very large incident laser fluences which result in very high electron temperatures, the electron–phonon coupling factors determined from TTR measurements deduced using traditional models are almost an order of magnitude greater than predicted from theory. By taking excess electron energy loss via electron-substrate transport into account with a proposed three temperature model, TTR electron–phonon coupling factor measurements are more in line with theory, indicating that in highly nonequilibrium situations, the high temperature electron system looses substantial energy to the substrate in addition to that transferred to the film lattice through coupling.

Signal analysis and characterization of experimental setup for the transient thermoreflectance technique

The transient thermoreflectance (TTR) technique is a powerful optical pump-probe technique often used to measure thermal properties and monitor ultrafast processes. The technique has been used to measure a range of properties including the thermal conductivities of thin films and electron-phonon coupling factors to mention a few. TTR measurements are sensitive to residual heating and misalignment, which can lead to erroneous analysis of TTR data. To minimize these errors, we have developed a simple phase correction technique to reduce errors associated with residual heating and other background noise. Besides its simple implementation, the technique also requires no previous knowledge of the transient reflectance response. The technique is verified with simulated experimental data. The importance of proper alignment of the pump and probe beams over the entire range of pump-probe time delays of interest is discussed, along with a means of quantifying error associated with misalignment.

Minimizing the uncertainties associated with the measurement of thermal properties by the Transient thermo-reflectance method

An approach for optimizing the transient thermo-reflectance (TTR) measurement of thermal properties is presented. The influence of the most important parameters of the system on the accuracy of the TTR measurements is investigated. An overall performance criterion is defined based on the responsivity of a given system and the thermoreflectance coefficient of the sample under test. It is shown that in order to obtain the smallest measurement uncertainty one should use a metallic absorption layer with the highest possible thermoreflectance coefficient and then compute the optimum thickness of that layer by maximizing the responsivity of the TTR system. The responsivity represents the sensitivity of the TTR method to the measurement of the thermal properties of a sample and relates the overall uncertainty of a TTR measurement to the uncertainties associated with the TTR apparatus.

Transient thermo-reflectance measurements of the thermal conductivity and interface resistance of metallized natural and isotopically-pure silicon

The transient thermoreflectance method has been used to measure the thermal conductivity of natural silicon and isotopically-pure silicon-28 layers that are epitaxially grown on natural silicon substrates. The measurements were performed at room temperature for both a low level (10 16) and a higher level (2×10 19) of Boron doping of the epitaxial layers. The results indicate a gain of approximately 55% in the thermal conductivity of Si 28 as compared to that of natural Si, at both low and higher levels of doping, and a loss of approximately 19% for both types of silicon due to the higher level of doping.

Electron-phonon relaxation rate in single-crystal lanthanum cuprate

We report the results of phonon relaxation processes obtained from picosecond transient thermoreflectance measurements of single-crystal La2CuO4+y (y∼0.06). Nonequilibrium heating was found to occur on a picosecond time scale. The time scale of the nonequilibrium heating is a sensitive measurement of the modest electron-phonon interaction (λ≤0.1) in the nonsuperconducting La2CuO4+y.

Transient thermoreflectance from thin metal films

This report describes the first demonstration of thermal diffusivity measurements using picosecond transient thermoreflectance (TTR). Although previously reported methods of measuring thermal transport properties of thin films require precise knowledge of the thermal properties of the substrate, this technique allows measurements on films as thin as 100 nm without any evidence of substrate interaction. The TTR measurement is modeled with a one-dimensional heat flow equation using a two-parameter fitting routine to determine the thermal diffusivity. The validity of our approach is confirmed by the TTR measured thermal diffusivity of single crystal nickel. We have also measured the thermal diffusivity of sputtered and evaporated single element metal films. Preliminary results from TTR measurements on compositionally modulated structures are also presented.

Generation of nonequilibrium electron and lattice temperatures in copper by picosecond laser pulses.

Picosecond laser pulses can be used to generate and probe nonequilibrium electron and lattice temperature differences in metals. This is demonstrated by transient thermoreflectance measurements on copper in the vicinity of the d-band absorption edge. Our measurements are in agreement with an energy-balance model based on a separation of the one-dimensional heat-flow equation into electron and lattice subsystems.