Transient Thermoreflectance Imaging Research Articles

(Invited) Sub-Bandgap Thermoreflectance Imaging of Ultra-Wide Bandgap Semiconductors

Over the past few decades, wide bandgap semiconductors (WBG) have been used to fabricate high power and high frequency devices. Device structures, such as Gallium Nitride (GaN) High Electron Mobility Transistors (HEMTs), have begun to be implemented in commercial products such as electrical vehicles and laptop power chargers. With the continuous increase in energy demand, however, extremely high voltage (>10 kV) applications are needed to increase the grid resiliency, storage, and transmission efficiency. Ultra-wide bandgap (UWBG) Semiconductors, with bandgaps >4 eV, could potentially be used to overcome this barrier. Pushing the maximum voltage limit of these devices, however, can lead to significant power dissipation and excessive joule heating.Unfortunately, the thermal conductivity of some UWBG semiconductors, such as Gallium Oxide, is inherently low and thus requires both device and package level thermal management. Innovative solutions, such as high thermal conductivity substrates, are required to reduce the active channel temperature and extend the device lifetime and reliability. In order to successfully implement these technologies, accurate measurement of the peak temperature and thermal distribution (both vertical and lateral) is necessary. Depth-averaged vertical temperature gradients have been demonstrated via Raman thermometry but this approach can become significantly time consuming if attempting to perform transient or lateral maps. On the other hand, Transient Thermoreflectance Imaging (TTI) has shown the potential to provide high throughput thermal images with high spatial resolution (100 nm) and temporal resolution (50 ns).Traditionally, TTI has been used to probe the temperature of metals due to their inherent high reflectivity. Nevertheless, recent studies have demonstrated the ability to probe the surface temperature of the semiconductor when using near-bandgap illumination sources. Due to the high bandgap of UWBG semiconductors, the maturity of high transmission UV sources, optics and detectors has not yet been fully developed. This study investigates the applicability of sub-bandgap wavelengths to probe the active channel temperature. The thermal properties of both Gallium Oxide transistors and Transfer Length Method (TLM) structures are investigated using TTI. The accuracy of the channel temperature is assessed by direct comparison to the adjacent metal contact temperature. A finite element model (FEM) is developed to understand the thermal transport in low thermally conductive semiconductors. Finally, a 3D analytical model is used to extract thermal properties such as the thermal conductivity of the thin film layers.Figure a) Optical CCD Image of Gallium Oxide Transfer Length Method (TLM) Structure b) Transient Thermoreflectance Image (TTI) of Gallium Oxide TLM using sub-bandgap illumination wavelength. Figure 1

Experimental Investigation of Thermal Actuation Crosstalk in Phase-Change RF Switches Using Transient Thermoreflectance Imaging

This article reports an experimental investigation of transient heat distribution and thermal actuation crosstalk in phase-change material (PCM) germanium telluride (GeTe)-based radio frequency (RF) switches. The RF switches are designed and optimized for efficient thermal energy transport from the embedded microheater to the PCM. Various multiport miniaturized monolithically integrated complex RF components require several switches to be integrated extremely close to each other. Thermal crosstalk is crucial in multiport complex devices to ensure that individual tuning elements actuate independently without influencing nearby switches’ performance. Thermal cross section simulations are performed using multiphysics finite-element modeling (FEM) to optimize the heat distribution within the PCM channel and are experimentally validated via transient thermoreflectance imaging technique with ultrafast temporal and spatial resolution. The devices are fabricated in-house using a custom eight-layer microfabrication process. The optimum device bias conditions, melt-quench sequence, and thermal actuation crosstalk limits are experimentally validated to develop miniaturized monolithic densely packed complex reconfigurable phase-change circuits. It is the first-ever demonstration of experimental transient thermal insights on phase-change RF switches.

Monitoring the Joule heating profile of GaN/SiC high electron mobility transistors via cross-sectional thermal imaging

The development of high-quality gallium nitride (GaN) high electron mobility transistors (HEMTs) has provided opportunities for the next generation of high-performance radio frequency and power electronics. Operating devices with smaller length scales at higher voltages result in excessively high channel temperatures, which reduce performance and can have detrimental effects on the device's reliability. The thermal characterization of GaN HEMTs has traditionally been captured from either the top or bottom side of the device. Under this configuration, it has been possible to map the lateral temperature distribution across the device with optical methods such as infrared and Raman thermometry. Due to the presence of the gate metal, however, and often also the addition of a metal air bridge and/or field plate, the temperature of the GaN channel under the gate is typically inferred by numerical simulations. Furthermore, measuring the vertical temperature gradient across multiple epitaxial layers has shown to be challenging. This study proposes a new cross-sectional imaging technique to map the vertical temperature distribution in GaN HEMTs. Combining advanced cross-sectioning processing with the recently developed near bandgap transient thermoreflectance imaging technique, the full transient thermal distribution across a GaN HEMT is achieved. The cross-sectional thermal imaging of the GaN channel is used to study the effects of biasing on the Joule heating profile. Overall, the direct measurement of the GaN channel, capturing both the vertical and lateral gradient, will provide deeper insight into the device's degradation physics and supply further experimental data to validate previously developed electrothermal models.

Thermal Performance of GaN/Si HEMTs Using Near-Bandgap Thermoreflectance Imaging

Superlattice (SL) structures have been used to reduce the stress in the GaN epilayer of high-electron-mobility transistors (HEMTs). This has led to an improvement in their properties such as the breakdown voltage. The increase in thermal resistance associated with these structures, however, causes elevated device temperatures which may outweigh the benefits of this approach. To verify this, the thermal performance of SL structures on HEMTs must, thus, be accurately characterized. Transient thermoreflectance imaging (TTI) is an optical technique that can map the temperature distribution across a surface. For materials, such as gold, TTI shows a high spatial and temporal resolution. Consequently, the technique has been primarily used to monitor the gate metal temperature distribution in HEMTs. The origin of the localized heating in HEMTs, however, is known to be in the active GaN layer and, thus, accurate thermal characterization of the channel is necessary. Using a UV LED excitation source with a wavelength near the bandgap of GaN, TTI of GaN channels in HEMTs is presented and verified by comparing the gate metal temperature. To ensure a strong thermoreflectance signal from the GaN surface, the importance of using an excitation wavelength near the bandgap of the GaN channel is highlighted. The effect of the bandgap on the magnitude and the linearity of the thermoreflectance coefficient is presented and discussed. Overall, the improvements to TTI discussed in this article make the technique an accurate method to measure the temperature distribution in GaN HEMT SL structures.

The Effects of AlN and Copper Back Side Deposition on the Performance of Etched Back GaN/Si HEMTs

The breakdown voltage of GaN/Si high-electron-mobility transistors (HEMTs) for power electronics has shown to be improved by removing the silicon substrate. The drawback to this approach is the increase in the device’s thermal resistance, which limits the power dissipation that the device can achieve before severe degradation. This letter shows the ability to improve the thermal dissipation of these devices by depositing copper (Cu) below aluminum nitride (AlN) filled etched back GaN-on-Si HEMTs. The device’s channel temperature is measured via Raman thermometry. The device’s transient thermal dynamics is investigated via transient thermoreflectance imaging, and the temperature profile across the gate metal is monitored. In addition to the device’s thermal properties, a residual stress analysis of the GaN channel is performed via photoluminescence. A notable decrease in the tensile residual stress is observed with the removal of the substrate and the addition of the AlN and Cu layers. Overall, the backside copper is shown to decrease the gate temperature of the etched backed AlN filled devices while maintaining a high breakdown voltage.

GaN-On-Diamond HEMT Technology With TAVG = 176°C at PDC,max = 56 W/mm Measured by Transient Thermoreflectance Imaging

Record DC power has been demonstrated in AlGaN/GaN high electron mobility transistors fabricated using a substrate replacement process in which a thick diamond substrate is grown by chemical vapor deposition following removal of the original Si substrate. Crucial to the process is a ~30 nm thick SiN interlayer that has been optimized for thermal resistance. The reductions obtained in self-heating have been quantified by transient thermoreflectance imaging and interpreted using 3D numerical simulation. With a DC power dissipation level of 56 W/mm, the measured average and maximum temperatures in the gate-drain access region were 176 °C and 205 °C, respectively.

Scalable Modeling of Transient Self-Heating of GaN High-Electron-Mobility Transistors Based on Experimental Measurements

This paper details an extraction procedure to fully model the transient self-heating of transistors from a GaN HEMT technology. Frequency-resolved gate resistance thermometry (f-GRT) is used to extract the thermal impedance of HEMTs with various gate widths. A fully scalable analytical model is developed from the experimental results. In the second stage, transient thermoreflectance imaging (TTI) is used to bring deeper insights into the HEMTs’ temperature distribution by individually extracting the transient self-heating of each finger. TTI results are further used to successfully validate the f-GRT results and the modeling of the thermal impedance. Overall, f-GRT is demonstrated to be a fast and robust method for characterizing the transient thermal characteristics of a GaN HEMT. For the first time to the authors’ knowledge, a scalable model of the thermal impedance is extracted fully from experimental results.

Transient Self-Heating at Nanowire Junctions in Silver Nanowire Network Conductors

Nanostructured transparent conducting electrodes (TCEs) may be suitable replacements for transparent conductive oxides due to their optical, electrical, and mechanical properties. Because nanowire (NW) nanowire or tube–tube junctions are the transport bottlenecks in network-based conductors, understanding the properties of these junctions and their connectivity within the network is crucial to understanding and controlling electrical conduction through these networks. Quantifying local self-heating within the network can provide information on the coupled electrothermal response, local conduction pathways, and potential reliability. In this study, self-heating thermal transients within a silver NW network are characterized using high-resolution transient thermoreflectance imaging that provides high temporal (∼200 ns) and spatial (∼200 nm) resolution. The self-heating induced by an applied voltage pulse results in distinct temperature changes at microscopic hotspots formed at individual NW–NW junctions. For both heating and cooling cycles, thermal time constants less than $\text{1 }\mu {\text{s}}$ are observed at various hotspots. For a representative hotspot, line scans along two crossing NWs, taken at different time instants ranging from 0 to 2 μ s, show the temporal and spatial evolution of the temperature profile. We estimate the van der Waals force (∼−4.0244 N), contact width (∼5 nm), and interface thermal resistance (∼1.6 × 105 K/W) between NWs and the underlying substrate. A heat transfer model that considers local power generated at a hotspot, local coupling between the NWs and substrate, heat conduction along the NWs and heat transfer into the substrate, is developed and used to interpret the experimental data. The heat transfer model and experimental temperature profile help to quantify the local power generated at the hotspot and the fraction of this power propagating along each wire. The ability to resolve the local self-heating with such temporal and spatial resolution uniquely enables understanding of electrothermal response and current pathways in the distributed conductors.

Transient Thermal Characterization of AlGaN/GaN HEMTs Under Pulsed Biasing

The development of steady-state thermal characterization techniques for AlGaN/GaN high-electron mobility transistors (HEMTs) has been used to measure the device’s peak temperature under DC conditions. Despite these methods enabling the accurate quantification of the device’s effective thermal resistance and power density dependence, transient thermometry techniques are necessary to understand the nanoscale thermal transport within the active GaN layer where the highly localized joule heating occurs. One technique that has shown the ability to achieve this is transient thermoreflectance imaging (TTI). The accuracy of TTI is based on using the correct thermoreflectance coefficient. In the past, alternative techniques have been used to adjust the thermoreflectance coefficient to match the correct temperature rise in the device. This paper provides a new method to accurately determine the thermoreflectance coefficient of a given surface and is validated via an electrical method: gate resistance thermometry (GRT). Close agreement is shown between the temperature rise of the passivated gate metal measured by TTI and the averaged gate temperature monitored by GRT. Overall, TTI can now be used to thermally map GaN HEMTs under pulsed conditions providing simultaneously a submicrosecond temporal resolution and a submicrometer spatial resolution.

Transient Thermal Response of Hotspots in Graphene–Silver Nanowire Hybrid Transparent Conducting Electrodes

Graphene-nanowire hybrid films have recently shown excellent performance as transparent conducting electrodes. However, due to their fundamental nonhomogeneity (grain boundaries, nanowire–nanowire junctions), self-heating induces hotspots along the co-percolating electrical conduction pathways. Although the steady-state characteristics of the hotspot temperature and location have been reported, the temporal response of the hotspots has not been studied in detail. In this paper, we use transient thermoreflectance imaging with high temporal resolution to quantify the transient characteristics of the hotspots. At local hotspots as well as nearby intermediate temperature regions, the temperature response in both heating and cooling phases exhibits two distinct time constants. We quantitatively determine the thermal time constants and associated amplitudes and show dependence of these parameters on distance from the contact. Based on solutions to heat diffusion equations, the short-time constant is attributed to the local self-heating within the hotspot region, and the long-time constant to diffusion of heat through the channel region. The knowledge of time evolution of hotspots and hence a more detailed understanding of the electrothermal conduction process through co-percolating networks could enable more reliable, advanced transparent conductors as well as one-dimensional/two-dimensional hybrid materials for other applications.

Electrothermal Evaluation of AlGaN/GaN Membrane High Electron Mobility Transistors by Transient Thermoreflectance

A novel wet etch process for fabrication of large-area AlGaN/GaN membranes is reported, along with an evaluation of membrane-high electron mobility transistor (HEMT) electrothermal performance up to 1.9 W/mm. Hall measurements showed negligible post-etch change in membrane-HEMT sheet resistance, Hall mobility and carrier concentration. Static (dc) current-voltage characteristics showed negligible change in on resistance ( ${R} _{\mathrm{ ON}}$ ), although ${I} _{\mathrm{DS,MAX}}$ was significantly reduced due to increased self-heating in the absence of the Si substrate. Pulsed output characteristics were similarly affected as self-heating was expected to be still present at ms pulse widths. In the off state, the drain leakage current was measurably lower by about an order of magnitude. Pulsed-mode off-state step stress showed a dynamic on resistance improvement by about a factor of 2 when both sides of the membrane were passivated by SiN. A peak temperature of 148.5 °C was measured on the membrane HEMT using transient thermoreflectance imaging. These initial results indicate that substrate removal does not necessarily cause device degradation, and can be a promising step in improving HEMT reliability in future generations of power devices.

Design and thermoreflectance imaging of high-speed SiGe superlattice microrefrigerators

ABSTRACTOver the past few years, thermoelectric (TE) materials have been receiving an increasing amount of attention owing to their promising potential for energy conversion and thermal management applications. Thermal characterisation techniques offer a powerful tool in investigating and optimizing the TE device performance. In addition, they can provide a better understanding of the underlying fundamental principles such as Peltier effects at the interfaces of the active medium. In this paper, we present the design and thermal characterisation of integrated highspeed microcoolers based on SiGe superlattices. The electrode metalisation is laid out as a coplanar waveguide, enabling to supply electrical pulses with short rise times to the coolers. We employ a variety of CCD-based transient thermoreflectance imaging methods to perform an extensive dynamic thermal analysis. These techniques provide 2-D temperature maps of the chip surface with ∼100ns temporal and submicron spatial resolution without the need to scan the sample. Net cooling in the 2 degree range is observed, with response times well below 1μs. This is almost two orders of magnitude faster compared to the best in the literature. The obtained images also confirm the previous observations that the Peltier cooling term responds faster than the Joule heating term, in agreement with their expected locality and associated thermal mass. This provides potential to study ultrafast electron-phonon interactions during Peltier effects.

Transient thermoreflectance imaging of active photonic crystals

Transient thermoreflectance imaging is used to study the dynamics of the temperature inside active two-dimensional photonic crystals (PhCs). We developed a pump-probe setup suited for optically pumped devices that presents submicrosecond time resolution and submicrometer spatial resolution. Characteristic thermal dissipation times of 429 ns in a PhC Bloch mode cavity and of 999 ns in a PhC membrane are measured. This technique gives also access to the diffusivity of the suspended PhC.

Transient Thermal Imaging of Si/SiGe superlattice and bulk Si microrefrigerators

AbstractIn this paper, we present a systematic study of the transient cooling in different Si/SiGe superlattices as well as bulk silicon microrefrigerators. Transient thermoreflectance imaging is used to obtain the temperature map of the device with sub micrometer spatial, 100ns temporal and 0.1C temperature resolution. It is shown that Peltier cooling dominates in the first 10-30 microseconds before Joule heating in the active and buffer layers reach the top surface. The transient characterization shows that at the optimum current for maximum steady-state cooling, the response of bulk silicon cooler is 25% faster than the 3 microns thick superlattice device and that of the 6 microns thick superlattice is 25% slower. However, it is possible to increase the cooling speed by a factor of two or three, down to 3.6 microseconds, by overdriving the current at the expense of the reduced steady-state cooling.