Thermal and electroluminescence (EL) imaging techniques are widely accepted as powerful tools for... more Thermal and electroluminescence (EL) imaging techniques are widely accepted as powerful tools for analyzing solar cells. We have identified and characterized various defects in photovoltaic devices with sub-micron spatial resolution using a novel thermoreflectance imaging technique that can simultaneously obtain thermal and EL images with a mega-pixel silicon-based CCD. Linear and non-linear shunt defects are investigated as well as electroluminescent breakdown regions at reverse biases as low as -5V. Pre-breakdown sites with electroluminescence are observed. The wavelength flexibility of thermoreflectance imaging is explored and thermal images of sub-micrometer defects are obtained through glass that would typically be opaque for infrared light. Image sequences show a 10μs thermal transient response of a 15μm defect in a polysilicon solar cell. Nanosecond reverse bias voltage pulses are used to detect breakdown regions in thin-film a-Si solar cells with EL.
Researchers have been extensively studying wide-bandgap (WBG) semiconductor materials such as gal... more Researchers have been extensively studying wide-bandgap (WBG) semiconductor materials such as gallium nitride (GaN) with an aim to accomplish an improvement in size, weight, and power of power electronics beyond current devices based on silicon (Si). However, the increased operating power densities and reduced areal footprints of WBG device technologies result in significant levels of self-heating that can ultimately restrict device operation through performance degradation, reliability issues, and failure. Typically, self-heating in WBG devices is studied using a single measurement technique while operating the device under steady-state direct current measurement conditions. However, for switching applications, this steady-state thermal characterization may lose significance since the high power dissipation occurs during fast transient switching events. Therefore, it can be useful to probe the WBG devices under transient measurement conditions in order to better understand the thermal dynamics of these systems in practical applications. In this work, the transient thermal dynamics of an AlGaN/GaN high electron mobility transistor (HEMT) were studied using thermoreflectance thermal imaging and Raman thermometry. Also, the proper use of iterative pulsed measurement schemes such as thermoreflectance thermal imaging to determine the steady-state operating temperature of devices is discussed. These studies are followed with subsequent transient thermal characterization to accurately probe the self-heating from steady-state down to submicrosecond pulse conditions using both thermoreflectance thermal imaging and Raman thermometry with temporal resolutions down to 15 ns.
Researchers have been extensively studying wide-bandgap (WBG) semiconductor materials such as gal... more Researchers have been extensively studying wide-bandgap (WBG) semiconductor materials such as gallium nitride (GaN) with an aim to accomplish an improvement in size, weight, and power of power electronics beyond current devices based on silicon (Si). However, the increased operating power densities and reduced areal footprints of WBG device technologies result in significant levels of self-heating that can ultimately restrict device operation through performance degradation, reliability issues, and failure. Typically, self-heating in WBG devices is studied using a single measurement technique while operating the device under steady-state direct current measurement conditions. However, for switching applications, this steady-state thermal characterization may lose significance since the high power dissipation occurs during fast transient switching events. Therefore, it can be useful to probe the WBG devices under transient measurement conditions in order to better understand the ther...
Self-heating in AlGaN/GaN high electron mobility transistors (HEMTs) negatively impacts device pe... more Self-heating in AlGaN/GaN high electron mobility transistors (HEMTs) negatively impacts device performance and reliability. Under nominal operating conditions, a hot-spot in the device channel develops under the drain side corner of the gate due to a concentration of volumetric heat generation leading to nonequilibrium carrier interactions and non-Fourier heat conduction. These subcontinuum effects obscure identification of the most salient processes impacting heating. In response, we examine self-heating in GaN-on-Si HEMTs via measurements of channel temperature using above-bandgap UV thermoreflectance imaging in combination with fully coupled electrothermal modeling. The methods together highlight the interplay of heat concentration and subcontinuum thermal transport showing that channel temperature cannot be determined solely by continuum scale heat transfer principles. Under conditions of equal power dissipation (PDISS = VDS × IDS = 250 mW), for example, a higher VDS bias (∼23 V...
Thermal and electroluminescence (EL) imaging techniques are widely accepted as powerful tools for... more Thermal and electroluminescence (EL) imaging techniques are widely accepted as powerful tools for analyzing solar cells. We have identified and characterized various defects in photovoltaic devices with sub-micron spatial resolution using a novel thermoreflectance imaging technique that can simultaneously obtain thermal and EL images with a mega-pixel silicon-based CCD. Linear and non-linear shunt defects are investigated as well as electroluminescent breakdown regions at reverse biases as low as -5V. Pre-breakdown sites with electroluminescence are observed. The wavelength flexibility of thermoreflectance imaging is explored and thermal images of sub-micrometer defects are obtained through glass that would typically be opaque for infrared light. Image sequences show a 10μs thermal transient response of a 15μm defect in a polysilicon solar cell. Nanosecond reverse bias voltage pulses are used to detect breakdown regions in thin-film a-Si solar cells with EL.
To achieve the required performance with high speed switching transistors, the gate feature lengt... more To achieve the required performance with high speed switching transistors, the gate feature length in communication devices is as small as a few tens of nanometers in multi finger configurations and transistors are arrayed in a Monolithic Microwave Integrated Circuit (MMIC). The technology therefore, makes thermal characterization more and more difficult. We employ a transient thermal imaging technique to characterize the surface temperature of such nano-featured circuits. The setup is for a non-invasive and indirect thermoreflectance method with external light illumination and CCD imaging. Due to the diffraction limit, that is set by the optical properties of the objective lens in the microscope, optical and thermal images of features smaller than 300 nm blur. We propose an algorithm to resolve this problem by using a Gaussian approximation for the diffraction function in order to blur the thermoreflectance map obtained from modeling, and further use it to reconstruct the true thermal map of sub-diffraction sized devices. Thermal expansion of the device under test is another challenge for such high magnification microscope imaging. We employ a three dimensional Piezo stage controller to take the pixel-by-pixel thermoreflectance coefficients. With this combination, thermal imaging for wires with one-pixel width ~100 nm is achieved. Transient thermal imaging of multi hotspots provides the information of thermal invasion to the neighboring circuit by the thermal diffusion from the hotspots in the MMIC. We will demonstrate the technology component, which combined, could gain the required information for a potential 3-D thermal structure analysis for practical multiple nano-featured hotspots on a chip.
International Workshop on Thermal Investigations of ICs and Systems, Sep 1, 2012
In the development of electronic devices, transient thermal information is necessary to validate ... more In the development of electronic devices, transient thermal information is necessary to validate whether the entire device or a specific part of the device is operating properly. This becomes even more important for higher frequency operation of such devices. Sometimes, this is more important for the circuit designers, process engineers, or chip architects, rather than the thermal or package engineers who mostly need just a few points or locations of time-averaged temperature data during the design phase. For the time domain, recent developments in thermoreflectance imaging allow us to achieve a wide range of time scales from 50 nanoseconds up to milliseconds. This study provides an example of transient thermal imaging on a test chip along with the thermoreflectance imaging technique. The discussion covers the relationship of spatial resolution and time resolution considering the `time constant' component of the chip. Spatial resolution for thermoreflectance is limited by the diffraction of the illuminating light, time resolution is limited by the high speed electrical signal management, and temperature resolution is limited by the signal-to-noise ratio. Time averaging, therefore, plays an important role in determining temperature resolution. The scientific inter-relationship between these factors adds to the complexity. The modeling work described above provides a guideline for the `scope' and `probe' depending on the dimensional scale of interest.
Transient thermal characterization by thermal imaging in our earlier work demonstrated its abilit... more Transient thermal characterization by thermal imaging in our earlier work demonstrated its ability to extract key information about the thermal response of transistors under operating conditions. We investigate a non-equilibrium response to a short time pulse for very high frequency devices, e.g. hetero-junction bipolar transistors (HBTs). Characterizing high speed thermal signals from devices with very small thermal mass has been an ongoing challenge, especially for the microwave and communication industries. A collaboration with a high time-precision pulsed I-V equipment with further advanced high speed thermoreflectance imaging technology enables a measurement of temperature response down to a sub microsecond pulse. Due to the short time pulse, the thermal response wave-form in rise and decay times shows differences from that expected for a typical thermal response, which is more symmetric in rise and decay. Modeling of the thermal diffusion considering the thermal boundaries helps to explain this phenomenon.
Transient thermoreflectance 2D thermal imaging is rapidly proving to be an effective technique fo... more Transient thermoreflectance 2D thermal imaging is rapidly proving to be an effective technique for meeting the thermal analysis challenges inherent with today's advanced high speed integrated circuits. Using near infrared light illumination is particularly suitable for the thermal imaging of flip chip mounted devices and silicon substrate chips. High speed transient imaging reveals the timing of heating the spots in the circuit to identify if it is designed or unintended heating. Time-dependent unusual thermal signals provide the information of the depth location of a failure by knowing the time delay underneath the opaque layers.
We present the high resolution thermal characterization of a GaAs MMIC. The thermal imaging techn... more We present the high resolution thermal characterization of a GaAs MMIC. The thermal imaging technique provides sub-microsecond temporal and sub-micron spatial resolutions. The results show that the gate area heats up in less than 3 us, much faster than the other area of the transistor. Also, the thermal cross talk between transistor arrays takes place in 100s us. This imaging method revealed unique thermal characteristics, not previously observed with traditional thermal measurement techniques.
Shrinking features and growing device complexity with today's advanced devices has led to inc... more Shrinking features and growing device complexity with today's advanced devices has led to increased challenges of gaining a full understanding of device thermal behavior. At the same time, with higher power densities having a full understanding of the device static and dynamic thermal behavior is essential for ensuring optimal tradeoffs between performance and device reliability. Thermal imaging based on the Thermoreflectance Principle can meet the challenges imposed by these advanced devices by providing sub-micron spatial resolution and temporal resolution in the picosecond range. This thermal imaging concept will be described in this paper and compared to traditional imaging techniques. Several case studies will be presented to further illustrate the advantages of the thermoreflectance technique for thermal imaging.
This paper presents a novel method for obtaining optimized, accurate, and fully calibrated images... more This paper presents a novel method for obtaining optimized, accurate, and fully calibrated images of the thermal behavior of complex semiconductor devices with submicron features. To thermally analyze the growing number of high power devices, such as microwave amplifiers for wireless mobile applications, a technique is required for high speed transient and high spatial resolution thermal characterization. Thermoreflectance imaging has been shown to have an advantage in measuring the time-dependent thermal response. A challenge, however, has been a noisy spatial response due to an optical artifact and/or the complex reflection of the layers of thin-films and geometries which comprise the transistor features. To intuitively understand the thermal profile, which is a great advantage of imaging, this nonphysical response can sometimes result in confusion. Wavelength dependent reflectance is a property of the material and the material's surface characteristics. A multiple wavelength or a full spectrum (hyperspectral) illumination, rather than a single wavelength, can be employed to achieve much greater accuracy and a clearer thermal image for all regions on a complex integrated circuit. A single heating wire deposited on a substrate is used to experimentally demonstrate how this technique works. The results show a very good hyperspectral thermoreflectance fitting for all materials on the test structure.
ABSTRACT Thermoreflectance imaging is used to obtain 2D temperature maps of encapsulated LED arra... more ABSTRACT Thermoreflectance imaging is used to obtain 2D temperature maps of encapsulated LED arrays and elements with sub-micron spatial resolution. Typical LED encapsulation is opaque for infrared light, which prevents direct measurement of the semiconductor die with infrared cameras and thermocouples. A lock-in transient imaging technique with a megapixel silicon CCD is used to obtain the thermoreflectance and electroluminescence signals simultaneously. Transient thermal response in different locations of the die is characterized. Different thermal time constants are observed which correspond to various heat transfer mechanisms.
Thermal diffusion in anisotropic and inhomogeneous materials are investigated by full-field therm... more Thermal diffusion in anisotropic and inhomogeneous materials are investigated by full-field thermal imaging using optical pump-probe thermoreflectance microscopy. Target sample material is a composite of nickel clusters, approximately 10 microns in size, distributed inhomogeneously within polydimethylsiloxane (silicone) resin. Such a material presents special challenges when characterizing thermal parameters. For example, the nickel clusters are similar in size to the optical excitation spot size when performing ultra-fast time domain thermoreflectance measurement. This introduces uncertainty as to whether one is measuring the material average response over areas larger than the embedded metal clusters, or the local response of individual clusters. The proposed optical pump probe imaging method can separate material average (macroscopic) and local (microscopic) properties by characterizing sample surface temperature change over both large and small excitation and probe regions. Experimental measurements are compared to both an analytic heat spreading model and finite element numerical simulation. Material anisotropy in the analytic solution is modeled using a well-known effective thickness method, which shows good agreement with the numerical simulation. Results suggest the proposed characterization method and associated analytical solution are applicable for thin or film-like materials with inhomogenous internal composite distribution and anisotropic thermal conductivity.
Hetero junction Bipolar Transistor (HBT) and High Electron Mobility Transistor (HEMT) arrays are ... more Hetero junction Bipolar Transistor (HBT) and High Electron Mobility Transistor (HEMT) arrays are commonly used for RF and microwave high speed and high power applications. Thermoreflectance imaging can be utilized to understand the transient thermal characteristics of a GaN HEMT device. A time resolution of 50 ns clearly shows the thermal location-dependent time constants for the device, which could be used for further analysis of the thermal structure. An array of GaAs HEMT devices on a Monolithic Microwave Integrated Circuit (MMIC) is also characterized to gain an understanding of the local thermal resistance distribution in comparison to a finite element analysis. Since thermoreflectance is sensing the light reflectance of the surface, hotspots underneath opaque layer(s) is discussed to illustrate the utilization of this method for such devices.
Thermal and electroluminescence (EL) imaging techniques are widely accepted as powerful tools for... more Thermal and electroluminescence (EL) imaging techniques are widely accepted as powerful tools for analyzing solar cells. We have identified and characterized various defects in photovoltaic devices with sub-micron spatial resolution using a novel thermoreflectance imaging technique that can simultaneously obtain thermal and EL images with a mega-pixel silicon-based CCD. Linear and non-linear shunt defects are investigated as well as electroluminescent breakdown regions at reverse biases as low as -5V. Pre-breakdown sites with electroluminescence are observed. The wavelength flexibility of thermoreflectance imaging is explored and thermal images of sub-micrometer defects are obtained through glass that would typically be opaque for infrared light. Image sequences show a 10μs thermal transient response of a 15μm defect in a polysilicon solar cell. Nanosecond reverse bias voltage pulses are used to detect breakdown regions in thin-film a-Si solar cells with EL.
Researchers have been extensively studying wide-bandgap (WBG) semiconductor materials such as gal... more Researchers have been extensively studying wide-bandgap (WBG) semiconductor materials such as gallium nitride (GaN) with an aim to accomplish an improvement in size, weight, and power of power electronics beyond current devices based on silicon (Si). However, the increased operating power densities and reduced areal footprints of WBG device technologies result in significant levels of self-heating that can ultimately restrict device operation through performance degradation, reliability issues, and failure. Typically, self-heating in WBG devices is studied using a single measurement technique while operating the device under steady-state direct current measurement conditions. However, for switching applications, this steady-state thermal characterization may lose significance since the high power dissipation occurs during fast transient switching events. Therefore, it can be useful to probe the WBG devices under transient measurement conditions in order to better understand the thermal dynamics of these systems in practical applications. In this work, the transient thermal dynamics of an AlGaN/GaN high electron mobility transistor (HEMT) were studied using thermoreflectance thermal imaging and Raman thermometry. Also, the proper use of iterative pulsed measurement schemes such as thermoreflectance thermal imaging to determine the steady-state operating temperature of devices is discussed. These studies are followed with subsequent transient thermal characterization to accurately probe the self-heating from steady-state down to submicrosecond pulse conditions using both thermoreflectance thermal imaging and Raman thermometry with temporal resolutions down to 15 ns.
Researchers have been extensively studying wide-bandgap (WBG) semiconductor materials such as gal... more Researchers have been extensively studying wide-bandgap (WBG) semiconductor materials such as gallium nitride (GaN) with an aim to accomplish an improvement in size, weight, and power of power electronics beyond current devices based on silicon (Si). However, the increased operating power densities and reduced areal footprints of WBG device technologies result in significant levels of self-heating that can ultimately restrict device operation through performance degradation, reliability issues, and failure. Typically, self-heating in WBG devices is studied using a single measurement technique while operating the device under steady-state direct current measurement conditions. However, for switching applications, this steady-state thermal characterization may lose significance since the high power dissipation occurs during fast transient switching events. Therefore, it can be useful to probe the WBG devices under transient measurement conditions in order to better understand the ther...
Self-heating in AlGaN/GaN high electron mobility transistors (HEMTs) negatively impacts device pe... more Self-heating in AlGaN/GaN high electron mobility transistors (HEMTs) negatively impacts device performance and reliability. Under nominal operating conditions, a hot-spot in the device channel develops under the drain side corner of the gate due to a concentration of volumetric heat generation leading to nonequilibrium carrier interactions and non-Fourier heat conduction. These subcontinuum effects obscure identification of the most salient processes impacting heating. In response, we examine self-heating in GaN-on-Si HEMTs via measurements of channel temperature using above-bandgap UV thermoreflectance imaging in combination with fully coupled electrothermal modeling. The methods together highlight the interplay of heat concentration and subcontinuum thermal transport showing that channel temperature cannot be determined solely by continuum scale heat transfer principles. Under conditions of equal power dissipation (PDISS = VDS × IDS = 250 mW), for example, a higher VDS bias (∼23 V...
Thermal and electroluminescence (EL) imaging techniques are widely accepted as powerful tools for... more Thermal and electroluminescence (EL) imaging techniques are widely accepted as powerful tools for analyzing solar cells. We have identified and characterized various defects in photovoltaic devices with sub-micron spatial resolution using a novel thermoreflectance imaging technique that can simultaneously obtain thermal and EL images with a mega-pixel silicon-based CCD. Linear and non-linear shunt defects are investigated as well as electroluminescent breakdown regions at reverse biases as low as -5V. Pre-breakdown sites with electroluminescence are observed. The wavelength flexibility of thermoreflectance imaging is explored and thermal images of sub-micrometer defects are obtained through glass that would typically be opaque for infrared light. Image sequences show a 10μs thermal transient response of a 15μm defect in a polysilicon solar cell. Nanosecond reverse bias voltage pulses are used to detect breakdown regions in thin-film a-Si solar cells with EL.
To achieve the required performance with high speed switching transistors, the gate feature lengt... more To achieve the required performance with high speed switching transistors, the gate feature length in communication devices is as small as a few tens of nanometers in multi finger configurations and transistors are arrayed in a Monolithic Microwave Integrated Circuit (MMIC). The technology therefore, makes thermal characterization more and more difficult. We employ a transient thermal imaging technique to characterize the surface temperature of such nano-featured circuits. The setup is for a non-invasive and indirect thermoreflectance method with external light illumination and CCD imaging. Due to the diffraction limit, that is set by the optical properties of the objective lens in the microscope, optical and thermal images of features smaller than 300 nm blur. We propose an algorithm to resolve this problem by using a Gaussian approximation for the diffraction function in order to blur the thermoreflectance map obtained from modeling, and further use it to reconstruct the true thermal map of sub-diffraction sized devices. Thermal expansion of the device under test is another challenge for such high magnification microscope imaging. We employ a three dimensional Piezo stage controller to take the pixel-by-pixel thermoreflectance coefficients. With this combination, thermal imaging for wires with one-pixel width ~100 nm is achieved. Transient thermal imaging of multi hotspots provides the information of thermal invasion to the neighboring circuit by the thermal diffusion from the hotspots in the MMIC. We will demonstrate the technology component, which combined, could gain the required information for a potential 3-D thermal structure analysis for practical multiple nano-featured hotspots on a chip.
International Workshop on Thermal Investigations of ICs and Systems, Sep 1, 2012
In the development of electronic devices, transient thermal information is necessary to validate ... more In the development of electronic devices, transient thermal information is necessary to validate whether the entire device or a specific part of the device is operating properly. This becomes even more important for higher frequency operation of such devices. Sometimes, this is more important for the circuit designers, process engineers, or chip architects, rather than the thermal or package engineers who mostly need just a few points or locations of time-averaged temperature data during the design phase. For the time domain, recent developments in thermoreflectance imaging allow us to achieve a wide range of time scales from 50 nanoseconds up to milliseconds. This study provides an example of transient thermal imaging on a test chip along with the thermoreflectance imaging technique. The discussion covers the relationship of spatial resolution and time resolution considering the `time constant' component of the chip. Spatial resolution for thermoreflectance is limited by the diffraction of the illuminating light, time resolution is limited by the high speed electrical signal management, and temperature resolution is limited by the signal-to-noise ratio. Time averaging, therefore, plays an important role in determining temperature resolution. The scientific inter-relationship between these factors adds to the complexity. The modeling work described above provides a guideline for the `scope' and `probe' depending on the dimensional scale of interest.
Transient thermal characterization by thermal imaging in our earlier work demonstrated its abilit... more Transient thermal characterization by thermal imaging in our earlier work demonstrated its ability to extract key information about the thermal response of transistors under operating conditions. We investigate a non-equilibrium response to a short time pulse for very high frequency devices, e.g. hetero-junction bipolar transistors (HBTs). Characterizing high speed thermal signals from devices with very small thermal mass has been an ongoing challenge, especially for the microwave and communication industries. A collaboration with a high time-precision pulsed I-V equipment with further advanced high speed thermoreflectance imaging technology enables a measurement of temperature response down to a sub microsecond pulse. Due to the short time pulse, the thermal response wave-form in rise and decay times shows differences from that expected for a typical thermal response, which is more symmetric in rise and decay. Modeling of the thermal diffusion considering the thermal boundaries helps to explain this phenomenon.
Transient thermoreflectance 2D thermal imaging is rapidly proving to be an effective technique fo... more Transient thermoreflectance 2D thermal imaging is rapidly proving to be an effective technique for meeting the thermal analysis challenges inherent with today's advanced high speed integrated circuits. Using near infrared light illumination is particularly suitable for the thermal imaging of flip chip mounted devices and silicon substrate chips. High speed transient imaging reveals the timing of heating the spots in the circuit to identify if it is designed or unintended heating. Time-dependent unusual thermal signals provide the information of the depth location of a failure by knowing the time delay underneath the opaque layers.
We present the high resolution thermal characterization of a GaAs MMIC. The thermal imaging techn... more We present the high resolution thermal characterization of a GaAs MMIC. The thermal imaging technique provides sub-microsecond temporal and sub-micron spatial resolutions. The results show that the gate area heats up in less than 3 us, much faster than the other area of the transistor. Also, the thermal cross talk between transistor arrays takes place in 100s us. This imaging method revealed unique thermal characteristics, not previously observed with traditional thermal measurement techniques.
Shrinking features and growing device complexity with today's advanced devices has led to inc... more Shrinking features and growing device complexity with today's advanced devices has led to increased challenges of gaining a full understanding of device thermal behavior. At the same time, with higher power densities having a full understanding of the device static and dynamic thermal behavior is essential for ensuring optimal tradeoffs between performance and device reliability. Thermal imaging based on the Thermoreflectance Principle can meet the challenges imposed by these advanced devices by providing sub-micron spatial resolution and temporal resolution in the picosecond range. This thermal imaging concept will be described in this paper and compared to traditional imaging techniques. Several case studies will be presented to further illustrate the advantages of the thermoreflectance technique for thermal imaging.
This paper presents a novel method for obtaining optimized, accurate, and fully calibrated images... more This paper presents a novel method for obtaining optimized, accurate, and fully calibrated images of the thermal behavior of complex semiconductor devices with submicron features. To thermally analyze the growing number of high power devices, such as microwave amplifiers for wireless mobile applications, a technique is required for high speed transient and high spatial resolution thermal characterization. Thermoreflectance imaging has been shown to have an advantage in measuring the time-dependent thermal response. A challenge, however, has been a noisy spatial response due to an optical artifact and/or the complex reflection of the layers of thin-films and geometries which comprise the transistor features. To intuitively understand the thermal profile, which is a great advantage of imaging, this nonphysical response can sometimes result in confusion. Wavelength dependent reflectance is a property of the material and the material's surface characteristics. A multiple wavelength or a full spectrum (hyperspectral) illumination, rather than a single wavelength, can be employed to achieve much greater accuracy and a clearer thermal image for all regions on a complex integrated circuit. A single heating wire deposited on a substrate is used to experimentally demonstrate how this technique works. The results show a very good hyperspectral thermoreflectance fitting for all materials on the test structure.
ABSTRACT Thermoreflectance imaging is used to obtain 2D temperature maps of encapsulated LED arra... more ABSTRACT Thermoreflectance imaging is used to obtain 2D temperature maps of encapsulated LED arrays and elements with sub-micron spatial resolution. Typical LED encapsulation is opaque for infrared light, which prevents direct measurement of the semiconductor die with infrared cameras and thermocouples. A lock-in transient imaging technique with a megapixel silicon CCD is used to obtain the thermoreflectance and electroluminescence signals simultaneously. Transient thermal response in different locations of the die is characterized. Different thermal time constants are observed which correspond to various heat transfer mechanisms.
Thermal diffusion in anisotropic and inhomogeneous materials are investigated by full-field therm... more Thermal diffusion in anisotropic and inhomogeneous materials are investigated by full-field thermal imaging using optical pump-probe thermoreflectance microscopy. Target sample material is a composite of nickel clusters, approximately 10 microns in size, distributed inhomogeneously within polydimethylsiloxane (silicone) resin. Such a material presents special challenges when characterizing thermal parameters. For example, the nickel clusters are similar in size to the optical excitation spot size when performing ultra-fast time domain thermoreflectance measurement. This introduces uncertainty as to whether one is measuring the material average response over areas larger than the embedded metal clusters, or the local response of individual clusters. The proposed optical pump probe imaging method can separate material average (macroscopic) and local (microscopic) properties by characterizing sample surface temperature change over both large and small excitation and probe regions. Experimental measurements are compared to both an analytic heat spreading model and finite element numerical simulation. Material anisotropy in the analytic solution is modeled using a well-known effective thickness method, which shows good agreement with the numerical simulation. Results suggest the proposed characterization method and associated analytical solution are applicable for thin or film-like materials with inhomogenous internal composite distribution and anisotropic thermal conductivity.
Hetero junction Bipolar Transistor (HBT) and High Electron Mobility Transistor (HEMT) arrays are ... more Hetero junction Bipolar Transistor (HBT) and High Electron Mobility Transistor (HEMT) arrays are commonly used for RF and microwave high speed and high power applications. Thermoreflectance imaging can be utilized to understand the transient thermal characteristics of a GaN HEMT device. A time resolution of 50 ns clearly shows the thermal location-dependent time constants for the device, which could be used for further analysis of the thermal structure. An array of GaAs HEMT devices on a Monolithic Microwave Integrated Circuit (MMIC) is also characterized to gain an understanding of the local thermal resistance distribution in comparison to a finite element analysis. Since thermoreflectance is sensing the light reflectance of the surface, hotspots underneath opaque layer(s) is discussed to illustrate the utilization of this method for such devices.
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Papers by Dustin Kendig