Search

Recent attacks using thermal laser stimulation (TLS) have shown that it is possible to extract cryptographic keys from the battery-backed memory on state-of-the-art field-programmable gate arrays (FPGAs). However, the professional failure analysis microscopes usually employed for these attacks cost in the order of 500k to 1M dollars. In this work, we evaluate the use of a cheaper commercial laser fault injection station retrofitted with a suitable amplifier and light source to enable TLS. We demonstrate that TLS attacks are possible at a hardware cost of around 100k dollars. This constitutes a reduction of the resources required by the attacker by a factor of at least five. We showcase two actual attacks: data extraction from the SRAM memory of a low-power microcontroller and decryption key extraction from a 20-nm technology FPGA device. The strengths and weaknesses of our low-cost approach are then discussed in comparison with the conventional failure analysis equipment approach. In general, this work demonstrates that TLS backside attacks are available at a much lower cost than previously expected.

For the first time a non-invasive localisation of a soft breakdown (SBD) is shown. The localisation is completed on fully functional back end of line (BEOL) test structures. The test structures used, provided by the interuniversity microelectronics centre (imec), are metal insulator semiconductor (MIS) structures. The low k dielectric within the test structures is SiOCH type with OSG 2.0 (k = 2.0), 45% porosity and 40 nm thickness. Contacless faul isolation methods have been evaluated for detecting a SBD on these structures. We evaluated photon emission microscopy (PEM) with two different signal detectors, the Si - CCD and the InGaAs camera. A proof of concept for detecting a SBD with themal laser stimulation (TLS) is presented. Using a Si - CCD and up to 2000 s integration time, photon emission (PE) signals from a 2 mm × 2 mm test structure with a leakage current less than 1 nA are presented. With the InGaAs detector a localised SBD from a 2 mm × 2 mm test structure with a leakage current of 100 pA is shown. The detected SBDs have a resistance of 33 GO and 260 GO respectively thus the level of degradation is several orders of magnitude lower compared to a hard breakdown (HBD). Up to now it was only possible to localise defects at higher levels of degradation. Due to the high energy at these levels, original defect signatures for SBD are usually destroyed. To better control the process of degradation, a way to nearly freeze the degradation process is shown. This method was used to detect a 1 nA leakage current of a 2 mm × 2 mm structure with a resistance of 35 GO using optical beam induced resistance change (OBIRCH) which is a similar contactless fault isolation (CFI) method to TLS. The presented SBD localisations allow to plan accurate physical preparations for the first time. Physical analysis of PEM localised SBD and HBD have been performed and compared. Possibilities to further improve the presented SBD detection levels are discussed for OBIRCH. Limitations for PEM with Si - CCD and InGaAs detectors as a CFI for SBDs are discussed.

This paper presents novel optical beam-based defect localization approaches for resistive and open failed wafer-towafer (W2W) bonding interconnects for 3-D integration. The use of an etch back process in combination with thermal laser stimulation (TLS) and light-induced capacitance alteration (LICA) using visible laser excitation enables us to accurately pinpoint defects in high-density W2W interconnect structures down to a pitch of 2.2 µm. We confirm our results by focusedion beam (FIB) cross sectioning.

Contactless Fault isolation (CFI) techniques of Microelectronic Debug and Failure Analysis (FA) are developed for the identification of anomalies. If not only anomalies can be detected but also regular logical functionality, the contactless approach opens up a path to hardware security attacks. Logical state information is typically more complicated to detect than anomalies, as the signal level is usually considerably smaller. Successful Logic State Imaging (LSI) has been published in the FA community using specific derivatives of photon emission (PE) and laser-based CFI techniques. This work investigates how the contactless LSI techniques can be used for reading sensitive data out of security-relevant circuitry. The qualification of PE, Thermal Laser Stimulation (TLS), and Laser Logic State Imaging (LLSI) to read out Static Random-Access Memory (SRAM) and battery-backed RAM (BBRAM) cells is demonstrated in two case studies. LLSI offers perspectives for low voltage technologies and feature size resolution by possibly expanding into the visible spectral regime. Therefore, the security risk for some technology generations to come will stay. Furthermore, when considering the use of artificial intelligence, CFI techniques can reach new dimensions of tracking sensitive on-chip data with unimagined consequences for security applications

Thermal laser stimulation (TLS) is a failure analysis technique, which can be deployed by an adversary to localize and read out stored secrets in the SRAM of a chip. To this date, a few proof-of-concept experiments based on TLS or similar approaches have been reported in the literature, which do not reflect a real attack scenario. Therefore, it is still questionable whether this attack technique is applicable to modern ICs equipped with side-channel countermeasures. The primary aim of this work is to assess the feasibility of launching a TLS attack against a device with robust security features. To this end, we select a modern FPGA, and more specifically, its key memory, the so-called battery-backed SRAM (BBRAM), as a target. We demonstrate that an attacker is able to extract the stored 256-bit AES key used for the decryption of the FPGA’s bitstream, by conducting just a single non-invasive measurement. Moreover, it becomes evident that conventional countermeasures are incapable of preventing our attack since the FPGA is turned off during key recovery. Based on our time measurements, the required effort to develop the attack is shown to be less than 7 hours. To avert this powerful attack, we propose a low-cost and CMOS compatible countermeasure circuit, which is capable of protecting the BBRAM from TLS attempts even when the FPGA is powered off. Using a proof-of-concept prototype of our countermeasure, we demonstrate its effectiveness against TLS key extraction attempts.

With the increasing complexity of circuits, it is difficult to carry out fault isolation in a short time. This is particularly true for mixed-signal circuits when using only standard fault isolation techniques (e.g. EMMI or TLS). It is often complicated to tell whether a spot is a consequence of a failure or points out the location of the defect itself. In this scenario, circuit simulators are a solution to resolve this ambiguity and ease this task. This work proposes a simulation method for circuits inspected using thermal laser stimulation techniques. Furthermore, simulations for the dynamic laser stimulation technique are proposed with encouraging results for failure analysis.

Thermal runaway is among the major failure mechanisms of power semiconductor packages. Thermal dissipation of power electronics depends on the quality of the solder die-attach, any defects such as voids, and delamination-impede heat dissipation that results in hot spots that can cause thermal runaway and/or a premature device failure. Therefore, investigation of thermal impact due to die-attach defects is indispensable for improved device performance and reliability. In this paper, transient dual interface measurement is used to characterize the thermal path of a power electronic package consisting of an insulated gate bipolar transistor (IGBT) and copack diode. The die-attach defects were identified by X-ray imaging. The impact of voids on thermal dissipation was quantified by junction-to-case thermal resistance and structure function analysis. The junction-to-case thermal resistance (IGBT) values were found to be increasing with the increase in void%. The largest void% instead of the total void% has a greater impact on the thermal dissipation of the devices. However, die-attach voids under the IGBT die have no impact on junction-to-case thermal resistance (diode), suggesting that the copack diode heat path is independent of the IGBT heat path.

We report a new non-destructive method to localize interconnection failures in 3-D devices. The scanning optical microscopy (SOM) technique is based on lock-in thermal laser stimulation (LI-TLS) and uses thermal waves to non-destructively map the current path in a 3-D device. We validate the method with test structures and show how the magnitude and phase of a propagating thermal wave may provide valuable 3-dimensional information on the failure location. We apply the technique on a short failed chain structure in a four level chip stack with an intensity modulated laser as a thermal wave injector and the structure under test as a detector. We confirm our results by physical failure analysis through a selective cross sectioning process.

We propose a method for determining the parameters of linear thermal circuits of semiconductor devices represented in the form of series-connected RC circuits. As a result of computer modeling, we obtain an estimate for errors in determining the indicated parameters based on the frequency dependence of the modulus of the thermal impedance.

Thermal Laser Stimulation (TLS) for static logic state analysis is applied to failure analysis. Three case studies of analyses of the digital logic in an automotive Application Specific Integrated Circuit (ASIC) are discussed. By analyzing the logic states of the circuit we were able to identify the mechanism and localize the site of irregular behavior non-destructively, both for a stuck-at fault and two weak interconnects. The approach measures the power supply current while applying TLS to the device back side. This allows an extremely high analysis coverage, because every transistor is connected to the power supply, making this method a universal tool for every digital circuit failure analysis (FA) workflow, because the gained understanding of the fault allows to replace multiple steps of alternative FA techniques by only a single technique, reducing time and cost for successful FA.

The errors in determining the thermal characteristics of materials with a finite duration laser pulse method are estimated. Handbook values of the characteristics of materials are compared with numerical solutions of the two-dimensional problem of heat conduction in a sample during exposure to a heat pulse of short duration under experimental conditions.

In this investigation, thermal transient measurements and analyses are used to study the thermal performance of a cascoded GaN power device. The method is based on the thermal characterization of the on-resistance (Ron) of the device and the synchronized current–voltage characteristics under continuous operation. The changes in Ron with temperature (25 °C–180 °C) are measured, statistically studied, and correlations investigated. The proposed method for estimating transient thermal impedance has the following characteristics: (1) it is robust and reproducible; (2) it yields a heating curve to prevent over-heating for the device under test (DUT); (3) it provides in-situ current–voltage characterization; (4) it includes a transient offset correction for initial transient electrical disturbances (such as current collapse); (5) it optimizes a compact thermal model; (6) it is sensitive to package structure and design variables. This monitoring of thermal impedance variation provides a simple and fast non-destructive method for analyzing power switching devices during thermal testing. Satisfactory experimental results confirm the feasibility of in-situ current–voltage characterization and the real power varies with the thermal impedance.

The thermoelectric effects in oxide materials are developed to the Seebeck effect for thermal sensing. Thermal sensors are the thermal converters, in which an electrical, magnetic, mechanical, radiation, chemical signal are measured by converting it into heat and measuring the resulting temperature changes in the device. This proposes an analysis the electrical converters and radiation sensors by using single−junction of p−Ca3Co4O9 and n−CaMnO3 materials are presented into possible thermal sensing. Several thermal converters are measured the signal electrical resistivity, and Seebeck coefficient depended on temperature. Thermal radiation sensors are included the sensitivity, electricity, and specific detectivity. For application, thermoelectric oxide cell can be fabricated crowbar circuit for protection an overvoltage condition of a power supply which it yield 1.2 s turn off switch.

Thermoelectric behavior in semiconductor materials is investigated for the purpose of converting thermal energy to electrical energy. This paper measures and quantifies the variations in the internal electrical and thermal resistances of a thermoelectric module with respect to the circuit’s electrical load in order to better characterize its behavior in a closed circuit. Since open-circuit measurements fail to accurately describe thermoelectric material properties in power generation mode, the load-bearing Seebeck coefficient and the load-bearing figure-of-merit are defined, measured, and discussed. It is shown that the load-bearing Seebeck and the load-bearing figure-of-merit are more accurate parameters in measuring a module’s ability to operate in thermoelectric power generation mode. In particular, there exist peak load-bearing characteristics that are not identified using the conventional open-circuit definitions of the Seebeck coefficient and the figure-of-merit. The results show that the peak load-bearing figure-of-merit is compared with the peak power output showing a misalignment between the two quantities, the significance of which is discussed.

Thermal Laser Stimulation (TLS) is an efficient technology for integrated circuit defect localization in Failure Analysis (FA) laboratories. It contains Optical Beam-Induced Resistance Change (OBIRCH), Thermally-Induced Voltage Alteration (TIVA), and Seebeck Effect Imaging (SEI). These techniques respectively use the principle of laser-induced resistance change and the Seebeck effect. In this paper, a comprehensive model of TLS technology is proposed. Firstly, the model presents an analytical expression of the temperature variation in Integrated Circuits (IC) after laser irradiation, which quantificationally shows the positive correlation with laser power and the negative correlation with scanning velocity. Secondly, the model describes the opposite influence of laser-induced resistance change and the Seebeck effect in the device. Finally, the relationship between the current variation measured in the experiment and other parameters, especially the voltage bias, is well explained by the model. The comprehensive model provides theoretical guidance for the efficient and accurate defect localization of TLS technology.

Thermal laser stimulation (TLS) is a failure analysis technique, which can be deployed by an adversary to localize and read out stored secrets in the SRAM of a chip. To this date, a few proof-of-concept experiments based on TLS or similar approaches have been reported in the literature, which do not reflect a real attack scenario. Therefore, it is still questionable whether this attack technique is applicable to modern ICs equipped with side-channel countermeasures. The primary aim of this work is to assess the feasibility of launching a TLS attack against a device with robust security features. To this end, we select a modern FPGA, and more specifically, its key memory, the so-called battery-backed SRAM (BBRAM), as a target. We demonstrate that an attacker is able to extract the stored 256-bit AES key used for the decryption of the FPGA’s bitstream, by conducting just a single non-invasive measurement. Moreover, it becomes evident that conventional countermeasures are incapable of preventing our attack since the FPGA is turned off during key recovery. Based on our time measurements, the required effort to develop the attack is shown to be less than 7 hours. To avert this powerful attack, we propose a low-cost and CMOS compatible countermeasure circuit, which is capable of protecting the BBRAM from TLS attempts even when the FPGA is powered off. Using a proof-of-concept prototype of our countermeasure, we demonstrate its effectiveness against TLS key extraction attempts., IACR Transactions on Cryptographic Hardware and Embedded Systems, Volume 2018, Issue 3

An efficient method of determining thermal parameters in high-power field-effect transistors has been developed and tested based on a study of transient processes during self-heating by direct current. With the developed relaxation spectrometer of the thermal processes, the differential distribution profiles of thermal resistance of KP723G transistors, which were selected in accordance with the regimes of the setting of their crystals, have been investigated. Thermal resistance spectra have been obtained from the analysis of time-dependent dynamic thermal impedance using a new nondestructive method of differential spectroscopy using higher order derivatives (order 3). We present both the continuous (integral) and discrete spectra of the distribution of the internal thermal resistance in the transistors and the value of the junction/case thermal resistance. The thermal characteristics of the KP723G transistors and their imported counterparts IRLZ44 and IRLB3036 have been determined. The method of determining the active region of the devices has been developed and its decrease during heating has been shown. The proposed methodology is useful in solving technological problems for forming the setting layers of the crystals and intermediate layers between the crystal and the heat sink, as well as for the development of thermal models in the SPICE modeling of powerful MOSFETs and diode emitters.

A method of identifying laser degradation mechanism is established based on thermal characteristic analysis of InGaN/GaN laser diodes (LDs). Both steady and transient thermal characteristics of LDs are calculated by the finite-element analysis method, the results show that thermal resistance and thermal time constant of each layer are determined by the corresponding structure and material. Experiments on thermal characteristics of LDs are carried out, degradation induced heat transport irregularities are localized by comparing the transient cooling curves of the virgin, and degraded LDs and further analyses give more information on degradation mechanisms of the LDs. The results confirm that this identification method is an effective method, which gives specific guidance for the analysis of the laser degradation mechanisms.

The high-power light-emitting diode (LED) lighting is especially essential in the high-temperature applications such as the street and automotive lighting. However, >70% of electrical power is converted into heat due to the low electrical-optical conversion efficiency. Because of the characteristics of semiconductor, the electrical property of LEDs sensitively varies with the operating temperature. These deviations affect the measurement of light and electricity. In this paper, the steady-state thermal measurement techniques developed by the National Institute of Standards and Technology are used to study the thermal resistances of multichip LEDs. The experimental results show that the measurements of thermal resistances with different connections are constant at the same power dissipation. The simulation results show that the detail of thermal interaction between powered and unpowered chips and confirm the experiment results.

A 3-D test circuit examining thermal propagation within a through-silicon via-based 3-D integrated stack has been designed, fabricated, and tested. Design insight into thermal coupling in 3-D integrated circuits (ICs) through both experiment and simulation is provided, and suggestions to mitigate thermal effects in 3-D ICs are offered. Two wafers are vertically bonded to form a 3-D stack. Intraplane and interplane thermal coupling is investigated through single-point heat generation using resistive thermal heaters and temperature monitoring through four-point resistive measurements. Thermal paths are identified and analyzed based on the metric of thermal resistance per unit length. The peak steady-state temperature due to die location within a 3-D stack is described. The reduction in peak temperature through fan-based active cooling is also reported. Thermal propagation from a heat source located on the backside of the silicon is examined with both back metal and on-chip thermal sensors. A comparison of thermal coupling between two different heat sources on the same device plane is also provided.

Light-current and current-voltage characteristics of a set of high-power laser diodes of the spectral region of 980 nm, fabricated by the authors, are experimentally studied. The dependences of their total efficiency and heat load on the pump current are constructed. The latter experimental dependence is used for calculations in the three-dimensional thermal model of the laser diode with a stripe contact width of 100 µm. Thermal fields are studied and isothermal surfaces are constructed for laser diodes fabricated on C-mount heat sink elements under heat loads corresponding to output powers of 10 W and higher in continuous mode.

We discuss the design and application perspectives of different crystal, ceramic and composite-type submounts with thermo-compensating properties as well as submounts from materials with high thermal conductivity for overcoming thermal problem in high-power laser diodes (LD) and improving thermal management of other high-power optoelectronic and electronic semiconductor devices. Thermal fields in high-power laser diodes were calculated in 3 D thermal model at CW operation for some heatsink designs taking into account the experimental dependence of laser total efficiency against pumping current in order to extend the range of reliable operation up to thermal loads 20-30 W and corresponding output optical power up to 15-20 W for 100 μm stripe laser diodes.

In this paper, we were investigated electrical properties of monocrystalline and polycrystalline silicon solar cells due to laser irradiation with 650 nm wavelength in two states, proximate irradiation and via optics setup. Thermal effect on the cell surface due to laser irradiation was investigated on electrical properties too. Electrical parameters investigation of solar cells illustrates cell excitement via laser irradiation and efficiency decreases due to cell surface temperature increase. Monocrystalline parameters change with uniform shape due to thermal effect and laser irradiation toward polycrystalline cells.

The semiconductor technologies evolution allows greatly reducing noise impact on products and many structures have been created to reduce its effect. However, this paper presents the apparition of a noise issue during the production of a mixed-mode device dedicated to automotive applications. The research investigations concerned the fact that failure was not detected at test level but at customer level; therefore, it was determinant to understand the root cause of this failure mode to drive corrective actions in order to secure customer. The challenge was to analyse noise in Failure Analysis (FA) without fault spatial localization results. Indeed, Light Emission Microscopy (EMMI) and Thermal Laser Stimulation (ex: Soft Defect Localization – SDL) were unable to provide any defective area in the product. The lack of failing device identification led us to combine electrical and design analyses in order to define hypothesis on the failure origin. It was then possible to drive physical investigations through different approaches, using physical cross-section, Secondary Ion Mass Spectrometry (SIMS) and Scanning Capacitance Microscopy (SCM) techniques. Finally, the obtained complementary results will be discussed and an explanation of the failure mechanism will be presented as the root cause issue, allowing defining the defective step in production process.

The local temperature plays a key role in resistive switching devices. We have previously presented a method to experimentally evaluate the filament temperature using metal-insulator-semiconductor bipolar transistor structure. In light of the experimental results, we discuss here the various possible heat dissipation mechanisms, and compare thermal simulations with the measured temperatures. Since the applied bias dropped across a tunneling gap, heat dissipation occurs at the interfaces. The simulations are consistent with our experimental data for a filament tip diameter of ~ 1 nm. A temperature-dependent thermal resistance of the interfaces and/or bulk was introduced to obtain a fit between the simulations and measured data. Thermal resistances imposed by interfaces dominated bulk material heat transport in the simulations.

The goal of this paper is to experimentally and numerically study the thermal behaviors of flip-chip (FC) light-emitting diode (LED) packages with and without underfills and, furthermore, to compare it with conventional wire-bonding LED packages in order to understand its thermal dissipation mechanism. In experimental analyses, the junction temperature Tj and surface temperatures are measured by a junction temperature tester, thermal couples, and an infrared thermal imager, whereas the numerical analysis is carried out by an ANSYS simulation. After the validation of the simulation model with experimental results, the effects of FC bump number and underfill thermal conductivity on both Tj and thermal resistance Rth of the packages are investigated by this validated model. Furthermore, a simple model of effective thermal conductivity for a composite material of bumps and underfills is proposed for thermal analysis of the FCLED packages. Thermal results for the packages are presented and discussed in terms of volume fraction and thermal conductivity for bumps and underfills in this paper.

In this paper, the analytical description of the nonlinear thermal model of the semiconductor device considering the influence of the selected factors on its thermal resistance is proposed. The worked out nonlinear thermal model considers the influence of such factors as length of metal leads, solder area, dimensions of the heat-sink, ambient temperature, and dissipated power on the efficiency of heat transfer between the chip and the surrounding. The correctness of the worked out thermal model is verified experimentally for the selected types of semiconductor devices operating in different cooling conditions. In all the considered cases, the good agreement of the results of calculations and measurements is obtained.

Thermal laser stimulation of the skin is an efficient exploratory tool to characterize the nociceptive system. In the present study, finite element simulations are done to calculate the intra-cutaneous spatio-temporal temperature profiles following the delivery of such laser stimuli. The proposed computer-aided modeling considers a number of important parameters that have been disregarded in previous approaches: (i) variability of water content across the skin in both hairy and glabrous skin, (ii) temperature dependency of optical and thermal skin parameters, (iii) laser wavelength and corresponding absorption coefficient, (iv) beam shape (Gaussian vs. flat top) and (v) power emission (closed vs. open loop). Numerical simulations allow determining at each instant of time the volume and area of skin tissue whose temperature exceeds a given nociceptor activation threshold. This knowledge allows a finer characterization of the subpopulations of primary afferents that encode and convey nociceptive signals to the central nervous system. As an example, the approach is used to obtain an estimate of intraepidermal nerve fiber density in both physiological and pathological conditions. Moreover, a better knowledge of the heat distribution also reduces the risk of injury to the skin. Finally, in order to make the finite element simulations accessible to investigators with no prior background in numerical analysis, a specific open-source user-interface has been developed with the ONELAB software.

Multichip LED arrays are widely used for lighting to provide high luminance. Luminous efficacy, lifetime, and color temperature are highly dependent on the temperature at p-n junction. This paper investigated the effects of distance, number of chips, and driving current on the thermal resistance of LED module. Thermal resistance dramatically increased as the distance between LED chips decreased due to significant thermal spreading impedance for heat dissipation from junction to ambient. The parallel-resistance formula substantially underestimated the junction temperature of the LED modules due to significant thermal crowding effect. Thermal boundary can also rise junction temperature as the distance to the board edge decreased in both the two-chip and four-chip modules. Infrared results showed that chip temperatures were highly consistent with thermal resistance measurements under different driving currents.

Two case studies concerning failure analysis on open via defects are presented in this paper. Both of them were performed on a mixed-signal product for automotive application and showed several analysis techniques used to find defective net. First example showed the use of well-known Emission Microscopy (EMMI) in combination with advanced electrical setup to highlight floating net. This case study showed the need to thoroughly interpret EMMI signatures in combination with extensive setup investigation in order to solve an analysis with a complex failure mode. Second example showed an innovative use of Dynamic Thermal Laser Stimulation to identify a floating gate.

The semiconductor laser is the important part in the semiconductor laser frequency standard. The quality of light frequency standard depends on line width and stability of laser. Semiconductor laser is very sensitive to environmental changes, such as the temperature, humidity and so on. The driving voltage and circuit and the temperature compensation are significant to laser output stability. The paper put forward a novel method to drive the semiconductor laser based on constant voltage source. The experimental results show that the circuit is simple, reliable, and high-precision. After improvement, the driving source of laser can promise parameters: constant-current range is from 0 to 250 mA, and the drift of current is less than 10 −6 , and stability is 10 −5 . The drift of frequency of laser drive with the circuit is within 10 −5 nm.

The transient thermal characteristics related to catastrophic optical damage (COD) in high power AlGaAs/GaAs laser diodes (LDs) are studied experimentally by means of thermal infrared imaging, laser scanning confocal microscopy, and transient thermal technique. The transient thermal technique is based on the diode forward voltage method in which the structure function theory is applied to determine the component thermal resistance and temperature rise of the LDs. We have recorded the dynamics of COD process using thermal infrared camera. Two melting spots are observed in the output facet after COD which location exactly matches the position of the thermal flash from the thermal imaging and even the temperature of COD seed has a sudden increase of 32 K when the COD event occurs. Furthermore, we find that the thermal resistance from chip to In solder increases remarkably after COD, meanwhile the thermal resistance of solder and package does not change apparently. The results are attributed to the heat power dissipation increase originating from the enhancement of nonradiative recombination after COD under constant input electrical power.

We explore a modified thermal resistance analysis by induced transient method applied to light emitting diodes (LEDs) to discretize the junction-to-package thermal resistance. The temperature response of LED and package configuration is evaluated for discrete contributions from identifiable spatial domains in the multilayered device and package structure to obtain their thermal resistances and thermal capacitances using a Laplace transform-based method. The technique successfully extracts the junction-to-package thermal parameters of a variety of LED package configurations from the experimental temperature transient measurements of the LED junction and provides a straightforward method by which these parameters can be obtained.

The thermal properties of AlGaAs/GaAs laser diode bars have been analyzed in detail by transient thermal technique based on the diode forward voltage method on the vertical and horizontal thermal conduction paths, respectively. On the vertical thermal conduction path, it is found that the steady-state temperature rise of the central emitter among operated emitters has a linear relationship with the logarithm of the total operating current. The time constant determined for chip decreases with the total operating current, while the time constant determined for solder/heat sink interface and package remains almost constant. In addition, the effective thermal diffusivity of chip increases with total operating current. On the horizontal thermal conduction path, the time constant varies linearly with the distance increasing, and the effective thermal diffusivity remains almost constant. The results suggest that the thermal crosstalk between emitters increases with the total operating current.

A steady thermal conduction model was presented to study the temperature field and thermal stress distribution in film irradiated by continuous wave laser. The thermal effects may arise from either the absorbing inclusions or the intrinsic absorption of film. Based on the plane thermal conduction assumption, the characteristics of damage resulted from local melting or evaporation and thermal stress were discussed. The damage region resulted from local melting or evaporation smaller than that from thermal stress. The circumferential stress σθis the main cause accounting for the stress damage. The normal stress perpendicular to the interface between film and substrate is the direct cause of drum type damage. The characteristics of damage in unsteady thermal conduction caused by pulse laser can be analyzed qualitatively by means of this model.

Thermal coupling effect between multi-heat sources makes that true junction temperature based on self-heating of chips adds to thermal coupling temperature. This paper focuses on methods of calculation of junction temperature of MCM: thermal resistance network model and thermal resistance matrix model, and making a comparison between these two models. They are established by different way, but this paper will prove the final calculation formulas of these two models represent same physical significance. Then a type of power PWM is as a research object. The two models are verified by infrared thermal imaging and finite element analysis. In the end of this study, thermal coupling laws of multi-heating sources MCM are obtained by above analysis, argument and experiments.

Heat generation in the active region leads to high junction temperature that significantly affects electrical and optical properties, reliability and lifetime of high power diode laser arrays. It is of great importance to understand the thermal behavior to improve the devices. Compared to conduction cooling techniques, diode laser arrays packaged on microchannel heat sinks have superior capability to dissipate the large amount of heat so as to deliver higher output power and ensure high reliability. In this paper, numerical approach based on finite element method (FEM) and computational fluid dynamics (CFD) was employed to investigate thermal properties of microchannel cooled (MCC) high power diode laser arrays. The static and transient thermal behavior of the devices operated in continuous wave (CW) mode at different water flow rates have been studied in detail. The thermal resistance contributed from the laser chip, solder interface and MCC heat sink was revealed. The correlation between thermal resistance and water flow rate was analyzed. The thermal time constants were derived to characterize the three distinct heating processes related to active region, copper heat sink and copper/water interface. Non-uniformity of junction temperature across the diode laser array was discussed by thermal crosstalk employing the independent emitter analysis. Understanding thermal phenomena in diode laser arrays could offer useful guidelines in optimizing the operating conditions, MCC heat sink structures and packaging architectures for enhanced performance and reliability.

Thermal resistance evaluation of silicon carbide (SiC) power module for high-temperature operation has been performed in order to define the precise thermal resistance in real package structure. Transient thermal analysis method using SiC-Schottky barrier diode (SBD) is applied to measure the thermal structure function in wide temperature range from 50°C to 250°C. The module structure consists of a SBD, a silicon nitride-active metal brazed copper (SiN-AMC) and Cu baseplate which are bonded each other by Gold Germanium (AuGe) solder. We found that it is difficult to define the thermal resistance components of the SiN-AMC and baseplate in the thermal structure function. The reference module structure without SiN-AMC is prepared for isolation of thermal resistances. The thermal resistance of SiN-AMC is successfully defined by comparing the structure functions without and with SiN-AMC. This phenomenon is attributed to thermal spreading effect in the high thermal conductivity material (Cu baseplate) based on present experiment. The thermal resistance in the module structure are successfully defined into 4 components. Furthermore, temperature dependency of the thermal resistances is also measured even in the middle of the package structure in wide temperature range from 50°C to 250°C.

Based on the non-Fourier law and thermo-elastic theory, the non-Fourier expressions of the temperature field and the thermal stress field of metal material under multi-pulse laser irradiation were deduced. Taken stainless metal and two pulse width (ω=4ps,4fs) as an example, The effects that distributions and variations of transient temperature and thermal stress are influenced by laser pulse width are studied. The results show that the magnitude and frequency of transient temperature and thermal stress are seriously affected by the laser pulse width. More narrower the laser pulse width, faster and higher the local temperature rise; as well as the thermal stresses.

Thermal transient measurement based on structure function evaluation was used to measure the thermal resistance. The study signifies the importance of dual interface method in determining the exact point of separation between the board and the LED package. For a constant ambient temperature which was maintained at 28.2 ± 1.0 °C at 700 mA, the junction to board thermal resistance obtained was 10.84 K/W. In addition, an experimental set up has been reported in this work having a constant water flow beneath the external heat sink. More emphasis has been given in studying the effect of change of such measurement environment on the junction to board thermal resistance. It was revealed that the junction to board thermal resistance was not affected but the total real thermal resistance from junction to ambient was reduced significantly by 55.6% upon cooling with water. A study on the effect of light output on the total thermal resistance was performed and it was revealed that the efficiency and the reliability of an LED are strongly dependent on the optical properties of the device.

This paper measures the LED thermal resistance characteristic by thermal transient tester (T3Ster) obtained the cumulative structure functions curve and differential structure functions curve. The actual K factor and optical power are obtained by testingⅠ. The thermal resistance characteristic of each layer for LED structure are analyzed by testingⅡ in different cases. The experimental results show that the bonding method and the heat sink size have tremendous influence on the LED thermal resistance’s reduce, and the contact thermal resistance takes up a great proportion in LED total thermal resistance. The contact thermal resistance may reduce by pressure on interface and by a change of the attach material and the roughness of interface.

For high power and repetitive Nd:Glass laser, there would appear serious thermal effect due to the significantly-enhanced pumping optical power density in laser rod, which would induce thermal double refraction, phase distortion, thermal focusing, thermal depolarization, etc. Thus, the laser output power and beam quality would be influenced severely. Influence of thermal lens effect and depolarization effect caused by thermal stress double refraction on laser beam were studied in this paper for high power and repetitive laser. In the experiment of thermal lens effect, change of the laser spot by thermal lens effect was recorded after a TEM00 He-Ne laser beam passing through the Nd:Glass rod pumped by Xe lamp, and the corresponding optical compensator was applied to eliminate the thermal lens effect to improve the beam quality. Configuration of polarizer was designed in our experiment to research the depolarization effect caused by thermal stress double refraction. First, emergent light distribution and loss factor caused by double refraction was calculated according to the theoretical model. Then the exiting cone-shaped polarized light intensity distribution was obtained through the experiment. It could be obtained from the experiments that thermal lens effect and depolarization effect caused by thermal double refraction would reduce the output laser beam quality to varied extents. As the number of laser pulses increased, thermal effects began to emerge and the output laser energy apparently decreased; when the thermal effects steadied, the output energy would be tending towards stability.