Miquel Vellvehi

SiC Schottky Diodes for Harsh Environment Space Applications

This paper reports on the fabrication technology and packaging strategy for 300-V 5-A silicon carbide Schottky diodes with a wide temperature operation range capability (between -170 °C and 300 °C). These diodes have been designed for harsh environment space applications such as inner Solar System exploration probes. Different endurance tests have been performed to evaluate the diode behavior when working at a high temperature and under severe thermal cycling conditions (ranged from -170 °C to 270 °C). The radiation hardness capability has been also tested. It has been found that the hermeticity of the package in a neutral atmosphere is a key aspect to avoid an electrical parameter drift. Moreover, the use of gold metallization and gold wire bonds on the anode allows reducing the diode surface and bonding degradation when compared to Al-containing technology. On the back-side cathode contact, the Ti/Ni/Au metallization and AuGe combination have shown a very good behavior. As a result, the manufactured diodes demonstrated high stability for a continuous operation at 285 °C.

Thermomechanical Assessment of Die-Attach Materials for Wide Bandgap Semiconductor Devices and Harsh Environment Applications

Currently, the demand by new application scenarios of increasing operating device temperatures in power systems is requiring new die-attach materials with higher melting points and suitable thermomechanical properties. This makes the die-attach material selection, die-attaching process, and thermomechanical evaluation a real challenge in nowadays power packaging technology. This paper presents a comparative analysis of the thermomechanical performance of high-temperature die-attach materials (sintered nano-Ag, AuGe, and PbSnAg) under harsh thermal cycling tests. This study is carried out using a test vehicle formed by four dice (considering Si and SiC semiconductors) and Cu substrates. Thermally cycled test vehicles have been thermomechanically evaluated using die-shear tests and acoustic microscopy inspections. Besides, special attention is paid to set up a nano-Ag sintering process, in which the effects of sintering pressure or substrate surface state (roughness and surface activation) on the die-attach layer are analyzed. As a main result, this study shows that the best die-attach adherence is obtained for nano-Ag when pressure is applied on the dice (using a specifically designed press) during the sintering process (11 MPa provided die-shear forces of 53 kgf). However, this die-attach presents a faster thermomechanical degradation under harsh thermal cycling tests than other considered high-temperature die-attach materials (AuGe and PbSnAg) and PbSnAg shows the best thermomechanical performances.

Long-Term Reliability of Railway Power Inverters Cooled by Heat-Pipe-Based Systems

This paper analyzes the impact of a nonuniform temperature distribution inside insulated-gate bipolar transistor (IGBT) power modules on the reliability of railway power inverters. The interaction between the chosen cooling system (a heat-pipe-based one) and the power module is considered in detail. After showing the experimental setup and thermal conditions, thermal mapping inside the power module is carried out. Then, the effects of the thermal cycles on the constitutive elements of the IGBT module are pointed out when considering a real mission profile. Finally, the experimental results from the thermal cycles are linked to problems on the power-inverter reliability. Concretely, the thermal-grease distribution is analyzed on failed IGBT modules coming from the field, and the solder-delamination pattern observed in IGBT modules after endurance cycling tests is also reported.

Internal infrared laser deflection system: a tool for power device characterization

In this paper, a set-up based on the internal IR-laser deflection technique is described. This technique allows measurement of the temperature gradient and free-carrier concentration inside power semiconductor devices with high spatial (35 µm) and time (less than 1 µs) resolution. The internal IR-laser deflection technique consists in the measurement of deflection and absorption of an IR-laser beam passing through a biased power device. After describing the operational principle and the experimental set-up, the postprocessing methodology followed to extract the temperature gradient and free-carrier concentration is detailed. In order to show the set-up functionality, the drift region of a 600 V PT-IGBT device has been studied. These measurements are very helpful for performing reliability or thermal management studies on power semiconductor devices.

Thermal resistance investigations on new leadframe-based LED packages and boards

In Solid State Lighting, thermal management is a key issue. Within the C-SSL consortium, we have developed an advanced leadframe based package to reduce the thermal resistance of the component. Numerical simulations have been implemented using Ansys® software and thermal measurements have been carried out using the forward voltage method to derive the thermal resistance. The T3ster® transient thermal analysis has been used to determine the different thermal resistance contributions in the package and in the board showing good correlation between experimental and simulation results. Low thermal resistances of 5.5 K/W have been obtained on our advanced leadframe based package and have been compared with standard Rebel LEDs on board.

SiC Integrated Circuit Control Electronics for High-Temperature Operation

This paper is an important step toward the development of complex integrated circuit (IC) control electronics that have to attend to high-temperature environment power applications. We present in premiere a prototype set of essential mixed-signal ICs on SiC capable of controlling power switches and a lateral power MESFET able to operate at high temperatures, all embedded on the same chip. Also, we report for the first time the functionality of standard Si-CMOS topologies on SiC for the master-slave data flip-flop (FF) and data-reset FF digital building blocks designed with MESFETs. Concretely, we present the complete development of SiC-MESFET IC circuitry, able to integrate gate drivers for SiC power devices. This development is based on the mature and stable Tungsten-Schottky interface technology used for the fabrication of stable SiC Schottky diodes for the European Space Agency Mission BepiColombo.

Power-Substrate Static Thermal Characterization Based on a Test Chip

Thermal simulation is, nowadays, a basic tool to predict temperature distributions and heat fluxes of complex packages and modules. These variables are of main importance in high-power assemblies to analyze and predict their reliability limits. Nevertheless, the simulation results can be inaccurate due to the uncertainty of the values of the physical parameters involved in the models, as it is the case for the thermal conductivity of the dielectric layers (ceramics and composites) of the main families of power substrates [direct copper bonded (DCB) and insulated metal substrate (IMS)]. We propose a methodology for the in situ determination of these thermal conductivities under true operation conditions. Three test assemblies based on a thermal test chip and different types of power substrates (two IMS and one DCB) have been characterized in order to deduce their thermal resistance. Three-dimensional numerical models of the assemblies have also been developed. Thereby, the thermal conductivity of the critical layers is derived by minimizing the error between the experimental and the simulated thermal resistances. From the subsequent simulation results, the vertical temperature distributions are analyzed in order to predict the thermal stresses of the different layers inside the substrates.

A heterodyne method for the thermal observation of the electrical behavior of high-frequency integrated circuits

The observation of spectral components of the power dissipated by devices and circuits in integrated circuits (IC) by temperature measurements is limited by the bandwidth of either the temperature transducer or the intrinsic cut-off frequency provided by the thermal coupling inside the chip. In this paper, we use a heterodyne method to observe the high-frequency behavior of circuits and devices by means of low-frequency lock-in temperature measurements. As experimental results, two applications of the technique are presented: detection of hot spots in ICs activated by high-frequency electrical signals and the observation of the frequency response of an integrated resistor through temperature measurements. The heterodyne method has been used in this paper with four different measurement techniques: embedded differential BiCMOS temperature sensor, laser reflectometer, laser interferometer and internal IR laser deflection meter.

Steady-state sinusoidal thermal characterization at chip level by internal infrared-laser deflection

A new approach is reported for thermally characterizing microelectronic devices and integrated circuits under a steady-state sinusoidal regime by internal infrared-laser deflection (IIR-LD). It consists of extracting the amplitude and phase Bode plots of the temperature profile inside the chip (depth-resolved measurements in the frequency domain). As a consequence, not only are the IIR-LD performances significantly improved (accuracy, robustness to noise, control of boundary conditions and heat flux confinement) but also the direct temperature measurement is feasible when thin regions are inspected and thermal parameters can be easily extracted (thermal diffusivity). In order to show the efficiency of this technique, a thermal test chip (TTC) is used. The TTC is thermally excited by imposing a cosine-like voltage waveform. As a result, a vertical temperature profile inside the die is obtained depending on the heating frequency. Repeating this procedure at several frequencies, the frequency response of the chip internal temperature profile is derived. By comparing the experimental results with the model predictions, good agreement is achieved. This technique allows evaluation of the thermal behaviour at the chip level; also it could be useful for failure analysis.

Reduced-Order Thermal Behavioral Model Based on Diffusive Representation

The virtual prototyping of power electronic converters requires electrothermal models with various abstraction levels and easy identification. Numerous methods for the construction of compact thermal models have been presented in this paper. Few of them propose state-space models, where the model order can be controlled according to the necessity of the virtual prototyping analyses. Moreover, the model reduction methods require the experience of the engineer and previous calibration. Diffusive representation (DR) is proposed here as an original and efficient method to build compact thermal models as state-space models. The model reduction is obtained through the model parameter identification and/or the time horizon of the measurement data provided for the identification. Instead of eigenvalue elimination, the method enables to specify adequately inside the model the frequency domain wished for the virtual analysis at hand. The proposed method is particularly dedicated to the system optimization phases. Experimental and simulation results are in good agreement. The advantages and limitations of the DR are discussed in comparison to published methods.