[Elenco soci]




Pubblicazioni scientifiche

[1] Calabretta M., Sitta A., Oliveri S.M., Sequenzia G., Copper to resin adhesion characterization for power electronics application: Fracture toughness and cohesive zone analysis, Engineering Fracture Mechanics, 266, (2022). Abstract
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The use of molding compound as encapsulant is nowadays increasing in semiconductor power module applications. The adhesion of package interfaces between copper components and molding compound is one of the key aspect for an improved durability. The presented activity proposes the fracture toughness characterization of copper–resin interface in a power semiconductor package due to different experimental tests and the cohesive zone method to describe interfacial fracture. Double Cantilever Beam (DCB) and Four Point Bending (FPB) tests have been executed on dedicated bi-material coupons. The scope of these trials has been to enhance two different propagation modes based on different ratio between mode-I (opening) and mode-II (sliding) according to a mixed-mode approach. Strain energy release rate (SERR) and mode-mixity have been estimated by a finite element analysis based on the virtual crack closure technique (VCCT) and the crack surface displacement method (CSD). The information about fracture toughness at two different mode mixity have been considered to predict the SERR for every arbitrary mode mixity. Finally, dedicated finite element models based on cohesive elements have been developed and calibrated considering the fracture toughness experimental values and the measured force–displacements behavior during the two considered tests. Dedicated physical analyses have been carried out to validate the proposed method.

Keywords: Cohesive zone method | Finite element analysis | Fracture toughness | Interface | Power semiconductor package

[2] Calabretta M., Sitta A., A novel integrated approach to optimize copper wire bonding processes and manufacturing, International Journal on Interactive Design and Manufacturing, 16(1), 371-379, (2022). Abstract
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Copper is nowadays replacing the traditional gold in wire bonding interconnections, due to lower cost, better thermal/electrical properties and reliability performances. The increased hardness of Cu imposes higher bonding force and ultrasonic power during the wire-bonding process, increasing the risk of stress-induced bondpad damage. The aim of the presented work has been the modeling and characterization of stress and deformations resulting from the ball-bonding phase in order to have a quantitative method able to optimize the process set-up and the manufacturing capabilities already at design level. A finite element model has been developed and benchmarked with experimental samples obtained by freezing the ball bonding process at different steps, on which the deformations occurred in the bonded copper ball and in the bondpad layers have been measured through Xe plasma focused ion beam (Plasma-FIB) cross sections.

Keywords: Copper ball bonding | FEM | Power semiconductor devices | Ultrasonic softening | Wire bonding

[3] Calabretta M., Sitta A., Sequenzia G., Silicon carbide die sintering layer: manufacturing process optimization and modeling, International Journal on Interactive Design and Manufacturing, 16(1), 167-176, (2022). Abstract
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The massive development of Hybrid and Electrical Vehicles is strongly impacting the semiconductor industry demanding for highly reliable Power Electronic components. These challenges mainly originate from Silicon Carbide MOSFET’s superior properties allowing high power, high temperature capability, fast switching transients and high electric field operations. All these features can be obtained in a significant reduced chip area. In order to benefit from the disrupting advantages of these wide band gap semiconductor based power devices, a strong focus on silver sintering, as one of the most promising die attach technologies, is needed to withstand these challenging requirements. The aim of this work is to develop an integrated methodology, numerical and experimental, to assess the Ag sintering die attach process for a SiC power MOSFET. Different process parameters have been benchmarked by means of physical analyses, performed at time zero and also after liquid-to-liquid thermal shock aging test. The sintering flakes densification process has been reproduced by Finite Element Analysis and the obtained morphological texture has been used for extracting the mechanical properties of the layer as a function of the thermo-compression process itself. A simulation method, based on the evaluation of the inelastic strain accounted per cycle has been used for matching the experimental results according to an aging model. Furthermore, it has been predicted the silver sintering performances considering an active temperature cycle. The proposed methodology has supported the optimization of silver sintering parameters and has calculated the reliability performances of the silver sintering joint due to costumer-like active temperature cycle. Negligible sintering degradation has been carried out with a predicted number of cycles over two millions, suggesting die attach failure is not a relevant reliability bottleneck.

Keywords: Active temperature cycle | Reliability | Silicon carbide | Silver sintering | Simulation

[4] Cioni M., Fiorenza P., Roccaforte F., Saggio M., Cascino S., Messina A., Vinciguerra V., Calabretta M., Chini A., Identification of Interface States responsible for V<inf>TH</inf>Hysteresis in packaged SiC MOSFETs, IEEE International Reliability Physics Symposium Proceedings, 2022-March, 5B31-5B35, (2022). Abstract
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We investigate the mechanism governing threshold voltage (VTH) hysteresis in packaged SiC MOSFETs. A double-ramp measurement method was employed for this scope, being able to accurately evaluate the time-dependent recovery of the positive VTH shift induced by the sweep-up of the gate voltage. Particularly, we studied the effect of the (i) gate driving voltage (VGH), (ii) recovery time (TOFF) and (iii) temperature (T) on the VTH hysteresis. No appreciable differences were observed among data collected at different VGH, whereas a recovery speed-up was observed at higher T values. Temperature dependent measurement of VTH recovery yielded a 0.3 eV activation energy, that has been associated to SiC/SiO2 interface traps located ~0.3 eV below the SiC conduction band.

Keywords: interface traps | SiC | V hysteresis TH

[5] Malta G., Anoldo L., Mazza B., Piccione G.G., Russo A., Calabretta M., Russo S., Sitta A., Messina A., Lionetto A., Patane S., High speed thermal mapping on six-pack SiC-based module for hybrid and electric vehicles, 2022 23rd International Conference on Thermal, Mechanical and Multi-Physics Simulation and Experiments in Microelectronics and Microsystems, EuroSimE 2022, (2022). Abstract
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In this work, the thermal characterization of a commercially available SiC-based power module named as ACEPACK Drive, specifically designed by STMicroelectronics for automotive applications, was carried out. In particular, we studied the dynamic thermal behaviour and temperature gradient of one leg of the ACEPACK Drive by a combination of high-speed thermal microscopy and conventional thermal mapping. As main result, the temperature distribution on the device turned out to be homogeneous, with a maximum at around ~99 °C in simulated operative condition.

[6] Vinciguerra V., Landi A., Hintermaier H., Badala P., Klug G., Sitta A., Calabretta M., Bassi A., Renna M., Messina A.A., Failure Strength Analysis and Young Modulus Assessment of 4H-SiC through a Ball on Ring Test, 2022 23rd International Conference on Thermal, Mechanical and Multi-Physics Simulation and Experiments in Microelectronics and Microsystems, EuroSimE 2022, (2022). Abstract
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Imperfections in SiC die can have an impact on their mechanical properties, affecting the target of the high reliability standard, required in power devices intended for automotive applications. In this work the failure strength of 4H-SiC square die of different thicknesses has been determined through a ball-on-ring (BOR) test and compared with previous three-point bending test results. Moreover, an interpretation of the experimental results on the net deflection d-d0 at the center of the die sample, according to a revisited Vitman and Pukh equation, has been reported. This allowed to achieve an assessment of the effective or equivalent Young's modulus of the die, with the BOR equipment on back-grinded 4H-SiC die, thinned up to 110 µm. The BOR provided consistent values for the 180 µm thin samples with the results from the previous the 3-PB test and a value of the equivalent Young's modulus of (391 ±90) GPa for the 110 µm, which is comparable with the theoretical limit measured in 4H-SiC.

Keywords: 4H-SiC | ball-on-ring | equivalent Young's Modulus | failure strength

[7] Calabretta M., Sitta A., Messina A.A., Oliveri S.M., Sequenzia G., Mechanical Characterization of Power Electronics Solder Materials, Lecture Notes in Mechanical Engineering, 712-720, (2022). Abstract
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One of the main bottleneck for power semiconductor durability is the solder joint reliability. A proper design of the interconnections between silicon chip and printed control board is needed to fulfill the strict industrial and automotive requirements. Considering that solders are alloys with melting temperature lower than 450∘C, high-temperature package processes and costumer profile condition enhances the visco-plastic solder degradation, affecting the joint dimensional tolerances and reliability. The mechanical characterization of solder compounds and processes results fundamental to achieve reliability and geometric dimensioning and tolerancing targets. The presented work proposes an analytical-experimental methodology to characterize the mechanical constitutive equation of a specific solder compound widely used in semiconductor industries that is SnAgCu. Visco-plastic solder behavior with respect to environment temperature is experimental detected employing different uniaxial tensile tests considering some scenarios in terms of strain rate and temperature conditions. These outcomes are numerically post-processed to find out the Anand parameters of the analyzed solder according.

Keywords: Anand model | Material characterization | Solder compound | Visco-plasticity

[8] Calabretta M., Sitta A., Oliveri S.M., Sequenzia G., Fracture Toughness Characterization of Copper-Resin Interface in Power Electronics Application, Lecture Notes in Mechanical Engineering, 549-556, (2022). Abstract
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The use of molding compound as encapsulating material is nowadays increasing in semiconductor industry. Such component guarantees excellent thermal and reliability performances than the current silicone-based gel, enabling higher working temperature for semiconductor device and mitigating the solder joint reliability bottleneck. The adhesion of package interfaces between copper components and molding compound is one of the key aspect for optimized durability. Dedicated experiments and theoretical framework based on fracture mechanic are needed for this purpose. The presented activity proposes the fracture toughness characterization of copper-resin interface in a power semiconductor package. Double Cantilever Beam (DCB) test has been executed on dedicated bimaterial coupon with an initial crack at interface. The aim of this test has been to enhance the fracture propagation mode-I (opening). Strain energy release rate (SERR) and mode-mixity have been estimated from this experiment developing a finite element analysis that is able to predict the crack length during the experimental DCB trials and to predict the energy release rate by virtual crack closure technique (VCCT). Mode-mixity has been estimated collecting displacements near the crack tip by crack surface displacement method (CSD). The proposed methodology for fracture toughness characterization represents a strong pillar to predict fracture behavior due to any load conditions and it is needed to describe interface adhesion by cohesive zone method (CZM).

Keywords: Finite element analysis | Fracture | Interfacial delamination | Power electronics | Virtual crack closure technique

[9] Calabretta M., Sitta A., Oliveri S.M., Sequenzia G., Structural characterization of semiconductor multi-layer pad, Proceedings of the Institution of Mechanical Engineers, Part C: Journal of Mechanical Engineering Science, 235(24), 7734-7742, (2021). Abstract
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Structural mechanics and mechanical reliability issues are becoming more and more challenging in the semiconductor industry due to the continuous trend of the device dimensional shrinkage and simultaneous increased operative temperature and power density. As main consequence of the downsizing and more aggressive operative conditions, the mechanical robustness assessment is now having a central role in the device engineering and assessment phase. The risk of mechanical crack in the brittle oxide layers, which are embedded in pad stacks, increases during the device manufacturing processes such as the electrical wafer testing and during wire bonding. This risk increases with the presence of intrinsic mechanical stress in individual layers resulting from the metal grain growth mechanisms, the stack layers’ interfacial mismatches in coefficients of thermal expansion and the temperature stress induced by doping diffusion and film deposition. The current trend of innovation in the electronic industry is going over the semiconductor material itself and it is now impacting the improvement of the Back-End of Line. Key actors are becoming the interactions between the semiconductor die and the device packaging such as adhesion layers, barriers and metal stacks. In the present work, different pad structures have been structurally analyzed and benchmarked. The experimental characterization of the pad structures has been done through a flat punch nano-indentation to investigate on the mechanical strength and the crack propagation. The considered mechanical load reproduces the vertical impact force applied during wire bonding process to create the bond-pad electrical interconnection. The obtained testing results have been compared to finite element models to analyze the stress distribution through the different layers’ stacks. Scope of this work is to demonstrate the validity of the proposed integrated numerical/experimental methodology, showing the impact of the metal connections layouts by the analysis of the stress notch factors and crack propagation behaviour.

Keywords: finite element | nanoindentation | Pad | stress analysis

[10] Musolino M., Xu X., Wang H., Rengarajan V., Zwieback I., Ruland G., Crippa D., Mauceri M., Calabretta M., Messina A., Paving the way toward the world's first 200mm SiC pilot line, Materials Science in Semiconductor Processing, 135, (2021). Abstract
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In recent years, the power electronics industry based on silicon carbide (SiC) has rapidly expanded, but suppliers are struggling to meet the market demand both for final devices and for the starting raw material, which nowadays consists of SiC wafers with a diameter of 150 mm (6 inches). For this reason, the top industrial players in the field of SiC power electronics are starting the development of next-generation wafers with a diameter of 200 mm (8 inches). This work describes the recent achievements in the implementation of the world's first industrial pilot line to produce power devices based on 200 mm SiC wafers. In particular, the crystal growth of the 200 mm SiC boules, the slicing and polishing of the wafers, the deposition of the epitaxial layer, and the first tests in the pilot lines are presented.

Keywords: 200 mm wafers | 4H SiC | 8-Inch silicon carbide | Epitaxial growth | Pilot line | REACTION

[11] Calabretta Phd M., Messina Phd A., Sitta A., Lifetime Modeling for Silicon Carbide Based Power Module, SAE Technical Papers, (2021). Abstract
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In the presented paper it has been proposed a methodology that translates the customer conditions in terms of electric power into thermo-mechanical variables correlated with power module failure mechanisms. An initial analysis of power module failure modes will be presented, then the module temperature during inverter load profile will be numerically estimated. The so-obtained time-temperature during inverter operation will be converted using Rainflow-like approach into a pre-defined number of loading conditions. According to the time and temperature dependency of the typical failure mechanism, the mentioned loading cases will be characterized by different temperature swing and time duration. The extrapolated equivalent history will be a starting point to set durability tests aimed to predict reliability performances.

[12] Mauromicale G., Sitta A., Calabretta M., Oliveri S.M., Sequenzia G., Integrated electromagnetic-thermal approach to simulate a gan-based monolithic half-bridge for automotive dc-dc converter, Applied Sciences (Switzerland), 11(18), (2021). Abstract
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New technological and packaging solutions are more and more being employed for power semiconductor switches in an automotive environment, especially the SiC-and GaN-based ones. In this framework, new front-end and back-end solutions have been developed, and many more are in the design stage. New and more integrated power devices are useful to guarantee the performances in electric vehicles, in terms of thermal management, size reduction, and low power losses. In this paper, a GaN-based system in package solution is simulated to assess the structure temperature submitted to a Joule heating power loss. The monolithic package solution involves a half-bridge topology, as well as a driver logic. A novel integrated electromagnetic and thermal method, based on finite element simulations, is proposed in this work. More specifically, dynamic electric power losses of the copper interconnections are computed in the first simulation stage, by an electromagnetic model. In the second stage, the obtained losses’ geometrical map is imported in the finite element thermal simulation, and it is considered as the input. Hence, the temperature distribution of the package’s copper traces is computed. The simulation model verifies the proper design of copper traces. The obtained temperature swing avoids any thermal-related reliability bottleneck.

Keywords: Automotive | Electromagnetic simulation | Finite element simulation | Gallium nitride | Half-bridge | Integrated package | Power devices | Thermal simulation

[13] Calabretta M., Sitta A., Oliveri S.M., Sequenzia G., Silver sintering for silicon carbide die attach: Process optimization and structural modeling, Applied Sciences (Switzerland), 11(15), (2021). Abstract
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The increasing demand in automotive markets is leading the semiconductor industries to develop high-performance and highly reliable power devices. Silicon carbide MOSFET chips are replacing silicon-based solutions through their improved electric and thermal capabilities. In order to support the development of these novel semiconductors, packaging technologies are evolving to provide enough reliable products. Silver sintering is one of the most promising technologies for die attach. Due to their superior reliability properties with respect to conventional soft solder compounds, dedicated reliability flow and physical analyses should be designed and employed for sintering process optimization and durability assessment. This paper proposes an experimental methodology to optimize the pressure value applied during the silver sintering manufacturing of a silicon carbide power MOSFET molded package. The evaluation of the best pressure value is based on scanning electron microscopy performed after a liquid-to-liquid thermal shock reliability test. Furthermore, the sintering layer degradation is monitored during durability stress by scanning the acoustic microscopy and electric measurement of a temperature sensitive electric parameter. Moreover, mechanical elastoplastic behavior is characterized by uniaxial tensile test for a bulk sample and finite element analysis is developed to predict the mechanical behavior as a function of void fraction inside sintering layer.

Keywords: Finite element analysis | Mechanical characterization | Physical analyses | Reliability | Silver sintering

[14] Anoldo L., Triolo C., Panarello S., Garesci F., Russo S., Messina A.A., Calabretta M., Patane S., Study of the Thermomechanical Strain Induced by Current Pulses in SiC-Based Power MOSFET, IEEE Electron Device Letters, 42(7), 1089-1092, (2021). Abstract
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Power SiC MOSFETs are going to substitute Si devices by to their significantly better performances that make them much suitable in power switching applications such as electric/hybrid vehicles. The increasingly use of these devices in critical mission profiles requires an ever-higher reliability, whereas the increase of the dissipated power during high-speed working cycling due to short current pulses leads to unavoidable thermal and mechanical stress. Here, we propose a direct method to evaluate the mechanical stress due to current pulses. This method highlights that high Power SiC-based MOSFET undergoes to almost two different thermomechanical processes with completely different time scale. The results allow a link between the thermo-mechanical stress and the device failure conditions, with special focus on the current pulses effects on metal surface, as this is a main power devices weakness.

Keywords: Coffin Manson | Power MOSFET | reliability | silicon carbide | strain wide band gap semiconductors

[15] Calabretta M., Sitta A., Oliveri S.M., Sequenzia G., Warpage behavior on silicon semiconductor device: the impact of thick copper metallization, Applied Sciences (Switzerland), 11(11), (2021). Abstract
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Electrochemical deposited (ECD) thick film copper on silicon substrate is one of the most challenging technological brick for semiconductor industry representing a relevant improvement from the state of art because of its excellent electrical and thermal conductivity compared with traditional materials, such as aluminum. The main technological factor that makes challenging the industrial implementation of thick copper layer is the severe wafer warpage induced by Cu annealing process, which negatively impacts the wafer manufacturability. The aim of presented work is the understanding of warpage variation during annealing process of ECD thick (20 µm) copper layer. Warpage is experimentally characterized at different temperature by means of Phase-Shift Moiré principle, according to different annealing profiles. Physical analysis is employed to correlated the macroscopic warpage behavior with microstructure modification. A linear Finite Element Model (FEM) is developed to predict the geometrically stress-curvature relation, comparing results with analytical models.

Keywords: ECD | FEM | Phase-Shift Moiré | Semiconductor | Warpage

[16] Sitta A., Mauromicale G., Sequenzia G., Messina A.A., Renna M., Calabretta M., Thermo-mechanical finite element simulation and visco-plastic solder fatigue for low voltage discrete package, 2021 22nd International Conference on Thermal, Mechanical and Multi-Physics Simulation and Experiments in Microelectronics and Microsystems, EuroSimE 2021, (2021). Abstract
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Nowadays, solder reliability in new power electronic packages is an important research topic. Therefore, it is of paramount importance to properly understand and model the material behaviour and to develop a calculation model to predict reliability performances. This work presents a thermo-mechanical analysis of different solder layers for a low voltage discrete package. The solder joint reliability between package and PCB is also considered in the simulation. This modelling activity is possible by employing the Anand visco-plastic model and by means of a finite element model implemented in COMSOL. The number of cycles to failure can be subsequently computed, with the Darveaux method, for fatigue life estimation purpose.

[17] Landi A., Mon A.A., Liaci L., Sitta A., Calabretta M., Sciuto A., D'Arrigo G., Renna M., Vinciguerra V., Failure Strength Weibull Analysis of 4H-SiC Die through a 3-PB test, 2021 22nd International Conference on Thermal, Mechanical and Multi-Physics Simulation and Experiments in Microelectronics and Microsystems, EuroSimE 2021, (2021). Abstract
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The paper reports the mechanical properties of 4H-SiC die with different thicknesses, that have been determined through a 3-point bend (3-PB) test. In particular, it reports 1) the measurement of the failure strength of thin 4H-SiC rectangular die; 2) the Weibull analysis of the failure strength of 4H-SiC die, exploited to determine the maximal load that can be applied to the die, without any breakage; 3) the measure of 244±15 GPa for the effective Young modulus E of SiC die, gained from the 3-PB test elaborations.

[18] Sitta A., Mauromicale G., Giuffrida V., Manzitto A., Papaserio M., Cavallaro D., Bazzano G., Renna M., Rizzo S.A., Calabretta M., Paralleling Silicon Carbide MOSFETs in Power Module for Traction Inverters: A Parametric Study, 2021 22nd International Conference on Thermal, Mechanical and Multi-Physics Simulation and Experiments in Microelectronics and Microsystems, EuroSimE 2021, (2021). Abstract
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The high power demanding in many emerging power electronic applications requires innovative solutions in semiconductor field. One of the possible keys to satisfy this requirement is the use of more paralleled devices. For traction applications, parallel connection of Silicon Carbide (SiC) MOSFETs is widely employed. In this framework, the power devices are housed in module packages. This paper presents a parametric analysis of a SiC power module for traction inverters, with a three-phase topology, containing many paralleled dice for each swich. The scope of the work is to study this phenomenon with a numerical investigation based on a circuit simulation, evaluating the junction temperature in a case study. The parameters varied in the simulations are the threshold voltage and the drain-source on-state resistance.

[19] D'Arrigo G., Scuderi M., Mio A., Favarò G., Conte M., Sciuto A., Buscema M., Li-Destri G., Carria E., Mello D., Calabretta M., Sitta A., Pries J., Rimini E., Mechanical characterization and properties of continuous wave laser irradiated Ge<inf>2</inf>Sb<inf>2</inf>Te<inf>5</inf> stripes, Materials and Design, 202, (2021). Abstract
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Crystalline micrometer size stripes in 2.2 μm thick Ge2Sb2Te5 phase-change material films were produced by irradiation with a Continuous Wave Laser of 405 nm wavelength. The shape and the dimensions of the crystallized regions were investigated by Transmission Electron Microscopy and then compared with simulations based on temperature-crystal growth velocity literature data. The temperature-time profile was determined taking into account the laser power, the optical and thermal properties of both the amorphous and crystalline phase. The mechanical properties of the amorphous and of the crystallized regions were characterized by an ultra nano-indentation technique. This procedure allows a direct and local measurement of hardness and Young's modulus in the amorphous and in the contiguous crystalline regions on the micrometer scale. The following values for Young's modulus and for hardness were obtained: 33±4 GPa and 2.00±0.3 GPa for the amorphous phase, and 51±8 GPa and 2.90±0.45 GPa for the crystalline phase. The stresses induced by the density increase in the crystallized region cause, on the irradiated surface, a series of fracture whose characteristic behavior depends on the laser power and on the spacing between two contiguous scans. These results are of relevance for the mechanical failure mechanisms in potential phase-change devices.

Keywords: Continuous wave laser irradiation | GST | GST mechanical properties | Ultra nano-indentation

[20] Calabretta M., Sitta A., Oliveri S.M., Sequenzia G., An experimental-numeric approach to manufacture semiconductor wafer using thick copper front metallization, International Journal on Interactive Design and Manufacturing, 15(1), 117-119, (2021). Abstract
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The presented work investigates about the deformation of semiconductor device induced by electrochemical deposited thick copper films. It enhances thermal and electric performances allowing to use copper interconnections without formations of intermetallic layers at the interfaces with consequent reliability improvement. Nevertheless, the induced deformation strongly affects manufacturability, criticizing the integration between different process steps. Experiment based on phase-shift Moiré principle has been performed to better understand the relation between warpage and temperature. Finite element model has been developed to reproduce the phenomenon in order to address the design and the process integration optimizing workability, electrical performances and reliability.

Keywords: Finite element model | Manufacturability | Power electronics | Process integration | Warpage

[21] Calabretta M., Sitta A., Oliveri S.M., Sequenzia G., Experimental characterization and modeling of power module warpage during assembly process, International Journal on Interactive Design and Manufacturing, 15(1), 113-115, (2021). Abstract
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Hybrid and full electric automotive market is strongly increasing the demand for power semiconductor modules. With respect to discrete packages, manufacturing of power modules is more complex and new process parameter, such as module deformation (warpage), assumes a key role for a robust design and to guarantee reliable application. The aim of this paper is to study the warpage behaviour during power module assembly flow by means of dedicated warpage measurement at different process steps. Once highlighted the most impacting process for warpage, a finite element model has been developed to reproduce phenomenology, predicting the induced deformation.

Keywords: Finite element modeling | Manufacturability | Planarity | Power module | Warpage measurement

[22] Anoldo L., Piccione G.G., Mauromicale G., Sitta A., Fazio E., Calabretta M., Lionetto A., Messina A., Di Guardo M., Patanè S., Thermal simulation of a 7kW interleaved module for fast automotive charger, 21st IEEE International Conference on Environment and Electrical Engineering and 2021 5th IEEE Industrial and Commercial Power System Europe, EEEIC / I and CPS Europe 2021 - Proceedings, (2021). Abstract
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Si- and SiC-based fast on-board chargers require very special thermal strategies, to ensure high reliability in order to assess long life in harsh environments, such as the automotive one. Indeed, the power losses in the switches may lead to a severe temperature rise in the enclosed cabinet, causing a reduce lifetime of the system. In this paper, we propose a scalable fast charger for automotive applications able to manage a 7 kW power flow. A thermal simulation based on COMSOL Multiphysics is performed in this work. The study allows us to get a realistic evaluation of the higher temperatures reached by the active Si and SiC devices of the converter during the normal operation, at the steady state and at the maximum allowed power.

Keywords: automotive | DC/DC converter | fast charger | LLC | OBC | PFC | thermal simulation

[23] Mauromicale G., Cascio A., Papaserio M., Cavallaro D.G., Bazzano G., Messina A.A., Patanè S., Calabretta M., Sitta A., Directly cooled silicon carbide power modules: Thermal model and experimental characterization, PCIM Europe Conference Proceedings, 2021-May, 1133-1137, (2021). Abstract
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The Silicon Carbide (SiC) semiconductor material is playing a fundamental role in the development of new power modules. Due to its excellent physical properties, the power devices based on this novel compound improve the traction inverter performance in electric vehicles. This work presents a 3D Finite Element Model (FEM) and fluid dynamics simulation to investigate the behavior of a directly cooled SiC module structure. Furthermore, a two-step experimental procedure to thermally characterize the module is also reported. It comprises a calibration and, subsequently, the thermal impedance computation. The proposed FEM model is compared with experimental test results in order to demonstrate its effectiveness.

[24] Pellitteri F., Busacca A., Martorana C., Miceli R., Stivala S., Messina A.A., Calabretta M., Vinciguerra V., Power losses comparison between Silicon Carbide and Silicon devices for an isolated DC-DC converter, 2021 IEEE 15th International Conference on Compatibility, Power Electronics and Power Engineering, CPE-POWERENG 2021, (2021). Abstract
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In recent years, new efficient power devices have been implemented. Silicon Carbide has replaced silicon as regards the production and the utilization of many devices, such as MOSFETs, diodes, IGBTs and many others. SiC devices are characterized by a low reverse recovery charge, high carrier saturation velocity, by which it is possible to work at high frequency, and high breakdown voltage. Thanks to the great thermal conductivity and the wide bandgap, these devices can operate at high temperature and reach high voltages and currents. What is important to stress is the fact that power losses in SiC devices are lower than the silicon ones. These are the reasons why these devices are utilized in a wide range of technological applications, including power electronics. In this paper, power losses in Si and SiC devices are evaluated in simulation and compared to each other. In order to demonstrate the remarkable advantages of the SiC devices over the silicon ones, a study which makes use of an isolated DC-DC converter has been conducted. As regards the proposed full-bridge converter, SiC and silicon MOSFETs and diodes, of whom some static and dynamic parameters are defined, are used in order to transfer power from a DC voltage supply to a load.

Keywords: DC-DC converters | Isolated power converters | Power losses | SiC devices

[25] Calabretta M., Sitta A., Oliveri S.M., Sequenzia G., Simulation of Dynamic Stresses on High Performance Engine Valve Spring System Considering Coil Clashing Effect, Lecture Notes in Mechanical Engineering, 108-113, (2021). Abstract
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The valve train plays a major role in the performance of internal combustion engines by controlling the combustion process and it is therefore one of the key aspects for increasing the efficiency of combustion engines. Considering the dynamics, the spring force must be high enough to reliably close the valve preventing from seating bouncing due to surge modes after the valve closure. On the other side, the spring force should be kept as low as possible in order to reduce the engine friction losses and consequently the fuel consumption. In the high-performance engines, the valve springs have to be designed and optimized for sustaining higher stresses with compact dimensions leading to critical material and manufacturing processes. This requires a reduction of moving masses and a strong focus on design and process optimization of the coil springs for reducing the mechanical load and the friction losses at low engine speed. At the same time, valve train should be reliable at high engine speed. The calculation of stresses and contact forces for moving parts under dynamic load is essential for durability analysis. A method to calculate the contact of moving masses is described and proposed to justify valve motions experimental results. To fully understand the failure mechanism of test bed reliability trials, the dynamic stresses have been calculated modeling the real springs’ shape. The contact forces have been reproduced considering the coil clash effects and the dynamic behavior of the flexible spring.

Keywords: Coil clash | FEM | Multibody | Valve springs | Valve train

[26] Calabretta M., Sitta A., Oliveri S.M., Sequenzia G., Analysis of Warpage Induced by Thick Copper Metal on Semiconductor Device, Lecture Notes in Mechanical Engineering, 55-60, (2021). Abstract
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Electrochemical deposited (ECD) thick film copper on silicon substrate is one of the most challenging technological brick for semiconductor industry representing a relevant improvement from the state of art because of its excellent electrical and thermal conductivity compared with traditional compound such as aluminum. The main technological factor that makes challenging the industrial implementation of thick copper layer is the severe wafer warpage induced by Cu annealing process, which negatively impacts the wafer manufacturability. The aim of presented work is the understanding of warpage variation during annealing process of ECD thick (~20 µm) copper layer. Warpage has been experimental characterized at different temperature by means of Phase-Shift Moiré principle, according to different annealing profiles. A linear Finite Element Model (FEM) has been developed to predict the geometrically stress-curvature relation, comparing results with analytical models.

Keywords: ECD | FEM | Phase-Shift Moiré | Semiconductor | Warpage

[27] Calabretta M., Sitta A., Oliveri S.M., Sequenzia G., Silicon Carbide Multi-Chip Power Module for Traction Inverter Applications: Thermal Characterization and Modeling, IEEE Access, 9, 76307-76314, (2021). Abstract
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Semiconductor power modules are the key hardware components of a traction inverter. It drives motor speed and torque, managing the energy exchange from battery to motor and viceversa. The increasing demand for electric and hybrid vehicle requests high performance power modules. Power semiconductor devices based on wide band gap compound, like silicon carbide (SiC), have excellent electrical properties in terms of on-state resistance, stray inductance and performance at high commutation frequency. One of the most promising solution is silicon carbide MOSFET power module in which each switch is made by several different dies placed in parallel. Embedded direct cooling system and novel materials with high conductivity (e.g., active metal brazed substrates) can be considered to enhance thermal performance. A robust method is needed to characterize and to predict power module temperature behavior considering the importance of the thermal performance to improve reliability and to optimize module weight and dimensions. According to several parallel dies inside each switch, classic method based on temperature electric sensitive parameter (TSEP) shall be validated with direct measurement. In this framework, it has been reported the thermal characterization of a power module for a traction inverter based on eight silicon carbide MOSFETs for each switch. Both TSEP and infrared measurements have been employed. Thermal behavior has been numerically reproduced, creating a simplified equivalent network and developing a predictive model by finite element method (FEM).

Keywords: numerical model | Power modules | SiC MOSFET | thermal measurements

[28] Sequenzia G., Calabretta M., Assenza I., Oliveri S.M., Dynamic behaviour modelling of an internal combustion engine water pump transmission belt drive, International Journal of Vehicle Performance, 7(1-2), 170-184, (2021). Abstract
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Belt drives are commonly used in various types of transmissions to link two or more rotating shafts. In order to transmit the motion, an effective grip on the pulley has to be set by imposing a pre-load on the belt. Moreover, the dynamics of the system is strongly affected not only by the geometrical and inertial properties but also by the imposed belt tension force as a functional parameter affecting the vibration characteristics. In the present work, it is presented an integrated methodology, experimental and numerical, to determine the dynamic behaviour of a water pump drive in a high-performance internal combustion engine.

Keywords: Belt | Internal combustion engine | Multibody model | Natural frequency | Vibration modes | Water pump transmission drive

[29] Calabretta M., Sitta A., Oliveri S.M., Sequenzia G., Power Semiconductor Devices and Packages: Solder Mechanical Characterization and Lifetime Prediction, IEEE Access, 9, 22859-22867, (2021). Abstract
X

Solder reliability is a key aspect for the packaging of low voltage power semiconductor device. The interconnections among package components, e.g. the silicon chip and copper leadframe, and between package itself and the external printed control board (PCB) should be properly designed to ensure the automotive durability requirements. In this framework, the proposed paper introduces an experimental-numeric characterization flow with the purpose to analyze solder visco-plasticity and fatigue during passive temperature cycle. The presented methodology has included solder mechanical characterization aimed to determine the parameters of Anand model which reproduces the solder visco-plastic behavior and the mechanical properties' temperature dependency. Finite element model has been employed to calculate the inelastic work which solder dissipates during each temperature cycle. Simulation results serve as input to predict solder lifetime according to an energetic method. Moreover, failure analyses have been performed to assess the failure mechanism and to check model correlation in terms of number of cycles to failure forecast.

Keywords: finite element model | material characterization | power semiconductor package | Reliability

[30] Messina A.A., Imbruglia A., Calabretta M., Vinciguerra V., Moise C.C., Sitta A., Enachescu M., Roccaforte F., The "first and euRopEAn siC eighT Inches pilOt liNe": A project, called REACTION, that will boost key SiC Technologies upgrading (developments) in Europe, unleashing Applications in the Automotive Power Electronics Sector, 2020 AEIT International Conference of Electrical and Electronic Technologies for Automotive, AEIT AUTOMOTIVE 2020, (2020). Abstract
X

REACTION is an EU Innovation Action Funded Project which aims to develop the first worldwide 200mm Silicon Carbide (SiC) Pilot Line for SiC based Power technology. The project, which mainly addresses the Smart Mobility societal challenge (car electrification), will allow to match the ever-increasing demand of requirements in terms of quality and cost constraint for next decade generation's power electronics. A major strength of the project is the complete value chain implementation of the Pilot Line, which integrates and optimizes the cooperation and partnership among producers of 8"SiC substrates and equipment developers, as well as SiC process technologists, RTOs and end-users partners, in order to deliver the final applications. Hence, to achieve innovative SiC power devices with improved performances, along with cost and size reduction, are the most relevant requirements addressed in the project that are expected to lead to a new stronger European supply chain for very compact SiC converters, ideal for the addressed Automotive application.

Keywords: Automotive | Package | Power Electronics | SiC | Smart Mobility

[31] Selgi L.M., Sciuto A., Calabretta M., Sitta A., D'Arrigo G., Electrical characterization of gate oxide current in a silicon power MOS subjected to uniaxial mechanical stress, Proceedings - 2020 2nd IEEE International Conference on Industrial Electronics for Sustainable Energy Systems, IESES 2020, 365-370, (2020). Abstract
X

This paper presents the results of the electrical characterization of a silicon Low Voltage Trench Gate charged-coupled Power MOSFET under external uniaxial mechanical stress. The stress was imposed by a 3 Point Bending system during electrical measurement of the oxide leakage current.

Keywords: 3 Point Bending | gate oxide | mechanical stress | Power device | Silicon MOSFET | Trench Gate

[32] Sitta A., Russo S., Torrisi M., Messina A.A., D'Arrigo G., Sequenzia G., Renna M., Calabretta M., An integrated approach to optimize solder joint reliability, 2020 21st International Conference on Thermal, Mechanical and Multi-Physics Simulation and Experiments in Microelectronics and Microsystems, EuroSimE 2020, (2020). Abstract
X

This work has investigated the impact of crystallographic structure on SnAgCu (SAC) solder reliability at print board circuit (PCB) level. A detailed reliability analysis has been performed on packages with different solder thickness. The correlation between experiments and Finite Element Model results explains how NiAu metallization and the reduction of solder thickness improve the solder joint reliability performances.

[33] Calabretta M., Guarnera D., Sitta A., Renna M., Sciuto A., D'Arrigo G., Oliveri S.M., Sequenzia G., Mechanical characterization and modeling of different pad structures, 2020 21st International Conference on Thermal, Mechanical and Multi-Physics Simulation and Experiments in Microelectronics and Microsystems, EuroSimE 2020, (2020). Abstract
X

Innovation on semiconductors technology requires enhancements of all actors like adhesion layers, barriers and metal stacks, beyond of semiconductor materials themselves. In general, metallic layers influence the whole die performances. The composition and the layout of these metal layers are fundamental for the signal transmission from the frame to the die and vice versa, and therefore their improvement contributes to the die development in terms of performances and reliability. In the present work, two pad structures have been benchmarked and analyzed under the structural strength standpoint. The experimental comparison among the different pads has been done through a flat punch nanoindentation to highlight the material strength and the crack propagation phenomena. Testing results have been compared to finite element models to analyze the stress through the different layers. The findings of the work demonstrate the validity of the methodology adopted and show the importance of the metallic connections layouts for the stress concentration and crack formation analysis.

[34] Sitta A., Renna M., Messina A.A., Sequenzia G., D'Arrigo G., Calabretta M., Thermal measurement and numerical analysis for automotive power modules, 2020 21st International Conference on Thermal, Mechanical and Multi-Physics Simulation and Experiments in Microelectronics and Microsystems, EuroSimE 2020, (2020). Abstract
X

The presented analysis has been aimed to evaluate the impact of die solder and sintering solution for automotive power modules in terms of thermal behavior. First, dedicated temperature measurements have been performed to evaluate the module thermal impedance in the two cases. Then, a lumped equivalent networks has been calculated, by means of a dedicated numeric, and finally function structures have been extracted.

[35] Sitta A., Renna M., Messina A.A., Mirone G., D'Arrigo G., Calabretta M., Power Module Ceramic Substrates: Mechanical characterization and modeling, 2020 21st International Conference on Thermal, Mechanical and Multi-Physics Simulation and Experiments in Microelectronics and Microsystems, EuroSimE 2020, (2020). Abstract
X

In this work the warpage of a power modules ceramic substrates due to temperature variation has been numerically calculated. It has been used a not linear finite element model, which account the experimentally characterized material properties, including the copper elastoplasticity. Model results have been compared with dedicated interferometric measurements to validate the model. As application example, it has been calculated the benefit in terms of temperature warpage realized with a different substrate design option.

[36] Sitta A., Landi A., Cafra B., Renna M., Calabretta M., Analysis of warpage behavior of electrochemical deposited thick copper on silicon, PCIM Europe Conference Proceedings, 1, 943-946, (2020). Abstract
X

The presented work shows the results of investigations with respect to the microstructural evolution and the thermo-mechanical characterization of thick Cu films (20 mm), electrochemical deposited on silicon substrates. Warpage measurements based on Phase-Shift Moir´e principle have been performed considering different temperature profiles. A theoretical explanation has been proposed to justify experimental warpage behavior, also proofed by Differential Scanning Calorimetry (DSC).

[37] Mancaleoni A., Sitta A., Colombo A., Villa R., Mirone G., Renna M., Calabretta M., Copper wire bonding process characterization and simulation, CIPS 2020 - 11th International Conference on Integrated Power Electronics Systems, 351-354, (2020). Abstract
X

Copper wires are nowadays replacing the traditional Au material in wire bonding interconnections, due to lower cost, better thermal/electrical properties and reliability performances. The increased hardness of Cu imposes higher bonding force and ultrasonic power during the wire-bonding process, increasing the risk of stress-induced bondpad damage. The aim of the presented work has been the modeling and characterization of stress and deformations resulting from the ball-bonding phase. A Finite Element Model has been developed and benchmarked with experimental samples obtained by freezing the ball bonding process at different steps, on which the deformations occurred in the bonded copper ball and in the bondpad layers have been measured through Plasma-FIB cross sections.

[38] Selgi L.M., Sciuto A., Calabretta M., Sitta A., D’Arrigo G., Experimental evaluation of oxide current on a low voltage trench gate power mos under mechanical bending conditions, PCIM Europe Conference Proceedings, 1, 1019-1025, (2020). Abstract
X

In the context of reliability of silicon Power Devices, we conducted a study on Power MOS Gate Trench devices. The stress tensor inside the device is characterized through experimental analysis and Finite Element calculation. A test bench is developed to investigate the impact of mechanical stress on the gate leakage current of examined devices.

[39] Calabretta M., Sitta A., Oliveri S.M., Sequenzia G., Design and Process Optimization of a Sintered Joint for Power Electronics Automotive Applications, Lecture Notes in Mechanical Engineering, 470-480, (2020). Abstract
X

The massive development of Hybrid and Electrical Vehicles (HEV) is strongly impacting the semiconductor industry demanding for highly reliable Power Electronic components. Within the engine compartment installation space is of major concern, therefore small size and high integration level of the modules are needed. Conventionally devices are typically soldered to ceramics substrates that are vacuum soldered to water-cooled base plates. The known reliability limitations of traditional solder joints are significantly limiting the power density increase, limiting the maximum operative temperature and representing a strong constrain for using high performances devices such as wide bandgap compound like Silicon Carbide (SiC). Silver sintering today has started to replace the solder joint from chips to carrier substrates, leaving one major reliability bottleneck. Combining properly temperature, time and pressure, a strong, highly electrically and thermally conductive bond is formed. The aim of this work is to develop an integrated methodology, numerical and experimental, to assess the Ag sintering die attach process for a SiC power MOSFET. Different process parameters have been benchmarked by means of physical analyses, performed at time zero and also after liquid-to-liquid thermal shock aging test. The sintering flakes densification process has been reproduced by Finite Element Analysis and the obtained morphological texture has been used for extracting the mechanical properties of the layer as a function of the thermo-compression process itself. A simulation method, based on the evaluation of the inelastic strain accounted per cycle has been used for matching the experimental results according to an aging model.

Keywords: Die attach | Finite Element Model | Reliability | Silicon Carbide | Thermal shocks

[40] Calabretta M., Sitta A., Oliveri S.M., Sequenzia G., An Integrated Approach to Optimize Power Device Performances by Means of Stress Engineering, Lecture Notes in Mechanical Engineering, 481-491, (2020). Abstract
X

In the present work it is shown how stress engineering can be used in semiconductor industry to improve Power MOSFET transistor’s performance beyond simple geometrical downscaling. The aim of this paper is to present an integrated methodology, coupling modelling and experimental results, focused on the structural optimization of a power device by means of final passivation mechanical stress tuning. The proposed approach is based on a Finite Element Model that describes and predicts the mechanical strain of a singulated power device (MOSFET) validated by the correlation with interferometric experimental warpage measurements (Topography and Deformation Measurements). Scope of the activity is to engineer Power Devices with reduced intrinsic stresses in order to optimize the reliability performances. Controlled stress into a single semiconductor crystal oriented substrate can be managed at manufacturing level by several methods, including the introduction of a layer on the top of the substrate or around the gate region. From the knowledge of the mechanical boundaries, as a function of temperature, it is possible to predict the stress conditions impacting on device fabrication and on reliability performances. Moreover, according to the piezoresistive model, it has been evaluated the electrical characteristics (on-resistance) in the operative working condition range. According to the proposed approach an optimized passivation layer solution has been proposed, simulated by Finite Element model and validated by experiments.

Keywords: FEM | Moiré topography | Reliability | Silicon MOSFET | Warpage

[41] Mirone G., Sitta A., D'arrigo G., Calabretta M., Corrections to: Material characterization and warpage modeling for power devices active metal brazed substrates (IEEE Transactions on Device and Materials Reliability (2019) 19:3 (537-542) DOI: 10.1109/TDMR.2019.2932971), IEEE Transactions on Device and Materials Reliability, 19(4), 796, (2019). Abstract
X

In [1] below, reference 1 is incorrect. It should read: A. Sitta, M. Calabretta, M. Renna, and D. Cavallaro, 'Solder joint reliability: Thermo-mechanical analysis on power flat packages' in Advances on Mechanics, Design Engineering and Manufacturing. Cham, Switzerland: Springer, 2017, pp. 709-716.Instead of Advances on mechanics, design engineering and manufacturing,in Proc. Int. Joint Conf. Mech. Design Eng. Adv. Manuf., Catania, Italy, Sep. 2016. [Online]. Available: https://rd.springer.com/book/10.1007/978-3-319-45781-9.

[42] Calabretta M., Renna M., Vinciguerra V., Messina A.A., Power Packages Interconnections for High Reliability Automotive Applications, European Solid-State Device Research Conference, 2019-September, 35-39, (2019). Abstract
X

In this paper an overview on Power Packages challenges and technology approaches is given. These challenges mainly originate from Silicon Carbide MOSFETs superior properties allowing high power, high temperature capability, fast switching transients and high electric field operations. All these features can be obtained in a significant reduced chip area. In order to benefit from the disrupting advantages of these wide band gap semiconductor based power devices, a strong focus on packaging and interconnection technologies is needed to withstand these challenging requirements. In addition one of the major and strong boost for Silicon Carbide technology development is given by the car electrification trend: The strict requirements for the Automotive Market are leading to a design and engineering oriented, since the preliminary stage of development, to optimize the reliability of Power Packages.

Keywords: Die Attach | Finite Element Model | Reliability | Silicon Carbide | Thermal Shocks

[43] Mirone G., Sitta A., D'Arrigo G., Calabretta M., Material Characterization and Warpage Modeling for Power Devices Active Metal Brazed Substrates, IEEE Transactions on Device and Materials Reliability, 19(3), 537-542, (2019). Abstract
X

The metallized insulating substrates work as mechanical supports for the circuitry of Power Module Packages. Due to their specific functions, substrates for power electronics are made by different materials. The conductive metal layers can assume different functions: the top metal serves as power circuitry routing while the bottom metal improves the mechanical robustness and thermal efficiency. Ceramic layer provides excellent electrical insulation. These features play an essential role in the operation of power modules, which are often operated at high voltage and high current density. The substrates, composed by materials with different thermal expansion coefficients, are subjected to cyclic stresses due to temperature variations induced by operative working conditions. The substrate layouts typically include differences in shape and/or thickness between the top and the bottom side; this generates asymmetrical distributions of stress/strain resulting in overall warpage. The variations of this warpage induce mechanical fatigue during lifetime and represent a limiting factor for reliability. The scope of the presented work is the characterization of the out of plane warpage of Active Metal Brazed substrates (AMB) by means of numerical approach. The elastoplastic properties of metal and ceramic have been measured, evaluating the thermal softening of the copper as well. These characteristics are needed to calculate AMB warpage through Finite Element Models (FEM), simulating the warpage induced by a passive temperature cycling. Warpage computed from numerical model have been benchmarked and validated with optical warpage measurements. The validated numerical model has been developed to optimize the substrate warpage variation during cycling improving the whole package reliability.

Keywords: Ceramic substrate | FEM | material characterization | warpage

[44] Santopa M., Russo S., Torrisi M., Renna M., Sitta A., Calabretta M., Fast transient thermomechanical stress to set a pressure-assisted sintering process, PRIME 2019 - 15th Conference on Ph.D. Research in Microelectronics and Electronics, Proceedings, 61-64, (2019). Abstract
X

High temperature application and long term reliability are the future trends for power electronics. A key factor to enable future applications is the interconnection durability improvement under high temperature and thermo-mechanical cycling loads. Nowadays, the standard solders cannot fulfill the reliability requirements of future power electronic devices, therefore interconnection technologies have to be developed. One of the most promising joining technique is Ag sintering. Combining properly temperature, time and pressure, a strong, highly electrically and thermally conductive bond is formed. The aim of this work is to develop a methodology to assess the Ag sintering die attach process for a SiC power MOSFET. Different process parameters have been benchmarked by means of physical analyses, performed not only on just assembled devices but also considering the aging effect induced by a liquid-to-liquid thermal shock test.

Keywords: Ag sintering | Power devices | Reliability | Thermal shock

[45] Russo S., Bazzano G., Cavallaro D., Sitta A., Calabretta M., Thermal analysis approach for predicting power device lifetime, IEEE Transactions on Device and Materials Reliability, 19(1), 159-163, (2019). Abstract
X

The purpose of this paper is to predict the device lifetime under a power cycling test by a simulation method that is based on a distributed self-heating SPICE model. Correlation between numeric extrapolation and experimental data is done by considering the Repetitive Avalanche test, a particular active temperature cycling that evaluates the ruggedness of a power device. The considered failure mechanism is due to front metal aging that produces contact resistance degradation. This phenomenon is strongly dependent on temperature, and by the proposed simulation tool, validated with experimental data, it is possible to evaluate the accurate temperature map and, consequently, predict the lifetime.

Keywords: active cycle | Metal aging | predictive simulation | reliability

[46] D'Arrigo G., Mio A., Favaro G., Calabretta M., Sitta A., Sciuto A., Russo M., Calì M., Oliveri M., Rimini E., Mechanical properties of amorphous Ge<inf>2</inf>Sb<inf>2</inf>Te<inf>5</inf> thin layers, Surface and Coatings Technology, 355, 227-233, (2018). Abstract
X

The working principle of a Phase Change Memory (PCM) cell exploits the repeated reversible transition between a crystalline and an amorphous phase of chalcogenide alloys typically Ge2Sb2Te5, that are characterized respectively by a high (SET) and a low (RESET) conductive state. The change in density between the two phases (6%) induces a very high compressive stress to the active amorphous region by the surrounding crystalline materials. Moreover, the physical iterative transformation between crystalline and amorphous phase transformation introduces a swelling and deswelling effect. This is one of the key failure mechanisms that are limiting the reliability of the final integration of the PCM system. Knowledge of the mechanical properties of the amorphous phase is then an important factor. Amorphous structure, i. e. its short-range order, depends on the adopted formation procedure. In this paper we analyze the mechanical characteristics of sputtered amorphous Ge2Sb2Te5 thin layers and the modification introduced by ion irradiation, a procedure adopted to simulate the amorphous state produced by melt quenching. Measurements of Young's Modulus and Hardness were performed using Ultra High-Nano Indentation on plane samples. The values of both quantities increase of about 10–20% in the 30 keV Ge+ irradiated samples. This trend is due to the reduction of homopolar wrong bonds (Ge–Ge and Te–Te) present in the as deposited film. Thermal spikes associated to the impinging ion cause a local atomic rearrangement that results in a structure similar to that of the crystalline phase. The investigation was extended to cantilevers of length in the range 10–200 μm, with a layer of 100 nm Ge2Sb2Te5 deposited on 280 nm thick SiN. The cantilever modal analysis and the out of plane deflection measurements were correlated using a Finite Element modeling, that makes use of the mechanical values measured by Ultra high Nano Indentation. After deposition the amorphous Ge2Sb2Te5 layer is subject to a compressive mechanical shrinkage, this internal stress is released after ion implantation.

Keywords: Cantilever | Nano-indentation | Phase change material

[47] D'Arrigo G., Christian M., Morandi V., Favaro G., Bongiorno C., Mio A.M., Russo M., Sitta A., Calabretta M., Sciuto A., Piccinini E., D'Urso L., Rizzoli R., Mechanical and electrical characterization of CVD-grown graphene transferred on chalcogenide Ge<inf>2</inf>Sb<inf>2</inf>Te<inf>5</inf> layers, Carbon, 132, 141-151, (2018). Abstract
X

In the present work, we investigate some mechanical and electrical properties of a Chemical Vapor Deposition (CVD)-grown graphene layer transferred on 50 nm-thick Ge2Sb2Te5 chalcogenide and discuss the possible application of graphene as a scaled contact replacing metal lines in phase-change memory devices. At first, the graphene-chalcogenide interface was extensively investigated. The expected chemical composition was confirmed by means of Electron Energy Loss Spectrometry (EELS), and the absence of bond distortions in the chalcogenide layer after graphene transfer was proved through Raman spectroscopy. The latter evidenced also the presence of defects, further confirmed by Transmission Electron Microscopy (TEM) investigations. The quality of the contact stack was evaluated by means of the adhesion among layers and by the sheet resistance of the layers themselves. Scratch tests and numerical simulations revealed a stress distribution compatible with a failure of the interface where the graphene layer features a 2 %-void. Next, the contact resistance proved an effective good Ohmic quality with a high graphene sheet resistance, about 1400 Ohm/square, which suggests that a better optimized transfer process should be applied to reduce it more than one order of magnitude.

Keywords: CTLM | CVD-Grown graphene | Nano-scratch | PCM

[48] Sitta A., Russo S., Bazzano G., Cavallaro D., Greco G., Calabretta M., Numerical approach to predict power device reliability, Proceedings - 2018 13th IEEE International Conference on Design and Technology of Integrated Systems In Nanoscale Era, DTIS 2018, 1-5, (2018). Abstract
X

The scope of this paper is to work out a predictive method to estimate the power device reliability under active cycle tests. The proposed method is able to predict, through a numerical model, the local maximum temperature during test. The results validation has been pursued correlating the numerical thermal maps results with the experimental temperature distribution obtained from an infra-red camera. Front metal ratcheting has been recognized as the main root cause of contact resistance degradation during the considered reliability test (Repetitive Avalanche). This failure mode is dependent on the temperature variation for cycle, by which it is possible to predict the device lifetime according to the Coffin-Manson fatigue model.

Keywords: Avalanche Test | Numerical Model | Power Device | Reliability

[49] Sitta A., Calabretta M., Renna M., Cavallaro D., Solder joint reliability: Thermo-mechanical analysis on power flat packages, Lecture Notes in Mechanical Engineering, 0, 709-716, (2017). Abstract
X

In the last decades the main focus for improvements in Power Electronics was mainly addressed on chip technology. Therefore, Power Electronic performance depends by a high ratio on package technologies and on their interconnections. In particular, the automotive industry has high requirements regarding cost efficiency, reliability and compactness. Increasing power densities, cost pressure and more stringent reliability target for modern power semiconductors are making thermal system optimization more and more important in relation to electrical optimization. This article will give an overview of the new methodological approach leaded by Finite Element (FE) simulation for new packages and interconnection solution ideas. A viscoplastic creep modelling is adopted for the solder taking into account time, temperature and stress dependences in Thermal Cycle. A parametric study is performed by changing geometrical solutions. The results obtained from the modelling has been used to form design guidelines that were also matched with experimental data.

Keywords: Finite Element Modeling | Power Electronics | Solder Joint | Viscoplasticity

[50] Sequenzia G., Oliveri S., Calabretta M., Fatuzzo G., Cali M., A new methodology for calculating and modelling non-linear springs in the valve train of internal combustion engines, SAE 2011 World Congress and Exhibition, (2011). Abstract
X

The valve return springs in the distribution chain of internal combustion engines constitute a fundamental component for the duration, efficiency and performance of the engine itself [1,2,3,4]. This is even more true for high-performance engines whose mechanical and thermal power leads to the premature deterioration of poorly designed components. The elevated forces in such engines necessitate, where the valve springs have not been substituted by alternative kinematic systems, progressive springs, i.e. springs with variable stiffness. Despite this fact, the literature does not contain any univocal methods for defining the geometry of this type of spring. In the present study, the question is approached on the basis of a numerical-iterative calculation, providing a general methodology which, starting from data regarding the functioning of the engine and the geometric volumes to be respected, leads to the definition of the optimal geometry of the helix, taking account of the trend of the stiffness, of the natural frequencies and of the loads over the entire operating range of the spring. Tests on springs calculated in this way were performed using multi-body software, in order to verify the correspondence between the initial design data and the real behaviour of the geometry generated. © 2011 SAE International.

[51] Calabretta M., Cacciatore D., Carden P., Plail J., Development of a Timing Chain Drive Model for a High Speed Gasoline Engine, SAE International Journal of Engines, 4(1), 432-440, (2011). Abstract
X

Engine efficiency and optimization are key aspects for automotive manufacturers. Lamborghini has particularly focus attention for reduction of time to market building up a synergic approach for new component's development using simulation, Know-how experiences, engine engineering expertise and experimental validation. In particular to reach the best results in the shortest time it is used, in the preliminary stage of development, a massive support of simulation analysis. In the Lamborghini approach analysis and simulation has become key aspects during concept and development of timing drives. This type of activity is used to support the development of better chain timing drives focusing on improving durability, lower friction, less noise and reduced cost in less time than conventional trial and processes. Even during the concept design phase it is useful to use a mathematical model to calculate dynamic forces and motions of a chain drive. These models are used to assess in detail the choice of drive layout, to check that component load limits are not exceeded and to make a first choice of the tensioner settings. Later, when the first prototype engines are available, measurements of timing drive sprocket motion, tensioner motion and tensioner force are typically taken. At this stage the model can be correlated to the measured data and then the correlated model can then be used to explore the potential for improvements to the timing drive during later phases of engine development. This paper describes the correlation of a mathematical model of a complex timing chain drive for a Lamborghini V12 gasoline engine to measurements made on a prototype engine. The depth of model required and the choice of stiffness and damping values needed to give excellent agreement between calculations and measured data are discussed. © 2011 SAE International.

[52] Sequenzia G., Oliveri S., Calabretta M., Fatuzzo G., Cali M., A new methodology for calculating and modelling non-linear springs in the valve train of internal combustion engines, SAE Technical Papers, (2011). Abstract
X

The valve return springs in the distribution chain of internal combustion engines constitute a fundamental component for the duration, efficiency and performance of the engine itself [1,2,3,4]. This is even more true for high-performance engines whose mechanical and thermal power leads to the premature deterioration of poorly designed components. The elevated forces in such engines necessitate, where the valve springs have not been substituted by alternative kinematic systems, progressive springs, i.e. springs with variable stiffness. Despite this fact, the literature does not contain any univocal methods for defining the geometry of this type of spring. In the present study, the question is approached on the basis of a numerical-iterative calculation, providing a general methodology which, starting from data regarding the functioning of the engine and the geometric volumes to be respected, leads to the definition of the optimal geometry of the helix, taking account of the trend of the stiffness, of the natural frequencies and of the loads over the entire operating range of the spring. Tests on springs calculated in this way were performed using multi-body software, in order to verify the correspondence between the initial design data and the real behaviour of the geometry generated. Copyright © 2011 SAE International.

[53] Corvino C., Calabretta M., Zanasi R., Modeling of variable valve timing on high performance engine using power-oriented graphs method, SAE Technical Papers, (2011). Abstract
X

Engine efficiency is one of the key aspects to reduce CO2 emissions. In order to improve the emission maintaining high performance capabilities several devices are introduced in the system; variable valve timing technology allows more flexibility for modern engines to meet peak performance, fuel economy and low emissions targets [7] while providing good driveability. This paper describes the Lamborghini continuously-variable cam phaser model using a graphical technique, called Power Oriented Graphs (POG), this uses an energetic approach for representing the physical systems. The generally accepted approach is to calibrate an engine on a dynamometer and to adjust the operation of the engine to meet performance targets. With the current build and test approach, these adjustments may not be made until well into the development program, and this calibration is a costly and time consuming step in the engine development process: the main purpose of this works is showing how was described the model in order to get more easy and fast the calibrating operations. Furthermore the usefulness to model the system consists of analyzing in simulation many more system configurations than those available for real experiments so it's important using a simple methodology that is able to analyze the whole system's dynamic in order to reach the performance expectations. The results obtained were validated demonstrating the effectiveness of the POG technique. Copyright © 2011 SAE International.

[54] Calabretta M., Cacciatore D., Carden P., Plail J., Development of a timing chain drive model for a high speed gasoline engine, SAE Technical Papers, (2011). Abstract
X

Engine efficiency and optimization are key aspects for automotive manufacturers. Lamborghini has particularly focus attention for reduction of time to market building up a synergic approach for new component's development using simulation, Know-how experiences, engine engineering expertise and experimental validation. In particular to reach the best results in the shortest time it is used, in the preliminary stage of development, a massive support of simulation analysis. In the Lamborghini approach analysis and simulation has become key aspects during concept and development of timing drives. This type of activity is used to support the development of better chain timing drives focusing on improving durability, lower friction, less noise and reduced cost in less time than conventional trial and processes. Even during the concept design phase it is useful to use a mathematical model to calculate dynamic forces and motions of a chain drive. These models are used to assess in detail the choice of drive layout, to check that component load limits are not exceeded and to make a first choice of the tensioner settings. Later, when the first prototype engines are available, measurements of timing drive sprocket motion, tensioner motion and tensioner force are typically taken. At this stage the model can be correlated to the measured data and then the correlated model can then be used to explore the potential for improvements to the timing drive during later phases of engine development. This paper describes the correlation of a mathematical model of a complex timing chain drive for a Lamborghini V12 gasoline engine to measurements made on a prototype engine. The depth of model required and the choice of stiffness and damping values needed to give excellent agreement between calculations and measured data are discussed. © 2011 SAE International.

[55] Calabretta M., Cacciatore D., Carden P., Valvetrain friction - modeling, analysis and easurement of a high performance engine alvetrain system, SAE International Journal of Engines, 3(2), 72-84, (2010). Abstract
X

Engine efficiency is one of the key aspects to reduce CO 2 emissions. Lamborghini S.p.A. has focused attention on the engine friction modeling, analysis and measurement to understand and control the phenomena. To reduce friction it is necessary to improve understanding of the behavior of the engine components and to pay attention to detail at every tribological contact. The valvetrain can make a significant contribution to whole engine friction especially at low engine speed and this is particularly true for a high speed sports car engine. Direct acting valvetrains are often used for this type of engine to minimise the moved mass and so enable high speed operation. However the sliding contact between the cam and tappet results in higher friction loss than the roller finger follower valvetrain used on many modern passenger car engines. In addition, the high maximum engine speed demands a large valve spring force to maintain contact between cam and tappet. The large spring force can lead to increased valvetrain friction at low engine speed when the inertia force is low. Thus the development of calculation methods to quantify friction of direct acting valvetrains and support the design of components is important. This paper describes the use of advanced mathematical models to quantify power loss at cam/tappet contact, tappet/ bore contact and camshaft bearings. The mathematical models are sufficiently detailed to capture the major influencing factors while being quick enough to use to enable engine designers to make decisions in the required time frame. This paper compares calculated and measured friction data for the valvetrain of a high speed passenger car engine as tested on a motored cylinder head test rig. The system friction was measured and calculated across the operating speed range with different oil supply temperature. The effect of diamond like carbon (DLC) coatings on the tappets was quantified by measurement and analysis. © 2010 SAE International.

[56] Calabretta M., Cacciatore D., Carden P., Valvetrain friction - Modeling, analysis and measurement of a high performance engine valvetrain system, SAE Technical Papers, 72-84, (2010). Abstract
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Engine efficiency is one of the key aspects to reduce CO 2 emissions. Lamborghini S.p.A. has focused attention on the engine friction modeling, analysis and measurement to understand and control the phenomena. To reduce friction it is necessary to improve understanding of the behavior of the engine components and to pay attention to detail at every tribological contact. The valvetrain can make a significant contribution to whole engine friction especially at low engine speed and this is particularly true for a high speed sports car engine. Direct acting valvetrains are often used for this type of engine to minimise the moved mass and so enable high speed operation. However the sliding contact between the cam and tappet results in higher friction loss than the roller finger follower valvetrain used on many modern passenger car engines. In addition, the high maximum engine speed demands a large valve spring force to maintain contact between cam and tappet. The large spring force can lead to increased valvetrain friction at low engine speed when the inertia force is low. Thus the development of calculation methods to quantify friction of direct acting valvetrains and support the design of components is important. This paper describes the use of advanced mathematical models to quantify power loss at cam/tappet contact, tappet/bore contact and camshaft bearings. The mathematical models are sufficiently detailed to capture the major influencing factors while being quick enough to use to enable engine designers to make decisions in the required time frame. This paper compares calculated and measured friction data for the valvetrain of a high speed passenger car engine as tested on a motored cylinder head test rig. The system friction was measured and calculated across the operating speed range with different oil supply temperature. The effect of diamond like carbon (DLC) coatings on the tappets was quantified by measurement and analysis. Copyright © 2010 SAE International.