Power Electronic Substrates Research Articles

Cold Gas Spraying Copper Metal On AlN Ceramics as an Alternative to Thick DBC Substrates for Power Electronics

Abstract This study investigates the use of cold gas spraying (CGS) as a low-temperature additive manufacturing method to bond copper onto aluminum nitride (AlN) substrates for electronic packaging of high-power applications. While direct bond copper (DBC) technique is commonly used, it has limitations due to the large mismatch in coefficient of thermal expansion, which affects substrate reliability. This work employed cold gas spraying (CGS) to mechanically bond Cu on AlN. The study explores the effect of multiple sprays, spray angle and spraying direction on deposition thickness, coating surface roughness, and deposited volume through a factorial design of experiments (DOE). The results, based on optical and scanning electron microscopy combined with profilometry data, showed that coatings sprayed at a 60° angle had a smoother profile topography and less surface roughness than those sprayed at a 90° angle. After depositing 10 layers, a surface roughness (Sa) of around 30 µm and a coating thickness of over 300 µm were successfully attained. These findings provide valuable insights into the processing factors affecting the growth and quality of copper coatings on AlN substrates via multiple sprays, thus enabling the realization of CGS technology as a potential solution to DBC substrates for electronic packaging of wide-bandgap semiconductors.

Quasi-direct Cu–Si3N4 bonding using multi-layered active metal deposition for power-module substrate

The advancement of power modules demands more reliable insulating circuit substrates. Traditional substrates, comprising Cu and Si3N4, are produced using active metal brazing (AMB). However, AMB substrates have reliability concerns owing to electrochemical migration and void formation from brazing filler metals. This study introduces a quasi-direct Cu–Si3N4 bonding technique using a Ti/Al bilayer active metal deposition at the bonding interface. A sputtered Ti/Al bilayer was formed on the Si3N4 surface, then heated and pressurized the sputtered Si3N4 substrate with Cu sheets in vacuum to bond each other without voids or delamination. The Ti/Al layers reacted with Si3N4 and Cu, forming a 300 nm intermediate layer. TEM observations show this layer contains segregated Ti–N and Cu–Al phases, with a good lattice match to Si3N4 and Cu–Al. Temperature-cycling tests on the Cu/Si3N4/Cu substrate revealed delamination caused by increased tensile stress at the periphery of the bonding area due to asymmetrical Cu patterns. This novel quasi-direct Cu–Si3N4 bonding technique addresses issues of electrochemical migration and void formation seen in AMB substrates, offering a reliable bonding interface for power electronic substrates.

Synergistically enhanced Si3N4/Cu heterostructure bonding by laser surface modification

A bonding approach based on laser surface modification was developed to address the poor bonding between Si3N4 ceramic and Cu. The bonding mechanism in Si3N4/Cu heterogeneous composite structure fabricated by laser modification-assisted bonding is examined by means of scanning/transmission electron microscopy and thermodynamic analysis. In the bonding process under laser modification, atomic intermixing at the interface is confirmed, as a result of the enhanced diffusion assisted by the dissociation of Si3N4 ceramic by laser. The dissociating Si precipitations on the surface, as well as the formation of micro-pores interfacial structure, would be the key concept of the bonding, by which the seamless and robust heterointerfaces were created. By controlling the laser-modifying conditions, we can obtain a reliable heterostructure via the optimization of the trade-off of the surface structure and bonding strength, as determined by the laser-modified surface prior to bonding. The maximum structure depth and S ratio at the Si3N4 surface were produced at a laser power of 56 W, corresponding to the maximal shear strength of 15.26 MPa. It is believed that the further development of this bonding technology will advance power electronic substrate fabrication applied in high-power devices.

Pyroelectric energy harvesting from power electronic substrates

Energy harvesting from available resources can lead to implementation of more controlling and actuating devices for industrial internet of things (IIoT). Energy harvesting from temperature fluctuations in power electronic devices by pyroelectric materials is a key solution to achieve self-powered sensor systems. In this study, performance of a pyroelectric energy harvester (PEH), utilized as a dielectric substrate in power electronic modules, is investigated. Accordingly, a comprehensive analytical model is developed in MATLAB software, and the theoretical results are validated with experimental data. The results of this study show that, the pyroelectric substrate can harvest an average power of 50 µW/cm2. Effect of amplitude and frequency of input heat, cooling density, and load resistance over PEH are studied. The results reveal that, frequency of the input heat rate higher than 2 Hz does not have a sensible impact on the power generation by the pyroelectric module. Convective cooling density with low heat transfer coefficients led to higher power generation. On the contrary, it can cause operating at higher temperature than the Curie temperature. The energy harvesting concept developed, for the first time, in this study shows significant potential of pyroelectric direct bonded copper substrate to provide self-powered sensors for IIoT in a vast range of power electronic applications.

Evaluation of Crystal Quality and Dopant Activation of Smart Cut<sup>TM</sup> - Transferred 4H-SiC Thin Film

The Smart CutTM process offers an advantageous opportunity to provide a large number of performance-improved SiC substrates for power electronics. The crystalline quality and the electrical activation of the 4H-SiC transferred layer are then at stake when it comes to the power device reliability. In this study, we find that the H+ ion implantation used for the Smart CutTM process leads to electrical deactivation of dopants and partially disorders the material. The transferred layer fully recovers its initial crystalline quality after a 1300°C anneal, with no further evolution beyond this temperature. At this point however, the n-type dopants are still inactive. The dopant reactivation occurs in the same temperature range than that of implanted nitrogen: between 1400°C and 1700°C. After 1700°C, the initial doping level of bulk SiC is recovered.

Interdiffusion and formation of intermetallic compounds in high-temperature power electronics substrate joints fabricated by transient liquid phase bonding

Power electronics packages typically comprise a dielectric substrate bonded to a metal layer and attached to heat sinks using a low-thermal-conductivity solder. These multiple layers increase the effective thermal resistance of the package and are responsible for package failures under cyclic thermal loads. Bonding the aluminum nitride dielectric layer (AlN) directly to a low coefficient of thermal expansion (CTE) aluminum silicon carbide (AlSiC) cold plate using copper‑aluminum (Cu-Al) transient liquid phase (TLP) bonding has been shown to improve the mechanical reliability of the power electronic packages while making the package more compact by bringing the cooling solutions closer to the devices. This study aims to characterize the Cu-Al bonds formed during the TLP bonding of AlSiC with three different power electronics substrates: pure aluminum nitride (AlN), aluminum (DBA), and copper (DBC). The material compositions and microstructures of the bonds were analyzed using scanning electron microscopy (SEM), x-ray spectroscopy (EDS), confocal scanning acoustic microscopy (C-SAM), and x-ray diffraction (XRD). α-Al solid compound was identified as the dominant phase in AlN-AlSiC and Al-AlSiC, with a notable presence of SiC particles. In contrast, three intermetallic phases – θ-CuAl2, η-CuAl, and γ′- Cu9Al4 – were observed in the Cu-AlSiC bond. A computational solid-state diffusion model was developed to predict the intermetallic compounds (IMCs) formed during Cu-Al TLP bonding in each system, which supported the observation that an initial Cu volume fraction of 20 % in the material layers produced a final bond composition of >95 % Al with minimal IMC growth. However, increased Cu concentrations produced higher concentration gradients, leading to increased growth of IMCs, Kirkendall voids, and interstitial cracks.

Aging effect on high heat dissipation DBA and DBAC substrates for high power electronics

Direct bonded aluminum (DBA) and direct bonded aluminum with copper (DBAC) substrates were fabricated successfully by transient liquid phase bonding. The 300 nm Ni metallization was deposited on the surface of AlN and joined with Al and Cu to form the DBA and DBAC substrates. After bonding, the discontinuous Al9(Fe,Ni)2 and Al3Ni intermetallic composites (IMCs) were formed at the interface of DBA substrate due to the diffusion of impurities (Fe) in Al and Ni metallization of AlN. As for DBAC substrate, three continuous Al4Cu9, AlCu, and Al2Cu IMCs were formed at the Cu/Al interface. After isothermal aging to 250 °C/2000 h, the bonding area of DBA and DBAC was greater than 98% and the interfacial shear strength of DBA was measured as 40 MPa and the DBAC decreased to 12.5 MPa due to the brittle Al–Cu IMCs at the Al/Cu interface. The unprotected Cu oxide and micrometers size cracks were formed at the surface of Cu in DBAC but no obvious Al oxide and crack in DBA. Even though the thermal conductivity of DBAC is 199 W/mK higher than DBAC substrates (184 W/mK) at room temperature, the thermal stability of DBA is better than DBAC for the applications in high-power electronics.

T‐ZrO2 toughened Al2O3 free‐standing films and as oxidation mitigating thin films on silicon nitride via colloidal processing of flame made nanopowders (NPs)

AbstractZirconia toughened aluminas (ZTAs) are one of the most important engineering ceramics with high melting points, excellent mechanical strength, and chemical stability, and are commonly used as wear resistant and high‐temperature components, as prosthetic implants, and electric circuit substrates. In this work, we explore methods of processing fine‐grained, dense, thin, free‐standing (ZrO2)x(Al2O3)1−x films (x = 0‐50 mol%, ~40 μm thick) by sintering flame made nanopowders (NPs) to optimize the t‐ZrO2 content, sinterability, and microstructures under select conditions (1120°C‐1500°C/5 h in O2 or 95%N2/5%H2). In all cases, the final sintered products retain t‐ZrO2 with average grain sizes (AGSs) of 0.1‐1 μm. ZTA film thicknesses were increased to ~200 μm to assess potential as electronic substrates. Excellent fracture toughness (24 MPa m1/2) and small AGSs of 0.7 μm were found for ~200 μm thick ZTA films sintered at 1500°C/5 h/N2/H2 using a three‐step binder burnout process. Furthermore, we show that homogeneous ZTA thin films (<5 μm thick) can be sintered on Si3N4 substrates (thickness ≈ 300 μm) to provide physical protection against oxidation under extreme conditions (1500°C/1 h/O2), offering additional practical utility for high‐temperature ceramics and power electronic substrates.

Synchrotron X-ray topography characterization of high quality ammonothermal-grown gallium nitride substrates

Ammonothermal growth of bulk gallium nitride (GaN) crystals is considered the most suitable method to meet the demand for high quality bulk substrates for power electronics. A non-destructive evaluation of defect content in state-of-the-art ammonothermal substrates has been carried out by synchrotron X-ray topography. Using a monochromatic beam in grazing incidence geometry, high resolution X-ray topographs reveal the various dislocation types present. Ray-tracing simulations that were modified to take both surface relaxation and absorption effects into account allowed improved correlation with observed dislocation contrast so that the Burgers vectors of the dislocations could be determined. The images show the very high quality of the ammonothermal GaN substrate wafers which contain low densities of threading dislocations (TDs) but are free of basal plane dislocations (BPDs). Threading mixed dislocations (TMDs) were found to be dominant among the TDs, and the overall TD density (TDD) of a 1-inch wafer was found to be as low as 5.16 × 103 cm−2.

Material Characterization and Warpage Modeling for Power Devices Active Metal Brazed Substrates

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.

Integration of Jet Impingement Cooling With Direct Bonded Copper Substrates for Power Electronics Thermal Management

The ability of packaging solutions to meet the high heat flux dissipation of power electronics relies heavily on effective thermal management strategies. During operation, power electronic devices generate high temperatures that can cause device degradation. To remove this heat from the devices, heat spreaders and cold plates are often used. However, the multiple layers that compose the module often lead to high thermal resistances that hinder heat dissipation and can give rise to interfacial failures. To address these challenges, this paper has investigated the direct integration of single-phase jet impingement cooling at the power electronic substrate level for improved thermal management. This method was evaluated using computational models, experiments, and analytical models. An optimization analysis was performed to determine the influence of various design parameters on the cooling system. Through the directly integrated cooling concept, a reduction of up to 25% in the total package thermal resistance compared to a conventional device package was achieved. In addition, the overall device temperature was reduced, and the thermal resistance was found to be between 0.21 °C/W and 0.47 °C/W for a heat dissipation of 100 W/cm2.

Prototyping and Production of High-temperature Power Electronic Substrates through Additive Manufacturing Processes

Abstract This paper reveals a study on Selective Laser Melting (SLM) as an alternative technology for producing power electronic substrates, and shows the possibility of producing a stable interface between alumina and copper through SLM technique. Additive Manufacturing (AM) has not yet been established in the manufacturing of electronic devices. The prevalent benefits of the generative manufacturing sector such as material efficiency, product customization/–flexibility, elimination of the usage of tools, constructional freedom and less process steps in contrast to the conventional fabrication methods of ceramic substrates for power electronic applications like DBC or AMB, are pointed out. Moreover, AM reduces energy costs due to the elimination of the necessary firing, etching and washing processes. The realized study focuses on the examination of adhesion strengths of copper structures, melted on different Al2O3 ceramics with and without pre-copper and -glass paste coating. The melting process was categorized for different laser parameters (1–3) based on the same energy input. Maximum shear values of the substrate probes reached were at about 30 N/mm2 for copper coated ceramic, and at 20 N/mm2 for conventional and glass paste coated substrates. All results were determined in a full factorial design of experiment (DoE) with 54 combinations and a sample size of six samples per parameter combination. Furthermore, several cross sections of the probes produced were illustrated to better understand the melting and joining behavior of the copper powder applied on the ceramic substrates. For improved mechanical adhesion, the ceramic substrates were roughened by laser radiation, with roughness values measured, and the cracking behavior of the exposed ceramics explained.

Properties of power electronic substrates based on thick printed copper technology

The aim of this work is to develop new metallisation procedures of 96% alumina substrates with copper using thick film technology and firing in a special batch furnace, designed for the process. Standard firing process of copper pastes is done in conveyor furnaces and it has been described in literature. The firing process in batch furnaces is characterised by different problems and it has not yet been published. The developed batch furnace can combine firing in a protective atmosphere e.g. that of high nitrogen purity gas, automated mixing of different gases, and evacuation of the furnace chamber using rotary pump. The work is concentrated on Heraeus pastes that make it possible to reach up to about 300μm film thickness and, if needed, high resolution pattern of lower film thickness on the same substrate, metallised through holes and application of ENIG metallisation. Principles and characteristics of particular firing phases in the batch furnace as well as morphology and film properties of the fired copper films are described in the paper. The whole metallisation process proved to be effective offering high quality substrates for power electronics at reasonable investment and production costs.

A Comprehensive Study on the Automation Potentials and Complexities of Advanced and Alternative Die-Attach Technologies for Power Electronic Applications

The field of power electronics packaging presents intricate and interdisciplinary challenges. System costs, reliability and performance are strongly determined by various aspects such as mechanical design, materials, thermal management and interconnect technologies. The overall costs of the product depend mainly on the complete process chain in the module development. Automation as well plays an important role and facilitates higher production rates, efficient use of materials, better product quality, and reduced factory lead times. This paper focuses on emerging interconnection technologies of bonding semiconductor components to power electronic substrates like diffusion soldering, conductive adhesive bonding and reactive multi-layer bonding. An overview on the automation potentials and complexities in individual technologies for the manufacturing of reliable and cost-effective power modules is given and discussed. Thus, a basis is created for choosing optimal die-attach technology depending on economic and technologic demand by comparing the state-of-the-art and advanced technologies.

Transition from Order to Configurational Disorder for Surface Reconstructions on SrTiO_{3}(111).

There is growing interest in ternary oxide surfaces due to their role in areas ranging from substrates for low power electronics to heterogeneous catalysis. Descriptions of these surfaces to date focus on low-temperature explanations where enthalpy dominates, and less on the implications of configurational entropy at high temperatures. We report here the structure of three members of the n×n (2≤n≤4) reconstructions of the strontium titanate (111) surface using a combination of transmission electron diffraction, density functional theory modeling, and scanning tunneling microscopy. The surfaces contain a mixture of the tetrahedral TiO_{4} units found on the (110) surface sitting on top of octahedral TiO_{5}[] (where [] is a vacant octahedral site), and TiO_{6} units in the second layer that are similar to those found on the (001) surface. We find clear evidence of a transition from the ordered enthalpy-dominated 3×3 and 4×4 structures to a configurational entropy-dominated 2×2 structure that is formed at higher temperatures. This changes many aspects of how oxide surfaces should be considered, with significant implications for oxide growth.

Effects of Extreme Temperature Swings ( <inline-formula> <tex-math notation="TeX">$-\hbox{55}\ ^{\circ}\hbox{C}$</tex-math></inline-formula> to 250 <inline-formula> <tex-math notation="TeX">$^{\circ}\hbox{C}$</tex-math></inline-formula>) on Silicon Nitride Active Metal Brazing Substrates

Reliability of power electronic substrates has been one of the main issues of high-temperature packaging technologies. Widely used direct bonded copper (DBC) substrates, if subjected to wide temperature swings, suffer from copper layers peeling off from a ceramic because of the large thermal stresses resulting from the difference in coefficient of thermal expansion (CTE) between the copper and the ceramic (e.g., $\hbox{Al}_{2}\hbox{O}_{3}$ or AlN). Recently, silicon nitride active metal brazing ( $\hbox{Si}_{3}\hbox{N}_{4}$ -AMB) substrate has been developed with the aim to utilize power electronic substrates for extreme environmental conditions. $\hbox{Si}_{3}\hbox{N}_{4}$ -AMB has superior reliability because of the high mechanical strength of $\hbox{Si}_{3}\hbox{N}_{4}$ . In this paper, the effects of extreme temperature swings ( $-\hbox{55} ^{\circ}\hbox{C}$ to 250 $^{\circ}\hbox{C}$ ) on $\hbox{Si}_{3}\hbox{N}_{4}$ -AMB substrates were investigated to evaluate their applicability to harsh environment. Their high resistance to the peeling off of copper layers was confirmed. However, the surface-roughening phenomenon was observed on the surface of copper layers after the temperature cycling test. The mechanism of surface roughening was analyzed. It was found that the out-of-plane displacement of single grains, which is called the orange-peel phenomenon, occurred on the surface of the copper layers.

Architectured bimetallic laminates by roll bonding: bonding mechanisms and applications

Manufacturing of bimetallic laminates (<2 mm) with an internal architecture by roll bonding allows to obtain a compromise of conflicting properties. Two applications are considered: copper–steel–copper sheets for high power electronic component substrates and lightweight aluminium–steel–aluminium laminates for structural, electromagnetic applications in power generation and for electromagnetic compatibility. They have in common an architecture, with three-dimensional percolating networks of two metals produced with a sufficient precision. The bonding mechanisms are investigated through microstructural and thermomechanical characterisation of the assemblies produced. Roll bonding fills the voids in a central component by plastic deformation and creates the bonds by cold welding. It may be followed or preceded by surface and heat treatments with the objective to improve the cold welding and to relieve the internal stress state. Architectural rules to optimise the properties of the two-phase laminates in view of the applications may be guided by finite element models combining physical and thermomechanical aspects.

DBC Technology for Low Cost Power Electronic Substrate Manufacturing

This paper deals with a method for low cost power electronic substratescreation, which are used for power electronic devices. Direct bond copper (DBC) technology is suitable for creating substrates with excellent thermal and electrical conductivity and good mechanical properties. Furthermore, DBC allows to create copper layers with the thickness of hundreds micrometers in one processing step. In this work, the muffle furnace is used for bonding copper to ceramic. Temperature profile, oxygen concentration during the bonding process and final treatment is described. The last part summarizes the results and presents advantages of this method.

Ammonothermal Bulk GaN Substrates for Power Electronics

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Ammonothermal Bulk GaN Substrates for Power Electronics

Soraa has developed a novel ammonothermal approach for growth of high quality, true bulk GaN crystals at a greatly reduced cost, known as SCoRA (Scalable Compact Rapid Ammonothermal). SCoRA GaN growth has been performed on seed crystals with diameters between 5 mm and 2" to thicknesses of 0.5-4 mm. The highest growth rates are greater than 40 um/h and rates in the 10-30 mm/h range are routinely observed. Two-inch diameter, crack-free, free-standing, n-type bulk GaN crystals have been grown. The crystals have been characterized by a range of techniques, including x-ray diffraction rocking-curve (XRC) analysis, optical microscopy, cathodoluminescence (CL), optical spectroscopy, and capacitance-voltage measurements. The crystallinity of the grown crystals is very good, with FWHM values of 15-80 arc-sec and average dislocation densities below 5x105 cm-2.