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This paper deals with the packaging of two III-V chips combined to form a System-in-Package (SiP) for RF base transceiver station applications. The first die consists of a high-power amplifier (HPA) and a switch made on GaN-on-SiC. The second one features a low-noise amplifier (LNA) and a driver built on GaAs. Both chips bring together the best of each substrate technology, namely high RF and power performances of GaN, and low-noise capability of GaAs. The SiP was built using fan-out wafer-level packaging (FOWLP) in chip-first face-down configuration. The gap between the chips is as low as $\mathrm{100}\ \mu\mathrm{m}$ . Electrical routing is secured by redistribution layer (RDL) and balls for flip-chip assembly on the PCB. Thermal dissipation has to be managed opposite to the PCB to avoid a too complex PCB design. It is managed by directly contacting the HPA backside with a Cu-liner acting as a heat spreader. This is achieved by opening the molding compound using laser ablation, and subsequently plating Cu on the SiP backside. The SiP has a final size of $\mathrm{4}\times \mathrm{4}\times \mathrm{0}.\mathrm{35}\ \text{mm}^{\mathrm{3}}$ , which eventually aims at fitting into the meshing size imposed by an active antenna array operating at 28 GHz. This paper addresses GaN and GaAs chips specific features which have an impact for the FOWLP process flow: low thickness ( $\sim \mathrm{100}\ \mu \mathrm{m}$ ) relative to the targeted $\mathrm{350}\ \mu\mathrm{m}$ -thick molding compound; chips backside coated with Au which shall not be removed; chip frontside with a relatively high topology (almost $\mathrm{20}\ \mu\mathrm{m}$ ). Signal losses were measured in an SiP-like environment at 0.1 dB/mm, 0.2 dB/mm and 0.4 dB/mm respectively at 30 GHz, 40 GHz and 60 GHz. These results are promising in anticipation of the SiP final testing.

International audience; The constant increase in power and heat flux densities encountered in electronic devices fuels a rising demand for lightweight heat sink materials with suitable thermal properties. In this study, discontinuous pitch-based carbon fiber reinforced aluminum matrix (Al-CF) composites with aluminum-silicon alloy (Al-Si) were fabricated through hot pressing. The small amount of Al-Si contributed to enhance the sintering process in order to achieve fully dense Al-CF composites. A thermal conductivity and CTE of 258 W/(m K) and 7.0 9 10-6 /K in the in-plane direction of the carbon fibers were obtained for a (Al 95 vol% ? Al-Si 5 vol%)-CF 50 vol% composite. Carbon fiber provides the reducing of CTE while the conservation of thermal conductivity and weight of Al. The achieved CTEs satisfy the standard requirements for a heat sink material, which furthermore possess a specific thermal conductivity of 109 W cm 3 /(m K g). This simple process allows the low-cost fabrication of Al-CF composite, which is applicable for a lightweight heat sink material.

Excitations of precursor molecules by different laser powers in laser-assisted growth of diamond films using a wavelength-tunable CO2 laser were studied. The wavelength of the CO2 laser was tuned to 10.532 μm to match a vibration mode of a precursor molecule, ethylene. The density of the incident laser power was adjusted to modify diamond crystal orientation, optimize diamond quality, and achieve high-efficiency laser energy coupling. It was observed that at incident laser power densities between 5.0 × 103 and 1.0 × 104 W/cm2, (100)-faceted diamond crystals were grown uniformly in the center areas of the diamond films. Higher incident laser powers, although further promoted growth rate, suppressed the uniformity of the diamond (100) facets. Best diamond quality was obtained within a laser power density range of 5.0 × 103 to 6.7 × 103 W/cm2, whereas the highest energy efficiency was achieved within a laser power density range of 3.3 × 103 to 6.7 × 103 W/cm2. The effects of the resonant laser energy couplin...

International audience; Direct formation of graphene with a controlled number of graphitic layers on dielectric surfaces is highly desired for practical applications but still challenging. Distinguished from the conventional chemical vapor deposition methods, a solid-state rapid thermal processing (RTP) method can achieve high-quality graphene formation on dielectric surfaces without transfer. However, little research is available to elucidate the graphene growth mechanism in the RTP method (heating rate ∼15 °C s−1). Here we show a solid-state transformation mechanism in which a metastable nickel carbide (Ni3C) intermediate phase plays a critical role in transforming amorphous carbon to two dimensional crystalline graphene and contributing to the autonomous Ni evaporation in the RTP process. The formation, migration and decomposition of Ni3C are confirmed to be responsible for graphene formation and Ni evaporation. The Ni3C-assisted graphene formation mechanism expands the understanding of Ni-catalyzed graphene formation and provides insightful guidance for controlled growth of graphene through the solid-state transformation process.

International audience; Diamond-dispersed copper matrix (Cu/D) composite materials with different interfacial configurations are fabricated through powder metallurgy and their thermal performances are evaluated. An innovative solution to chemically bond copper (Cu) to diamond (D) has been investigated and compared to the traditional Cu/D bonding process involving carbide-forming additives such as boron (B) or chromium (Cr). The proposed solution consists of coating diamond reinforcements with Cu particles through a gas–solid nucleation and growth process. The Cu particle-coating acts as a chemical bonding agent at the Cu–D interface during hot pressing, leading to cohesive and thermally conductive Cu/D composites with no carbide-forming additives. Investigation of the microstructure of the Cu/D materials through scanning electron microscopy, transmission electron microscopy, and atomic force microscopy analyses is coupled with thermal performance evaluations through thermal diffusivity, dilatometry, and thermal cycling. Cu/D composites fabricated with 40 vol% of Cu-coated diamonds exhibit a thermal conductivity of 475 W m−1 K−1 and a thermal expansion coefficient of 12 × 10−6 °C−1. These promising thermal performances are superior to that of B-carbide-bonded Cu/D composites and similar to that of Cr-carbide-bonded Cu/D composites fabricated in this study. Moreover, the Cu/D composites fabricated with Cu-coated diamonds exhibit higher thermal cycling resistance than carbide-bonded materials, which are affected by the brittleness of the carbide interphase upon repeated heating and cooling cycles. The as-developed materials can be applicable as heat spreaders for thermal management of power electronic packages. The copper-carbon chemical bonding solution proposed in this article may also be found interesting to other areas of electronic packaging, such as brazing solders, direct bonded copper substrates, and polymer coatings.

Crystallographic orientations determine the optical, electrical, mechanical, and thermal properties of crystals. Control of crystallographic orientations has been studied by changing the growth parameters, including temperature, pressure, proportion of precursors, and surface conditions. However, molecular dynamic mechanisms underlying these controls remain largely unknown. Here we achieved control of crystallographic orientations in diamond growth through a joint experimental and theoretical study of laser resonant vibrational excitation of precursor molecules (ethylene). Resonant vibrational excitation of the ethylene molecules using a wavelength-tunable CO2 laser steers the chemical reactions and promotes proportion of intermediate oxide species, which results in preferential growth of {100}-oriented diamond films and diamond single crystals in open air. Quantum molecular dynamic simulations and calculations of chemisorption energies of radicals detected from our mass-spectroscopy experiment provide an in-depth understanding of molecular reaction mechanisms in the steering of chemical reactions and control of crystallographic orientations. This finding opens up a new avenue for controlled chemical vapor deposition of crystals through resonant vibrational excitations to steer surface chemistry.

The increase in both power and packing densities in power electronic devices has led to an increase in the market demand for effective heat-dissipating materials, with high thermal conductivity and thermal expansion coefficient compatible with chip materials still ensuring the reliability of the power modules. In this context, metal matrix composites: carbon fibers, carbon nano fibers and diamond reinforced copper matrix composites among them are considered very promising as a next generation of thermal management materials in power electronic packages. These composites exhibit enhanced thermal properties compared to pure copper combined with lower density. This article presents the fabrication techniques of copper/carbon composite films by powder metallurgy and tape casting and hot-pressing; these films promise to be efficient heat-dissipation layers for power electronic modules. The thermal analyses clearly indicate that interfacial treatments are required in these composites to achieve high thermo-mechanical properties. Interfaces (through novel chemical and processing methods), when selected carefully and processed properly will form the right chemical/mechanical link between copper and carbon enhancing all the desired thermal properties while minimizing the deleterious effect. In this paper, we outline a variety of methods that are system specific that achieve these goals.The increase in both power and packing densities in power electronic devices has led to an increase in the market demand for effective heat-dissipating materials, with high thermal conductivity and thermal expansion coefficient compatible with chip materials still ensuring the reliability of the power modules. In this context, metal matrix composites: carbon fibers, carbon nano fibers and diamond reinforced copper matrix composites among them are considered very promising as a next generation of thermal management materials in power electronic packages. These composites exhibit enhanced thermal properties compared to pure copper combined with lower density. This article presents the fabrication techniques of copper/carbon composite films by powder metallurgy and tape casting and hot-pressing; these films promise to be efficient heat-dissipation layers for power electronic modules. The thermal analyses clearly indicate that interfacial treatments are required in these composites to achieve high thermo-mech...

Besides knowledge of thermal conductivities, information about the interfacial thermal resistances existing in layered systems such as power electronic packages is of primary importance. Indeed, thermal boundary resistances have a critical influence on the heat transfer process occurring between the layers. In this study, modulated infrared photothermal radiometry was employed to measure the thermal response of diamond films deposited on silicon substrates through laser-assisted combustion synthesis. The thermal resistance normal to the diamond/silicon interface was then estimated from the measurement of the phase and the amplitude of the thermal response. Preliminary results show that the layered diamond/Si system exhibits an interfacial thermal resistance of about 4×10-8 K.W-1. The technique developed in this study enables a precise evaluation of the thermal resistance at the diamond/silicon interface and is promising for various thermal management applications of diamond thin-films in optics, electronics, or mechanics.Besides knowledge of thermal conductivities, information about the interfacial thermal resistances existing in layered systems such as power electronic packages is of primary importance. Indeed, thermal boundary resistances have a critical influence on the heat transfer process occurring between the layers. In this study, modulated infrared photothermal radiometry was employed to measure the thermal response of diamond films deposited on silicon substrates through laser-assisted combustion synthesis. The thermal resistance normal to the diamond/silicon interface was then estimated from the measurement of the phase and the amplitude of the thermal response. Preliminary results show that the layered diamond/Si system exhibits an interfacial thermal resistance of about 4×10-8 K.W-1. The technique developed in this study enables a precise evaluation of the thermal resistance at the diamond/silicon interface and is promising for various thermal management applications of diamond thin-films in optics, electroni...

Highlights: Black-Right-Pointing-Pointer New process for deposition of copper nanoparticles onto various substrates. Black-Right-Pointing-Pointer Functionalization with phosphorus to enhance chemical bonding between Cu and substrate. Black-Right-Pointing-Pointer Control of size, number and location of the deposited particles. Black-Right-Pointing-Pointer Applicable to other metals, possibility to solve problems of assembling in many fields. -- Abstract: The efficiency of composite materials heavily relies on the ability of assembled materials to form strong interfaces, for allowing a good transfer of properties. High chemical affinity between the assembled materials is beneficial to the composite, whereas the assembly of materials with a weak mutual affinity is more difficult. When the interface is known to be non-cohesive, additional elements are commonly used to bond the matrix to the reinforcements. In the case of copper and carbon bonding, boron and chromium layers are frequently used . These additional bonding layers can, however, affect the properties of the composite, degrading for instance its mechanical or thermal properties. We report here an innovative process enabling the selective deposition of copper nanoparticles on substrates of various types (carbon fibers (CF) or nanofibers (CNF), diamond particles, silicon (Si) wafer, alumina plate) and shapes (1D, 2D, 3D). The deposition process involves a phosphate reagent usedmore » to functionalize the substrates. Cu nanoparticles deposit precisely onto the functionalized sites of the substrate and can then be used to bond the matrix to reinforcements with weak chemical affinity, such as copper with carbon fibers or diamond particles, therefore avoiding the use of interlayers which might be detrimental to the assembly properties. From a broader perspective, we anticipate that the method described here could enable the deposition of particles of many materials (Cu, Mn, Ti, Ni) onto substrates of various shapes and dimensions, creating bonding layers between materials of low chemical affinity and making easier their assembly without degrading its properties.« less

International audience; Diamond dispersed copper matrix (Cu/D) composite films with strong interfacial bonding were produced by tape casting and hot pressing without carbide forming additives. The tape casting process offers an original solution to obtain laminated materials with accurate thickness control, smooth surface finish, material net-shaping, scalability, and low cost. This study presents an innovative process of copper submicronic particles deposition onto diamond reinforcements prior to densification by hot pressing. Copper particles act as chemical bonding agents between the copper matrix and the diamond reinforcements during hot pressing, thus offering an alternative solution to traditionnal carbide-forming materials in order to get efficient interfacial bonding and heat-transfer in Cu/D composites. It allows high thermal performances with low content of diamond, thus enhancing the cost-effectiveness of the materials. Microstructural study of composites by scanning electron microscopy (SEM) was correlated with thermal conductivity and thermal expansion coefficient measurements. The as-fabricated films exhibit a thermal conductivity of 455 W m−1 K−1 associated to a coefficient of thermal expansion of 12 × 10−6 °C−1 and a density of 6.6 g cm−3 with a diamond volume fraction of 40%, which represents a strong enhancement relative to pure copper properties (λCu = 400 W m−1 K−1, αCu = 17 × 10−6 °C−1, ρCu = 8.95 g cm−3). The as-fabricated composite films might be useful as heat-spreading layers for thermal management of power electronic modules.

Fast growth of diamond crystals in open air was achieved by laser-assisted combustion synthesis through vibrational excitation of precursor molecules. A wavelength-tunable CO2 laser (spectrum range from 9.2 to 10.9 μm) was used for the vibrational excitation in synthesis of diamond crystals. A pre-mixed C2H4/C2H2/O2 gas mixture was used as precursors. Through resonant excitation of the CH2-wagging mode of ethylene (C2H4) molecules using the CO2 laser tuned at 10.532 Μm, high-quality diamond crystals were grown on silicon substrates with a high growth rate of ~139 μm/hr. Diamond crystals with a length up to 5 mm and a diameter of 1 mm were grown in 36 hours. Sharp Raman peaks at 1332 cm-1 with full width at half maximum (FWHM) values around 4.5 cm-1 and distinct X-ray diffraction spectra demonstrated the high quality of the diamond crystals. The effects of the resonant excitation of precursor molecules by the CO2 laser were investigated using optical emission spectroscopy.

International audience; The quality of diamond films deposited on cemented tungsten carbide substrates (WC-Co) is limited by the presence of the cobalt binder. The cobalt in the WC-Co substrates enhances the formation of nondiamond carbon on the substrate surface, resulting in a poor film adhesion and a low diamond quality. In this study, we investigated pretreatments of WC-Co substrates in three different approaches, namely, chemical etching, laser etching, and laser etching followed by acid treatment. The laser produces a periodic surface pattern, thus increasing the roughness and releasing the stress at the interfaces between the substrate and the grown diamond film. Effects of these pretreatments have been analyzed in terms of microstructure and cobalt content. Raman spectroscopy was conducted to characterize both the diamond quality and compressive residual stress in the films.

International audience; Diamond films were deposited on silicon and tungsten carbide substrates in open air through laser-assisted combustion synthesis. Laser-induced resonant excitation of ethylene molecules was achieved in the combustion process to promote diamond growth rate. In addition to microstructure study by scanning electron microscopy, Raman spectroscopy was used to analyze the phase purity and residual stress of the diamond films. High-purity diamond films were obtained through laser-assisted combustion synthesis. The levels of residual stress were in agreement with corresponding thermal expansion coefficients of diamond, silicon, and tungsten carbide. Diamond-film purity increases while residual stress decreases with an increasing film thickness. Diamond films deposited on silicon substrates exhibit higher purity and lower residual stress than those deposited on tungsten carbide substrates.

Laser-power-resolved excitations of precursor molecules in laser-assisted synthesis of diamond films using a wavelength-tunable CO2 laser were studied. The wavelength of the CO2 laser was tuned to 10.532 µm to match a vibration mode of a precursor molecule, ethylene (C2H4). The density of the incident laser power was adjusted to modify diamond crystal orientation, optimize diamond quality, and achieve high-efficiency laser energy coupling. It was observed that at incident laser power densities between 5×103 and 1.0×104 W/cm2, (100)-faceted diamond crystals were grown uniformly in the center areas of the diamond films. Higher incident laser powers, although further promoted growth rate, suppressed the uniformity of the diamond (100) facets. Best diamond quality was obtained within a laser power density range of 5×103∼6.7×103 W/cm2, whereas the highest energy efficiency was achieved within a laser power density range of 3.3×103∼6.7×103 W/cm2. The effects of the resonant laser energy coupling were investigated using optical emission spectroscopy.Laser-power-resolved excitations of precursor molecules in laser-assisted synthesis of diamond films using a wavelength-tunable CO2 laser were studied. The wavelength of the CO2 laser was tuned to 10.532 µm to match a vibration mode of a precursor molecule, ethylene (C2H4). The density of the incident laser power was adjusted to modify diamond crystal orientation, optimize diamond quality, and achieve high-efficiency laser energy coupling. It was observed that at incident laser power densities between 5×103 and 1.0×104 W/cm2, (100)-faceted diamond crystals were grown uniformly in the center areas of the diamond films. Higher incident laser powers, although further promoted growth rate, suppressed the uniformity of the diamond (100) facets. Best diamond quality was obtained within a laser power density range of 5×103∼6.7×103 W/cm2, whereas the highest energy efficiency was achieved within a laser power density range of 3.3×103∼6.7×103 W/cm2. The effects of the resonant laser energy coupling were investigat...

A wavelength-tunable CO2 laser (spectrum range from 9.2 to 10.9 f.m) was applied in a multi-energy process for growing diamond crystals in open air. A pre-mixed C2H4/C2H2/O2 gas was used as precursors for the diamond growth. Laser energy was coupled into the reactions through resonantly exciting the CHB2B-wagging mode of ethylene (C2H4) molecules by tuning the laser wavelength to 10.532 µm. Diamond growth rate and diamond quality were both promoted by the laser-induced resonant excitations. High-quality diamond crystals were grown on silicon substrates with a high growth rate of ∼139 µm/hr. Diamond crystals up to 5 mm in height and 1 mm in diameter were grown in open air in 36 hours. Sharp Raman peak at 1332 cm-1 with a full width at half maximum value around 4.5cm-1 and distinct X-ray diffraction spectra indicate the high quality of the diamond crystals.A wavelength-tunable CO2 laser (spectrum range from 9.2 to 10.9 f.m) was applied in a multi-energy process for growing diamond crystals in open air. A pre-mixed C2H4/C2H2/O2 gas was used as precursors for the diamond growth. Laser energy was coupled into the reactions through resonantly exciting the CHB2B-wagging mode of ethylene (C2H4) molecules by tuning the laser wavelength to 10.532 µm. Diamond growth rate and diamond quality were both promoted by the laser-induced resonant excitations. High-quality diamond crystals were grown on silicon substrates with a high growth rate of ∼139 µm/hr. Diamond crystals up to 5 mm in height and 1 mm in diameter were grown in open air in 36 hours. Sharp Raman peak at 1332 cm-1 with a full width at half maximum value around 4.5cm-1 and distinct X-ray diffraction spectra indicate the high quality of the diamond crystals.