View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by Open Archive Toulouse Archive Ouverte Open Archive Toulouse Archive Ouverte (OATAO) OATAO is an open access repository that collects the work of Toulouse researchers and makes it freely available over the web where possible. This is an author-deposited version published in: http://oatao.univ-toulouse.fr/ Eprints ID: 3845 To link to this article: DOI: 10.1007/s11664-008-0571-8 URL: http://dx.doi.org/10.1007/s11664-008-0571-8 To cite this version: Chasserio, N. and Guillemet-Fritsch, Sophie and Lebey, Thierry and DAGDAG, S. ( 2009) Ceramic Substrates for High- temperature Electronic Integration. Journal of Electronic Materials, vol. 38 (n° 1). pp. 164-174. ISSN 0361-5235 Any correspondence concerning this service should be sent to the repository administrator: [email protected] Ceramic Substrates for High-temperature Electronic Integration N. CHASSERIO,1 S. GUILLEMET-FRITSCH,1 T. LEBEY,2 and S. DAGDAG3,4 1.—Centre Inter-universitaire de Recherche et dÕInge´nierie des Mate´riaux-CIRIMAT-Baˆt. 2R1, 118 Route de Narbonne, 31077 Toulouse Cedex 04, France. 2.—Laboratoire LAPLACE, Universite´ Paul Sabatier. Baˆt. 3R3, 118, Route de Narbonne, 31062 Toulouse Cedex 9, France. 3.—Power Electronics Associated Research Laboratory, ALSTOM Transport, Rue du Docteur Guinier, 65 600 Semeac, France. 4.—e-mail: [email protected] One of the most attractive ways to increase power handling capacity in power modules is to increase the operating temperature using wide-band-gap semi- conductors. Ceramics are ideal candidates for use as substrates in high-power high-temperature electronic devices. The present article aims to determine the most suitable ceramic material for this application. Key words: High-temperature electronics, passive component, system integration, packaging, ceramic substrate devices designed for operation at high temperatures INTRODUCTION would require extensive alterations, affecting their The commercial availability of field-effect tran- entire environment, such as the passivation, sistors made of silicon carbide (SiC-JFET or MOS- encapsulation, and substrates.1–3 This therefore FET) has enabled studies to be initiated on the requires adaption of the technological choices to the industrial development of power converters working critical environmental constraints, and particularly at ambient temperatures exceeding 200°C. In aero- the temperature ones. This also implies modifica- nautic or automotive applications, the replacement tion of most of the converter components (active and of mechanical or hydraulic systems by electronic passive) for an environment where the ambient devices would result in increased performance and temperature can easily exceed 200°C, as is usually reduced weight of the system. However such novel studied.4 designs would imply placing electronics and power With the aim of developing power converters that electronics devices near the actuators or close to the can operate safely at an ambient temperature of engine—locations resulting in harsh environmental 300°C, this article focuses on one part of the constraints for the device, involving extreme thermal assembly—the substrate. The substrate has several excursions. The availability of high-temperature functions, including ensuring electrical insulation technology would enable spectacular gains in terms between the active components and the baseplate of accuracy, reduction of clutter, and optimization of (which is generally grounded) while favoring the efficiency. Classically a power device is a stack of removal of losses generated by the dies (during both several materials (semiconductors, ceramics, met- switching and conduction periods). Even though its allization, etc.). This combination is then fastened to thermal conductivity is of primary importance, a baseplate. The resulting complex and heteroge- other properties have to be taken into account. neous structure is subjected to a large number of Previous studies3 have shown that the main causes stresses when in service. New power electronic of failure result from thermomechanical aging or physicochemical stresses. The substrate (the ele- mentary part) must therefore have a coefficient of thermal expansion (CTE) close to those of the other parts of the whole assembly. Chemical stability of the materials with increasing temperature must also be ensured. Regarding its electrical properties, the substrate must present high resistivity to CERAMIC SUBSTRATES reduce leakage current, a high dielectric strength to withstand the voltage, and a low dielectric constant Various ceramic materials have been envisaged to limit the common-mode capacitance. Lastly, to for this application, namely alumina, aluminum resist the constraints of handling, its mechanical nitride, boron nitride, and silicon nitride. We first strength must also be as high as possible. recall their general properties. For the present investigation, the targeted ambi- ent temperature is 300°C, although hot spots up to Alumina 350°C to 400°C are possible. Such increases of Alumina is commonly used in power applica- temperature depend on the packaging technology tions since it is the cheapest material for use and on the thermal management of the power as a substrate. Its physical characteristics mainly devices. Therefore, the substratesÕ properties have to depend on its purity and its density. Samples under be characterized up to 450°C to 500°C. Taking into study are in the form of 50 mm 9 50 mm 9 0.635 mm account the aforementioned requirements, metal- plates (manufacturer 1, purity 99.6%). insulator-semiconductor systems can be dismissed due to their low electrical resistivity. On the other Aluminum Nitride hand, polymers are not serious candidates since their behavior in this high-temperature range is Compared to alumina, AlN offers a significant inappropriate. Considering the required specifica- increase in thermal performance for power circuit tions, ceramic substrates are the only suitable can- designers. However, its properties can vary greatly didates. Alumina, boron nitride, aluminum nitride, since the process of fabrication can change from one and silicon nitride are the most suitable ceramics supplier to another (especially the kind and the due to their high thermal conductivity. Despite its amount of additive used). Two types of AlN have excellent properties, beryllium oxide is not used been tested in this article (AlN1 manufacturer 2 and because of its toxicity. Since the mid 1980s direct AlN2 manufacturer 3). The samples were the same bonded copper (DBC) substrates have increasingly sizes as the alumina plates. been used in power electronics. The reason why Boron Nitride Al2O3 and AlN-DBC have become the preferred industry power substrates is their performance and Boron nitride is expected to have a thermal competitive prices. However their properties at conductivity higher than that of alumina, with a temperatures higher than 200°C still need to be lower CTE. For these reasons, this material may evaluated and/or new materials envisaged. The best substitute for alumina. In this article two thicknesses compromise between their various properties has to and two dimensions were studied namely: 50 mm 9 be reached not only to optimize and to increase the 50 mm 9 0.635 mm and 50 mm 9 50 mm 9 1.67 mm final performance of the converter but also to (manufacturer 4, purity >95%). guarantee reliability. This article presents the results of various char- Silicon Nitride acterizations (structural, microstructural, mechan- ical, thermal, and electrical) of alumina, boron Si3N4 substrates present high mechanical resis- nitride, aluminum nitride, and silicon nitride and tance. This could be of considerable interest for reli- aims to establish the best candidate among ability (particularly for thermomechanical aging) these materials for the application described. It also despite its low thermal conductivity (Table I). The reports a large study of the behavior of these cera- dimensions of the Si3N4 plates studied were mic substrates over a high-temperature range 50 mm 9 50 mm 9 0.635 mm (manufacturer 6). reaching 500°C. In the first part, the general prop- The most commonly reported physical character- erties of the ceramics tested are introduced. Then, istics of these materials, including the supplierÕs the different characterization methods are pre- values specified in the following, are given in sented. The results for the different materials under Table I. The large dispersion of values observed in study are then given and discussed. some cases is mainly due to the different processes Table I. Physical Characteristics of the Materials under Study at Room Temperature Thermal Conductivity CTE Flexural Strength Dielectric Strength (W/m K) (1026/°C) (MPa) (kV/mm) Alumina 26–35 6.8–9 300–400 10–20 Boron nitride 20–60 0.1–6 20–90 40–200 Aluminum nitride 150–180 4.3–6.2 300–350 14–17 Silicon nitride 20–30 2.6–3.6 500–800 10–14 and/or additives used by the different suppliers Thermal Conductivity during the synthesis of the materials. Since the maximum temperature of the elemen- tary semiconductor die directly influences the reli- EXPERIMENTAL ability of a semiconductor component, the thermal resistance has to be minimized to keep the temper- The main analytical techniques used investi- ature as low as possible. gated the electrical, mechanical, and structural The thermal conductivity (in W/m K) was mea- properties. sured using the hot disk method. To ensure the performance of the material at a working tempera- Structure and Microstructure ture of 300°C, which implies a component temper- The physical properties are related to the intrin- ature that can reach values up to 400°C, the sic characteristics of the materials such as the measurements were performed at temperatures purity or the microstructure. Hence the different from 25°C to 400°C (which corresponds to the phases present in the materials were characterized highest temperature that the probe can withstand by x-ray diffraction (XRD) analysis using a Bruker without damage). D4 Endeavour diffractometer working with the Ka ray of copper (k = 1.5418 A˚ ).