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Investigation towards the optimum of power capability, ageing stability and costs effectiveness on thick film resistor pastes for AlN ceramics
Keywords: thick film resistor paste, aluminum nitride, power electronics
Thick film technology is an established technology for the fabrication of both discrete electronic components and hybrid circuits on a variety of substrate materials. In dependence of the used materials it meets the demands of high reliability applications and thus suitable for the fabrication of sustainable, efficient and cost-effective power electronics. For this application, aluminum nitride (AlN) is an ideal substrate material because it combines a very high thermal conductivity, a thermal expansion coefficient close to silicon for high reliability packaging, good mechanical stability and dielectric properties as well as non-hazardous material properties. Thick film resistor pastes are usually composed of lead- and bismuth oxide containing glass frits and ruthenium dioxide (RuO2) as electrical conductive component. Not only the composition of the paste but also accompanying materials and –properties like substrate roughness and –quality cause interactions of the paste components resulting in deviating film structures and altered electrical properties [1]. Nevertheless, the main issue is the reaction of the above named base metal oxides and ruthenium dioxide with the ceramic substrate material. Thermodynamic calculations predict a reaction with AlN in a wide range of temperature, which was partly proven by experiments before [2]. Thus, a sophisticated lead- and bismuth oxide-free glass composition and well-engineered functional film materials are necessary for reproducible film formation on AlN ceramics. Fraunhofer IKTS developed a complete paste system for this substrate material in recent years. [3] The focus of this work was the optimization of a 10 Ω/sq thick film resistor (TFR) paste composition to obtain increased power capability, aging stability and minimum use of ruthenium oxide for cost savings without changing the defined narrow sheet resistance (Rsq) and temperature coefficient of resistance (TCR) specifications. In times of highly fluctuating precious metal costs, the use of a minimum of the precious metal ruthenium respectively ruthenium dioxide is one essential part for cost-effectiveness. The thick film paste formulation consists of the electrically conducting phase ruthenium dioxide, a lead-free glass phase and two inorganic additives for tuning thermo-mechanical and electrical properties of the formed films. A phthalate free organic vehicle with ethyl cellulose polymer was used to formulate a screen printable ceramic thick film paste. For this paper, RuO2 powders with various specific surface area values (Brunauer-Emmett-Teller method, BET) were prepared by thermal sintering of a precipitated fine ruthenium dioxide powder. All other solid and liquid components of the paste were the same as used for IKTS 10 Ω/sq TFR paste for AlN substrates. Furthermore, the content of ruthenium dioxide in the paste compositions was changed systematically around an assumed target content for the desired sheet resistivity. In exchange for the variation of the ruthenium dioxide content one of the two inorganic additives was varied substitutional. The influences of the variations of raw material and paste composition on the film properties were investigated by screen printing 24 resistors of 2 mm x 1 mm dimension in an 1” x 1” AlN substrate, firing at 850 °C for 10 minutes in air atmosphere and subsequently measuring Rsq, TCR, the change of resistance dR/R0 effected by artificial aging of the resistors after 100 up to 1000 hours and the maximum rated power dissipation (MRPD) as well as short term overload voltage (STOL). The results show that pastes with RuO2 powders having high values of specific surface area exhibit lower sheet resistivities and lower TCRs compared to these with powders of low specific surface area. This enables the use of lower RuO2 contents by maintaining the before named film properties. On the other hand, the resistance drift after aging shows a minor rise when powders with high surface area are used. Additionally, the power dissipation is supposed to fall adequately. These opposing results are discussed in regard to find an optimum between all demands of the most important electrical film properties.
Richard Schmidt,
Fraunhofer Institute for Ceramic Technologies and Systems
Dresden, Saxony

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