Synthesis of an effective interlayer between a functional oxide film and Si<sub>3</sub>N<sub>4</sub> substrate

Laminated CuO/Al2CuO4/CuO layers were developed to serve as an interlayer between the conducting oxide CaCu3Ru4O12 + 30 vol.% CuO and nitride substrate Si3N4. Screen-printing was used to fabricate these films. The Al2CuO4 layer impeded the chemical reaction between CaCu3Ru4O12 and Si3N4 and hence suppressed the decomposition of CaCu3Ru4O12. The CuO interlayers improved the adhesion between the Si3N4 substrate, Al2CuO4 layer, and CaCu3Ru4O12 + 30 vol.% CuO film. The main difference between the CaCu3Ru4O12 + 30 vol.% CuO film on the Si3N4 and Al2O3 substrates having the same interlayers was the presence of a small amount of RuO2 (formed from decomposition of CaCu3Ru4O12) and cracks in the film on the Si3N4 substrate. The difference in the coefficients of thermal expansion between the Si3N4 substrate, interlayers, and film caused cracks to appear when the temperature of the film decreased after sintering. The resistivities of the films on the Si3N4 and Al2O3 substrates were 7 and 3 mΩ·cm at 100 °C, respectively. The film on the Si3N4 substrate exhibited higher resistivity than the film on the Al2O3 substrate from room temperature to 600 °C. This is because the cracks on the film on the Si3N4 substrate scattered the charge carriers. A decrease in crack width with increasing temperature was also observed, which in turn decreased the resistivity of the film with temperature. The conducting properties of the film developed in this work are adequate for practical applications. However, the interlayer can be further improved to obtain films with better performance. The major achievement of the present study is the development of an interlayer that suppresses the chemical reactions between the functional oxide and nitride substrate during high-temperature sintering.