Bismuth Pyrochlore Films for Dielectric Applications
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Wei REN, Ryan THAYER, Clive A. RANDALL, Thomas R. SHROUT, and Susan TROLIERMcKINSTRY Materials Research Laboratory, Pennsylvania State University, University Park, PA 16802-4801
ABSTRACT Bismuth pyrochlore ceramics have modest temperature coefficients of capacitance, good microwave properties, and can be prepared at relatively modest temperatures (-900 - 1100 'C). This work focuses on the preparation and characterization of thin films in this family for the first time. A sol-gel procedure using bismuth acetate in acetic acid and pyridine, in combination with zinc acetate dihydrate and niobium ethoxide in 2-methoxyethanol was developed. The solution chemistry was adjusted to prepare (Bi 1 .5Zn0.5 )(Zn0.5 Nb1 .5)0 7 and Bi 2 (Znj/ 3Nb 2/3)20 7 films. Solutions were spin-coated onto platinized Si substrates and crystallized by rapid thermal annealing. In both cases, crystallization occurred by 550 'C into the cubic pyrochlore structure. (Bi 1 .5Zn0.5)(Zn0.5 Nb 1.5 )0 7 films remained in the cubic phase up to crystallization temperatures of 750 'C, while the structure of the Bi 2(Znl/ 3Nb2J3) 20 7 thin films is dependent of the firing temperature: cubic below 650 'C and orthorhombic above 750 'C. A mixture of cubic and orthorhombic structures is found at 700 'C. The resulting BZN films are dense, uniform, and smooth (rms roughness of < 5 nm). Cubic bismuth zinc niobate films show dielectric constants up to 150, a negative temperature coefficient of capacitance, TCC, (- - 400 ppm/PC), tan 8 < 0.01, and a field tunable dielectric constant. Orthorhombic films showed smaller dielectric constants (-80), low tan 8 (< 0.01), positive TCC, and field independent dielectric constants. TCC could be adjusted to new 0 ppm/°C using a mixture of orthorhombic and cubic material. INTRODUCTION The rapid development of communication technologies, especially mobile communication systems, is facilitated by miniaturization of devices. Dielectric materials for these applications must possess a large dielectric constant, low loss tangent (tan 8), high dielectric quality factor Q, and a small temperature coefficient of resonator frequency for resonators. The Bi 20 3-ZnO-Nb 20 5 (BZN) pyrochlore ceramic system with medium dielectric constants and high Q values has been developed for low firing multilayer capacitors [1-4]. Recent studies showed that some members of the BZN system also exhibit excellent microwave properties [5, 6]. BZN ceramics with dielectric constants >100, temperature coefficient of resonant frequency ITf I< 10 ppm/°C and Q-f > 5000 GHz have been reported [6]. The good dielectric properties shown by the BZN system suggest that thin films of this composition are potential materials for integrated microwave resonators and decoupling capacitors. Thin films may have the advantage of lower crystallization temperatures and smaller device size than bulk ceramics and can be integrated in microelectronic devices. To our knowledge, no BZN thin films have been reported previously. In this study, we report on the fabrication and properti
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