Ultra-Low Temperature Processing of Barium Tellurate Dielectrics

Open Access
Author:
Kwon, Do-Kyun
Graduate Program:
Materials Science and Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
November 15, 2005
Committee Members:
  • Michael T Lanagan, Committee Chair
  • Thomas R Shrout, Committee Chair
  • Clive A Randall, Committee Member
  • Gary Lynn Messing, Committee Member
  • Amar S Bhalla, Committee Member
Keywords:
  • ULTCC
  • Dielectric
  • Barium Tellurate
  • Microwave
Abstract:
Ceramics, metals and polymers have unique electrical properties that are combined for electronic devices and systems. It necessitates lower processing temperatures for ceramics to be compatible with metal and polymer systems. In this thesis, the synthesis, crystal structure, and dielectric properties of barium tellurate are studied for temperatures between 500 and 900 C. Barium tellurate dielectric ceramics (BaTe4O9, BaTe2O5, BaTe2O6, BaTeO3, BaTeO4, and Ba2TeO5) are extensively investigated as new LTCC (Low-Temperature Cofired Ceramics) dielectric systems integrated with low resistivity metal electrodes such as silver and aluminum for microwave application. Studies on the phase formation and crystal structure through thermal analyses (Differential Scanning Calorimetry and Thermogravimetric Analysis, DSC-TGA) and X-ray diffraction phase analysis attest that barium tellurates are formed in the temperature range of 500 ~ 900 C, through the sequential phase formations from Te-rich to Ba-rich phases. The oxygen coordination of the tellurium ion progresses from TeO4 to TeO6 via TeO3+1 and TeO3 with increasing barium content as confirmed by structural analysis using infrared spectroscopy. High density barium tellurate ceramics are achieved at temperatures as low as 550 C, which provides the potential to be co-fired with low-melting aluminum metal electrodes in LTCC processing. Dielectric permittivity, loss, and temperature stability of barium tellurate dielectric ceramics were measured from 100 Hz to 13 GHz. Barium tellurate ceramics exhibit excellent microwave dielectric properties with intermediate dielectric permittivities and high quality factors (Q). The dielectric properties at microwave frequencies are k = 17.5, Qxf = 54700 GHz, TCf = -90 ppm/C for BaTe4O9, k = 21, Qxf = 50300 GHz, TCf = -51 ppm/C for BaTe2O6, k = 10, Qxf = 34000 GHz, TCf = -54 ppm/C for BaTeO3, and k = 17, Qxf = 49600 GHz, TCf = -124 ppm/C for Ba2TeO5. Co-firing studies of barium tellurate ceramics with metal electrodes establish new LTCC systems for microwave devices. Chemical compatibility of barium tellurates with silver electrodes was achieved in the barium rich compositions. Ba2TeO5 was found to be co-fireable with silver electrodes at 850oC by adding CuO and B2O3 as fluxing agents. During the co-firing, a thin interfacial layer of AgTe is metastable according to the thermodynamic equilibrium between the Ba2TeO5-Ag/Ag2O pseudo-binary system. A breakthrough LTCC technology with aluminum is based upon the ultra-low processing temperature and chemical compatibility of BaTe4O9, which enables co-firing and fabrication of multilayer ceramic capacitors (MLCCs) with aluminum inner electrodes. The aluminum base metal electrode (BME) BaTe4O9 MLCCs provide good dielectric properties of k = 17.5, TCC = 100 ppm/C, and Q = 500 at 1 MHz, which are suitable for the class-1 MLCCs. Aluminum microstrip ring resonators on the BaTe4O9 substrates realized good electromagnetic performance of the new materials at microwave frequency exhibiting resonant frequency of 2.97 GHz and Q factor of 278.