Ultra-low permittivity ULTCC composite materials
A method to realize ultra-low temperature co-fireable ceramic composites with ultra-low permittivity is presented in this work. Hollow glass microspheres with size of 10—100 μm were used as a filler in a tape based on lithium molybdate (Li2MoO4) ceramic to introduce controlled porosity and reduce the relative permittivity of the sintered product. A lamination pressure of only 1.25 MPa was sufficient to produce samples with uniform structure and without delamination. Differential scanning calorimetry and thermogravimetric analysis were used to optimize the sintering temperature profile of the material. The microstructure of the samples was investigated with FESEM and the dielectric properties with a split post dielectric resonator (SPDR). Compatibility of the composite ceramic with silver was tested by applying thick film printed electrodes and post-firing them on the surface. Samples sintered at 540 °C exhibited a relative permittivity of 1.4—5.40 and a loss tangent of 10-3—10-4 at 5 and 10 GHz. The method shows interesting possibilities to significantly reduce processing temperatures compared to conventional LTCC materials and to obtain the extremely low permittivity that is especially required for future high frequency applications.