Properties of Porous AlN Multilayered Ceramic Sandwich Substrates

The development of denser and higher powered integrated circuits has led to a corresponding demand on the performance of dielectric substrates. This paper reports on the fabrication and properties of an AlN multilayered sandwich substrate comprising porous tape-cast layers sandwiched between nonporous layers. Tapes were produced by nonaqueous tape casting, with the porous tapes produced using polymer microspheres as sacrificial molds. Starting from initially Al₂O₃-rich tapes, the multilayered sandwich substrates were reaction sintered to produce AlN substrates. No interface cracking or delamination was observed in the substrates as a result of the processing. The added porosity resulted in a decrease in the substrate dielectric constant in correspondence to porosity volume. Mechanical strength of the sandwich substrates was improved over that of nonsandwich porous substrates, while substrates having noninterconnected pores showing higher mechanical strength than substrates with connected pores. Substrates with more than 2% porosity showed porosity-dependent thermal conductivity values, while thermal conductivity of substrates with less than 2% porosity was dependent on grain boundary effects. Thermal expansion coefficient of the substrates was unaffected by porosity levels.