The dielectric properties of alumina substrates for microelectronic packaging

Abstract

The dielectric characterization of alumina substrate materials used in high-performance microelectronic packaging is described. These materials included both pure and impure polycrystalline substrates and, as a reference standard, pure and chromium-doped single crystals of alumina. For each material the permittivity (ε′) and dielectric loss (ε″) has been measured over a frequency range of 0.5 kHz to 10 MHz, at room temperature, and correlated with the structure and composition as determined by supplementary techniques. At room temperature the pure substrates show the frequency independence of both ε′ and ε″, characteristic of pure single-crystal material. The permittivity (ε′= 10.1) agrees closely with the average of the anisotropic values for the single crystal but the dielectric loss is an order of magnitude higher than in the single crystal, giving tan σ ≈ 1.5 × 10⁻³. The impure substrates compared with the pure, show a small increase in ε′ and a marked, frequency-dependent increase in dielectric loss. Measurements have also been made in both the high- and low-temperature ranges (i.e. 20 to 600 ° C and 77 to 293 K, respectively) in order to establish the variation of permittivity with temperature and frequency. At temperatures below 200 °C the temperature coefficient of permittivity, [(ε′ −1)(ε′ + 2)]⁻¹ (∂ε′/∂T) _p is about 9 × 10⁻⁶ K⁻¹ for the pure materials but this increases rapidly with impurity addition.