Mullite ceramic substrate for thin-film application

The advantages of mullite ceramic for thin-film application are examined. Using homogeneous powder, a dense mullite substrate consisting of fine mullite grains was provided without the addition of a sintering aid. The properties obtained are superior to those of the mullite presented by M. Horiuchi et al. (1988) and a conventional alumina for application to packages and substrates: low dielectric constant (6.8), a thermal expansion coefficient (4.4×10^-6/°C) close to that of silicon, high flexural strength (302 MPa), and ease of polishing due to low hardness (10 GPa). Thin-film processes were applied to the mullite substrate. Chromium, copper, and nickel layers were sputter-deposited, followed by gold electroplating. The adhesion strength seems to be affected by the surface smoothness of the substrate. Pulse-propagation velocities and characteristic impedances of a coplanar line patterned on the substrates were measured. Tungsten via metallization was found to be cofireable with the mullite. This is advantageous for providing a package with high wiring density