modern production of thin film metallized devices requires thin, large area substrates with a highly smooth 'as fired' surface these properties are found in a new family of beryllia substrates produced by tape casting. flat substrates as large as 15*15 cm with a maximum thickness of 0.10 cm and with a surface finish of less than 0.13 mu m center line average were produced. the use of high purity beryllia powder insures a consistent high thermal conductivity (290 w/mk) and superior performance when compared to other common substrates such as alumina or aluminum nitride improved yields are derived from the smoothness of the surface beryllia tape substrates were compared to dry pressed and polished beryllia substrates. the quality of the coatings was evaluated by etching a standard test pattern using standard photolithographic processes. homogeneity of the deposition, thickness variation, and line uniformity were evaluated using sem/eds and atomic force microscopy. the quality of the coating was correlated to the ceramic surface finish. rf diode sputtering of these substrates and subsequent subtractive pattern processes yielded circuitry with less than 2.5 mu m absolute tolerances with about the same degree of difficulty found in similar processes with high quality 'as fired alumina'. resistors fabricated with tan had less flaws and hot spots than resistors fabricated on polished materials. reduction of these flaws and hot spots results in higher power handling capability hi-rel microwave conductor and resistor circuits can be fabricated