1997 Volume 105 Issue 1218 Pages 106-110
Polished aluminium nitride substrates have imperfections of surface roughness in the form of cavities due to the loss of grains. Measurements using the Image Processing System and the Dimensional Measurement method, revealed that cavities with diameters larger than 10μm existed in quantities of about 22.5×105 pieces on a substrate of 152mm2 and that their depths were about 1.3μm. The cavities also had sharp edges and steep cliffs. The application of a thin coating of silicon oxide was studied for obtaining a smoother surface of the polished aluminium nitride substrate. The depth of the coated cavity decreased with increasing thickness of the film, and the shape of the cavities, especially their sharp edge and steep cliff could not be seen clearly as the thickness of the film increased. However, even if the film thickness was as great as 1.2μm, cavities of about 0.21μm depth remained on the coated substrate. A 10μm comb electric pattern could be made on the surface of the substrate coated with 0.5μm of film. All patterns exhibited 1856 blocks (a block has 29 lines) and were uniform without any broken portions.