Dielectric breakdown of silicon nitride substrates with various thicknesses

Dielectric breakdown of silicon nitride substrates was evaluated using alternating current voltage. Two different kinds of Si3N4 ceramics with mainly small and large grains were used. Test specimens were prepared from both Si3N4 ceramics with thicknesses of 0.25, 0.32, and 0.64 mm. Average breakdown strength of 0.25, 0.32 and 0.64-mm-thick Si3N4 specimens with small grains were 36.8, 35.1 and 24.9 kV/mm, and those of Si3N4 specimens with large grains were 29.5, 27.1, and 21.4 kV/mm, respectively. At all thicknesses, average breakdown strengths of Si3N4 specimens with small grains were higher than those of Si3N4 specimens with large grains. Average breakdown strength of both Si3N4 ceramics increased with decreasing thickness. Both the top and bottom surfaces of the Si3N4 ceramics with small and large grains had holes of 50 to 250 µm in diameter after breakdown test. Concentric cracks were observed around the holes. Both Si3N4 ceramics with small and large grains showed a tortuous breakdown channel in the direction of thickness after breakdown test. One or both edges of the tortuous breakdown channel had a crater-like structure where the opening was larger than the channel. The channel openings showed two different types of morphologies. One channel opening had concentric deposits around the channel, and other opening showed bare grains and no deposits.