High Temperature Capacitors Based on [0001] and [1120] Sapphire Dielectrics

The test results of the dielectric properties of [0001] (C-plane) and [1120] (A-plane) sapphire (single crystalline Al2O3) at high temperatures indicate that these materials have very stable dielectric constants and low dielectric losses (compared with polycrystalline alumina) at low frequencies in the temperature range from room temperature to 550°C. Therefore, sapphire materials have become likely candidate dielectric materials for high temperature capacitors. This paper reports prototype low-volume (∼100pF) capacitors based on sapphire dielectrics for high temperature and low frequency applications. Low-volume parallel-plate capacitors using C-plane and A-plane sapphire as dielectric material were fabricated by stacking metallized sapphire substrates. These prototype capacitors were characterized in the temperature range from room temperature to 550°C by measuring the capacitance and parallel resistance of these devices at 120Hz, 1kHz, 10kHz, 100kHz, and 1MHz. The capacitance and equivalent parallel resistance of these capacitors were all directly measured by an AC LCZ impedance meter in controlled temperature environments. These prototype devices demonstrate stable capacitances over a wide temperature range, and therefore, have the potential to be integrated with silicon carbide (SiC) devices to enable high temperature electronics. The needs of thin-film metallization and encapsulation for these sapphire substrates are also discussed.