Effect of Space Radiation on the Leakage Current of MEMS Insulators

The effect of space radiation on the reliability of microelectromechanical systems (MEMS) devices is an important consideration for future upper atmosphere and space applications. MEMS capacitors with insulator materials of silicon nitride (Si₃N₄), silicon oxide (SiO₂), and ultrananocrystalline diamond (UNCD) were selected for radiation and leakage current studies. Leakage current was used as a measure of insulator performance and reliability, and is suggested here as a method to detect charge trapping, which also affects reliability. UNCD capacitors were orders of magnitude leakier than Si₃N₄ and SiO₂, with Si₃N₄ being leakier than SiO₂. SiO₂ devices exhibited unstable leakage current with accumulated electric field stress, and were not utilized in radiation studies. Si₃N₄ capacitors exhibited leakage current decay (with a time constant of 190 s) under constant voltage stress above 2 MV/cm due to charge injection from the electrodes and trapping in the insulator. Si₃N₄ and UNCD capacitors were more sensitive to ionizing gamma radiation than to displacement damage from fast neutrons. Both Si₃N₄ and UNCD devices survived total doses of radiation representative of 20-100 years in the Van Allen radiation belts with 4 mm Al equivalent shielding. Capacitor equivalent circuit and resistor capacitor (RC) circuit charging models are developed to explain leakage current behavior of Si₃N₄ capacitors subjected to constant voltage stress and/or irradiation. In situ monitoring of Si₃N₄ capacitors placed next to the nuclear reactor core did not yield any single event effects at electric field strength of 1 MV/cm with a fast neutron fluence of 2×10¹² n/cm². Si₃N₄ MEMS capacitors appear best suited for upper atmos- here and space applications with their relatively low leakage current (low power consumption) and apparent radiation hardness.