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 (Si
₃
N
₄
), silicon oxide (SiO
₂
), 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 Si
₃
N
₄
and SiO
₂
, with Si
₃
N
₄
being leakier than SiO
₂
. SiO
₂
devices exhibited unstable leakage current with accumulated electric field stress, and were not utilized in radiation studies. Si
₃
N
₄
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. Si
₃
N
₄
and UNCD capacitors were more sensitive to ionizing gamma radiation than to displacement damage from fast neutrons. Both Si
₃
N
₄
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 Si
₃
N
₄
capacitors subjected to constant voltage stress and/or irradiation. In situ monitoring of Si
₃
N
₄
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 2×10
¹²
n/cm
²
. Si
₃
N
₄
MEMS capacitors appear best suited for upper atmosphere and space applications with their relatively low leakage current (low power consumption) and apparent radiation hardness.