Enhanced Electron Emission from Positive Dielectric/Negative Metal Configurations on Spacecraft

Results are presented of an experimental study of UV-enhanced electron emission from solar cell samples on metal substrate. The dielectric cover glass is given a positive voltage with respect to the metal, either by use of an electron beam or with an applied bias voltage to the metal substrate, or by shining UV light (¿ .1 sun) on the sample. For certain samples tested, the current from the sample to the chamber walls exhibited what we refer to as "enhanced emission": The current was noisy (unsteady) with time, and the current was greater than that expected considering photoemission and secondary emission. Strong enhanced electron emission was observed for samples with fused silica cover glasses, while the effect was much reduced for samples with ceria-doped microsheet coverglasses. The critical difference in the two samples appears to be the resistivity of the coverglasses. The high resistivity of the fused silica (¿ ¿ 1017 ¿-cm as compared to 1013 ¿-cm for ceria doped microsheet) allows a large buildup of positive charge very close to the exposed negative metal (interconnects, soldering bar). This triggers field emission from the exposed metal. The noiseness of the current with time indicates the positive ions of the dielectric are contributing to the current which suggests an accelerated surface degradation. This emission effect appears below 1 kV.