Document Type

Conference Poster

Journal/Book Title/Conference

12th Spacecraft Charging Technology Conference

Publication Date

5-18-2012

Abstract

Charging of thin-film, multilayer dielectric materials subject to electron bombardment was found to evolve with time. The charging behavior was also highly dependent on the incident energy of the monoenergetic electron beams; this is driven by energy dependant processes including the electron penetration depth, electron emission, and material conductivity. The electron penetration depth is the average range to which incident electrons at a given incident energy penetrate into the material, thus defining the mean depth of an embedded charge layer. The secondary electron yield is the ratio of electrons emitted from the surface to the number of incident electrons; this ratio largely controls the magnitude of the surface charge layer. The material conductivity determines how easily charge layers can move with and out of the material, and hence the time evolution of the charge distribution. While range and yield are highly energy dependent, the conductivity depends more on beam power and is highly temperature dependent. Radiation induced conductivity becomes significant for high electron fluxes. Models based on the dependence of electron range, electron emission and conductivity on incident energy, flux and sample temperature allow the evolution of the internal charge distribution to be inferred from measurements of the net surface potential and displacement current.

Measurements were made for thin film SiO2 multilayer structures with a conductive middle layer charged, using 200 eV and 5 keV electron beams with regular 15 s pulses at 1 nA/cm2 to 500 nA/cm2. Results show that there are two basic charging scenarios. The first scenario, for incident energies between the crossover energies, is characterized by small positive net surface potentials, limited by electron emission and re-attraction of emitted electrons to the positive surface. The second scenario, for incident energies above the second crossover energy, is characterized by larger negative net surface potentials, limited by the conductivity and the breakdown voltage at which the material can no longer sustain electric fields produced by the deposited charge layers. Large negative net surface potentials led to electrostatic breakdown and large visible arcs, which can to lead to detrimental spacecraft charging effects. These measurements are shown to be quantitatively consistent with the simple charging models described above, using previous results of independent electron range, yields and conductivity measurements.

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