Charging regime of PMMA studied by secondary electron emission

Foils of Polymethyl methacrylate (PMMA) 2 mm thick were studied by measuring the total Secondary Electron Emission yield sigma (SEE yield) in a dedicated Scanning Electron Microscope especially equipped to study the fundamental aspects of the charge transport and trapping in insulating materials. The intrinsic SEE yield sigma(0), (yield of the uncharged material) and the charging kinetics were studied under low current density J = 10(+5) pA/cm(2). The curve of the primary beam energy variation of sigma(0) exhibits a maximum intrinsic yield sigma(0max) = 2.2 at 370 +/- 20 eV and two crossover energies E-I = 84 +/- 20 eV and E-II = 1465 +/- 20 eV for which sigma(0) = 1. For sigma(0) > 1 PMMA is positively charged and negatively for sigma(0) < 1. As electron injection is proceeding under the low current density used, the SEE yield varies from sigma(0) to the steady value sigma(st) = 1. This value that expresses the equality between the average number of emitted and injected electrons, characterizes the steady charge regime called "Self-Regulated Regime". The evolution of sigma during the injection process is due to the internal field that blocks or enhances the secondary electron emission, according to the positive or negative nature of the trapped charges. A current density effect, characterized by a steady SEE yield slightly higher than unity, sigma(st) = 1.03, instead of one, is observed at high energy (for example 4000 eV) for a strong current density J > 10(+6) pA/cm(2). It is interpreted by a field ionisation (Poole-Frenkel type) that enhances the secondary electron emission.