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Issue Eur. Phys. J. Appl. Phys.
Volume 37, Number 1, January 2007
Page(s) 79 - 86
Section Characterization of Materials: Imaging,\ Microscopy and Spectroscopy
DOI 10.1051/epjap:2006128
Published online 08 November 2006

Eur. Phys. J. Appl. Phys. 37, 79-86 (2007)
DOI: 10.1051/epjap:2006128

Charging regime of PMMA studied by secondary electron emission

M. Boubaya and G. Blaise

Laboratoire de Physique des Solides, Bât. 510, Université Paris XI, 91405 Orsay, France

blaise@lps.u-psud.fr

(Received: 3 May 2005 / Received in final form: 18 May 2006 / Accepted: 13 September 2006 / Published online: 8 November 2006 )

Abstract
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/cm2. The curve of the primary beam energy variation of $\sigma _{0}$ exhibits a maximum intrinsic yield $\sigma _{\rm0max}= 2.2$ at 370 $\pm $ 20 eV and two crossover energies EI=84 $\pm $ 20 eV and EII =1465 $\pm $ 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/cm2. It is interpreted by a field ionisation (Poole-Frenkel type) that enhances the secondary electron emission.

PACS
79.20.Hx - Electron impact: secondary emission.
68.37.Hk - Scanning electron microscopy (SEM) (including EBIC).
61.80.Fe - Electron and positron radiation effects.

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