Modelling energy deposition in polymethyl methacrylate with low-energy electron irradiation.

Energy deposition in dielectric materials by electron irradiation is important in evaluating irradiation effects in various applications. Herein, we developed a novel Monte Carlo model to calculate the actual distribution of energy deposition in polymethyl methacrylate (PMMA) by simulating low-energy electron transport, including secondary electron cascades. We compared the energy deposition calculated using this model with the distribution of energy loss based on the continuous slowing down approximation (CSDA). The difference in depth distribution between energy deposition and energy loss near the surface is attributed to the secondary electron emission. The characteristics of energy deposition distributions at various incident angles and primary energy were analysed. Energy depositions based on different energy loss mechanisms were classified. Approximately half of the total energy deposition was formed in paths of the secondary cascade at keV-electron irradiation. The temporal properties of energy deposition show that the fast process of energy deposition occurs first near the surface of the dielectric material, then deep inside and 1-keV electrons deposit their energy in 10-14 s.

Influence of surface topography on the secondary electron yield of clean copper samples.

Secondary electron yield (SEY) due to electron impact depends strongly on surface topography. The SEY of copper samples after Ar-ion bombardment is measured in situ in a multifunctional ultrahigh vacuum system. Increasing the ion energy or duration of ion bombardment can even enlarge the SEY, though it is relatively low under moderate bombardment intensity. The results obtained with scanning electron microscopy and atomic force microscopy images demonstrate that many valley structures of original sample surfaces can be smoothed due to ion bombardment, but more hill structures are generated with stronger bombardment intensity. With increasing the surface roughness in the observed range, the maximum SEY decreases from 1.2 to 1.07 at a surface characterized by valleys, while it again increases to 1.33 at a surface spread with hills. This phenomenon indicates that hill and valley structures are respectively effective in increasing and decreasing the SEY. These obtained results thus provide a comprehensive insight into the surface topography influence on the secondary electron emission characteristics in scanning electron microscopy.

Heating-induced variations of secondary electron emission from ion-cleaned copper samples.

Secondary electron (SE) emission due to electron impact depends strongly on surface conditions. The variations of SE yield and spectrum with the heating temperature of Ar-ion-cleaned oxygen-free copper samples are therefore measured in situ in a multifunctional ultrahigh vacuum system. The SE yield and the SE spectrum are observed to increase and to narrow, respectively, after sample heating. The maximum SE yield increases from 0.97 before heating to 1.25 after heating at ∼313 °C, and the corresponding full width at half maximum of SE spectrum decreases considerably from 9.3 to 5.5 eV. More CO2 and Ar ions are shown to desorb at a higher heating temperature by residual gas analysis, indicating their contribution to the reduction in work function and surface potential barrier. Ar-ion desorption appears to affect the SE spectrum more than the SE yield. The obtained results provide a new insight into complicated surface influences on SE emission in thermal applications of scanning electron microscopy.

Note: measuring effects of Ar-ion cleaning on the secondary electron yield of copper due to electron impact.

In a measurement system of total secondary electron yield (SEY) with in situ ion cleaning, we investigate SEY characteristics of the Cu samples cleaned at different Ar-ion energies and cleaning time. Measured SEY data are compared with those before cleaning and simulated with the Monte Carlo method for an ideal surface of copper. We find that weakening the cleaning intensity, i.e., the ion energy or cleaning time, in some circumstances, can further reduce both the maximum SEY and the SEY at the high-energy end (>0.3 keV) of primary electrons, though the SEY is increased somewhat at the low-energy end. Accompanied by the analysis on the opposing contributions of contamination elimination and surface morphology to the SEY, this study thus provides a comprehensive insight into the effects of ion cleaning on the SEY in the investigation and suppression of secondary electron emission.