电子辐照聚合物带电特性多参数共同作用的数值模拟

电子辐照聚合物样品的带电特性是扫描电子显微镜成像、电子束探针微分析以及空间器件辐照效应等领域的一个重要研究课题. 通过建立基于蒙特卡罗方法的电子散射和时域有限差分法的电子输运的数值模型, 并采用高效的多线程并行计算, 模拟了电子非透射辐照聚合物样品的带电特性, 得到了带电稳态下的样品底部泄漏电流密度、表面负电位以及样品总电荷密度等带电特征量受入射电子能量、入射电流密度、样品材料的电子迁移率、样品厚度等相关参数共同作用的影响. 结果表明, 一个参数的变化使表面负电位增强时, 其他参数对负电位的影响将增强. 样品的带电稳态特征量在同一个电流平衡的模式下受参数影响的变化是单调的. 当电流平衡模式发生变化时, 如在入射电子能量较低的条件下, 样品内部的总电荷量会随着样品厚度的增大而先增加后减小, 出现局部极大值. 样品底部的泄漏电流密度随着入射电流密度的增大而近线性成比例地增大. 研究结果对于揭示电子辐照聚合物的带电规律及微观机理、预测不同条件下的样品带电状态具有重要科学意义.

Numerical simulation of multi-combined effects of parameters on polymer charging characteristics due to electron irradiation

Charging characteristics of an insulator specimen due to non-penetrated electron irradiation have been attracting a great deal of attention in the fields such as scanning electron microscopy, electron probe analysis, and space irradiation. In this paper, we use a numerical simulation model based on Monte Carlo method for investigating the electron scattering. The elastic scattering is calculated with the Mott cross-section, and the inelastic scattering is simulated with Penn model and the fast secondary electron model according to the primary energy. The charge transport caused by the build-in electric field and charge density gradient is calculated with finite-difference time-domain method. Multi-combined effect of correlative parameters on charging characteristics is investigated by efficient multithreading parallel computing. During the irradiation, the landing energy of primary electrons decreases due to the negative surface potential, which makes the secondary electron yield increase. Variations of secondary electron current and sample current are presented to verify the validity of the simulation model by comparing with existing experimental results. Evolutions of leakage current, surface potential and internal space charge quantity are calculated under different conditions of incident electron current, primary energy and sample thickness. The results are presented in contour maps with different multi-parameter combinations, primary energy and sample mobility, primary energy and sample thickness, and primary energy and incident current. The balance state of charging will be determined by leakage current under conditions of a larger primary energy, sample mobility, incident current, or a less sample thickness, which is shown as the leakage current dominated mode. While in the cases of a lower primary energy, sample mobility, incident current, or a larger sample thickness, the balance state of charging is mainly dominated by secondary electron current, as the secondary electron current dominated mode. In other cases except the above two, the balance state will be determined by both leakage and secondary currents as the mixture mode. In the same mode, variations of charging characteristics with parameters are monotonic. When the change of a parameter makes the negative surface potential increase, the effect of this parameter on negative surface potential will be weakened, while the effects of other parameters on the negative potential will be enhanced. With the change of current dominated mode, the total charge quantity exhibits the local maximum with respect to the sample thickness, and the value of this maximum increases with primary energy. Moreover, the leakage current increases with incident current linearly. The presented results can be helpful for understanding regularities and mechanisms of charging due to electron irradiation, and estimating the charging intensity under different conditions of irradiation and sample material.