电子束照射集成电路样品时的表面电位均匀化机理/Homogenization Mechanism of the Surface Potential of IC S

2018-08-11 08:50:02

equilibrium potential surface 电位 二次电子



均匀的表面电位对集成电路样品的观测具有十分重要的意义。残留表面电位会在后续观测中引入误差,影响临界尺度扫描电镜法的测量精度,并在电子束光刻中造成位置误差。为了使表面电位均匀化,一个有效的方法是使用正带电电子束扫描样品表面。然而这种均匀化方法的物理机制及影响平衡电位的实际因素至今尚未明确。
本论文首先推导了平衡电位的解析表达式。在引入正表面电位下二次电子返回率后,建立了表面电位平衡所需的入射电子,出射二次电子和返回二次电子的电荷平衡条件。利用解析二次电子能量分布,得到了二次电子返回率和平衡电位间的关系,从而导出了平衡电位的解析表达式。该解析表达式表明,平衡电位值会随二次电子产额和二次电子的最可几出射能量而增大。
其次,在一个假设的残留表面电位分布下计算了二次电子轨迹,给出了残留表面电位均匀化过程的微观机制。结果显示二次电子的离开或返回改变了进入样品的净电荷流量,从而使低于平衡电位的表面电位上升,高于平衡电位的表面电位下降,实现表面电位的均匀化。
最后,基于一个大规模集成电路多层结构模型模拟了电子束照射下残留表面电位均匀化的动态过程。为了简化模拟算法,分别使用镜像法和格林函数法推导了模型的表面电位。对于均匀介质,模拟得到的平衡电位和平衡电位的解析结果一致,从而证明平衡电位表达式的正确性。对于不均匀介质,模拟结果证实了二次电子产额的波动会对平衡电位造成扰动。
本论文的研究结果对于深入了解电子束与样品相互作用和电子束照射下的残留表面电位均匀化方法的实用化具有学术意义和应用价值。

The homogenization of surface potential is important for the observation of IC specimens with a scanning electron microscope (SEM). The residual surface potential may introduce errors in following observations, affect measurement accuracy in critical dimension SEM and cause pattern placement errors in electron beam lithography. To homogenize the surface potential, an effective method is to scan the specimen with an electron beam (e-beam) that makes the specimen positively charged. However, the homogenization mechanism and practical factors that influence the equilibrium potential remains obscure.

This dissertation firstly deduces the analytical expression of the equilibrium potential. By introducing the secondary electron (SE) redistribution rate, the equilibrium condition of the surface potential is established, i.e. the charge balance condition among incident electrons, emitted SEs and redistributed SEs. The relationship between the SE redistribution rate and the equilibrium potential is obtained with an analytical SE energy distribution. Consequently, the expression of the equilibrium potential is obtained and the potential value is found to increase with the SE yield and most probable emission energy of SEs.

Secondly, SE trajectories are calculated with a given residual surface potential model to clarify the micromechanism of the homogenization process. Results show that SEs leave or return to the specimen surface to change the net charge flux into the specimen. This thereby increases the surface potential below the equilibrium potential and decreases that above the equilibrium potential, homogenizing the surface potential.

Thirdly, the dynamic homogenization process under e-beam irradiation is simulated with a multilayer LSI model. To simplify the simulation algorithm, the surface potential of the multilayer model is derived with the image method and the Green’s function method respectively. For a homogeneous specimen, the simulated equilibrium potential agrees with the derived expression of the equilibrium potential, which confirms the validity of the derived expression. For a heterogeneous specimen, the simulated result shows that the equilibrium potential is disturbed because of the fluctuation of the SE yield.

The results of this dissertation have scientific and practical significance for deep understanding of the interaction of e-beam with specimens and the homogenization of the residual surface potential under e-beam irradiation.