Zobacová J, Frank L
Institute of Scientific Instruments AS CR, Brno, Czech Republic.
Scanning. 2003 May-Jun;25(3):150-6. doi: 10.1002/sca.4950250307.
This paper concerns the problems connected with the observation of a nonconductive specimen in a scanning electron microscope (SEM) when incident electrons create a surface charge and a corresponding electric field. The special configuration of the cathode lens enables one to control the landing energy of primary electrons via the specimen bias. In the cathode lens, the accelerating electric field at the surface of the specimen combines itself with that of the surface charge in influencing the trajectories of the signal electrons and hence the detected signal level and the possible recapturing of slow secondaries. Recaptured electrons reduce the ultimate positive surface potential, which arises when working below the higher critical energy of electron impact. Computer simulations of electron trajectories were performed for the typical cathode lens configuration and for a model specimen characterized by emission yields similar to those for glass. The simulations brought an extensive set of data about the trajectories of both secondary and backscattered electrons. Furthermore, the data were processed in order to assess the charge balance between the emitted and recaptured electrons as well as the collection efficiency of the detector. The results include values of the ultimate positive surface potential and the detected signal level, both in dependence on the initial energy of the electron impact and the size of the field of view. Finally, the method for the determination of critical energy is reevaluated. This is based on the measurement of the time dependence of the detected signal.
本文关注的是在扫描电子显微镜（SEM）中观察非导电样品时出现的问题，即入射电子会产生表面电荷和相应的电场。阴极透镜的特殊结构使得能够通过样品偏压来控制一次电子的着陆能量。在阴极透镜中，样品表面的加速电场与表面电荷的电场相结合，影响信号电子的轨迹，进而影响检测到的信号水平以及慢速二次电子的可能重新捕获。重新捕获的电子会降低最终的正表面电位，该电位是在低于电子撞击的较高临界能量下工作时产生的。针对典型的阴极透镜结构以及一个发射产率与玻璃相似的模型样品，进行了电子轨迹的计算机模拟。模拟得出了关于二次电子和背散射电子轨迹的大量数据。此外，对这些数据进行了处理，以评估发射和重新捕获电子之间的电荷平衡以及探测器的收集效率。结果包括最终正表面电位的值和检测到的信号水平，二者均取决于电子撞击的初始能量和视场大小。最后，对确定临界能量的方法进行了重新评估。这是基于对检测信号的时间依赖性的测量。
