绝缘膜负带电时的表面局部电场与二次电子返回特性

为分析IC多层版图扫描电镜(SEM)对准检测所利用的负带电成像原理,采用Mott弹性散射截面和修正的Bethe非弹性碰撞公式,对点照射入射电子在绝缘膜中的散射过程进行了Monte Carlo模拟,得到负带电绝缘物表面的局部电位分布.在此基础上计算了二次电子从表面出射后在局部场作用下的运动轨迹,获得了SEM像的二次电子信号电流.结果表明,在弱负带电条件下,照射点处表面电位越低,返回表面的二次电子就越多,对应的二次电子信号电流越弱.此结果与SEM实验中图像亮度随照射时间的变化规律相符

Local Field of Surface and Redistribution Properties of Secondary Electrons Emitted from Insulating Thin Film with Negative Charge

To investigate the principle of imaging for the marker below an insulating thin film charged weak-negatively with a scanning electron microscope (SEM) in the updated integrated circuit overlay measurement,a Monte Carlo simulation of electron scattering in the insulating thin film is performed first.The Mott cross section and modified Bethe formula are used for calculating elastic and inelastic scattering processes,respectively.The potential profile of the local field near the irradiating area is then calculated from the spatial distribution of holes and electrons.Trajectories of secondary electrons (SE) emitted from the film surface are numerically simulated,and the SE signal current that produces the SEM image contrast is obtained.It is shown that the SE signal current decreases with surface potential,which is in good agreement with the experimental variation of the SEM image brightness with irradiating time.