'Collapsing rings' on Schottky electron emitters
'Collapsing rings' on Schottky electron emitters
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DOI：
10.1016/j.ultramic.2010.05.004
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发表时间：
2010-08-01
期刊：
影响因子：
2.2
通讯作者：
Kruit, P.
中科院分区：
文献类型：
--
作者：
Bronsgeest, M. S.;Kruit, P.
当发射极以提取电压在提取电压下操作时，使用Schottky发射极作为电子源的系统中的电子光束可以显示周期性波动，该发射器在尖端时具有相对较低的磁场强度。过去，这些波动与所谓的“崩溃环”有关，没有更多的信息。在本文中，通过记录schottky发射极的发射模式的演变，显示出在不同的操作条件下显示“崩溃的戒指”，从而更详细地研究了与这些光束不稳定性相关的尖端的几何形状变化。扫描电子显微镜（SEM）的不同Schottky发射器的图像已用于支持解释。束的不稳定性通常大部分时间发生。但是，发现发射极正在连续地和重复过程中更改其尖端末端几何形状。这样的循环以{1 0 0}的刻度末端开始。该方面的大小减小，然后形成一个大环形步骤，将尖端末端几何形状变成一个{1 0 0}高的纳米高的岛，在{1 0 0 0}刻面的顶部。当岛上的原子全部被运输到完全暴露的面时，周期就完成了。发现岛上的清洁本身是一个重复的过程，每次（最高）岛减少时大小，然后分成两个彼此上面的岛屿。我们发现，在实际的工作条件下，堆叠岛的几何形状通常是不对称的，并且仅在较高温度和较低的场上成为对称性。特征光束电流波动与岛边缘的波动有关光束。波动之间的相对恒定的光束电流是两个效应的净结果：刻面中心的电流密度随着局部强度的增加而增加，而小平面直径降低，但面部和萃取器之间的梁差异也增加，也增加了。由于几何形状的变化。尽管在波动之间的相对恒定的光束电流隐藏，但对于折叠发射极的折叠，随着亮度，能量传播和虚拟源的重要特性正在连续变化。针对梁不稳定性的已知补救措施是增加提取电压，但这是这样。通常是反应性响应：发生波动后电压增加。这项研究表明，通过监测尖端的总尖端端电流和/或场增强因子，可以尽早检测到不稳定的方面尺寸减小，以防止其发生。 （c）2010 Elsevier B.V.保留所有权利。
The electron beam in systems that use a Schottky emitter as the electron source can display periodic fluctuations when the emitter is operated at an extraction voltage that gives a relatively low field strength at the tip. In the past, these fluctuations have been associated with the so-called "collapsing rings" without much further information. In this paper, the tip's geometry changes associated with these beam instabilities are investigated in more detail by recording the evolution of the emission pattern of a Schottky emitter showing 'collapsing rings' for different operating conditions. Scanning electron microscope (SEM) images of different Schottky emitters have been used to support the interpretation.The beam instabilities generally occur with large intervals. It was found, however, that the emitter is changing its tip end geometry continuously and in a repetitive process. Such a cycle starts with a {1 0 0} facet at the tip end. This facet decreases in size, and then a large ring-shaped step is formed that changes the tip end geometry into one with a {1 0 0} island of tens of nanometers high, on top of a {1 0 0} facet. The cycle is finished when the atoms of the island all have been transported away and the facet underneath is fully exposed.The cleaning up of the island was found to be a repetitive process in itself, in which each time the (uppermost) island is reduced in size, and then splits into two islands lying on top of each other. We found that at practical operating conditions, the geometry of the stacked islands is typically asymmetric, and becomes symmetric only for higher temperatures and lower fields.The characteristic beam current fluctuations are associated with island edges moving near or in the area that delivers the electrons for the beam. The relatively constant beam current in between fluctuations is the net result of the two effects: the current density on the facet center increases due to the increasing local field strength with decreasing facet diameter, but the beam divergence between the facet and the extractor also increases, as a consequence of the changing geometry. Although concealed by the relatively constant beam current in between the fluctuations, for a collapsing emitter the important properties as the brightness, energy spread, and virtual source are thus changing continuously.The known remedy against beam instabilities is to increase the extraction voltage, but this is usually a reactive response: the voltage is increased after a fluctuation has occurred. This study suggests that by monitoring the total tip end current and/or the field enhancement factor of the tip, the facet size reduction that heralds an instability can be detected early enough to prevent it from happening. (C) 2010 Elsevier B.V. All rights reserved.