扫描电镜材料检测方法

SEM 的历史
1938年Ardenne用一个透射电镜(TEM)的光栅电子束第一次 推断了 SEM 1942年, Zworkin等人第一次为块状样品发展了SEM . 1965年，第一台商品SEM. SEM的分辨率被不断地提高从1942 年的50nm到今天的 0.5nm. SEM 检测信号用于 确定成分信息, 如 特征X-射线, 背散射电 子,阴极发光, 俄歇电子和样品电流等。
Always consider Interaction Volume
扫描电子显微术：例子
30 kV 10 kV
5 kV
3 kV
The effect of Accelerating Voltage on SEM
(a) 30 kV x 2,500
(b) 5 kV x 2,500
Specimen: Toner 墨粉 When high accelerating voltage is used as at (a), it is hard to obtain the contrast of the specimen surface structure. Besides, the specimen surface is easily charged up. The surface microstructures are easily seen at (b).
影响SEM分辨率的三大因素：电子束的束斑大小，检测信号的 类型以及检测部位的原子序数
SEM分辨率的测定方法： 在已知的放大倍数（一 般在10万倍）的条件下， 把在图像上测到的最小 间距除以放大倍数所得 数值就是分辨率。
加速电压对 SEM 像的影响
When theoretically considering the electron probe diameter alone, the higher the accelerating voltage, the smaller is the electron probe. However, there are some unnegligible demerits in increasing the accelerating voltage. They are mainly as follows: 1) Lack of detailed structures of specimen surfaces. 2) Remarkable edge effect. 3) Higher possibility of charge-up. 4) Higher possibility of specimen damage. In SEM, finer surface structure images can generally be obtained with lower accelerating voltages. At higher accelerating voltages, the beam penetration and diffusion area become larger, resulting in unnecessary signals (e.g., backscattered electrons) being generated from within the specimen. And these signals reduce the image contrast and veils fine surface structures. It is especially desirable to use low accelerating voltage for observation of low-concentration substances.
第一个扫描电镜装置
剑桥大学，1951年
第一台商用扫描电镜
High Resolution Field Emission SEM
SEM images have a natural 3D look
And now a look inside the SEM….
SEM-Scanning Electron Microscope (or microscopy) TEM- Transmission Electron Microscope AEM- Analytical Electron Microscope STEM- Scanning Transmission Electron Microscope EPMA-Electron Probe MicroAnalyzer SPM-Scanned Probe Microscope (STM, AFM) To see a VIRTUAL SEM, go to the following link: http://www.micro.magnet.fsu.edu/primer/java/electronmicroscopy/magnify1/index.html
(b) 25 kV x7,200 (a) 5 kV x7,200 Fig. 6 Specimen: Sintered powder. At low accelerating voltage, while surface microstructures can be observed, it is difficult to obtain sharp micrographs at high magnifications.
(a) 5 kV x 36,000
(b) 25 kV x 36,000
Specimen: Evaporated Au particles. The image sharpness and resolution are better at the higher accelerating voltage, 25 kV.
SE (secondary electron) imaging
1. High resolution (better than 5nm) is obtainable with most SEM’s 2. Better than 2 nm resolution is possible in some cases 3. 10 nm resolution is very routine (unless the sample limits the resolution, as is often the case)
成像原理 二次电子产额对微区表面的几何形状十分敏感，如图所示，随 入射束与试样表面法线夹角增大，二次电子产额增大。 因为电子束穿入样品激发二次电子的有效深度增加了，使表面 5-10 nm作用体积内逸出表面的二次电子数量增多。
根据上述原理画出二次 电子形貌衬度的示意图
对于实际样品，表面形 貌要比上面衬度的情况 复杂得多，但形成二次 电子衬度的原理是相同 的。
(a) 5 kV x 1,400
(b) 25 kV x 1,400
Specimen: Filter paper. At 5 kV, the microstructures of the specimen surface are clearly seen as the penetration and diffusion area of incident electrons is shallow.
(a) 5 kV x2,200 (b) 25 kV x2,200 Specimen: Paint coat. When a high accelerating voltage is used, more scattered electrons are produced from the constituent substances within the specimen. This not only eliminates the contrast of surface microstructures, but produces a different contrast due to backscattered electrons from the substances within the specimen.
Edge effect (secondary electron emission differing with surface condition).
Among the contrast factors for secondary electrons, the tilt effect and edge effect are both due to the specimen surface morphology. Secondary electron emission from the specimen surface depends largely on the probe’s incident angle on the specimen surface, and the higher the angle, the larger emission is caused. The objects of the SEM generally have uneven surfaces. There are many slants all over them, which contribute most to the contrast of secondary electron images. On the other hand, large quantities of secondary electrons are generated from the protrusions and the circumferences of objects on the specimen surface, causing them to appear brighter than even portions.
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二次电子： Baidu Nhomakorabea入射电子作用下被轰击出来并离开样品表面的样品原子 的核外电子。这也是一种真空自由电子 。 由于原子核和外层价电子间的 结合能很小，因此，外层的电子较容易和原子脱离，使原子电离。用IS表示 二次电子流。 一个能量很高的入射电子射入样品时，可以产生许多自由电子。其中 90%来自于外层价电子。 特 征： 1)二次电子能量较低。一般不超过50 ev，大部分几ev； 2)来自表层5—10nm深度范围； 3)对样品表面化状态十分敏感，因此能有效地反映样品表面的形貌； 4)其产额与原子序数间没有明显的依赖关系。因此，不能进行成分分析。
SEM 的构造
气动保险阀
扫描线圈
Signals available from SEM
Signals
SEM的主要性能：
1，分辨率
二次电子和俄歇电子的分辨率高，特征X射线 调制成显微图像的分辨率最低。 电子束进入轻元素样品 电子束进入重元素样品 滴状作用体积 半球状作用体积
SEM的分辨率即二次电子相的分辨率 电子束进入重元素样品后，立即向横向扩展， 因此在分析重元素时，即使电子束的束斑很 细小，也不能达到较高的分辨率，此时二次 电子和背散射电子之间的分辨率的差距明显 变小。
实际样品中二次电子的激发过程示意图
1)凸出的尖棱，小粒子以及比较陡的斜面处SE产额较多，在 荧光屏上这部分的亮度较大 2)平面上的SE产额较小，亮度较低。 3)在深的凹槽底部尽管能产生较多二次电子，使其不易被控 制到，因此相应衬度也较暗。
Influence of edge effect on image quality
扫描电子显微术 Scanning Electron Microscopy
电子显微的一个粗略年表
1930年末：第一台透射电子显微镜 1935年：德国的Knoll提出了扫描电 镜的概念(STEM的概念, 100mm 分 辩率) 1938年：Von Ardenne开始进行实 验室研究，SEM--50 mm分辨率 1942：Zworykin. Hillier, 制成了第 一台实验室用的扫描电镜 - 今天扫描电镜的全部基本原理 - 50 nm 分辨率 - 问题: 贵, 照相（曝光）时间长, 电子干扰（噪声） 结论: SEM 不实用! 1948：C.W. Oatley, 剑桥大学