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1、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,SEM 的历史,1938年Ardenne用一个透射电镜(TEM)的光栅电
2、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。,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 Micr
3、oscope(or microscopy)TEM-Transmission Electron MicroscopeAEM-Analytical Electron MicroscopeSTEM-Scanning Transmission Electron MicroscopeEPMA-Electron Probe MicroAnalyzerSPM-Scanned Probe Microscope(STM,AFM)To see a VIRTUAL SEM,go to the following link:,SEM 的构造,扫描线圈,Signals available from SEM,Signal
4、s,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 滴状作用体积电子束进入重元素样品 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。,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。,When theoret
5、ically 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 ed
6、ge 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(
7、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.,加速电压对 SEM 像的影响,Always consider Interaction V
8、olume,扫描电子显微术：例子,The effect of Accelerating Voltage on SEM Images,30 kV,10 kV,5 kV,3 kV,Specimen:Toner 墨粉When high accelerating voltage is used as at(a),itis hard to obtain the contrast of the specimen surfacestructure.Besides,the specimen surface is easilycharged up.The surface microstructures are
9、easilyseen at(b).,(a)30 kV x 2,500,(b)5 kV x 2,500,Specimen:Evaporated Au particles.The image sharpness and resolution are betterat the higher accelerating voltage,25 kV.,(a)5 kV x 36,000,(b)25 kV x 36,000,Specimen:Filter paper.At 5 kV,the microstructures of the specimensurface are clearly seen as t
10、he penetration anddiffusion area of incident electrons is shallow.,(a)5 kV x 1,400,(b)25 kV x 1,400,Fig.6 Specimen:Sintered powder.At low accelerating voltage,while surface microstructurescan be observed,it is difficult to obtain sharp micrographsat high magnifications.,(a)5 kV x7,200,(b)25 kV x7,20
11、0,Specimen:Paint coat.When a high accelerating voltage is used,more scatteredelectrons are produced from the constituent substanceswithin the specimen.This not only eliminates the contrastof surface microstructures,but produces a differentcontrast due to backscattered electrons from thesubstances wi
12、thin the specimen.,(a)5 kV x2,200,(b)25 kV x2,200,SE(secondary electron)imaging,High resolution(better than 5nm)is obtainable with most SEMsBetter than 2 nm resolution is possible in some cases10 nm resolution is very routine(unless the sample limits the resolution,as is often the case),二次电子：在入射电子作用
13、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510nm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。,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
14、。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。,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。,Edge effect(secondary electron emission differing with surface condition).,Influence of edge effect on image quality,Among the
15、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 probes incident angle on the specimen surface,and the higher the angle,the larger emission is caused.T
16、he 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 s
17、urface,causing them to appear brighter than even portions.,Specimen IC chip.The higher the accelerating voltage,the greater is theedge effect,making the edges brighter.,Influence of edge effect on image quality,The degree of the edge effect depends on the accelerating voltage.Namely,the lower the ac
18、celerating voltage,the smaller the penetration depth of incident electrons into the specimen.This reduces bright edge portions,thus resulting in the microstructures present in them being seen more clearly.Normally,secondary electron images contain some backscattered electron signals.Therefore,if the
19、 tilt direction of the specimen surface and the position of the secondary electron detector are geometrically in agreement with each other,more backscattered electrons from the tilted portions are mixed,causing them to be seen more brightly due to synergism.,(a)5 kV x720 Tilt Angle:50,(b)25 kV x720
20、Tilt Angle:50,Specimen:IC chip.5 kV x1,100The sides of patterns are viewed by tilting the specimen.The amount of signals is increased.,Use of specimen tilt：a)Dependence of image quality on tilt angle,1)Improving the quality of secondary electron images;2)Obtaining information different form that obt
21、ained when the specimen is not tilted,that is,observing topographic features and observing specimen sides.3)Obtaining stereo micrographs.Fig.13 shows a photo taken at a tilt angle of 0(a)and a photo taken at 45(b).Their comparison shows that the latter is of smooth quality and stereoscopic as compar
22、ed with the former.When the specimen is tilted,however lengths observed are different from their actual values.When measuring pattern widths,etc.,therefore,it is necessary to measure without specimen tilting or to correct values obtained form a tilted state.,(a)Tilt angle:0,(b)Tilt angle:45,Specimen
23、:Back sides of oleaster leaves.Moreinformation is obtained from stereo-pair photos.,Use of specimen tilt：b)Stereo micrographs,With SEM images it is sometimes difficult to correctly judge their topographical features.In such a case observation of stereo SEM images makes it easy to understand the stru
24、cture of the specimen.Besides,stereo observation allows unexpected information to be obtained even from specimens of simple structure.In stereo observation,after a field of interest is photographed,the same field is photographed again with the specimen tilted from 5 to 15.Viewing these two photos us
25、ing stereo glasses with the tilting axis held vertically provides a stereo image.,Use of specimen tilt：c)Detector position and specimen direction,The amount of secondary electrons produced when the specimen is illuminated with an electron beam,depends on the angle of incidence theoretically.However,
26、there arises a difference in the image brightness depending on whether the tilted side of the specimen is directed to the secondary electron detector or the opposite side.With a long specimen,for example,the brightness differs between the side facing the detector and the opposite side.In such a case
27、,directing the longitudinal axis of the specimen to the detector makes the brightness uniform.,Fig.16 Specimen:FiberDetector position and specimen direction.,Specimen:Fiber7kV x2,200Directing the longitudinal axis of the specimen to the secondary electron detector makes the right and left sides equa
28、lly bright.(An SRT unit is used to direct the image longitudinally.),Use of specimen tilt：c)Detector position and specimen direction,(a)Specimen directed as at 1,(b)Specimen directed as at 2,(c)Specimen directed as at 3,Backscattered electrons vary in their amount and direction with the composition,
29、surface topography,crystallinity and magnetism of the specimen.The contrast of a backscattered electron image depends on(1)the backscattered electron generation rate that depends on the mean atomic number of the specimen,(2)angle dependence of backscattered electrons at the specimen surface,and(3)th
30、e change in the backscattered electron intensity when the electron probes incident angle upon a crystalline specimen is changed.,Use of backscattered electron signals,背散射电子：背散射电子是指被固体样品中的原子核或核外电子反弹回来的一部分入射电子。用Ib示背散射电子流。弹性背散射电子：一般样品表面原子核反弹回来可达数千至数万ev。非弹性背散射电子：电子在固体中经过一系列散射后最终由原子核反弹的或由核外电子产生的，不仅方向改变，能
31、量也有不同程度的损失。其能量分布范围很宽，数十ev至数千ev。特征：1)弹性背散射电子远比非弹性背散射电子所占的份额多；2)能量高，例如弹性背散射，能量达数千至数万ev；3)背散射电子束来自样品表面几百nm深度范围；4)其产额随原子序数增大而增多；5)用作形貌分析、成分分析（原子序数衬度）以及结构分析（通道花样）。,Retractable BSE detector in chamber,Objectivelens,Secondary detector(ETD),BSE paths and detector locations,Objectivelens,BSE from adjacent re
32、gions on flat sample,SEM Compositional image,Backscattered SEM image of an PbSn alloy showing contrast based on the atomic number.The brighter areas are Pb-rich.,1 背散射电子的衬度原理及应用 背散射电子用于：1）形貌分析来自样品表层几百nm范围2）成分分析产额与原子序数有关3）晶体结构分析基于通道花样衬度1BE形貌衬度特点1）用BE进行形貌分析时，其分辨率远比SE像低。2）图象衬度很强，衬度太大会失去细节的层次。下图是检测器接收BE
33、和SE的情况BE能量高，以直线轨迹逸出样品表面，对于背向检测器的样品表面，因检测器无法收集到BE而变成一片阴影，因此，其图象衬度很强，衬度太大会失去细节的层次，不利于分析。因此，BE形貌分析效果远不及SE，故一般不用BE信号,2BE原子序数衬度原理 原子序数Z与背散射电子产额的关系如图。E40，对E十分敏感。进行分析时，样品上原子序数较高的区域中由于收集到的背散射电子数量较多，故在荧光屏上的图象较亮。因此，利用原子序数造成的衬度变化可以对各种金属和合金进行定性的成分分析。需注意以下二点：1）用BE进行成分分析时，为避免形貌衬度对原子序数衬度的干扰，被分析的样品只进行抛光，而不必腐蚀。2）既进行
34、成分分析又要进行形貌分析时，可采用对称分布的检测器收集信号（同一部位的BE），然后计算机处理分别得到形貌信号和成分信号。如图所示。a）成分有差别，形貌无差别b）形貌有差别，成分无差别c）成分形貌都有差别 相加为成分像相减为形貌相,The backscattered electron image contains two types of information:one on specimen composition and the other on specimentopography.To separate these two types of information,a paired se
35、miconductor detector is provided symmetrically with respect to the optical axis.Addition of them gives a composition image while subtraction gives a topography image.And with composition images of crystalline specimens,the difference in crystal orientation can be obtained as the so-called“channeling
36、 contrast,”by utilizing the advantage that the backscattered electron intensity changes largely before and after Braggs condition.,Use of backscattered electron signals,Principles of composition image and topography image,SE versu BSE images of alloy,Objectivelens,Cu/Zn Alloy,SE(left),BSE(right).0.1
37、 Atomic Number Difference,.6.2 吸收电子成像的衬度原理及应用 如前所述，吸收电子产生的电流 当样品很厚，It=0，则：所以，吸收电子象的衬度是与背散射电子和二次电子像的衬度是互补的。实际上，SE信号与原子序数没有明显关系，因此，可以认为，背散射电子像和吸收电子像衬度正好相反。,The generation region of backscattered electrons is larger than that of secondary electrons,namely,several tens of nm.Therefore,backscattered elec
38、trons give poorer spatial resolution than secondary electrons.But because they have a larger energy than secondary electrons,they are less influenced by charge-up and specimen contamination.,Use of backscattered electron signals,TOPO,X-ray(Si),X-ray(Al),BEI,SEI,COMPO,Specimen:Slug20kV x1,100The back
39、scattered electron image is important also as a supplementary means for x-ray analysis.,Influence of charge-up on image quality,Specimen:Resist.Charge-up can be prevented by properly selecting the accelerating voltage.,(a)1.0 kV x3,200,(a)4 kV,(b)10 kV,(b)1.3 kV x3,200,Specimen:Foreleg of vinegar fl
40、y.Charge-up can be reduced by using low accelerating voltage.,Specimen damage by electron beam,Specimen:Compound eye of fly.5 kV x1,00When a specimen area is irradiated with an electron probe for a long time at high magnification,it may be damaged as shown in(b).,(b),(a),Contamination,When the elect
41、ron probe is irradiated on a specimen portion fora long time,its image may lose sharpness and become dark.Thisis caused by the residual gas in the vicinity of the specimen beingstruck by the electron probe.This phenomenon is called specimencontamination.The conceivable residual gases in the specimen
42、 chamber,whichcause contamination are:1)Gas caused from the instrument itself.2)Gas that specimens bring into the instrument3)Gas that the specimen itself gives off.,Specimen:ITO.A x18,000 photo taken after a long-time electron probe scanning at x36,000.As compared with the clear image of peripheral
43、 region,the middle region shows reduced contrast and lacks image sharpness.5 kV x18,000,电子探针显微分析,X-ray Microanalysis,Comparison of Energy Dispersive Spectroscopy EDS能谱仪 and Wavelength Dispersive Spectroscopy spectra WDS波谱仪,能谱仪成分分析的特点:优点:1,探测X射线的效率高，(探头与样品近,不必通过分光晶体衍射)能谱仪的灵敏度比波谱仪高一个数量级。2,可在同一时间内对分析点内
44、所有元素x射线光子的能量进行测定和计数，几分钟内可得到定性分析结果,而波谱仪只能逐个逐个测量每种元素的特征波长。3,结构简单，没有机械传动部分，因此稳定性和重复性都很好。4,不必聚焦，对样品表面没有特殊要求，适合于粗糙表面的分析工作。缺点:1，分辨率低于波谱仪。EDS:160eV WDS:510eV2，只能测试Z大于11的元素,而波谱仪可测Z从4到92间的所有元素。EDS里Si(Li)探测器的铍窗口限制3，Si(Li)探测器必须要液氮冷却。,电子探针的主要功能就是进行微区成分分析。它是在电子光学和X射线光谱学原理的基础上发展起来的一种高效率分析仪器。其原理是：用细聚焦电子束入射样品表面，激发出
45、样品元素的特征X射线，分析特征X射线的波长（或能量）可知元素种类；分析特征X射线的强度可知元素的含量。其镜筒部分构造和SEM相同，检测部分使用X射线谱仪，用来检测X射线的特征波长（波谱仪）和特征能量（能谱仪），以此对微区进行化学成分分析。,EPMA的结构和工作原理：,电子探针的信号检测系统是X射线谱仪,6.2.2 能量分散谱仪（能谱仪EDS）1.工作原理 利用不同元素X射线光子特征能量不同特点进行成分分析 锂漂移硅能谱仪Si(Li)框图,X-ray Microanalysis,Overview of EDS system,电子绝缘体,场效应晶体管,能谱仪成分分析的特点:优点:1,探测X射线的效
46、率高，(探头与样品近,不必通过分光晶体衍射)能谱仪的灵敏度比波谱仪高一个数量级。2,可在同一时间内对分析点内所有元素x射线光子的能量进行测定和计数，几分钟内可得到定性分析结果,而波谱仪只能逐个逐个测量每种元素的特征波长。3,结构简单，没有机械传动部分，因此稳定性和重复性都很好。4,不必聚焦，对样品表面没有特殊要求，适合于粗糙表面的分析工作。缺点:1，分辨率低于波谱仪。EDS:160eV WDS:510eV2，只能测试Z大于11的元素,而波谱仪可测Z从4到92间的所有元素。EDS里Si(Li)探测器的铍窗口限制3，Si(Li)探测器必须要液氮冷却。,EDS X-ray spectrum of a
47、 sintered WC in a Ni matrix showing the presence of W,Ni,Cr and Co.,250 cpsFullScale,波谱仪,1)已知电子束入射样品表面产生的X射线是在样品表面下一个um量级乃至纳米量级的作用体积发出的，若该体积内含有各种元素，则可激发出各个相应元素的特征X线，沿各向发出，成为点光源。2)在样品上方放置分光晶体，当入射X波长l、入射角q、分光晶体面间距d之间满足2dsin=时，该波长将发生衍射，若在其衍射方向安装探测器，便可记录下来。由此，可将样品作用体积内不同波长的X射线分散并展示出来。上述平面分光晶体使谱仪的检测效率非常低
48、，表现在：固定波长下，特定方向入射才可衍射 处处衍射条件不同要解决的问题是：分光晶体表面处处满足同样的衍射条件 实现衍射束聚焦,解决的办法是：把分光晶体作适当的弹性弯曲，并使X射线源、弯曲晶体表面和检测器窗口位于同一个园周上，就可以达到把衍射束聚焦的目的。该园称为聚焦园，半径为R。此时，如果晶体的位置固定，整个分光晶体只收集一种波长的X射线，从而使这种单色X射线的衍射强度大大提高。X射线聚焦方式有两种：1)Johann 型聚焦法：弯曲单晶的衍射晶面的曲率半径为2R。近似聚焦方式。2)Johansson型聚焦法：衍射晶面表面的曲率半径为R，即晶体表面磨制成和聚焦园相合。全聚焦法显然，只要改变晶体在聚焦园的位置，即可改变入射角，从而可探测不同波长的X线。,定性分析点分析,线分析,面分析,SEM X-ray Maps of W,Ni and Cr,20 micronMaps,
