扫描电子显微术：例子

1、扫描电子显微术：例子The effect of Accelerating Voltage on SEM Images 30 kV10 kV 5 kV 3 kV2021/9/121Specimen: 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 microstructur

2、es are easilyseen at (b).(a) 30 kV x 2,500(b) 5 kV x 2,5002021/9/122Specimen: 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 36,0002021/9/123Specimen: Filter paper.At 5 kV, the microstructures of the specime

3、nsurface are clearly seen as the penetration anddiffusion area of incident electrons is shallow.(a) 5 kV x 1,400(b) 25 kV x 1,4002021/9/124Fig. 6 Specimen: Sintered powder.At low accelerating voltage, while surface microstructurescan be observed, it is difficult to obtain sharp micrographsat high ma

4、gnifications. In such a case, clear images canbe obtained by shortening the WD or reducing the electronprobe diameter.(a) 5 kV x7,200(b) 25 kV x7,2002021/9/125Specimen: Paint coat.When a high accelerating voltage is used, more scatteredelectrons are produced from the constituent substanceswithin the

5、 specimen. This not only eliminates the contrastof surface microstructures, but produces a differentcontrast due to backscattered electrons from thesubstances within the specimen.(a) 5 kV x2,200(b) 25 kV x2,2002021/9/126SE (secondary electron) imagingHigh resolution (better than 5nm) is obtainable w

6、ith 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)2021/9/127Edge effect (secondary electron emission differing with surface condition).Influence of edge effect on image qualityAmong the co

7、ntrast 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

8、. 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 spec

9、imen surface, causing them to appear brighter than even portions.2021/9/128Specimen IC chip.The higher the accelerating voltage, the greater is theedge effect, making the edges brighter.Influence of edge effect on image qualityThe degree of the edge effect depends on the accelerating voltage. Namely

10、, the lower the accelerating 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 si

11、gnals. Therefore, if the 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 Ti

12、lt Angle: 50(b) 25 kV x720 Tilt Angle: 502021/9/129Specimen: 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 angle1) Improving the quality of secondary electron images; 2) Obtai

13、ning information different form that obtained 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 latte

14、r is of smooth quality and stereoscopic as compared 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

15、tilted state.(a) Tilt angle: 0(b) Tilt angle: 452021/9/1210Specimen: Back sides of oleaster leaves. Moreinformation is obtained from stereo-pair photos.Use of specimen tilt：b) Stereo micrographsWith SEM images it is sometimes difficult to correctly judge their topographical features. In such a case

16、observation of stereo SEM images makes it easy to understand the structure 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 photograph

17、ed again with the specimen tilted from 5 to 15. Viewing these two photos using stereo glasses with the tilting axis held vertically provides a stereo image.2021/9/1211Use of specimen tilt：c) Detector position and specimen directionThe amount of secondary electrons produced when the specimen is illum

18、inated with an electron beam, depends on the angle of incidence theoretically. However, 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 br

19、ightness differs between the side facing the detector and the opposite side.In such a case, directing the longitudinal axis of the specimen to the detector makes the brightness uniform.Fig. 16 Specimen: FiberDetector position and specimen direction.2021/9/1212Specimen: Fiber7kV x2,200Directing the l

20、ongitudinal axis of the specimen to the secondary electron detector makes the right and left sides equally 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) Spe

21、cimen directed as at 32021/9/1213Backscattered electrons vary in their amount and direction with the composition, 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

22、the mean atomic number of the specimen, (2) angle dependence of backscattered electrons at the specimen surface, and (3) the change in the backscattered electron intensity when the electron probes incident angle upon a crystalline specimen is changed.Use of backscattered electron signals2021/9/1214R

23、etractable BSE detector in chamberObjectivelensSecondary detector (ETD)2021/9/1215BSE paths and detector locationsObjectivelens2021/9/1216BSE from adjacent regions on flat sample2021/9/1217SEM Compositional imageBackscattered SEM image of an PbSn alloy showing contrast based on the atomic number. Th

24、e brighter areas are Pb-rich.2021/9/1218Use of backscattered electron signalsPrinciples of composition image and topography imageThe backscattered electron image contains two types of information:one on specimen composition and the other on specimentopography. To separate these two types of informat

25、ion, a paired semiconductor 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 s

26、o-called “channeling contrast,” by utilizing the advantage that the backscattered electron intensity changes largely before and after Braggs condition.2021/9/1219SE versu BSE images of alloyObjectivelensCu/Zn Alloy, SE (left), BSE (right).0.1 Atomic Number Difference2021/9/1220The generation region

27、of backscattered electrons is larger than that of secondary electrons, namely, several tens of nm. Therefore, backscattered electrons 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 spe

28、cimen contamination.Use of backscattered electron signalsTOPOX-ray (Si)X-ray (Al)BEISEICOMPOSpecimen: Slug20kV x1,100The backscattered electron image is important also as a supplementary means for x-ray analysis.2021/9/1221Influence of charge-up on image quality Specimen: Resist.Charge-up can be pre

29、vented by properly selecting the accelerating voltage.(a) 1.0 kV x3,200(a) 4 kV(b) 10 kV(b) 1.3 kV x3,200Specimen: Foreleg of vinegar fly.Charge-up can be reduced by using low accelerating voltage.2021/9/1222Specimen damage by electron beamSpecimen: Compound eye of fly. 5 kV x1,00When a specimen are

30、a is irradiated with an electron probe for a long time at high magnification, it may be damaged as shown in (b).(b)(a)2021/9/1223ContaminationWhen the electron 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

31、 the vicinity of the specimen beingstruck by the electron probe. This phenomenon is called specimencontamination.The conceivable residual gases in the specimen 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 th