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Solutions of Chapter 4 4.1. Compare light microscopy, transmission electron and scanning electron microscopy in terms of optical arrangement, illumination source, working environment, imaging formation mechanism and specimen preparation; define their similarities and differences. Light microscopy TEM SEM Optical Light source → arrangement collector lens→ condenser lens → specimen (on stage) → objective lens → eye piece Electron gun → accelerator → 1st condenser lens → 2nd condenser lens → specimen → objective lens → intermediate lens → projector lens fluorescent screen → Electron gun →1st electron lens→2nd condenser lens→3rd condenser lens → specimen → detector → amplifier →display screen Illumination source High energy electron beams (over 100 kV) High energy beam electron beam (1 ~ 40 kV) Working Ambient, vacuum or environment liquid High Vacuum High Vacuum Imaging formation mechanism Transmitted or reflected visible light from a specimen. Transmitted and/or diffracted Electron beam from a specimen Secondary electrons or backscattered electrons emitted from a specimen. Specimen preparation For reflection: cutting, grinding, polishing and etching. For transmission: microtomy Pre-thinning, and final thinning (electrolytic, ion milling or ultra-microtomy) do be aware of problems such as surface charging, dehydration, coating. visible light (3 types): TEM and light microscope have similarities in the overall setup and structure. The main differences are the illumination sources, working environment, number of lens and apertures. Both TEM and SEM employ electron guns as their illumination source and vacuum as working environment. SEM is significantly different from TEM in image formation mechanism. Materials Characterization Yang Leng © 2008 John Wiley & Sons (Asia) Pte Ltd 1 4.2. Given a specimen with surface fracture features with high differences up to 50 μm, how do you select SEM operation parameters to ensure the whole field is in focus? The depth of the field Df should be greater than 50 μm. We should select a small aperture size, a long working distance and a low mangification. Table 4.1 in the text indicate that M = 100 × and α f = 3 × 10−2 that will provide a depth of field 67μm. 4.3. Estimate the effective magnifications of an SEM image showing on your computer screen, when the probe size varies from 1 nm to 1 μm. SEM image resolution is primarily controlled by the probe size of electron beam. The effective magnification for showing image details equal to the probe size on computer screen with pixel size of about 100 μm should be 100 μ m 100 μ m ≤ M eff ≤ ; that is 100 ≤ M eff ≤ 10000 . 1μ m 1nm 4.4. How can you obtain SEM images with the best resolution by adjusting operation parameters? The best resolution will result from optimizing the probe size, probe current density, and eliminate astigmatism. High resolution of SEM requires reduce probe size. The probe current should maintain at certain level to ensure the sufficient signal/noise ratio. Unfortunately, reduction of probe size is accompanied by probe current reduction. To obtain the best resolution, we should select a short work distance, a medium or small size of final aperture, a high acceleration voltage, and correct astigmatism. Also, we should adjust the probe current, which is often ignored during SEM operation. 4.5. When we find that noise level is too high in a SEM image, how can we reduce the noise level by adjusting operational parameters? High noise level often results from weak signal electrons during probe scanning. Slow scanning rates can effectively reduce the noise level. Also, increasing probe current can increase intensity of signal electrons in probe without reduce scanning rate. 4.6. Can backscattered electrons generate the same levels of resolution as the secondary electrons? Why? Materials Characterization Yang Leng © 2008 John Wiley & Sons (Asia) Pte Ltd 2 Backscattering electrons cannot generate the same levels of resolution as the secondary electrons. BSEs with higher energy than SEs can escape from a larger volume under a specimen surface than SEs as shown in Fig. 4.10. Thus, the lateral resolution of BSEs is worse than that of SEs. 4.7. In a SEM image with compositional contrast as shown in Fig. 4.16b, which areas contain the heavy metallic elements--bright or dark? Explain. The bright areas contain the heavy metallic elements. The compositional contrast is generated by backscattered electrons. The number of backscattered electrons increases with increasing atomic number. 4.8. Is it possible to eliminate surface charging of a polymer sample without conductive coating? Explain. It is possible to reduce the surface charging by reducing acceleration voltage of electron beam. Field emission gun can ensure high brightness of beam with acceleration voltage as low as 1 ~ 5 kV. Materials Characterization Yang Leng © 2008 John Wiley & Sons (Asia) Pte Ltd 3