The SEM is widely used to analyze the surface morphology and chemical composition of the tested
materials. Its working principle is mainly based on detecting signals from the interaction of the
incident electrons with the sample's surface. The signals gathered in an SEM include the secondary
electrons, backscattered electrons and characteristic x-rays etc. Quality of the images is greatly
affected by a number of operation parameters. In this assignment, you are required to discuss the
influence of these operation parameters based on the given SEM images/data.
1. Influence of Working Distance (WD) (2 marks)
Fig 1 (a) and (b) are SEM secondary electron (SE) images for a composite material with carbon fibres
taken with different WD. Interpret the images and explain the influence of WD.
a) b) a) Figure 1 a) WD= 8mm. b) WD= 24mm. 2. Influence of Acceleration Voltage (HV) (2 marks)
A copper grid with a polymer thin film coating on top is used to study the influence of acceleration
voltage (HV).Fig 2 (a) and (b) are SE images of the sample using different HV. Explain why different
images are observed on the same sample and discuss the influence of HV. a) Figure2, a) HV= 2kV. b) HV= 3kV b) 3. Influence of Aperture and Scan Rate (2 marks)
Iron oxide powder is used to look at the influence of aperture and scanning speed. Fig 3 (a) and (b)
are the SE images obtained using the aperture size of 140 µm to 70 µm with same scan rate.
Whereas the image in Fig 3 (c) is taken using a slow scan rate with an aperture size of 70 µm. Based
on the three images, discuss how the aperture and scan rate affect the quality of image.
a) b) c) Figure 3, a) R=140µm. b) R=70µm with fast scan rate. c) R=70µm with slow scan rate. 4. Influence of received electron type (2 marks)
In SEM, both secondary electron (SE) and backscattered electron (BSE) will render images of surface
morphology of the sample. One of the two images below is SE image and the other is BSE image.
Differentiate them with explanations. The sample is iron oxide powder with impurities.
a) b) Figure 4, SE and BSE images of iron oxide powder with impurities. 5. Chemical analysis with energy dispersive X-ray spectroscopy (EDS) (2 marks)
Apart from SE and BSE images, it is also possible to determine the chemical composition of a
material using SEM equipped with EDS. Three scan modes in EDS, including line scan, point scan and
area scan can be used to obtain the element’s profile within the samples. After the scanning, the
atomic percent and weight percent of the sample can be obtained. Based on the characteristic x-ray
spectra and data given below, 1) briefly describe how the EDS works out the chemical composition; 2)
which scan mode gives the most reasonable information of the composition in this particular case?
Why? Figure 5, point, line and area scan on the unknown substance LG10000 ClKa 2100 NaKa 1800 AlKa Counts 1500 1200 900 0
0.00 0.80 1.60 ClKb ClLn 300 SKa
SKb 600 2.40 3.20 4.00 4.80 keV Element (keV) mass% Error% At% Na 1.041 31.12 0.12 38.68 Al 1.486 21.73 0.13 23.01 S 2.307 3.65 0.11 3.25 Cl 2.621 43.5 0.13 35.06 Table 3.1: Result of the LG1 line scan 5.60 AlKa 720 LG10001 640 560 ClKa CaKa 480 320 ClLn 80 SKb 160 0
0.00 0.80 1.60 2.40 ClKb NaKa 240 CaKb SKa Counts 400 3.20 4.00 4.80 5.60 keV Element (keV) mass% Error% At% Na 1.041 4.74 0.32 6.70 Al 1.486 29.99 0.22 36.14 S 2.307 9.27 0.22 9.40 Cl 2.621 22.00 0.26 20.17 Ca 3.690 34.01 0.37 27.59 Table 3.2: Result of the 001 point scan 6.40 AlKa 900 001 800 700 600 400 ClKa Counts 500 0
0.00 0.80 1.60 2.40 3.20 CaKb CaKa SKb ClLn 100 NaKa 200 ClKb SKa 300 4.00 4.80 keV
Element (keV) mass% Error% At% Na 1.041 3.15 0.38 4.08 Al 1.486 53.05 0.29 58.45 S 2.307 10.55 0.40 9.78 Cl 2.621 31.37 0.45 26.30 Ca 3.690 1.87 0.65 1.39 Table 3.3: Result of the 002 area scan 5.60

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