Introduction to Scanning Electron Microscopy (SEM) PDF

Introduction to scanning electron microscopy (SEM) B Basic concepts and operating principle Goldstein et al. Scanning Electron Microscopy and X-Ray Microanalysis, 4th Edition “Scanning Electron Microscopy and Associated...

Introduction to scanning electron microscopy (SEM) B Basic concepts and operating principle Goldstein et al. Scanning Electron Microscopy and X-Ray Microanalysis, 4th Edition “Scanning Electron Microscopy and Associated Techniques: Overview” (pages VII-XIV) MOL-32228 Electron Microscopy Tampere University History of scanning electron microscopes Louis de Broglie 1925: Dual nature of matter Hans Busch 1927: First electromagnetic lens Knoll 1935: Concept demonstration of scanning electron microscope Manfred von Ardenne 1938: First scanning electron microscope Cambridge Scientific Instrument Company 1965: First commercial scanning electron microscope  Optical microscopy vs. scanning electron microscopy Optical microscope Scanning electron microscope  Operating principle of SEM Sample Image Electron beam stays at the first stop of the sample for the dwell time Detector detects signal during this time The signal level defines the grayscale of the corresponding pixel in the image Point-by-point correspondence  Operating principle of SEM Sample Image Electron beam moves to the next stop of the sample Detector detects the signal The signal level defines the grayscale of the corresponding pixel in the image  Operating principle of SEM Sample Image The first row is scanned through The greyscale of first row pixels is defined The electron beam moves to the next row Point-by-point correspondence [Goldstein et al. Scanning electron microscopy and X-ray microanalysis, 3rd Edition] Magnification and resolution Magnification: Resolution: The ability to resolve two lines which have a = (minimum) distance of Limage Lspecimen Depth-of-field Depth of field is the length around the plane of optimum focus at which the sample is still in focus For SEM the depth of field can be up to 600 μm, whereas for optical microscope only 20 μm can be achieved http://www.emal.engin.umich.edu  The structure of SEM 1. Formation and control of energetic electron beam Electron gun Lens system Associated electronics Vacuum unit 2. Scanning (position and movement) of the beam Scanning coil 3. Electron-specimen interactions 4. Detection of selected signal type Detectors 5. Image formation by point-by-point correspondence Electronics + Vacuum unit SEM operation parameters Beam electron energy Adjusted with acceleration voltage Typically 0.1 - 30 keV Beam size and focus Adjusted with lens current and apertures Associated electronics Diameter typically 0.5 nm – 1 μm Beam current Adjusted with lens current and apertures Typically 1 pA – 1 μA Beam convergence angle Adjusted with apertures and working distance Typically 0.001 - 0.05 rad Affects the depth-of-field Magnification Adjusted by controlling the scanned area Signal type Adjusted by selecting the detector to be used (+ detector parameters) + Vacuum unit Examples of the effects of operation parameters The SEM operator optimizes the operation parameters to achieve the best possible outcome for each case Simplified examples: Aim is good resolu on → operator selects small beam diameter = low signal level = poor contrast/visibility Aim is good depth of field → operator selects small aperture size = small beam divergence angle = good depth-of-field BUT low beam current = low signal level = poor contrast/visibility Selection of signal type: dominating topographic contrast OR compositional contrast (qualitative/quantitative) OR combination  20-60% of the power of SEM signals the electron beam converts to heat Introduction to scanning electron microscopy (SEM) B Basic concepts and operating principle Goldstein et al. Scanning Electron Microscopy and X-Ray Microanalysis, 4th Edition “Scanning Electron Microscopy and Associated Techniques: Overview” (pages VII-XIV) MOL-32228 Electron Microscopy Tampere University