Scanning Electron Microscopy Introduction to Scanning Electron Microscopy @CMSE

____________________________________________________________ What is an SEM? ____________________________________________________________ Scanning Electron Microscope is an instrument that investigates the surfaces of solid samples using a beam of electrons rather than light to form the image.

____________________________________________________________ Advantages of using an SEM instead of a light microscope ____________________________________________________________ Imaging High depth of field high resolution where features can be examined at high magnification. Elemental Analysis - EDS Structural Analysis - EBSD Ease of sample preparation since most SEMs only require the sample to be conductive.

____________________________________________________________ Advantages of using an SEM instead of a light microscope ____________________________________________________________ There are a number of differences between light and electron optics. light optics relies on the refraction or reflection of light. The wavelength of the visible light used in optical microscopes is between 400 and 700 nanometers (nm). Light microscopes are limited by the wavelength of imaging light to about 200 nanometers (0.2 microns). In electron optical systems, electrostatic or magnetic fields are used to influence the trajectories of beams of electrons. Scanning electron microscopy uses electrons with energies of a few thousand electron volts, imaging a thousand times greater than that of visible light (2 to 3 eV). The wavelength is 0.02 nanometers.

____________________________________________________________ What you observed? ____________________________________________________________ Surface topography of metals, ceramics and minerals Microstructural morphology of metals, ceramics and minerals

____________________________________________________________ Imaging in SEM ____________________________________________________________ High Magnification Range ~ 5x to 500,000x Resolution down to 1nm. point-to-point High depth of field Over 100x better than optical microscope All specimen heights in focus simultaneously Best at low magnification High brightness guns - LaB6, FEG Digital Storage ESEM Low keV, high resolution

____________________________________________________________ SEM Operation Mode ____________________________________________________________ High Resolution Mode: small beam, small current, optimum final aperture size after astigmatism correction, short working distance, high magnification High-Current Mode: High beam current, better contrast and microanalysis (more electrons and x-rays are emitted and collected), larger aperture-poorer resolution, use at low magnification under 1000x so that image blur is not noticeable Depth-of-Field Mode: small final aperture, long working distance, best at low magnification Low-Voltage Mode: increased surface sensitivity, reduced specimen charging, beam size larger than at higher kv-poorer resolution Resolution : small beam, small current, optimum final aperture size after astigmatism correction, short working distance, high magnification High beam current: better contrast and microanalysis (more electrons and x-rays are emitted and collected), larger aperture-poorer resolution, use at low magnification under 1000x so that image blur is not noticeable Depth-of-field: small final aperture, long working distance, best at low magnification Low voltage: increased surface sensitivity, reduced specimen charging, beam size larger than at higher kv-poorer resolution

____________________________________________________________ What kind of Data you get in SEM ____________________________________________________________ Topography (SE or GSE) The surface features of an object or how it looks, its texture, detectable features limited to a few manometers. These images give valuable information about sample's topography

____________________________________________________________ What kind of Data you get in SEM ____________________________________________________________ Morphology (BSE) The shape, size and arrangement of the particles making up the object that are lying on the surface of the sample or have been exposed by grinding or chemical etching, detectable features limited to several nanometers This image gives valuable information about sample's composition Dr Anthony Garratt-Reed

____________________________________________________________ What kind of Data you get in SEM ____________________________________________________________ Composition (EDS) The elements and compounds the sample is composed of and their relative ratios, in areas ~ 1 micrometer in diameter These X-Ray map images give valuable information about sample's composition SEM image Copper Lead Tin Dr Anthony Garrett-Reed

____________________________________________________________ What kind of Data you get in SEM ____________________________________________________________ Crystallographic Information (EBSD) The arrangement of atoms in the specimen and their degree of order; only useful on single-crystal particles >20 micrometers These Orientation Imaging Micrographs give valuable information about sample's crystallographic orientation Scanning Electron Microscope (SEM) strikes a crystalline material mounted at an incline around 70°, the electrons disperse beneath the surface, and diffract among the crystallographic planes. The diffracted beam produces electron backscatter patterns composed of intersecting bands, These patterns are imaged by placing a phosphor screen very close to the sample in the SEM sample chamber. The bands in the pattern are directly related to the crystal lattice structure in the sampled region. Analyzing of the pattern and bands can provide key information about the crystal structure for the measured phase such as: The symmetry of the crystal lattice is reflected in the pattern. The width and intensity of the bands are directly related to the spacing of atoms in the crystal planes. The angles between the bands are directly related to the angles between planes in the crystal lattice. Indexable patterns can be obtained from about 0.2 microns with a tungsten filament SEM and from about 0.05 microns with a field emission source. EDAX Inc.

_______________________________________________________ SEM Instruments Available in the EM Lab: _______________________________________________________ JEOL 5910: General-Purpose SEM Philips FEI XL30 : FEG-ESEM JEOL 6320: High-resolution SEM Secondary detectors (SE) Backscatter detectors (BSE) Energy-dispersive X-ray detectors (EDS)

JEOL 5910 General-Purpose SEM _______________________________________________________ Large sample chamber Easy to operate Use a tungsten filament Resolution down to 50 nm

_________________________________________________________ FEI XL30 FEG-ESEM _________________________________________________________ Large sample chamber with CCD camera Use a field emission gun (FEG) Gaseous secondary detector (GSE) used under poor vacuum in the environmental mode Backscatter diffraction patterns (EBSD) High resolution down to 20 nm

JEOL 6320 High-resolution SEM _________________________________________________________ Restricted sample chamber Use a field emission gun (FEG) Higher resolution down to 1.4 nm

MT-X Ultra microtome at Room Temperature ___________________________________________________________________________ This microtome is used to obtain ultra-thin sections for electron microscopy, semi-thin and thick sections for light microscopy. It can be used for both ultramicrotomy at room temperature sectioning and cryo-ultramicrotomy at low temperature sectioning

MT-X Ultra microtome cryo-ultramicrotomy at low temperature mode ___________________________________________________________________________ Cryo-ultramicrotomy at low temperature sectioning of polymers, elastomers, or any materials requiring processing down to -185°C. Each section of the sample can be between 5nm to 5um.

Introduction to Scanning Electron Microscopy Patrick Boisvert Thu Jan 24, 10-11:00am, 13-2137 The lecture will provide an introduction to the basic principles of Scanning Electron Microscopy with an approach to EDX, EBSD, and BSE. Contact: Patrick Boisvert, 13-1018, x3-3317, pboisver@mit.edu