Scanning electron microscopy

Tracy Furutani

November 13, 2008

Light microscope vs SEM

• Uses photons of visible light

• Resolution 1000 nm• Color images• Lenses of glass• Aberration in lenses

difficult to correct• Sample in air

• Uses electrons• Resolution 1 nm• Color meaningless• Lenses of solenoid

coils• Stigmator allows

control of beam• Sample in vacuum

Types of electron microscopy• Transmission electron microscopy -- first invented

(1931), works like light microscope but with electrons, needs thin samples

• Scanning electron microscopy -- have beam of electrons “scan” across a surface to image it

• Reflection electron microscopy -- detects the pattern of scattered electrons

• Scanning transmission electron microscopy -- combining SEM sample interaction with TEM imaging

• Scanning tunneling microscopy -- electrons “tunnel” through vacuum to “feel” surface

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Some definitions

• Stigmation -- correcting asymmetries in horizontal versus vertical focus. If the image has astigmatism there will be “streakiness” in the image.

• Collimation -- the creation of parallel path particles (in this case, electrons)

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Sample preparation

• Coated (sputtered) versus noncoated

• Stubs versus vises

• Carbon tape versus carbon paint

Some techniques to improve performance

• Spot size -- literally the size of the path of the focused electrons as they strike the sample

• Minimize this to improve focus at high magnification by a) decreasing the working distance or b) increasing the current on the focusing lens

• Trade off: smaller area of coverage, lower beam current means worse contrast

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More techniques to improve performance

• Depth of field -- how much relief can be on the sample with it still staying in focus

• Maximizing this allows you to see detail on rough surfaces by a) decreasing the aperture size, b) decreasing the magnification or c) increasing the working distance

• Trade off: lower magnification

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Even more techniques to improve performance

• Signal-to-noise ratio -- the contrast between interacting and noninteracting surfaces

• Maximize this to gain more fine detail by a) using a high beam current or b) using a slower scan rate

• Trade off: Much larger spot size

Energy dispersive X-ray spectroscopy

• Known as EDS, EDX or EDXRF

• Used for elemental analysis

• Electrons from SEM beam excite electrons in atoms in sample

• Atoms in sample emit characteristic energy X-rays as electrons drop back to ground state

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