Scanning electron microscope (SEM)

SEM obtains structural images by an entirely different method than that used by the TEM.

Explanation for basis image in the SEM

l An electron beam spot ~ 1 μm in diameter is scanned repeatedly over the surface of the sample. Slight variation in surface topography produce marked variations in the strength of the beam of secondary electrons – electrons ejected from the surface of the sample by the force of collision with primary electrons from the electron beam.

4.35 A commercial SEM. (Courtesy of Hitachi Scientific Instruments.)

l The secondary electron beam signal is displayed on a TV screen in a scanning pattern synchronized with the electron beam scan of the sample surface.

l The magnification possible with the SEM is limited by the beam spot size and is considerably better than that possible with the optical microscope with the optical microscope but less than that possible with the TEM.

l Important feature of an SEM image is that it looks like a visual image of a large-scale piece.

l SEM is especially useful for convenient inspections of grain structure.

Fig.4.36. SEM image of a 23-μm-diameter lunar rock from the Apollo 11 mission. The SEM gives an image with “depth” in contrast to optical micrographs. The spherical shape indicates a prior melting process.

Fig.4.37. SEM image of a metal (type 304 stainless steel) fracture surface, 180x.

(a) (b) (c)

Fig.4.38. (a) SEM image of the topography of a lead-tin solder alloy with lead-rich and tin rich regions. (b) A map of the same area shown in (a) indicating the lead distribution (light area) in the microstructure. The light area corresponds to regions emitting characteristic lead x-rays when struck by the scanning electron beam. (c) A similar map of the tin distribution (light area) in the microstructure.

2) Atomic resolution electron microscope

Resolution for this instrument is about 0.1 nm.

3) Scanning tunneling microscope (STM)

The first is in a new instrument capable of providing direct images of individual atomic packing patterns.

Fig.4.39. Scanning tunneling micrograph of an interstitial atom defect on the surface of graphite.

Explanation for basis image in the STM

The name of STM comes from the x-y raster (scanning) by a sharp metal tip near the surface of a conducting sample, leading to a measurable electrical current due to the quantum – mechanical tunneling of electrons near the surface.

For gap distance around 0.5 nm, an applied bias of tens of mV leads to nanoampere current flow.

The needle’s vertical distance (z-direction) above the surface is continually adjusted to maintain a constant tunnel current.

The surface topography is the record of the trajectory of the tip.

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