Sirion Scanning Electron Microscope (SEM) Laboratory

The instrument is an FEI XL30 Sirion SEM with FEG source, EDX detector located on the first floor of McCullough Building, Room 108.

Principles of operation:

In a scanning electron microscope (SEM), a fine beam of electrons is scanned across the surface of a specimen in synchronism with a spot on the display screen. A detector monitors the intensity of a chosen secondary signal from the specimen (for example, secondary electrons) and the brightness of the display spots controlled by an amplified version of the detected signal. If, for any reason, the intensity of the emitted secondary signal changes across the specimen then contrast will be seen in the image on the display screen. The resulting image is strikingly similar to what would be seen through an optical microscope; the illumination and shadowing show a natural-seeming topography.

It is important to remember that the image formed in an SEM is not necessarily that of the surface. As the electron beam penetrates the sample, the interaction causes excitation of secondary, backscatter, and Auger electrons; characteristic and Bremsstrahlung x-rays; and photons. It is possible, by choosing the electron energy, to control the depth to which the electrons penetrate and the type of emitted signal used to form the image. While this gives the microscopist a great deal of control over the nature of the final image, an understanding of how the image is formed is required to interpret it sensibly.

Available spot sizes and beam currents (UHR mode):

Spot

1 kV
WD = 2 mm

5 kV
WD = 4 mm

15 kV
WD = 5 mm

30 kV
WD = 6.2 mm

nm
pA
nm
pA
nm
pA
nm

pA

1
2.5
0.7
2.3
3.0
1.5
9.0
1.5
20
2
2.5
5.2
2.3
18
1.5
33
1.5
40
3
2.7
20
2.4
71
2.2
130
1.7
140
4
2.8
90
2.5
310
2.5
550
1.8
550
5
3.1
390
3.7
1400
3.5
2200
2.0
2200
6
3.6
1400
4.7
7200
5.0
9300
3.8
8500
Note: These spot sizes are approximate, and are highly dependent on instrument and specimen conditions. This chart is to be used as a guideline only.

Advanced techniques:

Energy Dispersive Spectroscopy (EDS)
Electron Backscatter Diffraction/Orientation Imaging Microscopy (EBSD/OIM)

Restrictions on samples:

The sample material must be able to withstand a high vacuum environment without outgassing. It must be clean. It may be attached to the sample holder using any suitable SEM vacuum-quality adhesive. The sample must be electrically grounded to the sample holder to minimize sample charging. If the sample is nonconductive (plastic, fiber, polymer, or other substance with an electrical resistance greater than 1E10 ohms), the sample may be coated with a 200 – 300 Å layer of carbon or other conductor. Rough surfaced samples must be evenly coated from every direction. Biological, cloth and powder samples may require carbon or other conductive painting on portions of the sample that are hard to coat. The workstation can accommodate up to 100 mm (4") wafers.

Training and Service:

Basic training for inexperienced SEM users requires a minimum of three two-hour sessions which can be done in groups of two, save the final training which is done solo. Training slots are regularly posted on the SEM calendar for up to two weeks in advance. Additional training in specific SEM techniques is available on an as-needed basis following basic training. SEM users with prior experience will be trained at the level required.