SEM introduction

Question 1

what is Magnification and Resolution

Answer 1

Magnification: Is how much an image is enlarged under a microscope.

Resolution (resolving power): Is the amount of detail you can see in an image. You can enlarge a photograph indefinitely using more powerful lenses, but the image will blur together and will be unreadable. Therefore, increasing the magnification will not improve the resolution. This is also known as the resolving power. Resolution, d, is the minimum distance at which two points are resolvable

Question 2

SEM vs. Optical Microscope

Answer 2

The SEM provides two outstanding improvements over the optical microscope:
• High Resolution
• Depth of Field

Question 3

High Resolution

Answer 3

SEM extends the resolution limits so that the picture magnifications can be increased from:
1000 - 2000X (light microscope) to 30,000 - 100,000X (SEM).
The different wavelengths of the radiation sources result in different resolution levels.
1. Wavelength of ultraviolet side of visible light > 2000 A
λ = 2000 A
Theoretical Maximum Resolution = 2000 A Max. Magnification = 2000X
2. Electrons have shorter wavelength than light photons. The wavelength of electrons is:
λ = 0.5 A
Theoretical Maximum Magnification = 1,000,000X
Practical Magnification ~ 100,000X
Resolution limit ~ 40 A (for commercial SEM)
Resolution limit ~ 10 A (for field-emission SEM)

Question 4

Scanning Electron Microscope Resolution

Answer 4

• In a SEM, an electron beam scans rapidly over the surface of the sample specimen and yields an image of the topography of the surface.
• The resolution of a SEM is about 10 nm (nanometers). The resolution is limited by the width of the exciting electron beam and the interaction volume of electrons in a solid.

Question 5

Transmission Electron Microscope Resolution

Answer 5

• In a TEM, a monochromatic beam of electrons is accelerated through a potential of 40 to 100 kilovolts (kV) and passed through a strong magnetic field that acts as a lens.
• The resolution of a TEM is about 0.2 nm (nanometers). This is the typical separation between two atoms in a solid.
• The resolution of a TEM is 1,000 times greater than a compound microscope and about 500,000 times greater than the human eye.

Question 6

Depth of Field

Answer 6

• Depth of field is defined as the ability to maintain focus across a field of view regardless of surface roughness.
• SEM improves the depth of field resolution by a factor approximately 300.
• Depth of field decreases as magnification increases.

Question 7

General Applications

Answer 7

1. The combination of high resolution, extensive magnification range, and high depth of field, makes the SEM uniquely suited for the study of surfaces.
2. A second use, perhaps just as important, is to provide chemical analysis of micron-sized areas of the structure revealed on the surfaces.
3. The SEM is also capable of examining objects at very low magnification. This feature is useful in forensic studies.

Question 8

The Scanning Electron Microscope Instrumentation

Answer 8

The various components of the SEM can be categorized as

1. The Electron Column
2. The Specimen Chamber
3. The Vacuum Pumping System
4. The Electronic Control and Imaging System

Question 9

The Electron Column

Answer 9

Contains the:

a. Electron Gun,
b. Magnetic Lenses,
c. Scan Coil, and
d. Apertures.