Corrosion, Failure Analysis and Materials Selection Specialists

 

 

 

 

 

Corrosion Testing / Failure Analysis

 

 

 

Corrosion Testing

Failure Analysis

Field Investigations

Litigation

Metallography

Technical Papers

CTL Profile

Pricing & Policies

Contact CTL

Quality Assurance

Return to CTL's Facilities

Scanning Electron Microscope (SEM)

Scanning electron microscopes have been commercially available since the late 1960's. The instrument has evolved from a highly specialized research laboratory instrument into an accepted part of analytical laboratories and production facilities. With the capacity of magnifying features from 10 to 100,000X, it can be found in the businesses of semiconductor and nylon fiber quality assurance, pollution particle characterization, and equipment failure analysis. The SEM also serves as a platform for micro-analytical techniques, such as Energy Dispersive X-ray Spectroscopy (EDS). In combination with this technique, the SEM becomes a powerful tool for ferreting out and characterizing evidence for root cause failure analysis. Both mechanical and corrosion related failures often have features that can be best discerned at high magnification and identified by x-ray microanalysis.

 

Operating Principles

In the SEM, a very finely focuses beam of electrons is scanned over the surface of the specimen. As the electron beam scans the specimen, it not only provides topographical information, but also, as it penetrates the surface, interacts with the sample to cause effects such as electron backscattering, x-ray emission, secondary electron emission, and cathode luminescence.

Cracking on surface

The imaging process of the SEM takes place when a cathode ray tube (CRT) is scanned simultaneously with the electron beam. The most common method of imaging utilizes secondary electrons. As the electron beam is scanned over the specimen, surface features in line of sight of the secondary electron detector will generate proportionally more electrons. The detector generates a signal that is proportional to the number of the electrons received as various surface features come under the electron beam. The intensity of the CRT beam is modulated proportionally to represent the magnitude of the signal arriving from the secondary electron detector. A picture is built up that represents the surface topographical features and are discerned by the effect that, for a "hill" on the surface, the side facing the detector will generate more secondary electrons that are likely to arrive at the detector, and consequently, that side will appear brighter than the side that is not in the line of sight of the detector.

Another commonly used image is produced by the detection of backscattered electrons. The intensity of these electrons is influenced by differences in atomic number. Microstructural phase with different average atomic numbers and grain boundaries in unetched specimens can be imaged with these detectors.

 

Limitations of the SEM

Feature resolution is limited to 70 to 100 Angstroms for most microscopes. Specimens must be resistant to vacuum; liquids with vapor pressures less than 10-3 torr cannot be analyzed. Since the scattering takes place in vacuum, the maximum size of the specimens to be examined is limited to the size of the chamber at the base of the microscope. Consequently, typical specimen dimensions are limited to less than 3 cm on a side.

 

Standardized Methods

The standard practice for the performance of a scanning electron microscope is covered in ASTM E 968, "Scanning Electron Microscope Beam Size Characterization."

Site Index

Site Copyright 1995 - 2007, All Rights Reserved,

Corrosion Testing Laboratories, Inc.

60 Blue Hen Drive

Newark, Delaware USA 19713

Phone: 1-302-454-8200

Fax: 1-302-454-8204

web@corrosionlab.com