Our Pulse

05 September 2014

DRP SEM/EDS Capabilities

A little over two years ago DRP acquired an FEI Quanta 250 Environmental Scanning Electron Microscope (SEM) equipped with an EDAX Apollo X silicon drift detecter energy-dispersive x-ray spectrometer (EDS).

The Quanta 250 is capable of operation in three different vacuum modes: high vacuum (< 6 e-4 Pa) for conductive or conventionally prepared specimens; low vacuum (10-130 Pa) for non-conductive specimens without preparation; and ESEM™ mode (10-2600 Pa) for specimens such as hydrous materials (cement paste) that are incompatible with high vacuum. The instrument is equipped with several detectors for imaging: an Everhardt Thornley secondary electron detector (SED), a Large Field, Low vacuum SED, a high- sensitivity, low kV solid state backscatter electron detector (BSED), and gaseous SED and BSED for ESEM conditions.

The instrument has a magnification range from 6 to more than 1,000,000 x. The Quanta 250 uses a Tungsten hairpin filament mounted within a tetrode gun assembly. The instrument operates with an accelerating voltage of 200 V to 30 kV with beam currents up to 2 μA. The spot size of the instrument controls aspects of the electromagnetic lenses that determine the crossover of the beam and its current. These parameters determine the lateral spatial resolution of the beam in any given material. The resolution of the instrument is as follows (measured as particle separation on a carbon substrate):

High vacuum mode: 3.0 nm at 30 kV and 8.0 nm at 3 kV for SED
4.0 nm at 30 kV for BSED
Low vacuum mode: 3.0 nm at 30 kV and 10.0 nm at 3 kV for SED
4.0 nm at 30 kV for BSED
Extended vacuum mode (ESEM): 3.0 nm at 30 kV for SED

Secondary electrons (SE) are collected with the detector that is appropriate for the different vacuum modes listed above. SE are emitted from the specimen itself when the electron beam strikes the sample. SE are low-energy electrons that originate from within a few nanometers of the sample surface. If the surface is flat and the beam is perpendicular to it, the activated region is uniform and a certain number of beams will escape and be detected. As the angle of incidence increases, the distance for the electrons to escape will decrease on one side of the beam and more electrons will emerge from that area. This results in steep surfaces and edges tending to appear brighter than flat areas, which provides images with great depth of field. Consequently, SE imaging is best for depicting the topography of the sample.

Backscatter electrons (BSE) are also collected with detectors that are appropriate for the various vacuum modes described above. BSE are high-energy electrons that originate within the electron beam; they are produced within the interaction volume of the specimen by elastic scattering with the atoms present in the sample. Heavy elements with higher atomic numbers have a stronger backscatter signal than lighter elements with a lower atomic number, such that materials or phases which contain heavy elements appear brighter than materials or phases which contain light elements. Consequently, BSE imaging is well-suited for showing compositional variations within a sample.

Combined BSE/SE images are produced by mixing the signal from both the SE and BSE detector. The blue and orange colors produced in this method are generated by the software embedded in the FEI Quanta. The SEM does not detect photons in the visible light spectrum.

The Quanta is also with an equipped with an EDAX® Apollo X Silicon Drift Detector for Energy Dispersive X-ray Spectroscopy (EDS) analysis. The Apollo X is equipped with a 10 mm2 window and has a resolution of 131 eV or better with peak to background ratios that are greater than 10,000:1. The detector is capable of handling input count rates up to 850,000 cps and throughput of more than 350,000 cps. The detector is capable of detecting all chemical elements down to Beryllium (Be, atomic number = 3) and is capable of quantitative analyses down to and including Boron (B, atomic number = 4). The EDS has a take-off angle of 35° at a 10 mm working distance.

EDS is used for the elemental analysis of materials. This analytical method uses an energy-dispersive spectrometer to measure the number and energy of X-rays that are produced when an energy source such as an electron beam excites the sample. The excitation of the sample causes the ejection of electrons from inner shells, which results in electrons from outer, higher-energy shells to fill the hole in the inner shell. The difference in energy states between the shells is released as an X-ray. X-rays that are characteristic of whatever elements are present in the analysis area are detected simultaneously and the software used to process the X-ray signal produces data in several forms. The most commonly used data output is an EDS spectrum, which is similar to a histogram. The elements are arranged on the x-axis in increasing atomic number, such that Be, the lightest element detectable by EDX, appears at the left of the spectrum and higher elements appear toward the right. The y-axis of the spectrum represents the number of counts for given element. When an element is present, the spectrum will show a peak for that particular element. The height of the peak is broadly consistent with the abundance of the element relative to other elements present in the sample. However, various factors affect the signal, such as the nature of the matrix (especially important in cement-based materials), the smoothness of the sample, the energy of the beam, and the configuration of the instrument, to name a few. Consequently, peak heights are not accurate quantitative measurements of elemental concentrations. Semi-quantitative analyses can be conducted with EDS using various normalization methods, such as ZAF, which corrects for matrix effects by considering the atomic number (Z), absorption (A) and fluorescence (F) of the material using a physical model. Under very favorable conditions, EDS analyses have an accuracy of ± 1%.