Meaning
Analytical x-ray emissions captured from a material surface during electron beam excitation allow for the precise identification and quantification of the elemental composition in a sample. This energy dispersive spectroscopy is a non-destructive technique typically integrated into a scanning electron microscope to provide chemical information alongside high resolution topography. When the electron beam strikes the sample, it dislodges inner shell electrons, causing higher energy electrons to drop into the vacancies and release characteristic x-rays.
The energy of these x-rays is specific to each element, allowing the system to determine exactly which atoms are present in the interaction volume. In the context of electronic manufacturing, this method is widely used for failure analysis, contamination identification, and the measurement of intermetallic layers. It provides a rapid and effective way to verify that the materials used in a production line meet the required specifications and are free from unwanted impurities.
Emission Mechanism
Interaction between the primary electron beam and the atoms in the sample is the physical basis for the generation of signal in energy dispersive spectroscopy. The incoming electrons must have sufficient energy to overcome the binding energy of the target atoms, which is why the accelerating voltage of the microscope is a critical parameter. Once an x-ray is emitted, it is captured by a solid state detector, typically a silicon drift detector, which converts the x-ray energy into an electrical pulse.
The magnitude of this pulse is proportional to the energy of the photon, and the system counts thousands of these events to build a spectrum. Each peak in the resulting spectrum corresponds to a specific electronic transition in a specific element, such as the K-alpha or L-alpha lines. By analyzing the position and height of these peaks, the software can identify the elements present and estimate their relative concentrations.
Elemental Mapping
Spatial distribution of elements across a surface can be visualized by scanning the electron beam in a grid pattern while simultaneously collecting x-ray data. This feature of energy dispersive spectroscopy allows for the creation of colorful maps where each color represents a different element, such as copper, tin, or nickel. This is particularly useful for examining the cross section of a solder joint, as it clearly shows the layers of different intermetallic compounds.
It also helps in identifying the source of contamination, such as chlorine or sulfur, by showing exactly where these elements are concentrated. Mapping provides a more comprehensive view of the sample than a single point analysis, as it can reveal gradients in composition or the presence of localized precipitates. The resolution of the map is limited by the size of the interaction volume, which is usually around one micrometer in diameter depending on the material and the beam energy.
Quantification Limit
Accuracy of the chemical analysis depends on several factors, including the surface roughness of the sample and the atomic number of the elements being measured. While energy dispersive spectroscopy is excellent for detecting major and minor elements, it has a detection limit typically around 0.1 weight percent. Light elements like carbon, nitrogen, and oxygen are more difficult to quantify accurately because their low energy x-rays are easily absorbed before they reach the detector.
To obtain reliable quantitative data, the sample surface should be flat and polished, which is why metallographic preparation is often required. The software uses mathematical models to correct for matrix effects, such as the absorption of x-rays by other elements in the sample. Despite these challenges, the technique remains the most common tool for routine chemical characterization in the electronics industry.
It provides the necessary evidence for root cause analysis during the investigation of field failures or production defects.
