Next to secondary electrons and backscattered electrons, the characteristic x-rays that are produced by the interaction of an electron beam with a sample are perhaps the most widely used signals in an SEM.
X-rays result from the incoming electrons knocking inner shell electrons out of atoms in the sample. As outer electrons drop into the vacancy, they are obliged to dispose of the excess energy, often as an x-ray photon. Since each element has its own unique set of energy levels, the emitted photons are indicative of the element that produced them. Analyzers are used to characterize the x-ray photons for their energy (or wavelength) and abundance to determine the composition of the sample.
Due to the small size of the electron probe, it is possible to obtain elemental analyses for volumes as small as a few hundred nanometers in diameter.
Energy Dispersive X-ray Spectroscopy (EDS)
This technique is also sometimes referred to as EDXA (energy dispersive x-ray analysis) or EDAX (energy dispersive analysis of x-rays). EDS or EDXA are preferred since EDAX has been adopted as a trademark by one of the suppliers of EDS systems.
EDS uses a crystal of silicon or germanium to detect the x-rays. Each photon generates multiple electron-hole pairs equal in total energy to the energy of the photon (each pair has a fixed energy determined by the crystal). A voltage is applied to the crystal to collect the charge as a small step-change in voltage. Pre-amplifiers and amplifiers process the signal and pass it to a multi-channel analyzer (analog-to-digital converter) so that the x-ray spectrum can be displayed as a histogram of x-ray intensity as a function of energy.
The detector crystal typically uses a thin window to isolate it from the SEM chamber. Early windows severely attenuated x-rays from elements lighter than sodium. Improvements in window technology now permit detection of x-rays from Beryllium. This is a great help when analyzing minerals (O) and organic compounds (C and O).
Types of X-ray Analyses
X-ray analyses may be broadly divided in to qualitative and quantitative analyses. Qualitative analyses are those that are concerned with determining the elements in a sample and rough measures of their abundance and distribution. Such analyses include survey analyses, line-scan profiles, and x-ray maps. Quantitative analyses have as their goal the determination of the elemental composition at one or more points. There are a number of requirements that must be met for quantitative analyses to succeed. These analyses are discussed below.
These analyses seek to answer the general question "What is that phase?" They are useful if there is no advance knowledge of the composition of a sample. These analyses seek to determine the elements present and their distribution. Spectra are collected for a number of points and the elements present are identified. Often digital beam control is used to select areas for analysis.
Mapping generates a two-dimensional image using the abundance of an element as the intensity of the image. It easily shows where an element is abundant and where it is not. Maps can often be collected in a few minutes so that users get a sense of the elemental distribution. Additional time reveals more detail and smaller changes in concentration.
Similar to x-ray maps, this technique shows the abundance of an element along a line rather than over a two-dimensional image. It is especially helpful for examining trends at interfaces and concentration gradients within a sample. Since the analysis is conducted over fewer points (e.g., 512 for a linescan vs.50,000 for a map) the counting statistics are much better and linescans quickly reveal subtle changes in composition.
This involves careful determination of the abundances of the elements present in a sample. Accuracy is often possible to tenths of a percent when standards are used. Standardless analyses can generally provide a compositional estimate, but with less accuracy.
Several requirements must be met to ensure the success of a quantitative analysis. Samples must generally be flat, homogenous, and thick to satisfy assumptions in the analysis software. Standards are advisable to account for particular characteristics of the microscope. All elements should be measurable. These are spelled out in greater detail below.
- Relative x-ray intensity varies with the angle of the surface of the sample. Therefore, the surface must be at some known, consistent angle. Typically, the sample is polished flat.
- Correction algorithms assume a uniform distribution of elements on an atomic scale. Therefore, it is not possible to guarantee results when the excited volume is not uniform as in the case of dendritic structures or mixtures of particles.
- Algorithms assume that electrons penetrate and are completely absorbed in the sample (i.e., not escaping from the underside). Likewise, samples must also be homogenous with depth so that the beam is entirely contained in a single phase.
- Standards help to account for peculiarities in geometry or detector, and even for chemical environment. Standardless analyses may produce results good to within 10% relative. They rely on standards collected on another SEM and then corrected for the geometry and settings of the present microscope. It is better to have standards collected on the same scope under identical conditions.
- Analyses require a full accounting of the elements. If a light element is not detectable, it is sometimes possible to account for it by difference, or by stoichiometry, or by a specified amount, but it will require standards for the other elements.
Special features of the MARL X-ray Systems
The Aztec system on the Quanta offers several special capabilities of note.
- The system uses Oxfords X-Max 80 detector. It is a silicon drift detector (SDD) with an active area of 80 mm2. This permits adequate x-ray counts even at low beam currents used in high resolution imaging. It also allows for fast acquisition at higher beam currents (typically 15-20 kcps).
- Data for all x-ray photons are saved for maps and linescans. Therefore, it is not necessary to specify all elements before collecting data. Elements can be added and removed during acquisition and afterwards.
- All EDS systems allow for deconvolution of overlapping peaks in x-ray spectra for quantitative analysis. Aztec also provides for deconvolution of overlapping peaks in maps and linescans to show the true distribution of the elements.