X-ray fluorescence is a naturally occurring phenomenon in which atoms subjected to sufficiently high-energy external radiation stimulus, emit x-rays. The energy of the emitted x-ray photons is unique to the element under stimulus and thus provides a “fingerprint” or source of identification that is highly advantageous to qualitative and quantitative analysis.
When sample materials are exposed to high-energy radiation sources, such as electron beams, or x-rays, some of this energy will directly strike individual atoms in the sample. The energy of this impact is absorbed by the atoms, and if it is high enough, a core electron (electrons occupying fully populated low-energy orbital shells, below the valence shell around the nuclear) is ejected from its orbit.
Energy Dispersive Spectrometer
When this ejection occurs, an electron from an outer shell drops into the unoccupied orbital and fills the hole left by the ejected electron. This electron transition from an outer shell to the core orbital hole, gives off an x-ray of fixed energy that is unique to the atom (and therefore the element) generating the radiation. This characteristic radiation can then be measured by a fluorescence detector. The precise energies being detected can be used to identify the various elements within the sample (qualitative analysis) and the number of detected x-rays of the characteristic energy can be used to identify the amount of material in the sample from each element (quantitative analysis).
The process of analyzing radiation (such as infrared, visible light, x-rays, etc.) in terms of frequency (or wavelength) and amplitude is called spectroscopy. For x-ray spectroscopy, there are two types of spectrometers in wide use today; 1) wavelength dispersive spectrometers (WDS or WDX) and energy dispersive spectrometers (EDS or EDX). WDS spectrometers separate photons of different wavelength or energy by diffracting the radiation through a single crystalline substance before detection and they provide very high resolution and sensitivity. EDS systems directly detect the energy of the radiation and are smaller and less expensive than WDS systems. Their resolution and sensitivity, however, is not nearly as good as WDS.
While x-ray fluorescence spectrometers are very good at determining the resolution better than the size of the irradiated volume quantities and amounts of elemental materials within a sample, they do not provide any direct means of imaging these materials. In other words, the location and physical distribution of various elements within a sample cannot be determined through wavelength or energy dispersive spectroscopic methods.
With the introduction of the nanoXFi™ X-ray Fluorescence Imager in 2005, Xradia introduced an entirely new of x-ray imaging tool that adds a powerful new analytical and diagnostic capability to that provided by EDS and WDS techniques. The nanoXFi places a high-resolution zone-plate x-ray lens in close proximity to samples under electron beam probe (such as in a scanning electron microscope). Rather than measuring the wavelength or intensity of e-beam induced x-ray fluorescence, the lens collects the fluorescing x-rays, much as a camera lens collects and focuses visible light. The collected x-rays are filtered and ultimately imaged onto a direct-detection CCD camera.
X-ray fluorescence image of copper layers in semiconductor IC
Resulting images provide a clear, highly detailed spatial map or picture of the fluorescent x-rays, one wavelength, or one element at a time. Even subsurface detail, such as the structure of multiple copper interconnect layers in a semiconductor IC, can be clearly resolved through fluorescence imaging. As such, the x-ray fluorescence imager can be thought of as an imaging spectrometer. It fills a critical information gap in the analytical capabilities of EDS and WDS technologies, and can be thought of as a highly complimentary tool.
Click here for links to the nanoXFi product data sheet and our white paper on X-ray fluorescence imaging.