Electron Probe Microanalysis

While SEMs are the workhorse method for using electron optics to image samples, there are other alternatives as well that offer their own advantages. Among these are electron probe microanalysis (EPMA), which may also be referred to as “microprobe” for short. EPMA is a surface analysis technique that probes the sample surface with a beam of electrons. The primary signal analyzed by the instrument is composed of x-rays that are emitted from the sample due to interactions with the electron beam. These x-rays are analyzed by energy-dispersive spectrometers (EDS) and/or wavelength-dispersive spectrometers (WDS). Both provide information on the elemental composition of the sample surface directly targeted by the electron beam.

Generally speaking, EDS detectors are fast, but WDS detectors provide more quantitative analysis and better coverage of lighter elements. Typical EPMA instruments have a single EDS detector and multiple WDS detectors. The electron beam penetrates a small distance into the sample, so the measurement provides elemental analysis of a small volume of the surface layer on the order of a micron in dimension. Field emission sources can help reduce the size of the spot by roughly a factor of 10.

By moving the electron beam across the surface, the instrument can build up elemental spectra for each point in a map of the sample surface. EPMA instruments are also outfitted with detectors for electrons that emerge or are scattered from the surface. This signal can be composed into an image of the surface, just as in a standard SEM. This demonstrates the complementary nature of EPMA and SEM with EDS or WDS detection. Both types of systems use electrons to develop an image and x-ray detectors to provide elemental information. The main difference is that the EPMA system is optimized for x-ray spectroscopy, while the microscope is optimized for imaging.

Academia is the largest market for EPMA, applying the technique to many advanced materials science and engineering projects. Sample types include geological samples, alloys, ceramics, semiconductor materials, microelectronics and thin films on a variety of substrates. Samples are exposed to vacuum in EPMA analysis, so it is generally not suited for life science samples. EPMA is also commonly found in industrial labs in the mining and metals, and semiconductors and electronics industries.

Including aftermarket and service, which are significant for these sophisticated instruments, total market demand for EPMA was about $70 million in 2015. However, not many vendors compete in the market. Cameca has taken over the leading position in the market from JEOL, but these two companies dominate the competitive situation, with Shimadzu as the remaining significant vendor. Other vendors support the market through various components: vacuum systems, electron optics and x-ray detectors.

EPMA at a Glance:

Leading Suppliers

• Cameca (AMETEK)

• JEOL

• Shimadzu

Largest Markets

• Academia

• Metals & Mining

• Semiconductors & Electronics

Instrument Cost

• $500,000—$2 million

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