Overview of Key Points on Electron Probe Micro-Analyzer (EPMA)

1、 What is Electron Probe Micro-Analyzer (EPMA)?

The Electron Probe X-ray Micro-Analyzer (EPMA) is an efficient analytical instrument developed based on the principles of electron optics and X-ray spectroscopy. It combines microanalysis and compositional analysis and is typically attached to the column of a scanning electron microscope (SEM) or a transmission electron microscope (TEM) as a wavelength dispersive spectrometer (WDS) and energy dispersive spectrometer (EDS).

EPMA uses a high-energy electron beam of 0.5~1 μm to excite the sample. The interaction between the electron beam and the sample produces characteristic X-rays, secondary electrons, absorbed electrons, backscattered electrons, and cathodoluminescence, which are used to analyze the composition, morphology, and chemical bonding state of the sample's micro-region (μm level).

Figure 1: Electron Probe X-ray Micro-Analyzer (EPMA)

2、 Basic Principles of EPMA

When high-energy electrons eject inner-shell electrons (low energy level) from atoms, X-rays are produced. High-energy electrons replace the ejected electrons, and the energy loss during their transition to a lower energy level is emitted as characteristic X-rays.

The wavelength of the characteristic X-ray spectrum lines is uniquely related to the atomic number (Z) of the sample material. By focusing a fine electron beam on the sample surface to excite the characteristic X-rays of the sample elements and measuring the wavelength (or characteristic energy) of the characteristic X-rays, the atomic number of the corresponding elements can be determined, allowing qualitative analysis of the elements present in the sample.

Since the intensity of a characteristic X-ray of a specific element is proportional to the concentration of that element in the sample, measuring the intensity of the characteristic X-ray allows for the calculation of the relative content of that element (quantitative analysis).

3、 EPMA Equipment

An EPMA instrument consists of three main parts: the electron column, sample chamber, and signal detection system. The electron column and sample chamber parts are similar to those of an SEM.

The signal detection system is an X-ray spectrometer. For the characteristic X-rays produced in the sample, there are two methods of spectrum dispersion:

1. Wavelength Dispersive X-ray Spectroscopy (WDS): The spectrometer measures specific wavelengths of characteristic X-rays.
2. Energy Dispersive X-ray Spectroscopy (EDS): The spectrometer measures the characteristic energy of the X-rays.

Typically, SEM and EPMA are combined to provide both morphological and compositional analysis functions.

Figure 2. Schematic Diagram of the Electron Probe Micro-Analyzer (EPMA) Equipment

4、 EPMA Application Scenarios

Point Analysis

Point analysis involves performing a full spectrum scan on selected micro-regions of the sample surface for qualitative or semi-quantitative analysis and determining the mass fraction of the elements present for quantitative analysis.

Figure 3. EPMA Point Analysis Results

Line Scan Analysis

Line scan analysis involves analyzing the mass fraction of elements qualitatively or semi-quantitatively along a selected linear trajectory on the sample surface.

Figure 4. EPMA Line Scan Analysis Results

Area Scan Analysis

Area scan analysis involves raster scanning the sample surface with the electron beam, using the signal intensity of the X-rays of specific elements to modulate the brightness of the cathode ray tube screen, resulting in a scanned image of the mass fraction distribution of the element. The distribution of elements on the sample surface can be displayed as brightness (or color) on the screen, with brighter areas indicating higher element content. This method is commonly used to study the distribution of impurities, phases, and elemental segregation in materials, typically in conjunction with microscopic morphology analysis.

Figure 5. EPMA Area Scan Analysis Results

5、 EPMA Analysis Examples

Quantitative Analysis of Matrix Phase Grains

Due to the changes in peak position and shape of characteristic peaks when super light elements combine with different elements, the characteristic peaks and background peaks of standard and unknown samples should be set separately. Extending the test time enhances sensitivity. For rare earth elements, interference effects from different elements' characteristic peaks and background peaks may occur, requiring confirmation of each element to avoid overlapping interference.

Alloying by adding rare earth elements such as Tb and Dy is an effective method to improve the magnetic properties of Nd-Fe-B magnets, achieved through grain boundary doping and grain boundary diffusion. EPMA elemental area analysis results show the distribution of doped elements mainly at the grain boundaries.

Figure 6. Element Distribution Characteristics of Grain Boundary Modified Nd-Fe-B Magnets:(a) Backscattered Image;(b) Element B Distribution Image;(c) Element O Distribution Image;(d) Element Tb Distribution Image;(e) Element Co Distribution Image;(f) Element Cu Distribution Image;(g) Element Ga Distribution Image;(h) Element Pr Distribution Image;(i) Element Nd Distribution Image.

6、 EPMA Sample Requirements and Preparation Methods

Sample Requirements

1. Sample size: Should be blocky or granular, with maximum size determined by the sample holder size of different instruments. Homogeneous samples are required for quantitative analysis, typically with a thickness greater than 5 μm.

2. Good electrical and thermal conductivity: Metal materials generally have good thermal/electrical conductivity, while silicate and other non-metallic materials usually have poor conductivity. A uniform layer of carbon or aluminum film of 20~400 Å is typically coated on the surface of these materials to increase surface conductivity.

3. Smooth and flat surface: The sample surface must be polished to ensure cleanliness and flatness. An uneven surface may cause irregular absorption of emitted X-rays.

Preparation Methods

1. Powder samples: Powder can be directly placed on double-sided conductive carbon tape on the sample holder, pressed with a flat object (glass plate), and loose particles blown off with a blower.

2. Block samples: Small block samples can be embedded in epoxy resin for grinding and polishing. Larger samples can be directly ground and polished. Porous or loose samples like sintered materials require vacuum embedding. Non-conductive samples need to be coated with conductive films like gold or carbon immediately after processing to improve conductivity.