Article

SXI: X-ray Beam Induced Secondary Electron Imaging

X-ray beam induced secondary electron imaging (SXI) has proved to be a valuable tool for micro-area XPS analysis. SXI produces a micrometer-resolved image of the sample surface by scanning a focused X-ray beam across it and recording the secondary electrons emitted at each position.

In many analysis situations, the feature of interest occupies only a small part of the sample surface. For instance, a particle on a medical device may be only tens of micrometers across. To characterize such a localized feature with XPS, one must first locate it and then place the analysis spot precisely on it. SXI can be used to achieve both of these goals.

In this article we will explain how an SXI image is formed, starting with the emission of secondary electrons from a sample under X-ray illumination and then describe why the lateral information (image) originates from rastering the X-ray beam across the sample rather than the electron detector. We will also touch briefly upon the sources of contrast in SXI.

How does SXI work?

Let's begin by understanding the working of SXI in a series of steps:

Step 1: Secondary electron emission

When an X-ray beam strikes a sample surface, it ejects photoelectrons and Auger electrons. As these energetic electrons travel through the solid, they lose their kinetic energy through inelastic scattering. These scattering events release a cascade of low-energy secondary electrons, most with kinetic energies of only a few electron-volts. As a result of their low energy, only those secondary electrons that are generated within the outermost few nanometers can escape the surface of the sample. Therefore, the secondary electron yield is a strong surface-sensitive signal. The emission of secondary electrons from the surface is dependent upon the composition as well as the topography of the X-ray irradiated surface.

Step 2: Rastering the beam

The emitted secondary electrons are then collected by the same energy analyzer and detector used for photoelectron spectroscopy. The detector counts every electron passing through the analyzer from anywhere within its acceptance cone, without the ability to register where on the sample they originated. Since the detector cannot tell where the electrons came from, forming an image requires controlling where they are generated, and this is the role of the scanning X-ray beam.

In a scanning X-ray microprobe, X-rays are generated by focusing an electron beam onto an anode, which emits characteristic X-rays from a small spot. A curved quartz crystal placed in the path of these X-rays acts as both a monochromator and a focusing element. The result is a focused, monochromatic X-ray micro-beam. Rastering the electron beam moves the X-ray source spot on the anode, and the monochromator translates that into a matching movement of the X-ray spot on the sample. The beam is thus rastered electronically, with no moving mechanical parts, and its position is known at every moment in time.

Step 3: Turning a sequence of counts into an image

With a beam that can be placed anywhere, the image can now be constructed one pixel at a time:

1. The X-ray beam is directed to a defined position (pixel) on the sample.
2. Emitted secondary electron yield is counted for a short dwell time.
3. That count is stored as the intensity of the pixel at that position.
4. The beam steps to the next position, and the loop repeats until the full raster is complete.

Since only one small spot is illuminated at any instant, every count recorded during a dwell period can be attributed to a single, known beam position. The scan control pairs each count with the position at which it was measured, so the lateral information is carried by the timing of the measurement rather than by the detector. Assembling these counts in raster order produces a grayscale map of secondary electron yield across the scanned area.

Additional considerations

• Resolution is set by the beam. Image resolution is governed by the X-ray beam size and the pixel step, not by the detector. SXI is therefore usually acquired with the smallest beam available, to resolve the finest features.
• Imaging and analysis beams may be different. Spectra are often acquired with a larger beam than the one used for imaging, trading lateral resolution for a higher count rate. The analysis beam is chosen to match the feature, in a way that it is small enough to stay on it yet large enough to give good signal.
• Image and analysis are inherently registered. Whatever beam size is chosen, it comes from the same X-ray source and optics and is positioned by the same deflection system that formed the image. Selecting a point on the SXI image therefore places the analysis beam on that point.

Compositional contrast

On a flat sample, SXI contrast is dominated by differences in material, which produce different secondary electron yields.

The yield is mainly governed by the following factors:

• Generation. Elements differ in how strongly they absorb characteristic X-rays, for example Al Kα X-rays. Stronger absorption near the surface creates more photoelectrons and Auger electrons and, in turn, more secondary electrons.
• Escape. Once created, a secondary must travel to the surface and overcome the work function to leave. How far low-energy electrons travel before losing their energy varies between materials, and surfaces with a lower work function generally emit more secondary electrons and appear brighter.

Topographic contrast

SXI images show local topographical contrast because the secondary electron yield changes with the local tilt of the surface and its orientation toward the analyzer. Two mechanisms are at work, one governing how many secondary electrons escape the surface, and the other governing how many of those reach the detector.

In practice, the two mechanisms act together. With the X-ray beam and the analyzer at a fixed angle to each other, tilting a facet changes both how many secondary electrons it emits and how many are collected, so the brightness of a facet usually reflects both.