GENERIC METHODOLOGIES
2.6 SURFACE ANALYSIS AND DEPTH PROFILING .1 AUGER ELECTRON SPECTROSCOPY
39 2.6 SURFACE ANALySIS AND DEPTH PROFILING
FIGURE 2.20
Schematic diagram of a refractively scanned interferometer.
From Peter R. Griffiths, James A. De Haseth, James D. Winefordner (Series Editor), Fourier Transform Infrared Spectrometry, 2nd Edition. Copyright © 2007, John Wiley and Sons
the FT calculations are done digitally (Griffiths, 1983). A third advantage is the use of interferences in the light, giving this technique a high signal-to-noise ratio which allows it to boost the amplitude of the targeted wavelengths. In addition, remote sensing, which enables the process to not have physical contact with the sample, ensures that the property of the sample remains unchanged.
The FTIR technique has gone through many developments in its instrumentation since the 1960s, mainly to improve the speed of the scanning process. It has provided a new way to test samples using infrared spectroscopy, making analyzing materials “virtually limitless” (Nicolet, 2001). However, there are some limitations to this technique, one of which is that FTIR spectroscopy focuses more on qualita- tive results as opposed to quantitative (Van de Voort, 2009). As an example, if the FTIR spectroscopy method was used to track relative changes in a substance, the results may be thrown off if the formula- tion of the substance changes or is covered by another substance.
2.6 SURFACE ANALYSIS AND DEPTH PROFILING
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Auger Process to give surface composition information with very high resolution (Grant, 2003). It can also be used to analyze characteristics such as surface roughness, impurities, and binding energies of a sample.
The Auger process was discovered by Pierre Auger in 1925 (Chourasia, 1997). When an atom is exposed to short pulses of X-rays, one of two things can happen: the emission of electromagnetic radia- tion or the ejection of an electron. The latter is called the Auger Process and the electron is the Auger Electron. As demonstrated in Fig. 2.21, when an atom is exposed to an X-ray pulse or electron beam, an initial electron is ejected to release the extra energy. This causes an empty electron hole in the inner orbital shell. To keep its stability, an outer electron will move in to fill the space and a second electron will be ejected. This second ejected electron is called the Auger electron.
Since different atoms will produce Auger electrons with different energies, AES uses this information to sort the Auger electrons and associate them with their parent atoms to determine the sample’s composition.
The instrumentation of AES is usually composed of an electron beam, a sample compartment, an analyzer, and a detector. Instead of X-rays, modern day AES uses electron beams to excite the surface molecules. The beam is usually around 10 nm in diameter, but the smaller the diameter, the higher the resolution produced, because it can analyze smaller area with the same details (Felton, 2003).
As discussed in an article, “On the Surface with Auger Electron Spectroscopy,” by Michael J.
Felton, there are two main types of analyzers. The analyzer is used to separate the electrons by energy level. The first one is a cylindrical mirror analyzer (CMA). It has a simple configuration, where the sample and electron beam all align in a straight line as shown in Fig. 2.22 (Felton, 2003). When the FIGURE 2.21
Two views of the Auger process. (A) It illustrates the Auger deexcitation. An incident electron/photon creates a core hole in the 1s level. An electron from the 2s level fills in the 1s hole and the transition energy is imparted to a 2p electron which is emitted. The final atomic state thus has two holes, one in the 2s orbital and the other in the 2p orbital. (b) It illustrates the same process using spectroscopic notation.
From Auger Effect, Wikipedia. Retrieved from https://en.wikipedia.org/wiki/Auger_effect.
41 2.6 SURFACE ANALySIS AND DEPTH PROFILING
electron gun is focused on the sample, the Auger electrons will be ejected and bounced against the outer mirror, eventually funneled to the analyzer and detector.
The second type of analyzer is the hemispherical analyzer (HSA). It has a more complex align- ment, but it can give energy resolution measurement information. Fig. 2.23 is an example of the HSA alignment (Felton, 2003). The electron beam is shot at an angle which allows the Auger electrons to travel through the instrument. Lenses and mirrors are used to focus the electrons so they can reach the analyzer and detector without losing their energy.
Finally, there is the detector in the Auger electron spectrometer. This component is used to count the number of each type of electron after passing through the analyzer. The ongoing trend now is to use multiple detectors (up to 16) within the machine. Less time is required to process and record the data (Felton, 2003). With this data, the types and amount of each atom can be calculated and can give the composition of the area analyzed.
Some advantages of using this technique are that it has high resolution, precise chemical sensitiv- ity, and it is very good in analyzing semiconductors with submicrometer features (Felton, 2003). The precision of the instrument is mainly determined by the diameter size of the electron beam, so AES can be used to analyze surfaces as small as 10 nm.
However, there are some issues researchers are still working on now to improve this technique to make it more efficient. One issue is the backscattering in high-energy electron beam (Felton, 2003).
At high energy and small diameter, the beam can sometimes lose focus because some of the ray gets deflected outward. Another problem is that when using high energy, the beam may affect the chemical properties of the sample. Therefore, the current focus now is to create beams that are lower in energy but still high in resolution (Felton, 2003). In addition, improvement on the Auger electron matching software is also useful to the advancement of this technique to make it more efficient.
2.6.2 SECONDARY ION MASS SPECTROSCOPY
The secondary ion mass spectroscopy (SIMS) is a surface analysis technique that analyzes the compo- sition of a sample at the atomic level (around the 100 nm range). It accomplishes this by using a beam FIGURE 2.22
AES experimental setup using a CMA.
From https://upload.wikimedia.org/wikipedia/commons/f/f7/AES_Setup2.JPG.
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infused with a primary ion to break off molecules from the material’s surface. The ejected ions are called secondary ions. These are then used to determine some characteristics of the material such as surface composition, surface impurities, thickness, and 3D profiling.
It started in 1910, when J. J. Thomson first discovered the release of positive ions and neutral atoms when a solid material is bombarded by ions. It was not until the 1960s that SIMS instruments were developed (Benninghoven, 1987). By the year 2002, scientists declared the method no longer a novelty and decided that this technology is at the end of its development. Fortunately, during the following years there were rapid advancements in the field of SIMS due to the incorporation of a charge beam and time-of-flight mass spectrometry (TOFMS) analyzers (Griffiths, 2008).
The SIMS technique works by shooting an ion beam composed of a primary ion, usually argon, gallium, or other alkali metal ions. When the ion hits the surface of the material, its charge and kinetic energy gets transferred over to the surface molecules in contact. This causes those molecules to become ionic and dislodge from the sample. These dislodged molecules are called “secondary ions.” In a way, as quoted by Fraser Reich of Kore Technology, “It’s like playing molecular pool, the cue ball goes into the material, and it sputters-it lifts off material that’s characteristic of the top surface” (Griffiths, 2008).
Fig. 2.24 is a pictorial interpretation of this effect.
FIGURE 2.23
An example of a spectrometer with a HSA.
From XPS Instrumentation, CasaXPS (2005).
43 2.6 SURFACE ANALySIS AND DEPTH PROFILING
Since each secondary ion will have a specific charge and mass, they can be sorted and counted depending on their mass-to-charge ratios. The result can be used to identify the composition of the material’s surface.
SIMS instrumentation involves four components: the ion beam, a vacuum, a mass analyzer, and a detector. As mentioned before, the ion beam is usually composed of a primary alkali metal ion that propels toward the surface of a sample material to slowly chip away its surface for analysis.
After the secondary ions are ejected, they are taken through the vacuum toward the analyzer. The analyzer will sort the ions according to their mass-to-charge ratio. When they reached the detector, the amount of each type of ions will be recorded. Fig. 2.25 shows the components of an SIMS as discussed above.
According to Jennifer Griffiths, there are three basic types of mass analyzers: quadrupole, magnetic sector, and time of flight mentioned. The quadrupole analyzer only allows selected mass to pass through and the ions are separated by resonant electric fields. The magnetic sector analyzer uses electrostatic and magnetic properties to separate the ions by their mass-to-charge ratio. The last one, time-of-flight analyzer, separates ions according to their kinetic energy.
Several significant advantages are established using this technique. The SIMS technique has a high sensitivity. Since the area analyzed is determined by the size of the ion beam and ions are very small in diameter, even at low concentration, SIMS is still precise enough to analyze every part of the sample.
It also does not require complex sample preparation, which makes it time efficient.
Compared to the early developments of this method, the SIMS today is no longer limited to surface analyzers, as advances have been made in examining samples in three dimensions, known as depth profiling. As the outer layer of the sample slowly chips away, depth profiling allows for a continuous, layer-by-layer analysis of its surfaces. However, it does require a considerable amount of computer data to record every single image of the material in three dimensions, which is still one of its ongoing issues.
Other limitations to using SIMS include the physical damage it causes to the sample, allowing only a FIGURE 2.24
An ion beam is shot toward the surface of a sample to break it apart.
From https://en.wikipedia.org/wiki/Secondary_ion_mass_spectrometry.
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one-time analysis of the particular sample specimen. In addition, the area analyzed on the sample is dependent on the size of the ion beam.
The future of SIMS is viable as many recent studies have been applied to further this technol- ogy. Some of these include the utilization of a cluster ion beam, which uses particles composed of multiple atoms as the primary ion for the spectrometer. With this beam, more complex molecules will be able to break away from the surface. This modification has already propelled SIMS research in the biological field, because scientists are able to study the material surface without reducing it to simple carbons.