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Before presenting the techniques discussed in this book, it would be helpful to find a common basic principle to describe them. No matter whether we are talking about infrared spectroscopy or TOF-SIMS or SEM, the main principle consists in probing a sample with radiation.

Figure I.4: General concept of probing the surface of a sample by primary radiation excitation

The sample is essentially analysed by radiation that probes for specific properties and characteristics of the material. This radiation, which is called the primary radiation, can consist of electrons, ions, neutral particles and photons, such as infrared waves and X-rays. The primary radiation triggers a reaction specific to the sample that may take the form of the emission of electrons, ions or X-rays. This “reaction” by the sample is detected by an electronic system composed of an analyser and a detector. The result can be displayed as a spectrum on a computer or be printed on paper. The last step of the process is data evaluation by an experienced analyst. The evaluation must include

 plausibility check

 comparison with databases

 interpretation with respect to the analytical problem

The nature of the interaction which occurs between the probing beam and the sample depends on the type, energy and angle of incidence of the probing radiation and, of course, the sample material.

The primary radiation interacts with the sample in a specific way. Each type of sample reaction can be detected separately and analysed to reveal the chemical and physical composition of the sample and its surface. The radiation emitted by the sample is called secondary radiation. Each type of primary radiation can produce a different type of secondary radiation. Probing with an electron beam, for example, may lead to the formation of:

 secondary electrons

 X-rays

 back-scattered electrons

 fluorescence

Figure I.5: Surface analysis techniques sorted by the excitation radiation type: photons, electrons and ions

Each type of radiation conveys different information about the sample that all adds up to a comprehensive understanding of the sample’s properties. Not only the primary radiation, but the secondary radiation emitted by the sample, can consist of electrons, ions, neutral particles and photons that result from sample excitation or reflection of the primary radiation.

The latter is a consequence of diffraction and dispersion that change the energy, angle and intensity of the primary radiation in accordance with the topography, structure and chemical composition of the sample. The secondary radiation emanating from the sample is detected, analysed and displayed in the form of an angle-, energy- or mass-resolved spectrum, which contains information about the sample and its surface.

The various types of probing primary radiation and detected secondary radiation have spawned more than 50 different analytical techniques over the decades. Some of them are useful for solving practical problems and have made their way into routine work. Many of them, however, never passed the experimental stage and have very limited application to technical samples outside of academia. Figure I.5 sums up the abbreviations of a few techniques sorted by the type of primary radiation. A sample excitation by photons (left part) can be performed e.g. by infrared radiation resulting in absorption and the “answer” of the sample can be analysed by reflection absorption spectroscopy (IRRAS), internal reflection spectroscopy (MIR or ATR) or diffuse reflection spectroscopy (DRIFT). A photon excitation can also be done using a laser resulting in the release of ions (LAMMA) or by X-rays that produce photoelectrons analysed by X-ray photoelectron spectroscopy (XPS).

Table I.1: List of analytical techniques
Primaryradiation	Secondary radiation	Technique	Abbreviation	Analysed area
Electrons	electrons	auger electron spectroscopy	AES	uppermost molecular layer
scanning electron microscopy	SEM	sample surface down to a depth of a few microns
X-rays	electron microanalysis	ESMAEDSWDX	sample surface down to a depth of a few microns
Infrared radiation	infrared radiation	surface infrared spectroscopy	FT-IRATRIRRAS	sample surface down to a depth of a few microns
infrared microscopy	IRM
X-rays	electrons	X-ray photoelectron	XPSESCA	sample surface down to a depth of a few nanometres
ions	ions	secondary ion mass spectroscopy	SIMSTOF-SIMS	uppermost molecular layer

In this book, the author covers those techniques which have proven to be very useful for routine work and can deliver data in a reasonable time and at reasonable cost.

The techniques mentioned in Table I.1 yield different data about the sample. Each has its particular strengths and weaknesses. It is very important to appreciate this when trying to find the right combination for the given analytical problem. It is commonly said that one technique on its own is no use and so a combination is the best way of achieving the right results. The important parameters of a technique are its

 information depth

 detection limits

 information content

 suitability for technical problems

Some techniques, for example, allow only very limited sample sizes, which sometimes renders the technique useless for “real world samples”. Others require vacuum conditions, and that excludes liquid or volatile samples. Only a handful of techniques have proven useful for routine work. The limiting features are:

 measuring time per sample

 suitability for technical samples

 comprehensive databases of reference materials

 sample preparation

Another important question is the sample area to be analysed. If, for example, a paint crater a few microns in diameter has to be analysed for possible surface contaminants capable of causing cratering, the technique to use must allow for spot analysis. This means the investigation of a very small spot with a lateral resolution of a few microns.

Figure I.6: Measuring modes in surface analysis

If, on the other hand, it is the general surface quality of the sample which is of interest, a larger area measurement must be performed in order that a representative image of the surface composition may be obtained.

Analysis of the distribution of a specific substance over a certain area calls for a scanning technique that generates a chemical map of the analysed area. On the assumption that not only the chemical composition of a surface area has to be analysed but also the depth distribution, a depth-profiling mode needs to be chosen. That entails sputtering the sample layer by layer and analysing the surfaces as they become exposed.