You are faced with low yields, field failure, customer complaints, and regulatory agency complaints. Avoiding poor quality, decreased profits, and losses to the competition requires that you find the source of the problem and fix it, immediately.
You suspect surface contamination; and perhaps you find a higher than expected particulate level, an increased non-volatile residue (NVR), or very high total organic carbon (TOC). These tests can tell you whether or not you have contamination, but do not tell you what kind of contamination it is or where it is on the surface of your component. Ok, then what? You need specifics. The most direct route is often to identify the contaminant. Then you can look for the source and determine how the contaminant is getting on your product or why your process is not removing it. So you search for an analytical test to identify the contaminant.
Analytical tests involve “somehow perturbing the area to be analyzed and observing what results”.1 There are three aspects to specific analysis: where is it, what is it, and how much of it there is. In identifying what it is, distinguishing between elements is part of the story. Chemical speciation, distinguishing between different molecules containing the same atoms (e.g., are the carbon bonds single or double) can give a much more complete story.
Spectroscopy is frequently used to characterize low levels of contaminants by surface perturbation. Among the large assortment of spectroscopic techniques,2 two of the most useful for locating, identifying, and quantifying trace surface contaminants are X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES). Each technique has both advantages and disadvantages depending on the problem at hand, so it is worth your time to understand the features, benefits, and limitations of both.
XPS and AES are examples of techniques based on the photoelectric effect, discovered by Heinrich Hertz in 1887. By explaining that the photoelectric effect was caused by light quanta (photons), Albert Einstein was awarded the 1921 Nobel Prize in Physics. In the photoelectric effect, energy and momentum from an incident light photon (the perturbation) is sufficient to dislodge an atomic electron (the effect). In many ways, the action is similar to the cleaningaction of dry media or CO2 snow that we discussed in last month’scolumn.3Another non-specific technique, optically stimulated electron emission (OSEE)4thatisalso based on the photoelectric effect can be used as a screening test for thepresence of contamination but that technique is neither element- nor chemical-specific.
XPS, often referred to as electron spectroscopy for chemical analysis (ESCA), has been a mainstay of surface analysis, notably in aerospace industries, since the early 1970s. It was as developed as an analytical tool largely through the research of Swedish physicist Kai Seigbahn, for which he sharedthe 1981 Nobel Prize in Physics.
XPS is useful in tracking down culprit contaminants because not only does it allow identification of chemical elements, it can also be used to identify molecules. When an incoming X-ray photon hits an atom near a surface, an electron from a core electron shell of that atom may be ejected. XPS analyzes the energies of these emitted electrons. Each element emits electrons with specific energies, thus allowing the identification of the specific chemical element. In addition, these energies are shifted by the presence of chemical bonds to other atoms in the molecule. By matching to known fingerprints, in many cases, the identification (speciation) of the molecular structure that contains the emitting atom can be made. XPS is a surface analytical technique because even though the incident X-rays penetrate far into the body of the object, only electrons emitted from atoms near the surface can escape and reach the analyzer.
The Auger effect is named for the French physicist, Pierre Auger. However, in a footnote to history, the effect was actually described about two years before Auger’s independent discovery by an Austrian physicist, Lise Meit-ner. Possibly due both to the political climate of the 1920s and reluctance to accept the contribution of women scientists, her discovery was largelyoverlooked and attributed to Auger.
AES provides another way of analyzing surfaces; it is especially useful when the location of contaminants need to be precisely specified as well as its elemental or chemical identity. With AES as with XPS, atomic core electrons are emitted. However, while XPS perturbs the surface with incident x-rays, AES employs a focused beam of electrons. This beam can be focused down to very small diameter of a few tens of nanometers. The energy level occupied by the ejected Auger electron distinguishes AES from XPS. The Auger electron is not directly ejected by the incident electron. Instead, in a sort of “musical chairs” scenario, a higher energy state electron moves in to take the place of the directly ejected electron. The excess energy associated with this transition is sufficient to cause the ejection of yet another higher energy state electron, the Auger electron. So in AES, one incident electron comes in and several electrons go out: the incident electron that may leave the surface or remain on the surface, the inner core electron that the incident one ejects, and the Auger electron that carries the element identification characteristics. The energies of the Auger electron are also shifted by molecular composition so can be utilized for speciating chemical identity.
AES is used for contaminants with very small surfaces; and it is most readily used with conductive substrates. When AES is performed with insulating surfaces, there are charging effects. Since the flux of incident electrons is much greater than the flux of emitted electrons, the surface acquires a negative charge. The repelling effect of this surface charge acts as a barrier and slows and can even stop the flux of emitted electrons.
With XPS, there is also a charging effect. On non-conducting surfaces, the emitted electrons carry away a negative charge and the surface gradually acquires a positive charge. This charge gives a boost to the energies of the emitted electrons causing a slight additional shift in the energies of the characteristic lines. However, this phenomenon can be readily compensated for by the skilled analyst and therefore does not does not prevent XPS from being used on insulating surface.
AES OR XPS?
Now that you have a flavor of the theory, which should you use? In our next column, we will provide some guidelines in terms of the surface, the suspectcontaminant(s), and your analytical resources.
Acknowledgement: The authors thank Dr. Ben Schiefelbein, RJ Lee Group, for many helpful comments and suggestions.
- B. Schiefelbein, personal communication.
- A fairly comprehensive review of spectroscopic techniques and accompanying alphabet soup of acronyms can be found in “A Descriptive Classification Of The Electron Spectroscopies”, Pure & Appl.
Chem., Vol. 59, No. 10, pp. 1343–1406, 1987. (found online at www.iupac.org/publications/pac/1987/pdf/5910×1343.pdf )
- B. Kanegsberg and E. Kanegsberg, “Contamination Control Inside and Outside the Cleanroom: Non-chemical cleaning”, Controlled Environments Magazine, Sept. 2007.
- M. Chawla, “How Clean is Clean?”, Handbook for Critical Cleaning (HBCC), CRC Press (2001), pg 423; C. LeBlanc, “Cleaning Metals: Strategies for the New Millennium”, HBCC, pg 513.
Barbara Kanegsberg and Ed Kanegsberg are independent consultants in critical and precision cleaning, surface preparation, and contamination control. They are the editors of The Handbook for Critical Cleaning, CRC Press. Contact them at BFK Solutions LLC., 310-459-3614; firstname.lastname@example.org; www.bfksolutions.com.