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Combining ESCA with SSIMS to achieve synergy.

Combining ESCA With SSIMS To Achieve Synergy Electron spectroscopy for chemical analysis (ESCA) is a powerful surface analysis technique, but it is limited in its ability to give the user molecular-specific information.

Static secondary ion mass spectroscopy (SSIMS), on the other hand, provides "fingerprint" characterization of molecular monolayers but lacks the quantitative and broader characterization aspects of ESCA> The complementary and synergistic use of SSIMS and ESCA can greatly enhance characterization of insulating surfaces.

Using this complementary approach, Surface Science Laboratories now employs an ESCA\SSIMS instrument developed in conjunction with Sub Monolayer Science to solve problems in the magnetic media, aerospace, semiconductor, and laser optics industries.

This tandem arrangement is particularly useful in polymer adhesion problems in thin-film metallization and biomaterials areas, as well as in chlorofluorocarbon (CFC) replacement investigations, where sensitivity to submonolayer organic contaminants is critical. Manufacturers considering CFC replacement in their cleaning processes must evaluate effects of such changes on cleanliness levels as well as on process parameters. The ESCA\SSIMS combination is ideal for detecting these differences.

ESCA uses monochromatic Al k [alpha] x-rays to excite the surface atoms in a specimen. The kinetic energy and number of electrons escaping from the material yield information about both elemental composition and chemical bonding. The "soft" nature of the x-ray irradiation allows nondestructive analysis of polymers and thin organic coatings (e.g., molecular monolayers).

The strengths of ESCA include its ability to ascertain the composition and chemistry of a surface (top 50 A), as well as the fact that it is nondestructive and can analyze insulative as well as electrically conductive surfaces.

However, spectral overlaps for important elements such as carbon, oxygen, and nitrogen in materials with multifunctional groups limit the specificity of chemical assignments. Furthermore, the analysis depth of about 50 A is often too deep for solving problems in catalysis, printing on plastics and adhesion failures. In addition, ESCA in insufficient for identification of the spectral "fingerprints" of submonolayer contaminants.

Static SIMS is an ultra-surface-sensitive tool which employs a low energy ion beam to desorb and ionize atomic and molecular fragments from the outermost atomic layer. These ions are then detected by a quadrupole or time-of-flight analyzer, allowing precise, molecular-specific structural assignments. SSIMS yields structural information about organic molecules in submonolayer concentrations and has very low detection limits for Li, Na, and K.

Given the strengths of each technique, teaming them opens the door to challenges like chemical species identification on surface-modified substrates, detection of thin-layer organic contamination on polymers, correct assignments of siloxanes on quartz and optics, and characterization of surface functionalized groups on silicon wafers.

Attachment of SSIMS to an ESCA instrument affords the user a second analytical technique for a fraction of the cost of an additional stand-alone instrument. A typical ESCA spectrometer contains a high-vacuum analysis chamber (usually with one or two extra ports), vacuum pumps a focusing flood gun charge neutralizer, and ion etching capabilities. Simple tuning of the ion gun for very low etch rates and proper focusing of the charge neutralize are all that is required for SSIMS analysis.

The real synergy is that a sample need only be pumped down once. The area of analysis is already defined, and the ESCA data give a head start for interpreting the key mass fragments from the SSIMS data.

The combination of ESCA and SSIMS has some interesting applications. Electropolishing of stainless steel is a critical step in the manufacture of high-purity gas lines, medical instruments, clean-room surfaces, and equipment for food processing and packaging.

The technique involves electrochemical removal of surface asperities, using acidic reagents and electric current to produce a smooth, uniform chromium-rich oxide layer. This adds to corrosion resistance and removes inclusions and other particulate generation. Contaminants of concern are alkali, phosphate, and sulfate residues, and organic materials including hydrocarbons, silicones, and esters.

While ESCA serves as a useful tool in measuring the amount of inorganic and organic contaminants, accurate characterization of specific organic material such as cutting oil residues, plasticizers, and mold release agents is often difficult.

In one study, ESCA and SSIMS were both used to measure the effectiveness of a new steel cleaning process in removing hydrocarbon, dioctyl phthalate (DOP) and organic silicon. While the ESCA data showed that much of the carbon had been removed from the steel surface, high-resolution data were not definitive in characterizing the outermost layer of remaining organics.

By contrast, SSIMS data indicated that although much of the organic layer was removed by the new cleaning process, a thin film of hydrocarbons and silicones was still present. The SSIMS data could be used as a direct measure of the cleaning process selectivity.

Evaluating changes in or the efficacy of clearing processes is an area where an ESCA/SSIMS combination is useful. Even minor contamination on a laser optic window surface may lead to absorption or deflection of light. Thus, cleanliness is vital to optics manufacture. CFCs are currently being replaced by aqueous-based cleaning methods which must be as effective in removing pervasive contaminants such as silicones. At the same time, detergent residues like [Na.sup.+] or [K.sup.+] should be avoided.

While ESCA can determine the surface abundance of carbon, it cannot characterize specific types of organic contamination. However, SSIMS spectra for common organic molecule like hydrocarbons, DOP, and silicone are unique and may be used as spectral "fingerprints" to identify them. Cations, such as [Na.sup.+] and [K.sup.+], can be detected with SSIMS at concentrations a hundredfold lower than by ESCA.

Trace levels of silicones are especially difficult to measure on SiO films and optics, because the large [SiO.sub.2] silicon peak dominates the ESCA region where silicone would be detected. Fortunately, the SSIMS spectrum for dimethyl silicone has strong peaks at 73 and 147 atomic mass units (AMU) from ([CH.sub.3])[sup.3.Si.sup.+] and its dimen which are absent in the spectrum of [SiO.sub.2]. With SSIMS, detection limits of one-tenth monolayer organic silicone are possible.

Checking adhesion of metal films to polymers is another area where the combination of the two technologies can serve. Sputtered metal thin films on polymers have found broad applications, from low-emissivity insulating window films, to electrical flex circuitry, to transparent conductive films for interactive video displays. The key to success is to achieve proper product adhesion between the sputtered film and the polymer substrate.

Adhesion failures may be examined by exposing fresh delamination surfaces on both sides of an interface. Each surface is then analyzed by ESCA to determine the surface chemistry of the interface. From these analyses, the failure mode may be determined as adhesive or cohesive.

In many cases, ESCA analyses reveal the two sides of the interface to appear to be different, suggesting an adhesive failure. In other words, the polymer delamination surface resembles the bulk polymer, while the metal surface has only a thin carbon layer.

In one such instance, however, SSIMS indicated that the thin carbon layer on the metal surface was identical to the polymer material. This information suggested a cohesive failure within the polymer very near the deposited metal film.

Solvent extract analysis via gas chromatography/mass spectrometry showed a loosely bound layer of oligomer to the polymer surface prior to metallization.

The thin oligomer layer could easily have broken away from the polymer, leading to an apparent adhesive failure. SSIMS was essential to solving the delamination failure.

Plasma modification has been used to increase wettability of Teflon for printing and adhesion. In one case, it was reported that Teflon could be "oxidized" by [O.sub.2]. While ESCA analyses showed a buildup of oxygeon on the treated polymer surface, it was suspected that oxidation of a thin surface layer of hydrocarbon contamination was occurring. To test this, Teflon was contaminated with a film of hydrocarbon and subjected to an oxygen plasma.

ESCA analyses showed a significant increase in oxygen on the treated surface. However, static SIMS analysis showed that this oxygen was bound to [CH.sub.2] rather than Teflon [(CF.sub.2)]. While the surface may appear more wettable, the oxidized hydrocarbon layer has no physical bond to the Teflon, and adhesion failure may result.
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Title Annotation:electron spectroscopy for chemical analysis; static secondary ion mass spectroscopy
Author:Cormia, Robert D.; Lipari, Robert J.
Publication:R & D
Date:Apr 1, 1991
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