Contaminants, surface

As a somewhat anecdotal aside, there has been an interesting question as to whether gold is or is not wet by water, with many publications on either side. This history has been reviewed by Smith [119]. The present consensus seems to be that absolutely pure gold is water-wet and that the reports of non wetting are a documentation of the ease with which gold surface becomes contaminated (see Ref. 120, but also 121). The detection and control of surface contaminants has been discussed by White [121] see also Gaines [122]. 

A laser pulse strikes the surface of a specimen (a), removing material from the first layer, A. The mass spectrometer records the formation of A+ ions (b). As the laser pulses ablate more material, eventually layer B is reached, at which stage A ions begin to decrease in abundance and ions appear instead. The process is repeated when the B/C boundary is reached so that B+ ions disappear from the spectrum and C+ ions appear instead. This method is useful for depth profiling through a specimen, very little of which is needed. In (c), less power is used and the laser beam is directed at different spots across a specimen. Where there is no surface contamination, only B ions appear, but, where there is surface impurity, ions A from the impurity also appear in the spectrum (d). 

By using a laser with less power and the beam spread over a larger area, it is possible to sample a surface. In this approach, after each laser shot, the laser is directed onto a new area of surface, a technique known as surface profiling (Figure 2.4c). At the low power used, only the top few nanometers of surface are removed, and the method is suited to investigate surface contamination. The normal surface yields characteristic ions but, where there are impurities on the surface, additional ions appear. 

Eventually, multipliers become less sensitive and even fail because of surface contamination caused by the imperfect vacuum in the mass spectrometer and the impact of ions on the surfaces of the dynodes. 

A second approach modifies the CA resist chemistry. Eor example, researchers have introduced basic additives into the resist formulation to minimize the impact of surface contamination of the resist film (82,83). A resist that already contains added base (and consequendy requites a larger imaging dose) should be less affected by the absorption of small amounts of basic contaminants. Systems of this type have been claimed to have improved resolution as well. The rationalization here is that the acid that diffuses into the unexposed regions of the resist film is neutralized and does not contribute to image degradation (84,85). 

Instmmentation for tern is somewhat similar to that for sem however, because of the need to keep the sample surface as clean as possible throughout the analysis to avoid imaging surface contamination as opposed to the sample surface itself, ultrahigh vacuum conditions (ca 10 -10 Pa) are needed in the sample area of the microscope. Electron sources in tern are similar to those used in sem, although primary electron beam energies needed for effective tern are higher, typically on the order of ca 100 keV. 

Ingots of EGS are evaluated for resistivity, crystal perfection, and mechanical and physical properties, such as she and mass. The iagots are sHced iato wafers usiag at least 10 machining and polishing procedures. These wafers are sHced sequentially from the iugot, and evaluated for the correct surface orientation, thickness, taper, and bow. As a final procedure, the wafers are chemically cleaned to remove surface contaminants prior to use. 

Similarly, airborne emission limits have been estabHshed by the NRC for nonrestricted areas. Limits of surface contamination must be estabHshed to provide a safe workplace for users (19). The appHcation of the as-low-as-reasonably-achievable (ALARA) principle to the above draws on the creative talents of the user to regard the limits as nonapproachable barriers and not as tolerable maxima for discharge. 

Strength. The fracture strength of vitreous sihca depends on its surface quaUty, which can be affected by thermal treatment and handling conditions. Microcracks, surface contamination, and crystallisa tion can reduce the strength from the value of pristine vitreous sihca by several orders of magnitude. 

CO oxidation catalysis is understood in depth because potential surface contaminants such as carbon or sulfur are burned off under reaction conditions and because the rate of CO oxidation is almost independent of pressure over a wide range. Thus ultrahigh vacuum surface science experiments could be done in conjunction with measurements of reaction kinetics (71). The results show that at very low surface coverages, both reactants are adsorbed randomly on the surface CO is adsorbed intact and O2 is dissociated and adsorbed atomically. When the coverage by CO is more than 1/3 of a monolayer, chemisorption of oxygen is blocked. When CO is adsorbed at somewhat less than a monolayer, oxygen is adsorbed, and the two are present in separate domains. The reaction that forms CO2 on the surface then takes place at the domain boundaries. 

Pitting corrosion may occur generaHy over an entire aHoy surface or be localized in a specific area. The latter is the more serious circumstance. Such attack occurs usuaHy at surfaces on which incomplete protective films exist or at external surface contaminants such as dirt. PotentiaHy serious types of corrosion that have clearly defined causes include stress—corrosion cracking, deaHoying, and corrosion fatigue (27—34). 

The inherent solderabiUties of selected alloys are Hsted in Table 9. Class IV and V ratings with this particular flux indicate the presence of oxides or other surface contaminants that may be removable with more aggressive flux or acid pickling. 

A widely used instmment for air monitoring is a type of ionization chamber called a Kaimn chamber. Surface contamination is normally detected by means of smears, which are simply disks of filter paper wiped over the suspected surface and counted in a windowless proportional-flow counter. Uptake of tritium by personnel is most effectively monitored by urinalyses normally made by Hquid scintillation counting on a routine or special basis. Environmental monitoring includes surveillance for tritium content of samples of air, rainwater, river water, and milk. 

Metal Preparation. Sheet-steel parts are formed by stamping, bending, and shearing. Many parts require welding (qv), which needs to be carried out in a uniform, smooth manner so that the welded joint can be enameled without defects. Cast-iron parts are formed by the usual cast-iron foundry methods however, additional care is given to prevent contamination of the surface. Surface contamination causes defects in the enamel, particularly bUsters and bubbles. Aluminum metal can be formed in sheets, extmsions, and as castings. 

Critical factors. In general, porosity is caused by the entrapment of gas during the welding process or during solidification of the weld metal. Surface contamination may provide a gas source during the welding operation. 

Silica [7631-86-9 (colloidal)-, 112945-52-5 (fumed)]. Purification of silica for high technology applications uses isopiestic vapour distillation from cone volatile acids and is absorbed in high purity water. The impurities remain behind. Preliminary cleaning to remove surface contaminants uses dip etching in HF or a mixture of HCl, H2O2 and deionised water [Phelan and Powell Analyst 109 1299 1984],... 

Tellurium [13494-80-9] M 127.6, m 450 . Purified by zone refining and repeated sublimation to an impurity of less than 1 part in 10 (except for surface contamination by Te02). [Machol and Westrum J Am Chem Soc 80 2950 1958.] Tellurium is volatile at 500°/0.2mm. Also purified by electrode deposition [Mathers and Turner Trans Amer Electrochem Soc 54 293 1928]. 

Surface preparation, e.g. polishing, tumbling, blasting, etching or a machine process, may be required due to the nature of the process or due to surface contamination. This can be an added source of variability. 

The degree of surface cleanliness or even ordering can be determined by REELS, especially from the intense VEELS signals. The relative intensity of the surface and bulk plasmon peaks is often more sensitive to surface contamination than AES, especially for elements like Al, which have intense plasmon peaks. Semiconductor surfaces often have surface states due to dangling bonds that are unique to each crystal orientation, which have been used in the case of Si and GaAs to follow in situ the formation of metal contacts and to resolve such issues as Fermi-level pinning and its role in Schottky barrier heights. 

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