Materials surface: sputtering

In the sputtering process, each surface atomic layer is removed consecutively. If there is no diffusion in the target, the composition of the vapor flux leaving the surface is the same as the composition of the bulk of the material being sputtered, even though the composition of the surface may be different from the bulk. This allows the sputter deposition of alloy compositions, which can not be thermally vaporized as the alloy because of the greatly differing vapor pressures of the alloy constituents. 

In other articles in this section, a method of analysis is described called Secondary Ion Mass Spectrometry (SIMS), in which material is sputtered from a surface using an ion beam and the minor components that are ejected as positive or negative ions are analyzed by a mass spectrometer. Over the past few years, methods that post-ion-ize the major neutral components ejected from surfaces under ion-beam or laser bombardment have been introduced because of the improved quantitative aspects obtainable by analyzing the major ejected channel. These techniques include SALI, Sputter-Initiated Resonance Ionization Spectroscopy (SIRIS), and Sputtered Neutral Mass Spectrometry (SNMS) or electron-gas post-ionization. Post-ionization techniques for surface analysis have received widespread interest because of their increased sensitivity, compared to more traditional surface analysis techniques, such as X-Ray Photoelectron Spectroscopy (XPS) and Auger Electron Spectroscopy (AES), and their more reliable quantitation, compared to SIMS. 

GP 2] [R 3a] The performance of one micro reactor with three kinds of catalyst -construction material silver, sputtered silver (dense) on aluminum alloy (AlMg3), and sputtered silver on anodically oxidized (porous) aluminum alloy (AlMg3) -was compared with three fixed beds with the same catalysts [44]. The fixed beds were built up by hackled silver foils, aluminum wires (silver sputtered) and hack-led aluminum foils (anodically oxidized and silver sputtered), all having the same catalytic surface area as the micro channels. Results were compared at the same flow rate per unit surface area. 

SIMS involves bombarding a material surface with a primary ion beam, with a typical energy in the keV range. Ion impacts on the surface induce a so-called collision cascade sputtering process, where the energy of the primary ions is transferred to the surface through nuclear collisions [Brunelle et al. 2005]. 

Certainly, the inherent lack of depth resolution of the BMP techniques minimizes the utility of sputter profiling combined with BMP analysis. However, gross comparisons of the exterior versus interior composition can be obtained by recording X-ray spectra before and after a minimum of several thousand angstroms of material are sputtered from the particle surface (13,44). Similarly, BSCA is not very suitable when used in conjunction with sputter profiling for reasons that include a) data acquisition rates are very slow, b) potential chemical information is lost since sputtering may alter the chemical forms of the elements present, and c) individual particles cannot be depth profiled (11, 14, 26). 

In plasma environment, the surface chemistry is modified with respect to the initial surface, and the threshold energy and the slope A are found to decrease [69,70], In this case the terminology chemical enhanced sputtering is often used to distinguish this process from the sputtering of a clean surface. Sputtering leads to the ablation of the material, and can concern any surface exposed to the plasma, not only the surface to be processed. 

The general theory discussed in Sect. 12.2 explains that most sputtering phenomena in elemental materials are based on a collision cascade picture The incident ion initiates collisions in a volume (the collision cascade) surrounding the ion track. The energy of the incident ion is shared among those atoms within that volume and then dissipated. Only collisions that occur near the surface of the material can knock atoms out of the material. Most sputtered atoms emerge only from the first few atomic layers. The more collisions that occur near the surface,... 

Sometimes researchers will resort to reinsertion of AC to try to circumvent this problem. This is, however, a process that can be very deceiving because, depending on the materials and sputtering energies involved, some freshly sputtered surfaces are extremely reactive centers. Thus, we have found cases where carbon deposition has led to carbide production. An example for silicon is shown in Figure 6. Other examples of materials for which this has occurred include Fe, Ti, Mo,... 

Reasonably, the corrosion form is typical at relatively high velocities between the material surface and flie fluid, and it is particularly intensive in cases of two-phase or multiphase flow, i.e. hquid-gas and liquid-solid particle flow. Components often liable to erosion corrosion are propellers, pumps, turbine parts, valves, heat exchanger tubes, nozzles, bends, and equipment exposed to liquid sputter or jets. Most sensitive materials are those normally protected by corrosion products with inferior strength and adhesion to flie substrate, e.g. lead, copper and its alloys, steel, and under some conditions aluminium/aluminium alloys. Stainless steel, titanium... 

---