Plasma- surface interactions

Nuclear Fusion Research Understanding Plasma-Surface Interactions... 

C. Parameter Control At present, the most serious impediment to routine use of plasma etching is the large number of parameters that affect the process. As noted, both gas phase considerations and plasma-surface interactions must be controlled. The problem is illustrated in Figure 7 32). Naturally, if the basic plasma parameters (A, /(e), r) could be con-... 

Emphasis will also be placed on approaches which lead to the removal of reactive species from the gas phase as well as the special role of energetic positive ions in plasma-surface interactions. Controlled scavenging of critical species from the gas phase and/or at specific surfaces and the degree of positive ion bombardment at a given surface can in fact result in simultaneous polymerizatidn at one surface and etching at another within the same apparatus. 

Fig. 2.1. Representation of the parameter problem in plasma-surface interaction. n,-electron density, f(E)-electron energy distribution, N-gas density, x-residence time for gas molecules in plasma volume...

Characterization of Plasma-Surface Interactions 2.2.1 Etch Rate or Polymerization Rate Measurements... 

O. Auciello, A. Gras-Marti, J. A. Valles-Abarca, and D. L. Flamm, eds., Plasma— Surface Interactions and Processing of Materials, Kluwer Academic... 

Plasma deposition and etch rates are affected by a large number of process parameters and physicochemical processes, illustrated schematically in Fig. 2, making the development and operation of plasma processes difficult. Moreover, given a particular process chemistry, it is not obvious how readily accessible parameters (e.g., feed rate, pressure, power, and frequency) should be manipulated to obtain the desired film uniformity and material properties. Glow discharge physics is complex, and the chemical mechanisms are not well known, in particular those underlying the plasma-surface interactions. Consequently, there is considerable incentive for gaining... 

These surface modifications were performed in "pure" micro-wave (2.45 GHz, "single-mode") or in combined microwave/ radio frequency (2.45 GHz/13.56 MHz, "dual-frequency") plasma. Important systematic changes of the surface composition, wettability, and adhesion of thin metal films were observed for different substrate bias values, and for the different gases. The modified surface-chemical structure is correlated with contact angle hysteresis of water drops this helps to identify which surface characteristics are connected with the wettability heterogeneity and with adhesive bonding properties, and how they are influenced by plasma-surface interactions. 

Fig. 2.1. Fusion triple product uTte, vs. temperature (KeV), and contours of constant plasma surfaces interaction intensity p (definition see text). Q is the ratio of output fusion power to input heating power. The upper left part (beyond the radiation limit line) is not accessible, due to unavoidable radiation losses (Bremsstrahlung) already exceeding fusion power production...

The main objective of this article is to summarize the work performed at the Max-Planck-Institute for Plasma Physics in Garching over the past few years relevant to plasma-surface interaction processes in the system hydrogen and carbon. This includes a short review of the properties of amorphous, hydrogenated carbon layers, further on abbreviated as a-C H, determination of reaction probabilities of reactive species such as atomic hydrogen and methyl radicals, and investigation of the simultaneous interaction of these species and low-energy ions with hydrocarbon surfaces. The reviewed ma-... 

V.S. Voitsenya, A.F. Bardamid, V.N. Bondarenko, W. Jacob, V.G. Konovalov, S. Masuzaki, O. Motojima, D.V. Orlinskij, V.L. Poperenko, I.V. Ryzhkov, A. Sagara, A.F. Shtan, S.I. Solodovchenko, M.V. Vinnichenko Some problems arising due to plasma-surface-interaction for operation of the in-vessel mirrors in a fusion reactor. J. Nucl. Mater. 290-293, 336 (2001)... 

Erosion during mixed impurity species bombardment of beryllium has also shown unexpected chemical effects that play a dominant role in determining the erosion rate of the substrate material. Bombardment of a beryllium sample with a CO+ ion beam produces an equilibrium surface state consisting of beryllium oxide, elemental carbon and C-0 compounds [13]. The chemical erosion of CO limits the carbon accumulation on the surface and therefore beryllium continues to be eroded. The complicated and interrelated nature of plasma-surface interactions requires measurements to be made in a situation that includes as many of the conditions of the final application as possible. 

The properties of the plasma, and the way the plasma interacts with the wall of the container or with any surface immersed in it (such as a sample) are described by a number of important parameters. A short review of some parameters is given hereafter, and typical values are reported in Table 1. In a first order approximation, parameters in the volume of the plasma control the formation of the active species and the chemical reactions in the gas phase, parameters at the plasma surface boundary control how these species interact with the surface. As described below this description is far too simple, and an important feedback exists between the plasma-surface interaction and the gas phase chemistry. 

The combination of analyses of both the plasma and the surface is probably the best strategy to improve the understanding of the plasma-surface interaction. A brief description of the most common techniques is given hereafter. Examples of applications are given here and later in Secs 14.2.4 and 14.2.5. 

An important feature of mass spectrometry is the detection of volatile plasma-surface interaction products. Figure 7 reports the evolution of the production rate of WF6 and WOF4 from a tungsten surface in SF,—02 plasma upon plasma exposure time [21], At a first glance, the rather surprising feature in these results is that during the surface oxide elimination the dominant product is WF6... 

Absorption spectroscopy and laser induced fluorescence (LIF), give access to the concentration of molecules, atoms, and ions in the ground state. LIF is enable to achieve highly spatial and time resolved analyses. This technique is thus particularly suitable to investigate composition changes in the plasma, and obtain spatial or time concentration profiles. Published results in fluorine plasmas using absorption [25-27] and LIF [28-32] mainly concern temperature measurements [25] or the quantification of CFV radicals [26-31] in fluorocarbon-based plasmas and SOx in SF6—02 discharges [32], Recently LIF has been used to measure plasma-surface interaction products [33]. 

Ricard, A. In Plasma-Surface Interactions and Processing of Material, Auciello, O., Ed. Kluwer Academic The Netherlands, 1990, Chap. 1. 

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