Effects of Ion Bombardment

Ion bombardment can be used in several different ways to alter the properties of sputter-deposited coatings. The simplest of these is unbalanced magnetron sputtering. 

An alternative is to carry out post-deposition bombardment of a sputter-deposited coating. This provides great flexibility by the selection of ion beam power and ion composition. A general result of the process is increased coating density and improved performance in humid air. 

Hirvonen and co-workers reported that high-energy neon ion bombardment resulted in amorphization of the crystal structure and increased film hardness. However, the friction remained low, and this suggests that the amorphous coating readily recrystallised with basal plane orientation under a sliding stress. 

It is clear that the variety of processes and conditions available in conjunction with sputtering is currently leading to an enormous empirical increase in the available range of sputtered coatings, which is rapidly improving the performance obtainable 

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Although there has been theoretical and experimental interest in the effects of ion bombardment on materials since about 1960 (153), the growth in ion implantation technology and appHcations since then is due almost solely to the semiconductor (integrated circuit) industry. The advantages of ion implantation for semiconductor doping were first pointed out in 1955 (154), but these advantages were not widely accepted until about 1970. 

It is difficult to isolate the effect of ion bombardment on material properties. An external bias can be applied to the discharge in an attempt to study the ef-... 

Enhancement of surface diffusion of the growth precursors is considered as one of the beneficial effects of ion bombardment [246,428]. The potential energy of ions, which is released when the ion is neutralized, is typically 10 eV. This energy can be a substantial fraction of the total energy transferred. The release of this ionization energy is sufficient to excite atoms into excited electronic states, thereby weakening their bonds and enhancing their mobilities [429]. 

Figure 5. Effect of ion bombardment on the decay rate of a 1-D grating of period 17 pm on An (100) at 993C. The ion damage greatly accelerates the decay either by formation of surface defects or by removing the impurities. Similar effects are produced by oxygen exposure. The ordinate is proportional to the grating amplitude.

The created ions, electrons, and neutral fragments participate in complex surface reactions that form the basis of the film growth. Positive-ion bombardment of surfaces in contact with the plasma plays a key role by modifying material properties during deposition. A direct-current (dc) bias potential may be applied to the excitation electrode to increase the ion energy and enhance the desired effects of ion bombardment (30). 

Equation for Electric Field Strength. To include electron-impact source terms in continuity equations for neutral species and to include the effect of ion bombardment on the rate of surface reactions, equations that predict electron and ion densities, momentums, and energy profiles are required. The profiles require an equation for the electric field strength. In general equations for electron and ion continuities (these equations yield electron density and ion density, respectively), electron and ion momentums (for electron and ion net or directed velocity), and electron and ion energies (for electron and ion random or thermal energy) must be solved. Finally, the electric field profile is obtained from Poisson s equation. 

Ion-assisted gas-surface chemistry mechanism is probably the best terminology to describe the plasma processing of a surface. A remarkable illustration of the effect of ion bombardment is reported in Fig. 16 [66]. The reaction rate of XeF2 with Si increases drastically upon the simultaneous combination of chemical species (XeF ) and ions (Ar+) on the surface. Obviously chemical reaction and some sputtering processes are expected to occur and to be responsible of the ablation of the material, but the combined effect of active neutral species and ion bombardment is more efficient than the sum of the individual processes. 

Unfortunately, it is very difficult to demarcate the area of ion bombardment from the surrounding area and to distinguish the more chemical effects from the other effects induced by bombardment. Therefore one has to try to find a definition of chemical effects of ion bombardment in the sense of the present article. 

Chemical Effects of Ion Bombardment 2.6.1 The Creation of Point Defects at Low Temperature... 

Chemical Effects of Ion Bombardment 3.2,5 Methods for the Evaporation of Solid Materials... 

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