domingo, 14 de marzo de 2010

ION-IMPLANTATION EFFECTS IN SOLIDS


Ion-surface interactions have already been discussed within several contexts in this book. Sputtering for film deposition and Rutherford backscattering for microanalysis are the most important examples; typical ion energies involved are 5 keV and 2 MeV, respectively. At ion energies between these extremes, i.e., tens to hundreds of keV, the probability is great that projectile ions will be implanted hundreds to thousands of angstroms deep beneath the surface.

As a surface modification technique, ion implantation has a number of important advantages as well as disadvantages. Among the advan-tages are controllable and reproducible subsurface depth concentrations, no sacrifice of bulk properties, low-temperature processing, no significant dimen¬sional change in implanted objects, extensión of solid solubility limits, forma-tion of metastable phases, and vacuum cleanliness. Significant limitations include line-of-sight processing, shallow penetration of ions, lattice damage, and, of course, very high capital equipment and processing costs. Despite the latter drawbacks, ion implantation is not only indispensible in VLSI process¬ing, but its use has been explored as a means of beneficially modifying virtually every surface property of interest.

Modification of surfaces occurs because the newly implanted distributions of chemical species are accompanied by considerable structural disorder. Some-times it is possible to induce compositional change without appreciable struc-tural modification, e.g., in the case of low-dosage implants followed by thermal annealing. Alternatively, crystalline targets can be disordered struc-turally and even made amorphous by implanting ions that are identical to matrix atoms. In this case no chemical change is effected. Frequently, how-ever, both compositional and structural changes are inseparably linked and serve to broaden the number of possible ways surfaces can be modified. Some choice over the extent of modification can be exercised through control of processing variables.

 Energy Loss and Structural Modification

The collisions of an individual energetic ion in a solid cause the motion of atoms and the excitation of electronic states. At the outset the ions primarily induce "gentle" electronic transitions that cause relatively little structural damage. Nevertheless, the electronic structure is excited by Coulomb interac-tions with moving ions. A relatively narrow lightning-like trail surrounds the ion track and defines a región of intense electronic excitation, e.g., ionization, secondary and Auger electrón production, electron-hole pair formation, lumi-nescence, etc. Through these mechanisms of energy loss the ion slows suffi-ciently until it begins to set in motion violent nuclear collision cascades along its trajectory. These cascades are the result of displaced atoms that dislodge yet other atoms so that a jagged branched trail is produced.


From the standpoint of structural modification, matrix damage and atomic relocation following nuclear collisions are cardinal issues. In addition to the cascades referred to earlier, which can affect a considerable portion of the matrix, depending on the extent of the branch overlap, there are also "spikes." When the density of energy deposition in either electronic or nuclear cascades exceeds a certain level, the result is a thermal spike. These are launched when bombarding particles transfer energies of a few hundred eV to lattice atoms with the virtually instantaneous liberation of heat. In Cu, calculation has shown that the spike will heat a sphere of approximately 20 A in diameter to the melting point within 5 X 10"12 sec. In another 3 X 10"" sec, the temperature decays to 500 °C, quenching the motion of some 1000 atoms in the process (Ref. 16). Small, highly energized cascade regions melt and quench too rapidly to grow epitaxially within the surrounding crystalline material. As a result, amorphous zones form. In silicon these have been found to be more resistant to formation of defect-free material on low-tem-perature annealing than thin amorphous-Si surface layers or films formed by láser melting or by physical vapor deposition. In this sense the amorphous cascade or spike regions have a different defective nature.



Ronellys Flores---CRF---libro the materials science of thin films




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