Two main approaches to improving or altering the surface properties of solids have evolved over the years. The more traditional one involves the deposition of films and coatings from solid, liquid, and vapor sources. Processing utilizing these methods has totally dominated our attention in the book until this point. But there is another approach of more recent origin based on modifying existing surfaces through the use of directed-energy sources. These include photon, electrón, and ion beams, and it is their interaction with surfaces.
Coherent (láser) and incoherent light sources, as well as electrón beams, tnodify surface layers by heating them to induce melting, high-temperature solid-state annealing or phase transformations, and, occasionally, vaporization. In the case of lasers, the relation between the required power density and irradiation time is depicted for a number of important commercial processing applications. However, the focus of this book is thin films and in the applications shown much thicker layers of material are modified. These materials processing techniques will, therefore, not be discussed in any detail, ñor will there by any additional mention of electrón beams.
Their heating effects are basically equivalent to those produced by lasers of comparable power. Furthermore, the great depth of the heat-affected zone is more typical of bulk rather than surface processing. The thin-film or layer-modification regime we shalJ be concerned with is characterized by approximate láser energies of ~ 0.1-2 J/cm2, interaction times of ~ 10~9 to 10~6 sec, and power densities of ~ 106 to 108 W/cm2. These conditions prevail in the indicated región. Surface layers ranging from 0.1 to 10 ¡im in thickness are correspondingly modified by melting under such conditions. The melting-solidificatión cycle frequently does not restore the surface structure and properties to their original states. Rather, interesting irreversible changes may occur. For example, one consequence of láser processing can be an ultrahigh quench rate with the retention of extended solid solutions, metastable crystalline phases, and, in some cases, amorphous materials. Directed thermal energy sources have also been employed to effect annealing, surface alloying, solid-state transformations and homogenization. The controlled epitaxial re-growth of molten Si layers over Si02 or insulators, is an important example of the great potential of such processing.
Like photon and electrón beams, ion beams play an indispensible role in surface analytical methods and have also achieved considerable commercial success in surface processing. In the very important ion-implantation process, ion beams have totally revolutionized the way semiconductors are doped. Depending on the specific ion projectile and matrix combination, dopants can be driven below the semiconductor surface to readily predictable depths through control of the ion energy. Unlike traditional diffusional doping where the highest concentration always occurs at the surface, ion-implanted distribu-tions peak beneath it. The reduction of the threshold voltage required to trigger Láser processing regimes illustrating relationships between power den-sity, interaction times and specific energy. D-drilling; SH-shock hardening; LG-laser glazing; DPW-deep penetration welding; TH-transformation hardening.
current flow in MOS transistors, by means of ion implantation, ushered in battery-operated, handheld calculators and digital watches. Today ion-implan-tation doping is practiced in MOS as well as bipolar transistors, diodes, high-frequency devices, optoelectronic devices, etc., fabricated from silicon and compound semiconductors. Achievements in microelectronics encouraged broader use of ion implantation to harden mechanically functional surfaces, improve their wear and fatigue resistance, and make them more corrosión resistant. Critical components such as aircraft bearings and surgical implant prostheses have been given added valué by these treatments. In addition, there are other novel ion-beam-induced surface-modification phenomena such as ion-beam mixing, or subsurface epitaxial growth, that may emerge from their current research status into future commercial processes.
The purpose of this chapter is to present the underlying principies of the interaction of directed-energy beams with surfaces, together with a description of the changes which occur and why they occur. Accordingly, the subject matter is broadly subdivided into the following sections:
Lasers and Their Interaction with Surfaces
Láser Modification Effects and Applications
Ion-Implantation Effects in Solids
Ion-Beam Modification Phenomena and Applications
Ronellys Flores---CRF---libro the materials science of thin films
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