Láser Sources
The intense scientific and engineering research associated with the develop-ment of lasers has resulted in much innovation and rapid growth of applica¬tions. Space limitations preclude any discussion of the details of the theory of láser construction, operation, and applications, which are all covered ad-mirably in other textbooks. Suffice it to say, that all lasers contain three essential components: the lasing médium, the means of excitation, and the optical feedback resonator.
The most common lasers employed in materials processing contain either gaseous or solid-state lasing media. Gas lasers include the carbón dioxide (C02:N2:He), argón ion and xenón fluoride excimer types. The solid-state varieties used are primarily the chromium-doped ruby, the neodymium-doped yttrium-aluminum-garnet and neodymium-doped glass láser. These solid-state lasers are excited through pumping by incoherent light derived from flash lamps. Gas lasers, on the other hand, are excited by means of electrical discharges. Láser excitation may be continuous or cw, pulsed, or Q-switched to provide the different output powers shown schemati-cally.
The distinctions in these power-time characteristics are important in the various materials processing applications. In the welding and drilling of metáis, for example, advantage is taken of the power-time profile in the pulsed and Q-switched lasers. Both the reflectance and the thermal diffusivity of metáis decrease with increasing temperature. Therefore, the high-power leading edge of these lasers is used to preheat the metal and enhance the efficieney of the photon-lattice phonon energy transfer.
the common lasers employed in surface processing together with their pertinent operating characteristics are listed. Among the important láser properties are spatial intensity distribution, the pulse width, and pulse repetition rate. The spatial distribution of emitted light depends on the cavity configuration with Gaussian (TEM^) intensity profiles common. Because a uniform láser flux is desirable in surface processing, methods have been developed to convert emission modes into the "top-hat" spatial profile. The dwell time or pulse length, rp, ranges from less than 10 nsec to 200 nsec for Q-switched lasers, and many orders of magnitude longer for other types of lasers. Repetition rates for pulsed and switched lasers range from one in several seconds to many thousands per second. Although the low repetition rates of Q-switched lasers may not be practical in industrial processing applications because the duty eyele, is low, they are useful for laboratory research.
It is the magnitudes of both the absorbed radiant power and rp that determine the effective depth of the surface layers modified through melting or redistribution of atoms. Generally, the smaller valúes of rp result in submicron melt depths. Melting and extensive interdiffusion over tens to hundreds of microns occur with the longer irradiation times possible with cw lasers. No single láser spans the total range of accessible melt depths.
Láser Scanning Methods
Practical modification of large surface áreas with narrowly focused láser beams necessarily implies some sort of scanning operation. For cw lasers the surface generally rotates past the stationary beam in a manner reminiscent of a phonograph record past a needle. Through additional x-y motion, radial positioning and choice of rotational speed, a great latitude in transverse velocities (v) is possible. This also means a wide selection of interaction or dwell times, íd, given by td = dm/v, where dm is the effective melt trail diameter. Typically, td ranges between tens of microseconds to hundreds of milliseconds. In this case the surface-modified región appears to consist of a chain of overlapping elliptical melt puddles.
The experimental arrangement for processing using pulsed or Q-switched lasers. In this case, discrete, overlapping circular-mod-ified (melted) regions are generated by a train of láser pulses. For área coverage larger than the individual melt spots, a mechanism for ráster scanning must be provided. This is usually accomplished by computer-controlled x-y stepping of substrates.
Thermal Analysis of Láser Annealing
The substrate heating caused by an incident láser pulse is due to electronic excitation processes accompanying the absorption of light. Typical pulse durations of 1 nsec or longer far exceed the relaxation time for electronic diffusion length, 2 y/KdTp.
Therefore, it is permissible to assume that the thermal history of the irradiated sample can be modeled by continuum non-steady-state heat-conduction theory. The fundamental equation for the tempera-ture T(x, t) that has to be solved is where the first two terms representing conventional one-dimensional heat conduction should be familiar to readers. The term A(x, t), in units of W/cm3, is the spatial and time-dependent power density absorbed from the incident láser pulse. Other quantities which appear are p, the density, c, the heat capacity, n, the thermal conductivity, and x and t, the distance measured from the surface into the interior and time, respectively. Depending on the relative valué of the absorption length, a~] cm, of the láser light within the specimen surface, two limiting regimes of thermal response can be distin-guished.
Ronellys Flores---CRF---libro the materials science of thin films
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