摘要:

The basic and dominant physical mechanism of laser interaction in absorbing media is heating. The temperature rise is determined by the laser power, pulse duration, absorption coefficient, specific heat and thermal diffusivity. It is possible to heat and melt thin surface layers of metals and semiconductors in a microsecond, or even nano‐ or picoseconds. The subsequent cooling rates may range from 108°C/sec to 1014°C/sec. Annealing of ion‐implanted surfaces, laser glazing, surface alloying and cladding are technological applications based on the fast heating and cooling rates obtainable with short time irradiation of good absorbers with intense laser beams.

ISSN:0094-243X    

DOI:10.1063/1.31659

出版商:AIP    

年代:1979

数据来源: AIP

摘要:

The physical mechanisms responsible for the annealing of ion‐implanted semiconductors have been identified and characterized for a wide range of materials and annealing conditions. Our approach emphasizes the use of in‐situ dynamical measurements of the optical properties in addition to post annealing measurements using Rutherford backscattering and channeling, optical and electron microscoy and conductivity. These measurements have enabled us to unambiguously identify two specific annealing mechanisms. For long exposure times, as exemplified by the use of a scanning cw argon laser, the recrystallization proceeds by accelerated solid‐phase epitaxy. For short exposure times, typical ofQ‐switched lasers, it is necessary to melt the entire amorphous layer, and regrowth occurs by liquid‐phase epitaxy. In the melt regime results are presented for melt thresholds, melt durations, annealing thresholds, regrowth velocities, and damage thresholds. Where possible, comparisons are made with theoretical estimates. For silicon, the wavelength and dose dependence has also been studied. A simple and efficient method of annealing with two pulses of different wavelengths is also demonstrated.

ISSN:0094-243X    

DOI:10.1063/1.31651

出版商:AIP    

年代:1979

数据来源: AIP

摘要:

When the energy from a powerful pulsed laser is trained on the surface of an absorbent material, a high‐amplitude stress wave is generated. If the surface is covered with a material which is transparent to the incident laser light, peak pressure environments are significantly enhanced compared to free surface conditions. Experimental pressure measurements using piezoelectric pressure transducers have demonstrated that peak pressures up to approximately 10 GPa can be generated in this manner. The rise time of these pressure waves, which is controlled by the temperature of the laser heated absorbent material, approximates the shape of the incident laser pulse. The decay time of the pressure waves is slower than the laser pulse because it is governed by the rate at which work is done on surrounding materials and the rate at which heat is conducted out of the heated vapor into colder adjacent materials. Theoretical calculations of the pressure environments using a one‐dimensional radiation hydrodynamic computer code are in good agreement with the experimental measurements.The stress wave propagates into the material and modifies the material’s substructure and properties in a way similar to shock waves generated by other means. This process has been successfully used to increase the strength and hardness of several aluminum alloys, titanium, and the hardness of stainless steel. The yield strength of the heat‐affected zones in welded aluminum structures has been increased to values up to the strength of the parent material. Recent studies have demonstrated that laser shock processing can also be used to improve the fatigue life in an aluminum alloy.

ISSN:0094-243X    

DOI:10.1063/1.31670

出版商:AIP    

年代:1979

数据来源: AIP

摘要:

The temperature aspect of laser‐induced crystal regrowth is treated with emphasis to the effect of slightly nonuniform irradiation. We show that it leads to very nonuniform heating if the absoprtion of the materials is weak but increases with temperature. This is the case for Si irradiated by Nd laser radiation. The difficulty can be overcome by sufficiently increasing the initial absorptivity of the sample. This is demonstrated with a simple mathematical model of the transient heating process as well as with experiments.

ISSN:0094-243X    

DOI:10.1063/1.31694

出版商:AIP    

年代:1979

数据来源: AIP

摘要:

An apparatus for studies of infrared laser induced processes at low temperatures under ultrahigh vacuum, particularly for measurements in the IR finger print region between 900 and 4000 cm−1, is described.The apparatus has been employed to investigate the interaction between MWcm−2CO2laser pulses of 200 ns length and solid SF6films at 67 K and base pressures in the low 10−10mbar range. After excitation of the &ngr;3(32SF6) vibration the pressure rise of all32SF6ion fragments, except F+and SF+2, was observed, the rise time being in the millisecond range. The time constants of the decay of the individual ion fragments differ significantly.

ISSN:0094-243X    

DOI:10.1063/1.31706

出版商:AIP    

年代:1979

数据来源: AIP

摘要:

We present a theoretical model for the heating of metal targets by CO2laser pulses, with air plasma ignition. Such modeling is necessary for deduction of the details of the thermal transient at the target surface, direct measurement of the transient being difficult or impossible, and the thermal flux being determined by the plasma coupling efficiency rather than the laser flux. Our model permits calculation of the thermal transient from simple time‐integrated thermal fluence and energy deposition data.

ISSN:0094-243X    

DOI:10.1063/1.31717

出版商:AIP    

年代:1979

数据来源: AIP

摘要:

In order to investigate the dynamic behavior of pulsed‐laser annealing, we have measured the time‐dependent optical reflectivity of ion‐implanted silicon surfaces under the mode‐locked Nd:YAG‐laser pulse‐train irradiation (pulse width of 30 psec, time separation of 10 nsec between pulses, and pulse train duration of about 150 nsec) over the wide ranges of laser‐energy densities. By the laser irradiation the reflectivity increased at first and next decreased to that of crystalline Si. The reflectivity enhancement was also observed for low‐laser energies which induced no appreciable amorphous to crystalline (a‐c) transition. The reflectivity enhancement was found to be due to melting of Si surface and, in the case of no a‐c transition, mainly due to the high temperature rise. The differences of the dynamic behaviors between the mode‐locked Nd:YAG andQ‐switched Nd:YAG lasers are discussed.

ISSN:0094-243X    

DOI:10.1063/1.31727

出版商:AIP    

年代:1979

数据来源: AIP

摘要:

Time‐resolved measurements of the specular reflectivity of pure metals and alloys subjected to 60 ns CO2laser pulses under approximately homogeneous illumination conditions are reported. The laser irradiations were conducted in hard vacuum with peak power density in the range of 0.2–5.4×108W/cm2. Generally, the results indicate an anomalous transient drop in specular reflectivity with full recovery late in the pulse for metal surfaces exposed to peak beam power densities above a threshold level. This threshold value is material dependent and is near that power density calculated to produce melting for most metals. Evidence of melt was observed on the irradiated surfaces using Nomarski microscopy, but examination of taper sections through the surfaces did not always confirm melting.

ISSN:0094-243X    

DOI:10.1063/1.31737

出版商:AIP    

年代:1979

数据来源: AIP

摘要:

The principles of forming metastable solids by rapid melt quenching are reviewed with primary emphasis on the growth of crystals into melts and glasses. Experience is reviewed which indicates that the rate of growth of crystals in pure metal melts may be limited only by the impingement frequency of atoms from the melt on to the crystal face. However, growth requiring impurity distribution will be governed by the more highly correlated atomic transport processes within the melt. We have suggested that the shapr increase in the resistance to this redistribution with falling temperature is an important requirement for melt quenching alloys to the glassy state.Growth of crystals into covalently bound melts and glasses is, in contrast with that in pure metal melts, limited by thermal activation barriers with magnitudes near those of the covalent bond energies. The formation of ledges on densely packed crystal faces and the role of such ledges in crystal growth are discussed.The experience on the regrowth of crystalline silicon into amorphous overlays is discussed in terms of the concepts presented.

ISSN:0094-243X    

DOI:10.1063/1.31738

出版商:AIP    

年代:1979

数据来源: AIP

摘要:

In this review we present experimental evidence for the existence of two regimes, liquid‐phase and solid‐phase, for epitaxial regrowth of laser and electron beam annealed silicon that had been implanted to form an amorphous layer on the underlying crystal substrate. Liquid‐phase growth is generally found with short duration (t<100 ns) and solid‐phase with long duration (10−3to 10−2sec) laser and electron beam pulses. Much of the insight into pulsed annealing is based on early measurements of furnace annealing of implanted‐amorphous Si at temperatures well below the melting point. Solid‐phase epitaxial growth was found to be strongly dependent on the orientation of the underlying crystal substrate and on the presence of impurities at concentrations greater than 0.1 at. %.

ISSN:0094-243X    

DOI:10.1063/1.31739

出版商:AIP    

年代:1979

数据来源: AIP