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  • 1
    Electronic Resource
    Electronic Resource
    On the dual role of the Knudsen layer and unsteady, adiabatic expansion in pulse sputtering phenomena (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 92 (1990), S. 5047-5056 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: When a surface is bombarded with pulses as from a laser and the number of monolayers sputtered per pulse is sufficiently small, the emitted particles fly freely from the target surface. For yields comparable to 0.5 monolayer in 10 ns, a limited number of gas-phase interactions is known to occur, leading to a Knudsen layer. As a result the particles develop moderate forward peaking (∼cos4 θ) and begin to flow (Mach number, M≈1). It is common, however, for yields to exceed 0.5 monolayer in 10 ns. We show that the resulting gas-phase interactions cause the Knudsen layer to evolve into an unsteady, adiabatic expansion which is formally like a gun which fires a finite charge into an infinite, one-dimensional barrel. An explicit solution can be obtained which has the form a=f1(x/t) and u=f2(x/t), where a is the speed of sound and u is the flow velocity. The solution is assumed to be valid up to a distance where the gas density ρ falls to the critical value ρc where free flight sets in. The particles are then characterized by extents of forward peaking much stronger than ∼cos4 θ (e.g., ∼cos25 θ in work by Namiki et al.) and correspondingly high M (cos25 θ is equivalent to an M near 4). In describing these effects, M and γ (the heat-capacity ratio) take the role of the basic parameters, the knowledge of which is essential for understanding what is happening.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.458540
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  • 2
    Electronic Resource
    Electronic Resource
    Thermodynamic effects in the ion-beam mixing of Fe-Al and Mo-Cr multilayers (1996)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 80 (1996), S. 2702-2711 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Ion-beam mixing of Fe-Al and Mo-Cr multilayers was studied by alternately sputtering with 5 keV Ar+ and then analyzing with Auger electron spectroscopy. The target temperature was variously room temperature, 450, 525, or 600 K. The two systems were chosen on the basis of having heats of mixing (ΔHm) which were opposite in sign, namely H1:1m=−0.25 eV for solid Fe-Al and ΔH1:1m=+0.075 eV for solid Mo-Cr. (ΔH1:1m applies to a 1:1 composition.) With Fe-Al a well-defined peak broadening was observed as the profiling temperature increased, while with Mo-Cr there was a slight narrowing of the peaks with increasing temperature. The results were analyzed in terms of numerical solutions of a diffusion equation which contained terms appropriate not only to ballistic motion but also to the motion of bombardment-induced residual defects. The latter were assumed to be in part "chemically guided,'' as quantified with the parameter "qp'' (qp∼∝ΔHm). A major difference between Fe-Al and Mo-Cr was found. The peak changes with Fe-Al scaled with what we have termed the effective diffusion coefficient Deff=Dt(1−qp/4). Here Dt is the total diffusion coefficient, i.e., it describes the total diffusivity for all mixing processes. With Mo-Cr, on the other hand, the peak changes scaled approximately with 1/qp. The mixing behavior of Fe-Al, with negative ΔHm, could thus be said to be mainly kinetic (with Deff playing a major role), while that of Mo-Cr, with positive ΔHm, is more nearly thermodynamic (with qp playing a major role). © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.363187
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  • 3
    Electronic Resource
    Electronic Resource
    Does normal boiling exist due to laser-pulse or ion bombardment? (2000)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 87 (2000), S. 3177-3179 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We consider here the process of normal boiling as induced by laser-pulse or ion bombardment. Normal boiling (thenceforth "boiling") refers to the appearance of heterogeneously nucleated bubbles which diffuse towards the outer surface of a liquid and, if the surface is reached, may possibly escape. We will here present evidence that boiling, whether the distance scale is atomically small (5–15 nm, as for laser-pulse or ion impact on a metal in the absence of thermal diffusion) or much larger, has a prohibitive kinetic obstacle because it requires bubble diffusion. That is to say: boiling will never be a significant process for bombardment with laser pulses which are short (〈1 μs) or with ions in general. This leaves vaporization and phase explosion as the only possible thermal-spike processes capable of expelling material from a laser-pulse or ion bombarded surface in a significant quantity. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.372319
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  • 4
    Electronic Resource
    Electronic Resource
    Novel geometrical effects observed in debris when polymers are laser sputtered (1992)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 61 (1992), S. 2784-2786 
    Details
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: When polymers are sputtered with 248 or 308 nm laser pulses there are two generically different responses. The most straightforward is where the emitted particles expand outwards away from the target surface obeying, in so doing, the laws of one-dimensional adiabatic flow. The other is where the particles expand both outwards and sideways and, because a certain fraction recondenses on the target surface, there is a prominent deposit of debris lying around the bombarded spot. For spots with other than circular shape the debris show interesting symmetry in which there is rotation with respect to the spot. We show that this rotation occurs wholly through the laws of flow and we conclude, therefore, that the debris phenomenon is a purely gas-dynamic effect. It follows that the elimination of debris can be approached in gas-dynamic terms. We also show that the numerical extent of debris formation increases with the complexity of the particles involved, a result which suggests additional methods to control debris.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.108090
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  • 5
    Electronic Resource
    Electronic Resource
    Direct imaging of the fragments produced during excimer laser ablation of YBa2Cu3O7−δ (1991)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 59 (1991), S. 1302-1304 
    Details
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The evolution dynamics of the fragments produced during KrF excimer laser (248 nm, 25 ns) ablation of YBa2Cu3O7−δ has been observed by ultrafast photography using a synchronized dye laser beam (∼1 ns) to probe above the target surface. The images show that fragment removal is initiated near the beginning of the laser pulse ((approximately-greater-than)10 ns), continues for a maximum of a few hundred nanoseconds, and has an expansion front velocity suggesting a target temperature varying from about 1500 to 4000 K or greater. The shock wave formed due to interaction of the ablated fragments with background oxygen gas has also been photographed, and its temporal evolution as a function of oxygen pressure has been shown to agree better with a planar than spherical model. The overall picture is that of an ablation process showing pronounced but understandable gas-dynamic effects.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.105481
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  • 6
    Electronic Resource
    Electronic Resource
    Thermodynamic effects in depth profiling and ion-beam mixing without invoking thermal spikes (1994)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 64 (1994), S. 2649-2651 
    Details
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Various studies on ion-beam mixing suggest that the extent of mixing is sensitive to the sign and magnitude of the heat of mixing, ΔHm. This implies a role, not only for random motion, but also for chemical driving forces of the type where the total diffusion flux is modified by the factor [1−αi(1−αi) 2hmp/RT(1+p)]. Here α1 is the atomic fraction of component i, αi(1−αi)hm is the heat of mixing of a regular solution, and p is the ratio of the diffusivities for chemically guided defect motion to those for random motion of all types. The parameter p has never been evaluated for any system and we wish to evaluate it first by analyzing the profiling experiments of Marton, Fine, and Chambers on multilayers of Ni-Ag. We then obtain further values of p from ion-beam mixing experiments on bilayers and multilayers. It is shown that it is possible to understand a variety of experimental results relating to profiling and to ion-beam mixing in terms of chemical driving forces and, moreover, to do so without invoking thermal spikes.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.111480
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  • 7
    Electronic Resource
    Electronic Resource
    Critical assessment of thermal models for laser sputtering at high fluences (1995)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 67 (1995), S. 3535-3537 
    Details
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A critical assessment of thermal models for laser sputtering at high fluences is presented. It is argued that the model explaining such sputtering by involving a subsurface superheating effect misinterprets the meaning of "vaporization'' and "boiling''. As a result inappropriate boundary conditions are used, including those for both the surface temperature and for the surface temperature gradient. On the contrary, it is shown that explosive boiling (also termed phase explosion) in the sense pioneered by Martynyuk and by Fucke and Seydel remains the only thermal mechanism able to explain laser sputtering at high fluences. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.114912
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  • 8
    Electronic Resource
    Electronic Resource
    On the debris phenomenon with laser-sputtered polymers (1992)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 60 (1992), S. 2980-2982 
    Details
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The sputtering of polymer films with 248 or 308 nm laser pulses causes two groups of particles to be emitted. Light particles are emitted more rapidly and escape to form a shock wave. Heavy particles are emitted more slowly and under some conditions are impeded by the light ones, expand sideways, and leave debris on the target surface. Debris formation can be understood from ordinary gas dynamics where it is equivalent to recondensation beyond the bombarded spot. This is shown by numerical solution of the flow equations.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.107444
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  • 9
    Electronic Resource
    Electronic Resource
    Criteria for the occurrence of fission-fragment tracks in crystalline substances (1976)
    Springer
    Journal of materials science 11 (1976), S. 406-416 
    Details
    ISSN: 1573-4803
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract The tracks left in minerals by fission fragments arising from the spontaneous decay of uranium impurities serve as an important method for geological dating. If it is assumed that fission tracks in crystalline substances have a structure similar to that of an amorphous phase, then four criteria concerning whether such substances will be useful for dating can be made. These are the existence of amorphization following heavy-ion impact, the identification of an etchant, a high enough crystallization temperature, and the position of the substance on a scale of amorphizability. The criteria are illustrated by applying them to apatite, cobalt titanate, and columbite. Amorphization occurred with all three following bombardment with 10 to 35 keV Kr+ ions. Amorphous apatite was more successfully etched with aqueous KOH than with the commonly used HNO3, while amorphous columbite could be etched with an HF-HNO3 mixture. The crystallization temperatures, which lay in the interval 810 to 905 K for experiments of short duration, were judged to be sufficiently high to insure track retention at the temperature of the surface of the earth over a geological time scale. The positions on a scale of amorphizability were established by noting the bombardment doses at which there was sufficient amorphization to cause the loss of one-half of an inert-gas marker when specimens were heated or etched. Considering all four criteria, it is concluded that apatite and columbite but not cobalt titanate should be useful for dating, with a similar conclusion holding for other substances which are structurally or chemically similar.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00540921
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  • 10
    Electronic Resource
    Electronic Resource
    The structure and stoichiometry of anodic films on V, Nb, Ta, Mo and W (1977)
    Springer
    Journal of materials science 12 (1977), S. 1673-1684 
    Details
    ISSN: 1573-4803
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract Anodic films on Nb were found to have the expected amorphous structure for all formation voltages starting at 6 volts. Films on V, Ta, Mo, and W, on the other hand, were amorphous only when thick, with films below 9 to 30 nm (6 to 10 volts) showing a polycrystalline pattern by reflection electron diffraction. Although the patterns of the thin films were difficult to identify because of their poor quality, the following assignments are proposed: not V2O5, TaO z plus δ — Ta — O, MoO2 · H2O, WO2. To get information on the stoichiometry of thick anodic films on V, Mo, and W, specimens were crystallized by heating them at 300 to 500° C in air, but it was shown that the resulting observation of V2O5, MoO3, and WO3 has no relevance, as oxidation is both thermodynamically and kinetically possible. Vacuum crystallization was shown to be quite unsatisfactory with V and Mo, though is possibly acceptable with W; the information obtained from a comparison of film weights with metal-removal weights, namely that the stoichiometries were close to V2O5, MoO3, and WO3, is more relevant. The stoichiometries TaO z , MoO2 · H2O, and WO2 in the thin films are thermodynamically understandable. A greater problem arises in explaining the state of crystallinity of these as well as other anodic films. A parallel is proposed between anodic and ion-bombarded oxides, for in both cases the state of crystallinity tends to be the same and, furthermore, correlates with the ratio (crystallization temperature)/(melting temperature). A model based on crystallization due to energy deposition is discussed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00542819
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