Ablation process

The disadvantage of lasers with nanosecond-picosecond pulse duration for depth profiling is the predominantly thermal character of the ablation process [4.229]. For metals the irradiated spot is melted and much of the material is evaporated from the melt. The melting of the sample causes modification and mixing of different layers followed by changes of phase composition during material evaporation (preferential volatilization) and bulk re-solidification [4.230] this reduces the lateral and depth resolution of LA-based techniques. 

Several characteristics of the metal beam have been studied in detail. It is well known that metal clusters and metal oxides are formed as a result of the ablation process. However, these potentially interfering species have been studied in detail130 and it has been concluded that they do not introduce any doubt as to the validity of the experimental results. Much more important than cluster or oxide formation are the atomic electronic state populations of the metal beams. For each metal reactant, these have been characterized using laser-induced fluorescence (LIF) excitation spectroscopy. For Y, only the two spin-orbit states of the ground electronic state (a Dz/2 and a D-3,/2) were observed.123... 

The latest advancement in femtosecond (fs)-based micromachining technology has opened a new window of opportunity for fabrication of microdevices. Direct exposure of most solid materials (including fused silica glass) to high power fs laser pulses may lead to the ablation of a thin layer of materials at the laser focal point13. Due to the multiphoton nature of the laser-material interaction, the ablation process can be conducted on the material surface as well as within its... 

Molecular Studies on Laser Ablation Processes of Polymeric Materials by Time-Resolved Luminescence Spectroscopy... 

Substance that forms a protective surface on a spacecraft or a missile, and is consumed in an ablation process. 

In 1998, U.S. Department of Energy (DOE) researchers estimated that using the F2 Associates, Inc., laser ablation process would cost 9.92/ft for a mobile robotic unit, and 6.77/tf if a hand-held unit was used. This estimate was for the D D of paint 1 mil thick (D189031, p. vii). This compared favorably with conventional D D technologies. Details of this estimate are summarized in Table 1. 

Laser ablation ToF MS analysis, using IR lasers, has also been performed to characterize ILs [15]. It could be shown that intact ILs are ablated by this method. The neutral species ablated could be further investigated by vacuum UV postionization. Due to the faster heating rates achieved by this ablation process compared to the UV-LDI analysis, less fragmentation caused by the thermal decomposition of the analytes was observed. 

Figure 2.13 b) Schematic diagram of photon-solid interaction by the laser ablation process. 

Although Eq. (2) is not adequate to fully describe ablation processes, the absorption coefficient is still a useful parameter as it provides a quantitative evaluation of the level of interaction between a given medium and photons of a specific wavelength and, to a first-order approximation, their propensity to laser ablation. 5 different materials can have different chemical compositions and structural arrangements, it is not surprising that they have different absorption coefficients and thus can exhibit different ablation characteristics. This is indeed found to be the case. For instance, PMMA is readily structured at 193 run ( 193 2 X 10 cm I) although it is essentially transparent and unaffected at 308 nm (a 308 < 1 x 10 cm i). However, for an aromatic polyimide such as the system whose dianhydride/diamine components are pyromelletic dianhy-dride/oxydianiline (PMDA-ODA), ablation occurs readily not only at 193 nm... 

Recently, the VLS growth method has been extended beyond the gas-phase reaction to synthesis of Si nanowires in Si-containing solvent (Holmes et al, 2000). In this case 2.5-nm Au nanocrystals were dispersed in supercritical hexane with a silicon precursor (e.g., diphenylsilane) under a pressure of 200-270 bar at 500°C, at which temperature the diphenylsilane decomposes to Si atoms. The Au nanocrystals serve as seeds for the Si nanowire growth, because they form an alloy with Si, which is in equilibrium with pure Si. It is suggested that the Si atoms would dissolve in the Au crystals until the saturation point is reached then they are expelled from the particle to form a nanowire with a diameter similar to the catalyst particle. This method has an advantage over the laser-ablated Si nanowire in that the nanowire diameter can be well controlled by the Au particle size, whereas liquid metal droplets produced by the laser ablation process tend to exhibit a much broader size distribution. With this approach, highly crystalline Si nanowires with diameters ranging from 4 nm to 5 nm have been produced by Holmes et al. (2000). The crystal orientation of these Si nanowires can be controlled by the reaction pressure. 

To obtain accurate, quantitative results, either the amount of material ablated per laser pulse must be similar for the standards and samples or the relative ablation rates must be experimentally measurable (through use of an internal standard, for example). The size distribution of the ablated particles must be similar enough for the standards and samples so that that transport efficiency of ablated material is similar or again can be accounted for accurately. Ideally, the ablation process should produce particles and sample vapor that have the same chemical composition as the sample. However, elemental fractionation can occur, particularly if the ablation process is predominantly thermal [61]. 

The utilily of PLD for Ihin film synlhesis is due in large part to Ihe unique characteristics of Ihe laser ablation process. As indicated above, laser ablation is a nonequilibrium process that enables stoichiometric evaporation of elements from a target source. In addition, it is also possible to control the energy of evaporated species in PLD, and thus control film growlh on Ihe subslrale surface. The underlying basis for these features of laser ablation and their utihty in thin film synthesis are described below. 

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