Ablation threshold

So far powerful lasers with picosecond to nanosecond pulse duration have usually been used for the ablation of material from a solid sample. The very first results from application of the lasers with femtosecond pulse duration were published only quite recently. The ablation thresholds vary within a pretty wide interval of laser fluences of 0.1-10 J cm , depending on the type of a sample, the wavelength of the laser, and the pulse duration. Different advanced laser systems have been tested for LA ... 

We first experimented with the Quartz Crystal Microbalance (QCM) in order to measure the ablation rate in 1987 (12). The only technique used before was the stylus profilometer which revealed enough accuracy for etch rate of the order of 0.1 pm, but was unable to probe the region of the ablation threshold where the etch rate is expressed in a few A/pulse. Polymer surfaces are easily damaged by the probe tip and the meaning of these measurements are often questionable. Scanning electron microscopy (21) and more recently interferometry (22) were also used. The principle of the QCM was demonstrated in 1957 by Sauerbrey (22) and the technique was developed in thin film chemistiy. analytical and physical chemistry (24). The equipment used in this work is described in previous publications (25). When connected to an appropriate oscillating circuit, the basic vibration frequency (FQ) of the crystal is 5 MHz. When a film covers one of the electrodes, a negative shift <5F, proportional to its mass, is induced ... 

The use of PLD to deposit these materials is complicated by the fact that their structures cannot be subjected to a laser-induced pyrolyhc decomposition and re-polymerizahon process that works for addition polymers. In fact, the perfect reconstruction upon deposition of the organic molecular structures for these materials is never observed once ablated at laser fluences higher than their ablation threshold. However, PLD is possible at laser fluences near the ablahon thresholds of these materials and some examples are given next. 

PhotoDecomposition (APD) that occurs above a well-defined ablation threshold usually leads to removal of polymer material leaving a new fresh "clean" surface with the same composition as the untreated material. However, in the case of PET (JL) both composition and structural modifications occur even for fluences below the ablation threshold. 

From all these XPS informations in order to take into account the fact that after the laser treatments several species like isoimide, C=N-,-CO,0-C-0, and carboxylic groups are present at the surface of polyimide, we propose the following surface reaction under UV laser radiation above the ablation threshold in presence of air moisture ... 

Interestingly, a sample of poly(di-n-pentylsilane), which absorbs only weakly at 248 nm, was ablated very slowly at a fluence of 75 mj/cm per pulse. However, ablation increased markedly with a XeCl source (308 nm), whose spectral output is near the absorption maximum of the sample (313 nm). For this sample at 308 nm, an ablation threshold was also observed at —50 mj/cm per pulse. This experiment suggests that the energy deposited per unit volume is an important feature of the ablation process. 

The ablation threshold is the irradiance value at which the material starts to separate from the sample. As a rule, the material separates through vaporization to a depth where the energy supplied by the laser beam equals the energy of vaporization. The material can separate in other forms including solid or liquid fragments and products of the photochemical decomposition of the sample. Because the energy deposited over the sample must exceed its latent heat of vaporization (//, J/lcg), the threshold fluence J/m-) is given by... 

As with laser ablation, a number of borderline situations (defined via the so-called damage threshold , ablation threshold and plasma threshold ) can be considered in describing the interaction of a laser pulse with a surface to induce the ablation and plasma formation associated to laser-induced breakdown. Only the plasma threshold is discussed here, however, as the other two are shared by laser ablation and are dealt with in Section 9.2.2. 

The plasma threshold is the irradiance required to produce the optical breakdown of the vapour. It depends on the nature of the surface (particularly on various optical, thermophysical and thermodynamic properties of the material). The plasma threshold is usually higher than the ablation threshold. Observing optical emission entails applying more energy than that required to reach the plasma threshold. 

Because the laser-desorption method avoids the need for high fluences above the ablation threshold, the beams generated are much more intense at the desired mass than are those generated by laser ablation of graphite and are much more stable as well. We have made no attempt to quantify the enhancement in intensity. However, Haufler et al. have quoted intensities from a similar source several orders of magnitude higher , and our qualitative observations are consistent with this. 

Hemmerlin M. and Mermet J. M. (1996) Determination of elements in polymers by laser ablation ICP AES effect of the laser beam wavelength, energy and masking on the ablation threshold and efficiency, Spectrochim Acta, Part B 51 579-589. 

Fig. 11 Laser-induced fluorescence spectra recorded from Napl-doped PMMA samples (1.2 wt%) after their irradiation with a single pump pulse at 248 nm at laser fluences below a, and above b the corresponding ablation thresholds. For comparison purposes, the spectra have been scaled. The figure also illustrates the approximate deconvolution of the probe spectrum into the emission bands of the suggested species (the Nap2 spectrum is recorded in the photolysis of high-concentration NapI solution, while the NapH/PMMA spectrum is recorded from PMMA doped with 0.08 wt% NapH)...

Fig. 12 Laser-induced fluorescence spectra recorded from Napl-doped PMMA samples (1.2 wt%) after their irradiation with a single pump pulse a at 248 nm and b at 308 nm at laser fluences about 1.5 times the corresponding ablation thresholds...

Fig. 15 Pulse evolution of the intensities of C6H5C1 and of the various photoproducts formed in the 248 nm irradiation of condensed films of C6H5C1 (Flaser=150 mj/cm2, whereas the ablation threshold is 100 mj/cm2). It is clear that HC1 is nearly constant with successive laser pulses, whereas for the other indicated species, accumulation with successive laser pulses is significant, until finally an equilibrium between amount of photoproduct formed and photoproduct ejected is attained. A more detailed description of the observed effects is given in [35]...

Ablation with femtosecond pulses is comprehensively reviewed by Kruger and Kautek in this issue. The present discussion focuses exclusively on photoproduct formation in the irradiation of Arl-doped systems with fs pulses. This examination indicates several subtle mechanistic possibilities. Upon irradiation with 500-fs pulses at 248 nm, only ArH-like product formation is observed as the fluence is raised above the ablation threshold (Fig. 17). Recombination (e.g., Nap2-type) or other by-products are not observed even for dopant concentrations as high as 4 wt% [83-84]. In contrast, at low flu-ences, a number of by-products are observed after irradiation with few laser pulses. In this case, the accumulating radicals evidently react with each other to produce ill-defined products. Most interestingly, ill-defined species are not observed above the threshold even after extensive irradiation (Fig. 17). It is clear that photochemical processes in the ablation with fs pulses must differ distinctly from those in ns ablation. 

A third source of stress waves derives from the expansion of any gases (CO, CN, N2, CH3 etc.) produced by thermal or photochemical decomposition within the substrate [104]. This factor, for instance, has been invoked to account for the transient stresses of about 0.1 MPa detected in the UV irradiation of polyimide below the ablation threshold [106]. In the case of doped PMMA, irradiation with 150-ps pulses at 1064 nm, Hare et al. [104] estimate that at the ablation threshold, the thermoelastic mechanism and the expansion of the decomposition products contribute about equally to the generated pressure. For specifically designed polymers that upon irradiation form a high enough concentration of volatile products, the generated pressure has been suggested to be the primary cause of material ejection [68-69]. 

Generally, peak pressure amplitudes range from a few MPa at the ablation threshold up to several hundred MPa at high laser fluences [104-108]. During propagation through the substrate, these high-amplitude waves may induce structural modifications at areas away from the ablation spot. Thus, in contrast to the photochemical effects which are confined to the laser-irradiated area, the photomechanical effects of UV ablation can be much more delocalized. 

Photochemical volume models [56, 57, 72-74], reveal sharp ablation thresholds and lead to logarithmic dependence of the ablated depths per pulse. Such models may also result in a linear dependence if the movement of the ablation front is taken into account, and if the screening by ablation products is ignored. These models cannot explain the previously described Arrhenius tails observed in mass loss measurements. 

Thermal surface models [79, 80, 82, 87], (developed mainly for metal ablation [88]) do reveal smooth Arrhenius tails, due to the Arrhenius dependence of the recession velocity on temperature. These models cannot describe the sharp ablation threshold of polymers. 

This variation is one of the reasons why other investigations have applied a quartz microbalance (QMB) [63, 176] technique to measure the threshold with a much higher precision. As an alternative technique, conventional UV spectroscopy is used in this study to determine the ablation threshold with single pulses. The absorbances of thin polymer films cast on quartz wafers were measured before and after irradiation with various fluences, as shown in Fig. 24. 

Laser Ablation of Kapton with 308-nm Irradiation. The laser ablation of Kapton at 308 nm has been studied previously in detail [276]. An ablation threshold of 40 mj cm-2 has been determined for the same Kapton samples and the same experimental setup as in this study. The term ablation threshold is defined in this study as the laser fluence necessary to remove polymer material in the irradiated area, measured by a profilometer. For this study we used laser fluences below, at, and above the threshold of ablation of Kapton films. 

---