# Current Progress in Femtosecond Laser Ablation/Ionisation Time-of-Flight Mass Spectrometry
## 4 Performance of fs-LIMS Instruments in the Element and Isotope Analysis
### Depth Profiling, Chemical Imaging, and 3D Chemical Analysis
These test beds were prepared under galvanostatic conditions in the presence of certain plating additives (e.g., polymers and surfactants), which is based on industrially applied protocols in the semiconductor industry for the fabrication of Cu interconnects in microprocessors. Some Cu films were prepared at slightly different plating conditions in which an embedment of the additives inside the Cu deposit is observed in form of spatially confined contamination layers with few nm thicknesses (further detailed discussion is  
Figure 10: (**A**) Ion signal vs. laser shot number at various pulse energies. The data were obtained by ablating a \(\sim\)330 nm Ta\({}_{2}\)O\({}_{5}\) film deposited on Ta foil [19]. (**B**) Comparison of a single pulse (SP) (**a–c**), double pulse (DP) with both pulses at the same energy (72 nJ) and–73 ps pulse delay (**d–f**), and DP with the first pulse at 72 nJ, the second pulse at 79 nJ, and \(\sim\)73 ps pulse delay (**g–i**). Top, middle and bottom panels describe the recorded depth profiles of Cr, Ni and Si, respectively [82]. A clear improvement in the depth profiling resolution is seen between SP and DP operation. Reprinted with permission [19; 82]. given in Section 5.3). The femtosecond laser system was used as an ionisation source with a spot diameter of about 15 \(\upmu\)m. At higher laser pulse energies, an increase in the ablation rate was observed. The mean ablation rate showed a logarithmic dependence on the laser pulse energy, which however was not affected by the presence of impurities. In contrast to traditional SIMS measurements, where a certain area of the sample is typically scanned to monitor these impurities, in LMS, it was sufficient to probe single locations (size of the ablation crater) to identify and measure the embedded nm-thick impurity layers. Whereas the impurity intensities recorded by SIMS expanded over a broader depth range and attenuate with progressive ion erosion, LMS allowed for a smooth ablation procedure with negligible roughening effects at the crater bottom (Cu film thickness of about 15 \(\upmu\)m). Surface roughening is provoked by the ion sputtering of the SIMS, which smears and dilutes the embedded contaminants in the subjacent Cu layers. The smoother ablation enabled the identification of a specific elemental pattern for the individual layers characterising the chemical composition of the contaminants at the location of their embedment. Another drawback of the commonly applied SIMS technique for this type of samples is the presence of strong matrix effect that makes the stoichiometric analysis of the embedded impurities impossible. Fundamental laser ablation studies were also carried out on Si, a dielectric material with different ablation behaviour, and were compared to Cu. To study the size and depth of craters created by the laser ablation process, an anisotropic etching method was applied to Si to cross-section the craters. Cu craters were analysed by applying a PDMS casting procedure which creates an imprint of the craters using specifically developed multi-component polymers [173]. Both techniques demonstrated the formation of cone-shaped craters with depths from 1 to 70 \(\upmu\)m, depending on the number of laser pulses. The ablation depth was larger for copper and exceeded the ablation depth for Si by roughly a factor of 3. The high sensitivity of the instrument allows studies on the ablation threshold. At these conditions, the removal of 1 nm layer/laser shot can be conducted on average [173; 174]. Recently, an improved depth profiling study was conducted with LMS by applying a double pulse ion source [88]. NIST SRM 2135c depth profiling standard reference material characterised by alternating layers of Cr and Ni (5 Cr layers, and 4 Ni layers) was used to conduct the depth profiling performance studies [82]. Comparison of single pulse (SP) and double pulse (DP) UV-laser (258 nm, pulse duration -190 fs) ablation showed improved depth resolution for the latter irradiation method.