Material Analysis with Ultra-Short-Laser-SIMS and -SNMS

A key issue in all analytical bulk and surface methods is the quantitative information which can be obtained from the signals. Qualitative information is relatively easy to deduce from the signals, but quantitative information is sometimes very hard, or even impossible to obtain. Widely used analytical methods are based on secondary emission of surface (bulk) particles as Secondary ion mass Spectroscopy (SIMS) or Secondary Neutral Mass Spectroscopy (SNMS). These methods consist of basically three steps: In a first step particles are removed from the surface, usually by ion bombardment. In a second step the particles have to be ionized. This can be a inherent part of the particle removal process (SIMS) where during the emission process ionization takes place. Alternatively, the particles have to be ionized in the gas phase by a specific ionization step (plasma post ionization, laser post ionization etc.). In a third step the ionized particles are detected. In particular, the second step is often responsible for the loss of quantitative information, since the ionization during the emission process strongly depends on the surface parameters. Furthermore, beside atomic particles complex molecules and clusters are emitted, complicating the quantitative information obtained from a mass signal, ionic or neutral. The use of laser radiation for the second step (instead of ion beams) has been tried for many years, but many problems have been encountered. The use of ultra-short laser radiation might overcome the deficiencies of conventional laser radiation for this purpose and establish a new quantitative surface analytical method. We have noticed in recent preliminary experiments that the secondary ion emission after ultra short laser ablation has very promising quantitative aspects. The neutral emission seems to be even better suited in that respect. The aim of this project is to investigate ion and neutral emission during ultra-short laser ablation of metals, semiconductors and insulators in detail for various laser parameters, and to evaluate this method for quantitative purposes. Key issue for achieving these goals is the innovative use of two new techniques: ultra short laser post-ionization of neutrals and pulse shaping of the ablation pulses to optimize the quantitative signal information. We call the methods UL- LSIMS (Ultra Short-Laser Secondary Ion Mass Spectroscopy) and UL-LSNMS (Ultra Short-Laser Secondary Neutral Mass Spectroscopy).

The use of lasers in today`s life is present everywhere, sometimes even unrecognized.. Lasers with extremely short pulse lengths (10 -15s) offer many new areas for applications ranging from creating and modifying structures and materials on extremely small scales (nano-engineering) to precise laser surgery. These new areas of application become possible, because enormous intensities of laser light can be deposited in materials in very short times. New types of processes can then take place in the material. We have succeeded in applying ultra short lasers for analyzing materials: the laser is used to remove few atoms from the material in a very precises and non-damaging manner and then the lasers is used to identify the atoms with extremely high sensitivity. In many cases this offers several advantages to more established methods (the quantitative information can be taken as a good example), but as we have shown, many questions with regard to the physics of the processes and their parameters (for example the velocity of the released atoms) have to be answered. Therefore, we have put substantial effort on investigating these processes and we have identified the general form of processes which take place, when ultra-short lasers interact with metals and semiconductors. This knowledge will help to better make use of the various potentials of ultra short lasers.