Femtosecond laser ionization mass spectrometric analysis of layers grown by pulsed laser deposition

Femtosecond laser-induced charged particle emission and detection by time-of-flight mass spectrometry are used for elemental analysis and in-depth profiling of multi-layered metal-oxide-semiconductor (MOS) structures grown by pulsed laser deposition (PLD): thin films of a high-k material - praseodymium oxide, Pr6O11 - deposited on a silicon(1 0 0) surface and covered by an aluminum contact layer. An efficient emission of singly charged positive ions to large clusters was found at low incident fluence. The simultaneous monitoring of the main ion yields with the number of pulses exhibits pronounced anti-correlations, revealing several interfaces. This indicates not only that the layers were resolved but also additional layers were detected. Surface morphology investigation shows that, upon single- or multi-pulse exposure, localized holes and nano-particles were created on the irradiated area. The multiple interfaces may account for in-homogeneities within the PLD layer and/or a surface rearrangement due to the nano-particles formation. We demonstrate that, with this method, a depth resolution in the nanometer range can be achieved. Fundamental mechanisms of femtosecond laser ablation from hetero-structures are addressed.