Femtosecond laser ablation (fs-LA) and sputter XPS depth profiling of a tantalum nitride thin film – a comparative study

Tantalum nitride (TaN x ) is a commonly used material for thin films in semiconductor devices and as a protective coating for engineering applications. X-ray photoelectron spectroscopy (XPS) depth profiling is widely employed to analyse the composition of thin films and coatings, but use of traditional ion beam sputtering to remove material during profiling can lead to chemical damage and incorrect compositions/chemical state information being derived from the XPS results. XPS analysis of a 435 nm thick, reactively sputtered TaN 1.5 thin film has been performed following different methods to remove material either for surface cleaning or depth profiling purposes. For surface cleaning, XPS analysis of the bulk TaN 1.5 thin film was performed following removal of a 2.2 nm thick surface oxide through monatomic (500 eV Ar + ) sputtering and pulsed femtosecond laser ablation (fs-LA) using a single 250 μJ pulse from a 160 fs, 1030 nm infrared laser. Employing the 500 eV Ar + beam at a current density of 1.0 μA/cm 2 , the etch rate was 2.1 nm/min. Preferential sputtering of nitrogen and the presence of a distorted Ta 4f peak occurred following surface cleaning and during depth profiling. The steady state composition was TaN 0.85 and a total etch time of 213 mins was required to profile through the 435 nm TaN 1.5 layer. In contrast, fs-LA allowed retention of the correct chemical composition and chemical state information after surface cleaning and during depth profiling. At a laser energy of 250 μJ (0.52 J/cm 2 ), the ablation rate for TaN 1.5 was found to be approximately 19 nm/pulse. fs-LA offers many advantages as a new approach to surface cleaning and depth profiling of inorganic thin films and coatings, with the avoidance of chemical damage and rapidity of profile acquisition being particularly significant advances in the thin film and coatings field.