Development of laser based procedures for stoichiometry analysis of ternary boride thin films

Abstract

The application of thin films via physical vapor deposition (PVD) offers enormous potential in modifying the surface of bulk materials to reach the requirements of various scientific and industrial applications. Traditional materials for hard protective coatings include nitrides and carbides. A rather new class of hard materials are borides of transition metals, which allow to combine hardness with ductility as well as chemical resistance. However, the mechanical/chemical properties of such boride coatings are closely linked to their elemental composition, control of the PVD process is therefore a prerequisite for the preparation of coatings with tailored properties. A difficult task since elemental fractionation effects can occur during sputter deposition, optimization of the PVD procedure requires therefore accurate determination of derived thin film composition.

Measurement of the composition of boride thin films is challenging, boron is hard to measure with X-Ray diffraction and X-Ray fluorescence analysis, the need of appropriate matrix matched standards or certified reference materials for quantitative determinations limits the applicability of most other solid sampling techniques. In literature also liquid analysis of sample stoichiometry is reported, for conversion of the solid material into a liquid procedures such as acid digestion, fusion or dry ashing are applied. These methods are well established but require a significant investment in sample handling, are prone for sample contamination and/or analyte losses, and provide only information on the average composition of the material.

Here we present two laser based approaches for the characterization of boride thin films, enabling analysis of sample composition but also homogeneity. Ternary boride thin films based on tungsten diboride alloyed with tantalum, aluminum of vanadium were prepared by PVD, determination of sample stoichiometry was accomplished using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) and laser induced breakdown spectroscopy (LIBS). For signal quantification the concept of self-aliquoting micro-grooves has been applied, which represents a further development of the "dried-droplet-approach" described in literature. This method allows to easily adapt matrix composition and dopant levels in account to the needs of the application thanks to the vast variety of available liquid standard solutions. Thus, with this approach the need of certified reference materials with similar composition could be circumvented. Conventional wet chemical analysis was used for verification of derived LA-ICP-MS and LIBS data, confirming accuracy and applicability of the proposed laser based procedures.