  In this paper, we report on the use of tantalum and iridium as adhesion
layers for platinum electrodes used in high temperature SAW devices based on
langasite substrates (LGS). Unlike iridium, tantalum exhibits a great adhesive
strength, and a very low mobility through the Pt film, ensuring a device
lifetime of at least half an hour at 1000{\textdegree}C. The latter is limited
by morphological modifications of platinum, starting by the apparition of
crystallites on the surface, and followed by important terracing and breaking
of the film continuity. SNMS and XRD measurements allowed us to show that these
phenomena are likely intrinsic to platinum film, whatever be the nature of the
adhesion layer. Finally, after having outlined a possible scenario leading to
this deterioration, we consider some solutions that could replace platinum in
order to increase the lifetime of LGS-based SAW devices in high temperatures
conditions.
