Investigation of a Ta-Si-O/Ta-Si-N bilayer system for embedded SAW finger structures

This paper presents results of a study on an insulating bilayer system for the application of capping surface acoustic wave (SAW) structures with embedded finger electrodes. It consists of a stack of two different layers. A Ta-Si-O film followed by a Ta-Si-N film is deposited by physical vapor deposition (PVD) on top of different substrate materials. Due to the deposition parameters both layers contain large amounts of the reactive gas species (N or O) resulting in insulating behavior especially for the Ta-Si-O film. Properties and applicability of the bilayer were evaluated by different analytical techniques, e.g. electrical measurements, X-ray techniques, depth profiling, atomic force microscopy, and scanning electron microscopy. The breakdown strength and the relative permittivity of the bilayer were found to be respectable compared to conventional used dielectric materials. The barrier properties offered by the bilayer system are poor when heat-treated at high temperatures. The failure is mainly attributed to a Cu diffusion through the bilayer stack towards the surface.