Verification of layered structures in Sno2/metal-based gas sensors by X-ray microanalysis: Comparison with X-ray photoelectron spectroscopy

The depth profile of thin film layers on bulk substrate, avoiding the cross -sectioning of samples, is commonly performed by techniques such as X-ray p hotoelectron spectroscopy (XPS), Auger electron Spectroscopy (AES), and sec ondary ion mass spectroscopy (SIMS). Techniques based on X-ray emission int ensity measurements by energy dispersive spectroscopy (EDS), with conventio nal matrix or ZAF correction, are normally applied to cross-sectioned sampl es. This article compares XPS with surface Xray intensity measurements by E DS, carried out with a more realistic X-ray generation and absorption model , known as the phi(rho Z) model. The phi(rho Z) approach has been adopted t ogether with Monte Carlo simulation for the proper selection of SEM acceler ating voltages, in conjunction with the analysis of SEM morphological image s for thin film density correction. The method discussed hereafter and comp ared with the XPS technique, has advantages of higher lateral resolution, n on-destructive elemental analyses, morphological visualization, low cost, a nd faster performance. This methodology has been followed to verify the lay ered structure of SnO2/metal-based gas sensors. X-ray intensities were meas ured using an EDS ultra-thin window detector. Two different porous layers, 25-nm thick of SnO2 and 10-nm thick of Cu, were detected, showing better ag reement with their nominal thickness compared to results obtained using XPS measurements where porosity affects XPS data. If confirmed to be reliable and as effective as XPS depth profiling, this technique may be adopted for process quality control purposes.