Characterization of a heat-insulating coating on floatglass by sputter-assisted EPMA

Abstract

Heat-insulating coatings on window glass consist of thin metallic films embedded in thin dielectric films. Such multilayer systems are produced commercially by physical vapour deposition in a continuous technique. For process control as well as for the development of new types of multilayer systems, surface-sensitive methods of analysis are essential to characterize the structure of the coatings in terms of the thickness and chemical composition of each layer. The present work reports on the first application of electron probe microanalysis (EPMA) in depth profiling of a complex heat-insulating coating on floatglass. It is well known that conventional EPMA is a powerful tool in microbeam analysis to quantify chemical compositions with high accuracy and to detect rather low concentrations even for light elements (B-F). However, the large information depth of x-rays (∼0.1-1 μm) limits the applicability to depth profiling. A great step forward in depth profiling in the submicrometre range was achieved by combining EPMA with surface removal by a sputtering procedure. Two various techniques can be applied: measurement of the emitted x-rays by scanning the electron beam across a sputter crater edge; or sputtering the material in situ with an integrated ion gun. In this work we succeeded in quantifying the film thickness (mass coverage) and chemical composition of each single component of the heat-insulating structure. The great advantage of the new method is its ability to determine the depth scale, as well as its remarkable detection limit. For example, in a depth of 100 nm a thin NiCr layer ∼1 nm thick could be detected. The Ni/Cr ratio of the layer was determined reproducibly to be 85: 15. The minimal thickness that could have been detected was estimated to be 0.15 nm.