X-ray photoelectron spectroscopy investigation of MgAl 2 O 4 thin films for humidity sensors
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Giampiero Montesperelli and Enrico Traversa Dipartimento di Scienze e Tecnologie Chimiche, Universitd di Roma "Tor Vergata ", Via delta Ricerca Scientifica, 00133 Rome, Italy (Received 9 August 1993; accepted 9 February 1994)
l2O4 thin films, to be studied as active elements for humidity sensors, were deposited on Si/SiC>2 substrates by radio-frequency sputtering. This paper discusses the x-ray photoelectron spectroscopy (XPS) investigation of these films. XPS demonstrated that the thin films had a stoichiometry close to that of MgAl 2 O 4 . The evaluation of the modified Auger parameter a1 for Al gave structural information about the order of the crystalline structure of the thin films. The combination of Ar+ ion etching and XPS analysis showed the simultaneous presence of Mg, Al, and Si at the film-substrate interface. The thicknesses of the interfaces were calculated between 7 and 10 nm. The analysis of the binding energy (b.e.) values of the XPS peaks at different etching depths showed that O Is and Si 1p b.e. values were characteristic of a silicate at the interface, whereas in the substrate they were typical of silica. This suggests a chemical interaction took place between film and substrate with the formation of a silicate layer at the interface, which may be the cause of the good adhesion of MgAl 2 O 4 films to silica, as observed by peel tests with Scotch tape.
I. INTRODUCTION Monitoring and controlling ambient humidity is becoming ever more important, mainly for human comfort and for industrial processes.1 Particularly, in the last years the industrial use of humidity sensors has greatly increased in the areas of the production of electronic devices, precision instruments, and foodstuffs.2 For automated controlling systems, which are more widely diffused because of the lower cost of microprocessors and interface circuits and their improved reliability, humidity sensors using changes in electrical parameters are needed.3 For this purpose, a wide variety of materials have been used as sensing elements in humidity-measurement devices.4 The materials for commercially developed humidity sensors are mainly organic polymer films and porous ceramics, and require external signal processing circuitry. However, at the present time there are no universally suitable materials.1 Nevertheless, ceramic oxides have shown advantages in terms of mechanical strength, and thermal, physical, and chemical stability.5'6 The recent development of ceramic deposition technologies is an important step forward in ceramics for humidity sensors. In fact, recent trends in sensor production are toward silicon micromachining and on-chip integration technology.7 Miniaturization of sensors is thus required, which in turn requires microelectrodes. At the present time, chemical and physical technologies are 1426 http://journals.cambridge.org
J. Mater. Res., Vol. 9, No. 6, Jun 1994 Downloaded: 25 Mar 2015
available to deposit ceramic films.8 Greater miniaturization, ease of operation, and microprocessor compatible readouts are offered by
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