Ultrathin metal oxidation for vacuum monitoring device applications

Abstract

The oxide growth on thin metal films at room temperature has been investigated in terms of resistance change during oxidation. These data have been interpreted using the extended Cabrera-Mott theory of oxidation by Boggio. The resulting oxide thickness as well as the oxidation kinetics was found to depend on pressure. According to this dependence, oxidation of ultrathin metal films can be applied for monitoring the vacuum quality inside an evacuated environment. The performance of aluminum and copper sensing layers are compared with respect to sensor lifetime and response. Furthermore, the theoretically evaluated and resistively measured oxide thicknesses are verified by TEM studies.1 Introduction Ultrathin metal films are of great interest for many fields of application. Their electrical properties [1,2], morphology [3] and oxidation kinetics [4] were studied in terms of suitability as sensor. The latter aspect was summarized by Tellier [5] concerning sensing of gases, magnetic properties, thermal effects etc by thin metal films. Furthermore, great attention was dedicated to metal oxidation to understand the mechanisms and kinetics [6][7][8][9][10][11]. The early stages of oxidation were studied by ellipsometry, electron microscopy and electron diffraction as well as resistance measurements. The resistance-based method relies on the change in resistance due to the growth of a non-conductive oxide layer which reduces the electrical conductive cross section.The use of this resistance change as an indicator for an air leak is not state-of-the-art, but previous investigations [12,13] demonstrated the functionality of thin aluminum layers such as a sensor. A possible application area is as a quality monitor for a vacuum insulation panel (VIP). A VIP consists of evacuated microporous fumed silica bounded by a metallized polymer barrier film. The maintenance of low pressure inside the VIP is essential due to the strong influence on the insulation properties, in particular on the heat transfer coefficient. For example, if a VIP is integrated into a refrigerator, a mechanical damage of the sensitive barrier film increases the heat conduction and convective

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